Chemistry of Nanomaterials: Fundamentals and Applications [1 ed.] 0128189088, 9780128189085

Table of contents :
Cover
Chemistry of Nanomaterials: Fundamentals and Applications
Copyright
Contents
List of Contributors
Preface
Part 1: Introduction to nanomaterials
1 Introduction
1.1 What is nanoscience and nanotechnology?
1.1.1 Nanoworld
1.1.2 Nanoscience
1.1.2.1 Nanoscience in nature
1.1.3 Nanotechnology
1.1.3.1 From nanoscience to nanotechnologies
1.2 History of nanotechnology
1.2.1 Feynman talks on small structures
1.2.2 Emergence of nanotechnology
1.2.2.1 First generation (beginning ∼2000)
1.2.2.2 Second generation (beginning ∼2005)
1.2.2.3 Third generation (beginning ∼2010)
1.2.2.4 Fourth generation (beginning ∼2015–20)
1.3 Nanometer scale
1.3.1 Special at nanoscale
1.3.1.1 Quantum effects
1.3.1.2 Surface area-to-volume ratio
1.4 Nanoparticles
1.4.1 Types of nanoparticles
1.4.1.1 Natural nanoparticles
1.4.1.2 Anthropogenic nanoparticles
1.5 Nanomaterials
1.5.1 What are nanoparticles, nanotubes, and nanoplates?
1.5.2 Classification of nanomaterials
1.5.2.1 Zero-dimensional nanomaterials
1.5.2.2 One-dimensional nanomaterials
1.5.2.3 Two-dimensional (2D) nanomaterials
1.5.2.4 Three-dimensional nanomaterial
1.6 Applications and challenges in nanotechnologies
1.6.1 Applications
1.6.1.1 Nanotechnology in electronics
1.6.1.2 Nanotechnology in the production of energy
1.6.1.3 Nanotechnology in automobile industries
1.6.1.4 Nanotechnology in cosmetics
1.6.1.5 Nanotechnology in space technology
1.6.1.6 Nanotechnology in medicine
1.6.1.7 Nanotechnology in the textile industry
1.6.1.8 Nanotechnology in home appliances
1.6.1.9 Nanotechnology in the food industry
1.6.1.10 Nanotechnology in sports equipment
1.6.2 Challenges in nanotechnology
References
2 Quantum effects
2.1 Wave–particle duality
2.2 Electromagnetic waves
2.3 Energy quanta
2.4 The de Broglie hypothesis
2.4.1 Derivation
2.4.2 Implications of de Broglie hypothesis
2.5 Evidence for the wave nature of electrons
2.5.1 Davisson–Germer experiment
2.5.2 G. P. Thomson’s experiment
2.6 Heisenberg’s uncertainty principle
2.7 Quantum dots
2.8 Moore’s law
2.8.1 Moore’s second law
2.8.2 Ultimate limits of the law
2.9 Quantum tunneling
2.9.1 Tunneling through a single potential barrier
2.9.2 Applications
2.10 Exercise
References
Further reading
3 Interfaces and surfaces
3.1 Introduction
3.2 Surface physics and chemistry
3.3 Surface and interface
3.4 Surface modification
3.4.1 Methods of surface modification
3.4.1.1 Surface scratching/roughening
3.4.1.2 Surface patterning
3.4.1.3 Chemical surface modification
3.4.1.4 Thin films and surface coatings
3.4.1.5 Pharmaceutical attachment to surfaces
3.4.1.6 Drug delivery assistance by porous surface
3.5 Thin-film deposition
3.5.1 Deposition techniques
3.5.1.1 Physical vapor deposition
3.5.1.1.1 Vacuum deposition
Thermal evaporation
Electron beam evaporation
3.5.1.1.2 Cladding
Laser cladding
Explosion cladding
3.5.1.1.3 Sputtering
Magnetron sputtering
3.5.1.1.4 Arc welding
3.5.1.1.5 Thermal spraying
3.5.1.2 Chemical vapor deposition
3.5.1.2.1 Atmospheric pressure chemical vapor deposition
3.5.1.2.2 Low-pressure chemical vapor deposition
3.5.1.2.3 Metal organic chemical vapor deposition
3.5.1.2.4 Plasma-enhanced chemical vapor deposition
3.5.1.2.5 Atomic layer deposition
3.5.1.2.6 Electroplating
3.6 Self-assembly
3.6.1 Molecular self-assembly systems
3.6.2 Idea of molecular self-assembly
3.6.3 Equilibrium and nonequilibrium self-assembly
References
4 Properties of nanomaterials
4.1 Background history of subatomic particles
4.2 Subatomic physics to chemical systems
4.2.1 Types of chemical bonds
4.2.1.1 Ionic bonds
4.2.1.2 Covalent bonding
4.2.1.3 Metallic bonds
4.2.1.4 Van der Waals interactions
4.3 Properties of nanomaterials
4.3.1 Electrical properties
4.3.2 Mechanical properties
4.3.2.1 Hardness
4.3.2.2 Elastic modulus
4.3.3 Thermal properties
4.3.3.1 Heat capacity
4.3.3.2 Melting point
4.3.3.3 Coefficient of thermal expansion
4.3.4 Magnetic properties
4.3.5 Optical properties
References
Further reading
5 Tools and instrumentation
5.1 Microscopy
5.1.1 Brief history
5.1.2 Concept of microscopy
5.1.3 Optical microscopy
5.1.3.1 Simple microscope
5.1.3.1.1 Magnification of simple microscope
5.1.3.2 Compound microscope
5.1.3.2.1 Magnification of compound microscope
5.1.3.3 Limitations
5.1.3.4 Advantages and disadvantages
5.1.4 Various optical microscopic techniques
5.1.4.1 Bright-field microscopy
5.1.4.2 Dark-field microscopy
5.1.4.3 Phase contrast microscopy
5.1.4.4 Differential interference contrast microscopy
5.2 Electron microscopy
5.2.1 Electron interaction with material sample
5.2.2 Working of electron microscopy
5.3 Types of electron microscopy
5.3.1 Scanning electron microscope
5.3.1.1 Working of scanning electron microscope
5.3.1.2 Advantages of scanning electron microscope
5.3.1.3 Disadvantages of scanning electron microscope
5.3.1.4 Limitations
5.3.2 Transmission electron microscope
5.3.2.1 Working of transmission electron microscope
5.3.2.2 Advantages of transmission electron microscope
5.3.2.3 Disadvantages of transmission electron microscopes
5.3.2.4 Applications of transmission electron microscope
5.3.3 Dissimilarities between scanning electron microscope and transmission electron microscope
5.4 Scanning tunneling microscope
5.4.1 Components and workings
5.4.1.1 Various features of scanning tunneling microscope
5.5 Atomic force microscopy
5.5.1 Construction of atomic force microscope
5.5.1.1 Laser
5.5.1.2 Cantilever
5.5.1.3 Scanner
5.5.1.4 Photodetector
5.5.1.5 Feedback electronics
5.5.1.6 Sample
5.5.2 Working principle of atomic force microscope
5.5.3 Modes of operation
5.5.3.1 Contact/repulsive mode
5.5.3.2 Noncontact/attractive mode
5.5.3.3 Tapping/intermittent mode
5.5.4 Advantages and disadvantages
5.5.5 Applications
5.6 Fluorescence method
5.7 Synchrotron radiation
5.8 Atom probe instrument
5.8.1 Construction
5.8.2 Working of atom probe field ion microscopy
5.8.3 Mathematical analysis
5.8.4 Limitations of atom probe
5.8.5 Comparison with tunneling electron microscope and SIMS
References
6 Fabricating nanostructures
6.1 Introduction
6.2 Lithography
6.2.1 Photolithography
6.2.1.1 Steps of photolithography
6.2.1.1.1 Surface preparation
6.2.1.1.2 Oxidation
Types of photoresist
6.2.1.1.3 Masking
6.2.1.1.4 Exposing to UV light
Contact printing
Proximity printing
Projection printing
6.2.1.1.5 Etching
6.2.2 Electron beam lithography
6.2.2.1 Procedure
6.2.2.1.1 Coating by resist
6.2.2.1.2 Deposition of metallic layer
6.2.2.1.3 Aggressive solvent mixture
6.3 Molecular beam epitaxy
6.3.1 Molecular beam epitaxy process
6.3.1.1 Epitaxial growth of crystalline layers on substrate
6.3.1.2 Epitaxy types
6.3.1.2.1 Homoepitaxy
6.3.1.2.2 Heteroepitaxy
6.3.2 Working principle
6.3.3 Molecular beam epitaxy layout
6.3.4 Features of molecular beam epitaxy
6.3.5 Advantages and disadvantages of molecular beam epitaxy
6.3.6 In situ growth monitoring techniques
6.4 Self-assembled masks
6.4.1 Distinctive features
6.4.2 Order
6.4.3 Interactions
6.4.4 Building blocks
6.4.5 Examples
6.4.6 Properties
6.4.7 Self-assembly at the macroscopic scale
6.5 Focused ion beam
6.5.1 The construction of focused ion beam
6.5.1.1 The vacuum system
6.5.1.2 The liquid metal ion source
6.5.1.3 The ion column
6.5.1.4 The sample stage
6.5.1.5 The imaging detectors
6.5.1.6 Gas source usage or deposition
6.5.1.7 Dual platform
6.5.2 Principle
6.5.3 Applications of FIB
6.6 Stamp technology stamping
6.6.1 Operations
6.6.2 Stamping lubricant
6.6.3 Industrial applications
References
Part 2: Interactions in nanomaterials
7 Electrons in nanostructures
7.1 Introduction to electrons
7.1.1 Importance of electrons in bonding
7.2 Emission of electrons
7.2.1 Thermionic emission
7.2.1.1 Dependence of thermionic emission
7.2.2 Field emission
7.2.3 Photoelectric emission
7.2.4 Secondary electron emission
7.3 Variations in electronic properties of materials
7.3.1 Electrical properties
7.3.2 Optical properties
7.4 Electrons in nanostructures
7.4.1 Quantum effects of electrons in nanostructures
7.5 Free electron model
7.6 Bloch’s theorem
7.6.1 Implications of Bloch’s theorem
7.7 Band structure
7.7.1 Energetic bands
7.7.2 Band gaps
7.8 Single electron transistor
7.8.1 Operation of single electron transistor
7.8.2 Applications
7.8.2.1 Supersensitive electrometer
7.8.2.2 Single electron spectroscopy
7.8.2.3 Detection of infrared radiation
7.8.2.4 Charge state logics
7.8.2.5 Programmable single electron transistor logic
7.9 Resonant tunneling
References
8 Molecular electronics
8.1 Molecular electronics
8.2 Lewis structures
8.2.1 Limitations
8.3 Variational approach to calculate molecular orbitals
8.4 Hybridization of atomic orbitals
8.5 Donor acceptor properties
8.6 Electron transfer between molecules
8.7 Charge transport in weakly interacting molecular solids
8.8 Single molecule electronics
8.8.1 Theoretical background
8.8.2 Examples
References
9 Nanomaterials
9.1 Introduction of nanomaterials
9.1.1 Dimensionality
9.1.1.1 Zero-dimensional nanomaterials
9.1.1.2 One-dimensional nanomaterials
9.1.1.3 Two-dimensional nanomaterials
9.1.1.4 Three-dimensional nanomaterials
9.2 Quantum dots
9.2.1 Applications
9.2.1.1 Optical applications
9.2.1.2 Quantum computing
9.2.1.3 Biological applications
9.3 Nanowires
9.3.1 Synthesis
9.3.2 Properties of nanowires
9.3.2.1 Magnetic properties
9.3.2.2 Thermoelectric properties
9.3.2.3 Electron transport properties
9.3.2.4 Optical properties
9.3.3 Applications of nanowires
9.4 Nanophotonics
9.4.1 Optoelectronics and microelectronics
9.4.2 Basic principles
9.5 Magnetic nanostructures
9.5.1 Synthesis
9.5.2 Properties of magnetic nanostructures
9.5.3 Applications of magnetic nanostructures
9.6 Nano thermal devices
9.7 Nanofluidic devices
9.8 Biomimetic materials
References
Part 3: Applications of nanomaterials
10 Nanobiotechnology
10.1 Introduction to Nanobiotechnology
10.2 DNA microarrays
10.2.1 Principle
10.2.2 Applications
10.3 DNA assembly of nanoparticles
10.3.1 Uses
10.4 Protein and DNA assembly
10.4.1 Protein assembly
10.4.2 DNA assembly
10.5 Digital cells
10.6 Genetic circuits
10.7 DNA computing
References
11 Nanotechnology: the road ahead
11.1 Nanostructures
11.1.1 Nanoscaled biomolecules
11.2 Structure of carbon nanotubes
11.3 Quantum dots (QDs)
11.3.1 Properties of quantum dots
11.3.2 Fabrication of quantum dots
11.4 Energy harvesting and storage
11.4.1 Piezoelectric nanogenerators
11.4.2 Solar cells
11.4.3 Electrochemical energy storage
11.4.3.1 Rechargeable batteries
11.4.3.2 Supercapacitors
11.4.3.3 Fuel cell
11.5 Quantum informatics
11.5.1 Nanostructures in quantum informatics
11.5.1.1 Semiconductor nanostructures in quantum computation
11.5.1.2 Nanostructures for quantum information processing
References
Glossary
Index
Back Cover

Citation preview

Chemistry of Nanomaterials Fundamentals and Applications

Chemistry of Nanomaterials Fundamentals and Applications

Authored by

Tahir Iqbal Awan Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan

Almas Bashir Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan

Aqsa Tehseen Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2020 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-12-818908-5 For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Susan Dennis Acquisitions Editor: Anneka Hess Editorial Project Manager: Lena Sparks Production Project Manager: Debasish Ghosh Cover Designer: Miles Hitchen Typeset by MPS Limited, Chennai, India

Contents List of Contributors Preface

xi xiii

Part 1 Introduction to nanomaterials 1.

Introduction

3

Tahir Iqbal Awan, Aqsa Tehseen and Almas Bashir

2.

1.1 What is nanoscience and nanotechnology? 1.1.1 Nanoworld 1.1.2 Nanoscience 1.1.3 Nanotechnology 1.2 History of nanotechnology 1.2.1 Feynman talks on small structures 1.2.2 Emergence of nanotechnology 1.3 Nanometer scale 1.3.1 Special at nanoscale 1.4 Nanoparticles 1.4.1 Types of nanoparticles 1.5 Nanomaterials 1.5.1 What are nanoparticles, nanotubes, and nanoplates? 1.5.2 Classification of nanomaterials 1.6 Applications and challenges in nanotechnologies 1.6.1 Applications 1.6.2 Challenges in nanotechnology References

3 4 4 5 6 7 9 10 10 13 14 15 16 17 20 20 25 25

Quantum effects

29

Tahir Iqbal Awan, Almas Bashir and Aqsa Tehseen 2.1 2.2 2.3 2.4

Wave particle duality Electromagnetic waves Energy quanta The de Broglie hypothesis 2.4.1 Derivation 2.4.2 Implications of de Broglie hypothesis

29 30 30 32 32 35 v

vi

3.

Contents

2.5 Evidence for the wave nature of electrons 2.5.1 Davisson Germer experiment 2.5.2 G. P. Thomson’s experiment 2.6 Heisenberg’s uncertainty principle 2.7 Quantum dots 2.8 Moore’s law 2.8.1 Moore’s second law 2.8.2 Ultimate limits of the law 2.9 Quantum tunneling 2.9.1 Tunneling through a single potential barrier 2.9.2 Applications 2.10 Exercise References Further reading

35 36 37 37 39 39 40 41 41 43 46 47 47 49

Interfaces and surfaces

51

Almas Bashir, Tahir Iqbal Awan, Aqsa Tehseen, Muhammad Bilal Tahir and Mohsin Ijaz

4.

3.1 3.2 3.3 3.4

Introduction Surface physics and chemistry Surface and interface Surface modification 3.4.1 Methods of surface modification 3.5 Thin-film deposition 3.5.1 Deposition techniques 3.6 Self-assembly 3.6.1 Molecular self-assembly systems 3.6.2 Idea of molecular self-assembly 3.6.3 Equilibrium and nonequilibrium self-assembly References

51 53 54 55 55 65 67 78 79 80 81 81

Properties of nanomaterials

89

Muhammad Rafique, Syeda Hajra, Muhammad Bilal Tahir, Tahir Iqbal Awan, Almas Bashir and Aqsa Tehseen 4.1 Background history of subatomic particles 4.2 Subatomic physics to chemical systems 4.2.1 Types of chemical bonds 4.3 Properties of nanomaterials 4.3.1 Electrical properties 4.3.2 Mechanical properties 4.3.3 Thermal properties 4.3.4 Magnetic properties 4.3.5 Optical properties References Further reading

89 90 91 97 97 100 103 109 113 115 117

vii

Contents

5.

Tools and instrumentation

119

Aqsa Tehseen, Tahir Iqbal Awan, Almas Bashir, Sumera Afsheen and Muhammad Yaqoob Khan

6.

5.1 Microscopy 5.1.1 Brief history 5.1.2 Concept of microscopy 5.1.3 Optical microscopy 5.1.4 Various optical microscopic techniques 5.2 Electron microscopy 5.2.1 Electron interaction with material sample 5.2.2 Working of electron microscopy 5.3 Types of electron microscopy 5.3.1 Scanning electron microscope 5.3.2 Transmission electron microscope 5.3.3 Dissimilarities between scanning electron microscope and transmission electron microscope 5.4 Scanning tunneling microscope 5.4.1 Components and workings 5.5 Atomic force microscopy 5.5.1 Construction of atomic force microscope 5.5.2 Working principle of atomic force microscope 5.5.3 Modes of operation 5.5.4 Advantages and disadvantages 5.5.5 Applications 5.6 Fluorescence method 5.7 Synchrotron radiation 5.8 Atom probe instrument 5.8.1 Construction 5.8.2 Working of atom probe field ion microscopy 5.8.3 Mathematical analysis 5.8.4 Limitations of atom probe 5.8.5 Comparison with tunneling electron microscope and SIMS References

120 120 120 122 125 127 127 129 129 129 132

Fabricating nanostructures

153

135 135 136 137 139 141 142 143 144 144 145 146 147 147 149 149 149 150

Tahir Iqbal Awan, Muhammad Irfan, Mohsin Ijaz, Almas Bashir, Aqsa Tehseen and Sumera Afsheen 6.1 Introduction 6.2 Lithography 6.2.1 Photolithography 6.2.2 Electron beam lithography 6.3 Molecular beam epitaxy 6.3.1 Molecular beam epitaxy process

153 155 156 159 161 161

viii

Contents

6.3.2 6.3.3 6.3.4 6.3.5

Working principle Molecular beam epitaxy layout Features of molecular beam epitaxy Advantages and disadvantages of molecular beam epitaxy 6.3.6 In situ growth monitoring techniques 6.4 Self-assembled masks 6.4.1 Distinctive features 6.4.2 Order 6.4.3 Interactions 6.4.4 Building blocks 6.4.5 Examples 6.4.6 Properties 6.4.7 Self-assembly at the macroscopic scale 6.5 Focused ion beam 6.5.1 The construction of focused ion beam 6.5.2 Principle 6.5.3 Applications of FIB 6.6 Stamp technology stamping 6.6.1 Operations 6.6.2 Stamping lubricant 6.6.3 Industrial applications References

162 162 163 164 164 164 165 165 165 166 166 166 166 167 167 172 172 173 173 174 174 175

Part 2 Interactions in nanomaterials 7.

Electrons in nanostructures

179

Tahir Iqbal Awan, Almas Bashir, Aqsa Tehseen and Saliha Bibi 7.1 Introduction to electrons 7.1.1 Importance of electrons in bonding 7.2 Emission of electrons 7.2.1 Thermionic emission 7.2.2 Field emission 7.2.3 Photoelectric emission 7.2.4 Secondary electron emission 7.3 Variations in electronic properties of materials 7.3.1 Electrical properties 7.3.2 Optical properties 7.4 Electrons in nanostructures 7.4.1 Quantum effects of electrons in nanostructures 7.5 Free electron model 7.6 Bloch’s theorem 7.6.1 Implications of Bloch’s theorem 7.7 Band structure 7.7.1 Energetic bands

179 180 181 182 183 184 184 185 185 186 187 188 190 193 194 194 196

Contents

8.

ix

7.7.2 Band gaps 7.8 Single electron transistor 7.8.1 Operation of single electron transistor 7.8.2 Applications 7.9 Resonant tunneling References

197 198 199 200 202 204

Molecular electronics

207

Khalid Nadeem Riaz, Zainab Israr, Tahir Iqbal Awan, Almas Bashir and Aqsa Tehseen

9.

8.1 Molecular electronics 8.2 Lewis structures 8.2.1 Limitations 8.3 Variational approach to calculate molecular orbitals 8.4 Hybridization of atomic orbitals 8.5 Donor acceptor properties 8.6 Electron transfer between molecules 8.7 Charge transport in weakly interacting molecular solids 8.8 Single molecule electronics 8.8.1 Theoretical background 8.8.2 Examples References

207 209 210 212 213 215 216 217 217 218 220 222

Nanomaterials

225

Tahir Iqbal Awan, Anam Ahmad, Saliha Bibi, Aqsa Tehseen and Almas Bashir 9.1 Introduction of nanomaterials 9.1.1 Dimensionality 9.2 Quantum dots 9.2.1 Applications 9.3 Nanowires 9.3.1 Synthesis 9.3.2 Properties of nanowires 9.3.3 Applications of nanowires 9.4 Nanophotonics 9.4.1 Optoelectronics and microelectronics 9.4.2 Basic principles 9.5 Magnetic nanostructures 9.5.1 Synthesis 9.5.2 Properties of magnetic nanostructures 9.5.3 Applications of magnetic nanostructures 9.6 Nano thermal devices 9.7 Nanofluidic devices 9.8 Biomimetic materials References

225 226 227 228 229 229 230 234 235 236 240 240 241 246 248 251 253 261 266

x

Contents

Part 3 Applications of nanomaterials 10. Nanobiotechnology

273

Sumera Afsheen, Muhammad Irfan, Tahir Iqbal Awan, Almas Bashir and Mohsin Ijaz 10.1 Introduction to Nanobiotechnology 10.2 DNA microarrays 10.2.1 Principle 10.2.2 Applications 10.3 DNA assembly of nanoparticles 10.3.1 Uses 10.4 Protein and DNA assembly 10.4.1 Protein assembly 10.4.2 DNA assembly 10.5 Digital cells 10.6 Genetic circuits 10.7 DNA computing References

11. Nanotechnology: the road ahead

273 274 275 277 278 278 278 278 279 281 283 284 286

289

Muhammad Bilal Tahir, Muhammad Abrar, Aqsa Tehseen, Tahir Iqbal Awan, Almas Bashir and Ghulam Nabi 11.1 Nanostructures 11.1.1 Nanoscaled biomolecules 11.2 Structure of carbon nanotubes 11.3 Quantum dots (QDs) 11.3.1 Properties of quantum dots 11.3.2 Fabrication of quantum dots 11.4 Energy harvesting and storage 11.4.1 Piezoelectric nanogenerators 11.4.2 Solar cells 11.4.3 Electrochemical energy storage 11.5 Quantum informatics 11.5.1 Nanostructures in quantum informatics References Glossary Index

289 290 292 293 293 295 296 296 297 299 301 302 305 309 311

List of Contributors Tahir Iqbal Awan Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan Almas Bashir Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan Aqsa Tehseen Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan Sumera Afsheen Department of Zoology, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan Muhammad Bilal Tahir Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan Saliha Bibi Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan Mohsin Ijaz Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan Muhammad Rafique Department of Physics, University of Sahiwal, Sahiwal, Pakistan Syeda Hajra Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan Muhammad Yaqoob Khan Department of Physics, Kohat University of Science and Technology, Khyber Pakhtoonkhwa, Pakistan Muhammad Irfan Department of Biochemistry and Biotechnology, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan Khalid Nadeem Riaz Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan Zainab Israr Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan Anam Ahmad Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan Muhammad Abrar Department of Physics, Hazara University, Mansehra, Pakistan Ghulam Nabi Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan

xi

Preface The subject of Chemistry of Nanomaterials is an area of great interest for researchers and provides an opportunity to talk about the exclusive phenomena of nanoscience. Consequently, the topics of introduction, history of nanotechnology, and origin of nanotechnology are of unique interest for the readers. This book is divided in three sections, that is, an introduction to nanomaterials, interactions, and associated aspects in nanomaterials and applications of nanomaterials. Part 1 (Introduction to nanomaterials) contains six chapters. Chapter 1, Introduction, provides a comprehensive and integrated introduction to the chemistry of nanomaterials and presents various examples of nanodevices and their applications in the real world. Starting from the fascinating idea presented by Richard Feynman related to nanoscience on December 29, 1959, and in his article “There are plenty of rooms at bottom” in 1960 in which he come up with the idea of nanotechnology. The history of nanotechnology depicts that it has emerged as a helpful technology in several fields of science due to the unique properties of nanomaterials compared to bulk materials because at nanolevel scale, the principles of quantum mechanics govern. As nanotechnology brings a revolution in the industries so this book presents some important industries that are based on nanotechnology, including cosmetic, textile, medicine, food, space, and automobile manufacturers. Chapter 2, Quantum effects, presents the basic concept related to wave-particle duality and how de-Broglie gave the clue for the wave-like nature of electrons. A brief introduction of quantum dots and their applications are given, which are of great interest for researchers and scientists. The development of nanotechnology is based on Moore’s law that predicts the rate of miniaturization and the costs of nanostructured components. Surfaces and interfaces are the focus of Chapter 3, Interfaces and surfaces, since material properties can be desirably tuned by surface modification which leads to many applications in real life due to their great significance. Chapter 4, Properties of nanomaterials, describes that nanomaterials exhibit diverse electrical, optical, mechanical, and magnetic properties compared to bulk materials. The diversity in properties is due to having unique structures and bonding that depends on the size and dimensions of the nanomaterials. Chapter 5, Tools and instrumentation, describes the main tools and instrumentations that are required in the field of nanoscience and

xiii

xiv

Preface

Chapter 6, Fabricating nanostructures, presents the fabricating techniques of such nanostructures. Part 2 (Interactions in nanomaterials) contains three chapters. Chapter 7, Electrons in nanostructures, is about electrons in nanostructures and describes quantum effects of electrons. The variations in electronic properties such as electrical and optical properties of materials are also discussed. Bloch’s theorem is used to study the behavior of electrons and eigen energy states present in the periodic potentials of metal crystal. In Chapter 8, Molecular electronics, some important and basics concepts are discussed to understand the multidisciplinary subject of molecular electronics. Chapter 9, Nanomaterials, describes the various types of nanomaterials and their synthesis processes. Part 3 (Applications of nanomaterials) contains the final two chapters. Chapter 10, Nanobiotechnology, provides the tools constructed from nanotechnology to study biological systems. The development of nanoparticles and nanodevices serving as sensors for the detection of minor changes in the contentment or composition in the adjacent media, vehicles for drug delivery to the targeted sites inside the body, probes to investigate specific molecules, and molecular imaging techniques to diagnose diseases well before the time to prevent harm. Chapter 11, Nanotechnology: the road ahead, describes the road ahead toward the applications of nanostructures. Nanostructures play a major role in recent developments in the field of science and industry due to their potential for use in a vast number of applications. The applications of different nanostructures in quantum informatics, such as quantum computation and quantum information processing, are also presented. The applications of nanomaterials will open the new areas for readers and researchers to enter this field. This will provide a link among different disciplines and different fields. This book will enhance the understanding of readers in this emerging field of science. Tahir Iqbal Awan Almas Bashir Aqsa Tehseen

Chapter 1

Introduction Tahir Iqbal Awan, Aqsa Tehseen and Almas Bashir

Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan

Chapter Outline 1.1 What is nanoscience and nanotechnology? 1.1.1 Nanoworld 1.1.2 Nanoscience 1.1.3 Nanotechnology 1.2 History of nanotechnology 1.2.1 Feynman talks on small structures 1.2.2 Emergence of nanotechnology 1.3 Nanometer scale 1.3.1 Special at nanoscale 1.4 Nanoparticles

1.1

3 4 4 5 6 7 9 10 10 13

1.4.1 Types of nanoparticles 1.5 Nanomaterials 1.5.1 What are nanoparticles, nanotubes, and nanoplates? 1.5.2 Classification of nanomaterials 1.6 Applications and challenges in nanotechnologies 1.6.1 Applications 1.6.2 Challenges in nanotechnology References

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What is nanoscience and nanotechnology?

Nanoscience involves the phenomena and manipulation of materials at extremely small scales (comparable to the atomic and molecular scale) [1]. It provides the laws and principles for understanding the properties of materials at the atomic level. Nanoscience and nanotechnology are important globally for advanced and novel applications. Specifically, those dealing with scientific phenomena and efficient technologies in small size domains. Nanotechnology has an unbelievable ability to manipulate extremely small objects that cannot be seen through the normal optical microscope; there are advanced nanoscale machines, computers, and electron microscopes for this purpose [2]. It deals with materials existing in nanometers (nm) and with nanomaterials that exhibit unique and expressively enhanced physical and chemical properties. The relation between nanoscience and nanotechnology is clear from Fig. 1.1. Chemistry of Nanomaterials. DOI: https://doi.org/10.1016/B978-0-12-818908-5.00001-9 © 2020 Elsevier Inc. All rights reserved.

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Nanoscience

Physics

Providing basic laws and principles

Chemistry

Biology

Nanotechnology FIGURE 1.1 Illustration of the relationship between nanoscience and nanotechnology. Nanoscience provides the basic laws and principles at nanoscale for nanotechnology for different disciplines like physics, chemistry, biology, and so forth, and the aim of nanotechnology is to use materials at nanoscale to design different nanodevices.

Nanomaterials show novel properties in contrast with bulk materials [3,4]. They have higher surface-to-volume ratios than structures of equal volume but lesser surface area. Likewise, quantum phenomena become more vital when deciding on the qualities and properties of materials when materials are taken at nanoscale. The sizes, phases, and morphologies of nanomaterials have great impact on their properties and applications. To overcome this difficulty, many researchers have concentrated on governing the size, phase, and shape of nanomaterials[5]. Nanobiotechnology is considered as remarkable in its treatment approach because of its small molecular size, biochemical properties, physical properties, and wide range of applications [6]. This field is emerging day by day.

1.1.1

Nanoworld

Basically, the term nanoworld is a mixture of two different terms, i.e., nano and world. The nanoworld consists of four different fields, namely nanomaterials, nanometrology, electronic nanotechnology and nanobiotechnology. These fields have led scientists to the development of many new inventions [2].

1.1.2

Nanoscience

Nanoscience deals with objects of extremely small sizes ranging between 1 and 100 nm, which shows important progress in the past few years. Nanoscience studies and manipulates the materials of different scales such as molecular, macro molecular and atomic scales. Nanoscience pays particular attention to the effects, properties, and structures of materials at the

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nanoscale [7]. The production of nanoelectronic devices below 1 µm has remarkably changed our daily lives. So, old technologies could be made to work more efficiently if nanoscience is used [8].

1.1.2.1 Nanoscience in nature Nanoscience relates to numerous particles and phenomena in nature. Nanoscience involves tiny and common things. For example, water contains many tiny particles and is a natural part of this universe [7]. In a sense, nanoscience prevails everywhere from the bottom of the ocean to upper space, that is, as dust particles, microlens arrays, shark skin, sea spray, and fruit flies’ eyes. 1.1.3

Nanotechnology

It is the technology for governing and manipulation of nanoscale matter to fabricate unique products and materials with enormous great potential to change the world. It is basically the implementation of technology at the nanoscale (i.e., ,100 nm). This field includes biosciences, chemistry, physics, and mechanical engineering. It is the study of the design and characterization of different structures, devices, and systems by managing the size and shape of the materials used at the nanoscale [9]. The purpose of nanotechnology is to fabricate different types of structures with unique properties due to their small size. Researchers collaborate to share ideas, techniques, and tools relating to nanotechnology in different countries [8]. It is the main area of present research of material science, and deals with the manipulation, design, and synthesis of nanoparticles (NPs) or nanostructures with dimensions in the range of 1 100 nm. Nanotechnology is mushrooming daily in all domains of life such as food, cosmetics, chemical industries, light emitters, photoelectrochemical, nonlinear optical devices, catalysis, space industry, energy science, single electron transistors, reprography, drug gene delivery, electronics and so forth [8]. In the past few decades, many remarkable developments have been achieved in the field of nanotechnology with various techniques being developed to fabricate NPs of particular morphologies. Nowadays, research based on the synthesis of NPs gains much attention due to their novel properties (magnetic, mechanical, and optoelectronic). NPs can be manufactured using various methods including biological, chemical, and physical methods [8,10]. NPs show novel and improved properties that differ from their corresponding bulk materials due to their size. The intrinsic properties of nanostructures commonly depend upon their crystalline nature, morphology, and composition [11]. NPs have a greater fraction of surface atoms, which enhances surface-to-volume ratio. Nanomaterials, especially NPs, have a growing number of applications in various fields. NPs have unique physicochemical properties such as optical

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properties, antibacterial properties, catalytic properties, and magnetic and electronic properties. In the development of novel technologies, the synthesis of metallic (especially noble metals) and semiconductor NPs plays a key role in their potential applications [12]. NPs have strained the consideration of researchers due to their tremendous uses in the development of new technologies such as in medicine, electronics, and material sciences. They also have a wide range of uses in the domains of diagnostics, therapeutics, and microelectronics, as well as in antimicrobial applications and in high sensitivity bimolecular detection [8]. They have an inhibitory effect toward many microorganisms and bacterial strains usually present in industrial and medical routes. In the medical field, NPs have potential applications such as for the treatment of burns and open wounds, in creams containing Ag to prevent infection, and in skin ointments, implants, and medical instruments with Ag-impregnated polymers. In sport equipment, NPs-embedded fabrics are utilized [13].

1.1.3.1 From nanoscience to nanotechnologies In the past two decades, a lot of research work has been done on nanomaterials ranging from material science to biotechnologies and genetics. Nanoscience and nanotechnology are two versatile fields that connect structures and components that have unique properties as compared to the properties of the bulk materials used because of their size reduction. The reason for these exceptional properties involves the large number of surface atoms and the 3D confinement of electrons. The collaboration of these two fields has led to new inspiring scientific discoveries. Nanotechnology and nanoscience are linked with each other. Any research that involves objects with dimensions not more than 100 nm is related to nanoscience as well as nanotechnology. The applications of these fields lie in different areas of science, for example, physics, engineering, chemistry, and biology. The domains of nanoscience involve the identification of the structure of molecules and the description of their properties, while nanotechnology involves the study of methods for controlling the mass that produces molecules in order to get the required Teflon coating [14]. This facilitates the emergence of new and essential perceptions. Thus it can be said that nanotechnology is actually the result of nanoscience. Two advanced technologies that assisted in the advancement of nanoscience are the scanning electron microscope (SEM) and the scanning tunneling microscope (STM). These are capable of providing images with remarkable results, even when conditions are not satisfactory.

1.2

History of nanotechnology

It is interesting to note that nanoparticles were used a long time ago by craftsmen who did not know what they were doing, and one example of this

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is the “Lycurgus Cup,” which was created in the 5th century. This cup shows two different colors because one is under reflected light and the other one is under transmitted light. In the 1990s, some scientists were doing research on this important artifact. They discovered that its interesting color and properties were due to embedded nanoparticles of silver, gold, and copper in the glass. This glass continued to be used from the 6th century through to the 15th century. The word “nano” is a Greek word that means “dwarf.” Almost 50 years ago, an American physicist named Richard Feynman predicted the future of nanotechnology in a conference held in 1959. This talk was about describing molecular machine building with atomic precision. In this conference, he made an interesting comment, “Why is it not possible to write the entire 24 volumes of the encyclopedia Britannica on the head of a pin? [13],” which means that we can write the entire library onto the tip of a pen and a lot of opportunities came out of this statement by Feynman and, thus, started the history of nanoscience. His talk “There’s plenty of room at the bottom” became a roadmap for nanotechnology, which means that this type of technology can store a large amount of data in a small area. After that, nanoscience became a subject and it has developed enormously. In 1974, a Japanese scientist, Taniguchi, used the term nanotechnology in a journal based on his work on ion sputtering machines. After the term nanotechnology came about, people began trying to discover certain machines to see nanomaterials. In 1981, a machine called a scanning tunneling microscope (STM) was developed for this purpose. In 1985, a form of carbon and its structure called buckyballs were discovered; to see this type of material an STM can be used. In 1986, another instrument, the atomic force microscope (AFM) was invented [14,15]. Electron beam lithography, in the early 1970s, was utilized to make nanodevices and nanostructures in the extremely small range of 40 70 nm. IBM have been doing a lot of nanomaterial research using individual atoms; they made their logo using nanomaterials. Finally, in 1997, Cybex nanometer was founded, which was the first company producing nanomaterials. Afterwards, many novel devices were designed on the basis of this technology [15].

1.2.1

Feynman talks on small structures

In 1959, a scientist named Richard Feynman predicted the future of nanotechnology. He gave a talk called “There’s plenty of room at the bottom,” which became a roadmap for nanotechnology. In the past, glass windows with small quantities of different colored metal particles were prepared, for instance, gold particles produce a red color, silver gives glassy yellow color, and so forth. During the past two decades, major developments have taken place in nanotechnology and nanoscience. After his talk in 1960, Richard P. Feynman published an article (There’s plenty of room at the bottom) in which he

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provided a hypothetical idea about nanotechnology and nanomaterials [15,16]. All encyclopedias, ever written, could be stored in a cube with each side 0.02 inch long, if a bit of information consists only on 100 atoms. He also pointed out that all the encyclopedias ever written could be stored on a single dust particle. In 1974, the term “nanotechnology” was first defined by Norio Taniguchi to the scientific world in Tokyo. Nanotechnology describes the process of the consolidation, separation, and deformation of materials by one molecule or by one atom. Richard P. Feynman put forward the idea of nanotechnology and K. Eric Drexler promoted it. In 1986, he published a book entitled “Vehicles of Creation: The Coming Era of Nanotechnology.” A number of important inventions and discoveries were made for the further development of nanotechnology in the second half of the 1980s [16]. The number of applications and publications in the field of nanotechnology are increasing very sharply. In the United States, the first national scientific fund for nanotechnology started to operate in 1991. The National Nanotechnology Initiative of the United States was approved in 2001. In the first half of the 21st century, it should become a foundation for the national security and economy of the United States. In Japan, significant importance is given to the development of nanotechnology as it is in the United States. The Economic Association of Japan arranged a department of nanotechnology in support of their technical and industrial committee, in 2000. An agenda for nanotechnological research was established in 2001 [17]. The basic purpose of this agenda is to provide substantial capital investment for fabrication based on nanotechnology and to designate the basic direction of nanotechnology such as material technology, energy and environmental studies, biotechnology, and information technology [18]. To organize the effective cooperation between scientific, industrial and different state organizations, the national strategy of nanotechnology plays an important role for future development. According to the agenda of national programs, western European countries carry out research in nanotechnology. Research in nanotechnology under the auspices of the Federal Ministry of Education and Research, in technology and science is in progress in Germany. The development of nanotechnology is supervised by the National Physical Laboratory, for technology and physics research in England. The development of nanotechnology in France is defined by the National Center of Scientific Research. South Korea and China give special importance to the development of nanotechnology as an emerging field. C.I. S. countries started research on nanotechnology according to the agenda of state scientific program. Hence 1980 and 1990 are the starting years of the development of nanotechnology and the paradigm of nanotechnology was formed at the turn of 1960. The entire period up to 1960 was just the initial stages of green nanotechnology [19].

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1.2.2

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Emergence of nanotechnology

The primary idea was displayed in 1959 by the teacher of material science, Dr. Richard P. Feynman [20]. The term “nanotechnology” had been introduced by Norio Taniguchi in 1974 [21]. A nanometer is defined as a 1,000,000,000th part of a meter or the width of a human hair B80,000 nm. Innovation is actually the making, utilization, and learning of instruments, machines, and procedures, keeping in mind the end goal to take care of an issue or play out a particular ability. Nanoscience suggests different physical frameworks that can do wonders for the measurements at the nanoscale. The emergence of nanotechnology is portrayed point by point in this section. Fig. 1.2 highlights the generations of nanostructures. Starting from 2000, which was the first generation of nanostructures, it then reaches active nanostructures in 2010, which was the second generation on nanostructures. Passing through systems of nanosystems, that is, the third generation, it reaches molecular nanosystems, that is, the current and fourth generation of nanostructures.

1.2.2.1 First generation (beginning B2000) Passive nanostructures: This includes nanoparticles, nanocomposites, and massive nanomaterials. Nanostructures can be made from metals, polymers, and different bio-building pieces. Nanotech-based sunscreens and golf balls are recent developments in the field of cosmetics and athletes. Enhancements can be made through the utilization of zinc oxide or titanium oxide in sunscreen [22]. 1 : Passive nanostructures

(1st generation products)

a. Dispersed and contact nanostructures—e.g., aerosols, colloids b. Products incorporating nanostructures—e.g., coatings, nanoparticle reinforced compositesnanostructuredmetalspolymersceramics ~2000

2 : Active nanostructures a. Bioactive, health effects—e.g., targeted drugs, biodevices b. Physicochemical active—e.g., 3D transistors, amplifiers, actuators, ~ ~2005

3 : System of nanosystems e.g., guided assembling, 3D networking and new ~2010

4 : Molecular nanosystems

~2015–20

e.g., molecular devices ‘by design’,

FIGURE 1.2 Four generations of nanostructures with some examples. Reprinted from W. Goddard et al., Handbook of Nanoscience, Engineering, and Technology, vol. 51, 2003, with permission from Elsevier.

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1.2.2.2 Second generation (beginning B2005) Active nanostructures: This includes transistors, enhancers, coordinated prescriptions and chemicals, regular and non-natural sensors, actuators, and flexible structures. 1.2.2.3 Third generation (beginning B2010) System of nanosystems: In this generation, nanoparticles in a system perform an extensive number of operations [22]. These are called as system of nanosystems. 1.2.2.4 Fourth generation (beginning B2015 20) Molecular nanosystems: Each particle in a nanosystem has a unique structure and can perform various operations. The atoms and molecules can be used as devices in this generation.

1.3

Nanometer scale

Nanoscale is an extremely small scale where objects in the range of 1 100 nm are taken into consideration. For something to officially be a nanomaterial, at least one of its dimensions must be smaller than 100 nm [22,23]. In fact, most nanoscale materials are too small to be seen, even with the help of conventional microscopes. To look at a nanomaterial one has to use a better microscope, that is, an electron microscope, which can magnify samples up to 1 million times. Even then, to actually work on the nanoscale, you need something like the STM, which not only allows individual atoms and molecules to be seen, but also to be moved around. Compared to their larger counterparts, they often have better properties like increased strength, chemical reactivity, and conductivity [23]. That kind of precision engineering is a perfect application of nanotechnology. Fig. 1.3 shows the range of sizes in which nanotechnology plays a role in providing excellent properties to different materials at the nanometer scale. As the size reduces to the nanoscale, new properties of matter emerge, and many new challenges appear in measuring and fabricating matter on that scale. This field enables us to know and control the exclusive properties of matter and living systems at the nanoscale. The developments in nanoscience are forwarding many new inventions in communications, medicines, quantum computing, environmental science, and forensics [23,24].

1.3.1

Special at nanoscale

From the size of our airplanes to the height of our skyscrapers, feats of engineering just keep getting bigger and bigger. But bigger is not always better. Sometimes, one wants things to be small. If one wants things to be really

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FIGURE 1.3 Size range in which nanotechnology plays a role. Reprinted from J. Majoros et al., Progr. Molecul. Biol. Translat. Sci. (2010) 193 236, with permission from Elsevier.

small then one needs to make them out of small materials, that is, nanomaterials. These are used everywhere from the healthcare industry to electronics, and even though they are small in size, they pack a big punch. The most important problems of today are actually microscopic. Think of things like cancer or autoimmune diseases. These are issues at the cellular level; far too small to solve with any conventional tools [24]. This can be solved using nanomaterials. Different researchers deal with nanoparticles at different scales.

1.3.1.1 Quantum effects In order to understand the quantum confinement size effects on the properties of nanostructures, one has to know the field of nanotechnology. After studying this, you will be able to learn (1) electronic organization of bulk semiconductors; (2) electronic transitions in bulk semiconductors; and (3) the electronic structure of semiconductor nanostructures, that is, a top-down approach. This module discusses the evolution of quantum size effects and their effects on the electronic structure of semiconducting nanostructures. The quantum confinement effect has been introduced via two approaches, that is, a top-down and a bottom-up approach. So, this confinement refers to the spatial confinement of electrons in a nanomaterial, rather than their free existence within the bulk materials. The effects of spatial confinement are different for different materials and their properties because they vary with the characteristic length scale of the property that is eventually shaped by a material’s structure and composition [25]. The significant length scale here is the exciton or electron hole pair, and Bohr’s radius, ao , is a dimension that describes the spatial extent of the excitons in a semiconductor that is between 2 and 15 nm based on the type of material. This spatial extension of the excitons inside semiconducting nanostructures results in the effect of quantum confinement. So, appreciating its cause and impact on the electronic

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structure of a semiconductor nanostructure is a vital aspect of nanoscience. The understanding of the electronic structure of bulk materials is a prerequisite. This confinement begins at different nanocrystal sizes for different semiconductors as ao varies broadly within semiconductor nanomaterials. It should be noted that ao and band gap, Eg , are interrelated. On the contrary, insulators are described by extremely confined excitons and, therefore, possess short ao and, thus, get influenced by the quantum confinement effect for the sizes already in the cluster reach.

1.3.1.2 Surface area-to-volume ratio A ratio gives the relationship between two quantities and can be used for comparisons. As an object increases in size, its outside volume and surface area also increase. However, its inside (volume) increases at a faster rate than its outside (surface area). Therefore the ratio of surface area to unit volume decreases with increasing size. Alternatively, in nanotechnology, as an object gets smaller, its surface area gets larger compared to its volume. Nanomaterials have relatively large surface areas and surface atoms in contrast with bulk materials [26]. The strength and electrical properties of materials are also affected at the nanoscale because the materials become more chemically reactive at this scale as compared to in bulk in which these are nonreactive. A clear difference in surface area between bulk and nanolevel materials is shown in Fig. 1.4. Nanomaterials are certainly small; but they actually have a comparatively large surface area. a. The ratio 6:1 means that a total of 6 square surfaces per unit volume are exposed for interaction with the outside environment for every unit of cube volume.

FIGURE 1.4 The surface area for nanoparticles is extremely large as compared to a bulk particle. Consider a bulk particle (on the left) with a volume and surface area of 1 m3 and 4.836 m2 respectively, there is a huge difference in total surface area having the same total volume (on the right), that is, 4836 km2. Reprinted from G.L. Hornyak et al., Nanosci. Dermatol. (2016) 15 29, with permission from Elsevier.

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6 surfaces per unit volume per unit time

b. The ratio 3:1 means that a total of 3 square surfaces per unit volume are exposed for interaction with the outside environment for every unit of cube volume. Rate of Interaction 5

3 surfaces per unit volume per unit time

c. The ratio 2:1 means that only a total of 2 square surfaces are exposed per unit volume for interaction with the outside environment for every unit of cube volume. Rate of Interaction 5

2 surfaces per unit volume per unit time

If the largest cubes were placed in an environment where their surfaces could interact, the surface area-to-volume ratio would indicate their rate of interaction with the environment per unit volume per unit time [26].

1.4

Nanoparticles

NPs are particles with a diameter size less than 100 nm. Therefore they are zero dimensional (0D) nanomaterials. Here a noteworthy term is introduced, namely “colloids” having one dimension between 1 nm and 10 µm NPs are taken from the subdivision of colloids. NPs are particles that are not exactly 100 nm in measurement, there sizes differ between 1 and 100 nm. But some analysts consider NPs to be under 10 nm. NPs show unique properties from the same type of bulk materials [27]. The properties differ as the particle size varies. In order to control the extent of NPs, one thing that is necessary to determine is the end goal in order to enable materials to have unique properties. The exceptional properties of materials at the nanolevel are because of their large surface area and quantum effects. Nanostructured materials and NPs have acted as a source of inspiration for a long time in the universe of science and technology because of their applications in photonics, imaging, photography, and for surface analysis. Moreover, the optical properties of NPs also differ from that of bulk crystal despite them having the same structure [28]. NPs are classified into two groups, namely organic NPs having carbon NPs such as fullerenes and a small fraction of inorganic NPs that are composed of metal NPs like silver, gold, magnetic NPs, and semi-conductors NPs such as zinc oxide and titanium. In the 16th century, gold NPs were utilized for different purposes, one of them involving recoloring [29].

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1.4.1

Types of nanoparticles

The types of nanoparticles are shown in Fig. 1.5.

1.4.1.1 Natural nanoparticles Naturally occurring NPs are those that exist naturally within the size range of 1 100 nm and are found in organic as well as inorganic materials [30]. Organic: Fungi, coal, bacteria, and many others. Inorganic: Carbonates, silicates, metal sulfides and oxides, and so on. 1.4.1.2 Anthropogenic nanoparticles Anthropogenic NPs are those that do not occur naturally in the universe. In order to have them, different chemical reactions are performed in a laboratory. In both organic and inorganic materials these NPs are found. Organic: Soot, fly ash, and carbon nanotube Inorganic: SiO2, TiO2, and ZnO. Anthropogenic NPs are further divided into two types, that is, incidental and engineered NPs [11]. Incidental NPs are formed unintentionally as a result of different manmade industrial processes. Usually, they are made from the different elements and they do not have well-defined shapes and sizes. In daily life, the accidental NPs are formed by the following examples [12,30]. Examples G G G

Cooking smoke Diesel exhaust Welding fumes

Nanoparticles

Anthropogenic nanoparticles

Incidental nanoparticles FIGURE 1.5 Classification of nanoparticles.

Natural nanoparticles

Engineered nanoparticles

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Industrial effluents Sandblasting

Engineered NPs are intentionally created NPs for different purposes. Engineered NPs have size dimensions ,100 nm. Engineered NPs can also exist in different dimensions with reference to their size, that is, 0D, 1D, 2D, and 3D. Nanomaterials having all three dimensions (length, width, and height) less than 100 nm are called 3D nanomaterials, for example, NPs and nanoshells. Nanomaterials having two dimensions (e.g., length, width or height, length, etc.) less than 100 nm are called 2D nanomaterials, for example, nanowires and nanotubes [30]. Nanomaterials having one dimension (length, width, or height) less than 100 nm are called 1D nanomaterials, for example, nanosheets, thin films, and layers. Proteins, nucleic acids, and carbohydrates are also included in 1D nanomaterials. Examples G G G G

Quantum dots (0D) NPs, nanoshells, and microcapsules (3D) Nanotubes, fibers, and nanowires (2D) Nanosheets, thin films, layers, and coatings (1D)

1.5

Nanomaterials

Nanomaterials are materials that are studied and synthesized at the nanoscale. Nanoscale materials are those whose at least one dimension is greater than the nanoscale (,100 nm). They often require entirely different methods from that of simple materials. To change the dimensions of nanomaterials researchers are adopting different methods. These methods can change the chemical as well as physical properties of nanomaterials in order to utilize them in different fields of daily life sciences. Nanomaterials can be synthesized using two approaches, that is, a bottom-up and a top-down approach. The methods of the mentioned approaches are biological, chemical, and physical synthesis, using which, low dimensional NPs can be fabricated. Nanomaterials have the ability to be developed in different ways, through which various products and materials are created. Another fascinating aspect of nanomaterials is that they enhance several properties of the materials used, that is, electrical, optical, and magnetic [31,32]. Nanomaterials can be standalone materials, amorphous, polycrystalline, metallic, polymeric, or ceramic. Due to these promising properties, they have the ability to provide excessive control in medicine, electronics, and many other fields. Some nanomaterials arise naturally and other nanomaterials are engineered due to the need for relevant properties for numerous profitable products and processes. To fabricate nanomaterials of various dimensions, some physical and chemical methods are given in Table 1.1.

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TABLE 1.1 Types of nanomaterials dimensionally and physical/chemical methods to fabricate the different nanomaterials (0D, 1D, 2D, 3D). Nanomaterials

Fabrication (physical method)

Fabrication (chemical method)

Nanorods

Radiofrequency thermal vibration

By solvothermal method

Quantum dots

By evaporation

Synthesize by wet chemical method

Nanotubes

Chemical vaporization thermally

By electrochemical process

Nanoflower

Evaporate thermally

By solvothermal process

Nanomaterials are being successfully synthesized by researchers because of their astonishing properties and applications. They have been used for electronic, antimicrobial, and photocatalytic applications. Food products can be saved using NPs having antimicrobial and antibacterial activity. In other words, nanomaterials increase the efficiency of appliances and are beneficial for the environment and human health. They provide a clean environment by cleaning water and impurities with their photocatalytic application. Also, they are being used for new applications to enhance the efficiency and reduce the cost of daily objects. Further, nanomaterials are proven to possess toxic effects for microorganisms that are harmful to human health. Nanocomposites and nanocoatings are finding uses in various consumer products like sports equipment, windows, bicycles, and automobiles [32].

1.5.1

What are nanoparticles, nanotubes, and nanoplates?

Particles having a size less than 100 nm are usually considered to be NPs. The properties of nanomaterials are entirely different from the bulk materials. NPs are known as important in the research field because of their astonishing properties. The properties of NPs include high reactivity, large surface area, photocatalytic activity, photoluminescence, antimicrobial activity, anticorrosive activity, UV-protection, antioxidant activity, and low reduction potential. These important properties have many applications in solar cells, gas sensors, photodiodes, photocatalysts, transparent electrodes, and photoelectric devices, and so forth [33 37]. Materials are made up of small particles that may further consist of a large number of particles. The size of these particles is extremely important because we cannot see them with the naked eye. There are many types of materials, but nanomaterials are of great interest today. Nanomaterials are

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defined as those materials with at least one dimension in the nanometer range (1 100 nm). These nanomaterials have low weight and high strength. Some nanomaterials are naturally present and some are now fabricated. The surface-to-volume ratio of nanomaterials is extremely large. These materials have great importance in many applications today like in electronics, energy production, the textile industry, sports, diagnostic techniques, drug delivery, cosmetics, paints, varnishes, stain resistor, and so forth. Nanocomposites and nanocoatings are used in naturally occurring nanomaterials like protein, viruses, bacteria, wing of bee, etc. [32], but nanomaterials are artificially synthesized for commercial usage. These nanomaterials are helping mankind in many ways and are very useful for the welfare of mankind. Nanomaterials are basically classified on the basis of whether they are naturally occurring nanomaterials or manmade nanomaterials. Nanomaterials that are present in nature are known as natural nanomaterials, for example, viruses, antibodies, protein, bones, blood, milk, and so forth. Materials that are fabricated in laboratories with a well-organized method are known as artificial nanomaterials, for example, nanowires, nanotubes, quantum dots, semiconductor nanomaterials, and so forth. There is another way to classify nanomaterials that is based upon their families, for example, metal-based nanomaterials, dendrimers, nanocomposites, and carbon-based nanomaterials. Metal-based nanostructures contain metals and/or oxides of metals. Dendrimers are materials that have a branched 3D structure like a tree. It can be synthesized by two methods, namely a divergent and a convergent method. Nanocomposites are materials that are represented in the form of repeating units and each unit has a dimension less than 100 nm. Carbonbased materials played a pivotal role in the emergence of nanotechnology [32,38]. There is another classification that depends upon dimensions, namely 0D material, 1D material, 2D material, and 3D material.

1.5.2

Classification of nanomaterials

Nanomaterials can be classified into different types on the basis of their dimensions as shown in Fig. 1.6. All the dimensions are within nano range. The idea of the classification of nanomaterials was first given by Gleiter in 1995, and in 2000, it was reinforced by Gleiter and Skorokhod, but their concept did not illustrate a classification with respect to dimensions, which was later described by Pokropivny and Skorokhod [39] and is described as follows.

1.5.2.1 Zero-dimensional nanomaterials The main discrimination between nanomaterials is based on their dimensions. In zero dimension, all the dimensions lie within the nanoscale, but less

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FIGURE 1.6 The classification of nanomaterials with respect to dimensions (0D, 1D, 2D, and 3D). Reprinted from V. Shukla et al., Nanoelectronics (2019) 399 431, with permission from Elsevier.

than 100 nm. These are also known as NPs due to their small size. Today, many procedures and methods are adopted to artificially prepare 0D nanomaterials because it is an emerging field of science. Zero-dimensional nanomaterials could be prepared by various chemical and physical procedures. Examples of 0D nanomaterials include quantum dots, nanoclusters, nanospheres, and so forth [40]. These nanomaterials are used in medicine, energy, and so forth. Some 0D nanomaterials, for example, NPs and nanospheres are sketched in Fig. 1.7A C. Zero-dimensional materials are comprised of these properties: 1. Present both in crystalline and amorphous form. 2. Are of many shapes and kinds. 3. Should be in ceramic, polymeric, and metallic form.

1.5.2.2 One-dimensional nanomaterials Over the past few years, 1D nanomaterials have become necessary in many research fields and have various applications in society. They are mostly used in electronic and optical devices. The most common examples of 1D nanomaterials are nanowires, nanorods, nanoribbons, and so forth. The main thing in 1D nanomaterials is that there are two dimensions in the nanoscale and one lies in the macroscale. These materials have a pointed needle-like structure. One-dimensional nanomaterials are of great importance in many

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FIGURE 1.7 (A C) show 0D nanomaterials, for example, nanoparticles and nanospheres; (D F) show 1D nanomaterials, for example, carbon nanotubes and nanowires; (G I) show 2D nanomaterials, for example, graphene-based composites, carbon-coated nanoplates, and carbon nanobelts; and (J L) show 3D nanomaterials, for example, some composite electrodes. Reprinted from R. Liu et al., Heterogeneous nanostructured electrode materials for electrochemical energy storage, Chemical Commun. (1996), with permission from the Royal Society of Chemistry.

industries [38]. Some 1D nanomaterials, for example, carbon nanotubes and nanowires are sketched in Fig. 1.7D F. One-dimensional nanomaterials should have these properties: 1. They are found in both impure and pure form. 2. They are present as individual crystals or in matrix form. 3. They are present in metallic polymeric and crystalline forms.

1.5.2.3 Two-dimensional (2D) nanomaterials The materials whose at least two dimensions are in the nanoscale are called 2D nanomaterials. Their shape is like a disk. Examples of these types of nanomaterials include nanomaterial prisms, nanomaterial disks, nanomaterial deposited sheets, and so forth. It has become an emerging field in material sciences. Their properties are different from bulk matters. This opens a new research field for researchers. Their practical applications include 2D nanomaterial sensors, nanomaterial reactors, nanomaterial containers, nanophotocatalysts, and so forth. Some 2D nanomaterials, for example, graphene-based composites, carbon-coated nanoplates, and carbon nanobelts are sketched in Fig. 1.7G I.

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PART | 1 Introduction to nanomaterials

A 2D nanomaterial is comprised of these properties: 1. Their structure should consist of many layers. 2. They can be deposited on any material. 3. They should be ceramic polymer or crystalline [41].

1.5.2.4 Three-dimensional nanomaterial In this type of nanomaterial, every dimension is out of the nanoscale region, that is, .100 nm. No dimension of 3D nanomaterials is confined to the nanometer scale and they are considered to be bulk materials with the features of nanomaterials. In a bulk material, individual particles must lie on the nanometer scale, for example, a bunch of nanowires, nanotubes, or nanorods. Table 1.1 describes which nanomaterials are fabricated physically and which are fabricated chemically. These structures should be different from each other. They consist of multiple nanolayers. Some scientists consider these to be nanomaterials and some do not. Their practical applications are in energy and medicine, and so forth. They have been fabricated for 10 years [42]. The rate of absorption is greater because of their large surface area. They are also used in electronic devices. Some 3D nanomaterials, for example, some composite electrodes are sketched in Fig. 1.7J L.

1.6 1.6.1

Applications and challenges in nanotechnologies Applications

Nanotechnology has a wide field of applications; some of these are described in Fig. 1.8. Some applications of nanotechnology are described briefly in this section 1.6. Nanomaterials are utilized in sporting equipment, boat exteriors, and locomotive parts as light weight and strong materials can be manufactured [43]. Nanomaterials are also used in cosmetic products like sunscreens, makeup, and so forth. When weight and size are concerned, nanostructured products can be used. For example, while trying to decrease heat loss from an antique and insulating long pipelines in distant places. By reducing waste and saving electricity, chemical manufacturing processes can be made more proficient with the help of nanostructured catalysts [44]. Using nanoceramics, it becomes possible for dentists to fill holes in teeth as their chemical and mechanical properties can be modified to fascinate other bones from nearby to build an original new bone. Various pharmaceutical products have been developed again with nanosized particles to enhance captivation and make them easy to manage [45]. The medical field has been improved by the use of NPs in the drug delivery process. The delivery of drugs is possible to specific parts of an organism using

Introduction Chapter | 1

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Electronics

Energy production

Stain resistors

Textile industries

Varnishes Applications of nanotechnology Paints

Sports

Cosmetics Drug delivery

Diagnostic techniques

FIGURE 1.8 Nanotechnology finds applications in every field of life. Some of these applications are given here.

NPs. The reproduction of damaged tissue can be done by injecting the necessary drug into the required area of the body and is only possible using nanotechnology. This method is also cost effective and has no side effects. The repair of damaged tissue, known as tissue engineering, has replaced the traditional methods of implantation or transplantation; one example of such a process is the use of carbon nanotubes for the growth of bones [46,47].

1.6.1.1 Nanotechnology in electronics In electronics, nanotechnology plays a pivotal role. A revolution in this field comes by reducing the size of electronic devices. Nanotechnology is enhancing the capabilities of the devices that are used in electronics [48]. This increase in capabilities comes by decreasing the size, power consumption, and weight. Some areas of electronics have space for work and still require the attention of researchers. The production of nanomaterials by controlling their size is vital for their use in chips, lighting displays, lasers, batteries, fuel cells, and photovoltaic cells. Such nanomaterials are prepared to enhance the storage capabilities of microchips of computers. NPs are used in the display technology for the bright display in computers and televisions, for example, cadmium sulfide, nanocrystalline lead telluride, and zinc selenide utilized in LEDs. The separate plate of NPs in batteries can store more energy as compared to the other ordinary batteries [49].

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PART | 1 Introduction to nanomaterials

1.6.1.2 Nanotechnology in the production of energy Energy is necessary for all human activities. In the present era, energy consumption is greater than energy production from different sources. It is the need of the hour to use more effective methods for energy production such as nano-based technology. The use of nanotechnology can provide a higher efficiency. It is environment-friendly, so there are minimum chances of environmental pollution. The properties of materials are changed at the nanoscale, which is useful for the production of renewable energy. The application of nanotechnology includes fuel cells, lithium-ion batteries, LEDs, and solar cells. Nanotechnology-based energy production methods have the lowest production costs, but are higher in efficiency. It is the need of the hour to utilize this technology for the benefit of human beings because 20% of the world’s population suffer from energy availability problems. This can be solved using nano-based techniques, therefore, it should be given priority in the energy sector. As most countries are trying to produce more and more energy, nanomaterials are helping society to produce energy such as hydrogen energy, solar energy, and so forth. A schematic of nanostructured solar cells is given in Fig. 1.9. 1.6.1.3 Nanotechnology in automobile industries Nanotechnology in the automobile industry is used in several ways, for example, for the formation of long lasting and strong parts for vehicles. For good and symmetric paint work on cars and other vehicles, NPs are used to spray through a nozzle.

FIGURE 1.9 A schematic representation of nanostructured solar cells for the production of energy. Reprinted from E. Serrano et al., Nanotechnol Sustain. Energy (2009) 2373 2384, with permission from Elsevier.

Introduction Chapter | 1

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1.6.1.4 Nanotechnology in cosmetics Nanotechnology is widely used in cosmetic materials. NPs are capable of absorbing unwanted radiations. For example, zinc oxide NPs are extremely small in size so they are used as ultraviolet radiation absorber for skin protection. NPs of titanium oxide work the same as zinc oxide; they are used in antiaging creams. Some cosmetics nanoproducts that are available on the market are shown in Fig. 1.10. 1.6.1.5 Nanotechnology in space technology Nanotechnology plays an important role in the aerospace industry. One requirement is to build structures that are lighter in weight and can withstand any situation. It is possible through nanotechnology as NPs are small in weight and have high strength. They can remain firm under low temperatures and high pressures. 1.6.1.6 Nanotechnology in medicine Nanotechnology is widely used in the field of medicine for both surgeries and diagnostic purposes as well as in drug delivery. Nanorobots are used for diagnostic purposes and for surgeries. They are highly used in cancer treatment

FIGURE 1.10 Use of nanotechnology in different cosmetic products that are available on the market. Reprinted from S. Nanda et al., Nanobiomater. Galenic Formulat Cosmet. (2016) 47 67, with permission from Elsevier.

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PART | 1 Introduction to nanomaterials

and to check blood sugar levels, and so forth. Nanotechnology is being used in diagnostics as well as treatment areas. Houston-based Nanospectra Biosciences has been building up another treatment utilizing a blend of gold nanoshells. Lasers are utilized to annihilate growing tumors with heat. In the diagnostics region, nanosensors can distinguish, recognize, and evaluate the organic substances in the body [50].

1.6.1.7 Nanotechnology in the textile industry Textile industries are now trying to prepare cloth that has anti-dirt capabilities and so dust particles cannot be trapped in it. Stain-proof clothes are also prepared, and for this purpose, silver nanoparticles are used as antibacterial particles. Stains can also be removed from clothes by using NPs as shown in Fig. 1.11. The properties of natural materials are being modified via producers who are including nanosized parts to regular materials to enhance execution. For instance, several garment producers are producing water and stain repellent clothes by utilizing nanosized hairs in the texture. 1.6.1.8 Nanotechnology in home appliances There is an effective influence of nanotechnology in households. The applications of nanotechnology in households consists of antibacterial furniture, antibacterial coatings in appliances, solar cells, antiseptic sprays, the use of NPs in refrigerators, air purifiers, smoothening of household equipment, washing machines, dishwashers, and so forth. In washing machines, a silver plate releases silver ions through an electrolysis process during the wash cycle. This machine was launched in 2003. Silver ions also kill bacteria to detoxify wastewater [51]. The variety of NPs has increased through technological advancement and human activity. Silver NPs suppress the respiration

FIGURE 1.11 A glance at a stain-proof shirt. Reprinted from Ali K. Yetisen et al., Nanotechnol Textiles (2016), with permission from American Chemical Society.

Introduction Chapter | 1

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of bacteria during interaction with bacteria. This affects bacterial cell growth and metabolism. There is a coating of silver NPs in the inner surfaces of refrigerators that controls airborne bacteria when air circulates due to its antibacterial and antifungal property [52].

1.6.1.9 Nanotechnology in the food industry In the food industry, nanotechnology plays an important role. It is used in dietary supplements and for packing purposes. Nanotechnology makes it possible to save food for some months. For packing, silver NPs are widely used because they have an antibacterial nature. Silver (Ag) NPs have antimicrobial and antibacterial properties. Ag NPs are toxic to algae [53]. Other antimicrobial NPs consist of TiO2 and ZnO. The antimicrobial activity of TiO2 NPs is only active in the presence of ultraviolet light, but not in the dark. Food packaging is also done by combining different antimicrobial incorporation. Packaging systems include chlorine dioxide and sulfur dioxide, which release antimicrobials [54]. 1.6.1.10 Nanotechnology in sports equipment Nanotechnology is also helpful in the sport industry as it uses carbon nanotubes to make strong equipment, for example, cricket bats, tennis racquets, and so forth. 1.6.2

Challenges in nanotechnology

There are many challenges in the field of nanotechnology, but a few of them are more important than others. We know that nanomaterials are beneficial due to their unique properties and applications, but if they are misplaced or mishandled then they can be extremely harmful and destructive. Therefore a challenge is to develop such instruments that can measure the quantity of mishandled nanomaterials in the environment. Some nanomaterials are highly toxic and can cause serious environmental and health problems. To develop different methods and techniques to measure the toxicity of nanomaterials is also a challenge. Nanomaterials that are newly fabricated must become a prime focus for researchers in order to know and predict their potential impact.

References [1] M.C. Roco, Nanoparticles and nanotechnology research, J. Nanopart. Res. 1 (1) (1999) 1 6. [2] W.A. Goddard III, et al., Handbook of Nanoscience, Engineering, and Technology, CRC Press, 2007. [3] N. Feltin, M. Pileni, New technique for synthesizing iron ferrite magnetic nanosized particles, Langmuir 13 (15) (1997) 3927 3933.

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[4] M. Pileni, L. Motte, C. Petit, Synthesis of cadmium sulfide in situ in reverse micelles: influence of the preparation modes on size, polydispersity, and photochemical reactions, Chem. Mater. 4 (2) (1992) 338 345. [5] C. Rao, et al., Synthesis of inorganic nanomaterials, Dalton Trans. 34 (2007) 3728 3749. [6] A.P. Alivisatos, Semiconductor clusters, nanocrystals, and quantum dots, Science 271 (5251) (1996) 933 937. [7] C. Rao, A. Cheetham, Science and technology of nanomaterials: current status and future prospects, J. Mater. Chem. 11 (12) (2001) 2887 2894. [8] A. Mamalis, Recent advances in nanotechnology, J. Mater. Process. Technol. 181 (1 3) (2007) 52 58. [9] N. O’Brien, E. Cummins, Recent developments in nanotechnology and risk assessment strategies for addressing public and environmental health concerns, Hum. Ecol. Risk Assessment: An. Int. J. 14 (3) (2008) 568 592. [10] H. Singh, J. Du, T.-H. Yi, Biosynthesis of silver nanoparticles using Aeromonas sp. THG-FG1. 2 and its antibacterial activity against pathogenic microbes, Artif. Cells, Nanomedicine, Biotechnol. (2016) 1 7. [11] K.M.A. El-Nour, et al., Synthesis and applications of silver nanoparticles, Arab. J. Chem. 3 (3) (2010) 135 140. [12] B.S. Kim, J.Y. Song, Biological synthesis of gold and silver nanoparticles using plant leaf extracts and antimicrobial applications, Biocatalysis Biomolecular Eng. (2010) 447 457. [13] S. Lindsay, Introduction to nanoscience, Oxford University Press, 2009. [14] J. Corbett, et al., Nanotechnology: international developments and emerging products, CIRP Ann. 49 (2) (2000) 523 545. [15] A.G. Davies, J. Thompson, Advances in nanoengineering: electronics, materials, assembly, Vol. 3, Imperial College Press, 2007. [16] R.P. Feynman, There’s plenty of room at the bottom, Eng. Sci. 23 (5) (1960) 22 36. [17] C. Toumey, Plenty of room, plenty of history, Nature Publishing Group, 2009. [18] D.R. Bassett, Notions of identity, society, and rhetoric in a speech code of science among scientists and engineers working in nanotechnology, Sci. Commun. 34 (1) (2012) 115 159. [19] C.P. Toumey, Reading Feynman into nanotechnology: a text for a new science, Techne´: Res. Philosophy Technol. 12 (3) (2008) 133 168. [20] J. Ramsden, Essentials of nanotechnology, Book Boon, 2009. [21] N. Taniguchi, On the basic concept of nano-technology., in: Proc. Intl. Conf. Prod. Eng. Tokyo, Part II, Japan Society of Precision Engineering, 1974. [22] N. Meert, et al., Comparison of removal capacity of two consecutive generations of high-flux dialysers during different treatment modalities, Nephrology Dialysis Transplant. 26 (8) (2011) 2624 2630. [23] L. Ma, et al., Nanometer-scale recording on an organic-complex thin film with a scanning tunneling microscope, Appl. Phys. Lett. 69 (24) (1996) 3752 3753. [24] E. Gnecco, E. Meyer, Fundamentals of friction and wear on the nanoscale, Springer, 2015. [25] S.N. Khanna, A.W. Castleman, Quantum phenomena in clusters and nanostructures, Springer Science & Business Media, 2013. [26] J. Eastman, et al., Enhanced thermal conductivity through the development of nanofluids, MRS Online Proc. Library Archive 457 (1996). [27] G. Cao, Nanostructures and nanomaterials: synthesis, properties and applications, World Scientific, 2004.

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[28] C. Tan, et al., Recent advances in ultrathin two-dimensional nanomaterials, Chem. Rev. (2017). [29] T.-H. Shin, J. Cheon, Synergism of nanomaterials with physical stimuli for biology and medicine, Acc. Chem. Res. 50 (3) (2017) 567 572. [30] D. Astruc, Nanoparticles and catalysis, John Wiley & Sons, 2008. [31] Y. Zhang, et al., Nanomaterials-enabled water and wastewater treatment, NanoImpact 3 (2016) 22 39. [32] M. Khalil, et al., Advanced nanomaterials in oil and gas industry: design, application and challenges, Appl. Energy 191 (2017) 287 310. [33] M.-H. Kim, Y.-U. Kwon, Semiconductor CdO as a blocking layer material on DSSC electrode: mechanism and application, J. Phys. Chem. C. 113 (39) (2009) 17176 17182. [34] R. Gupta, et al., Low temperature processed highly conducting, transparent, and wide bandgap Gd doped CdO thin films for transparent electronics, J. Alloy. Compd. 509 (10) (2011) 4146 4149. [35] X. Liu, et al., Synthesis and electronic transport studies of CdO nanoneedles, Appl. Phys. Lett. 82 (12) (2003) 1950 1952. [36] F. Yakuphanoglu, et al., Electrical characterization of nanocluster n-CdO/p-Si heterojunction diode, J. Alloy. Compd. 506 (1) (2010) 188 193. [37] H. Hirata, K. Higashiyama, Analytical study of the lead ion-selective ceramic membrane electrode, Bull. Chem. Soc. Jpn. 44 (9) (1971) 2420 2423. [38] S. Gong, W. Cheng, One-dimensional nanomaterials for soft electronics, Adv. Electron. Mater. 3 (3) (2017) 1600314. [39] P.N. Sudha, et al., Nanomaterials history, classification, unique properties, production and market, in Emerging applications of nanoparticles and architecture nanostructures, Elsevier, 2018, pp. 341 384. [40] Z. Zeng, et al., Unraveling the cooperative synergy of zero-dimensional graphene quantum dots and metal nanocrystals enabled by layer-by-layer assembly, J. Mater. Chem. A 6 (4) (2018) 1700 1713. [41] H. Zhang, Ultrathin two-dimensional nanomaterials, ACS Nano 9 (10) (2015) 9451 9469. [42] K. Shehzad, et al., Three-dimensional macro-structures of two-dimensional nanomaterials, Chem. Soc. Rev. 45 (20) (2016) 5541 5588. [43] A.K. Hussein, Applications of nanotechnology in renewable energies—a comprehensive overview and understanding, Renew. Sustain. Energy Rev. 42 (2015) 460 476. [44] N. Dasgupta, S. Ranjan, C. Ramalingam, Applications of nanotechnology in agriculture and water quality management, Environ. Chem. Lett. 15 (4) (2017) 591 605. [45] P. Formoso, et al., Nanotechnology for the environment and medicine, Mini Rev. Medicinal Chem. 16 (8) (2016) 668 675. [46] S.R. Mudshinge, et al., Nanoparticles: emerging carriers for drug delivery, Saudi Pharm. J. 19 (3) (2011) 129 141. [47] A.P. Nikalje, Nanotechnology and its applications in medicine, Med. Chem. 5 (2) (2015) 081 089. [48] W.J. Parak, A.E. Nel, P.S. Weiss, Grand challenges for nanoscience and nanotechnology, ACS Publications, 2015. [49] D.A. Press, et al., Fluorescent protein integrated white LEDs for displays, Nanotechnology 27 (45) (2016) 45LT01. [50] W. Vogelsberger, J. Schmidt, Studies of the solubility of BaSO4 nanoparticles in water: kinetic size effect, solubility product, and influence of microporosity, J. Phys. Chem. C. 115 (5) (2010) 1388 1397.

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[51] Andrea Porcari, A.I., Elvio Mantovani, I.C.f. AIRI/Nanotec IT—Italian Association for Industrial Research (AIRI), and N.N. IT, Silver nanoparticles: situation and perspective for industrial application in the Lombardia region, 2013. [52] K. Mavani, M. Shah, Synthesis of silver nanoparticles by using sodium borohydride as a reducing agent, 2013. [53] C. Vidya, M.N.C. Prabha, M.A.L.A. Raj, Green mediated synthesis of zinc oxide nanoparticles for the photocatalytic degradation of Rose Bengal dye, Environ. Nanotechnology, Monit. Manag. 6 (2016) 134 138. [54] P. Appendini, J.H. Hotchkiss, Review of antimicrobial food packaging, Innovative Food Sci. Emerg. Technol. 3 (2) (2002) 113 126.

Chapter 2

Quantum effects Tahir Iqbal Awan, Almas Bashir and Aqsa Tehseen

Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan

Chapter Outline 2.1 2.2 2.3 2.4

Wave
particle duality Electromagnetic waves Energy quanta The de Broglie hypothesis 2.4.1 Derivation 2.4.2 Implications of de Broglie hypothesis 2.5 Evidence for the wave nature of electrons 2.5.1 Davisson
Germer experiment 2.5.2 G. P. Thomson’s experiment

2.1

29 30 30 32 32 35 35 36 37

2.6 Heisenberg’s uncertainty principle 2.7 Quantum dots 2.8 Moore’s law 2.8.1 Moore’s second law 2.8.2 Ultimate limits of the law 2.9 Quantum tunneling 2.9.1 Tunneling through a single potential barrier 2.9.2 Applications 2.10 Exercise References Further Reading

37 39 39 40 41 41 43 46 47 47 49

Wave
particle duality

In ancient times, Galileo considered light as fast-moving particles that take time to cover a definite distance, but he failed to measure the speed of light. Huygens proposed, in 1678, that light travels in the form of waves through space. In 1801, Young performed his famous double slit experiment to prove that light is a wave because it shows the interference property of waves. In this way, Young was able to prove Huygens’s idea [1,2]. In the 20th century, a series of experiments (Planck’s quantum theory, the photoelectric effect, and the Compton effect, etc.,) proved the opposite; that is, the particle nature of light. The concept of wave
particle duality is the interface between classical and quantum mechanics, according to which, light has two natures simultaneously, that is, a wave nature and a particle nature. Classically, it was considered that waves and particles are different things, but in quantum mechanics, they are interconnected. Particles carry energy and momentum in Chemistry of Nanomaterials. DOI: https://doi.org/10.1016/B978-0-12-818908-5.00002-0 © 2020 Elsevier Inc. All rights reserved.

29

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PART | 1 Introduction to nanomaterials

localized small mass and waves are the motion of disturbance that transfers energy [3].

2.2

Electromagnetic waves

In the classical domain, it is thought that electric and magnetic waves consist of electric and magnetic fields with specific wavelengths as shown in Fig. 2.1. These types of waves propagate at the speed of light in space, that is, 3 3 108 ms21 . The speed of light can be expressed by the electric permit1 tivity, εo , and magnetic permeability, μo , of a vacuum such as c 5 pffiffiffiffiffiffiffi εo μ . o

Examples of these waves include visible, infrared, gamma rays, x-rays, microwaves, and radio waves, and so forth [4
6]. These waves show two types of behavior in different experiments, that is, first as a wave and second as a particle. In experiments that exhibit the optical phenomena of light matter interaction, it acts as particles. On the other hand, it behaves as waves when traveling through a vacuum. Hence, electromagnetic waves show a dual behavior [7].

2.3

Energy quanta

The discrete packet of energy emitted from the atom due to transition of excited electron to a lower energy level is known as energy quanta, which

FIGURE 2.1 An electromagnetic wave showing electric and magnetic field components and wavelength. Adapted with permission under the terms of the CC BY-SA 4.0 (https://creative commons.org/licenses/by-sa/4.0/deed.en).

Quantum effects Chapter | 2

31

cannot be further divided into parts and given by Eq. (2.1). On the other hand, it is more useful to consider electromagnetic waves in the quantum domain. Max Planck proposed that electromagnetic radiations interact with matter in the form of discrete quanta of energy. Einstein also proposed that light propagates as a wave and is absorbed as quanta, where photons are known as light quanta that have an energy given by following the Planck relation [8]. E 5 hv 5

hc λ

ð2:1Þ

where λ 5 wavelength of light, c 5 speed of light, and h 5 Planck0 s constant. When this energy quantum is propagating from the source to the target, it acts like a wave, but when it interacts with any material object then it transfers momentum, therefore, it is considered to be behaving like a particle [9]. Example 2.1: (a) Prove that a wavelength of 1,243 nm for photons of an infrared beam is equal to 1.00 eV energy. (b) Find out the wavelength of blue light with a photon energy of 2.76 eV. (c) An energy of 3.50 eV is dangerous to humans as it causes skin burn. Calculate its equivalent wavelength in nanometers. a. from Eq. (2.1) E 5 hv 5

E

5

hc λ

    m 6:63 3 10234 Js 2:998 3 108 s 1243 3 1029 m 5 1:602 3 10219 J 5 1:00 eV

E

b. from Eq. (2.1) λ 5 hc E Since E 5 2:76 3 1:6 3 10219 5 4:43 3 10219 J     m 6:63 3 10234 J s 2:998 3 108 s λ 5 4:42 3 10219 5 450 3 1029 m 5 450nm

λ c. from Eq. (2.1) λ 5

hc E

 λ λ

5

6:63 3 10234 J s



  m 2:998 3 108 s

ð3:50eVÞð1:602 3 10219 J=eVÞ 5 3:54 3 1027 m 5 354nm

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PART | 1 Introduction to nanomaterials

2.4

The de Broglie hypothesis

In 1924, Louis de Broglie proposed the dual behavior of matter, that is, the wave
particle duality of matter [10]. He thought that since light has a wave
particle duality and nature should be symmetrical, matter should have a particle nature as well as a wave nature. Matter can behave as a particle as well as a wave, for example, electrons, which are considered to be particles, can be diffracted like a beam of light or any wave of water. This proposal was astonishing to researchers and there was no experimental evidence at that time. Now, matter wave is a concept that is referred to as the de Broglie hypothesis and it is a basic component of the theory of quantum mechanics [11].

2.4.1

Derivation

The wavelength associated with any moving particle can be derived by considering that nature is symmetrical, and that light has a wave as well as a particle nature, and taking Einstein’s mass
energy relation and Planck’s relation as depicted in Eq. (2.1) for light into account, then E E

5 mc2 hc 5 λ

Comparing these two equations gives, mc2 mc p

hc λ h 5 λ

5

5 mc 5

h λ

which shows that the momentum of a photon is coupled with the wavelength of light by Planck’s constant. But for material particles, the speed of light “c” can be replaced by the speed of particle “v” λ5

h h 5 p mv

ð2:2Þ

where λ is the de Broglie wavelength and p is the momentum of the particle. Since the kinetic energy of the moving particles is K:E: 5 12 mv2 , which can be written as follows: rffiffiffiffiffiffiffiffiffiffiffiffi 2ðKEÞ v5 m

Quantum effects Chapter | 2

33

using for the momentum of the particle, p p

sffiffiffiffiffiffiffiffiffiffiffiffi 2ðKEÞ 5 mv 5 m m pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffi 5 2mðKEÞ 5 2meV

By using this equation in Eq. (2.2), the de Broglie wavelength can also be expressed as h 5 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2mðKEÞ h 5 pffiffiffiffiffiffiffiffiffiffiffiffi 2meV 12:24 5 pffiffiffiffi A˚ V

λ λ λ

ð2:3Þ

Eq. (2.3) is also the de Broglie wavelength of a moving particle, which depends inversely on the square root of the applied potential to the particle [12]. Example 2.2: Calculate the wavelength of a photon when the momentum of a fast-moving electron with a speed of 2:01 3 105 m=s is equal to the momentum of the photon. The condition is that ðmvÞe 5 λ λ

h

λ p.

So, 5

5

h mv

6:63 3 10234 J s ð9:11 3 10231 kgÞð2:01 3 105 m=sÞ 5 3:64 3 1027 m 5 364nm

λ

Example 2.3: Find the de Broglie wavelength for (a) an electron, (b) a proton, and (c) a 0.20 kg ball if all these have a speed of 2:5 3 106 m=s. a. for the electron, λ λ λ

5 5

h mv

6:63 3 10234 J s ð9:11 3 10231 kgÞð2:5 3 106 m=sÞ 5 2:91 3 10210 m

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PART | 1 Introduction to nanomaterials

b. for the proton, λ λ

5 5

h mv

6:63 3 10234 J s ð1:67 3 10227 kgÞð2:5 3 106 m=sÞ 5 1:59 3 10213 m

λ c. for the 0.20 kg ball λ λ λ

5 5

h mv

6:63 3 10234 J s ð0:20kgÞð2:5 3 106 m=sÞ 5 1:33 3 10239 m

Example 2.4: Calculate the de Broglie wavelength of an accelerated electron through a potential of 120 V. Since 12 mv2 5 Ve, v v

sffiffiffiffiffiffiffiffi 2Ve 5 m vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u u2ð120VÞð1:60 3 10219 CÞ 5 6:49 3 106 m=s 5t 9:11 3 10231 kg

and λ5

h 6:63 3 10234 Js 5 5 0:112nm mv ð9:11 3 10231 kgÞð6:49 3 106 m=sÞ

Example 2.5: Calculate the potential required to accelerate electrons for an ˚ electron microscope if the wavelength is 0:5 A. Using the de Broglie relation, λ 5 KE of electron KE of electron

h mv

 2 1 1 h2 h 5 mv2 5 m mv 5 2 2 2mλ2  2 6:63 3 10234 5  2 2 3 9:1 3 10231 3 0:5 3 10210

Quantum effects Chapter | 2

35

So, KE 5 9:66 3 10217 J. V5

KE 9:66 3 10217 J 5 5 600V e 1:60 3 10219 C

Example 2.6: Calculate the kinetic energy and wavelength for a thermal neutron for 293 K. KE KE

5

3 3 kT 5 3 1:38 3 10223 J=K 3 293K 5 6:07 3 10221 J 2 2 5

1 2 m 2 v2 p2 mv 5 5 2 2m 2m

Thus λ5

2.4.2

h h 6:63 3 10234 J s 5 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 5 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi   ffi 5 0:147nm p 2mðKEÞ ð2Þ 1:67 3 10227 kg 6:07 3 10221 J

Implications of de Broglie hypothesis

According to this hypothesis, all moving particles, either microscopic or macroscopic, are associated with the de Broglie wavelength. This wavelength is observable in the case of microscopic particles, but for macroscopic objects, it is so smaller that it becomes unnoticeable. From Eq. (2.2), it is evident that if v 5 0, then λ 5 N, and if v 5 N, then λ 5 0, which shows that waves are associated with each material particle if they are in motion [13]. This association of wave behavior with material particles is quite independent from whether the material particles are charged or uncharged. So, de Broglie waves cannot be electromagnetic (EM) waves in nature because EM waves are neutral and can be generated by accelerated charged particles. Since microscopic particles have a dual nature and due to their wave nature, the exact location of these particles cannot be measured, which implies an uncertainty in the position of these particles [14].

2.5

Evidence for the wave nature of electrons

After the de Broglie hypothesis, Davisson and Germer showed the wave behavior of electrons by studying the diffraction pattern of electrons after striking with them nickel crystal. This experiment is described in Section 2.5.1. The waves associated with particles are called material waves and were also confirmed by G.P. Thomson’s experiment. Both experiments are discussed here one by one [15].

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PART | 1 Introduction to nanomaterials

2.5.1

Davisson
Germer experiment

In 1927, Davisson and Germer observed an interference pattern of an electron beam scattered by a single crystal of nickel. The evidence for the wave nature of electrons was given by this experiment performed by them. They put forward wave
particle duality on experimental footing and this experiment proved to be a major step in the development of quantum mechanics. A schematic diagram of the experimental apparatus used is shown in Fig. 2.2. A beam of electrons was emitted through a heated filament. This beam of electrons passed through accelerating anodes, which increased the kinetic energy (KE 5 eV) of the electrons. The anode voltage, V, could be adjusted using a rheostat. These high velocity electrons interacted with the target nickel crystal and were diffracted through it at an angle θ, which could be detected by movable detector. This whole apparatus was kept in a vacuum chamber to avoid any disturbing effects in the path of the beam of electrons [15
17]. The potential difference V 5 54V was applied to te accelerated electrons, so utilizing de Broglie’s hypothesis λ λ

12:3 5 pffiffiffiffi A˚ V 12:3 5 pffiffiffiffiffi A˚ 5 1:66A˚ 54

Vacuum chamber Movable detector Heated filament

θ

Nickel target

Electron beam Accelerating anode

FIGURE 2.2 Schematic diagram of the experimental arrangement of the Davisson
Germer experiment. Adapted with permission under the terms of the CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/).

Quantum effects Chapter | 2

37

X-ray diffraction from the planes of nickel crystal, which have an inter˚ gives the scattering angle θ 5 50o , which correplanar distance of d 5 0:91A, sponds to first order diffraction, so n 5 1. Hence by Bragg’s law   θ nλ 5 2dsin 90 2 2 ð1Þλ λ

5 2 3 0:91cos25o 5 1:65A˚

The results are close to each other, which confirms the de Broglie hypothesis [17].

2.5.2

G. P. Thomson’s experiment

In this experiment, a fine beam of electrons was transmitted through a thin polycrystalline material film. These electrons were diffracted by crystals by making some angles and produced a diffraction pattern on a photographic plate. Several rings were formed on the plate for different values on n, which confirms the wave nature of electrons [18]. The schematic is shown in Fig. 2.3.

2.6

Heisenberg’s uncertainty principle

It is impossible to simultaneously determine the momentum and position of a particle with perfect accuracy. So, there is always a fundamental uncertainty in the measurement of these physical quantities [19]. If Δx is the uncertainty in the measurement of position and Δp is the uncertainty in the measurement of momentum then this principle is stated as Δx:Δp $

ħ 2

FIGURE 2.3 Schematic diagram of G. P. Thomson’s experiment. Reproduced with permission from Fundamentals of Physics Extended, 10th Edition, D. Halliday et al., 1, (2013), with permission from John Wiley and Sons.

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PART | 1 Introduction to nanomaterials

The uncertainty principle is a consequence of wave
particle duality. Therefore it is associated with extremely small particles, not with macroscopic particles [20]. The stream of light photons scattering from a flying tennis ball hardly affect its path, but when a photon strikes an electron, it drastically affects the path of electrons. Let a wavelength λ is used to locate a microparticle moving along x-axis, the uncertainty in its position measurement is Δx  λ

ð2:4Þ

At most, the photon of light can transfer all of its momentum to microparticles whose own momentum will be uncertain by the amount Δp 

ħ 2

ð2:5Þ

From Eq. (2.4) and Eq. (2.5) it can be concluded that: G

G

In order to reduce momentum, light of a larger wavelength should be used. In order to reduce position, light of a smaller wavelength should be used. Multiplying Eq. (2.4) by (2.5) we get

  ħ ðΔxÞ:ðΔpÞ  λ λ

ðΔxÞ:ðΔpÞ

ħ

This is one form of the uncertainty principle. Its second form is given in terms of energy and time. ðΔEÞ:ðΔtÞ  ħ According to this, the product of uncertainty in measured amount of energy ΔE and time Δt available for this measurement in a particular state is approximately equal to Planck’s Constant. It can be concluded that it is impossible to know everything about a particle simultaneously. There will be uncertainty about one or another property. It helps in understanding the dual behavior of light and material particles. As the value of Planck’s constant is very small so when dealing with macroscopic particle like tennis ball or billiard ball, the uncertainty principle is not applicable. With the help of this principle, we can show that an electron can’t exist inside the nucleus [21]. Example 2.7: The size of nucleus is approximately 5:0fm in radius. Calculate the uncertainity in the momentum of for an electron. Since Δx 5 5:0 3 10215 m,

Quantum effects Chapter | 2

Δp $

2.7

39

ħ 1:054 3 10234 Js  $ 1:1 3 10220 kg m=s $  2Δx 2 5:0 3 10215 m

Quantum dots

Researchers are taking great interest in the optical and electronic properties of quantum dots due to their importance in novel applications over the past two decades. A semiconductor crystal with tunable optical and electronic properties by controlling its three dimensional size less than 100 nm is called quantum dot (QD) [22]. In general, QDs are atomic clusters or nanocrystallites that consist of about 102
106 atoms having less than 100 free electrons. QDs are classified into three types based on their electron confinement, that is, planar, vertical, or self-assembled QDs. In the first two types, the size of QDs has dimensions around 100 nm due to electrostatic confinement, whereas in third type QDs have dimensions about 10 nm due to structural confinement [23]. Quantum confinement in different dimensions results in different types of nanostructures. Two-dimensional (2D) structures, that is, quantum wells/ films are formed due to one-dimensional (1D) confinement. 2D confinement creates 1D structures, for example, quantum wires. 3D confinement leads to the formation of zero-dimensional (0D) structures, for example, quantum dots or quantum boxes [24]. Due to limitations in the number of electrons in a 0D confined region, discrete quantized energies are resulted in the density of states (DOS) for QDs [25]. It is worth mentioning that the sizes of QDs are generally subjected to the material of which they are made up because at this small level, quantum confinement effects (QCEs) are very crucial. More importantly, due to QCEs at small size, QDs show different properties from bulk solids and find many applications due to their attractive properties. On the other hand, QDs are toxic, which prohibits their use in medical applications in future. QDs can damage DNA and disturb the normal activity of cells [26,27].

2.8

Moore’s law

Moore’s law is an observational law in which it was predicted that the number density of transistors would double on integrated circuits (ICs) every year [28]. It was predicted by Gordon Moore, cofounder of Intel, in 1965, that this growth rate would remain so for at least 10 years [29]. It is worth mentioning that the power of the computer was predicted years prior. This law was based on calculated data, use of ICs within past years. Accurate use of the circuits and transistors in upcoming years was very important for it. The annual growth of transistors on a microchip is shown in Fig. 2.4. This prediction of the doubling of transistors was updated for

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PART | 1 Introduction to nanomaterials

FIGURE 2.4 Annual transistor count on a microchip. Reproduced with permission from Springer Nature: Springer Device Physics, Modeling, Technology, and Analysis for Silicon MESFET by Iraj Sadegh Amiri et al, copyrights (2019).

the subsequent ten years in 1975 [30]. The new definition of Moore’s law is, the number of micro-components that could be placed in an integrated circuit or microchip and lowest manufacturing cost was doubling every 18 months which accounts for the improvement in the speed of the computers. This trend would likely continue accurately into the future for a few decades [31].

2.8.1

Moore’s second law

It states that the cost of a transistor chip or IC manufacturing factory doubles every four years. This law is also called Rock’s law named after Arthur Rock [32]. In 2015, the worth had reached around US$14 billion [33]. When the price of ICs to the customer reduces, the price of factory for manufacturers to satisfy Moore’s law shows a reverse inclination. Test costs, manufacturing, research and development have amplified the price progressively by individually IC production. Increasing industrial prices are a significant reflection of the nourishing of Moore’s law [34].

Quantum effects Chapter | 2

41

TABLE 2.1 Annual journey of the development of intel microprocessor chips with increase in the number of transistors. Intel microprocessor chips

Number of transistors

Year

4004

2,300

1971

8085

6,500

1976

80286

134,000

1982

80486

1,180,000

1989

Pentium pro

5,500,000

1995

Pentium III

9,500,000

1999

Pentium IV

42,000,000

2000

Core 2 Duo

291,000,000

2006

Corei7 (1st generation)

731,000,000

2008

Corei7 (2nd generation)

1,160,000,000

2011

Corei7 (3rd generation)

1,400,000,000

2012

Corei7 (4th generation)

1,400,000,000

2013

Corei7 (5th generation)

1,900,000,000

2015

AMD Ryzen

4,800,000,000

2017

2.8.2

Ultimate limits of the law

Later, on April 13, 2005, Gordon Moore said, “It can’t continue forever. The nature of exponentials is that you push them out and eventually disasters happens.” Transistors would ultimately reach the restrictions of shrinking at atomic levels until 2025 [35]. The annual growth in the number of transistors in intel chips doubles as shown in Table 2.1.

2.9

Quantum tunneling

The theory of quantum tunneling was initiated from the study of radioactivity [36]. In 1927, Friedrich Hund first observed quantum tunneling by studying double-well potential [37]. Mikhail Leontovich and Leonid Mandelstam noticed same thing while dealing particle motion confined in a potential well [38]. In 1928, George Gamow first solved the mathematical application of quantum tunneling for alpha decay and the same was done independently by Ronald Gurney and Edward Condon [39
41]. Max Born contributed by making it a general result of quantum mechanics and not restricted to nuclear

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PART | 1 Introduction to nanomaterials

physics. The development of transistors and diodes, which are great inventions, is due to electron tunneling in solids. Tunneling plays an essential role in modern physical, chemical, and biological phenomena like the effect of large kinetic isotopes in chemicals reactions, radioactive decay, superconductor devices, and semiconductor devices. Tunneling is a quantum mechanical phenomenon by which small particles like electrons can pass through a barrier of some potential without sufficient energy, which they classically would not be able to do because they do not have sufficient energy [36]. To understand the concept of tunneling, consider that particles are just like rolling balls that are trying to roll over a mountain as shown in Fig. 2.5. Classically, it is easy to understand that particles with less kinetic energy than the potential energy of an object at the height of a mountain cannot go through the mountain. But according to quantum mechanics, the probability of the existence or transmission of particles of the incident beam is not zero on the other side of the barrier [42]. Tunneling through this barrier becomes possible only when particles of the incident beam steal energy from their environment and returned to more energetic reflected electrons. Similarly, a ball can go the other side of the hill through tunnel instead of climbing the hill. Here the hill represents a barrier that may be a vacuum, a potential energy barrier, or an insulator [43]. If particles of energy, E, are bombarded on a potential step with a height, Vo , then classically, there is no transmission at all for E , Vo and no reflection for E . Vo . But in the quantum regime, transmission probability is not

Passing through tunnel (Quantum picture)

FIGURE 2.5 A ball rolling over or tunneling through a hill. Adapted with permission under the terms of the CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

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FIGURE 2.6 Classically, incident electron with less kinetic energy than the potential of the barrier will reflect back to the same region and there is no transmission through the barrier at all, but in the quantum picture, an electron wave can tunnel through a potential barrier and the transmission through it is not zero. Adapted with permission under the terms of the CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

exactly zero when energy of incident particle is less than the height of the potential step. The classical and quantum picture of reflection and transmission of electrons from a barrier is shown in Fig. 2.6, this barrier has greater potential than the incident electrons. Some of the particles from the incident beam can go through the potential step to the other side, although their energy is not sufficient to overcome Vo . This purely quantum phenomenon is called tunneling. When the wavelength of the tunneling particle is greater than the dimensions of the potential barrier, it cannot tunnel through the barrier. So the particle should have a small wavelength for tunnelling. This phenomenon is valid for small particles for which classical laws cannot be applicable, for example, electrons [44].

2.9.1

Tunneling through a single potential barrier

A potential barrier of width “L” and potential U ðxÞ can be written as:  Uo 0 # x # L U ðxÞ 5 0L , x , 0 A schematic of a potential barrier is shown in Fig. 2.7. When the energy of the incident of particles is E , Uo , then general the solution to the Schrodinger wave equation for each of the three regions will be: 5 Aeikx 1 Be2ikx Region Iðx , 0Þ Ψ1 ðxÞ Ψ2 ðxÞ 5 Ce2γx 1 Deγx Region IIðL . x . 0Þ Ψ3 ðxÞ 5 Eeikx 1 Fe2ikx Region IIIðx . LÞ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffi 2mðV 2 EÞ where k 5 2mE 2 and γ 5 ħ2 ħ At the boundary with the boundary conditions, x 5 0 and x 5 L,

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PART | 1 Introduction to nanomaterials

FIGURE 2.7 A schematic diagram of a potential barrier with three regions. The left region has zero potential, the middle region has Uo potential, and the right region has zero potential. The energy of incident particles is E and the width of the potential barrier is L along the x-axis. After tunneling, the amplitude and, hence, probability of finding the particle both decrease exponentially. Adapted with permission under the terms of the CC BY 4.0 (https://creativecommons. org/licenses/by/4.0/).

Ψ1 ð 0 Þ 0 Ψ1 ð0Þ Ψ 2 ð LÞ 0 Ψ 2 ð LÞ

5 Ψ2 ð0Þ-A 1 B 5 C 1 D  5 Ψ2 ð0Þ-ikðA 2 BÞ 5 2 γðC 2 D 2γL γL ikL 2ikL 5 Ψ3 ðLÞ-Ce  2γL 1 De γL 5 Ee  1ikLFe  0 5 Ψ3 ðLÞ- 2 γ Ce 2 De 5 ik Ee 2 Fe2ikL 0

From this equation, the relation formulated using a matrix such as: 2   1 γ 62 11ik  6 A  56 6 1 γ B 4 12i Þ 2 k 2   1 k ðik1γÞL 62 12iγ e  6 C   56 61 k ðik2γÞL D 4 11i e 2 γ

between various constants can easily be  3 1 γ 12i   2 k 7 7   7 C 5 M1 C 1 γ 7 D 5 D 11i 2 k 3   1 k 2ðik2γÞL 11i e 7  2 γ 7 E E 7   5 M 2 1 k 2ðik1γÞL 7 F F 5 12i e 2 γ

So, the connection between constants A, B, and constants E, F is through matrix M, which consists of entries mij as:     C E E A 5 M1 M2 5M 5 M1 D F F B For constants A and E, the transmission coefficient ‘T’ can be given by using A 5 m11 E as (using F 5 0, which is an asymptotic condition):

2

E

1 T 5



5 A jm11 j2

Quantum effects Chapter | 2

45

Then: "

#21  2 2 2 γ 1k T 5 11 sinh2 ðγLÞ 2kγ The transmission coefficient for a weak barrier, that is, γL , , 1 is: 1 T 2 1 1 ðkL=2Þ The transmission coefficient in the case of a strong barrier, that is, γL, is large,   4kγ 2 T 2 expð 22γLÞ k 1γ 2 When the kinetic energy of incident beam is greater than potential height of the barrier, that is, E . Uo , use γ 5 ik2 . "  #21 2 k2 2k2 2 2 T  11 sin ðk2 LÞ 2kk2 Note that the transmission maxima (T 5 1) takes place when k2 L 5 nπ. Example 2.8: The energies of electrons are 1.0 eV and 2.0 eV, which have fallen on a potential barrier of 10.0 eV with a width of 0.50 nm. (a) Calculate their individual probabilities of transmission. (b) What would happen if the width of the barrier is doubled? a.

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2mðUo 2 EÞ 5 k2 5 ħ

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi    ffi 2 9:1 310231 kg ð10:0eV 21:0eV Þ 1:63 10219 J=eV 1:054 310234 Js 51:6 31010 m21

Since L 5 0:50nm 5 5:0 3 10210 m,   10 21 210 5:0 3 10 m 5 16 2k2 L 5 2 1:6 3 10 m T1 5 e22k2 L 5 e216 5 1:1 3 1027 If the energy is 2.0 eV, then T2 5 2:4 3 1027 b. When the width is doubled, then T 01 T 02

5 1:3 3 10214 5 5:1 3 10214

The results of the transmission coefficient “T” of tunneling with varying ratios of kinetic energy of the incident particle to the barrier height by keeping the barrier dimension L is shown in Fig. 2.8. For constant E and Uo , the transmission coefficient “T” is associated with the width “L” of the barrier,

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PART | 1 Introduction to nanomaterials

FIGURE 2.8 Transmission coefficient “T” variation with the ratio of kinetic energy of the incident particle to the barrier height for fixed barrier thickness L. Here the dashed line shows the classical situation and the solid lines show quantum pictures. Adapted with permission under the terms of the CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/).

the dashed line shows the transmission of the particles through the barrier in a classical view, but, on the other hand, the solid lines show the transmission of the particles on a quantum level. It can be concluded that the classical picture shows that if the kinetic energy of the incident particles is greater than the potential height then all the particles should transmit through the barrier, whereas the quantum picture discloses that total transmission takes place just for some discrete energies. So a particle having E . Uo and following the condition k2 L 5 nπ may also have the probability to reflect back. This probability of reflection diminishes quickly by the growing energy of the incident particle E.

2.9.2

Applications

There is a wide range of applications of quantum tunneling such as in tunnel diodes, in quantum conductivity, STM, in kinetic isotope effect, and in tunnel field-effect transistors. Nuclear fusion in stars is a result of the quantum tunneling of hydrogen nuclei that overcome the coulomb repulsion barrier, although the temperature is not enough for thermonuclear fusion [45]. The process of radioactive decay comprises the release of energy and particles by unstable nuclei. In beta decay, electrons tunnel out from the nucleus. The tunneling of electrons is highly significant in numerous biochemical reactions

Quantum effects Chapter | 2

47

such as photosynthesis and cellular respiration, while the tunneling of protons is important for impulsive changes of DNA. In cold emission, electrons, in spite of having less energy as compared to the attraction of metallic nuclei, can tunnel out of the barrier. For this purpose, a high electric field is established, and as a result, the thickness of the barrier reduces to a small length. Hence, electrons can easily tunnel out from the barrier to provide current, which depends exponentially on electric field. To study tunnel junctions, quantum tunneling is important to understand. Tunnel junctions are formed when two conductors are separated by thin insulating air as a barrier [46].

2.10 Exercise 1. Find the energy of a photon if the wavelength is 460 nm in the blue region. 2. Calculate the wavelength of 610 eV energy for a photon beam. 3. If a sodium lamp of 25 W emits photons of a yellow color with a wavelength of 591 nm, then calculate the number of photons per second from it. 4. For a beam of 520 nm wavelength of photons, find the speed and momentum of these photons. 5. Prove that the wavelength of an accelerated electron by potential differpffiffiffi nm: ence V is 1:226 V 6. An accelerated electron beam by a potential of 8 kV is produced through a heat gun, calculate its de Broglie wavelength. 7. A potential difference of 0.6 MV is used to accelerate electrons, calculate the wavelength associated with these electrons. 8. The radius of an atom of hydrogen is 5:3 3 1011 m. Using the uncertainty principle, calculate the least energy of electron in this atom.

References [1] L.G.L. Nicolini, Paper 1: taking a pragmatic position for describing objects, time, space, and making an extra-model of them, Cosm. History: J. Nat. Soc. Philosophy 13 (1) (2017) 361
420. [2] A. Plotnitsky, Niels Bohr and complementarity: an introduction, Springer Science & Business Media, 2012. [3] D. Cassidy, Quantum mechanics 1925
1927: triumph of the copenhagen interpretation, Werner Heisenberg, 2008. [4] P.A. Tipler, G. Mosca, Physics for scientists and engineers: electricity, mognetism, light & elementary modern physics, Recording for the Blind & Dyslexic, 2004. [5] B. Thide´, Electromagnetic field theory, Upsilon Books Uppsala, Sweden, 2004. [6] A. Richards, Alien vision: exploring the electromagnetic spectrum with imaging technology, Vol. 9, SPIE Press, Bellingham and Washington, DC, 2001. [7] W.D. Kimura, What are electromagnetic waves?, in Electromagnetic waves and lasers, Morgan & Claypool Publishers, 2017. p. 1-1-1-33.

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[8] J. Norton, The logical inconsistency of the old quantum theory of black body radiation, Philosophy Sci. 54 (3) (1987) 327
350. [9] F. Selleri, Wave-particle duality, Springer, 1992. [10] J.-P. Vigier, Louis de Broglie—physicist and thinker, Found. Phys. 12 (10) (1982) 923
930. [11] M.-C. Combourieu, H. Rauch, The wave-particle dualism in 1992: A summary, Found. Phys. 22 (12) (1992) 1403
1434. [12] G. Ekspong, The dual nature of light, as reflected in the nobel archive, Proc. Am. Philos. Soc. 143 (1) (1999) 42
49. [13] R. Horodecki, De Broglie wave and its dual wave, Phys. Lett. A 87 (3) (1981) 95
97. [14] W. Heitler, The quantum theory of radiation, Courier Corporation, 1984. [15] J. Navarro, A history of the electron: JJ and GP Thomson, Cambridge University Press, 2012. [16] C. Calbick, The discovery of electron diffraction by Davisson and Germer, Phys. Teach. 1 (2) (1963) 63
91. [17] C. Davisson, L.H. Germer, Diffraction of electrons by a crystal of nickel, Phys. Rev. 30 (6) (1927) 705. [18] R.Y. Abhang, Making introductory quantum physics understandable and interesting, Resonance 10 (1) (2005) 63
73. [19] S. Kakani, Modern physics, Anshan Pub, 2007. [20] H.P. Robertson, The uncertainty principle, Phys. Rev. 34 (1) (1929) 163. [21] C.L. Fefferman, The uncertainty principle, Bull. Am. Math. Soc. 9 (2) (1983) 129
206. [22] D. Bera, et al., Quantum dots and their multimodal applications: a review, Materials 3 (2010) 2260
2345. Crossref Google Scholar. [23] S. Bednarek, et al., Modeling of electronic properties of electrostatic quantum dots, Phys. Rev. B 68 (15) (2003) 155333. [24] A. Yoffe, Semiconductor quantum dots and related systems: electronic, optical, luminescence and related properties of low dimensional systems, Adv. Phys. 50 (1) (2001) 1
208. [25] L. Chico, M.L. Sancho, M. Munoz, Carbon-nanotube-based quantum dot, Phys. Rev. Lett. 81 (6) (1998) 1278. [26] Y. Chen, et al., Two-dimensional graphene analogues for biomedical applications, Chem. Soc. Rev. 44 (9) (2015) 2681
2701. [27] C. Frigerio, et al., Application of quantum dots as analytical tools in automated chemical analysis: a review, Analytica Chim. acta 735 (2012) 9
22. [28] G.E. Moore, Cramming more components onto integrated circuits, Proc. IEEE 86 (1) (1998) 82
85. [29] G.E. Moore, Cramming more components onto integrated circuitsReprinted from Electronics, volume 38, number 8, April 19, 1965, pp. 114 ff IEEE Solid-State Circuits Soc. Newsl. 11 (3) (2006) 33
35. [30] Moore, G.E. Progress in digital integrated electronics. in Electron devices meeting. 1975. [31] Benmayor, L., Dimensional analysis and similitude for microassembly design and assembly. 2000. [32] K. Rupp, S. Selberherr, The economic limit to Moore’s law, IEEE Trans. Semiconductor Manuf. 24 (1) (2011) 1
4. [33] E. Brynjolfsson, A. McAfee, The second machine age: work, progress, and prosperity in a time of brilliant technologies, WW Norton & Company, 2014. [34] B. Schaller, September The origin, nature, and implications of “MOORE’S LAW”, the benchmark of progress in semiconductor electronics, 26, Microsoft Corporation, 1996.

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[35] Spacey, J. Moore’s law vs Rock’s law. 2016; Available from: https://simplicable.com/new/ moores-law-vs-rocks-law. [36] M. Razavy, Quantum theory of tunneling, World Scientific, 2003. [37] Nimtz, G. and A. Haibel, Zero time space. How quantum tunneling broke the light speed barrier. 2008. [38] L. Mandelstam, M. Leontowitsch, Zur Theorie der Schro¨dingerschen Gleichung, Z. fu¨r Phys. 47 (1
2) (1928) 131
136. [39] R.W. Gurney, E.U. Condon, Quantum mechanics and radioactive disintegration, Phys. Rev. 33 (2) (1929) 127. [40] R.W. Gurney, E.U. Condon, Wave mechanics and radioactive disintegration, Nature 122 (3073) (1928) 439. [41] G. Friedlander, J.W. Kennedy, E.S. Macias, Nuclear and radiochemistry, John Wiley & Sons, 1981. [42] G. Muga, R.S. Mayato, I. Egusquiza, Time in quantum mechanics, Vol. 734, Springer Science & Business Media, 2007. [43] T. Miyazaki, Atom tunneling phenomena in physics, chemistry and biology, Vol. 36, Springer Science & Business Media, 2013. [44] S. Keshavamurthy, P. Schlagheck, Dynamical tunneling: theory and experiment, CRC Press, 2011. [45] F. Trixler, Quantum tunnelling to the origin and evolution of life, Curr. Org. Chem. 17 (16) (2013) 1758
1770. [46] J.P. Velev, et al., Magnetic tunnel junctions with ferroelectric barriers: prediction of four resistance states from first principles, Nano Lett. 9 (1) (2008) 427
432.

Further reading Available from: http://www.physics.brocku.ca/PPLATO/h-flap/phys11_1.html#top. Clark, D. Moore’s law is showing its age. 2015; Available from: https://www.wsj.com/articles/ moores-law-is-showing-its-age-1437076232. Tunneling. 2017; Available from: https://chem.libretexts.org/Bookshelves/Physical_and_ Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_ Chemistry)/Quantum_Mechanics/02._Fundamental_Concepts_of_Quantum_Mechanics/ Tunneling. Vasileska, D. and G. Klimeck, Tunneling. 2011.

Chapter 3

Interfaces and surfaces Almas Bashir, Tahir Iqbal Awan, Aqsa Tehseen, Muhammad Bilal Tahir and Mohsin Ijaz Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan

Chapter Outline 3.1 3.2 3.3 3.4

Introduction Surface physics and chemistry Surface and interface Surface modification 3.4.1 Methods of surface modification 3.5 Thin-film deposition 3.5.1 Deposition techniques

3.1

51 53 54 55 55 65 67

3.6 Self-assembly 78 3.6.1 Molecular self-assembly systems 79 3.6.2 Idea of molecular selfassembly 80 3.6.3 Equilibrium and nonequilibrium self-assembly 81 References 81

Introduction

Interfaces are thin borderline regions differentiating a piece of matter from its surroundings or two media from one another [1]. Any phase boundary can be termed as an interface, while condensed phase and a gas has a boundary between them, which can be referred as the surface. It is desirable to use the term interface for all conditions due to the dependence of the properties on each phase. When materials are made up thinner and thinner, the properties of their surfaces vary uniquely from that of their bulk counterparts. Every system feels the influence of interfaces, therefore, in numerous cases, it is important to know the interfacial properties along with the bulk properties. For example: G

G

G

The shape of small fluid masses or menisci is governed by surface tension. Sticking, sliding over, or repelling one another and the conditions of adjacent systems can be determined by their surfaces. Normally what we see is the interaction of light with the interfaces.

Chemistry of Nanomaterials. DOI: https://doi.org/10.1016/B978-0-12-818908-5.00003-2 © 2020 Elsevier Inc. All rights reserved.

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52 G

G

PART | 1 Introduction to nanomaterials

As surface tension depends on the composition of the system, the formation of froths or foams is made possible. The chemistry of a reaction with nearby phases of a system is governed by the accumulated trace elements at the surfaces.

The material at surfaces is quite different in properties to those of the bulk material. At the interfaces, the mechanical stress state differs from that in bulk phases. The difference in mechanical stress states for the interfaces of fluids is obvious due to the tension at these surfaces or interfaces. The reason behind the existence of boundary tension is mechanical stresses, which are localized internally in a material. For an unstrained bulk fluid at rest, a single scalar quantity can be used to describe the stress at any point in the interior by pressure. However, the specifications of the interfacial layer of fluid demand a stress tensor whose components under the same conditions are equal to “1”. These are parallel to the layer, but smaller than normal component [1
3]. Boundary tension is led by the deficit of this tangential pressure in the layers of the interface. Capillarity deals with the analysis of fluid interfaces. Solid
solid and solid
fluid boundaries are also usually in tension, though this is not freely computable. These boundary layers are usually in a state of nonequilibrium and have more complex stress fields as compared to fluid interfaces. Thus besides their dependence on thermodynamic state, it must also be considered whether the process behind the formation of the interface is a consequence of the breakage of the bulk phase, the precipitation out of solution, the stretching of a preexisting surface, or due to any other means [3]. The chemical composition is another dissimilarity between the interfacial layer and the bulk phases. For example, the interface between metal and air is shown in Fig. 3.1. Except noble metals, metal substrate binds with a single oxide layer which is at least partly hydroxylated with the interaction of water in the air. The acid or base nature of the -OH groups have a dependence on the metal. At the top, a layer of water molecules is tightly bound with the addition of a Air Any contamination Water molecules loosely bound Water molecules tightly bound Oxide layer

Oxygen atom

Metal substrate

Hydrogen atom

FIGURE 3.1 The chemical composition at the metal
air interface.

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FIGURE 3.2 The separation of charges at the surface of a solid negatively-charged metal from a bulk liquid to form a double layer of opposite charges. Adapted with permission under the terms of the CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/).

loosely bound water layer [4]. The toppest layer contains some organic impurity like adsorbed scum of grease. As the interfaces contain such components that are usually not carried by the bulk phases it introduces high complexity into the structure. Electrical charge separation is often exhibited by the interfacial layer. The electrically neutral interface of bulk may get charges because the separation of negative and positive charges is perpendicular to the interface. As a result electrical double layer is formed at the interface. Charge separation appears due to various mechanisms and its presence has numerous effects. For example, the accumulation of positive charges from a bulk liquid on a negatively-charged solid surface is displayed in Fig. 3.2, which may provide the opportunity to attach negative charges to form a double layer [5].

3.2

Surface physics and chemistry

The investigation of the chemical and physical phenomena happening between two phases at the interface, for example, fluid
solid, gas
solid, vacuum
solid, or gas
liquid interfaces, is known as surface science. The significance of surface science is evident in the areas of geochemistry, electrochemistry, and heterogeneous catalysis [6]. The physics and chemistry of surfaces are studied under the umbrella of surface science. G

G

The investigation of interactions of a chemical nature occurring at interfaces is termed surface chemistry. The investigation of interactions of a physical nature occurring at interfaces is termed surface physics.

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Condensed matter physics Nanotechnology

Bulk solid-state physics

Surface and interface physics

Computer calculations

Particle beam optics

Vacuum technology

Molecular physics

Corrosion and surface protection

Semi conductor technology and devices

Real surfaces and thin films

Atomic clusters

Electrochemistry

FIGURE 3.3 The relationship of surface and interface physics with various fields of research.

There exists a strong and remarkable relationship between interfacial physics and chemistry and various fields of research. Fig. 3.3 focuses on the fields that are prerequisites and provide inputs into interfacial and surface sciences (such as condensed matter physics, nanotechnology, vacuum technology, and molecular physics) as well as on such fields that emerge as applications (such as corrosion and surface protection, semiconductor technology and devices, real surfaces and thin films, and electrochemistry) of surface and interface science [7].

3.3

Surface and interface

An interface is the thin border between two localized phases of matter. This thin boundary is called an interface for any physical state of mater. Specifically, when this interface exists between any condensed state and a

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gas or vacuum then it is called a surface. Interface is a more general term that can be used rather than surface. G

G

G G

G

G

Those regions that interact with each other at equilibrium are called phases and the common boundary between them is termed as a phase boundary. Polycrystalline matter has a grain boundary at equilibrium that is an example of an interface. The width of interfaces and thin films controls the interacting properties. The presence of interfaces affects the overall properties of nearby bulk materials. The importance of systems that have large surface area to volume ratios increases, for example, colloids. The shape of interfaces in liquids may be curved or flat depending upon a physical quantity called surface tension. The thin films of liquids on rough interfaces are flat shaped due to surface tension.

The outermost layer of a piece of condensed state matter (solid or liquid) is called a surface. So it is the region that is perceived first by the senses and further it could be said that it is the portion that interacts first during a reaction between any two substances. The outer area to the surface of a piece of matter is called its surroundings, whereas the inside is called the bulk matter [8,9].

3.4

Surface modification

The surface morphology of solid materials is a significant property. This morphology is linked with effective surface area (ESA), which is greater than geometrical macroscopic area (GMA) at all times. There are numerous ways that are employed for rising the ESA above the GMA. The frequent synthesis of solid materials from liquids provides their smooth surfaces due to surface energy. But materials with large ESAs are favored in many applications. Since rough surfaces have larger ESAs than smooth ones do, they offer better adhesion to the coating of a material. Surfaces can be made rough through deposition and etching. Surface modification (SM) comprises of artificial alteration at the surface of materials to change the surface properties from the previous ones. The surfaces of polymers are nonreactive. In order to make the surfaces significantly reactive SM is used.

3.4.1

Methods of surface modification

SM should be shallow and influence just the uppermost coating of a surface as deep modifications may unfortunately vary the properties of the bulk material and create troubles in sticking with surfaces, although shallow

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FIGURE 3.4 Some steps to develop multilayer films from untreated substrate. Adapted from Z.-Y. Qiu, et al., Advances in the surface modification techniques of bone-related implants for last 10 years. Regen. Biomater 1(1) (2014) 67
79, with permission under the terms of the CC BY.

layers are liable to disintegration; regardless of these necessities, there are various approaches for the modification of the outer layers of a material to improve its usefulness [10,11]. Numerous strategies can be utilized to accomplish ideal properties [12]. In the case of polymer-derived stents, the techniques used for the modification of surfaces with the true objective of accomplishing better blood compatibility, improved reendothelialization, or both are divided into some important steps. These steps are grafting, coating, gradient coating, roughening, patterning, chemical modification, pharmaceuticals attachment, and the formation of multilayer films or porous surfaces; a number of which are presented in Fig. 3.4.

3.4.1.1 Surface scratching/roughening Surface area can be increased using the surface roughing technique and by this method we can restrict the motion of cells through which cell attachment can be enhanced. But cells can still relocate on rough surfaces; however, no critical rises or falls in movement have been observed in comparison with flat surfaces [14,15]. The roughening step changes the outer look of a surface without changing it chemically, which may have material-dependent advantages and preferable uses [16]. In the case of metals, sputtering by TiN (which is a well-known roughening technique) can successfully upgrade endothelial cell connection. So, endothelial nitric-oxide-synthase (eNOS) is shown less by these cells, which can increase the dysfunction of endothelial cells, and this decreased activity of eNOS alter the metallic property. [17]. To enhance cell attachment, reactive-ion etching after microblasting on metal tops produces roughened and high energy surfaces [18]. Surface topology can be altered using plasma deposition. For the case of polymers, the plasma deposition of Ar/O2 enhances the rough surface area

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FIGURE 3.5 Scanning electron microscope (scale bar 5 200 nm) image of silicon surface textured using Cl2, CF4, and O2 gas plasma. Reprinted from H.G. Craighead et al., Chemical and topographical patterning for directed cell attachment, Curr. Opin. Solid State Mater. Sci. 5 (2001) 177
184, with permission from Elsevier.

and hydrophilicity, and cell connection has been shown to be increase by both; plasma deposition used for stents provide improved biocompatibility and curing of injury. Due to the processes of melting and recrystallizing using plasma processing, the surface topography is changed, bringing about greater roughness as showed in Fig. 3.5 [19,20]. The roughness of the surface can also be improved by sanding and etching along with microblasting and polishing [21]. Shadpour et al. roughened the surface of a polymer through utilizing an alumina particle slurry with the high expectation of upgrading endothelial cell connection without chemically changing the surface of the polymer. For roughening surfaces and enhancing surface area utilizing this process, there is another patterning purpose for which it can be used and both of these empower cell and biomolecule connection [16]. This process has been shown to enhance cell connection by changing the surface of polymer without affecting the bulk material and it would be worth exploring for future versions of stents because of the probability for enhanced biocompatibility. Chemical and plasma-based etching can be done on surfaces that are exposed to etched gas, which is a sort of plasma. Etching can degrade the surfaces of polymers because etching can be used to break old bonds, and through this, new bonds are created on the upper-layer that has chain of bonds. The surface topology can be altered through this procedure and it can influence the

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wettability of the surface, potentially becoming compatible with progressive biocompatible surfaces. Etching can also be done on materials that have coatings, but it should be taken into account that coating is accurate. The use of specific acids that have an effect of etching may likewise be used to support the connection and relocation of endothelial cells, particularly polymeric hydrogels. Grafting of distinct chains of polymers can change the roughness of a surface, especially on the nanoscale. To improve biocompatibility and upgrade cell attachment, roughening at this scale is a remarkable technique. A typical method utilized for patterning and that can also be used for roughening the surfaces of polymers is printing transfer. There are still numerous techniques that cannot yet been utilized for the shape memory polymers (SMPs), their utilization on polymers proves the fruitful use of SMPs, provided that strategies keep on modifying just the upper layers of materials.

3.4.1.2 Surface patterning Surface designing offers an ordered way for roughening to change the outer layer of a material. Patterning may suppress the unclear interactions of a surface and proteins because these cause the failure of devices [22]. Techniques of pattering are regularly used to upgrade endothelial cell connection, which sequentially boosts vessel-wall curing and encourages an antithrombotic condition. A nanopillar array produced as a result of plasma processing is shown in Fig. 3.6 that provides support to cell proliferation or drug delivery [19]. Patterning on the surfaces of metals, essentially on the nanolevel, has been shown to encourage further endothelial cell connection contrasted with irregular nanopatterning or microlevel patterning [12,18]. These nanopatterned surfaces further increase endothelial cell connection as compared to smooth cell connection, which is needed in vessel healing, assisting higher densities of the cells superficially, and even for improving the spreading of this type of endothelial cell [12,23]. The cells present in their local condition come into contact with topographies on the nanoscale, which could be the cause for improved cell connection [24
26]. A few patterning techniques try to copy local endothelium for biomimetic effect with the expectation of progressively improving reendothelialization and vessel healing without the existence of an extracellular matrix or plasma proteins [12,22,26,27]. Biomimetic patterning has big consequences for stents due to the expanded biocompatibility gains by stent polymerizing of local blood vessels and precisely moving local endothelial patterns on the stent surface. Patterning can be attained by diblock-copolymer grafts, which can form designs on the surface of nanometer-sized solids. Diblock copolymers are attached through chemical or physical connection and they can produce domains of nanosize when microphase separation has been experienced

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FIGURE 3.6 By setting the scale bar to 20 μm the image of scanning electron microscope has been produced via processing of plasma. Reprinted from H.G. Craighead et al., Chemical and topographical patterning for directed cell attachment, Curr. Opin. Solid State Mater. Sci. 5 (2001) 177
184, with permission from Elsevier.

by them. This type of pattern either increases or decreases the adsorption of protein or cell adhesion and is governed by the polymers used. Consequently, diblock copolymers have been studied in relation to topography or the energy of the surface, and they are being utilized potentially for bioactivity [28]. Polymers that experience phase separation, for example, the mixture of poly(4-bromostyrene) and polystyrene, can produce a variety of surface topographies just through changes in the concentrations and proportions of the polymers [29,30]. As a result of changing the ratio of the polymers, variations in shape occur, for example, islands, pits, and ribbons; consequently, the alteration in concentration directs the variation in feature sizes [30]. The multiplication and spreading of cells differ depending on feature height, with smaller feature heights giving remarkable results in improved multiplication and the spreading of cells [31]. With respect to the patterning of the polymer surface, lithography is a the most regularly utilized method, a normal method in the field of electronics, primarily for designing wafers of silicon. Patterns can contain anything from pillars and dots to ridges and grooves, where ridges and grooves are the most regularly investigated because of the expanded inclination of cells to join and multiply these features [31,32]. Lithography can also be utilized to make tilted patterns or hierarchical patterns, whenever needed [33]. A couple of studies have endeavored to

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determine the reason that cells adjust near ridges and grooves, but various cells have various tendencies related to shape and size of the produced pattern. Photolithography has been commonly utilized on the surfaces of polymers, and this procedure specifically opens the outside layers to photoirradiation, thus, making a surface pattern [34
36]. This permits controlled features of topography, leading to cell connection [34,37]. Lithographic methods are the most obvious surface changing techniques for polymers and the application of these techniques to SMPs, especially stents based upon SMP, is also advantageous. The channels created by microfluids offer another way to direct cell adhesion by means of patterning. Proteins that are adsorbed on the surface subsequently flee by elastomeric channels in the shape of a solution. Such proteins are utilized for particular cell adhesion. This technique is also utilized to yield a co-culture cell when two dissimilar sorts of cells are requires to follow to the same exterior [38]. Monolayers that are self-assembled, a typical SM strategy, have additionally been investigated in making patterns on the surfaces of biomaterials [31,39]. SelfAssembled Monolayers (SAMs) support the adhesion and placement of cells; characteristics that are beneficial for the biocompatibility of stents by governing the captivation of protein on the surface [19,40]. They are also utilized for smaller-scale contact printing; one more approach for designing that is regularly utilized to support cell connection [19]. With respect to some specific SMP methods for patterning, the ways in which balls (i.e., lime or steel glass) create spaces superficially have been investigated [41]. In addition, patterns of wrinkling over the SMPs can be be utilized by the abilities of shape memory and if wrinkling is controlled than the various surface properties increase biocompatibility along with wetting, bonding, and roughness [42]. Printing based on transfer includes the exchange of a pattern from a growth form to a substrate of polymer, bringing about a thin film at the exterior of the polymer [33]. These types of films are basically polymers on their own and have the ability to support cell adhesion by presenting nanopatterns that support cell connection. Printing based on transfer can also make hydrophobic and hydrophilic surfaces, directing cell connection to specific zones [43]. Zhao et al. found that micro-transfer molding utilizing a mold of polydimethylsiloxane (PDMS) makes micro-sized patterns, which can be used to increase the attachment of endothelial cells [44]. Like printing based on transfer, stencil-assisted printing includes utilizing a single stencil to engrave a required structure or pattern on the surface of a polymer. The patterns is created superficially by the stencil which will remain uncovered. This method does not need any alteration after the manufacturing of the stencil, thus, it is a pleasing way to upgrade the biocompatibility of a material along with being a potential method for SMPs-based designing of surfaces [22]. Nanopatterning by dip-pin lithography utilizes the tip of atomic force microscopy (AFM) to make a design on the surface of a material. The tip is immersed in a polymer solution and

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contacts the outside of the material, thus, modifying the outlook of the surface in a composed way and making a pattern [45]. Based on polymer deposited on the surface, an increase in blood similarity and additionally cell connection can be accomplished. The utilization of a warmed tip to make designs on the outside of SMPs has been investigated, and may lead the way to designing SMPs to support cell connection [46]. Spaces can also be created utilizing an scanning force microscopy (SFM), however, there may be potential for SM as well [47,48]. With the intention to actually copy the designs found in local vessels, solutions based on prepolymers are polymerized within a harvested, local vessel [33]. A polymer takes the surface features for blood vessel, but the primary confinement of this strategy is that the tissue of the vessel must be melt down to evacuate the polymer, interpreting reproducibility troublesome. Since SMPs need unchanged shape for the start of polymerization, that technique which is connected to SMPs might be enough for further examination. In spite of the fact that SMPs are the most widely utilized patterning techniques, the achievements related to the patterning of materials of polymers recommend that designing SMPs within systems might have constructive results.

3.4.1.3 Chemical surface modification Methods of chemical SM alter the properties of materials chemically, but there is no pointed effect on the properties of the bulk material. Plasma vapor deposition (PVD), chemical vapor deposition (CVD), grafting, and SAMs are used for this purpose [12,49]. Many processes are used for the SM of metals like the plasma immersion ion implementation process in which different gases are used like O2 , N2 , and acetylene, for carrying away corrosion, strength of material, the discharge of metal into the environment, and also for wearing [50]. The ion implementation method can be conducted on polymers to decrease blood clot generation, platelet collection by the expansion of hydrophilicity, and the adsorption of protein by the surface [51]. CVD uses some reactive plasma to set thin films on the top of material texture, a little change in the surface permits film coating [37]. Because of nonfouling characteristics related to film coating, plasma-related CVD methodologies are commonly used for blood compatibility [52]. Commercially, on stents and different blood contacting devices a CVD form is used by the name of parylene. It helps in biocompatibility and gives a mean for continuous drug emit from permeable matrix [53]. A covering does not have useful associations to append biomolecules, thus, dealing with chemicals or plasma to present fastening molecules is needed for biomolecule connection [54]. A reduced amount of pressure plasma actions that utilize ions, radicals, electrons, UV rays, or meta-stables inspire responses on the surfaces of

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polymers. Ammonia plasma action can energize cell connection further by acidic group interaction on plasma film and amide/amine associations on top of the polymer, which show a basic character for endothelial cell attachment [55,56]. Some investigations have demonstrated that cornea cells show better connection and development at plasma-treated surfaces as compared to untreated surfaces [54]. Ho et al. verified that polymer samples that experience vapor plasma action can inspire more increased cell connection as compared with untreated samples because of the development of hydroxyl groups on the interface, thus, permitting the formation of a hydrogen bond between the cells and surface [57,58]. Indeed, even as studies have been ordinarily uncertain regarding well surfaces helping cell attachment and development, surfaces that are lightly hydrophilic or lightly hydrophobic seem to help cell growth; these light circumstances can be done by plasma action utilizing reactive gases involving materials of an organic nature [31,38,59
61]. Polymer materials have plasma action of viable outcomes on cell attachment as well as enhancement on materials, specifically by improving wettability and hydrophilicity [11,59,62,63]. The deposition of plasma may even be utilized to diminish thrombogenicity [11]. PVD methods, for example, evaporation by matrix-aided pulsed laser, coat biological and organic materials on the outside of blood-linking gadgets, thus, changing the exterior [12]. The deposition by ionic plasma has been demonstrated to develop endothelial cell attachment [64]. Many surfaces of polymer revealed to O2 and N2 in “He” show increasing connection characteristics with the degree of surface alteration governing on the surface of the polymer itself [55,65]. Distinctive surfaces presented to N2 gases have been perceived to show diminished thrombotic characteristics [66,67]. Similarly as with scratching, plasma approaches root the arrangement of free radicals on the texture surface [68]. Surfaces that are highly reactive can be utilized to energize inclusion with a thin coating and can encourage the connection of molecules. Photografting of polymers uses highly accelerated UV, gamma radiation, visible light, and electrons to alter the exterior of polymers for better compatibility with blood as well as to increase endothelialization [12,55]. Bilek et al. reveal that polymer surface action with ions to make a free radical surface supports protein immobilization, at the same time structure of protein remains unchanged without a doubt of improving biocompatibility [51]. Photooxidation (PO), a way to present hydrophilic associations with polymer surfaces in which PO and grafting time are controlled, has been shown to be useful for endothelial cell improvement at the material surface [31]. The techniques of chemical grafting, for example, polyethylene glycol grafting to the outside of biomaterials, may diminish the connection of steric repulsion by erythrocytes, consequently reducing the danger of thrombosis [69,70]. Polyethylene glycol (PEG) is mainly hydrophilic as well as having a

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huge elimination size, adding to this process with nonimmunogenic and nonharmful characteristics for necessary parts of biocompatible substances [69]. The technique of grafting copolymerization that joins polymers of hydrophilic nature with hydrophobic surfaces with the aim of removing hydrophobicity can likewise motivate cell attachment. UV and plasma grafting on polymer surfaces can likewise advance antibacterial and anticoagulation characteristics [31]. SAMs methods change the surfaces of substances for improving purpose hydrophilicity/hydrophobicity and for adding functional or reactive groups to the exterior side [71]. SAMs also alter the wettability and surface energy of polymer surfaces by vigilant decision of functional groups used for single layer, substantially enhancing biocompatibility of substance [72]. SAMs give the advantage of their simple fabrication and controlling ability over orientation and order, permitting the action of a selected group at the changed surface, building up the capacity to provide biocompatibility of substance to uniform explicit requirements [11]. Chemical alteration methods endeavor to change the surface of a material while in transit to enrich the ability of that material. Polymer substrates are a revelation to those distinctive strategies, giving a substance with an enhanced surface and the excessively unaltered properties of the bulk material. Whenever achieved well, these chemical alteration procedures may be used for SMPs, permitting a good surface without influencing the bulk material.

3.4.1.4 Thin films and surface coatings Thin films and surface coats are further methods to transform the surfaces of metals and polymers as an approach to enhance biocompatibility. These methods do not include straight joining of chemical groups or surface changes, the way customary chemical alteration methods do, anyway still change the surfaces for enhanced biocompatibility. A couple of covering and film methods that have been demonstrated to enhance endothelial cell connection or decrease thrombosis and blood coagulation are discussed. Concerning the wet films/dissolvable films of stents, dimethyl-sulfoxide (DMSO) has been shown to avoid vascular simple muscle cell action on the surface of stents, bringing down the probability for restenosis while also avoiding tissue component action, subsequently not supporting thrombosis [73]. Investigations illustrate that DMSO does not impart a poisonous quality to vascular endothelial cells, setting this method as a suitable choice for SMPs and polymers [74]. Dip coating, utilized to shape nanostructures at polymer surfaces, makes super-hydrophobic surfaces that stop blood coagulation [75]. Polymers for coatings with polyelectrolyte multilayers make available a dynamite stage for endothelial cells on the surfaces of polymers [71]. Langmuir
Blodgett (LB) films closely stuffed structures of recognized thickness deposited. These also permit for cell attachment, diminished

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platelet bond and increased hemocompatibility [55,76]. Such LB films may be coated on the surfaces of polymers by reacting with it to fascinate LB films and presenting polymer to LB trough, letting a single layer to shape before exposure to endothelial cells [77]. These LB films have still not been investigated broadly in 3D contexts; applying them on 3D assemblies may be worth further study because of the amplified biocompatibility given through this method. Polymer films in layers have been demonstrated to decrease platelet attachment on nitinol, which is a commonly utilized stent material. A coating of chitosan on the exterior of certain polymers presented better cell compatibility [12]. Investigations with certain carbon films have also shown a good enhancement of blood compatibility on the surfaces of polymers [78]. The enhanced blood compatibility in several polymer surfaces is evidently investigated with diamond-like carbon (DLC) films in several studies [79].

3.4.1.5 Pharmaceutical attachment to surfaces The connecting ability of an element to a material surface without affecting the properties of the bulk material is an interesting way to deliver pharmaceuticals. In order to connect bioactive molecules to the surface, polymer surfaces should be functionalized during the process as they have normally inert surfaces. These surface techniques can be used for SMPs and polymers; although many such techniques have been applied to polymers, they may be beneficial to use for SMPs as well. The attachment of bioactive composites may be due to ligand receptors, covalent bonding, or electrostatic interactions where covalent bonding is really stable and common. CVD is not only utilized to improve the level of biocompatibility, but it can also create tethering clusters at the surface of polymers for proteins and different biomolecules to connect by means of covalent bonding. Some of those bioactive molecules assist to create much less thrombotic surroundings by stopping on the surface of polymers inside blood vessel. For connecting pharmaceuticals onto the surface of polymers and metals, the plasma deposition method is the most stable option to produce coatings that can help in functionalization positions and corrosion resistance. 3.4.1.6 Drug delivery assistance by porous surface As mentioned earlier, stents are normally used for drug transportation by stimulating vessel restoration and permitting much improved integration of stents without the usage of altered anticoagulant drug pills. The drugs for stents can be joined straightforward to the surface of stents as shortly mentioned above, or to house the drugs until delivery they can be included into the surface of the stents by using pores. Absorbent stents permit for the combination of drugs without a further coating of polymer, which is normally present in drug eluting stents. Various SMs are used to tune surfaces for the

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persistence of covering the delivery of drugs. For long time, etching the surfaces of polymers may produce pores in the surfaces, that can be used in the localized housing of drugs. The usage of photolithography can also be considered for the creation of pores or fabrication of embedding porous micro- or nanoparticles onto the surface of a sample. In metals, sandblasting has been used to efficiently produce porous surfaces.

3.5

Thin-film deposition

The process of adding an exceptionally thin layer ranging from 5 nm to 100 μm in thickness on a substrate surface is known as thin-film deposition. The normal growth process of a thin film by depositing atoms is given in Fig. 3.7. Some features of a normal growth process are as follows: 1. Thin films start to grow with arbitrary nucleation process followed by growth stages and nucleation. 2. Both of these depend on different deposition conditions (temperature, chemistry of the substrate surface, and growth rate). 3. The nucleation stage can be changed by exterior actions (the bombardment of electrons or ions). Thin films are placed onto a bulk substrate to attain exceptional properties that are certainly not possible in the case of the substrate. These properties such as structural properties, chemical composition, and film thickness generally vary with the physical parameters of the material in bulk form and the deposition conditions [80,81]. These unique properties can be due to the small thickness of between a few atomic layers up to micrometer values. This will change the optical, magnetic, electrical, thermal, mechanical, and chemical properties to the desired level. Table 3.1 divides thin-film properties into five groups and provides examples of the distinctive applications in the case of each group [82]. The characteristics of thin films can also be influenced by the surface to volume ratios of the films. Several cases have reported that the preparation and

FIGURE 3.7 The thin-film deposition process. Reprinted from H. Adachi, K. Wasa, Thin films and nanomaterials, in: Handbook of Sputtering Technology, 2012, pp. 3
39, with permission from Elsevier.

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TABLE 3.1 Thin-films properties and their typical applications. Property category

Applications

Electrical properties

Conduction Insulation Piezoelectric drivers Semiconductor devices

Magnetic properties

Memory disks (hard disks and tapes)

Optical properties

Interference filters Antireflective/reflective films Optical waveguides Decoration (luster, color) Optical memory disks (DVDs, CDs)

Thermal properties

Barrier layers Heat sinks

Chemical properties

Gas/liquid sensors Protection against oxidation or corrosion Barrier to alloying or diffusion

Mechanical properties

Micromechanics Adhesion Hardness Wear-resistant coatings

growth of a thin film are affected by the properties of the basic material of the substrate. The effectiveness of the optical nature of metallic films as well as scientific interest regarding the performance of 2D solids is the main reason behind the increasing interest in the study of thin-film technology. The unique characteristics of thin films like their geometry, thickness, and structure are the reasons of getting indirect or direct advantages in exploring new areas of chemistry and solid-state physics. There are several revolutionary devices such as microelectronic solid-state devices and computers in which major credit goes to thin-film deposition technology. In 1957, Faraday, in a series of experiments, exploded metallic wires in a vacuum vessel for the fabrication of thin metallic films. In the past, thin-film deposition techniques was developed for example, vacuum deposition (chemical and physical), arc welding, thermal spraying, cladding, sputtering, thermal deposition, electron

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beam deposition, metal organic, atmospheric pressure, plasma-enhanced, low-pressure CVDs, and atomic layer deposition (ALD). Amongst various deposition techniques, a few of them are discussed in detail in the next section 3.5.1.

3.5.1

Deposition techniques

There are mainly two techniques for the deposition of thin films. These techniques are classified on the basis of the processes that occur in the procedure, that is, physical or chemical.

3.5.1.1 Physical vapor deposition PVD consists of different methods of vacuum deposition to develop thin coatings and films. In PVD, a material first changes to vapor phase from a condensed one and then back again to its original phase in the form of thin films. The PVD process is shown in Fig. 3.8. The key processes during PVD are evaporation and sputtering. It is used in the production of items that require thin coatings for optical, mechanical, electronic, or chemical operations. These coatings are long lasting due to being corrosion resistant, harder, and having high impact strength. PVD coatings are more environmentfriendly than earlier existing coatings. PVD provides multiple techniques for film deposition. On the other hand, there are constraints in each technique, some only work at elevated temperatures and some require a vacuum for their operation. They also require cooling systems using water to cope with

FIGURE 3.8 The physical vapor deposition process. Adapted with permission under the terms of the CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/).

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large heat loads. Its applications include semiconductor devices, for example, solar panels with thin films for energy production purposes, in aluminized polyethylene terephthalate (PET) films for food packaging, and in cutting tools coated with titanium nitride for metalworking. 3.5.1.1.1 Vacuum deposition In PVD, the material to be deposited is vaporized from the solid phase. The vapor reaches the substrate by passing through a low-pressure region (vacuum) preceded by film growth on a substrate. The ions reaching on the growing surface are necessary for the development of the structure of the film and the arrival rate and surface mobility must be balanced, providing time in which the equilibrium sites are approached by the atoms [83]. Thermal evaporation Thermal evaporation is a well-known method for coating a thin layer in which the source material evaporates in a vacuum due to high temperature heating, which facilitates the vapor particles moving and directly reaching a substrate where these vapors again change to a solid state. In this method, a charge holding boat or resistive coil is used in the form of a powder or solid bar. In order to get the high melting points necessary for metals, the resistive boat/coil is exposed to a large direct current (DC), where the high vacuum (below 1024 Pa) supports the evaporation of the metal and further carrying it to the substrate. This technique is specially applicable for material with low melting points [83,84]. A schematic of the thermal evaporation system is exhibited in Fig. 3.9. Electron beam evaporation In electron beam evaporation, the source material can be evaporated using high energy electrons in the form of an intense beam. A hot filament causes the thermionic emission of electrons, which can, after acceleration, provide sufficient energy for evaporating any material. In a typical case involving 1 A of emission accelerated through a 10 kV voltage drop, 10 kW is delivered upon impact [85]. To avoid melting the filament in the arriving evaporant, the filament is located out of sight of the evaporant as shown in Fig. 3.10 and the electron beam is pulled around to the surface by a magnetic field, B, the point shows the direction in this figure. The combined force, F, on an electron in electric (E) and magnetic field is known as Lorentz force and is given by [86]: F 5 FE 1 FB 5 qe E 1 qe ðv 3 BÞ

ð3:1Þ

where F is in N, qe in C, E in V/m, B in webers/m2 5 tesla, and the electron velocity v is in m/s. The cross-product vector, FB, is oriented perpendicularly to both v and B as shown in Fig. 3.10. The first force term in Eq. (3.1) accelerates the electrons away from the filament or cathode. The speed so acquired causes the electrons to be deflected sideways as they cross the

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FIGURE 3.9 The phenomenon of thermal evaporation in a schematic. Reprinted from R.J. Mart´ın-Palma, A. Lakhtakia, Vapor-deposition techniques, in: Engineered Biomimicry, 2013, pp. 383
398, with permission from Elsevier.

magnetic field lines in accordance with the second force term. The centrifugal force of the electrons curving at radius r balances the second force. 3.5.1.1.2 Cladding Laser cladding It is also called laser deposition, which is a technique in which one material is added onto the surface of another, base material [87]. A laser cladding system is shown in Fig. 3.11. In the laser cladding mechanism, one material in wire form or powdered form is selected and is fed under the action of a laser beam, when this is done, the laser beam is scanned through the surface of the target material. The scanned laser beam will leave a deposited coating after scanning [87,88]. A schematic of the laser cladding process using wire, a powder stream, radially symmetric injection nozzles, and a conical nozzle is shown in Fig. 3.12A
D. It provides the opportunity to use a material with minimal heat input, accurately and selectively where it is needed. Deposits should be perfectly fused onto the surface of the target or base material, and in that case, the measure of porosity will

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FIGURE 3.10 The phenomenon of the electron beam evaporation system in a schematic. Reprinted from John X.J. Zhang, Kazunori Hoshino, Fundamentals of nano/microfabrication and scale effect, in: Molecular Sensors and Nanodevices, 2019, pp. 43
111, with permission from Elsevier.

FIGURE 3.11 A laser cladding system. Adapted with permission under the terms of the CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/).

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FIGURE 3.12 A schematic of the laser cladding process using (A) wire, (B) powder stream, (C) radially symmetric injection nozzles, and (D) a conical nozzle. Adapted with permission under the terms of the CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/).

be slightly less, which is responsible for the corrosion resistance of the deposited coating. Explosion cladding It is a process also known as explosive welding in which an inclined impact is performed at high velocity. In the explosive process, energy is used to gain a low-pressure welding. A schematic of the explosive welding process before explosion and after explosion is illustrated in Fig. 3.13A and B. In the mechanism of explosive welding, two plates are present, one is known as the flyer plate and the other plate is called the base plate. The flyer plate is present on the base plate at a smaller angle. The flyer plate may be made up of sand or it may have a metallic nature and the top

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FIGURE 3.13 Explosive welding setup: (A) before and (B) after explosion. Reprinted from Zong-Xian Zhang, Special blasting techniques, in: Rock Fracture and Blasting, 2016, pp. 483
491, with permission from Elsevier.

surface of this plate is bound with some layers or buffer, which can be rubber or a coating with a greater thickness, or plastic. An explosive material, which is in the form of a sheet or in powder, is also laid down on the top surface of the buffer and from the lower side, it explodes. To attain the desired acceleration of the flyer plate, a special distance known as the standoff distance and the initial angle α between both plates are important. The collision or dynamic angle, which is also called the impact angle β, may be dependent on the arrangement of the plates. If both plates are lying parallel to each other, then the initial angle α will be zero and the impact angle β should be constant. But if both plates have inclined or curved the setup impact angle β will be changed during the process [89]. 3.5.1.1.3

Sputtering

Sputtering is a process for the deposition of a thin film largely applied nowadays in optical devices including disk drives, semiconductors, and CDs. Considering the atomic scale, it is the process of bombarding a source material with high energy particles causing the ejection of atoms, which further coats on a substrate like a silicon wafer, solar panel, or optical device. At the start of the sputtering process, a vacuum is created by a vacuum pump in a chamber enclosing any inert gas—mostly argon. A substrate for coating is placed in this chamber and a negative charge is given by a radiofrequency/direct current (RF/DC) battery to the target material. In a plasma environment, electrons from the argon atoms from the outermost shell are

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FIGURE 3.14 A schematic showing the sputtering process. Reprinted from John X.J. Zhang, Kazunori Hoshino, Fundamentals of nano/microfabrication and scale effect, in: Molecular Sensors and Nanodevices, 2019, pp. 43
111, with permission from Elsevier.

removed by the high negative charge that is applied to the target and this causes the glowing of the plasma. The positively charged inert atoms are further attracted toward the target at high speeds, which causes particles of atomic size to sputter off due to the collision momentum as shown in Fig. 3.14. These particles deposit a thin coating on the surface to be coated by crossing the vacuum chamber. For sputtering process, the kinetic energy of the bombarding particles should be high than usual thermal energies. So this can permit a more precise and pure atomic level coating of a thin film as compared to conventional thermal energies obtained by melting. The sputter yield can be referred to as the number of ejected atoms from the source material. This can be governed by the angle of incidence and the energy of the ions that are bombarded along with the surface binding energy and the relative masses of the target atoms and ions. There are numerous ways of sputtering that are extensively used including gas flow, magnetron sputtering, and ion beam sputtering. Here we shall describe only magnetron sputtering. Magnetron sputtering Due to the charge nature of ions, their behavior and velocity can be controlled using magnetic fields. The first planar source of magnetron sputtering loaded with a high field was invented by John S. Chapin in 1974. The conventional diode sputtering tends to be more effective and slower with tiny substrates that can deposit ultrathin films reaching the atomic scale. The object which is to be coated can be damaged due to overheating caused by the bombardment of the substrate. Magnets are utilized in magnetron sputtering at the back of the negative cathode to confine electrons over the target material so that the substrate is not bombarded by them, permitting the deposition rates to be faster. The rectangular magnetron is a widely used “inline” system for larger scales, whereas in “confocal,” smaller-scale

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batch systems, circular magnetrons are more common. In this type of sputtering, several methods are used to induce a high energy state with RF, alternating current (AC), and DC magnetron sources. Magnetron sputtering has advanced quickly and is now recognized as the process of choice for the deposition of various important coatings in industry. In various cases, PVD deposited films are now outperformed by magnetron sputtered films and these provide the same features as films generated by other techniques of surface coating. Therefore magnetron sputtering now has an important role in applications like low-friction coatings, corrosionresistant coatings, decorative coatings, wear-resistant coatings, and coatings with special optical or electrical properties [90]. 3.5.1.1.4 Arc welding Gas metal arc welding (GMAW) is a broadly used and successful industrial method for welding metallic pieces. The energy involved in this process is related to various phenomena such as heat flow, plasma physics, fluid flow, etc. [91]. The concept of GMAW was first presented in 1920, and then it became commercial in 1948 [92]. First, for shielding purposes, inert gas was used and, therefore, it was named metal inert gas welding (MIGW). After that, different reactive gases were used with inert gases as mixtures, which is the reason it became known as gas metal arc welding or GMAW. It is used in nearly every industry because of its advantages and versatility. GMAW is a process of welding in which heat is produced through electric arc. This arc is resulted between continuously used electrode and metal piece. A schematic of the GMAW process is presented in Fig. 3.15. Along with the use of a welding

FIGURE 3.15 A schematic of the GMAW process. Reprinted from D.S. Naidu et al., Gas metal arc welding: modeling, in: Modeling, Sensing and Control of Gas Metal Arc Welding, 2003, pp. 9
93, with permission from Elsevier.

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gun, there are some other important components, that is, a feed unit of electrode wire, a power supply, and a shielding gas source. The shielding gas tube, current wire, and electrode wire is led by the welding gun. There exist various types of electrical arc welding. In the case of manual welding, these include gas tungsten arc welding (GTAW) or sometimes called tungsten inert gas (TIG), stick welding, or manual metal arc (MMA), and GMAW. Every process has its own advantages depending on the task of the welding to be done. 3.5.1.1.5 Thermal spraying In this technique, a spray gun generates a heat source in which either wire or powder form of a selected coating material is melted. A high velocity gas jet is then used to inject the molten particles onto the surface to be coated as shown in Fig. 3.16. In comparison to other techniques, thermal spraying can provide comparatively thin layers (0.1
0.8 mm) and is also not limited in the selection of coating materials. Contrary to other techniques, thermal spraying techniques can work at lower cost, either offsite or onsite [93,94]. Presently, there are a few kinds of thermal spraying techniques including flame spray, electric arc spray, high velocity oxy-fuel spray, detonation spray, high velocity air-fuel spray, and plasma spray, which can be used for avoiding several plants problems [95]. Some other PVD techniques are defined as: G G

G

G

An intense laser is used to sputter materials in pulsed laser deposition (PLD). A plasma discharge is used to sputter a material for consequent sputtering deposition. A pulsed electron beam (PEB) removes a material by vaporization from a target and deposits it in the form a thin coating in a process called pulsed electron deposition. Artificial crystals are created by sublimation method.

FIGURE 3.16 The technique of thermal spraying and coating formation on substrate. Reprinted from C.J. Li, Thermal spraying of light alloys, in: Surface Engineering of Light Alloys, 2010, pp. 184
241, with permission from Elsevier.

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3.5.1.2 Chemical vapor deposition The method for the synthesis of materials in which a solid film is formed at a substrate surface due to the reaction of vapor phase constituents is referred to as chemical vapor deposition. By altering the experimental circumstances such as composition of gas, substrate material, total pressure gas flow, substrate temperature, etc., materials can be fabricated with variable properties. In line with the altering of these variables, it is important to completely get the reaction of reactants as it is a vital property of this method. The kinds of chemical reactions that are used in CVD for the creation of solid films are oxidation, reduction, synthetic chemical reaction, pyrolysis, hydrolysis, etc. [83]. 3.5.1.2.1

Atmospheric pressure chemical vapor deposition

Atmospheric pressure-CVD (APCVD) is a method performed at normal pressure that is utilized for depositing undoped and doped oxides. Because of its comparatively low temperature, the density of the deposited oxide is low and ˚ /min can be the coverage is normal. Films ranging between 2 and 3000 A  grown by reaction on heated wafer, typically at 400 C [96]. The high wafer throughput and simple reactor design are huge advantages of this process. APCVD suffers from poor step coverage, particle contamination, fast precursor flow, and the need for frequent cleaning, which can be minimized by the injection of reactants and a good understanding of the reaction mechanisms. 3.5.1.2.2 Low-pressure chemical vapor deposition Another vacuum based technique is low-pressure CVD (LPCVD). This process enables a conformity as high as almost 1. This is due to the nonuniform movement of particles caused by the low pressure (10
100 Pa), while the high (900 C) temperature supports conformity [96]. The stability and density are very high as compared to APCVD. Films ranging from a few nanometers to many micrometers can be deposited on semiconductors using LPCVD. 3.5.1.2.3 Metal organic chemical vapor deposition Extremely thin atomic layers can be deposited on a semiconductor wafer using the metal organic-CVD (MOCVD) technique utilizing the injection of ultrapure gases into a reactor and fine dosing. It is a highly difficult process for developing crystalline layers to produce complex semiconductor multilayer structures [97]. 3.5.1.2.4 Plasma-enhanced chemical vapor deposition A structure of choice can be created on the substrate by adding plasma along with the reaction gases in the deposition chamber in a method known as plasma-enhanced chemical vapor deposition (PECVD). The temperature range for the PECVD process is 250
350 C so there will be no thermal

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decomposition of the process gases. The preferred film quality can be achieved by controlling the parameters of the process like power input, gas flow rate, chamber pressure, inter electrode spacing, reactor geometry, substrate temperature, etc. [98]. The PECVD process is favorable because of the high film density, ease of cleaning the chamber, and low temperature, while the stress of plasma bombardment and the expense of the equipment must be taken into account [99]. 3.5.1.2.5 Atomic layer deposition Another CVD modified process to manufacture thin films is atomic layer deposition (ALD). It is a thin-film deposition technique that is grounded on the progressive use of a gas phase chemical process. Each gas reacts in such a way that the current surface is saturated and, therefore, equilibrium is achieved. The substitute gas is also allowed to react with this surface [100]. 3.5.1.2.6

Electroplating

Electroplating of metallic objects may be done when one metallic object is positioned in an aqueous solution of the salt of a similar metal or in a different one that is provided by strong electronic potential. Basically, through cathodic reactions, metallic objects are reduced because of the removal of oxygen or the addition of hydrogen, which is provided by an aqueous solution of salt [101]. In most examples, substrates used for electroplating are iron- or steel-based and Ni, Cr, and Cd are used as plating metals. This process is performed in larger tanks in which an aqueous solution of the salt of a metal is present and a power supply is attached through this. In this process, a lead sheet acts as an anode. A limited range of metals can be electroplated. Metal alloys can also be electroplated, but this is a considerably difficult process. In the aqueous solutions, to deposit a metallic film onto the substrate is a highly complicated process that is almost incomprehensible. Most of time, the process of the formation of thin films works well in the absence of external interventions, which is usually ignored. Generally, it is believed that coatings that are produced by electroplating begin as smaller nodules of the deposited material then with the passage of time it grows and develops like a coating. This type of coating has flaws where one nodule joins another nodule, that is, at the boundary, in the process of the growth of the coating. The electroplating principle and a copper coating produced as a result of electroplating are shown in Fig. 3.17. This cell contains metal ions and an electrode, that is, an anode and cathode. In this mechanism, a current is passed through a solution of liquid in a cell so that metals will be extracted and deposited onto the surface of the cathode, which is obtained as a result of the electroplating process. There is a large amount of hydrogen present because of the cathodic reactions in coatings, which are formed as a result of the electroplating process. This hydrogen

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FIGURE 3.17 Process of electroplating and electroplated coatings of copper. Reprinted from John X.J. Zhang, Kazunori Hoshino, Fundamentals of nano/microfabrication and scale effect, in: Molecular Sensors and Nanodevices, 2019, pp. 43
111, with permission from Elsevier.

may cause microstructural problems, for example, brittleness, in the coatings. One of the major problems of these coatings is that they are not uniform. Coatings produce by electroplating depend upon various factors such as the local strength of the electric field between the anode and cathode [101,102]. Practically, it is said that for the growth of all parts of the substrate; a uniform time limit is required. The thickness of deposited coating will have greater extent on the edges, corners and projections as compared to other parts because of high intensity of electric field. Advancements in electroplating are combined with conventional methods. In older methods, to deposit a coating onto the surface of a metal or onto the surface of an insulator, spontaneously occurring redox reactions were used. More current techniques of electroplating allow for coatings to be deposited which have insulating-based substrates with metals that are able to change the thickness, grain size, and the speed of growth. This method provides metal growth by the electrode connected to substrate [103].

3.6

Self-assembly

Here the term “self-assembly,” in which “assembly” means “to put together” and the “self” indicates “without outside help or on its own,” refers to a noteworthy tool in the field of supramolecular chemistry. Self-assembly is a method in which the components (macroscopic particles, molecules, or colloids) of a system arrange orderly into functional patterns or structures as an outcome of certain, local interactions of the constituents themselves, without outside guidance. It has two types, that is, dynamic or static [104], depending on the thermodynamic picture of the resultant assemblies as shown in Fig. 3.18. The arranged state reaches an equilibrium by decreasing free energy

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FIGURE 3.18 Static self-assembly and dynamic self-assembly. Reprinted from M. Varga, Self-assembly of nanobiomaterials, in: Fabrication and Self-Assembly of Nanobiomaterials, 2016, pp. 57
90, with permission from Elsevier.

in a static self-assembly (SSA) system. Once the developing assemblies are formed, they remain well-organized, but stationary; different functions and reconfigurations cannot be done governed by alterations in the outer parameters. In dynamic self-assembly (DSA), different structures are formed, changed, and worked outside the boundaries of thermodynamic equilibrium. A chaotic assembly of constituents develops into a well-organized assembly by the involvement of an external source of energy, which disintegrates, for example, as heat. The system can attain dissimilar arrangements subject to input energy rate, and if any energy is not driven by the system, it cannot work efficiently and is broken down. There are many examples of DSA including in cells, motor-powering bacteria, and fibers containing cytoskeleton. Selfassembly motivates many sorts of molecular assemblies, for example, amphiphilic fibers [105,106], SAMs [107
109], LB films [110], and higher-order designs made from nanotubes [111], nanoparticles [112], or nanorods [113].

3.6.1

Molecular self-assembly systems

Molecular self-assembly systems are at the boundary of chemistry, materials science, molecular biology, engineering, and polymer science [114
116]. Molecular self-assembly has tried to be an easy approach supported by multiple weak unit forces resulting in the creation of huge, ordered, and discrete structures from comparatively easy units. The creation of lipid layers, molecular crystals [117], self-assembled monolayers [71], and colloids [118] are exemplars of the molecular self-assembly system. It is supposed that the use of versatile and simple systems of molecular self-assembly can offer us new prospects to review a few complicated and already unknown biological development.

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Molecular engineering using self-assembly and molecular design of biological building blocks is a supporting technology that will possibly play a progressively vital part in the forthcoming technology. Larger systems like synthetic molecules also provide a level of governing over the characteristics of the components and interactions between them, which allows for the tracking of basic exchanges. Hence it is important to study the self-assembly of living systems. A wide range of examples related to functional self-assembly are offered by cells due to the presence of a large range of complex structures [119]. Molecular self-assembly offers routes to a variety of sequenced structures, for example, liquid crystals, molecular crystals [120], phase-separated [71], and semicrystalline polymers. It offers a general strategy for creating ensembles of nanostructures. There is prospective for its application in electrochemistry, smart materials, protein binding, nonfouling properties, corrosion resistance, DNA assembly, molecular electronics, cell interactions, and biological arrays [121]. Hence self-assembly is significantly useful in various areas such as physics, chemistry, biology, nanoscience, and materials science.

3.6.2

Idea of molecular self-assembly

Molecular self-assembly (MSA) is the assembly of molecules within an ordered structure with the help of multiple intermolecular forces comprising relatively weak noncovalent interactions, such as electrostatic, π
π stacking, Van der Waals forces, hydrogen bonding, coordination, and ion-dipole interactions [114,115]. The new microscale structures can be developed by controlled and accurate use of intermolecular forces. With a lot of nature-based examples, MSA is an extensively studied phenomenon that is yet to obtain comprehensive understanding. The noncovalent interactions of covalently prefabricated building blocks like protein tertiary structures, bilayer lipid liposomes, complex biological processes as well as DNA double helices are the foundations behind the supremacy of nature to accurately construct diverse complex biological functions. MSA is abundant in nature and now appears as a new approach in nanotechnology, chemical synthesis, materials, engineering, and polymer science. A new class of materials can possibly be created at the molecular level using MSA. Taking into account the technological and fundamental point of views, the functional hybrid materials that are dependent on this method are significant. For example, organic and inorganic systems that are prepared noncovalently have played major roles in the designing of various hybrid materials and these are worthwhile for biological and optoelectronic applications [115,122,123]. The morphological and optical properties of these soft materials ultimately depend on the packing of the molecules. Unique hybrid materials with boosted optical, mechanical, electronic, and thermal properties can be constructed using self-assembled architectures fabricated from biomolecules/organics/polymers.

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3.6.3

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Equilibrium and nonequilibrium self-assembly

Self-assembled systems are confined to a local minimum in the energy landscape. Its time evolution is influenced by the shape of the energy landscape and two situations can be foreseen. If the energy barrier for a pathway leading to the thermodynamic equilibrium is low enough, that is, on the same order of magnitude as kBT (where kB is the Boltzmann constant and T the temperature), the system will slowly relax to a more stable structure. Such a system is in a so-called metastable state. Note that multiple metastable configurations can exist along the pathway to the global minimum. However, when the energy barrier is much higher than kBT, the system will remain captured in the local minimum for a period much longer than the experimental observation. This state is commonly referred to as a kinetically trapped state. In the latter case, suitable experimental procedures have to be undertaken to ‘‘help’’ the system to escape this trap [124]. Dissipative self-assembled systems require a constant influx of energy or matter (e.g., a chemical fuel or light) and for the removal of waste products to be kept steadily in a dissipative nonequilibrium state. If the energy supply stops, the system relaxes spontaneously to the thermodynamic state or to a nondissipative nonequilibrium state encountered on the way. The term “dissipative structures” as formulated by Prigogine, refers to emergent structures or patterns that are formed on length scales much larger than the individual molecules, the latter of which do not form such structures or patterns at equilibrium. Instabilities occurring far from equilibrium such as those due to reaction
diffusion phenomena, can lead to dissipative structures even on the millimeter scale, that is, far beyond the length scale of typical intermolecular interactions (e.g., hydrogen bonding, ionic interactions, p
p stacking, etc.) [125]. In supramolecular chemistry, well-ordered structures often already exist in nondissipative states, and to this day, it is unclear how dissipation in self-assembly is related to dissipative structures in the Prigogine sense. What is clear, is that dissipative self-assembly is an exciting new direction [126
129] where challenges such as obtaining nonequilibrium steady states (NESS) or oscillations are abundant.

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1079.

Chapter 4

Properties of nanomaterials Muhammad Rafique1, Syeda Hajra2, Muhammad Bilal Tahir2, Tahir Iqbal Awan2, Almas Bashir2 and Aqsa Tehseen2 1

Department of Physics, University of Sahiwal, Sahiwal, Pakistan, 2Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan

Chapter Outline 4.1 Background history of subatomic particles 4.2 Subatomic physics to chemical systems 4.2.1 Types of chemical bonds 4.3 Properties of nanomaterials 4.3.1 Electrical properties

4.1

89 90 91 97 97

4.3.2 Mechanical properties 4.3.3 Thermal properties 4.3.4 Magnetic properties 4.3.5 Optical properties References Further reading

100 103 109 113 115 117

Background history of subatomic particles

Nanostructure technology is a wide area of development and research activity that has been developing day by day over the past few years. It has important commercial influence, which will certainly grow in the future. Fig. 4.1 shows the evolution of nanotechnology in terms of patent and journal records yearly [1]. Nanoscale materials can be defined as substances that have dimensions less than 100 nm, for example, surface coatings and thin films. Nanoobjects and nanoparticles have 2 and 3 dimensions less than 100 nm, for example, Carbon Nanotubes (CNTs). Nanomaterials have different classifications, for example, nanotubes, fullerenes, quantum dots, and dendrimers. In nature, nanostructures were already formed like skeletons and seashells, moreover, early humans formed nanoscale smoke particles during the use of fire. But the scientific story of nanomaterials began much later. In 1857, Michael Faraday reported on colloidal gold particles for the first time. After 70 years, nanostructured catalysts were investigated [1,2]. In 1991, the discovery of CNTs opened a new concept in the field of nano and materials sciences. It was observed that CNTs have great electrical, Chemistry of Nanomaterials. DOI: https://doi.org/10.1016/B978-0-12-818908-5.00004-4 © 2020 Elsevier Inc. All rights reserved.

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FIGURE 4.1 Evolution of nanotechnology for patent and journal records annually. Reprinted from M. J. Pitkethly, Nanomaterials
the driving force, Materials Today (2004) 20
29, Copyright (2004), with permission from Elsevier.

electronic, mechanical, tensile strength, and magnetic properties. For example, CNTs have 100-times more tensile strength than other hard metals like steel and iron. Moreover, CNTs are great conductors of electricity and heat; they can conduct more heat than copper and other polymer materials [3]. Another type of solid nanomaterials is nanowires, which have a diameter smaller than 100 nm. There are three types of nanowires and each has different properties. The first type is called insulting nanowire, for example, Titanium dioxide (TiO2) and Silicon dioxide (SiO2). The second type is called metallic nanowire, that is, Gold (Au), Nickel (Ni) and Platinum (Pt) ; while, the third type is semiconducting nanowire, for example, GaN, InP, and Si [4]. Nanometer-sized crystals with sizes below 10 nm are called quantum dots (QDs), which can vary from 2 to 10 nm. QDs are a precise type of semiconductor in which electrical conductivity is tunable through exposure to light and by changing the voltage used. In QDs, small numbers (#100) of electrons are free, so they are also known as atomic clusters. Electrons are confined in different directions, so on the basis of these confinements, QDs are classified into three types, namely self-assembled, vertical, and planar QDs. Mostly structures are lens-shaped or pyramidal and roughly 10 nm in size in the case of self-assembled QDs. While planar and vertical QDs are 10 nm in size with dimensions about 100 nm [5].

4.2

Subatomic physics to chemical systems

When two or more atoms are combined, the energy of two single atoms is greater than the combined energy of two atoms and then chemical bonds are

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formed. In other words, we can say that the probability of finding electrons between two atoms becomes greater as a result of the charge transfer that occurs between two atoms. Nanomaterials have properties in microscopic dimensions and these properties are different from bulk properties. In nanostructures, a single bond or molecules effect the functions and properties of the materials [6
8].

4.2.1

Types of chemical bonds

Chemical bonds are categorized on the basis of the rearrangement of the probability density that occurs on bonding, namely (1) covalent bonds, (2) ionic bonds, (3) metal bonds, and (4) Van der Waals interaction, the details of these bonds are given in Sections 4.2.1.1
4.2.1.4.

4.2.1.1 Ionic bonds In ionic bonds, the complete transfer of one or more electrons occurs between the donor and acceptor elements. There are few factors that cause the formation of ionic bonds; one of them is the large differences in electronegativity of atoms, which attract other atoms for the transfer of their electrons. This chemical interaction of electrons creates a strong bonding between the atoms as compared to other types of bonds. For example, in the case of Sodium chloride (NaCl) or Potassium chloride (KCl), an electron is transferred between the donor (Na) and acceptor (Cl). As a result, an Na1Cl2 salt is formed as shown in Fig. 4.2. A large amount of energy is required to transfer the electrons from the sodium to the chlorine atom. After the transfer of electrons, sodium loses 3 s electrons and becomes sodium ions (Na1), while the chlorine element gains an electron and becomes chlorine ions (Cl2) [8]. In nanotechnology, pure electrostatic interactions of electrons between ionized atoms such as salts (NaCl) are of less interest. As compared to salts, poly ions as well as molecular ions are of great interest in this field. Macromolecules have a large amount of parallel functional groups, so when these macromolecules are ionized then polyionic macromolecules are

FIGURE 4.2 Ionic bonding between sodium and chlorine atoms. Reprinted from E. Stauffer, J. A. Dolan, R. Newman, Review of basic organic chemistry, Fire Debris Analysis (2008) 49
83, Copyright (2008), with permission from Elsevier.

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formed. When polyionic macromolecules interact with small oppositely charged ions, stable multiple thin layers are formed as a result. Electrostatic bonds, surface charges, and electrostatic repulsion are necessary for the operation of nanoparticles, micelles, and macromolecules in the liquid phase. Moreover, by controlling the surface charges we can create and stabilize nanoheterogeneous systems [7,8]

4.2.1.2 Covalent bonding In covalent bonds, the sharing of electrons occurs between two elements. Double occupied binding orbitals are formed due to strong interactions between two atoms occurring with unpaired electrons. As compared to other bonds, coolant bonding mostly occurs between two atoms, for example, nitrogen (N2) and oxygen (O2). For example, hydrogen (H2) gas cannot exist as a single atom in normal conditions, it shares its unpaired electrons with another H atom and forms nonpolar, covalent, diatomic molecule H2. The covalent bonding of homoatomic H2 molecule, heteroatomic HCl, and Ethene (C2H4) molecules are shown in Fig. 4.3. The polarity for covalent bonding is provided by differences in the electronegativity of two atoms. There are many factors that affect covalent bonding such as a valence or a large number of atoms. When the number of atoms increases, disk-shaped or 3D-structured solids are formed. In nanotechnology, it is important to fix the rules for the distribution of electron density. These rules may include the angle between the bonds, the number of nonbinding outer electrons, and the number of bonds per atom [7,9]. Valency and the direction of atoms greatly affect the geometry of the bonds. For example, bent or linear structures are formed by bivalent atoms, while trigonal pyramidal or trigonal planar geometries are formed by trivalent atoms as shown in Fig. 4.4. Moreover, four valences of atoms form square, tetragonal, or planar geometries, which can be distorted in the case of asymmetric substitutions. Five and six valences form square pyramids as well as trigonal or octahedral pyramids as shown in Fig. 4.5. The topology and mobility of the atoms of bonds are determined by the arrangements and orientations of the bonds. So when the bonds are formed, then the topology of the bonds is used to control the degrees of freedom, molecular geometry, and external pressure effects on the bonds. The free rotation of bonds can be narrow without intermolecular bridges due to the presence of double bonds. So in nanotechnology, double bonds and bridged structures of covalent units are important themes for molecular structural design [7,9]. 4.2.1.3 Metallic bonds Metallic bonds are formed when the charge is spread over a larger distance as compared to the size of single atoms in solids. Mostly, in the periodic

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FIGURE 4.3 Covalent bonding by electron sharing for H2, HCl, and C2H4 molecules. Reprinted from E. Stauffer, J. A. Dolan, R. Newman, Review of basic organic chemistry, Fire Debris Analysis (2008) 49
83, Copyright (2008), with permission from Elsevier.

table, left elements form metallic bonds, for example, zinc and copper. Because metals are solid, their atoms are tightly packed in a regular arrangement. They are so close to each other so valence electrons can be moved away from their atoms. A “sea” of free, delocalized electrons is formed surrounding a lattice of positively charged metal ions. These ions are held by strong attractive forces to mobile electrons; in this way, metallic bonds are formed as shown in Fig. 4.6 [8]. Metallic bonds are also formed by the exchange of binding electrons without asymmetric dispersal of the electron density. A single covalent bond takes place when the exchange of electrons follows only one direction. While a metallic bond is formed when the exchange of electrons occurs in different spatial directions and is also combined with a high mobility of the binding electrons. A 3D structure of equal bonds is formed when there is a simultaneous existence of bonds in several asymmetric directions. Moreover, clusters are formed when the smallest quantity of atoms is involved in the bonding. Solids are mostly conductive in nature due to the high movement of the binding electrons as shown in Fig. 4.7 [7,10].

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FIGURE 4.4 Relation between molecular geometry and valences for two, three, or four valent atoms. Adapted with permission under the terms of the CC0 1.0 Universal (https://creative commons.org/publicdomain/zero/1.0/).

In micro- and nanotechnology, metal bonds are of great interest due to the wide-spread application of semiconductors and metals as electronic or electrical materials in different devices. Moreover, metal bonds enable the connection of both thermal and electrical conductivity at boundaries between several alloys and metals. In nanotechnology, metal bonds serve as tunneling barriers recognized by local restrictions of electron mobility. Additionally, for magneto-resistive sensors, different arrangements of ultrathin magnetic layers have the ability to change the magnetic properties with constant electrical conductivity [7,11].

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FIGURE 4.5 Relation between molecular geometry and valences for five and six valent atoms. Adapted with permission under the terms of the CC0 1.0 Universal (https://creativecommons. org/publicdomain/zero/1.0/).

4.2.1.4 Van der Waals interactions This type of interaction is formed due to the electrical attraction between two or more dipoles or atoms that are very close to each other [8]. This type of binding occurs due to the interaction of the shells of atoms with each other. When atoms of different elements attract each other, the electrons of one atom disturb the electron distribution of the other atom. During this interaction, the sum of the energies of the two atoms is lower than the sum of the energies of the isolated atoms. The strength of the bond depends upon the difference in the initial and final energies of the interacting atoms.

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FIGURE 4.6 Schematic of metallic bonding in a piece of metal.

FIGURE 4.7 Comparison of energy levels of metal nanoparticles (right) and molecules (left). Reprinted from M. Kohler et al., Molecular Basics, Wiley Books, 2007, with permission from John Wiley and Sons.

Van der Waals interaction is a weak bond and can be broken easily at room temperature [12]. Among nonpolar and polar molecules when they lie large distances apart, the Van der Waals interaction creates attractive forces between them. When these molecules lie at short distances apart, the interaction is repulsive due to the repulsion of the electron shells. There are three types of Van der Waals interactions, namely: 1. Induced dipole-induced dipole (London dispersion) interaction, in which instantaneous charge fluctuations create the interaction of the moments that arise in the classical model. 2. Dipole-dipole (orientation) interaction, which occurs between molecules that have permanent dipole moments called polar molecules.

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3. Permanent dipole-induced dipole (induced dipole) interaction, this type of interaction occurs between polar and nonpolar molecules. In this case, the first molecule permanent dipole moment interacts with the moment induced by its field into the second molecule [13,14]. In nanotechnology, Van der Waals interactions are not of much importance due to the presence of solids consisting of a large number of molecules instead of individual atoms. Van der Waals bonds are mostly used for the combination of electron shells with molecules and surfaces when two or more atoms are combined by metal, covalent, or ionic bonds. In this way, the number of atoms in molecules increases, which enhances the attraction of molecules for binding on the substrate. These bonds are also important for cell structure as they form a 3D structure of proteins. Moreover, in hydrophobic interactions, Van der Waals interactions play a vital role and this is important for microlithography technology [7,15].

4.3

Properties of nanomaterials

Some nanomaterials are naturally occurring, but many nanomaterials are artificially designed through different methods. These nanomaterials have many unique electrical, optical, mechanical, and magnetic properties. On the basis these properties, nanomaterials have been greatly used in different nanotechnology applications, in healthcare, for electronic applications, for environmental protection, and also in information technology [16].

4.3.1

Electrical properties

The electrical conductivity of materials is measured by taking the inverse of their electrical resistivity. The specific electrical resistivity of conventional coarse-grained (CG) Copper (Cu) is 7
20 times smaller than that of nanocrystalline (NC) Cu with a grain size of 7 nm measured at temperatures below 275 K. As shown in Fig. 4.8, at temperatures .100 K, the specific electrical resistivity of CG Cu and NC Cu increases linearly with increasing temperature, but the temperature coefficient of resistivity for NC Cu is equal to 17 3 10 2 9 Ω cm K21, which is higher than the 6.6 3 10 2 9 Ω cm K21 of conventional Cu. The coefficient of electron scattering at the grain boundaries in NC Cu is higher by a factor of two in comparison to CG Cu. This difference created between CG and NC Cu is due to the different widths and structures of the materials and the developed volume fraction of grain boundaries. The high electrical resistivity of NC Cu is caused primarily by electron scattering at the grain boundaries and the short mean free path (λ) of electrons, that is, λ  4.7 nm for NC Cu in comparison to λ  44 nm for CG Cu. Moreover, a decrease in the crystallite size increases the degree of localization, decreases the concentration of charge carriers and, hence, increases

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FIGURE 4.8 Comparison of resistivity of normal crystalline Ni and nanocrystalline Ni. Reprinted from U. Erb, Electrodeposited nanocrystals: synthesis, properties and industrial applications, Nanostructured Materials (1995) 533
538, Copyright (1995), with permission from Elsevier.

the specific electrical resistivity. As compared to CG metals, Cu, Ni, and Iron (Fe) submicrocrystalline have specific electrical resistivities 15%, 35%, and 55% higher at 250 K respectively [17,18]. Another nanomaterial, graphene, is considered a zero-gap semiconductor, which shows semiconducting behavior even at room temperature with a resistivity range of 100
300 K. Resistivity is greatly affected by temperature, as at room temperature thermal conductivity is (5.30 6 0.48) 3 103 WmK21 [19]. While on the other hand, the resistivity of layered graphene decreases with increases in temperature and number of layers. Novoselov et al. investigated the resistivity of graphene field-effect transistors whereas these transistors depend upon the gate voltage and Ferroelastic Point Group (FG). Moreover, by increasing the layers of graphite, the mobility of charge carriers also increase [20]. The intrinsic mobility of graphene depends upon the charging impurities on the substrate surface or top of the graphene, phonon-scattering, and corrugation/ripples in the graphene sheet. Phonon scattering, however, limits the mobility and the theoretically calculated value of intrinsic mobility at room temperature is above 105 cm2(Vs)21. The results revealed that graphene does not show superconducting behavior itself because of the Josephson effect. The transmission of

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the superconductor
graphene
superconductor junction has been investigated using the nonequilibrium Green’s function method and the results predicted that it is possible to construct a superconducting switch. Moreover, making a sandwich of palladium (Pd) sheets between graphene sheets increase the superconducting transition by 3.6 K with superconductivity occurring in the Pd sheets [21,22]. Structural as well as electrical properties of different materials are carefully considered for the fabrication of oxygen pumps, oxygen-separation, water electrolysis, fuel cells, ceramic membrane, oxygen sensors and oxideion conductors. For example, microstructural defects such as interstitial and vacancies play a vital role in the nickel oxide crystallites and they are responsible for the electrical conductivity of nickel oxide. It was observed that a perfect crystalline structure is formed by increasing the substrate temperature. As a result, the concentration of charge carriers (electrons and holes) decreases and, thus, the electrical conductivity decreases [16]. In the case of Tin oxide (SnO2), electrical conductivity increases when the grain size decreases. It was also observed that conductivity increases with grain size. This may produce charged states of O2 by causing a higher fraction of the grain boundary volume. Due to the migration of charged particles of SnO2 (e.g., O22 and O2) a low-frequency reduction was observed. It is also reduced when bulk species like O2 defects VO21 cross the grain boundaries [23]. La-deficient La2GeO5 is another example of ions conductor which has become interesting due to its high oxide ion conductivity and unique structure. It has a high conductivity value that is equivalent to that of fast oxide ion conductors. Fig. 4.9A and B shows the film thickness effect on the oxide ionic conductivity of La1.61GeO5-δ. It shows that with a decreasing thickness of thin films, oxide ion conductivity is increased. It also shows that La1.61GeO5-δ is a function of the partial pressure of O2 and temperature [24]. At low temperatures, the conductivity of the film increased, for example, with a thickness of 373 nm its value is 0.05 S/cm21 at 573 K. Moreover, conductivity of the film increased by increasing the temperature. This is because of direct relation of temperature with local stress, which increased the movement of oxide ions within the film that improves the value of conductivity [24,25]. The electronic conductivity of cerium oxide is increased by doping it with hexa- or penta-valent donor ions. Moreover, the electrical conductivity of polycrystalline cerium oxide also depends on the grain size of the donor materials. It was observed that cerium oxide with a large grain size, has a high electrical conductivity value due to ionic contribution. The ionic partial conductivity decreases with decreases in the grain size due to a defect in accumulation in space charge layers. Moreover, electronic conductivity increases, resulting in a transition from largely ionic to electronic conductivity at a grain size of about 60 nm. The absolute value of electronic conductivity for a bulk cerium oxide is about 1 3 1027 S/cm [16].

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FIGURE 4.9 (A) Graph of thin films of La1.61GeO5-δ and bulk La1.61GeO5-δ. (B) PO2 dependent electrical conductivity in thin film of La1.61GeO5-δ with different thickness values at 873 K. Reprinted from T. Ishihara, J. Yan, H. Matsumoto, Extraordinary fast oxide ion conductivity in La1.61GeO5-δ thin film consisting of nano-size grain, Solid State Ion. 177 (2006) 1733
1736, Copyright 2006, with permission from Elsevier.

4.3.2

Mechanical properties

4.3.2.1 Hardness Conventionally, the plastic distortion of crystalline materials is due to the movement of the dislocations. The movement of dislocations is obstructed by the grain boundaries in polycrystalline materials. A decrease in grain size of given materials causes an increase in the volume fraction of grain boundaries (as high as 1013
1014 cm23 for 10 nm nanocrystallite), which in turn increases the hardness and strength as represented by the Hall
Petch Eq. (4.1): σ 5 σ 1 kxd21=2

ð4:1Þ

Where flow stress is denoted by σ and grain size by d, whereas σ0 and k are constant parameters and depend upon nature of the materials. The σ0 is the friction stress and the constant k reflects the difficulty in slip transfer from one grain to another. Equation 3.1 indicates a rise in strength by reducing the grain size of the particles. Hence materials with grain sizes ,100 nm exhibit substantially higher strengths than conventional materials with grain sizes .10 μm. The yield stress of Pd with a grain size of 14 nm is about 259 MPa, which is approximately five-times that of CG (grain size B50 μm) Pd (i.e., 52 MPa). Moreover, NC Cu shows an ultimate tensile and yield strength of around 1100 MPa and 800 MPa respectively, which are much higher than those of an annealed high-purity bulk Cu [18]. The microhardness of CG Cu increases with deceases in the grain size from 25 to 5 μm. The microhardness of a 5 μm grain size Cu is about 2.5-times lesser than a 16 nm grain size NC Cu.

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Similarly, in the case of titanium aluminum (TiAl), an intermetallic compound, with grain sizes above B20 nm, an enhancement in hardness was found with a decreasing grain size. But when the grain size of TiAl was further decreased to below 20 nm, a decrease in the hardness was found [26]. It has been theoretically as well as experimentally shown that Frank-Read sources for dislocation generation are almost impossible in most of the cases when nanomaterials have grain size range between 10 to 50 nm. Hence the deformation of these nanomaterials via dislocations is not possible. Therefore further decreases in the grain size of NC Cu from 16 to 8 nm decreases the microhardness by about 25%. A similar decrease of microhardness was also found with a reduction in the grain size of NC Pd from 13 to 7 nm. Similarly, a decrease in the microhardness of NC alloys of Titanium Aluminide (TiAl), Niobium Alumnide (NbAl3) TiAlNb, and NickelPhosphorous (Ni-P), was found with a reduction in the grain size from 60
100 nm to 6
10 nm. This smallest range of grain size follows an inverse Hall
Petch relationship [27]. Graphene has sp2 hybridization, so it is considered stronger and stiffer than CNT. For a defect-free monolayer, excellent values for graphene have been calculated for intrinsic strength and young modulus, that is, 130 GPa and B1 TPa respectively and a breaking strength of about 42 N m 2 1. Different results revealed that by adding 0.6
1 wt.% of graphene to Polymethylmethacrylate (PMMA) and poly(vinyl alcohol) leads to increases in elastic modulus and tensile strength of between 20% and 80% [28
30]. It was found that the hardness of micro-Ni or bulk Ni is four-times less than the hardness of nano-Ni. It was observed that 20 nm sized grains have less hardness compared to 10 nm sized grains, indicating grain-boundary strengthening in the NC range. Similarly, at 300 K, CG materials have 2
7 times less microhardness compared with NC materials [26]. A NC electrodeposited Ni with an average grain size of 26 nm shows a tensile strength .2.2 GPa and a Vickers hardness of about 600 MPa. However, below 26 nm, a downward deviation from the values predicted from the Hall
Petch equation can be seen clearly in Fig. 4.10. A negative slope is seen for Ni with a grain size below B11 nm. However, below a grain size of 6 nm, the measured value of the hardness is decreased by rapid room temperature creep and roughly follows the inverse Hall
Petch equation [17]. This could have happened due to the fact that below the critical grain size (dc), grain boundaries sliding and/or diffusion flow may become important deformation mechanisms even at room temperature, greatly increasing the deformation rate. These deformation mechanisms cause decreases in the hardness or saturation and also decrease the strength with decreasing grain size (d) in the range of d less than dc. In brief, there is a critical size where the competition between the grain boundary (GB) diffusional creep (Coble creep) balances and lattice dislocation slips cause saturation and give the inverse Hall
Petch relationship that have a value less than the dc [18,26].

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FIGURE 4.10 Hall
Petch plot for electrodeposited Ni. Reprinted from U. Erb, Electrodeposited nanocrystals: synthesis, properties and industrial applications, Nanostructured Materials (1995) 533
538, Copyright (1995), with permission from Elsevier.

The GB density increases in polycrystalline materials when the grain size decreases from tens of microns to the critical grain size (dc). This is considered as a positive factor because they act as barriers to the dislocation motion. But generally, on the basis of structure and material parameters, the critical value of dc differs from 10 to 30 nm. At the GBs, the stress concentration is enough to nucleate slip in the next grain. While a large applied stress will be needed to yield the boundaries, that is, the material is braced. The stress needed to yield a GB is based on the GB energy state and structure, that is, the force needed to yield a high-angle GB as compared to a small-angle boundary and to start a successive motion of dislocations in neighboring grains [9]. However, the microhardness of submicrocrystalline Cu and Pd with mean grain sizes of 200
300 nm and 150 nm respectively, and calculated at room temperature after annealing at the temperature range of 500
600 K, decreases rapidly by almost a factor of 3 due to the grain growth and partial annealing of dislocations [26].

4.3.2.2 Elastic modulus A quantity that is capable of determining the resistance of a substance being deformed by applying stress is known as elastic modulus. The value of the

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elastic modulus will be higher when bond strength is higher. The elastic properties of crystalline materials are mostly microstructure independent. However, a large increase in defect concentration and vacancy are expected to decrease the elastic modulus. Due to their high defect concentration, the elastic modulus of nanomaterials was found to be reduced by B30%
50% in comparison to bulk materials. This significant drop in the elastic modulus of NC materials occurs because of the large volume fraction of GBs with a thickness of 1 nm or more. The elastic modulus of GB is only 12% of the bulk counterpart. However, in the case of NC materials having grain sizes smaller than 5
10 nm, the resulting number of atoms situated close to the GBs is extremely large and these materials may have an atomic structure close to amorphous samples. The elastic modulus of amorphous materials is approximately half the modulus of similar CG crystalline materials. Therefore the elastic modulus of nanomaterials having grain sizes of 5
10 nm or less is considerably lower in comparison to bulk materials [26]. If we compare CNTs and nanograined materials, then the elastic modulus of CNTs becomes high when the diameter of the tube is decreased due to the effects of surface tension [1].

4.3.3

Thermal properties

4.3.3.1 Heat capacity The heat capacity of a substance is thoroughly associated to its configurational and vibrational entropy, which is considerably affected by the neighboring configurations. The heat capacity of bulk materials is 1.2
2 times lesser than nanopowders within the temperature range of 10 K # T # Debye temperature. The higher value of heat capacity of nanopowders is due to the wide surface area of these materials [26]. The heat capacity of CG Ni was found to be about 1.5
2 times smaller than the heat capacity of nano-Ni at temperatures less than 22 K. The heat capacity value of metallic glass such as Pd72Si18Fe10 is higher than that of polycrystalline Pd by about 8%. This can be obtained from the different atomic structure and the deviation in the chemical composition from the Pd. The high heat capacity of NC materials has been attributed to a large fraction of GBs, which contain free volumes and the presence of some impurities like H2 [31]. Moreover, the heat capacity of compacted NC samples of nano-Cu with a grain size of 8 nm and nano-Pd with a grain size of 60 nm was studied in the temperature range of 150
300 K. The comparative density of the nano-Pd and nano-Cu samples was found to be 80% and 90% density of pore-less polycrystalline CG Cu and Pd respectively. The heat capacity of bulk Cu and bulk Pd was found to be 9%
11% and 29%
53% lesser as compared to that of the nano-Pd and nano-Cu specimens correspondingly. Similarly, the

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heat capacity at low temperatures for CG Cu was 5
10 times smaller than that of the bulk NC compacted Cu with a grain size of 8.5 nm or 6.0 nm in temperature range of 0.06
10.0 K. The low relative density of nano-Pd as compared to nano-Cu suggests an open atomic structure of GB component in the nano-Pd. In this way, the coupling between the atoms becomes weak, which enhanced the value of heat capacity because of the GB component [26,31].

4.3.3.2 Melting point The melting point is referred to as the temperature at which the molecules, atoms, or ions of a crystalline material change their periodic ordered state to a disordered state. A number of studies reveal that the melting points of metals such as Pb, Sn, Au, Cd, Bi, Al, Ag, and In decrease with decreases in their size, particularly below 30 nm. When the mobility of atoms increases on material surfaces then melting also starts in the materials. The diffusion coefficient associated with these atoms leads to liquid-like values at temperatures lower than those for bulk materials [32
34]. The reason for this is the large surface area to volume ratio of nanoparticles, which in turn have high surface energies; hence, the activation energy required for the melting of the surface atoms is lesser than for bulk materials. An example of a decrease in the melting point of aluminum (Al) as a function of Al clusters is shown in Fig. 4.11. There is a reduction in the melting point seen when the size of the clusters becomes small. A reduction of 140 C for the Al clusters has been reported with radii of B2 nm [34].

FIGURE 4.11 Melting point as a function of Al cluster size, where R is the radii and Tm is the melting point of Al cluster. Reproduced from S. L. Lai, J. R. A. Carlsson, L. H. Allen, Melting point depression of Al clusters generated during the early stages of film growth: nanocalorimetry measurements, Appl. Phys. Lett. 72(9) (1998) 1098
1100, Copyright (1998) with permission AIP Publishing.

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FIGURE 4.12 Enthalpy and melting point of fusion of Al nanoparticles versus reciprocal of grain size (A) Melting point and (B) enthalpy of fusion of Al nanoparticles versus reciprocal of grain size . Reprinted from J. Eckert et al., Melting behavior of nanocrystalline aluminum powders, Nanostructured Materials (1993) 407
413, Copyright (1993), with permission from Elsevier.

Several researchers have taken care to avoid the oxidation of Aluminium (Al) and Iron (Fe) by isolating these nanoparticles using an inert material. NC powder manufactured by mechanical erosion under different atmospheres (oxygen, hydrogen and argon) has different melting behaviors. The grain size attained in the oxygen atmosphere was 13 nm, while it was 22
25 nm for the Al nanoparticles synthesized in the argon and hydrogen atmospheres. Fig. 4.12A and B shows that both the enthalpy of fusion as well as the melting point of nanoparticles are linearly proportional to the reciprocal of grain size. For a grain size of 13 nm, the melting point decreases gradually until it reaches its minimum value of 836 K (563 C). The impurity of a Fe due to attrition after 80 h of milling was ,0.1 wt.%. There was a negligible effect on melting point of Al nanoparticles due to a very small fraction of Fe as an impurity. The enthalpy of fusion is also found to drop significantly with decreases in the grain size of the samples. The stored enthalpy of cold work is also a possible source of the decrease of melting point. However, the bulk melting point of Al was not recovered even after remelting. Thus it is supposed that the melting point depression was produced by storing enthalpy of cold work, so this effect should be removed upon melting of the samples. A decrease in the enthalpy of fusion indicates that the nanoparticles of metals have a higher free energy relative to that of liquid metals, that is, the nanoparticles become more like liquid [35]. However, Germanium (Ge) nanocrystals show interesting phenomena in silica glass as they melt at a temperature of almost 200 C, which is greater than the melting point of bulk Ge. Similarly, lead nanoparticles also show different behavior when inserted in an Al matrix; they exhibit superheating. This is probably due to an increase in volume during the heating of the nanoparticles, which increases pressure on the particles of the matrix. Hence with

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FIGURE 4.13 Normalized melting point (Tm/TmB) versus particle size for Au nanoparticles, where TmB is the melting point of bulk Au and Tm is the melting point. Adapted with permission under the terms of the CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/).

decreases in the size of particles (nanoparticles), the melting point of these particles injected in a bulk matrix increases [35]. Moreover, the melting point of gold (Au) nanoparticles becomes dramatically lower as the particle size decreases, particularly below 10 nm. This is the reason that sub-10 nm particles sinter at temperatures much lower than those required for larger particles [16]. Fig. 4.13 shows the normalized melting point (Tm/TmB) versus particle size for Au nanoparticles, where the melting point is denoted by Tm and TmB is the melting point of bulk Au [7]. For example, the melting point of 3 nm sized Au nanoparticles is more than 300 C, which is lower than the melting point of bulk Au. Bulk Au has a melting point of about 1336 C [8]. The approximate melting point of nanoparticles may be obtained using the equation: Tm 5 TmB ½1
ðα=dÞ; where α is a constant which depends on the melting temperature as well as the surface energy of the material. The d denotes the diameter of the nanoparticles. The melting point and heat of fusion of Tin-Silver (Sn-Ag) alloy nanoparticles decreases to about 195 C (versus the B220 C for bulk) and to about 10 J/g respectively for nanoparticles with a radius of about 5 nm [36]. Similar to nanoparticles, Zn nanowires also show a shift in endothermic peak to lower temperatures, indicating a decrease in melting point with a decrease in nanowire diameter [33]. Table 4.1 summarizes the depression in melting points of different metals and alloys as a function of their sizes. From Table 4.1, the following important observations are drawn. 1. In the case of Al clusters, ΔT of 160 C was reported for particle sizes varying between 5 and 20 nm. In contrast to this, for the same range of

TABLE 4.1 Summary of size-dependent melting behavior of metal/alloy nanoparticles and nanowires. Metal/ alloy

Atomic radius (nm)

TmBa ( C)

Tmb ( C)

ΔTc ( C)

Onset particles diameter (nm)

Endpoint diameter (nm)

Bulk particles diameter (nm)

Au

0.114

937

187

750

20

5

200

Al

0.143

660

500

160

20

5

60

Zn

0.133

420

409

11

50

25

225

Bi

0.150

271

150

121

10

2

100

Sn 5 0.151

220

195

25

20

5

35

Sn
Ag

Ag 5 0.144 a

TmB is bulk melting point, Tm is the melting point for NPs or nanowires (NWs), ΔT 5 ( TmB 2 Tm).

b c

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particle size (5
20 nm), a small depression of 25 C was found for Sn
Ag alloy. For Bi nanoparticles, a depression of 121 C was reported for particle sizes varying between 2 and 10 nm. 2. Zn nanowire arrays (diameter: 50
25 nm) show a small (B11 C) decrease in melting point. 3. It is interesting to see a decrease of 750 C in the melting point of Au nanoparticles when their size is 5
20 nm diameters. This is a point of concern for Au used for interconnection applications in the electronics industry. 4. The author of this book could not correlate the relationship between the atomic radii of different elements and the depression in the melting point of metals. For example, Al and Au atoms have almost similar atomic diameters, FCC structures, and coordination numbers, but for the same size (B20 nm) of nanoparticles, Al shows a depression in melting point of 160 C, while Au shows 750 C. Based on data obtained by various authors, it can be concluded that the melting point of bulk crystals and small particles with a size .10 nm is almost the same. However, a strong decrease in melting point was found for nanoparticles when the size is below 10 nm. The largest decrease in the melting point of clusters of Hg, Sn, and Ga was 95, 152, and 106 K respectively, whereas no melting of In, Pb, and Cd clusters was detected. A decrease of several hundred degrees in the melting point was found in colloidal CdS nanoparticles with a radius of 1
4 nm. In the case of Au nanoparticles, the linear relationship between the melting temperature of nanoparticles and inverse of particle radius breaks down when the particle size becomes B1.6 nm or less than this. However, one can realize a significant deviation from the inverse linear relationship at larger sizes of 6.5 nm for lead. A depression or decrease in melting point with a decrease in the grain size of metals can be explained in a number of ways: According to the classical thermodynamics model, when the temperature is below the melting point of a bulk material then the surface of a solid cluster produces a liquid-like shell adjacent to a solid core. The whole cluster melts homogeneously when the thickness of the liquid layer surpasses a critical value [30]. According to some researchers, a surface layer of roughly a few nanometers thick melts first, followed by the melting of the core in a second step. This means that smaller particles with a radius smaller than that of the surface layer may melt all at once and particles thicker than this surface layer will melt in two steps. This may be attributed to the different degrees of orders of atoms in the interior and exterior of the particles, that is, they decrease from center to surface [26]. Increases in specific surface area as a result of reductions in the grain size of nanoparticles leads to increased surface energy, which makes nanoparticles “unstable” or “metastable.” The volume fraction and grain size of GBs have an inverse relationship; when grain size decreases, the volume

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fraction of the GB increases or vice versa. GBs are regions of disordered atomic arrangement because the atomic orientation across two different grains changes at the GB. This may cause vibrational uncertainty on heating (also produce melting) due to increase in free energy that nucleated at the GBs. The number of the closest neighbors of an atom in the core or surface of a crystal is known as the coordination number. The maximum coordination number of an atom on the surface is 9, whereas it is 12 for an atom at the core of a crystal. As the surface area increases with a decrease in grain size, more and more atoms are brought to the surface. This results in a decreased average coordination number for the crystal and, hence, leads to an increase in dangling bonds of the atoms. As a result, the cohesive energy, that is, the energy required to break all the bonds associated with an atom, decreases. With decreased cohesive energy, nanoparticles become thermally unstable as compared to their bulk counterparts and the melting point is also decreased. The depression in the melting point of metals and alloys can be exploited for some useful applications. For example, Tin (Sn) and Sn alloys are used to connect materials in off-chip and on-chip applications. Owing to the high melting point (220
240 C) of bulk Sn, a high reflow temperature is required in the development process of electronics, which may have adverse effects like warpage and thermal stresses on the components being fabricated. A depression in the melting point and, thus, the processing temperatures of nano-Sn and its alloys can alleviate these problems. Similarly, the sintering of silver paste can be done at a much lower temperature than that of bulk silver [1].

4.3.3.3 Coefficient of thermal expansion Nanocrystalline coarse-grained has high coefficient of thermal expansion (CTE) due to the large amount of interfacial volume. For example, the volume CTE and linear CTE of NC selenium increase by 31% and B21% respectively. This is due to a decrease in crystallite size from 46 to 13 nm as shown in Fig. 4.14. As another example, NC Cu with a mean crystallite size of 8 nm has a CTE of 31 3 1026 K21. This obtained value is twice the value of CG Cu, which is 16 3 1026 K21. The high value of CTE for NC materials has been used to the large fraction of GBs. A high portion of GBs increased the thermal expansion coefficient as compared to micrometer-sized crystallites. For example, the CTE value of CG Cu is about 2.5
5 times lesser than GBs with a value of 40.80 3 1026 K21 [10,15]. Similarly, an increase in CTEs was found in NC TiO2, Pd, Fe78B13Si9, and Ni
P samples given in Table 4.2 [37]. 4.3.4

Magnetic properties

Materials can be classified on the bases of their response to a magnetic field into, namely ferromagnetic, diamagnetic, and paramagnetic. Du et al.

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FIGURE 4.14 Grain-size dependence of the linear CTE (A) along the a-axis, (B) c-axis, and (C) the volume CTE of selenium. Reprinted with permission from Y. H. Zhao, K. Lu, Grain-size dependence of thermal properties of nanocrystalline elemental selenium studied by x-ray diffraction. Phys. Rev. B 56(22) (1997) 14330
14337, Copyright (1997) by the American Physical Society.

calculated the magnetic properties of ultrafine Ni particles having sizes within the 2
99 nm range. The specific saturation magnetization (σ) and the coercivity (Hc) of fine Ni particles measured at 7 kOe are shown in Fig. 4.15A and B. It can be observed that the specific saturation magnetization first decreases slowly with decreasing particle size, and below 15 nm, the magnetization drastically reduces, which implies that most particles change to superparamagnetism at room temperature. Ni particles with a size of 85 nm exhibit specific saturation magnetization of near 43 emu/g, which is less than that of bulk Ni (54 emu/g) at room temperature. Hc increases to the highest value of 250 Oe (for single domain sized Ni nanoparticles) and then decreases on reduction of the particle size from 85 nm to less than 15 nm. Its value approaches 0 when the particle size is close to the critical particle size of superparamagnetic particles (B15 nm). The value of effective anisotropy constant for Ni nanoparticles is 5.8 3 105 erg/cc, which is greater than the value for bulk Ni (3.4 3 104 erg/cc). The reason for the higher value of effective anisotropy constants for Ni nanoparticles has been attributed to the change of crystal anisotropy as well as surface and shape anisotropy. The Curie temperature for 9 nm sized Ni nanoparticles is about

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TABLE 4.2 CTE (in 3 1026 K21) of coarse-grained, amorphous, and nanocrystalline materials. Material

Temperature range (K)

Condition Coursegrained 6.9

Amorphous 7.4

Nanocrystalline

Fe78B13Si9

300
500

14.1

Cu

110
293

16




31

Ni
P

300
400

13.7

14.2

21.6

FIGURE 4.15 (A) Specific saturation magnetization and (B) coercivity as a function of average diameter of nickel particles. Reprinted with permission from Y. W. Du et al., Magnetic properties of ultrafine nickel particles, J. Appl. Phys. 70 (1991) 5903
5905, Copyright 1991, American Institute of Physics.

300 C, which is less than that of bulk Ni (TcB358 C). This is due to the contraction of bond length with a decreasing cluster size of Ni. This decreases the interval between the Ni atoms, thus, resulting in a smaller exchange integral. This, in turn, causes a reduction in the Curie temperature [38]. Nanostructured magnetic materials are mostly used as magnetic recording materials in the read and write heads and also in the information storage media. Thin-film tape media such as disks have high bit densities and thicknesses of 10
100 nm. Moreover, perpendicular and longitudinal recording media are about 50
250 nm and 10
50 nm respectively. The magnetic tapes used in high-density applications (in an acicular) has iron core passivated by an oxide surface, where iron nanoparticles have diameter and length of 20 and 200 nm, respectively. For both tapes and disks, magnetic single domains should have smaller sizes in order to attain greater densities with suitable signal-to-noise ratios. But one should note that domains that are not as small as those of magnetic materials become paramagnetic [37].

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TABLE 4.3 Some important features of metals in coarse-grained and nanostructured states. Properties

Value

Young’s modulus (GPa) Ultimate solubility at 293 K (%)

Materials

CG

NS

Cu

128

115

C in α
Fe

0.06

1.2

Diffusion coefficient (m /s)

Cu in Ni

1 3 10

1 3 10214

Debye temperature (K)

Fe

467

240

Ni

375

293

2

2

220

Saturation magnetization (Am /kg)

Ni

56.2

38.1

Curie temperature (K)

Ni

128

595

Table 4.3 shows important properties of CG and nanostructured (NS) metals/alloys. The value of the Curie temperature of NS Ni handled by severe plastic torsion straining (SPTS) is 595 K, while in bulk form its 631 K, which is less than that of CG Ni by B36 K. The saturation magnetization of NS Ni after spectral power distribution (SPD) is about 38.1 Am2/ kg, while for the bulk material its value is 56.2 Am2/kg, which is less than that of CG Ni by 31%. The decreases in the saturation magnetization and in the Curie temperature have been attributed to the reduced grain size and significant distortion of the crystal lattice. Both values recover to a value near to that of the bulk material when the NS Ni is heated from 300 to 600 K. The Debye temperature of the NS Ni obtained by SPD is 293 K, which is considerably less than the value of 375 K for bulk Ni. Similarly, the Debye temperature of NS Cu is 233 K, which is lower than that of to the bulk Cu by 23%. The Debye temperature of NS iron at its near-boundary regions is 240 K, whereas for CG iron it is 467 K. The lower Debye temperatures for NS metals are due to increased amplitude of the thermal vibrations of atoms in the GBs as well as to increase the point defects near the region of GBs. Moreover, nonequilibrium GBs and long-range elastic stress fields are also responsible for decreases in the Debye temperatures for NS substances managed by SPD. It is well studied that the value of Debye temperature near the boundary regions of NS Ni is about 127 K, which is almost 200 K lower than the conforming value for bulk Ni. As shown in Table 4.3, the Debye temperature of the GB phase in CG iron is 227 K, which is high as compared to the NS iron, which is about 240 K. Due to lower Debye temperatures, increases in the dynamic activity of atoms occurred in regions that are near to the boundary of NS SPD materials. This resulted in a significant increase

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in diffusion rates, for example, the diffusion coefficient of Cu in NS Ni is almost six orders of magnitude more than in CG Ni due to a reduced diffusion path length in NS Ni [39]. The Young’s modulus of NS Cu is about 115 GPa and bulk Cu has a value of 128 GPa, which is below the value of CG Cu, that is, approximately lower by 10% than the bulk value. This behavior of materials is due to a difference of 15%
17% between the elastic modulus values of CG metal and NS metal in the near-boundary regions. Transmission electron microscopy confirmed that the value of the moduli of NS materials having nonequilibrium GBs is improved to a particular value close to that of bulk materials by transformation present in the equilibrium GBs of bulk material [39].

4.3.5

Optical properties

The size effects in optical properties are important for nanoparticles .10
15 nm, which is significantly below the wavelength (λ) of light. In semiconductors, the energy of interatomic interactions is high. The electronic excitation of semiconductor crystal may lead to the production of a weakly bonded electron
hole pair, that is, the Mott
Vanie exciton. The region of delocalization of these excitons is considerably greater as compared to the semiconductor lattice constant. The optical properties of the particles are affected significantly when the size of semiconductor crystal is equal to the size of the Bohr radius (exciton). For Copper chloride (CuCl) and Gallium arsenide (GaAs) semiconductors, the Bohr radius of the exciton diverges from 0.7 to 10 nm. The energy of the electronic excitation of a molecule is usually higher than the energy gap in macroscopic semiconductors. In other words, absorption band shows a blue shift for semiconductor nanoparticles with a decrease in their size. For example, Cadmium sulfide (CdS) showed blue shift when size decreased from 10
12 nm. For another semiconductor, CdTe nanoparticles, the energy band increases when its size reduces from a bulk crystal to 4
2 nm in size. The energy of the maximum of the absorption band varies inversely to the square of the radius of nanoparticles. Moreover, the distance between the peaks has a tendency to enhance with declines in particle size due to the dispersal of energy levels. The range of blue shift depends upon the particle shape, size distribution, and aspect ratio of the nanoparticles. To a certain extent, the presence of impurities may also affect it. In Chapter 7, Figure 7.4 clearly shows how the energy band gap changes with respect to decreasing particle size. Another factor, that is, quantum size confinement, also shows great effect when the size of a particle is comparable to the Bohr exciton radius (αB). For example, CdS (the Bohr radius is B2.4 nm and the radius of the particle is .2.4 nm), CdSe, and GaA (the binding energy of exciton is 4.6 meV and the exciton Bohr radius is B11.8 nm), they show strong quantum confinement effects (blue shift) with decreases in the particles size [40].

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Quantum confinement and electronic band structure are also used to differentiate between semiconductors and metals. In semiconductors, bandgap plays an important role, but in the case of metals, electrons are free to move in their half-filled conduction band. With the enhancement of particle size in semiconductors, the center of the band grows first and the edge grows last. In metals, the energy level spacing is small and in the band center, the Fermi level lies. So, in smaller sizes of semiconductor nanoparticles the quantum size confinement effect has an important role as compared to metals. When metal nanoparticles show insulator or semiconductor behavior, this is due to the large bandgap for small metal nanoparticles. These phenomena happen at temperatures where bandgap energy leads the thermal energy. For example, Au is known for its yellow color, noble nature, and metallic shine, which do not discolor. It melts at 1336 K and also has a face-centered cubic lattice structure. However, a small particle of the same Au with a size of about 10 nm absorbs green light and, thus, gives the impression of a red color. Moreover, 2
3 nm sized nanoparticles show exceptional catalytic behavior and significant magnetism. For low temperatures, .2 nm sized metallic particles become insulators. For example, silver clusters show a nonmetallic bandgap that decreases with increases in cluster size. However, this bandgap is closed for clusters containing less than 300 atoms. An important threshold is reached when the gap between the lowest unoccupied (Kubo gap) and the highest occupied state has equal thermal energy, where the electrons get thermally excited across the Kubo gap. In other words, insulators can be changed into semiconductor by increasing their temperatures. Blue shift is the most feasible reason for using colloidal Au nanoparticles in palaces as dyes and stained-glass windows in churches. For example, 10 nm sized Au particles absorb green light, but seem red due to surface plasmons [40]. This size-dependent optical behavior can be explained by the increase in surface plasmon resonance (SPR) for metals and in energy level spacing for semiconductor nanoparticles. When metallic particles interact with an electromagnetic field, SPR is produced as a result of the coherent motion of all available free electrons. An SPR is also generated when the size of the nanocrystal is below that of the wavelength of the incident radiation. Free electrons get polarized due to the electric field of incoming light rather than cationic lattice. So a restoring force is produced once the net charge difference arises at the surface of a nanoparticle. Thus a certain frequency is created by a dipolar oscillation of free electrons. SPR is a dipolar excitation of the entire particle in between the positively charged lattice and the negatively charged electrons. Moreover, the energy that is associated with SPR depends on the free electron density as well as the surrounding dielectric medium covering the nanoparticle. In this way, resonance width changes with relaxation time before the scattering of electron. For noble metals, the resonance frequency lies in the visible light

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range, while for larger nanoparticles, the resonance improves due to an increase in scattering length [41]. A strong absorption in UV-visible range occurs when the size of nanoparticles becomes lesser than the electron mean free path which is called the surface plasmon resonance (SPR). According to Mie, bigger particles have much importance in the higher order modes as light cannot separate nanoparticles homogeneously. These higher order modes show the peak at relatively smaller energies or higher wavelengths. Thus the plasmon band shifts toward a higher wavelength (a redshift) with increasing particle size [33,34]. For example, a suspension of Au nanoparticles in a solvent will show a red solution if this solution strongly absorbs lights of a blue wavelength (B450 nm) and a green wavelength (B520 nm). In general, the color of the solution corresponds to the color of light absorbed by the solution. The characteristic SPR bands for usual silver and Au nanoparticles are around 400 nm and 520 nm respectively. The SPR peak position and bandwidth are both sensitive to the aspect ratio of nanorods, which exhibit two distinct extinction bands corresponding to electron oscillations across (known as transverse mode, TM) and along the long axis (known as longitudinal mode, LM). In such anisotropic nanoparticles, transverse plasmon resonance produces the blue band, while longitudinal plasmon resonance creates a redder band. The redder band shifts with increases in aspect ratio or in length of nanorods [41,42].

References [1] A, A., Chapter
Introduction to nanomaterials. 2011. p. 76. [2] V.B. Sutariya, Y. Pathak, Biointeractions of nanomaterials, CRC Press, 2014. [3] Z. C¸aldıran, et al., Space charge limited current mechanism (SCLC) in the graphene oxide
Fe3O4 nanocomposites/n-Si heterojunctions, J. Alloy. Compd. 631 (2015) 261
265. [4] J.I.D.S. Filho, Study of the interactions between particles based in paraquantum logic, J. Mod. Phys. 3 (2012) 362
376. [5] K. Aruna, K.R. Rao, P. Parhana, A systematic review on nanomaterials: properties, synthesis and applications, I-Manager’s J. Future Eng. Technol. 11 (2) (2015) 25. [6] M. Kearnes, P. Macnaghten, Introduction:(Re) imagining nanotechnology, Sci. Cult. 15 (4) (2006) 279
290. [7] M. Ko¨hler, W. Fritzsche, Nanotechnology: an introduction to nanostructuring techniques, John Wiley & Sons, 2008. [8] S. Lindsay, Introduction to nanoscience, Oxford University Press, 2010. [9] P. Atkins, J. De Paula, Elements of physical chemistry, Oxford University Press, USA, 2013. [10] M.J. Mayoral, N. Bilbao, D. Gonz´alez-Rodr´ıguez, Hydrogen-bonded macrocyclic supramolecular systems in solution and on surfaces, ChemistryOpen 5 (1) (2016) 10
32. [11] J. Kotz, P. Treichel, G. Weaver, Chemistry & chemical reactivity, Thomson Learning. Inc., Australia, 2006. [12] R.P. Schwarzenbach, P.M. Gschwend, Environmental organic chemistry, John Wiley & Sons, 2016.

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[13] J.W. Steed, J.L. Atwood, Supramolecular chemistry, John Wiley & Sons, 2013. [14] L. Dai, Intelligent macromolecules for smart devices: from materials synthesis to device applications, Springer Science & Business Media, 2004. [15] C. Joachim, Nanotechnology—an introduction to nanostructuring techniques. By Michael Ko¨hler and Wolfgang Fritzsche, Chem. Phys. Chem 5 (11) (2004). p. 1806-1806. [16] R.K. Goyal, Nanomaterials and nanocomposites: synthesis, properties, characterization techniques, and applications, CRC Press, 2017. [17] S.I. Sadovnikov, A.A. Rempel, A.I. Gusev, Nanostructured lead, cadmium, and silver sulfides: structure, nonstoichiometry and properties., Vol. 256, Springer, 2017. [18] U. Erb, Electrodeposited nanocrystals: synthesis, properties and industrial applications, Nanostruct. Mater. 6 (5
8) (1995) 533
538. [19] K. Lu, Nanocrystalline metals crystallized from amorphous solids: nanocrystallization, structure, and properties, Mater. Sci. Engineering: R: Rep. 16 (4) (1996) 161
221. [20] T. Ohno, et al., Size effect for barium titanate nano-particles [Translated], KONA Powder and Particle Journal, 22, 2004, pp. 195
201. [21] C.C. Koch, Probing deformation mechanisms of nanostructured Mg alloys for unprecedented strength and good ductility, North Carolina State University Raleigh United States, 2018. [22] T.C. Lowe, R.Z. Valiev, Investigations and applications of severe plastic deformation, Vol. 80, Springer Science & Business Media, 2012. [23] W. Shen, et al., Effects of high pressure on the electrical resistivity and dielectric properties of nanocrystalline SnO2, Sci. Rep. 8 (1) (2018) 5086. [24] T. Ishihara, J. Yan, H. Matsumoto, Extraordinary fast oxide ion conductivity in La1. 61GeO5 2 δ thin film consisting of nano-size grain, Solid. State Ion. 177 (19
25) (2006) 1733
1736. [25] M. Aliofkhazraei, Nanocoatings: size effect in nanostructured films, Springer Science & Business Media, 2011. [26] A. Camposeo, et al., Local mechanical properties of electrospun fibers correlate to their internal nanostructure, Nano Lett. 13 (11) (2013) 5056
5062. [27] A. Gusev, A. Rempel, Nanocrystalline materials, Cambridge intern, 89, Science Publ, Cambridge, 2004. [28] Wang, M., C. Yan, and L. M, Graphene Nanocomposites. 2012. [29] M.N. Chong, et al., Recent developments in photocatalytic water treatment technology: a review, Water Res. 44 (10) (2010) 2997
3027. [30] M.-H. Liao, D.-H. Chen, Preparation and characterization of a novel magnetic nanoadsorbent, J. Mater. Chem. 12 (12) (2002) 3654
3659. [31] J. Rupp, R. Birringer, Enhanced specific-heat-capacity (c p) measurements (150
300 K) of nanometer-sized crystalline materials, Phys. Rev. B 36 (15) (1987) 7888. [32] H.S. Shin, J. Yu, J.Y. Song, Size-dependent thermal instability and melting behavior of Sn nanowires, Appl. Phys. Lett. 91 (17) (2007) 173106. [33] X.W. Wang, et al., Size-dependent melting behavior of Zn nanowire arrays, Appl. Phys. Lett. 88 (17) (2006) 173114. [34] S. Lai, J. Carlsson, L. Allen, Melting point depression of Al clusters generated during the early stages of film growth: nanocalorimetry measurements, Appl. Phys. Lett. 72 (9) (1998) 1098
1100. [35] J. Eckert, et al., Melting behavior of nanocrystalline aluminum powders, Nanostruct. Mater. 2 (4) (1993) 407
413.

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[36] Y. Li, D. Lu, C. Wong, Isotropically conductive adhesives (ICAs), in Electrical conductive adhesives with nanotechnologies, Springer, 2010, pp. 121
225. [37] C. Suryanarayana, C. Koch, Nanocrystalline materials
current research and future directions, Hyperfine Interact. 130 (1
4) (2000) 5. [38] Yw Du, et al., Magnetic properties of ultrafine nickel particles, J. Appl. Phys. 70 (10) (1991) 5903
5905. [39] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Bulk nanostructured materials from severe plastic deformation, Prog. Mater. Sci. 45 (2) (2000) 103
189. [40] E. Roduner, Size matters: why nanomaterials are different, Chem. Soc. Rev. 35 (7) (2006) 583
592. [41] S. Link, M.A. El-Sayed, Shape and size dependence of radiative, non-radiative and photothermal properties of gold nanocrystals, Int. Rev. Phys. Chem. 19 (3) (2000) 409
453. [42] N. Srinivasan, et al., Investigation of MALDI-TOF mass spectrometry of diverse synthetic metalloporphyrins, phthalocyanines and multiporphyrin arrays, J. Porphyr. Phthalocyanines 3 (04) (1999) 283
291.

Further reading Q. Sun, L. Yang, The adsorption of basic dyes from aqueous solution on modified peat
resin particle, Water Res. 37 (7) (2003) 1535
1544.

Chapter 5

Tools and instrumentation Aqsa Tehseen1, Tahir Iqbal Awan1, Almas Bashir1, Sumera Afsheen2 and Muhammad Yaqoob Khan3 1

Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan, 2Department of Zoology, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan, 3Department of Physics, Kohat University of Science and Technology, Khyber Pakhtoonkhwa, Pakistan

Chapter Outline 5.1 Microscopy 120 5.1.1 Brief history 120 5.1.2 Concept of microscopy 120 5.1.3 Optical microscopy 122 5.1.4 Various optical microscopic techniques 125 5.2 Electron microscopy 127 5.2.1 Electron interaction with material sample 127 5.2.2 Working of electron microscopy 129 5.3 Types of electron microscopy 129 5.3.1 Scanning electron microscope 129 5.3.2 Transmission electron microscope 132 5.3.3 Dissimilarities between scanning electron microscope and transmission electron microscope 135 5.4 Scanning tunneling microscope 135 5.4.1 Components and workings 136

5.5 Atomic force microscopy 137 5.5.1 Construction of atomic force microscope 139 5.5.2 Working principle of atomic force microscope 141 5.5.3 Modes of operation 142 5.5.4 Advantages and disadvantages 143 5.5.5 Applications 144 5.6 Fluorescence method 144 5.7 Synchrotron radiation 145 5.8 Atom probe instrument 146 5.8.1 Construction 147 5.8.2 Working of atom probe field ion microscopy 147 5.8.3 Mathematical analysis 149 5.8.4 Limitations of atom probe 149 5.8.5 Comparison with tunneling electron microscope and SIMS 149 References 150

Chemistry of Nanomaterials. DOI: https://doi.org/10.1016/B978-0-12-818908-5.00005-6 © 2020 Elsevier Inc. All rights reserved.

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5.1

Microscopy

5.1.1

Brief history

The role of microscopy is integral to study the micro-sized objects. The origin of microscopy dates back to the 17th century, before that, in the 13th century, a single lens of limited magnification was used as a magnifying glass. The Kepler ocular was designed to get the desired magnification with a convex eyepiece and objective lens by Johannes Kepler in 1611. Giovanni Faber, in 1625, was the first person to introduce the term microscope in the context of the ability to visualize small things. Huygens was the first person who designed the initial eyepiece with two convex lenses, one gives a smaller and brighter image of an object and the second focuses this image. Today, this design with a magnification of x10 or less is still being used in eyepieces. With the passage of time, the quality of lenses and knowledge of magnification and resolution have upgraded, so different kinds of microscopes exist today. In the 1600s, the first optical microscope was developed, after that, in the 1920s, the electron microscope and in the 1980s, the atomic force microscope were developed.

5.1.2

Concept of microscopy

The resolution ability of the naked human eye is nearly 150 µm. The intention of using a microscope is to enhance that resolution. The minimum distance that is visible between any pair of points, aided or unaided by a microscope, is called resolution. The microscope has the ability to differentiate between two images that are close together as being separate. The ability of an instrument to see images distinctly is called resolving power (RP). In simple terms, the RP defines how well a microscope differentiates between two objects that are close together at the microscale. RP as a function, which depends on the aperture of the objective lens and wavelength of light being used in the optical system is given in Eq. (5.1) and this is able to differentiate between any two points separately. Light microscopes have an RP value equal to 0.2 µm, which is 750-times greater than a normal human eye. RP is the smallest separation that can be noticed by any optical system and is given as: RP 5

wavelength of light 2 3 numerical aperture of objective lens

ð5:1Þ

Example 5.1: If the numerical aperture is 1.25 for an objective lens of a microscope and a 500 nm wavelength is being used, calculate the resolving power of the microscope. Solution: Numerical aperture of objective lens 5 1:25

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Wavelength of light used in microscope 5 500 nm Resolving power ðRPÞ 5 ? RP 5

500 nm 5 200 nm 5 0:2 µm 2 3 1:25

In microscopes, an aberration means that a point is failed to focus all the rays at a point. Five different types of aberrations occur in optical microscopes, that is, coma, spherical, distortion, field curvature, and astigmatism. An aberration of a serious nature can cause a decrease in the resolution. So to obtain the theoretical resolution and remove aberrations, corrective lenses are used in optical microscopes. The type of aberration that is most common is the spherical aberration. In this type of aberration, microscope lenses lack of ability to focus all incident rays on one point, and as a consequence, this gives diverse focal-lengths as shown in Fig. 5.1A. A light diaphragm focuses the rays on one point and blocks those rays that do not have the direction to reach that point as shown in Fig. 5.2B, so can be used as a solution to spherical aberration. The interaction of light with the specimen is the most central feature of microscopy. The visualization of the specimen cannot be realized without this interaction of light. In optical microscopy, reflected or transmitted light can be utilized. White or ultraviolet (UV) light may be utilized in the illumination source. But it should be taken into account that light manipulation in all the techniques of microscopy affects the RP of optical devices. The ability to increase the size of an apparent image is known as magnification, which depends on the RP of the microscope and the eye as given by:

FIGURE 5.1 (A) Inborn spherical aberration of a convex lens. (B) Light diaphragm removes the spherical abberation by allowing only those rays that meet at a single point. Reprinted from T.M. Roane, I.L. Pepper, R.M. Maier, Microscopic techniques, in: Environmental Microbiology, pp. 157172, Copyright (2009), with permission from Elsevier.

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FIGURE 5.2 (A) Ray diagram of simple microscope. (B) Ray diagram of compound microscope. Adapted with permission under the terms of the CC BY-SA 3.0 (https://creativecommons. org/licenses/by-sa/3.0/).

Magnification 5

Limit of resolution by eye limit of resolution of microscope

Magnification 5

150 µm 5 750 3 0:2 µm

There is no upper limit to enlarge images using optical devices, however, mostly blurred images are obtained by magnification with effect to limited resolution. There is another important feature which states that ability to distinguish the object from the medium surrounded it is referred as contrast. This could be understood more clearly as, the resolved points can be visualized unless they are in contrast with respect to each other and the medium surrounding them.

5.1.3

Optical microscopy

An optical microscope is a device that enables us to view small particles in the microscale range using visible light; this is why it is usually called a light microscope. It is basically of two types, namely (1) simple microscopes and (2) compound microscopes.

5.1.3.1 Simple microscope A device that provides a magnified image of an object using a single lens, for example, a magnifying glass is known as a simple microscope. A ray diagram of a simple microscope is shown in Fig. 5.2A. A few low-priced simple microscopes consisting of a single lens are available in digital version for commercial purposes. 5.1.3.1.1

Magnification of simple microscope

Angular magnification is stated as, the ratio of angle formed by image of lens to the angle formed by object kept at nearest distance of distinct vision of eye.

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M5

123

β image size 5 α object size

Using trigonometric ratios and approximation, it is obtained as: M5

d p

Using lens formula, it can also be expressed as: M511

d f

It shows that the magnification of a simple microscope depends on the focal length of the lens. These three expressions can be used to calculate the magnification of a simple microscope.

5.1.3.2 Compound microscope A compound microscope has several lenses including condenser, ocular, and objective lenses as shown in Fig. 5.2B. There are different kinds of compound microscopes that can used for research purposes. Generally, the objective lens, in these types of microscopes, is placed nearest to the object being observed for gathering light and it forms a real image (image 1) inside the focal length of the second lens. Due the presence of image 1 in the focal length of the end lens, called the eyepiece, a larger, virtual, and erected image (image 2) is formed by the eyepiece [1]. The eyepiece/objective pair of lenses in a compound microscope provide much greater magnification. Such microscopes have replaceable objective lenses that permit the handler to rapidly fine-tune the magnification [1]. More innovative setups of illumination (e.g., phase contrast) are also available nowadays in compound microscopes. 5.1.3.2.1 Magnification of compound microscope The magnification of a compound microscope “M” is defined as the product of the magnification of the eyepiece, M1 , with the magnification of the objective lens, M2 , that is, M 5 M1 3 M2 where M1 and M2 are expressed as: M1 5 M can also be expressed as:

q d and M2 5 1 1 p fe


 q d 11 M5 p fe

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This shows that the magnification of a compound microscope depends on the focal length of the eyepiece lens. A typical transmission optical compound microscope with its optics is given in Fig. 5.3.

5.1.3.3 Limitations The main limitation of an optical imaging system (microscope, telescope, camera, etc.) is resolution due to various factors like imperfections and the misalignment of lenses. The resolution of any optical system is principally limited due to diffraction. If the resolution performance of an optical system is the same as the theoretical limit then this is known as the diffraction limit of the system. λ=2 defines the diffraction limit, where λ is the wavelength of incident light on any object. Visible light is used as a source in optical microscopes with a diffraction limit of around 250 nm or 0.25 µm. For example, if λ 5 500nm, the diffraction limit is 250 nm, which means we cannot resolve more than 250 nm or 0.25 µm by means of an optical microscope.

FIGURE 5.3 A common compound transmission microscope and its internal optics. Reprinted from T.M. Roane, I.L. Pepper, R.M. Maier, Microscopic techniques, in: Environmental Microbiology, pp. 157172, Copyright (2009), with permission from Elsevier.

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It also limits the magnification practically to nearly 1500 3 . Light coming from outside the focal length also decreases resolution. To dig deep into the micro world, we use electrons as a source since the waves associated with them are of much smaller wavelengths, providing us with high RP, which will be discussed in next section.

5.1.3.4 Advantages and disadvantages Optical magnifying instruments give direct imaging without the need for pretreating the samples being used. It is the main form of microscopy that gives genuine color images. It can be adapted to any kind of sample (solid, liquid, or gas), even living objects can be used. Images taken from it can be stored using a digital camera system. Disadvantages include its comparatively low resolution and low magnification. 5.1.4

Various optical microscopic techniques

There are five different techniques of optical microscopy, which are based upon different types of light sources and lenses such as dark-field, brightfield, differential interference, and phase contrast. A comprehensive comparison of the magnification, resolution, and important features of various optical as well other microscopic techniques is given in Table 5.1.

5.1.4.1 Bright-field microscopy In this technique, images are formed due to transmitted light from a specimen. Some light is absorbed by the specimen, and the remaining light transmits by an ocular lens. So a specimen appears dark against the bright surrounded background. This technique is mostly used to observe morphology; but in order to obtain the required magnification, it frequently needs staining to enhance the contrast, especially for relatively smaller objects. Other optical microscopic techniques, as discussed here, are used to enhance contrast. 5.1.4.2 Dark-field microscopy For transparent specimens, this technique can be used to enhance contrast. Using central stop earlier to condenser, little light is allowed to reach the objective through the condenser. Only scattered light from boundaries of specimen is observed. So a specimen appears bright in the image against the dark surrounded background. Live specimens are visualized by this technique that cannot be stained or fixed. 5.1.4.3 Phase contrast microscopy It increases the contrast of transparent specimens, which helps to see images in a high-contrast nature. The benefit of this technique is that it can evaluate

TABLE 5.1 Comparison of different types of microscopy. Microscope

Magnification

Resolution

Significant features

Visible light as illumination source Bright field

2000 3

0.2 µm 5 200 nm

Conventional multiuse light microscope for live and well-preserved stained samples; dark sample, white field; offers quite good cellular features

Dark field

2000 3

0.2 µm

For best live observation, unstained samples; bright sample, black field; gives framework of sample with diminished inner cellular aspects

Phase contrast

2000x

0.2 µm

Live samples can be viewed; sample is distinguished in contrast to gray backdrop; inner cellular aspects can be studied

Differential interference

2000 3

0.2 µm

Gives glowingly colored, extremely contrasting, 3D images of live samples

2000 3

0.2 µm

Fluorescent dyes that stain samples or are intermixed by fluorescent antibodies release simple light; it has become one of the best diagnostic tools due to its specificity

Transmission electron microscope (TEM)

1,000,000 3

0.5 nm

Segments of samples are observed under large magnification; the smallest details of viruses and inner cell structures can be seen through it; utilized only on well-preserved samples

Scanning electron microscope (SEM)

100,000 3

10 nm

Complete samples can be observed under large magnification; the cellular order and exterior structures are shown; usually utilized on well-preserved samples

1,000,000 3

0.5 nm

Can observe preserved or live samples;

Illumination source is ultraviolet rays Fluorescence Source is electron beam

Source of surface forces Atomic force microscope (AFM)

gives details of a surface with high resolution

Source: Reprinted from T.M. Roane, I.L. Pepper, R.M. Maier, Microscopic techniques, in: Environmental Microbiology, pp. 157172, Copyright (2009), with permission from Elsevier.

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structures that are transparent as well as have variations in densities. The varying density portions interact with light differently, thereby producing contrast between the desired images and the background. It utilizes diaphragm series for splitting and rejoining diffracted and direct light rays.

5.1.4.4 Differential interference contrast microscopy This type of microscopy offers colored and highly contrasting 3D images of living samples. In this from of microscopy, an enlightening beam splits up into two further isolated beams and strikes the sample. One of them passes through the sample by creating a phase difference among these two upcoming beams of electrons, which acts as a reference beam. After that, both of these combine to interfere and permit small variations in the elevation or depth of the sample surface to be found for 3D images.

5.2

Electron microscopy

The electron microscope (EM) allows us to view an image of a specimen at nanoscale dimensions by the use of an electron beam. It was first developed in 1931 by Ernst Ruska. He used the idea of waveparticle duality and found that the wavelengths of electrons are much smaller than those of light. So an extremely high magnification and resolution can be achieved using an electron beam as compared to a light beam. Even today, modern electron microscopes use the prototype/basic principle created by Ernst Ruska. So unlike an ordinary optical microscope, an electron beam is used instead of light rays in the electron microscope. Some differences between electron and optical microscopes are summarized in Table 5.2. Ernst Ruska first made an electromagnetic lens and then used an electron beam to attain high magnification and resolution. Although the results were not as good as was expected, they were still far better than optical microscopes, which motivated him to move further. Nowadays, a modern electron microscope has a magnification power of about 10,000,000 3 and a resolution power of 0.05 nm. Ernst Ruska was awarded a Nobel Prize because of his remarkable innovation.

5.2.1

Electron interaction with material sample

There are a variety of ways in which electrons can interact with a sample, which form the basis of electron microscopy to produce the specimen image. We classify electrons in two types, that is, primary and secondary electrons (SE). Primary electrons: The electrons present in the incident beam and used to strike the specimen. Secondary electrons: The electrons emitted from the surface of the sample due to the interaction between the primary electrons with the sample material.

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TABLE 5.2 Difference between characteristics of optical and electron microscopes. Characteristics

Optical

Electron

Medium

Atmosphere

Vacuum

Wavelength

˚ (visible) 7500 A

˚ (20 kV) 0.086 A

˚ (ultraviolet) 2000 A

˚ (100 kV) 0.037 A

Magnification

Up to 2000 3

Up to 1,000,000 3

Resolving power

˚ 2000 A

˚ 3A

Lens

Glass lens

Electrostatic lens

Focusing

Mechanical

Electrical

Samples needs treatment or staining

Yes/no

Always

Colored image produced

Yes

No

Illuminating beam

Light

Electron

Viable specimen

Yes

No

Source: Reprinted from T.M. Roane, I.L. Pepper, R.M. Maier, Microscopic techniques, in: Environmental Microbiology, pp. 157172, Copyright (2009), with permission from Elsevier.

Each primary electron follows its own trajectory while passing from the specimen. Due to their interaction with the atoms of the specimen, the direction of these electrons is changed from the original one and as a result a scattering occurs [1,2]. The chance of a scattering interaction of a single electron depends upon the hurdles present in the path of the electrons within the sample including path length, types of encountered elements, and arrangement of atoms. The interactions between electrons and the specimen are mainly divided to two types, that is, elastic and inelastic interactions. When no energy of incoming electrons is lost during an interaction then this is called an elastic scattering. Only the direction of these electrons is changed with no change in the wavelength of incident electrons. An electron diffraction effect is produced by a coherent elastic interaction allowing the structure of crystals to be studied [3,4]. When there are energy losses of incoming electrons during an interaction then it is called an inelastic scattering. In this type of scattering, the wavelength of scattering electrons is longer as compared to that of incident ones. It occurs in many ways as shown in Fig. 5.4A, and a number of signals are produced as a result of energy transfer in inelastic scattering. These signals are used for material characterization. Thickness of specimen: There will be more scattering events the thicker a specimen is. It also becomes improbable that electrons will scatter elastically. So the elastic to inelastic scattering ratio decreases for specimens of

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greater thicknesses. As a result of more inelastic scattering events, electrons lose their kinetic energy faster and when the energy of the electrons is finished, then they will be absorbed by the sample. Interaction volume: In a specimen, the volume of interaction, which can be calculated by the trajectory distribution of electrons in 3D, is called the interaction volume as shown in Fig. 5.4B. The maximum penetration depth of an electron after it comes to rest and losses all its energy is called the range of electron. Electron transparency: If incident electrons can pass from a specimen then this specimen is transparent for these electrons. This transparency mainly depends on the KE of electrons by controlling the accelerating voltage. In the dependency of transparent materials, chemical composition, thickness of specimen, and atomic number of elements of the specimen are also key factors.

5.2.2

Working of electron microscopy

The working of EM is also similar to the ordinary microscope. The electron source provides a beam of fast-moving electrons that acts like the light coming from the light source in an ordinary microscope. This fast-moving electron beam is made incident on a specimen where electrons interact with the specimen. For an electron microscope, the specimen must be placed in a vacuum chamber because the beam of electrons would be deflected in the presence of air particles, thus, will not give the desired results. Before examining the specimen in an electron microscope, it must have passed through some processes (like fixation, embedding, dehydration, staining) depending on the type of specimen. A series of coil shaped electromagnets are used for focusing the electrons into a beam shape just like lenses focus the light beam in an optical microscope. A common problem in both microscopes is poor contrast, which is overcome by staining the samples. As a result of this, images from electron microscopes are produced in gray shades and color can be added through the use of computers.

5.3

Types of electron microscopy

EM has two types, which are described here one by one.

5.3.1

Scanning electron microscope

SEM creates highly magnified sample images by bombarding highly energetic electrons to scan the surface of the sample used. When these electrons strike the sample surface, they eject secondary electrons. Detectors produce signals by collecting these secondary electrons, which carry information about various features of the sample. These signals are then used to produce a magnified image on an electron micrograph. The main working principle of the scanning electron microscope is described here.

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FIGURE 5.4 Shows the interaction of primary electrons with a sample. (A) Various signals are produced by this interaction in a thin sample. (B) Interaction of electrons with thick samples and the absorption of SE, BSE, and X-rays due to inelastic scattering. Reprinted from B.J. Inkson, Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for materials characterization, in: Materials Characterization Using Nondestructive Evaluation (NDE) Methods, pp. 1743, Copyright (2016), with permission from Elsevier.

5.3.1.1 Working of scanning electron microscope In SEM, first samples are passed through various preparation phases, then samples are placed in a sample holder and electrons emitted from an electron gun strikes on the sample (Fig. 5.5). When these highly energetic electrons strike the surface of the specimen, different types of electrons are emitted, which are classified into several categories that provide information about the specific features of the specimen. The specific features include topography, morphology, composition, and crystallography. Incident electrons that scatter in a backward direction after the interaction are known as backscattered electrons and are used for imaging samples, that is, they are helpful in providing the final magnified image on screen. Electrons emitted just from the surface of the sample are known as secondary electrons. These electrons provide information about the topography and morphology of the samples. The incident electrons after diffraction from the crystalline structure of specimen scatter in backward direction are called diffracted/backscattered electrons which provide information about crystal structure of sample. Various detectors are also used to differentiate among these different types of electrons. X-rays are also emitted from the collision of highly energetic electrons, because on collision with a solid sample, electrons go from a high energy state to a low energy state. These

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FIGURE 5.5 Schematic of SEM. Reprinted from B.J. Inkson, Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for materials characterization, in: Materials Characterization Using Nondestructive Evaluation (NDE) Methods, pp. 1743, Copyright (2016), with permission from Elsevier.

electrons then pass through electromagnetic lenses to focus them into a beam shape and provide detailed information about the different features of a sample in the form of an image on a screen.

5.3.1.2 Advantages of scanning electron microscope The main advantages of SEM include: 1. It gives a high resolution and a magnification power. 2. It has extremely high magnification power, which helps a lot in researches and thus leads to the development of different fields, especially biologically. 3. It has brought a great revolution in technological and industrial applications. 4. It can provide information about surface shape, feature, and structure.

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5. It has a wide-array of applications. 6. It is operated by software and controlling it can be easily learnt. 7. It has a fast scanning speed.

5.3.1.3 Disadvantages of scanning electron microscope The main disadvantages of SEM include: 1. 2. 3. 4. 5.

It is highly expensive. A special environment is required for its working. It is hard to operate without proper training. It results are highly affected by stray magnetic fields. Various costly processes have to be done for the preparation of specimens. 6. A major disadvantage of SEM is its size. It is extremely large.

5.3.1.4 Limitations A few limitations of the electron microscope are: 1. Images produced by electron microscopes are black and white, so one can’t distinguish between different colors. For different biological specimens this causes a problem, so the digitization of the image is then done to produce coloring effects, which itself is a costly and time-consuming process. 2. In an electron microscope, living specimens can’t be viewed because they have to be placed in a vacuum. 3. In an electron microscope, researchers can only see a small part of the specimen with a high resolution. Because when the resolution is increased, the examining portion of the specimen visible on screen will be reduced. 4. Especially for SEM, samples must be solid and their size should be adaptable with the microscope chamber, which is small in size.

5.3.2

Transmission electron microscope

TEM was first developed in 1931 by Max Knoll and Ernst Ruska. It works by passing an energetic ray of electrons through samples of less than 100 nm in thickness. After that, this transmitted beam hits a fluorescence screen to form an image. This process could be understood by considering the operation of a movie projector using a negative image for projection. White light shines on the negative film and transmitted light produces the image present in the negative. TEM can produce 2D images of samples, but for 3D images, it produces projections on 2D planes. The main working principle of TEM is described here.

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5.3.2.1 Working of transmission electron microscope First, the specimen to be examined is passed through various preparation processes then it is placed in sample stage inside a vacuum. The schematic of TEM is shown in Fig. 5.6. Electron beam is bombarded on a thin sample with the help of electron gun. The electrons reflect back which encounter heavy atomic nucleus while other electrons which face lighter atomic particles passed through specimen and form specific pattern in the form of image. The electron beam is first passed through a condenser lens and then after

FIGURE 5.6 Schematic of TEM. Reprinted from B.J. Inkson, Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for materials characterization, in: Materials Characterization Using Nondestructive Evaluation (NDE) Methods, pp. 1743, Copyright (2016), with permission from Elsevier.

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passing through the specimen, an image is formed by the transmitted electrons magnified by electromagnetic fields. In TEM, the final magnified image is seen on a photographic plate made of phosphorus compounds instead of an electron micrograph. Electrons coming out of electromagnetic lenses strike the plate and form a magnified image on the photographic plate. In this way, a black and white magnified image is produced in which lighter areas indicate lighter parts of the sample through which more electrons pass and dark areas represent the dense areas of the sample.

5.3.2.2 Advantages of transmission electron microscope There are many advantages of TEM such as: 1. It has a powerful magnification that is a million times more than that of other conventional microscopes. 2. It has a broad area of applications in various fields like science, education, and industry. 3. It gives information about the structure of elements and compounds. 4. It provides detailed and fine quality images. 5. Very easy to operate and gives information about surface shape, features, structure, and size.

5.3.2.3 Disadvantages of transmission electron microscopes Disadvantages of TEM are: 1. The instrument is bulky and expensive. 2. Samples need special treatment before use. 3. Produces black and white color images.

5.3.2.4 Applications of transmission electron microscope 1. TEM has a broad range of applications in different areas of science like nanotechnology, biology, forensic analysis, and for research work in almost all fields of science. 2. In biology, TEM is used to see the structure of cells and tissues in order to study the performance of different organs under different conditions for the diagnosis of different diseases. 3. Industries use TEM to see different features of micro-sized objects to solve different problems. 4. The high resolution and detailed image obtained by TEM also helps in morphology.

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5.3.3 Dissimilarities between scanning electron microscope and transmission electron microscope 1. Scattered electrons are used in SEM, while transmitted electrons are used in TEM for the imaging of a specimen. 2. SEM provides information only about the surface, whereas TEM focuses on the entire sample and gives data about the composition and internal structure of the sample. 3. TEM has a much better and higher magnification power than SEM. 4. In TEM, fluorescent screens are used for display purposes, whereas in SEM, monitors are used. 5. SEM gives a 3D image, whereas TEM gives a 2D image of the specimen being placed.

5.4

Scanning tunneling microscope

The scanning tunneling microscope (STM) has a roughly 35-year history and IBM has been very much involved in every step of the way. So it started with Gerd Binning and Heinrich Rohrer who are IBM scientists. They were scientists in a Switzerland research facility in Zurich. They had the crazy idea that you could actually hold a metal needle close to a surface and use it for imaging as shown in Fig. 5.7. In this regard, they both received a Nobel Prize in physics in 1986. Then this particular laboratory in Zurich at IBM

FIGURE 5.7 A schematic diagram of STM. Adapted with permission under the terms of the CC BY-SA 2.0 AT (https://creativecommons.org/licenses/by-sa/2.0/at/deed.en).

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Almaden was the first to be able to move atoms. So Gerd and Heinrich used it to see atoms and then Don Eiger used it as his hands and positioned atoms for writing the famous IBM logo. We also have a long history of using this tool to build structures and look at these structures. The STM is significantly used in many industrialized applications and is vital to study atomic-scale portraits of metal exteriors. It gives a 3D shape of a surface, which is extremely valuable for the characterization of surface roughness, the observation of surface defects, noting sizes, molecules conformation, and combinations on skin. The new reviews in electron physics and IBM laboratories offer different examples of advanced studies using STM. Numerous diverse scanning microscopes also make use of the technology of scanning tunneling. Electronic cloud related to atoms at the top of metallic surface extends an extraordinarily short gap. In STM, tip is scanned over the top of sample.

5.4.1

Components and workings

The STM works on the principle of the tunneling effect, which is the quantum mechanical phenomenon in which a particle having less energy than the height of an “energy barrier” crosses the barrier. The STM has a small nanosized metal tip with a high conductance that comes extremely close to the sample as shown in Fig. 5.7. When an electric potential is applied between the tip and the sample, the electrons from the sample are tunneled toward the tip and, hence, a small electric current is produced. This current produced by the tunneled electrons is then amplified and sent to a computer based on the recordings of the tunneling current. The information about the top of the sample is known as the magnitude of the tunneling current obtained, which depends on the gap between the tip and the sample. The tunneling current is high when the distance is small and the tunneling current decreases as the distance increases. The STM gives a magnification of around 100 million and a resolution of about 0.1 nm. It allows for the imaging of surfaces at the atomic level and also gives a 3D profile of surfaces. The inventers of STM were given a Nobel Prize in 1986. The first STM image was mapped by Nobel laureates Rohrer and Binnig of a silicon 1 3 1 surface. Later, with a lot of improvements, they tried to improve the topographic imaging of the same silicon surface by reconstructing the 1 3 1 silicon into a 3D 7 3 7 surface.

5.4.1.1 Various features of scanning tunneling microscope 1. STM gives a 3D image of a surface profile. 2. It has a higher resolution than other electron microscopes. 3. In STM, the probe is not directly connected with the surface of the specimen, but is kept at a shorter distance.

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4. It is expensive and only produces grayscale images. 5. It is only applicable to conducting surfaces. 6. It operates under an ultra-high vacuum.

5.5

Atomic force microscopy

Atomic force microscopy (AFM) is the most recent technique used to obtain surface characterizations from micro- to nanoscale. Gerd Binning designed AFM in 1986 to overcome the deficiencies of STM. This is an analytical, powerful, and nondestructive technique that is useful in liquid, air, or vacuum [58]. AFM provides topographic images with nanolevel resolution of a surface. Nanoscale properties, for example, mechanical (frictional, viscoelastic, stiffness, and modulus), chemical, magnetic, and electrical properties can be studied by AFM [5,9,10]. 3D images of surface scribes, defects, corrosion pits, gouges, scratches, etc., can also be obtained by it. AFM is small in size and easy to handle, which makes it a highly useful tool for surface characterizing and the study of the small features and properties of a surface. Fig. 5.8 shows a typical AFM (tabletop). Materials that allow a tunneling current to pass through them can be used for STM images. But in AFM, other materials can also be used for imaging that do not allow a current to pass through them. Sometimes, STM offers a better resolution than AFM as a tunneling current depends exponentially on the distance. The dependence between force and distance in AFM is more complicated if properties, for example, contact force and tip shape, are

FIGURE 5.8 A typical AFM (tabletop). Reprinted from M.K. Khan, Q.Y. Wang, M.E. Fitzpatrick, Atomic force microscopy (AFM) for materials characterization, in: Materials Characterization Using Nondestructive Evaluation (NDE) Method, pp. 116, Copyright (2016), with permission from Elsevier.

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considered. SEM and TEM are far behind AFM in characterizing the purpose of materials at smaller scales. AFM has the main advantage of producing 3D images of probed surfaces. Table 5.3 displays the differences between AFM and SEM/TEM. A comparison between different microscopies and AFM is shown in Fig. 5.9 with respect to cost and the range that can be observed through them.

TABLE 5.3 Differences between AFM and SEM/TEM. Parameter

AFM

SEM/TEM

Surface height

Possible

Not possible

Measurement environment

Air, water, gas, vacuum, etc.

Vacuum

Size of equipment

Extremely small

Large

Measurement dimension

3D

2D

Usage

Easy to use

Skilled operator required

Cost

Cheap

Expensive

Measurement speed

Slow

Fast

FIGURE 5.9 Comparison of different microscopies and AFM with respect to cost and minimum visible range. Reprinted from M.K. Khan, Q.Y. Wang, M.E. Fitzpatrick, Atomic force microscopy (AFM) for materials characterization, in: Materials Characterization Using Nondestructive Evaluation (NDE) Method, pp. 116, Copyright (2016), with permission from Elsevier.

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5.5.1

139

Construction of atomic force microscope

A list of the fundamental parts of an AFM such a photodetector, a cantilever, laser diode, and a sample stage, which can move along three axes, is given below. A simple schematic is shown in Fig. 5.10 for the construction of an AFM. 1. Laser: To form a scanned version of the sample using a laser deflection system. 2. Cantilever: Having a reflective back and a sharp tip that scans the surface of the sample. 3. Scanner: xy translational stage to place the sample. 4. Photodiode: To convert the measured difference signal into an electrical signal. 5. Feedback electronics: A computer with software control of the cantilever and lead zirconate titanate (PZT) stage. 6. Sample: AFM can scan any type of substrate or sample, but suitable substrates are flat and rigid.

5.5.1.1 Laser Here a laser is used just for the purpose of reflection. The laser is deflected from the reflective back of the cantilever onto the position-sensitive photodiode for surface topography. So its laser energy is kept smaller than the work function of the cantilever material. The question is, why use a laser instead of other light? The answer is simple, because of its coherency and

FIGURE 5.10 Simple schematic of AFM. Reprinted from M.K. Khan, Q.Y. Wang, M.E. Fitzpatrick, Atomic force microscopy (AFM) for materials characterization, in: Materials Characterization Using Nondestructive Evaluation (NDE) Method, pp. 116, Copyright (2016), with permission from Elsevier.

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unidirectionality. Therefore a scattering-controlled imaging can be obtained. It is important to adjust the laser to the right spot on the cantilever’s back, otherwise it may produce unwanted results and create confusion. So the first goal is to set the beam of the laser to the tip of the cantilever.

5.5.1.2 Cantilever Cantilevers with a sharp tip are generally made from silicon nitride or just silicon with a reflective back. Typical cantilevers are 1 µm thick, and 100 µm long. The spring constant, kspring , is B0.0120 N/m. The radius of the tip, Rtip , is B120 nm and its resonant frequency, fres , is B4400 kHz. A reflective back cantilever is used for better signal. Beside silicon nitride and silicon, tips are also made from nanotubes and diamond. The maximum resolution depends on the sharpness of the AFM tips. 5.5.1.3 Scanner It is the piezoelectric transducer that controls the movement of the sample during scanning below the AFM tip. When the applied voltage changes then the piezoelectric scanner moves the sample in predefined steps forth and back. The maximum area of scan that is possible by it depends on the size of the bottom. There is a sample holder to hold the tube of the piezoelectric scanner and the sample in a straight line below the tip. 5.5.1.4 Photodetector It detects deflections of laser light by measuring the difference in laser intensities and changes it to a map of the surface. The position-sensitive photodiode is adjusted before every measurement and for different modes. This is set to zero before any experiment. 5.5.1.5 Feedback electronics This is the main part of AFM. It includes an error corrector, a feedback loop, and a computer. The error corrector detects and corrects any errors in the photodiode output signal (Fig. 5.11). This corrected signal travels back to the PZT scanner through the feedback loop and finally to the computer system. The system continuously changes in response to an experimental output (cantilever deflection) only with the help of the feedback loop. The computer system has software control over both the cantilever and PZT scanner and it adjusts the z-piezo distance to keep the cantilever deflection constant and equal to the setpoint value. 5.5.1.6 Sample There is no need for sample preparation, but what is taken as a sample or substrate? What does it look like? AFM can scan any type of substrate or sample, but suitable substrates are flat and rigid. For example, SiO2, glass (nanometer roughness, hydrophobic), ultra-flat gold (stripped gold).

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FIGURE 5.11 Insight of an AFM showing prominence of feedback system. Reprinted from Mengzhen Ding, Cuiping Shi, Jian Zhong, Atomic force microscopy for food quality evaluation, in: Evaluation Technologies for Food Quality, pp. 715741, Copyright (2019), with permission from Elsevier.

5.5.2

Working principle of atomic force microscope

The sharp tip of the cantilever scans the sample surface, and during this scan, it examines the sample surface by identifying the short-range attractive or long-range repulsive force between the tip and the sample surface. The way atoms will respond to the Van der Waals forces is dependent on the contact type. This force can be described with the spring constant and the distance of the tip from the surface, which is known as Hooke’s law (F 5 kx). When the tip reaches near to the sample surface, the cantilever bends down toward it due to the attractive nature of Van der Waals forces, but when the tip touches the sample surface, the repulsive nature of these forces deflects the cantilever away from it. These deflections are sensed by the laser beam, which is incident on the cantilever top, which reflects it to the photodetector (position sensitive). This photodetector records the cantilever deflections. Ceramic crystals that contract or expand due to applied voltage changes are called piezo crystals. Image data in digital form is collected from the height of the scanner by the scanner motion along the scanning path. The step size of the scanning is important for data resolution and it is found by the quantity of data points on a single line and the total scan area. Usually the quantity of lines is equal to the quantity of data points on a single line. For quality results, scanner motion along the z-axis is confined to the range of some nanometers.

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Modes of operation

In AFM, there are three modes of operation, which are discussed one by one here. The type of interaction between the cantilever tip and the surface determines which mode of operation will be used.

5.5.3.1 Contact/repulsive mode In this mode, the tip touches the surface of the sample below 0.5 nm of separation. This mode is considered good only when the surface of the sample is not considerably rigid from the tip. If it is not so, the cantilever is bent and broken down due to the greater stiffness of the surface of the sample. A surface image is attained by the deflections of the cantilever due to repulsive constant force. The feedback circuit regularly adjusts the height of the tip so that the force remains constant. Hooke’s law is used to calculate the force of deflection, which is examined by a detector. The schematic in Fig. 5.12 shows the contact mode during the scanning of a surface. 5.5.3.2 Noncontact/attractive mode In this mode, the cantilever does not touch the surface of the sample and maintains a distance range of 0.110 nm. It shakes close to the surface of the sample with a frequency higher than a resonant one. The amplitude and resonant frequency reduce due to the attractive force in this mode. The schematic in Fig. 5.13 shows the noncontact mode during the scanning of a surface. In different environments, a surface can be scanned in this mode and it offers the possibility of imaging those surfaces, which is not possible with electron microscopes. 5.5.3.3 Tapping/intermittent mode In this mode, the tip of the cantilever alternates contact to the surface of a sample at resonant frequency. This mode is identical to contact mode. At a range of 20100 nm of amplitude, the tip of the cantilever shakes with a lower frequency than a resonant one. The tip of the cantilever makes contact

FIGURE 5.12 A schematic representation of the contact mode during scanning in AFM. Reprinted from M.K. Khan, Q.Y. Wang, M.E. Fitzpatrick, Atomic force microscopy (AFM) for materials characterization, in: Materials Characterization Using Nondestructive Evaluation (NDE) Method, pp. 116, Copyright (2016), with permission from Elsevier.

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FIGURE 5.13 A schematic representation of the noncontact mode during scanning in AFM. Reprinted from M.K. Khan, Q.Y. Wang, M.E. Fitzpatrick, Atomic force microscopy (AFM) for materials characterization, in: Materials Characterization Using Nondestructive Evaluation (NDE) Method, pp. 116, Copyright (2016), with permission from Elsevier.

FIGURE 5.14 A schematic of the tapping mode during scanning in AFM. Reprinted from M.K. Khan, Q.Y. Wang, M.E. Fitzpatrick, Atomic force microscopy (AFM) for materials characterization, in: Materials Characterization Using Nondestructive Evaluation (NDE) Method, pp. 116, Copyright (2016), with permission from Elsevier.

with the surface when it comes to its lowest point during the scanning swing. This contact is for a very short time. This mode offers better quality for soft films and materials. The schematic in Fig. 5.14 shows the tapping mode during the scanning of a surface.

5.5.4

Advantages and disadvantages

AFM has several advantages including: 1. It provides a 3D image of the sample. 2. In this method, samples do not need any special treatment for proper operation. 3. It offers a higher resolution than other techniques. 4. It is a quick and easy way to characterize.

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AFM has several disadvantages including: 1. It has a slow scanning time, which results in thermal drift on the specimen being used. 2. It is an expensive method. 3. The size of a single scan image is of the order of 150 3 150 µm2: 4. It damages soft samples (i.e., silicon, polymers).

5.5.5

Applications

AFM has several applications in different fields including: 1. 2. 3. 4.

Digital imaging of topographical surfaces. In biology, imaging of nonconducting surfaces of samples like DNA. Determining the roughness and thickness of a crystal layer. It is useful in different binding studies like antibodyantigen and ligandreceptor.

5.6

Fluorescence method

In this section, we are going to explore the science and effects of fluorescence. Some basic terms related to fluorescence need to be understood first. For example, it is already known that the electrons present in the atom have a specific spin associated with them. These electrons pair themselves with electrons having opposite spins. As a result, the net spin of the pair of electrons is zero. When the electrons are paired with electrons having opposite spins then they are said to be in a singlet state. However, when both the paired electrons have the same spin associated with them, then such a state is called a triplet state as shown in Fig. 5.15. Now we can try to understand how fluorescence works by looking at the energy level diagram as shown in Fig. 5.16. The energy level diagram helps one to understand the production of fluorescence. As seen in Fig. 5.16, there are different states of molecules at ground level where the molecules have constant energies and possess the lowest value. When external electromagetic radition (EMR) falls on a molecule, it absorbs the energy from these radiations. This absorption of energy results in an electronic transition and molecules transit to electronic states of higher energy like first and second electronic singlet states. It should be taken into account that transitions of electrons will occur just for definite molecules when struck by definite wavelengths of EMR. It is due to definite energy being absorbed by the radiation in the characteristic structure of the atom. In Fig. 5.16, λ1 denotes the wavelength of absorbed light by molecules transit to the first singlet state of electrons. The certain time range for the excited singlet state is 10281024 seconds. After this short range of time, electrons return to ground singlet state and the transition energy is emitted in the form of light emissions. This phenomenon is called

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FIGURE 5.15 Singlet and triplet excited states of molecules. Adapted with permission under the terms of the CC0 1.0 (https://creativecommons.org/publicdomain/zero/1.0/).

FIGURE 5.16 Energy level diagram for the production of fluorescence. Reprinted by permission from Springer Nature: W.S. Chow, Photosynthesis: from natural towards artificial [11], J. Biol. Phys., Copyright (2003).

fluorescence. These are low energy transitions as compared to the precious absorptions. So wavelengths emitted during this phenomenon are larger than wavelengths involved in absorption, that is, λ3 is greater than λ1 . One can perform both qualitative and quantitative analyses using fluorescence.

5.7

Synchrotron radiation

Synchrotron radiation is the process in which light gets emitted by accelerated particles. Usually these are charged particles like electrons. An example

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FIGURE 5.17 Production of synchrotron radiation by electrons under strong magnetic fields. Adapted with permission under the terms of the CC BY 3.0 (https://creativecommons.org/ licenses/by/3.0/).

from daily life is radio waves, where electrons are accelerated up and down in a radio antenna, which in turn emits light in the form of radio waves. This is a very low frequency light. This is actually EM radiation released by the accelerating charged particles radially. In this way, operators can choose the wavelength needed for a certain experiment where photons of high intensity permit fast researches using weak scattering crystals. When highly energetic particles are moving swiftly together with electrons, synchrotron radiations are produced if these are compelled to move in a curved path by a magnetic field (Fig. 5.17). It is the same method used in radio antennae, the only difference is the change in the observed frequency by relativistic speed and Lorentz factor. The contraction of length and increase in frequency is noted by additional factor γ, so increasing the frequency (GHz) of the resonant cavity that fasttracks the electrons in the X-ray range. The power radiated is expressed by the relativistic Larmor formula, while force on releasing electrons is represented by AbrahamLorentzDirac force. The radiation is distorted by an isotropic dipole pattern into very advance directing funnel of radiation. These synchrotron radiations are intense artificial X-ray sources. The geometry of planar acceleration seems to produce linearly polarized radiation if seen in an orbital plane and circularly polarized if seen at a small angle to that plane.

5.8

Atom probe instrument

This chapter includes a refined tool for the characterizations of nanoparticles. In this chapter, we discuss the atom probe instrument, its construction, working, and interpretations. This chapter includes the basic purpose of the use of the atom probe instrument.

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Atom probing is a combination of instrumentation, which was introduced by Erwin Mu¨ller in 1967, who titled that instrument the atom probe field ion microscope [12,13]. Atom probe instruments were developed over time. Atom probes are a technique to study the 3D imaging of each single atom, the chemical composition of the material under observation, to study multilayer structures [14,15] and the atomic network, and the poisoning of atoms within the material up to 100 nm. It also gives the imaging of the network of the atoms within a material. The atom probe is usually associated with the AFM, but this is not true, there are some basic differences between the atom probe and the AFM. The first and most important difference is that the AFM gives the characteristics of the surface of a material, but the atom probe gives comprehensive 3D views of the atoms contained in a material. The second difference is that the AFM only gives a surface imaging, but the atom probe gives an imaging as well as the compositions of a material and the positioning of the atoms within the material.

5.8.1

Construction

It consists of two electrodes. The observable material is attached onto one electrode and a detector acts as the second electrode. A voltage or a laser pulse is applied to the Sharpe needle of the material. The detector used is a position-sensitive detector [16] as shown in Fig. 5.18.

5.8.2

Working of atom probe field ion microscopy

A high voltage is applied between the material and the opposite electrode. A laser pulse beam is used to ionize the material as shown in Fig. 5.19. An electric field is produced at the other end, then the ionized atoms of the material are absorbed and move toward the detector, and the imaging of the atoms is collected and the compositions of the material are studied. The high electric field produced and the sharpness of the tip of the material are the basic key points of the whole functioning of the atom probe. For example, if we have a very sharp tip of material under 100 nm, then 10 kV of potential applied will create enough of an electric field at the tip to pull the atom from the surface of the material toward the position-sensitive detector [17,18]. The electric field is a highly sensitive parameter in controlling the whole working of the atom probe. If we cannot control the electric field at the tip, then there is a possibility of damaging the material. The position-sensitive detector will measure the time of flight as well as the position of the ions absorbed from the tip [19]. The time of flight will correspond to the mass of the ion. After the determination of the mass of the ion, the chemical composition of the material can be identified. One thing is noticeable, the departure

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FIGURE 5.18 Schematic diagram of the atom probe. Reprinted from M.K. Miller, T.F. Kelly, K. Rajan, S.P. Ringer, The future of atom probe tomography, in: Materials Today, pp. 158165, Copyright (2012), with permission from Elsevier.

FIGURE 5.19 Schematic diagram of the mechanism of the atom probe. Reproduced with permission from Cambridge University Press, B. Gault, M.P. Moody, F. De Geuser, A. La Fontaine, L.T. Stephenson, D. Haley, S.P. Ringer, Spatial resolution in atom probe tomography, Microsc. Microanal. 99110, Copyright (2010).

time of the ion from the tip is known by which detector gives the time of flight of the ion.

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5.8.3

149

Mathematical analysis

If “m” is the mass, v is the velocity, “t” is the time of flight, and “e” is the charge on the ion being absorbed from the tip of the material, the amount of kinetic energy of the material corresponds to the potential applied to the ion. 1 2 mv 2 KE 5 neV KE 5

Comparing above equations. 1 neV 5 mv2 2 sffiffiffiffiffiffiffiffiffiffiffiffiffi 2 neV v5 m As we know v5

S t

where S is the distance traveled by the ion from the tip to the detector. The final expression shows that the mass of the ion depends on the time of flight, that is,
2 sffiffiffiffiffiffiffiffiffiffiffiffiffi S 2 neV 5 t m
2 t m 5 2neV S

5.8.4

Limitations of atom probe

This process will be continuing to work until the following parameters involved and these are the limitation of the atom probe at which the working of the atom probe stops. That limitation are, tip of material become unsharpened and less potential applied to the tip which cannot produce enough electric field to work the atom probe.

5.8.5

Comparison with tunneling electron microscope and SIMS

Atom probe field ion microscopy does not give ideal results regarding the imaging and composition of a material, but it gives highly refined and nearaccurate results. TEM does not give the composition of the material, but the average composition of the material. secondary ion mass spectrometry

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(SIMS) uses mass spectrometry to study atoms, which cannot give accurate positioning of the material, but atom probe gives the exact composition of the material and the positioning of the absorbed ion can be defined. The resolution of the atom probe is usually less than 0.5 nm. An atom probe field ion microscope gives more refined structural properties and composition of materials than the SIMS and TEM respectively. The results of an atom probe field ion microscope are collected as graphical representations in an oscilloscope. The graphs are explained on the basis of the involved parameters. The atom probe field ion microscope is used to study composition, positioning as well as exact 3D structures at the atomic scale with a resolution of less than 0.5 nm. The atom probe instrument detector measures the time of flight and calculates the mass of the ion, then the composition of the material under characterization can be determined. The output of the atom probe instrument is in the form of a graphical representation on an oscilloscope. The limitation of the atom probe instrument is to provide enough amount of voltage so that it can produce the required strength of electric field.

References [1] V. Bellitto, Atomic Force Microscopy: Imaging, Measuring and Manipulating Surfaces at the Atomic Scale, BoDBooks on Demand, 2012. [2] P.J. Goodhew, J. Humphreys, Electron Microscopy and Analysis, CRC Press, 2000. [3] B. Fultz, J.M. Howe, Transmission Electron Microscopy and Diffractometry of Materials, Springer Science & Business Media, 2012. [4] J. Zuo, J. Spence, Electron Microdiffraction, Springer Science & Business Media, 2013. [5] D.B. Williams, C.B. Carter, Transmission Electron Microscopy: Diffraction, Springer, 2009. [6] T. Okajima, et al., Self-oscillation technique for AFM in liquids, Appl. Surf. Sci. 210 (12) (2003) 6872. [7] M. Kageshima, et al., Noncontact atomic force microscopy in liquid environment with quartz tuning fork and carbon nanotube probe, Appl. Surf. Sci. 188 (34) (2002) 440444. [8] Y. Song, B. Bhushan, Finite-element vibration analysis of tapping-mode atomic force microscopy in liquid, Ultramicroscopy 107 (1011) (2007) 10951104. [9] A. Hendrych, R. Kub´ınek, A. Zhukov, The magnetic force microscopy and its capability for nanomagnetic studies-the short compendium, Mod. Res. And. Educ. Top. Microscopy 2 (2007) 805811. [10] F. Zhang, et al., Investigation on the magnetic and electrical properties of crystalline Mn 0.05 Si 0.95 films, Appl. Phys. Lett. 85 (5) (2004) 786788. [11] W. Chow, Photosynthesis: from natural towards artificial, J. Biol. Phys. 29 (4) (2003) 447. [12] E.W. Mu¨ller, J.A. Panitz, S.B. McLane, The atom-probe field ion microscope, Rev. Sci. Instrum. 39 (1) (1968) 8386. [13] E.W. Mu¨ller, Das feldionenmikroskop, Z. fu¨r Phys. 131 (1) (1951) 136142. [14] E. Marquis, et al., Evolution of tip shape during field evaporation of complex multilayer structures, J. Microscopy 241 (3) (2011) 225233.

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[15] B. Gault, et al., Advances in the reconstruction of atom probe tomography data, Ultramicroscopy 111 (6) (2011) 448457. [16] M.K. Miller, et al., The future of atom probe tomography, Mater. Today 15 (4) (2012) 158165. [17] T.F. Kelly, et al., On the many advantages of local-electrode atom probes, Ultramicroscopy 62 (12) (1996) 2942. [18] G. Kellogg, T. Tsong, Pulsed-laser atom-probe field-ion microscopy, J. Appl. Phys. 51 (2) (1980) 11841193. [19] A.A. Gribb, T.F. Kelly, Atom probe analysis, Adv. Mater. & Process. 162 (2) (2004) 3134.

Chapter 6

Fabricating nanostructures Tahir Iqbal Awan1, Muhammad Irfan2, Mohsin Ijaz1, Almas Bashir1, Aqsa Tehseen1 and Sumera Afsheen3 1

Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan, 2Department of Biochemistry and Biotechnology, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan, 3Department of Zoology, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan

Chapter Outline 6.1 Introduction 153 6.2 Lithography 155 6.2.1 Photolithography 156 6.2.2 Electron beam lithography 159 6.3 Molecular beam epitaxy 161 6.3.1 Molecular beam epitaxy process 161 6.3.2 Working principle 162 6.3.3 Molecular beam epitaxy layout 162 6.3.4 Features of molecular beam epitaxy 163 6.3.5 Advantages and disadvantages of molecular beam epitaxy 164 6.3.6 In situ growth monitoring techniques 164 6.4 Self-assembled masks 164

6.1

6.4.1 6.4.2 6.4.3 6.4.4 6.4.5 6.4.6 6.4.7

Distinctive features 165 Order 165 Interactions 165 Building blocks 166 Examples 166 Properties 166 Self-assembly at the macroscopic scale 166 6.5 Focused ion beam 167 6.5.1 The construction of focused ion beam 167 6.5.2 Principle 172 6.5.3 Applications of FIB 172 6.6 Stamp technology stamping 173 6.6.1 Operations 173 6.6.2 Stamping lubricant 174 6.6.3 Industrial applications 174 References 175

Introduction

The phrase, the fabrication of nanostructures, means to make the dimensions of a material between 1 nm and 100 nm. There are basically two approaches to fabricate nanoparticles and nanomaterials, that is, bottom-up and top-down approaches as indicated in Fig. 6.1. In the bottom-up methodology, we use small things like atoms, molecules, or nanoparticles gathered in a group to form

Chemistry of Nanomaterials. DOI: https://doi.org/10.1016/B978-0-12-818908-5.00006-8 © 2020 Elsevier Inc. All rights reserved.

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FIGURE 6.1 A schematic for the two approaches for nanosynthesis. Reprinted from M.C. Garc´ıa, F. Quiroz, Nanostructured polymers, Nanobiomaterials (2018) 339 356, Copyright (2018), with permission from Elsevier.

nanostructures. In the bottom-up method of fabrication, the arrangement of molecules is complex and unique, that is, in the form of atom by atoms or in the form of molecules by molecules and this develops a bulk material. In this method, nanoparticles, which are the building blocks of any bulk material, are firstly identified and produced by natural methods and then finally these nanoparticles are assembled to produce the required material. Nanomaterials such as nanocoatings are constructed using atoms, molecules, and by their grouping. In the top-down method, nanomaterials are formed by successively removal of material from bulk. Particles are removed for achieving dimensions in the range of 1 100 nm. This is done to achieve novel properties in a material. In the top-down method of fabrication, specific tools are used to cut, grind, and crush the bulk material into the desired shape and order, which has a size range approximately equal to the nanometer scale. These nanostructures and nanoparticles are obtained using a focused ion beam (FIB). An example is the world’s shortest wine glass. According to The Guinness Book of World Records, the smallest glass of world has a diameter 20-times smaller than that of normal human hair. The typical diameter for human hair is around 80,000 nm. Although this wine glass is quite a big structure on the nanoscale, it actually is the smallest-sized feature known to date. It is produced using FIB. These two approaches or methods of fabrication for nanomaterials are complementary because these two methods enable us to measure the magnetic and electric properties of nanoparticles using a top-down method. Nanoelectrodes can be produced through a top-down approach and attached to nanoparticles whose properties have to be studied. Printing methods and statue manufacturing from marble are also examples of it. In the top-down approach, three techniques are used, namely: 1. Photolithography 2. Electron beam lithography 3. Focused ion beam

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Techniques of top-down fabrication were firstly used in the semiconductor industry to build various parts of computer chips. These methods are collectively called lithography, which uses electrons or light beams to remove microscale structures from bulk materials also called resist. In the past few years, many efforts have been made to minimize the size of and add more features to electronic devices. It became possible through lithography that the size of transistor in latest processor is 45 nm

6.2

Lithography

The process of lithography is based on the immiscibility of oil and water. The technique is based on the principle, ‘like dissolve like’ and the repulsion of water for oil. The method is cost effective. It is a quick method that produces sharp and clear images and fast print runs are possible with this method. The drawbacks include the poor quality image and small prints are not possible with this method. There is complexity in using the materials for lithography. Stone lithography is based on a simple idea. Samples of lithography are shown in Fig. 6.2. 1. Painting on the stone with a greasy substance: 2. Humidifying the stone using water: 3. Oil-based ink is rolled onto the stone: 4. A piece of paper is pressed onto the stone:

The artist draws with greasy paints on stone. The second step includes humidifying the stone using water; the parts of the stone that are blank absorb water. The next step is to pour oily ink on the stone, the ink adheres only on the oily parts. In the last step paper is pressed against the stone and the ink adheres onto the stone and the pattern is drawn.

FIGURE 6.2 Samples for the visualization of lithography. Adapted with permission under the terms of the CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/).

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Photolithography

Photolithography pertains to the science of transferring shapes at the surface of wafer made of silicon. The word lithography is derived from the Greek words, “photo” (light), “litho” (stone), and “graphy” (write), which together means printing with light. “Photolithography is optical process in which required pattern is transferred onto the substrate” [1].

6.2.1.1 Steps of photolithography The step by step process of photolithography is given in Fig. 6.3. Several steps are involved in photolithography namely: 1. 2. 3. 4. 5. 6. 7.

Substrate cleaning Forming a silicon dioxide (SiO2) layer Layer of photoresist Removal of solvent Arrangement of mask Development of pattern on substrate Hardening The processes involved in photolithography include:

1. 2. 3. 4. 5.

Surface preparation Oxidation Masking Exposure to UV light Etching

6.2.1.1.1 Surface preparation The chemical cleaning of the silicon wafer used is the first step; this is for removing any impurities of organic matter stuck to it. The substrate used is

FIGURE 6.3 The step by step process of photolithography. Adapted with permission under the terms of the CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

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usually made up of glass or a semiconductor, that is, silicon (Si) or germanium (Ge). The surface of the substrate is prepared by chemical means. First of all, the substrate is chemically treated to clean and remove any contamination, that is, oil, grease, or dust particles from the surface of the substrate. For that purpose, hydrogen peroxide (H2 O2 ) is used. 6.2.1.1.2

Oxidation

After the cleaning of the substrate, it is ready for coating. A layer of silicon dioxide is applied as a barrier layer called photoresist. For the application of the coating in integrated circuit manufacturing, high speed whirling of the silicon wafer is used. This method is also known as spin coating. Now, silicon dioxide is added as a layer onto the substrate or wafer, also called oxidized wafer. The oxidized wafer is covered with photoresist by a spin coater. The application of a photo-resistant solution is applied on the wafer and the wafer is then spun rapidly to produce a uniform layer. The spin coater usually runs for the duration of 30 60 s and produces a layer of 0.5 0.25 µm in thickness. The photoresist is then baked to dry out the excess solvent typically at 90 100 C for 5 30 min. The photoresist usually used is poly methyl methacrylate (PMMA). Types of photoresist There are two major types of photoresist, that is, one is positive and the other is negative. Positive photoresist

Negative photoresist

When it comes into contact with light, its structure alters and its ability to form a mixture is enhanced Same drawing appears on the mask

When light falls on it, a polymer forms and it becomes hard to dissolve in solvent A drawing inverse of the original is obtained on the mask

After the interaction with light, unexposed parts of the photoresist are removed by introducing an aggressive solvent. It is used to remove solvent from photoresist. A soft baking can impart images on the coating. If too much baking is done, it will result in the destruction of the solubility of the resist, while under baking will hinder light from reaching the resist. 6.2.1.1.3 Masking In this process, the photoresist is covered with a mask of predetermined shape and pattern. A mask is a glass plate with a print on one side. The next step is to transfer this print from the mask to the wafer. After the correct alignment of the mask, a high light intensity can be used to expose the photoresist on the mask.

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6.2.1.1.4

Exposing to UV light

The wafer is then exposed to UV light through a photomask. UV light rays pass through the photomask and reach the photoresist and softening this layer. The photoresist is soluble due to the exposure to UV light in the presence of air resulting in bond breakage of the exposed area of the photoresist and it is removed by hydrofluoric acid (HF). The three primary methods for exposure are shown in Fig. 6.4. Contact printing In this type of printing, the silicon wafer that is resist coated, is attached to the mask. The photoresist is exposed to a direct contact with UV light while the substrate is attached to the mask. Because the resist and mask are close to each other, this results in a high resolution printing (e.g., 1000 nm features in 500 nm of positive resist). Waste materials are defect causing agents in contact printing, which can be found between the mask and the resist; this could cause the destruction of the mask resulting defected products. Proximity printing This method resembles contact printing, the difference is in the space, which could be of 25 µm, between the silicon photoresist and the mask. This gap can reduce the chances of mask damages. Proximity printing can produce a resolution of about 2 4 µm. Projection printing In this method, there is no chance of mask damage. The resist coated wafer is used for the projection of the image from many centimeters away. If a small portion is considered, it results in high quality images with good resolution. For this purpose, only small surface is deposited upon wafer. These are termed strap and heat printers, which are capable of giving a resolution of 1 µm.

FIGURE 6.4 The three methods for primary exposure. Reprinted by permission from Springer Nature: M. Leester-Scha¨del, T. Lorenz, F. Ju¨rgens, et al. Fabrication of microfluidic devices, in Microsystems for Pharmatechnology, Springer, 2016, pp. 23 57, COPYRIGHT (2016).

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6.2.1.1.5

159

Etching

In this process, HF acid is used to remove the upper-most layer of the substrate of the area unprotected by the photoresist. Beware of HF; it is corrosive to human skin. The unexposed area of photoresist remains protected from UV light. The rest of the photoresist is removed and the wafer is now ready for doping. Electron beam lithography works on the same principle as photolithography. In this case, infrared (IR), visible, and UV light are used. The steps involved in photolithography including the preparation of the substrate and the exposure to UV light to fabricate nanostructures of a required pattern have been illustrated in Fig. 6.5. Light is made to fall on a photoresist through a mask and the photoresist is removed and nanoholes are produced. Nanodevices are prepared using this method.

6.2.2

Electron beam lithography

Electron beam lithography (EBL) is the technique of using a scanned beam of electrons to print a pattern on a substrate by using a resist that is sensitive to electron interactions and its solubility changes as shown in Fig. 6.6.

6.2.2.1 Procedure 6.2.2.1.1 Coating by resist A substrate is coated with a thin layer of resist. The resist is sensitive to electrons. The resist is a polymer that changes its solubility pattern in solvent upon interaction with electrons, ions, and photons. A heterogeneous pattern leads to different pattern on the resist. There are two types of resist. Positive resist: Where a chemical change takes place, and because of which, the areas of the resist exposed to electrons become soluble. Negative resist: Where a physical enhancement of the molecular weight of the resist takes place, which renders the exposed areas insoluble. When the exposed areas can be dissolved in solvent it is called positive lithography, while if the nonexposed areas are dissolved in solvent it is called negative lithography. This process is called development. 6.2.2.1.2

Deposition of metallic layer

After this metallic layer is deposited on the substrate, the metallic layer adheres to the substrate exposed to electrons and to the nonexposed parts of the resist. 6.2.2.1.3

Aggressive solvent mixture

The nonexposed resist is dissolved in an aggressive solvent. The metal sticking to the resist loses its footing and the resist stuck on the substrate sustains.

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FIGURE 6.5 (A) A schematic for the process of photolithography. (B) Different steps of photolithography. (A) Reprinted from MEMS for Biomedical Applications, C-W. Li, G-J. Wang, MEMS manufacturing techniques for tissue scaffolding devices, 192 217, Copyright (2012), with permission from Elsevier. (B)Adapted with permission under the terms of the CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0).

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FIGURE 6.6 The step wise process for the creation of a patterning by electron beam lithography (A) Exposure of electron beams (B) Development and rinse (C) Etching of silicon dioxide (D) Etching of silicon (E) Removal. Reprinted from T. Doi, Promising future processing technology, in: Advances in CMP Polishing Technologies, 2012, pp. 229 295, Copyright (2012), with permission from Elsevier.

6.3

Molecular beam epitaxy

Epitaxy can be defined as the deposition of monocrystalline layers and their growth. Crystalline layers are deposited onto a substrate in this process. The deposited layer is called an epitaxial layer or film. This term has been derived from the Greek word “epi” meaning “above” and “taxis,” which means “an ordered manner” [3]. This method involves the deposition of a thin single crystal film. At Bell Telephone Laboratories, J. R. Arthur and his coworker, Alfred Y. Cho, invented this method in the late 1960s. Electronic devices and semiconductors are manufactured using this technique.

6.3.1

Molecular beam epitaxy process

In the molecular beam epitaxy (MBE) process, the atoms or molecules are in gaseous state so that they cannot react with each other. The substrate is placed in vacuum. Where the atoms find a suitable place on the substrate, they adhere on it.

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6.3.1.1 Epitaxial growth of crystalline layers on substrate MBE can be defined as a technique of epitaxial growth that occurs through the interaction of atoms or molecules with a substrate 6.3.1.2 Epitaxy types There are two types of epitaxy. 6.3.1.2.1

Homoepitaxy

In homoepitaxy, the material and substrate are of the same kind like in Si Si deposition. 6.3.1.2.2

Heteroepitaxy

In heteroepitaxy, the substrate and material are of different kinds like in Ga As deposition.

6.3.2

Working principle

The MBE instrumentation or device is, in principle, simple; the sample is placed in an ultra-high vacuum chamber after preparing and cleaning a single crystal face. For the deposition of an even layer of substrate, the beam is kept far away from the sample. The exposure of various materials is controlled by an elaborate control system for precise timing and accuracy. Oscillations in the reflections of diffracted materials from the surface layers are implemented to measure the thickness of the deposited material; and to perform this, reflection high-energy electron diffraction (RHEED) is applied. This yields a good quality monolayer with enhanced precision of deposited material.

6.3.3

Molecular beam epitaxy layout

When atomic/molecular beams interact with the surface of the heated crystalline substrate, the absorbed specie causes MBE to occur under ultra high vacuum (UHV). In a stainless steel growth chamber, these beams are subjected to evaporate in crucible within the cells. Prior to the main growth of the cells, the samples are subjected to a degassing chamber where a high temperature causes degassing in a few hours. The growth chamber is composed of a stainless-steel vessel that contains material sources, a heatable substrate holder, and mechanical shutters. The substrate is placed, facing downward toward the source cells, on the substrate handling system. In the next stage, the holder is heated up to the required temperature. The high temperature is just for the activation of the diffusion of the absorbed

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FIGURE 6.7 Schematic of practical apparatus for molecular beam epitaxy. Reprinted from R.J. Mart´ın-Palma, A. Lakhtakia, Vapor-deposition techniques, in: Engineered Biomimicry, 2013, pp. 383 398, Copyright (2013), with permission from Elsevier.

species. To enhance the uniformity of the growth over the wafer, the holder is rotated. A schematic of MBE apparatus is shown in Fig. 6.7.

6.3.4 1. 2. 3. 4. 5. 6. 7.

Features of molecular beam epitaxy 

Low deposition rates, typically 1 µm/h or 1 A /s Typically in an ultra-high vacuum Uses high-purity elemental charge materials Well-controlled growth With good crystalline structure, it produces films To grow alloy films, multiple sources are used High substrate temperatures are not required as deposition rate is low

The gas evolution rate should be kept minimum as this chamber is an ultra-high vacuum (UHV); and for this purpose, pyrolytic boron nitride (PBN) is a good fit for its characteristic of giving a low rate for the evolution of gas at 1400 C, the shutters and heaters used are made of tantalum and molybdenum. For the characterization of tools, the UHV like reaction high energy electron diffraction (RHEED) is used to obtain a sufficiently clear

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layer of epitaxy. The RHEED oscillation produces signal exactly equal to the time required to grow a monolayer, while the diffraction pattern of RHEED indicates the state of the surface.

6.3.5

Advantages and disadvantages of molecular beam epitaxy

Advantages Clean surfaces, free of an oxide layer Deposition of metal seeds, dopants, and semiconductor materials occurs in situ Low growth rate of 1 µm/h Precisely controllable thermal evaporation Each component evaporates separately High substrate temperature not required Profiles are ultra-sharp

6.3.6

Disadvantages Expensive (US$106 per MBE chamber) Asaro Tiller Grinfeld (ATG) instability Highly complicated system Epitaxial growth under ultra-high vacuum conditions It gives slow deposition rates

In situ growth monitoring techniques

There are two analysis techniques used in order to monitor real time growth and these are RHEED and quadruple mass spectrometer (QMS). The former finds its application in the study of surface morphology and calibrating growth rate by measuring the diffraction pattern on a photoluminescent screen, while the latter (QMS) is applied to monitor contamination and for leak detection, which is done by analyzing residual gases. Furthermore, the desorbing specie can also be measured by reflected beam fluxes.

6.4

Self-assembled masks

Micromachines of 0.1 100 nm are of great interest as these could be utilized in electronic devices, sensors, etc. Nanometer-sized materials are tough to analyze and utilize efficiently. Lithographic techniques based on mask have some challenges. These interfere with the reliable design of a mask of nanometer size. To cope with such challenges, self-organizing polymer monolayers are used for mask preparation. This can be done with 2D self-ordered arrays of self-assembling diblock copolymers using gold, Silicon dioxide (SiO2), and silicon or silicon oxides (Si/SiOx) surfaces. This imparts accurate mask features in underlying surface by generating 50 nm diameter-size and 110 nm periodicity with distinct topological arrays. These could be examined using lateral force microscopy (LFM). For the creation of surface nanostructures, specific pattern-forming materials were tested as candidate masks and these were found to be successful. Scientists have experimented to show that block copolymer reverse micelles deposited on a surface have the capability

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FIGURE 6.8 The process of self-assembly. Reprinted with permission from Y. Masuda, et al., Self-assembly patterning of colloidal crystals constructed from opal structure or NaCl structure. Langmuir 20(13) (2004) 5588 5592. Copyright (2004) American Chemical Society.

of self-ordering, hence, these can be derivatized selectively for providing etch contrast. The process of self-assembly is shown in Fig. 6.8. Molecular self-assembly is defined as the process by which disarranged systems consisting of preliminary components are used to make unique and arranged systems with no external support. In this process, these components are molecules. Self-assembly could be categorized into two types, either static or dynamic. In the static case, a system can be present in an organized state and this state can also be considered as equilibrium state.

6.4.1

Distinctive features

In this case, anyone can argue that large structures like precipitation are made up of the assembly of smaller atoms and molecules, which may assemble with the driving force of a chemical reaction. This process will lie under the classification of a self-assembly approach. However, for unique concepts, there should be, at minimum, three well-defined features present.

6.4.2

Order

This is also one of the most important features of self-assembled systems. It can be seen that isolated systems have a lower level of order as compared to self-assembled systems. Self-assembled systems can be present in specific shapes to perform their specific functions, but sometimes this is not acceptable, especially during chemical reactions in which one organized state performs its function according to the applied thermodynamic conditions.

6.4.3

Interactions

Slack interactions are a kind of interaction like hydrogen bonding that plays an important role during self-assembly or synthesis in self-assembled systems. These are weak bonds as compared to ionic, covalent, and other bonds that are present in metals. Slow interactions have eminent place, especially

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during biological synthesis of materials. Some important properties of slack interactions are discussed here such as them being used to identify the apparent features of liquids, how solvable solids are, and the arrangement of components in organic membranes.

6.4.4

Building blocks

One of the third most important characteristic of self-assembled systems is building blocks. It is said that building blocks not only consist of atoms and molecules, but also of nanostructured materials, which contain various chemicals within their own structures, compositions, physical properties, etc. This term “micrite” was introduced by Defense Advanced Research Projects Agency (DARPA) in reference to microrobots of submicron size. Their selfassembling traits can be compared to slime mold. Some examples of these multidisciplinary building blocks include polyhedra and patchy particles. These novel nano-sized building blocks (NBBs) may be synthesized by conventional chemical methods or through many other self-assembling techniques like directional entropic forces (DEF).

6.4.5

Examples

Some prominent examples of self-assembled materials are emergence of crystals, colloidal solutions, lipids, etc. The conversion of polypeptide chains into proteins and nucleic acids conversion into their own functional groups are examples of biologically synthesized self-assembled materials. Drug delivery systems have also begun using the self-assembly processes.

6.4.6

Properties

Self assembled materials elaborated their important features to synthesize materials with specific properties, their bonding extends them into some weak bonds like slow interactions and nano-sized building blocks will be considered in all self-assembled systems. Recently, self-assembled systems find best bonding properties between smaller atoms and molecules.

6.4.7

Self-assembly at the macroscopic scale

Mostly, self-assembled systems can be seen macroscopically in which the self-assembly process is followed. These self-assembled macroscopic parts are usually automotive. In the 1950s, some self-assembled systems, which are in centimeters, were developed by some scientists. These self-assembled systems are actively used in passive mechanical devices and robots. At this stage, self-assembled systems, their components, sizes, designs, etc., may be controlled accurately.

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6.5

167

Focused ion beam

Modern scientific and engineering research has entered into an era of innovative technology, which has far reaching effects in the field of imaging, fabrication, and many applications at the nanoscale. To achieve such high-level research and industrial progress, the development of nanoscience and nanotechnology is an essential requirement. This technology has led us to understand the smallest scale of materials, their structure, and their composition etc., through imaging. Furthermore, fabrication at the micro- and nanoscale has become an interesting tool to study material science deeply. To meet all these requirements and future demands, the focused ion beam (FIB) tool has its own worth in nanoscale studies. One can make a direct deposition on a material surface or can remove a material from a surface [5]. Technology was not as advanced as it is today when the FIB technique was developed. Levi-Setti was the developer of this technique in 1975, which was based on field emission technology [6]. Later on, Orloff and Swanson modified it using gas field ionization sources [7]. The FIB tool is almost analogous to the scanning electron microscope (SEM). Krohn developed this technique when working on liquid-metal ion source (LMIS) in the exact same year. The FIB tool furnished semiconductor research techniques by its commercialization in 1980 [8]. In the FIB technology, ions of a high energy are directed toward a sample surface for imaging, milling, etching, or the deposition of a material [9]. Nowadays, gallium ion (Ga1) ions from LMIS are used, which are then focused by an electric field and can be scanned on a metal surface. When ions pass through a sample surface, they collide with atoms there that can be elastic or inelastic. Elastic collision results in sputtering or milling, while inelastic collision results in ion transfer, which results in secondary electron and ion emission. The main uses of FIB are in the sputtering process, material deposition, ion implantation, and imaging techniques. One can make important modifications using the FIB technique as it has many unique properties, like making of probe hole, cutting of 3D cross section very precisely. The FIB tool is particularly used in materials science, semiconductors, and largely in the biology of material ablation, site-specific analysis, and deposition. The scientific setup of a FIB resembles the setup of an SEM. The FIB technology uses a focused ion beam, making it different from SEM, which utilizes a focused electron beam. Furthermore, the beauty of FIB is that both ion and electron beam columns can be incorporated, which make it useful for dual study with any of the beams.

6.5.1

The construction of focused ion beam

The components of the FIB system include a vacuum chamber, an ion column, an LMIS, a sample stage, a gas delivery system, detectors, and a

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FIGURE 6.9 A schematic of the FIB system. Reprinted by permission from Springer Nature: F.A. Stevie, L.A. Giannuzzi, B.I. Prenitzer, The focused ion beam instrument, in: Introduction to Focused Ion Beams, Springer, 2005, pp. 1 12, COPYRIGHT (2005).

computer. The FIB can be used individually, but it is also implanted with other instruments like SEM, Auger electron microscopy, and TEM. When FIB is operated individually then its ion column is vertically adjusted with respect to the sample while in dual beam systems (FIB/SEM) vertical column is of SEM and FIB column is adjusted at some angle with respect to vertical ion column. A schematic of the FIB system has been presented in Fig. 6.9 [10].

6.5.1.1 The vacuum system To avoid unwanted losses, a vacuum system should be attached. The most common FIB systems have a vacuum system, which is categorized into three parts. A vacuum of about 1 3 1028 Torr is maintained to prevent against impurities and electrical discharges due to the high voltage used. The sample chamber vacuum is about 1 3 1026 Torr and when this pressure is increased up to 1 3 1024 Torr, then ion interactions with gas atoms are detected clearly because of a decrease in the mean free path of ions. 6.5.1.2 The liquid metal ion source The most common instruments are utilizing sources like gallium ions, which are characterized as LMIS. In such sources, the heated gallium metal wets a tungsten needle due to their physical contact and causes the formation of a

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FIGURE 6.10 LMIS schematic illustrating tungsten needle in gallium reservoir. Reprinted by permission from Springer Nature: F.A. Stevie, L.A. Giannuzzi, B.I. Prenitzer, The focused ion beam instrument, in: Introduction to Focused Ion Beams, Springer, 2005, pp. 1 12, COPYRIGHT (2005).

Taylor cone (cusp-shaped tip) due to an electric field and surface tension from the gallium. The diameter of the tip of this cone is extremely small (B4 nm). The field emission and ionization of the gallium atoms are caused due to the large electric field (higher than 108 V/cm) at this small tip. The milling or sputtering at a small scale is made possible by LMIS, which can provide ions of about 5 nm in diameter. A schematic of LMIS is presented in Fig. 6.10. Commercially, gallium is used nowadays as it has a low melting point (29.8 C), which reduces the interaction between the liquid and the tungsten needle attached to the reservoir that holds the ion source, its low volatility at melting point, which results in a long source life, and its viscous behavior due to its low surface free energy and low vapor pressure [11]. This ion source can produce Ga1 in two ways. First, when Ga is heated it is molten and flows toward the small-sized tip of the tungsten needle, which has a radius of about B2 5 µm. Then an electric field of 108 V/cm is applied at the wetted tip of the tungsten needle, which creates a point source with a radius of 2 5 nm in diameter having Taylor cone-like shape. This shape develops due to the electrostatic force of the applied electric field, which balances the surface tension force. Second, the galium Ga ions are pulled out from the tungsten tip and are ionized with the density of B1 3 108 A/cm2. A low voltage on the order of 20 µA/kV is enough to create the Tayler cone shape. As a high energy can spread the beam, a low

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value of current is needed, which reduces the beam energy. With the passage of time, the applied suppressor voltage at the source must be increased to maintain the current beam because the work output of the source decreases with time. Also, when the work output is seeming zero, then the source should again be heated, but it must only happen when it is necessary. This enhances the lifetime of the source.

6.5.1.3 The ion column A voltage in the range of 5 50 keV is applied to accelerate the emitted ions of Ga1. A schematic of the ion beam column is shown in Fig. 6.11. There are two types of lenses, namely objective and condenser lenses. The condenser lens is used to generate a probe and the main purpose of the objective lens is to focus the ions on the sample surface. The range and probe size are defined by apertures having different diameters. The aperture can be adjusted manually as well as automatically. The unwanted decomposition of the sample is controlled by a beam blanker in the middle of the ion column. Beam deflection, alignment, and stigmation correction is made possible using cylindrical octopole lenses. The energy spread by the ion beam is much larger than that of the electron beam because ions are larger than

FIGURE 6.11 The focused ion beam column. Reprinted by permission from Springer Nature: F.A. Stevie, L.A. Giannuzzi, B.I. Prenitzer, The focused ion beam instrument, in: Introduction to Focused Ion Beams, Springer, 2005, pp. 1 12, COPYRIGHT (2005).

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electrons. Therefore the main defect that is produced is chromatic aberration in the FIB system.

6.5.1.4 The sample stage Five axis movements, which are X, Y, Z, rotation, and tilt, are provided by the sample stage of the automatic system. The size of the stage should be such that it can handle 300 mm slices. To avoid the repeated alignment of the sample stage, modern sample stages have the capability of eccentric motion. Thermal stability during the processing of a sample like milling or deposition is required. 6.5.1.5 The imaging detectors A lot of particles are generated when the ion beam falls on the sample surface, therefore, to collect electrons for the purpose of imaging from that surface, two detectors are used. One is a multichannel plate and the other is an electron multiplier. The electron multiplier is placed on one side of the ion column, while the multichannel plate is framed over the sample. The incident beam and electron multiplier have an angle of 45 . The main purpose of the electron multiplier is to detect the secondary electrons or positive ions from the sample surface when the ion beam is allowed to fall on the metal surface. As sputtering is the main effect when the FIB imaging technique is used, a low value of current (,100 pA) is usually used. This reduces material removal from the sample surface by sputtering. Images formed by secondary electrons are finer. Also, the penetration depth of ions is different for different materials. This limits the use of the FIB system. For example, 30 50 keV Ga1 are limited to few tens of nanometers [12]. 6.5.1.6 Gas source usage or deposition Whenever a particular point is required for deposition, then gas delivery systems are used along with the ion beam. The gas entering into the chamber can be controlled from a valve placed at 100 µm above the sample surface. Much of these gas molecules are absorbed by the gas inlet, but decomposition occurs only when the ion beam strikes the sample surface. This process of decomposition repeats itself to produce the desired results in a specific region of the sample. For better material deposition, the precursor must possess a strong sticking capability and rapid decomposition as compared to sputtering. Different deposited materials have different results at the surface of the sample. 6.5.1.7 Dual platform The capabilities of this technique are enhanced by dual beam usage. The ion beam column and an electron column are both used at the same time for imaging without the sputtering of the sample surface. This creates a highly

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precise milling. Low energy deposition is made possible by the use of electron beam deposition. The dual platform also provides 3D information as well as that which can be obtained by SEM.

6.5.2

Principle

The operation of the FIB system is similar to that of SEM and differs only in the type of beam used, which is also clear from the name of the FIB. The beam current required for the operation of FIB varies as per the application. Lower and higher beam currents are used for the imaging and sputtering/ milling applications respectively. As clearly presented in Fig. 6.12, by hitting the sample surface, the Ga 1 primary ion beam sputters a small amount of material, which leaves the surface as either neutral atoms (n0) or secondary ions (i 1 or i 2 ). An image can be formed by collecting the signals generated from the sputtered ions.

6.5.3

Applications of FIB

Due to the striking of the Ga1 ions (highly energized) on the sample surface, the surface atoms will sputter. The implantation of a few nanometers of Ga 1 atoms on the surface will make it amorphous. Due to this sputtering ability, the FIB tool is used as a machining tool at the micro- and nanolevel, which is helpful in the modification of surfaces at this small scale. This activity has gained importance and become regular at the microlevel, but it is still in the developing phase for nanolevel modifications. Generally, 2.5 6 nm is the smallest beam size for imaging.

FIGURE 6.12 The operation of FIB. Adapted with permission under the terms of the CC0 1.0 (https://creativecommons.org/publicdomain/zero/1.0/).

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The purpose of the FIB tool is divided into three categories, namely etching, deposition, and imaging, depending upon the sample surface. 1. Etching occurs by sputtering, which is due to the momentum transfer from the incident ion beam to the sample surface. As a result, a sputtered particle with an energy above the binding energy is obtained. By collecting the sputtered electrons and ions, an image is formed. Sometimes back sputtering occurs, which causes surface degradation and should be highlighted to control the quality of etching. 2. Material deposition is a major commercial use of the FIB technique for the deposition of conductors and insulators for integrated circuit (IC) editing, mask layering for the preparation of the sample surface during cross-section analyses and transmission electron microscopy, and a lot of other applications. A precursor must have two properties. Its sticking ability should be great, and second, its decomposition must occur faster than it is sputtered away by the ion beam. 3. Imaging is the third main application of the FIB tool, which is much like scanning electron microscopy. The only difference is that, in FIB, ions are used, while in SEM electrons are used. The main parts of a general FIB system have been discussed in detail, which are LMIS, sample stage, computer system, gas source, and detectors. Different modifications can be done to enhance the capabilities of a FIB system like chemical vapor deposition (CVD) deposition or enhanced etching. Imaging is one of the major applications of the FIB technique nowadays. A multibeam focused ion beam system has also been developed along with the single beam system. FIB combined with SEM imparts many other advantages as well.

6.6

Stamp technology stamping

The use of stamped parts is not new, as in the 1890s, they were used for mass-produced bicycles. In order to reduce the cost, die forging and machining were replaced by stamping, which was not as strong, but could produce a reasonable quality. In 1890, the United States imported stamped bicycle parts from Germany. Stamping, which is also known as pressing, is the practice of placing a flat sheet of metal, in either blank or coil form, into a stamping press where it is formed into a net shape by a tool and die surface. It comprises several processes for the formation of sheet-metal such as stamping press (punching), embossing, blanking, coining, flanging, and bending.

6.6.1

Operations

1. Bending—The bending or deformation of the material along a straight line. 2. Flanging—The curved line bending of the material.

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3. Embossing—The shallow depression stretching of the material. 4. Blanking—The formation of a blank (a piece cut out of a sheet of the material) for further processing. 5. Coining—Squeezing or compressing a pattern into the material. 6. Drawing—The formation of an alternate shape by stretching the surface area of the blank. 7. Stretching—The use of tension for the incrementation of the surface area of the blank. It may be used for the creation of autobody parts. 8. Ironing—The reduction and squeezing in the thickness material along a vertical wall which results in the reduction of the diameter of the open end of the tube/vessel. 9. Reducing/Necking—The reduction in the diameter of the tube or a vessels. 10. Curling—Formation of tubular profile by the deformation of the material, for example, door hinges. 11. Hemming—For the addition of thickness, the edge is folded over onto itself. 12. Piercing and cutting—All the listed methods can be performed over a strip by placing several dies in a row and passing the strip over several steps one by one with time.

6.6.2

Stamping lubricant

Lubrication is required for the protection of the surface of the die from galling or scratching and to overcome the friction generation through the tribology process. The other advantage of a lubricant includes the protection of the metal sheet from wrinkles and rips.

6.6.3

Industrial applications

Due to its unique application in metal working, stamping has gained a lot of attention and tremendous attraction in several industries around the globe. In a few industries, the thermal or electrical conductivity of beryllium copper is necessary for applications in defense or aerosols. Furthermore, the automotive industry demands the high strength application of steel and its many alloys. Industries in which metal stamping is used include agriculture, aerospace, appliances, ammunition, construction, automotive, commercial, electronics, lighting, heating, ventilation, and air conditioning (HVAC) lawn care and equipment, marine, lock hardware, medical, power storage, plumbing, small engines, and power tools. This technology can be used to make fluidic devices with liquid channels in nanoscale dimensions.

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References [1] S.E. Jorgensen, Introduction to Systems Ecology, CRC Press, 2012. [2] M. Leester-Scha¨del, et al., Fabrication of microfluidic devices, Microsystems for Pharmatechnology, Springer, 2016, pp. 23 57. [3] Basics of Molecular Beam Epitaxy (MBE) Fernando Rinaldi, A.R., Optoelectronics Department, University of Ulm. [4] Y. Masuda, et al., Self-assembly patterning of colloidal crystals constructed from opal structure or NaCl structure, Langmuir 20 (13) (2004) 5588 5592. [5] N. Yao, Focused Ion Beam Systems: Basics and Applications, Cambridge University Press, 2007. [6] W.H. Escovitz, T.R. Fox, R. Levi-Setti, Scanning transmission ion microscope with a field ion source, Proc. Natl Acad. Sci. U S Am. 72 (5) (1975) 1826 1828. [7] J. Orloff, L. Swanson, Study of a field-ionization source for microprobe applications, J. Vac. Sci. Technol. 12 (6) (1975) 1209 1213. [8] J. Orloff, et al., High resolution focused ion beams: FIB and its applications, Phys. Today 57 (1) (2004) 54 55. [9] N. Yao, Z.L. Wang, Handbook of Microscopy for Nanotechnology, Springer, 2005. [10] F. Stevie, L. Giannuzzi, B. Prenitzer, The focused ion beam instrument, Introduction to Focused Ion Beams, Springer, 2005, pp. 1 12. [11] L.A. Giannuzzi, et al., FIB lift-out specimen preparation techniques, Introduction to Focused Ion Beams, Springer, 2005, pp. 201 228. [12] J. Orloff, Handbook of Charged Particle Optics, CRC press, 2008.

Chapter 7

Electrons in nanostructures Tahir Iqbal Awan, Almas Bashir, Aqsa Tehseen and Saliha Bibi

Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan

Chapter Outline 7.1 Introduction to electrons 7.1.1 Importance of electrons in bonding 7.2 Emission of electrons 7.2.1 Thermionic emission 7.2.2 Field emission 7.2.3 Photoelectric emission 7.2.4 Secondary electron emission 7.3 Variations in electronic properties materials 7.3.1 Electrical properties 7.3.2 Optical properties 7.4 Electrons in nanostructures

7.1

179 180 181 182 183 184 184 of 185 185 186 187

7.4.1 Quantum effects of electrons in nanostructures 188 7.5 Free electron model 190 7.6 Bloch’s theorem 193 7.6.1 Implications of Bloch’s theorem 194 7.7 Band structure 194 7.7.1 Energetic bands 196 7.7.2 Band gaps 197 7.8 Single electron transistor 198 7.8.1 Operation of single electron transistor 199 7.8.2 Applications 200 7.9 Resonant tunneling 202 References 204

Introduction to electrons

Electrons are the constituent particles that make up an atom and, thus, these are known as subatomic particles. Other than electrons, protons and neutrons are also subatomic particles. Electrons are also called elementary particles because these particles have no further constituents in them. [1,2]. Electrons have a negative charge (1e 5 1.602 3 10219 C, first numerically calculated by R. A. Millikan in 1909) and a mass of about 9.109 3 10231 kg. Electrons are basically beta particles and that’s why they are sometimes represented by β2 or e2. In comparison to other subatomic particles, electrons are the lightest in mass (1/1836 mass of a proton) [3
7]. Quantum mechanically, electrons have a half-integral spin momentum of 21/2 ћ. Since an electron is classified as a fermion, two electrons cannot occupy the same state with the Chemistry of Nanomaterials. DOI: https://doi.org/10.1016/B978-0-12-818908-5.00007-X © 2020 Elsevier Inc. All rights reserved.

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same spin as stated by the Pauli exclusion principle [8]. Electrons also exhibit wave
particle duality, that is, they behave both as a wave and as a particle. They collide like a particle and diffract like a wave. By the de h Broglie wavelength (λ 5 mv ), electrons have a long wavelength as compared to protons and neutrons due to their light mass and, thus, their wave-like properties are easy to examine (like in the Davisson
Germer experiment). The antiparticles of electrons are positrons that have a mass exactly equal to the mass of electrons and an equal but opposite charge. The electron was discovered by J. J. Thomson in 1897, and the positron was predicted by Dirac in 1928; later on discovered by Carl Anderson in cosmic rays in 1932. Cosmic rays are the radiation that comes toward the Earth from its outer sphere like from galaxies, stars, etc. [9]. Electrons play an important role in many naturally occurring physical phenomena like electricity and magnetism, thermal and electrical conductivities, and electromagnetic and weak nuclear interactions [10]. It is worth mentioning that an electron at rest produces an electric field in its vicinity, and the same electron in linear motion gives rise to a spiral magnetic field. According to Lorentz force, ‘‘F 5 qðE 1 v 3 BÞ; ’’ electrons are influenced by the electromagnetic fields produced by other sources in their surroundings. Using electromagnetic fields (toroidal or poloidal magnetic fields) like in Tokamak, different laboratory instruments can be utilized to trap a single electron or a cascade of electrons. Many of the investigation and characterization techniques available are possible and successful only due to the practical involvements of electrons like in cathode ray oscilloscopes, electron microscopes, accelerators, and gaseous ionization detectors, etc. [11].

7.1.1

Importance of electrons in bonding

The interaction of an electron with other electrons and subatomic particles is of huge interest in the fields of physics and chemistry. As an example, the existence and composition of an atom is due to the Coulomb’s electrostatic attraction between electrons and positive nuclei. Chemical bonding, which is defined as the mutual sharing or exchange of electrons between two or more atoms is one of the most important types of bonding [12]. Only a few of atoms can exist independently as the majority of them reside as a combination due to the chemical bonding between two or more atoms. The nature of bonding between two similar or different atoms can affect the properties of a material substance. Atoms form bonds either due to the sharing or the exchange of electrons, and in this way, they attain a stable configuration of electrons in their valence shells. A bond or a chemical bond is usually defined as a force that holds two or more atoms together so that they are present as a unit. Some bonds are hard to break while others can be easily broken. The energy required to make or break a bond is called the bond energy. The electrostatic attraction between

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a cation (i.e., a metal ion) and an anion (a nonmetallic ion) results in the formation of ionic bonds. Thus an ionic compound is formed when a metallic atom reacts with a nonmetallic one. During this reaction, one or two electrons are exchanged or shared from a metal to a nonmetal. In this type of bonding, there is a large electronegativity difference between the reactants. Another type of bonding is covalent bonding, in which electron pairs of nonmetallic atoms are shared. One of both of the atoms may contribute in this mutual sharing. In such a type of bonding, atoms have an electronegativity difference equal to zero. The shared pairs of electrons are considered as localized because this state works as a metastable state for them. Covalent bonding is further classified into two types, which are nonpolar and polar. When two identical nonmetallic atoms share their electrons equally, nonpolar covalent bonds come into being. As an example, the bond formed between hydrogen and carbon is nonpolar. In this bonding, the electronegativity of both atoms is almost identical. Other examples include O2, Cl2, N2, etc. In contrast, polar bonds result from the sharing of electron pairs between two atoms unequally. The shared pairs of electrons get shifted from the core, making one end of the molecule positive and the other end negative. It is important to mention here that polar molecules are overall neutral. As a result of unequal sharing, partial charges are formed that are not the same as ionic charges, which are caused by the actual transfer of electrons between atoms [13,14].

7.2

Emission of electrons

The process of the giving out of electrons from the surface of a metal is called as the emission phenomenon. When a small piece of metal is positioned at room temperature, then free electrons perform random motions in it due to thermal energy. Inside an atom, there exists a core that is positively charged and known as the nucleus, which is surrounded by negatively charged electrons. Sometimes the force of attraction or the bonding between these oppositely charged entities is loose and a little push or tap can remove the negatively charged electrons from their orbits. The emitted electrons are termed as the free electrons inside that metal. One may ask the question, why don’t these electrons escape from the metal if they are free? The answer is that as a whole, a piece of metal is neutral, but positive nuclei collectively attract electrons. Those electrons that reach the surface of the metal are attracted by the potential of these positively charged nuclei and this prevents them from escaping. Due to this phenomenon, a barrier known as the surface barrier forms near the surface of the metal. Thus free electrons are “free,” but only inside the metal [15,16]. The electrical conductivity of different metals is a characteristic that proves that some electrons are not bound, but are able to move freely inside the metal. A characteristic binding energy also known as the work function

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still binds the electrons to its atomic lattice. The binding energy is defined as the energy required to take an electron off the surface of a metal and it varies from metal to metal [17,18]. The work function, for some pure metals, is in the range of 2
6 eV and this value depends on three basic factors including the purity of the metal, its nature, and the condition of the surface. It is always desirable to have a metal with a low work function due to which a lesser amount of energy could liberate a large number of electrons. Electron knockout from a metallic surface is only possible when a sufficient amount of surplus energy is provided by some kind of external source. In order to supply this external energy, a number of methods including thermal processes, storing energy in an electric field, using the kinetic energy of charges, or light energy can be used. Depending upon the nature of the source, there are four principle ways for achieving the emission of electrons from the metal surface, namely (1) hot emission, (2) cold emission, (3) field emission, and (4) secondary emission.

7.2.1

Thermionic emission

Thermionic emission is the process by which the electron emission from the surface of a metal takes place by providing thermal energy to the metal surface. This process involves the heating of the metal to a high temperature of about 2500 C, resulting in the release of electrons from the surface of the metal. A schematic of unheated and heated metal is shown in Fig. 7.1, in which electrons are bound within the surfaces of the unheated metal, but in heated metal, electrons can escape out of the metal surface by gaining heat energy. The number of charged particles emitted in this case is totally temperature dependent, that is, there will be a greater number of emitted electrons with a higher temperature. A vacuum tube is required for this kind of electron emission [19]. When the temperature is low, the electrons do not have enough energy to come off of the metal surface. By heating such a surface, a certain amount of thermal energy changes to kinetic energy and leads to accelerated electron motion. When there is enough of a temperature increase, an energy equal to that of the work function becomes acquired by the electrons. Resultantly, the electrons cross the surface barrier and come off the metal surface. Metals having a smaller work function need less energy, thus, resulting in electron emission at lower temperatures. Such materials include tungsten, metallic oxides of barium and strontium, and thoriated tungsten. It should be kept in mind here that thermionic emission takes place at high temperatures. As an example, for the emission of electrons, pure tungsten is required to be heated to a temperature of about 2300 C, whereas in the case of oxide-coated emitters, the required temperature is only 750 C for thermionic emission.

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Electron

Unheated metal Electron

Heated metal Heat FIGURE 7.1 Electrons in the case of (A) unheated metal and (B) heated metal.

7.2.1.1 Dependence of thermionic emission There are many factors that affect the thermionic emission of electrons, for example, the heat supplied to the metal and its surface area, nature, and particular work function. The number of thermions that escape during the emission process directly depends on the applied heat to the metal surface. Below a certain temperature and work function called as the threshold temperature and the threshold work function, no electrons escape from the metal. Another important factor is the emitter temperature. The greater the emitter temperature, the greater the amount of thermionic emission will be. According to the Richardson-Dushman equation as depicted in Eq. (7.1) [20
22], the emission current density (Js) measured in ampere per square meter (Am22) is given by: 211600φ T

Js 5 AT 2 e

ð7:1Þ

Here A is a constant measure in Am22 K22, T is known as the absolute temperature, which is measured in K, and φ is the work function of a given metal. This equation clearly depicts the effect of temperature on electron emission. By doubling the temperature of the emitter, the emission of electrons is enhanced up to 107 times. As an example, the emission of electrons from a small piece of pure tungsten at 1300 C is about 1026 Acm22, but this is increased to a remarkable value of 100 Acm22 at 2900 C [23].

7.2.2

Field emission

Field emission is a method for electron emission using a strong electric field near to a metal surface. When a high positively-charged metal plate is set close to the metal surface, the positive charges inside that conductor will pull the free electrons out of the metal surface. If the positive potential on the conductor is strong enough, it will overcome the restrictive forces and free electrons will be liberated from the surface of the metal as shown in Fig. 7.2.

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Electrons

+++++++++++++++++++++++++ +++++++++++++++++++++++++ Electric field

Metal FIGURE 7.2 The emission of electrons by a strong positively-charged plate.

The field and potential applied on the conductor are interconnected through the equation V 5
Ed. To produce field emission, a strong field of a few megavolts/centimeter distance between the conductor and the metal surface is necessary. In contrast to thermionic emission, field emission can take place at low temperatures, even at room temperature, and that is why it is often called auto-electronic emission or cold-cathode emission [15]. The field emission process is used in field emission microscopy.

7.2.3

Photoelectric emission

This is a method in which electron emission occurs due to the application of light on a metal surface. When a filtered light beam falls on the surface of a particular metal, usually sodium, potassium, cesium, etc., the momentum and the energy of the photons are shifted to the free electrons inside the metal. In the case of high energy photons fall on metal surface, electrons will come out from metal surface as shown in Fig. 7.3. The electrons emitted in this process are known as photoelectrons and this phenomenon is known as a photoelectric effect. The incident photon has energy as E 5 hf where “h” is planks constant, which is equal to 6:63 3 10234 Js. The amount of electrons emitted in this process depends mainly on the intensity of light falling. The greater the intensity, the greater the electron emission will be. It is used in phototubes of sound films and television.

7.2.4

Secondary electron emission

Secondary electron emission is a method of electron emission from the surface of a metal. When primary electrons or some ionizing radiation is injected into a metal, secondary electrons come off its surface. These are sputtered due to the high kinetic energy of incident particles. This energy is shifted to free electrons present in the metal. When the energy becomes sufficient, these electrons can escape from the attractive potential of the nuclei.

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Anode Light

+++++++++++++++++++++++++ +++++++++++++++++++++++++ Electric field

Electrons

Metal

FIGURE 7.3 Photoelectric emission of electrons.

The incident electrons on the metal are called primary electrons and the emitted electrons from the surface are called secondary electrons. The number of incident electrons of high kinetic energy directly effects the number of secondary electrons.

7.3

Variations in electronic properties of materials

Electron confinement in nanostructures leads to the tremendous variations in the properties of these materials. Nanostructures are much smaller as compared to the distance between the scattering events of electrons [24]. The electronic configuration of materials changes as a result of confinement. For any material, the spacing between energy levels changes with the variation in temperature and there will exist a particular size below which a considerable difference in the electrical and optical properties of material is observed.

7.3.1

Electrical properties

Electrical conductivity is an important parameter for describing the electrical properties of nanomaterials. The effect of size reduction on these properties is based on different mechanisms. These mechanisms are generally assembled into categories given as the scattering at the surface comprises of scattering between electrons at the grain boundaries, Coulomb interactions, enhancement of bandgap, and increment in the nanostructures and tunneling [25]. Moreover, improved perfection by minimizing the dislocations, defects, and impurities in the structures is capable of influencing the electrical conductivity of nanostructures. In metals, the conduction phenomena can be described in terms of various scatterings of electrons. The total resistivity, ρT , of a metal involves contribution due to individual and independent scattering called Mathessiens’s rule given as: ρT 5 ρTh 1 ρD

ð7:2Þ

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Where ρTh represents the thermal resistivity and ρD represents the resistivity due to defects. When the electrons collide with the vibrating atoms give rise to thermal fluctuations or phonons, due to dislocation produce relative to their mean position. The presence of impurities and defects gives rise to disturbances in the lattice periodic potential and results in electron scatterings. Consider electrical resistivity due to individual scattering relative to mean free path (λ), now the Mathessiens’s rule is given as: 1 1 1 5 1 λT λTh λD

ð7:3Þ

Where the range of λT lies between several tens and hundreds of nanometers. The impact of size reduction on the electrical resistivity would involve an enhancement in crystal perfection, which would lead to a reduction in defect scattering. But a lesser contribution is given by defect scattering to the total electrical resistivity. Due to the surface scatterings add extra contribution into the total resistivity relation. This will play an important role in the measurement of whole electrical resistivity for a nanometer-sized particle. If the mean free path (λS ) is considered to be at its smallest, this results in the domination of the total resistivity. 1 1 1 1 5 1 1 λT λTh λD λS

ð7:4Þ

When nanowires or thin films are taken under consideration, the decrement in electrical conductivity takes place due to the scatterings of electrons at the surface. Due to their size reduction in comparison to mean free path, an influence on electron motion is found due to the surface collisions that leads to inelastic or elastic scattering. If the collision is elastic, the reflection of electron occurs with no loss of velocity or momentum. This results in there being no change in the electrical conductivity. But for inelastic scattering, the mean free path for an electron will be dismissed while the collisions occur and it gets scattered randomly after collision. Subsequently, due to the loss of energy and momentum, the electrical conduction decreases [26].

7.3.2

Optical properties

The influence of size reductions on optical properties is dominant for nanoparticles having sizes below 10
15 nm, that is, considerably smaller compared to the wavelength (λ) of light. For semiconductors, the energy that is associated with interatomic interactions is large. This results in the formation of electron
hole pairs that are not strongly bonded in the semiconductor crystal called Mott
Vanie excitons. The delocalization region related with these excitons is larger in comparison to the lattice constant of semiconductors [27]. When the semiconductor crystal and exciton size are such that they are comparable, a significant difference in the optical properties of particles

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is observed. Due to the reduction in particle size, the declination of the absorption band of semiconductors toward the low wavelength region, that is, blue shift, occurs. The factors with which the blue shift varies include shape, aspect ratio, and size distribution of nanoparticles. For example, the blue shift for Cadmium sulfide (CdS) semiconductor is found for particles with sizes less than 10
12 nm. The optical behavior varies with smaller dimensions and can be described by the surface plasmon resonance for metals and advancement in distance between the energy levels for semiconductors. When the interaction of electromagnetic light and metal surface takes place, the resonant oscillations of the electrons in the metal occurs nearby the boundary in between the two mediums. For this to happen, the nanoparticle size should be smaller compared to the corresponding light wavelength. The electric field component of the incoming light ray induces an electron polarization relative to fixed positively-charged ions. This leads to net charge formation that occurs at the boundary in between the two mediums, acting as a restoring force [28]. Inphase oscillations of conduction electrons occur at a particular wavelength. The dependence of energy is associated with surface plasmon resonance (SPR) which includes: 1. The density of free electrons 2. The medium that is used as a dielectric near the nanoparticle With time, the extent of resonance differs before the scattering of electrons. It was found that in the case of metals, the resonant frequency lies in the visible light range and it becomes sharper for larger nanoparticles owing to the increment in scattering length. According to Mie, when larger-sized particles are taken, then the higher order modes become more significant. This is the reason that the polarization of nanoparticles by light cannot occur consistently. The resonant peaks shown by these higher order modes are in a higher wavelength range. Hence the shifting of the plasmon band occurs with an increment in the size of the particle. In the case of nanorods, the bandwidth and the position of the resonance peak are both delicate to their aspect ratio and they show two unique bands relative to the oscillations of electrons across that is, the transverse mode and in the axis direction, that is, the longitudinal mode. For such nanoparticles, the blue band occurs owing to Transverse Magnetic (TM) plasmon resonance and the red band owing to LM plasmon resonance. Moreover, the shifting of the red band occurs with an increase in nanorod length or aspect ratio [29].

7.4

Electrons in nanostructures

In bulk materials, the electronic properties are described in terms of the scattering of electrons, which behaves as a frictional force. The electrons move

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with drift velocity in such a way that the force which is provided by an electric field given as: E5

V ; F 5 qE d

ð7:5Þ

This force becomes equivalent to the friction. As the current directly relates with the drift velocity of electrons, this verifies the direct relation of current and voltage for most conductors, that is Ohm’s law is confirmed. When the size of the particle is reduced, then scattering events that lead to resistance arise, in which the mean free path is in the nanometer scale at appropriate temperatures in most metals. Therefore when the scale of the mean free path related to an electron becomes similar to the structure size, Ohm’s law may not be verified [30]. Electron transportation at nanometer scale can be completely explained in terms of quantum effects.

7.4.1

Quantum effects of electrons in nanostructures

The most considerable effect of particle size reduction to the nanometer range is the emergence of quantum effects owing to the confinement of electron motion, and these start dominating below a certain particle size in nanometers. Here, the term, “confinement” refers to the restriction of randomly moving electrons to specific discrete energy levels and the word “quantum” describes the properties of particles at the atomic level. Quantum confinement effects appear when the dimensions of a particle become extremely small in comparison with the electron wavelength. This leads to an enhancement in the bandgap as well as the energy of these materials [31]. The main difference between metals and semiconductors associated with quantum confinement is the variation in their electronic structures. In semiconductors, bandgap plays a significant role as compared to metals. As in the case of metals, electrons are free to move between the valence and partially-filled conduction bands even at low temperatures. A bandgap change owing to a decrease in dimensions is depicted in Fig. 7.4. By increasing the particle size, the band center develops first and then the edge. In the case of metals, there is no band gap. Fermi energy levels occur at the band center. Moreover, the spacing in between the energy levels is minor, reflecting that the properties of even a small particle are analogous to the bulk properties [28]. If small metal nanoparticles are considered, an appreciable bandgap can be obtained and they may behave as insulators or semiconductors when the bandgap energy becomes equal to or greater than the thermal energy (kBT). For example, gold is recognized owing to its unique properties such as its invariant yellow color, metallic nature with the crystal structure of a facecentered lattice, and a melting temperature of 1336 K. Nevertheless, when the same gold metal is reduced in size to less than 10 nm, it behaves differently by absorbing green light and so seems to be red in color.

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FIGURE 7.4 Change in the bandgap with a size variation. Republished with permission of Royal Society of Chemistry (Great Britain), from E. Roduner, Size matters: why nanomaterials are different. Chem. Soc. Rev. 35 (2006); permission conveyed through Copyright Clearance Center, Inc.

Metallic nanoparticles with sizes less than 2 nm can act as insulators at low temperatures as well as exceptional catalysts [32]. A significant threshold, which is known as the Kubo gap (δ), appears when the bandgap between the energy levels of the highest occupied level and the lowest unoccupied level matches with the thermal energy resulting in electron excitation through the δ. For metals, the spacing (δ) has a dependence on the Fermi energy of the metal, that is, EF as well as on the electrons number (N) given by: δ5

4EF 3N

ð7:6Þ

The Fermi energy is mostly 5 eV in the case of metals. An insulator at low temperatures can become a semiconductor with increasing temperatures when the Kubo gap becomes greater than the thermal energy, that is, δ . KBT. Therefore it is anticipated that the quantum confinement effects become considerable at lesser sizes for metals and at larger sizes for semiconductors. The ability to control the dimensions and the

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configurations of materials makes it valid to make amendments to the properties of materials according to the requirements.

7.5

Free electron model

The free electron model is considered to be the simplest model for interpreting the behavior of valence electrons in the case of metallic solids. In this model, valence electrons turn into conduction electrons and are capable of moving freely throughout the solid. This model views conduction electrons as a gas comprising of free and noninteracting elements with fixed positive ions in the crystal lattice [33]. It was developed by Arnold Sommerfeld who fused the classical Drude model with the Fermi
Dirac statistics, thus, called the Drude
Sommerfeld model. The long-range electrostatic interactions between electrons are ignored. The sole medium for interaction is the scattering of conduction electrons with other electrons in which energy is exchanged and electrons are not deflected via ion cores that are arranged on the periodic crystal lattice. This leads to the direct significance of ‘Pauli exclusion principle’. Thus in this case, the free electron Fermi gas leads to the free electron gas that is subjected to the Pauli exclusion principle [34]. The Fermi liquid model was proposed by Landau. It is known for effective description of electrodynamics of metals. Fig. 7.5 shows a plot depicting the average occupation number of fermions corresponding to a temperature of zero and temperatures for 0.1 μ, 0.5 μ, 1 μ where μ denotes chemical potential.

1

kBT = 0.1 m

0.8

0.6 kBT = 0.5 m 0.4

kBT = 1 m

0.2

0 –8

–6

–4

–2

0

2

4

6

8

FIGURE 7.5 Fermi liquid theory of a metal. Adapted with permission under the terms of the CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/).

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When the temperature is zero, T 5 0, the chemical potential becomes equal to the Fermi energy level showing that all the energy levels are filled and the energy at the Fermi level represents the energy of the state of being highest occupied [24]. When the temperature starts varying, the electrons lying below the Fermi level will move to the higher level leading to mobile particles, that is. quasiparticles. This occurs in the creation of electron
hole pairs, that is, a net positive and a net negative charge below the Fermi energy level and above the Fermi level respectively, and is considered as a reason for neglecting the Coulomb’s interactions. Moreover, this excitation process of carriers leads to the fact that if the energy of the quasiparticles that are generated closely relate with the Fermi energy, then they are known as stationary states of system. Owing to the fact that near the Fermi level, the number of existing states that can be occupied by an excited electron becomes negligibly small [35]. And if the difference of energy between the quasiparticles and the Fermi level is zero, then the validation of conservation laws will not be possible. Second, quasiparticles obey the Fermi
Pauli distribution in the case of an ideal gas. The energy of quasiparticles relative to Fermi energy is given as: E5

ћ2 K 2 2m

ð7:7Þ

Where for the electrons, the mass m is substituted by m , known as an effective mass. Now, 1. For the Drude model of free electron metals, assume ,τ . is the mean time among the collisions resulting in electron scattering and ,v. is the averaged drift velocity. Using Newton’s second law: m

,v. 5 2 eE ,τ .

ð7:8Þ

And Ohm’s law: J 5 σE

ð7:9Þ

Where σ is known as the electrical conductivity of a material and is a reciprocal of resistivity. The current density J is given as: J 5 ne , v .

ð7:10Þ

Where n refers to the density of electron. From Eqs. (7.8), (7.9), and (7.10) the result that can be implied is: σ5

1 ne2 , τ . 5 ρ m

ð7:11Þ

Which is the Drude model result for conductivity. 2. In the Sommerfeld model, electron is a quantum particle with a spin. When two electrons are placed (each with spin up as well as spin down)

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inside a particular dn energy state per unit wave vector. Then the number of electrons, N, inside volume, V, satisfy the relation: N523

ð kF 0

Vk2 dk k3F 5 V 2π2 3π2

ð7:12Þ

Where kF is the wave vector of the topmost filled energy state. By taking the ration of momentum and mass, it is possible to calculate the velocity: vF 5

ћkF m

ð7:13Þ

In the form of electron density, that is, n 5 N V, Eq. (7.12) can be given as: n5

k3F 3π2

ð7:14Þ

Therefore from the electron density, n, the important features related to free electron gas can be interpreted [34]. The free electron model was presented by Paul Drude in the 1900s in order to describe the transportation properties related to electrons in metals. It is considered as an application of kinetic theory for electrons in the case of solids. He assumes that materials are made up from positive fixed ions surrounded by a free electron gas having electron density ‘n’. The postulates of this model are given as: 1. The electrons in the valence band of metallic atoms move freely throughout the volume that is contained by a metal just as gas molecules do inside a container. These valence electrons are known to form a free electron gas. 2. These electrons that are known to be free, move randomly and start colliding with other electrons or with the fixed positive ions in a lattice. The collisions between the electrons are elastic, which leads to energy conservation. 3. The laws of classical kinetic theory associated with gases is obeyed by the movements of these free electrons. 4. The velocities with which these free electrons move conform to the classical Maxwell
Boltzmann law of velocities distribution. 5. As fixed positive ions are present, there exists a uniform potential being periodic under which the motion of free electrons happens. 6. By applying an electric field to a metal, an opposite direction movement of electrons (headed to positive terminal of source) in comparison to an applied electric field occurs. The average velocity acquired by an electron due to the applied electric field is called the “drift velocity” (vd); also, the average time taken by an electron to attain a steady state velocity from zero velocity is called the “relaxation time” (τ r).

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7. When there is no electric field present, then the covered distance between two consecutive collisions is known as the “mean free path” (λ) and the time taken by an electron between two successive collisions is known as the “collision time” (τ c). The average velocity of free electrons is called the “root mean square velocity,” (vrms) or thermal velocity of electrons. 8. In this model, it is assumed that the collision time (τ c) is equal to the relaxation time (τ r) and the thermal velocity (vrms) is equal to the drift velocity (vd).

7.6

Bloch’s theorem

Bloch’s theorem identifies the type of wave function that is accountable for characterizing the energy levels of electrons in a periodic crystal. This theorem makes it possible to determine the different properties related to crystalline solids comprising of a large number of atoms. It diminishes the difficulty of involving the whole solid by utilizing merely the atoms in the repeated unit of the solid. In a periodic lattice, the physical quantities must be periodic, that is, in the case of the lattice translation vector, R, any property that is explicated by means of a function, U(r), must satisfy:  U ðr Þ 5 Uðr 1 R ð7:15Þ Therefore it is required to generalize the condition given in Eq. (7.15) for including the chance of phase factor multiplication with the wave function. We can select any value until the square of ψ ψ is 1 which satisfies the wavefunction given as: Ψn:k ðr Þ 5 exp½ik:r Un:k ðrÞ

ð7:16Þ

Here, Un;k ðrÞ is the periodic function in the repeated lattice over the distance R, that is: Un:k ðrÞ 5 Un;k ðr 1 RÞ Which justifies the above constraint as: exp½ik:r  3 exp½ 2ik:r  5 1 Now by the replacement of r 1 R with r, the above equation becomes: ψn;k ðr 1 RÞ 5 exp½ik:Rψn;k ðrÞ

ð7:17Þ

The types of wave functions that are explained in Eqs. (7.16) and (7.17) are called Bloch states [24]. Where k and n in the index represent wave vector value and band to which the wave function is connected respectively.

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7.6.1

Implications of Bloch’s theorem

Bloch’s theorem includes wave vector, k, which is considered important for playing a significant role in the problems of motion related to periodic potential just as the wave vector in the case of free electrons in the Sommerfeld theory. The latter wave vector is equal to p/ћ with p being the momentum associated with an electron. However, the Bloch wave vector is known as the crystal momentum. It does not vary proportionally with the electronic momentum, rather it represents a mathematical epitome for the variation in the wave function phase in real crystal. When the crystal is infinite, the Bloch wave vector magnitude leads to zero (the infinite wavelength) and it becomes negligibly small for small wavelengths owing to the fact that physical quantities must be capable of having the same periodicity as that of the lattice. In the case of an infinite crystal, the wave vector k lies in between 0 and 2π/a, which is known as the first Brillouin zone. This is due to the fact there is an equal change in positive and negative wave vector and each time when wave vector k enhances by a factor of 2π/a, the repetition of the lattice happens. Thus it is feasible to explain the properties related to an infinite crystal on the basis of the properties associated with the repeated unit cells of the lattice along with an additional parameter, that is, the wave vector k [36].

7.7

Band structure

Consider an atomic wave function ψsðrÞ; by employing this wave function it is possible to make the trial Bloch function, that is, ΨT ðr Þ: For this to happen, the form of Bloch’s theorem given in Eq. (7.17) is manipulated as: X exp½iknaψS ðr 2 naÞ ð7:18Þ ψT 5 n

Here, ψS ðr 2 naÞ is denoted as a wave function having the same periodicity as that of the lattice and with n being an integer. In the case of perturbation theory, the energy difference among the crystal lattice (i.e., no interactions) and the atomic case (i.e., interactions only with the nearest neighbors) by Hamiltonian, ΔU op , in the first order can be given as: EK 2 ES 5 , ψT jΔU op jψT .

ð7:19Þ

In this equation, the Dirac notation is used and ψT is the wave function for crystal. If the Hamiltonian related matrix elements have nonzero values, then the resulting terms that arise by putting Eq. (7.18) into Eq. (7.19) are: EK 2 ES 5 , ψS jΔU op jψS . 1 , ψS jΔU op jψS ðr 2 aÞ . exp ika 1 , ψS jΔU op jψS ðr 1 aÞ . exp 2 ika

ð7:20Þ

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By defining the relations: ES 1 , ψS jΔU op jψS . 5 εo And: , ψS jΔU op jψS ðr 6 aÞ . 5 τ Eq. (7.20) becomes: EK 5 εo 1 2τcoska

ð7:21Þ

When τ , 0, the comparison of the above wave vector, which is dependent 2 2 on energy and the result of free electron energy, that is, Ek 5 ћ2mk , is depicted in Fig. 7.6. In the case of small wave vectors, the value of EðkÞ in the region that is close to the center of the Brillion zone is quadratic just as for the free electrons. Although, when the value ka reaches the value 6 π, a change in the curve is found along with dEdkðkÞ -0 and it has an important physical significance. The energy change with the wave vector leads to group velocity. For wave packet, the velocity of particle is defined by the group velocity. So, we have

FIGURE 7.6 Energy dependence on the wave vector in the case of free electrons (solid line) and for interacting electrons (dashed line). Republished with permission of McGraw-Hill Education, from D.A. Neamen, Semiconductor physics and devices: basic principles, 2011; permission conveyed through Copyright Clearance Center, Inc.

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PART | 2 Interactions in nanomaterials

vg 5

dω 1 dEðkÞ 5 dk ћ dk

ð7:22Þ

represents the group velocity of the electrons and it becomes zero when ka 5 6 π. The wavelength that directly links with this wave vector value is λ 5 2kπ 5 2a giving the condition for Bragg reflection. The collaboration of both forward and Bragg reflected waves gives rise to the existence of standing waves. There is no electron propagation at these wave vector values. As the energy, EðkÞ, starts flattening, an advancement in the density of states found close to ka 5 6 π. In one-dimension, there is a constant change in the number of states per unit wave vector. Thus this change per unit energy is required to enhance because the energy function, EðkÞ, becomes feebler close to the Bragg reflection point.

7.7.1

Energetic bands

Band structure is responsible for the explanation of the electrical properties of a material. The way in which the energy states are filled is the same as it was found for the free electron case. For energy function, E(k), the integral as given in Eq. (7.12) must range for E(k) instead of k over the energy levels that are allowed. Consider a solid consisting of a specified number of electrons that are considered to be noninteracting. It is possible to create the ground state of system by creating lowest energy states at the bottom. The particular set of points related to the Brillion zone with Ek 5 EF, is known as the Fermi surface [35]. The important results are explained as: 1. Fermi level in an allowed energy band: For materials, mostly metals, in which the half-filled energy bands exists, unoccupied energy states are present having energy closer to that of the Fermi level energy and the occupied states are having energy less than EF [37]. For the onedimensional case, for each unit cell there exists one valence electron resulting in a half-filled band as shown in Fig. 7.7A. This leads to the existence of a Fermi level inside an allowed band. By continuing to fill the energy states with electrons, at a certain point the energy band becomes filled completely as shown in Fig. 7.7B. 2. Fermi level on top of an allowed band: In the nearly free electron model, by assuming ΔU like the perturbed potential when 2ΔUckBT, there will be no energy states available inside an allowed band. Such a material is referred to as an insulator. In order to excite the electrons to enable them to jump into a higher band, a strong electric field is required that results in the conversion of insulators to conductors. 3. When 2U  kBT, electron excitation becomes possible. Such materials are called semiconductors. In these materials, when the temperature is

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197

(B)

(A)

Fermi energy level

– /

0

/

0

/

FIGURE 7.7 (A) Partially filled band for one-dimensional system, and (B) Completely filled band for one-dimensional system.

increased, the electrons in the filled valence band get enough energy and migrate to the unoccupied or empty conduction band [38]. This leads to a flow of current in these semiconductors.

7.7.2

Band gaps

In the band theory of solids, bandgaps are referred to as the forbidden energy levels, that is, the levels that are not occupied by electrons. For different materials, the bandgap varies such as it being wider for insulators, smaller for semiconductors, and negligible in the case of metals (i.e., the Fermi level lies at the center in between two bands) and it shows the energy difference in between the valence band top and the conduction band bottom [39]. A single atomic state, that is, ψs, is taken into account. If the range of atomic states were considered, then a large number of bands would arise and each band would refer to a single atomic state. In this way, it becomes easy to understand the presence of the bandgap in between the allowed levels. In the tight-binding model, it is difficult to interpret the bandgap size between the two allowed bands as it is a function of both τ and EN (energy for an nth-band) [37]. In the nearly free electron model, it is relatively easy to introduce bandgaps. In this case, when the free electrons are perturbed due to periodic potential U then as a result, a parabolic band originates at the values ka 5 6 nπ along with a 2U magnitude of bandgap. In both cases, it is possible for bands (each single band refers to a specific atomic energy level) to get folded back in the first Brillion zone. The space or gap in between the different bands responds to the range of values of the wave vector k being forbidden to periodic potential [40].

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7.8

Single electron transistor

Diminishment in dimensions with the time fallouts in scaling the sizes of existing electronic devices in such a way that their operations are dominated by quantum effects, resulting in the alteration of the entire properties [41]. The single electron transistor (SET) is found as the most captivating nanodevice that exploits the quantum tunneling effect and is capable of performing as a switch or as an amplifier. This device comprises of three terminals just as field effect transistors [42]. The source and drain (reservoirs) terminals are the same; the only variation that occurs in the schematic of the SET is the replacement of the channel with quantum dots (generally metallic quantum dots are used for the construction of a SET) or island as depicted in Fig. 7.8. There exists a weak coupling between reservoirs and the island [43]. For a strong coupling, the two tunnel junctions are connected, which act as a barrier between the island and source-drain terminals. In this case, the coupling between a metallic quantum dot and gate is accomplished by using capacitor Cg for controlling the potential of the metal plate. The transistor becomes on, when the value of potential is adjusted in such a way that they lead to significant conductance through small metal particles. In quantum mechanics, the quantum dot or island is supposed to have energy levels that are separated by a small distance and each level comprises of a particular number of electrons comparative to potential well [44]. When the size of the island shrinks, the spacing of the energy levels increases, which leads to an indirect relation between the island and the square of the diameter of the energy level given in Eq. (7.23). In each energy level, there exists either zero or one electron.   1 hπN 2 En 5 ð7:23Þ 2m d

FIGURE 7.8 A schematic diagram of single electron transistor. Adapted with permission under the terms of the CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/).

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The electron transport from the dot or island happens by the tunneling effect through coupling junctions

7.8.1

Operation of single electron transistor

In the circuit diagram for SET, the separation of small metallic particle acting as quantum dot from source-drain electrodes is done by incorporating a thin insulator along with the capacitive coupling between quantum dot and gate as depicted in Fig. 7.9A. The current that tunnels through the tunneling barriers is controlled by the gate component that works on the Coulomb 2 blockade principle [45]. The energy required for charging the capacitor is e2 with a typical value of 80 meV. This amount of energy is necessary for the transportation of electrons, otherwise at low temperatures, electron transport blocks, known as a Coulomb blockade as seen in Fig. 7.9B. Here, the working of SET is explained in terms of biasing. When there is no biasing, electron transport will not take place as the Fermi levels of source-drain electrodes will not change as in quantum mechanics, all the energy levels are quantized and each level contains an appropriable number of electrons along with a fixed particular energy or an electron to move from one energy level to other, it has to lose or gain its energy based on its movement, which can either be from a higher to lower energy level or from a lower to higher energy level [47]. This quantization gives the Coulomb blockade in the quantum dot as shown in Fig. 7.10A. When the bias volatge is applied in between the drain and source, that is, VDS, shifting of Fermi levels for both the source and drain ocuurs in such a way that the source levels move in an upward direction and for the drain, the Fermi levels move downward by an energy-amount/2 that cause off the Fermi levels in between drain and source and this state is shown in Fig. 7.10B. Now, for making electron transfer possible from the source to the quantum dot (QD), a gate voltage, that is, VG is required [48]. The

FIGURE 7.9 (A) Circuit diagram of SET and (B) Coulomb blockade. Adapted with permission under the terms of the CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/) [46].

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(A)

(B)

Source Q.Dot

Drain

Source Q.Dot

(C)

Drain

Source Q.Dot

Drain

FIGURE 7.10 (A) Without biasing (energy levels of quantum dot up-down with gate voltage), (B) with biasing, VDS, and (C) with gate voltage, VG.

reason for this is the relative enhancement in QD energy for an excess electron, which will ultimately violate the energy conservation principle of the system. Therefore when the gate voltage is sweeped for a fixed VDS, one electron tunneling through QD happens in a typical way, that is, from source to QD and then QD to the drain terminal. When a change occurs in the gate voltage by an amount ΔVg 5 Ceg , all the QD energy levels move down below the source Fermi level region, but stay above the drain Fermi level region as shown in Fig. 7.10C. When this condition is maintained, the hopping of the electron occurs from the source to the QD and then for the QD to the drain without violating the energy conservation law in the QD region ensuring that the tunneling of the electron is controlled by the gate voltage. This completes the basic working principle of SET. Now, when the graph between the gate voltage and the drain current is plotted, after every e=Cg , current spikes are obtained as depicted in Fig. 7.11A. When the drain-source voltage is changed for several values of gate voltage, the current will be increased as shown in Fig. 7.11B and by changing VSG and VG at the same time, the plot obtained shown in Fig. 7.11C will be Coulomb diamond.

7.8.2

Applications

Different applications of SET are explained as:

7.8.2.1 Supersensitive electrometer The SETs are capable of behaving as electrometers owing to their great sensitivity to distinctive physical experiments like in superconductors, the explicit observations of parity effects are made possible by them. Whole measurements for small dc currents have been verified.

Electrons in nanostructures Chapter | 7 (A)

(B)

201

(C)

I

I

Vg Vb

FIGURE 7.11 Plots of (A) I versus Vg, (B) I versus Vb, and (C) VDS versus Vg. Reprinted figure (C) from Y.V. Nazarov, Electronic states and transport in quantum-dots, in: Comprehensive Semiconductor Science and Technology, 2011, pp. 1
22, Copyright (2011), with permission from Elsevier.

7.8.2.2 Single electron spectroscopy One considerable application of this electronic device lies in it providing the opportunity for determining the electron addition energies along with the distribution of energy levels in nanometer-sized objects and QDs. 7.8.2.3 Detection of infrared radiation The single electron electrometry provides a significant photoresponse toward electromagnetic radiations. This response relatively varies from Tien
Gordon theory for electron-assisted tunneling, centered on the supposition of tunneling events that are independent. However, the case alters in SET. This makes single electron systems with multiple junctions and small cotunneling rate an important candidate for heterodyne detection of electromagnetic radiations with larger frequencies like superconductor-insulator-superconductor junctions as well as arrays. The key advantages of the single electron array compared to its superconductor-insulator-superconductor (SIS) counterparts includes less noise and easy adjustability of the threshold frequency [49]. 7.8.2.4 Charge state logics In order to solve the problem of current leakage, a device that is capable of presenting information in small single bits by means of the absence or presence of single electrons, known as charge state logic. There is no current and power change as in a static state, there is no dc current [42]. 7.8.2.5 Programmable single electron transistor logic SETs have a nonvolatile memory function and are capable of functioning as typical N-type metal-oxide-semiconductor (n-MOS) type SETs as well as

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P-type metal-oxide-semiconductor (p-MOS) type SETs. The function of the SET circuit is to be programmed based on the functions stored by the memory function. The Coulomb oscillations phase is changed by the island of the SET. In order to tune the Coulomb oscillations, the operations of writing or erasing associated with the memory functions are capable of injecting or shifting the charge from or to the memory node close to the island of the SET used. The change in the phase shift is a half period of Coulomb oscillations, if the charge that is injected is appropriate [50].

7.9

Resonant tunneling

The resonant tunneling phenomena deals with tunneling, which incorporates a structure through which the electron transmission coefficient is sharply peaked around certain specific energies. To explicate the appearance of these peaks qualitatively, the walls can be considered as the boundary conditions. If an electron exhibits sufficient energy that corresponds to the resonant level of the quantum well, the attained transmission coefficient will be near to unity. This means that an electron having resonant energy is capable of crossing the barrier, that is, tunneling instead of being reflected. The model for double gap junction is depicted in Fig. 7.12. For central dominant particle modeling, a square well is introduced with a width of 2 R and first bound state energy of EO magnitude [24]. The tunneling rate is given by ΓL for left side tunneling on the localized state and ΓR for right side tunneling from the localized state.

FIGURE 7.12 Illustration of one-dimensional potential energy model for resonant tunneling. Reprinted with permission from V. Kalmeyer, R.B. Laughlin, Phys. Rev. B 35 (1987) 9805
9808, Copyright (1987) by the American Physical Society.

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In this figure, the electrons that are coming from the left side confront the barrier that has a localized state at EO along with height VO . The expression for the conductance of the junction with zero biasing is given as: G5

4e2 ΓL ΓR h ðE 2 EO Þ2 1 ðΓL 1 ΓR Þ2

ð7:24Þ

This shows a significant result. When the incoming electron energy E becomes equal to the localized state energy EO , that is, E 5 EO , Eq. (7.24) becomes: G5

4e2 ΓL ΓR h ðΓL 1 ΓR Þ2

ð7:25Þ

Now, we consider the symmetric structure case in which ΓL 5 ΓR , that is, the localized state being located in the middle of the barrier. Then the conductance in Eq. (7.25) becomes: G5

e2 h

ð7:26Þ

The result shown in Eq. (7.26) leads to half of the Landauer conductance. Therefore at resonance, that is, E 5 EO , it is appropriable to consider the localized state as a metallic channel in order to connect the two electrodes fixed at opposite sides. Here the values of ΓL and ΓR are not specified yet. The relation shown in Eq. (7.24) was actually derived by Breit and Wigner for nuclear scattering and it utters a resonant state halfway between Earth and the Moon and this often leads to a metallic tunneling on the astronomical level. This incredible result is in fact an illusion that is caused by the fact that localized state charging is neglected. The smaller the tunneling rates between the localized state and electrodes, the larger the charge growth on the localized state will be. When the device is small enough, electrons inside the device will create a strong Coulomb repulsion preventing other electrons to flow which results in coloumb blockade. For resonant tunneling, the conductance between any electrode and middle state needs to be equal or half of the Landauer conductance [51]. Therefore metallic point contact is desired between electrodes and the center state for the occurrence of resonant tunneling. A resonant structure can be made between two electrodes by placing a metal atom near the atoms at the top of each electrode. This leads to resonant tunneling just as the delocalization of electrons on the lattice. For tight coupling, a molecule can be considered as interceding resonant tunneling in the metallic gap with the condition that the states of the molecule should lie near the metal Fermi energy level. For solving this problem, the band structure model is utilized and transmission
energy curves are shown in Fig. 7.13. Here, curve a and curve c represent molecules having two states and that lie in the conduction band of wire with ΓL 5 ΓR for curve a and ΓL 5 4ΓR

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FIGURE 7.13 Transmission versus energy curves for tight binding model of resonant tunneling. Reprinted with permission from S.M. Lindsay, et al., Pressure and resonance effects in scanning tunneling microscopy of molecular adsorbates. J. Phys. Chem. 94 (11) (1990) 4655
4660 [52], Copyright (1990) American Chemical Society.

for curve c. Curve b shows a molecule with the single state available in the conduction band. The dashed lines show the transmission required through the gap to both sides of the molecule and for no molecule, the portion of curves over the dashed lines depicts the advancement in the vacuum tunneling as a result of resonant tunneling. The peak transmissions are greater as compared to the vacuum tunneling through the gap, which shows a way in which resonance improves tunneling [24].

References [1] L.J. Curtis, Atomic Structure and Lifetimes: A Conceptual Approach, Cambridge University Press, 2003. [2] K.T. Compton, The electron: its intellectual and social significance, Nature 139 (3510) (1937) 226. [3] R. Laming, J. Thomson. Mean lifetime. in Spin. [4] M.V. Simkin, V.P. Roychowdhury, A mathematical theory of citing, J. Am. Soc. Inf. Sci. Technol. 58 (11) (2007) 1661
1673. [5] G. Gra¨ff, H. Kalinowsky, J. Traut, A direct determination of the proton electron mass ratio, Z. fu¨r Phys. A At. Nucl. 297 (1) (1980) 35
39. [6] D.L. Farnham, R.S. Van Dyck Jr, P.B. Schwinberg, Determination of the electron’s atomic mass and the proton/electron mass ratio via Penning trap mass spectroscopy, Phys. Rev. Lett. 75 (20) (1995) 3598. [7] E. Reinhold, et al., Indication of a cosmological variation of the proton-electron mass ratio based on laboratory measurement and reanalysis of H 2 spectra, Phys. Rev. Lett. 96 (15) (2006) 151101.

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[8] J. Newman, Physics of the Life Sciences, Springer Science & Business Media, 2010. [9] L.G.H. Huxley, Electrons, Nature 136 (3435) (1935) 320
321; R. A. Millikan. Revised edition, University of Chicago Press, Chicago; University Press, Cambridge, London, 1947, pp. x 1 642. [10] C. Anastopoulos, Particle or Wave: The Evolution of the Concept of Matter in Modern Physics, Princeton University Press, 2008. [11] C. Henkel, et al., Resonant tunneling induced enhancement of electron field emission by ultra-thin coatings, Sci. Rep. 9 (1) (2019) 6840. [12] K. Pitzer, The nature of the chemical bond and the structure of molecules and crystals: an introduction to modern structural chemistry, J. Am. Chem. Soc. 82 (15) (1960). p. 4121
4121. [13] J. Owen, J. Thornley, Covalent bonding and magnetic properties of transition metal ions, Rep. Prog. Phys. 29 (2) (1966) 675. [14] S. Wemple, M. DiDomenico Jr, Behavior of the electronic dielectric constant in covalent and ionic materials, Phys. Rev. B 3 (4) (1971) 1338. [15] R.H. Fowler, L. Nordheim, Electron emission in intense electric fields, Proc. R. Soc. London. Ser. A, Containing Pap. A Math. Phys. Character 119 (781) (1928) 173
181. [16] S. Anisimov, B. Kapeliovich, T. Perelman, Electron emission from metal surfaces exposed to ultrashort laser pulses, Zh. Eksp. Teor. Fiz. 66 (2) (1974) 375
377. [17] H. Seiler, Secondary electron emission in the scanning electron microscope, J. Appl. Phys. 54 (11) (1983) R1
R18. [18] A. Modinos, Secondary electron emission spectroscopy, Field, Thermionic, and Secondary Electron Emission Spectroscopy, Springer, 1984, pp. 327
345. [19] E.B. Podgorˇsak, Radiation Physics for Medical Physicists, Springer, 2006. [20] K.L. Jensen, M. Cahay, General thermal-field emission equation, Appl. Phys. Lett. 88 (15) (2006) 154105. [21] O. Richardson, Electron emission from metals as a function of temperature, Phys. Rev. 23 (2) (1924) 153. [22] K.L. Jensen, Introduction to the Physics of Electron Emission, Wiley Online Library, 2018. [23] W. Dolan, W. Dyke, Temperature-and-field emission of electrons from metals, Phys. Rev. 95 (2) (1954) 327. [24] S. Lindsay, Introduction to Nanoscience, Oxford University Press, 2010. [25] R.J.B. Balaguru, B. Jeyaprakash, Quantum Size Effect, Electrical Conductivity and Quantum Transport, NPTEL, India, 2013. [26] B. Kramer, et al., Interactions and transport in nanostructures, Semiconductor Sci. Technol. 9 (11 S) (1994) 1871. [27] E. Roduner, Size matters: why nanomaterials are different, Chem. Soc. Rev. 35 (7) (2006) 583
592. [28] G. Cao, Y. Wang, Nanostructures and nanomaterials: synthesis, Prop. Appl. 2 (2004). [29] Z.J. Zhong, Optical Properties and Spectroscopy of Nanomaterials, World Scientific, 2009. [30] J. Van Ruitenbeek, Quantum point contacts between metals, Mesoscopic Electron Transport, Springer, 1997, pp. 549
579. [31] K. Pedersen, Quantum size effects in nanostructures, Org. Inorg. Nanostructures (2006). [32] S. Link, M.A. El-Sayed, Shape and size dependence of radiative, non-radiative and photothermal properties of gold nanocrystals, Int. Rev. Phys. Chem. 19 (3) (2000) 409
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[33] N.W. Ashcroft, N.D. Mermin, Solid State Physics (Holt Rinehart Winston, New York), Google Scholar, 1976. [34] C. Kittel, P. McEuen, P. McEuen, Introduction to Solid State Physics, Vol. 8, Wiley, New York, 1976. [35] A. Abrikosov, Fundamentals of the Theory of Metals, 1988. [36] O. Madelung, Introduction to Solid-State Theory, Vol. 2, Springer Science & Business Media, 2012. [37] W. Hanke, Solid state physics, High Performance Computing in Science and Engineering’04, Springer, 2005, pp. 79
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1375. [48] K.E. Drexler, Nanosystems: Molecular Machinery, Manufacturing, and Computation, Wiley, New York, 1992. [49] A. Cleland, et al., An extremely Low noise Photodetector based on the single electron transistor, J. Low. Temp. Phys. 93 (3
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1630. [51] M.H. Bukhari, RF field-driven electron tunneling through mesoscale junctions, J. Mod. Phys. 8 (12) (2017) 1950. [52] S.M. Lindsay, et al., Pressure and resonance effects in scanning tunneling microscopy of molecular adsorbates, J. Phys. Chem. 94 (11) (1990) 4655
4660.

Chapter 8

Molecular electronics Khalid Nadeem Riaz, Zainab Israr, Tahir Iqbal Awan, Almas Bashir and Aqsa Tehseen Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan

Chapter Outline 8.1 Molecular electronics 8.2 Lewis structures 8.2.1 Limitations 8.3 Variational approach to calculate molecular orbitals 8.4 Hybridization of atomic orbitals 8.5 Donor acceptor properties

8.1

207 209 210 212 213 215

8.6 Electron transfer between molecules 8.7 Charge transport in weakly interacting molecular solids 8.8 Single molecule electronics 8.8.1 Theoretical background 8.8.2 Examples References

216 217 217 218 220 222

Molecular electronics

Molecular electronics is related to the assembling and utilization of basic individual atoms and molecules, which are considered as the basic elements of many chemical and biological processes [1]. The area of organic compounds and their exceptional macroscopic properties for the development of organic devices has its origins in material sciences and is also known as molecular materials for electronics. Organic devices such as organic photovoltaic (PV) devices, organic light emitting displays, liquid crystal displays (LCD), organic transistors, infrared imaging, and biochemical sensors are the most successful commercial products of this area [1]. Molecular electronics involves the designing of molecular systems that are capable of long-distance electron transport through donor-bridge-acceptor (D-B-A) systems. In these systems, charge transport has been extensively studied with different bridge molecules such as DNA, proteins, porphyrins, and saturated and unsaturated hydrocarbons. Three scientists, namely Alan Heeger, Hideki Shirakawa, and Alan MacDiarmid have made noteworthy contributions to the growth in electric

Chemistry of Nanomaterials. DOI: https://doi.org/10.1016/B978-0-12-818908-5.00008-1 © 2020 Elsevier Inc. All rights reserved.

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conductive polymers and were also awarded the Nobel Prize in Chemistry for their significant work in the field of molecular electronics. Many interesting areas, for example, molecular switches [2], molecular rectifiers [3], DNA electronics [4], and negative differential-resistance junctions [5] have also been described in molecular electronics, but it is still uncertain whether these can find commercial-level applications or not. Electron movement by single molecules is the central focus of molecular electronics. The concept of the moving of electrons through single molecules is described in two different ways. The first involves the movement of electrons through a single molecule also known as electron transfer [6]. The second one involves the movement of a current passing through a single molecule. These two methods are interlinked and have a common purpose of discerning how electrons move through molecules [7,8]. In the early 1970s, Hans Kuhn and his team conducted studies related to the molecular electronics field. Particularly in 1971, Kuhn and coworkers directed conductivity measurements of single layers of fatty acids and cadmium salts. These studies reported that the conductivity of monolayers is directly linked with the thickness of the layers; conductivity decreased as the thickness of the layers increased, which also showed the tunneling effect of electrons in organic monolayers of cadmium salts and fatty acids. For the purpose of increasing interest in the field of molecular electronics, several conferences were organized in Washington DC that majorly focused on unusual observations and experiments related to the molecular transport behavior of electrons and possible mechanisms for electron transport behavior-based devices. The first theoretical study on electron transport through single molecules was published in 1974 by Mark Ratner and Arieh Aviram. They suggested an ad hoc scheme for calculations and also proposed that single molecules can be exploited as a device [9]. The most important advancement in this field occurred in the 1980s. The development of scanning tunneling microscopy (STM) and atomic force microscopy (AFM) allowed for the measurement of single molecule conductance. Mark Reed with his team and James Tours with his team at the University of South Carolina published significant research work that majorly focuses on attempts to measure single electron transport [10]. Their early 2000s research work emphasizes the transport properties of electrons in different molecules. Large fluctuations in their experimental data convinced us to treat single molecule transport and single molecular spectroscopy as being similar. The achievement of early experiments exploded the interest in the field of molecular electronics [8]. During the past three decades, an indicator of the evolution of citations in the field of molecular electronics is presented by a study by Aviram and Ratner. This indicator illustrates that late the 1990s and the early 2000s was a period of great interest for scientists in the field of molecular electronics as shown in Fig. 8.1 [9].

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FIGURE 8.1 Indicator of the evolution of the field of molecular electronics by the number of citations per year. Reprinted by permission from Springer Nature, G. Cuniberti, G. Fagas, K. Richter, Introducing Molecular Electronics: A Brief Overview, 2006.

8.2

Lewis structures

It is known that due to the lowering of quantum mechanical kinetic energy, the electrons of an atom tend to form bonds to be placed in a larger quantum box. For single electron attractive interactions, the force of electrons is balanced by the Coulomb force of repulsion between atomic nuclei, but in the case of many electrons, constraints are imposed by the Pauli exclusion principle. G.W Lewis presented a model for better understanding the electronic properties of molecules qualitatively. This model consists of remarkable rules that every chemical bond should obey. Lewis proposed that multielectron particles are inclined to settle in the states associated with bonds and participate in the formation of closed-shell electronic states. For example, two electrons are required for hydrogen atom corresponding to 2 s2 closed shell state of He atom. Eight electrons are required for p-states, six electrons for p-states, and two electrons for s-states. Lewis structures guess the formation of bonds and shared electron in them along with shape of particle. Each sharing of one electron is represented by a single line between pairs of atoms known as a single bond. The sharing of two or three electrons between a pair of atoms is represented by double and triple lines between the pair of atoms and these are also known as double and triple bonds respectively. Single bonds hold two electrons, while double and triple bonds hold four and six electrons respectively.

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For a better understanding of Lewis structures, let us consider the simplest examples of the formation of H2 and Cl2 molecules.

Now moving toward a slightly more complicated example of the formation of the ammonia molecule. Here three valence electrons of nitrogen form three single bonds with hydrogen atoms and the remaining electrons of nitrogen act as lone pairs of electrons. The Lewis structure for the ammonia molecule is given as:

Taking another example of CO2 molecule. It is impossible to put two oxygen atoms with carbon atoms by the formation of single bonds. The solution is to form double bonds instead of single bonds. The Lewis structure for the CO2 molecule will be given as:

For triple bonds, take the example of the structure of the hydrogen cyanide molecule. It is impossible to generate octet rule without the formation of a triple bond between the hydrogen, carbon, and nitrogen atoms where the remaining electrons of nitrogen will behave as lone pairs. The Lewis structure for the hydrogen cyanide molecule will be given as:

8.2.1

Limitations

To draw Lewis structures of molecules, one should follow these basic steps: Step 1: First, for the formation of the molecule, one must calculate the addition of all valence electrons present in the atoms. Let us consider the example of the PCl3 molecule: PCl3

5 1 3 3 7 5 26

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Step 2: One has to consider the least negative atom among all the atoms of the molecule and place it in the center of the structure, and with the help of single bonds, connect the remaining atoms with it as shown here:

Step 3: Now complete the octet rule for the outer atoms of the molecule. Each single bond holds two electrons:

Step 4: Now complete the octet rule for the central atom of the molecule:

Step 5: Keep forming double and triple bonds until the central atom forms the octet rule:

Step 6: The last and final step is to assign formal charges. For example, the formal charges on the example provided here will be calculated as:

6
7 5 21 4
4 5 0 6
5 5 11

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Consider another example of CO2. The formal charges on CO2 will be calculated as:

6
6 5 0 4
4 5 0 6
6 5 0 According to Lewis structure, the minimization of the formal charges is necessary for the development of fine structure. These simple rules are in good account with a wide range of compounds. Lewis structures are even applicable to structures of ions as well, but they also have some limitations. Since the octet rule is not followed by some elements, the Lewis structure doesn’t tell us about the bonding of molecules. The Lewis structure is helpful for predicting molecular structures [11].

8.3

Variational approach to calculate molecular orbitals

For building molecular orbitals from atomic orbitals, a variational approach is used. The variational formula for calculating molecular orbitals can be derived from the time-independent Schrodinger equation. Multiplying the  time-independent Schrodinger equation by ψ and integrating both sides gives: E0 5

, ψjHjψ . , ψjψ .

Here E0 is defined as ground state energy. Using this equation, we can find the energy of other states, say Eφ . The value of Eφ will be greater than that of E0 because here, E0 is defined as ground state energy. This speech was shown by increasing φ in the form of wave function ψ. Now starting with the initial trial wave function: X φ5 cifi i

By minimizing the calculated energy with respect to coefficients we can find the value of coefficients ci and fi. P P dEφ , i cifijHj i cifi . P P 5 50 , i cifij i cifi . dci For simplicity, we consider that trial function only consists of two functions, namely f1 and f2: φ 5 c1 f1 1 c2 f2

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By inserting the value of φ into the ground state energy equation we can get the value of Eφ . Eφ 5

c21 H11 1 2c1 c2 H12 1 c22 H22 c21 S11 1 c1 c2 S12 1 c22 S22

For simplification, taking derivative on both sides with respect to c1 and dE c2 and for the minimum value we require, dc1φ 5 0, gives us: ðc1 S11 1 c2 S12 ÞEφ 5 c1 H11 1 c2 H12 By repeating the calculations for c2, we get another liner equation for c2. Linear equations for c1 and c2 are:     H11 2 Eφ S11 c1 1 H12 2 Eφ S12 c2 5 0 H12 2 Eφ S12 c1 1 H22 2 Eφ S22 c2 5 0 These linear equations will only be nontrivial solutions when the secular determinant of these linear equations is zero.   H11 2 Eφ S11 H12 2 Eφ S12 50 H12 2 Eφ S12 H22 2 Eφ S22

8.4

Hybridization of atomic orbitals

Fig. 8.2 shows a comparison of the room-temperature conductivity value of different conductive polymers like germanium, silicon, glass, etc. [12]. Due

FIGURE 8.2 Comparison of room-temperature conductivity value of different conductive polymers. Reprinted by permission from Springer Nature, M.C. Petty, T. Nagase, H. Suzuki, et al., Molecular Electronics, 2017.

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FIGURE 8.3 (A) 2s orbital; (B) 2py, 2px, and 2pz orbitals. Reprinted by permission from Springer Nature, M.C. Petty, T. Nagase, H. Suzuki, et al., Molecular Electronics, 2017.

to the many possible electronic state configurations of the carbon atom it has properties in many aspects. It is known that the atomic number of carbons is six and it has four electrons in its valence shell. It has electronic configuration 1s2 ; 2s2 ; 2p2 , its 1s shell is fully filled and for bonding in 2s and 2p orbitals, four valence electrons are divided in them. 2s orbitals are spherical in symmetry and may be directed in any way, while 2p orbitals are different in symmetry to 2s orbitals as shown in Fig. 8.3. 2p orbitals are orthogonal in symmetry and make bonds in specified directions. When these valence electrons (two or more) are involved in bonding, these bonds can be accurately described by the creation of the hybridization of atoms of orbitals involving 2s and 2p orbitals. Different combinations of orbitals result in different hybrids. As an example, consider one 2s orbital and two 2p orbitals, this combination of orbitals yields three equal sp2 hybrid orbitals where s orbitals consist of 33.3% and p orbitals consist of 66.7%. These three construct hybrid orbitals directed 120 C and lies in the xy plane. Also, sp3 hybridization consists of one s and three p orbitals forming tetrahedron, each one is directed at 109.5 between the corners of the tetrahedron. In sp3 hybridization, each orbital consists of 25% s and 75% p orbitals. Another type of hybridization is sp, where each orbital consists of 50% s and 50% p content. Different types of chemical bonds can be formed by different types of hybridization (sp, sp2, and sp3) as shown in Fig. 8.4. These types of hybridization are considered as limiting cases. Types of bonds/hybridization having more s character tend to be stronger as compared to other types of hybridization due to the lower energy of electron in s orbitals. Depending upon the orientation of the orbitals, another type of bonding exists in the carbon atom. The type of bonding in which electron clouds are oriented in such a way that these electron clouds are lying above and below the nodal plane is called pi (π) bonding. Another type of bonding is termed sigma (σ) bonding in which electron clouds are overlapping with each other,

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FIGURE 8.4 Illustration of sp3, sp2, and sp hybrid orbitals in carbon. Adapted with permission under the terms of the CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/).

FIGURE 8.5 (A) Sigma (σ) bonding and (B) pi (π) bonding. Reprinted by permission from Springer Nature, M.C. Petty, T. Nagase, H. Suzuki, et al., Molecular Electronics, 2017.

both sigma and pi bonding are shown in Fig. 8.5. Sigma bonds are relatively stronger than pi bonds because the pi bond has a lower density of electrons as compared to the sigma bond. Carbon compounds that consist of pi bonds are considered as saturated compounds, whereas compounds that only consist of sigma bonds are known as unsaturated compounds.

8.5

Donor acceptor properties

Systems like D-B-A play an important role in the research area because of their potential application in the formation of molecular wires. The length variation of a bridge allows the measurement of charge transfer depending on distance. It shows the use of rigid donor and accepter in D-B-A systems which explains the hopping mechanism and super exchange. Consider as an example a D-B-A system in which 3,5-dimethyl-4-(9-anthracenyl)-julolidine (DMJ-An) acts as a donor, naphthalene-1,8:4,5-bis(dicarboximide) (NI) acts as an acceptor and -oligophenylene (Phn), 2,7-fluorenone (FNn), and pphenylethynylene (PEnP) act as bridge units. By nanosecond interest spectroscopy it was observed that the photoexcitation of DMJ-An gives a charge transfer to NI for the formation of DMJ 1 -An-Bridge-NI, which quickly suffer pair intersystem passing RP-ISC to give triplets RPs [13]. The D-B-A system used and charge transfer mechanism for better understanding is shown in Fig. 8.6.

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FIGURE 8.6 (A) Donor-bridge-acceptor system used in study and (B) charge transfer scheme for FN1
3, PE1
3P, and PH1
5. Reprinted with permission from A.M. Scott, et al., Spin-selective charge transport pathways through p-oligophenylene-linked donor 2 bridge 2 acceptor molecules. J. Am. Chem. Soc. 131 (48) (2009) 17655
17666 [14], Copyright (2009) American Chemical Society.

8.6

Electron transfer between molecules

The Wasielewski research group have developed the quantum interference model, which is used to predict the rate of bridge-mediated transportation of charge rates over cross-conjugated bridges. Cross-conjugation is a term for defining a compound consisting of three unsaturated groups, two of them are conjugated along the third central unsaturated group and are not conjugated with each other. This model predicts that the charge transfer rate through these molecular bridges can be varied over many orders of magnitude with the help of an appropriate choice of injection energy [15
17]. By adopting the quantum interference model, computed electron transmission through these cross-conjugated bridges bound between metallic electrodes is shown.

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FIGURE 8.7 (A) Atomic structure of gold
dithiol molecular junction and (B) orbital shape and energy of frontier molecular orbitals of phenyl dithiol (PDT), biphenyl dithiol (BPD), and terphenyl dithiol (TPD). Reprinted from Yongqiang Xue, M.A. Ratner, Theoretical principles of single-molecule electronics: a chemical and mesoscopic view. Int. J. Quantum Chem. (2005), with permission from John Wiley and Sons.

8.7

Charge transport in weakly interacting molecular solids

To study charge transfer in weakly interacting molecular solids let us consider the example of a prototype molecular device formed by a gold
dithiol-molecular
gold junctions as shown in Fig. 8.7A. This type of junction results from the combination of the benzene molecule with gold conductors over the sulfur end of atoms. The atomic structure, shape of molecular orbitals, and four frontier molecular orbitals are shown in Fig. 8.7B. As compared to other nanostructured devices, single molecular electronic devices require much more appropriate attachment of atomic groups, which are different in chemical process from core particles to create stable interactions with electrodes. The introduction of two sulfur electrodes has two different consequences. Either it introduces molecular states or results in modified metal-molecule contact over the bonding of end-metal. Charge transfer and induced electrostatic potential charge is shown in Fig. 8.8 as a function of the position in the XY plane of the gold
PDT
gold and gold
TPD
gold junctions. For the PDT particle, the XY plane is calculated by benzene ring, while for the TPD molecule the XY plane is calculated by the left-most and right-most benzene rings [18].

8.8

Single molecule electronics

Single molecular electronic devices are considered as the building blocks of molecular electronics. Assembling and exploiting biological molecules for the construction of electronic devices has been the exclusive goal of

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PART | 2 Interactions in nanomaterials

FIGURE 8.8 (A) Charge transfer and (B) induced electrostatic potential charge of gold
PDT
gold junction and gold
TDP
gold junction. Reprinted from Yongqiang Xue, M.A. Ratner, Theoretical principles of single-molecule electronics: a chemical and mesoscopic view. Int. J. Quantum Chem. (2005), with permission from John Wiley and Sons.

researchers and scientists. The first single molecular electronic device was reported by Aviram and Ratner in 1974. They recommended that molecules having a donor
acceptor bridge structure may be able to act as a molecular diode when connected with electrodes. In the past few decades, many scientists investigated the electronic and molecular behavior of organic materials in various potential applications. They also reported that these biological molecules have a vast range of applications in the field of chemical sensors, microelectronics [19], and biosensors [20]. In molecular electronics, many early proposals had major focus on the basic properties of biological particles. Today, scientists have realized that a thorough concept of the electronic and charge transport properties of biological molecules plays an important role in the development of single moleculebased electronic devices. Electron transport through single molecules has been studied by adopting different strategies such as crossed junctions, scanning nanoprobe microscopes, and nanowires. Theoretical approaches such as the semiempirical method, statistical analysis, density functional theory (DFT) studies, and the ab initio method were also adopted by many theoretical physicists for the development of single molecular electronic devices [21
23].

8.8.1

Theoretical background

As compared to semiconductor molecular electronics, the modeling of the transport of charge carriers in single molecular electronics is much more

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difficult to understand. Therefore the theoretical formation for solving problem requires integrating the basic concepts of surface science, quantum transport, the theory for the structure of electrons, and the modeling of a device using a single outline adopting first principle calculations. Yongqiang Xue and Mark A. Ratner introduced the theoretic outline for single molecular electronics using reliable matrix of Green’s function combined with the theory of quantum transport in a nonequilibrium Green’s function at the atomic level for the structure of electronic devices. They considered single particle devices as heterostructures consisting of chemically distinct groups of atoms and also examined the characteristics of devices in the form of charge and response of potential. Fig. 8.9 shows a schematic illustration of (A) two terminal single molecule device and (B) the atomic structure of the extended molecule. Researchers also demonstrated this concept by using examples of devices fabricated with different size benzene-based particles and gold conductors using sulfur as end atoms. Researchers also demonstrated extended molecule atomic structure consist of phenyl di thiol molecule with two gold surfaces at end of atoms of Sulphur [18]. This Green’s method function allows for the molecular states of molecules to be plugged with the electronic states of electrode materials. Also, other calculations have been carried out with the help of matrix elements using studies of DFT. Such wave functions are not like real functions, but lead to astonishingly good estimates in some cases [24].

FIGURE 8.9 (A) Schematic illustration of two terminal single molecule device and (B) atomic structure of extended molecule. Reprinted from Yongqiang Xue, M.A. Ratner, Theoretical principles of single-molecule electronics: a chemical and mesoscopic view. Int. J. Quantum Chem. (2005), with permission from John Wiley and Sons.

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FIGURE 8.10 (A) Twist angle and conjugation between the rings of benzene molecules and (B) histogram of conductance of benzene molecule measured in repeated break junction and (C) a plot of measured conductance versus twist angle between the rings of the benzene molecule. Reprinted by permission from Springer Nature, L. Venkataraman, et al., Dependence of singlemolecule junction conductance on molecular conformation (2006).

Another technique of break junction along with statistical analysis has found extensive use in the field of single molecular electronic devices. For this, a bi-phenol molecule such as benzene was considered as an example and a series of measurements were taken to find the effect of molecular conductance in the form of twist angle in the molecules of benzene rings as shown in Fig. 8.10A. The conductance of the molecules was measured and recorded individually as a histogram of conductance calculated in frequent pause junction measurements as shown in Fig. 8.10B. Also a plot of the measured conductance along with twisting angle was also recorded, which showed good agreement with previously reported theoretical studies given in Fig. 8.10C [24,25].

8.8.2

Examples

To design D-B-A systems scientist took inspiration from nature and designed a structurally efficient and long-distance “wire-like” charge transport system. This designed molecular wire is described as molecular bridges, which effectively transport charge through chemical bond lengths. Consider the example of the photosynthesis reaction. In the photosynthesis reaction, a center protein transfers charge through a step-wise hopping process. However, in most synthetic processes, charge is transferred through a tunneling process, which can be determined by the electronic structure of the molecule through a super exchange mechanism [13]. Both the super exchange and hopping processes can be understood by Fig. 8.11. Another example of D-B-A is a proposed chemical structure of molecular rectifier as shown in Fig. 8.12. Here tetracyanoquinodimethane (TCNQ) acts as an electron acceptor that can easily charge having low unoccupied electronic states, and tetrathiafulvalene (TTF) acts as an electron donor with the highest occupied electronic states. Three parallel chains of methylene groups act as a bridge between the electron acceptor and electron donor groups and

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Super exchange: Coherent ET

B1

D

B2

B3

A

Hopping: Incoherent ET

D

B1

B2

B3

A

FIGURE 8.11 Schematic diagram of the hopping and super exchange mechanisms for the photosynthesis process (D-B-A system).

FIGURE 8.12 Chemical structure of an acceptor-bridge-donor system proposed as a molecular rectifier. Reprinted with permission from C.A. Nijhuis, W.F. Reus, G.M. Whitesides, Mechanism of rectification in tunneling junctions based on molecules with asymmetric potential drops. J. Am. Chem. Soc. 132 (51) (2010) 18386
18401 [27], Copyright (2010) American Chemical Society.

isolates them. The energy transfer is always from the donor to the acceptor group via the molecular bridge. This molecular structure and charge transfer mechanism was proposed by Aviram and Ratner in a seminar paper that was specifically arranged to spark the interest of new researchers in the field of molecular electronics [26]. The study of molecular electronics leads scientists to modify the electrical properties of materials that behave biologically such as DNA. DNA played an important role in the development of the field of molecular electronics. According to reports, DNA is a small resistant molecular wire that usually behaves as an insulator. There are some contradictory reports regarding the conductivity of DNA due to different molecular sequences. DNA

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PART | 2 Interactions in nanomaterials

FIGURE 8.13 DNA double-helix showing the position of four bases, namely guanine (G), cytosine (C), adenine (A), and thymine (T). Reprinted by permission from Springer Nature, M.C. Petty, T. Nagase, H. Suzuki, et al., Molecular electronics, 2017.

consists of a double-helix stand having repeating sugar and phosphate groups, which are further attached with four bases, namely guanin (G), cytosine (C), adenine (A), and thymine (T) as shown in Fig. 8.13. Experimental and theoretical studies recommend that positive charge (hole) is much more stable than G
C and A
T base pairs. The distance between a G
C pair is so small that a hole can easily tunnel through it. Studies have confirmed that DNA consists of specific sequences. DNA chips exploit this fact and are used to find specific genetic codes in DNA. Microfabricated chips have a wide range of applications in the field of medical physics. Also if strings of DNA are attached in a specific order, it can also be used to resolve many combinational problems [4].

References [1] M. Petty, Molecular Electronics, Springer, 2007. [2] A.K. Flatt, J.M. Tour, Synthesis of thiol substituted oligoanilines for molecular device candidates, Tetrahedron Lett. 44 (35) (2003) 6699
6702. [3] R.M. Metzger, Monolayer rectifiers, J. Solid. State Chem. 168 (2) (2002) 696
711.

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[4] C. Dekker, M. Ratner, Electronic properties of DNA, Phys. World 14 (8) (2001) 29. [5] J. Chen, et al., Room-temperature negative differential resistance in nanoscale molecular junctions, Appl. Phys. Lett. 77 (8) (2000) 1224
1226. [6] R.A. Marcus, Electron transfer reactions in chemistry. Theory and experiment, Rev. Mod. Phys. 65 (3) (1993) 599. [7] A. Nitzan, Chemical Dynamics in Condensed Phases: Relaxation, Transfer and Reactions in Condensed Molecular Systems, Oxford university press, 2006. [8] A. Nitzan, Electron transmission through molecules and molecular interfaces, Annu. Rev. Phys. Chem. 52 (1) (2001) 681
750. [9] A. Aviram, M.A. Ratner, Molecular rectifiers, Chem. Phys. Lett. 29 (2) (1974) 277
283. [10] M.A. Reed, et al., Conductance of a molecular junction, Science 278 (5336) (1997) 252
254. [11] S. Lindsay, Introduction to Nanoscience, Oxford University Press, 2010. [12] M.C. Bo¨hm, Siegmar Roth: One-Dimensional Metals, VCH, Weinheim, 1995. ISBN 3527-6875-8, 247 Seiten, Preis: 148,-DM. Berichte der Bunsengesellschaft fu¨r physikalische Chemie, 1997. 101(8): p. 1194
1195. [13] S. Saari, and A. Moilanen, International Evaluation of Research and Doctoral Training at the University of Helsinki 2005
2010: RC-Specific Evaluation of ASP-Astronomy and Space Physics. 2012. [14] A.M. Scott, et al., Spin-selective charge transport pathways through p-oligophenylenelinked donor 2 bridge 2 acceptor molecules, J. Am. Chem. Soc. 131 (48) (2009) 17655
17666. [15] G.C. Solomon, et al., Understanding quantum interference in coherent molecular conduction, J. Chem. Phys. 129 (5) (2008) 054701. [16] D.Q. Andrews, et al., Quantum interference: the structural dependence of electron transmission through model systems and cross-conjugated molecules, J. Phys. Chem. C. 112 (43) (2008) 16991
16998. [17] G.C. Solomon, et al., Quantum interference in acyclic systems: conductance of crossconjugated molecules, J. Am. Chem. Soc. 130 (51) (2008) 17301
17308. [18] Y. Xue, M.A. Ratner, Theoretical principles of single-molecule electronics: a chemical and mesoscopic view, Int. J. Quantum Chem. 102 (5) (2005) 911
924. [19] Z. Cai, C.R. Martin, Electronically conductive polymer fibers with mesoscopic diameters show enhanced electronic conductivities, J. Am. Chem. Soc. 111 (11) (1989) 4138
4139. [20] B.R. Eggins, Chemical Sensors and Biosensors, Vol. 28, John Wiley & Sons, 2008. [21] M.A. Reed, Molecular-scale electronics, Proc. IEEE 87 (4) (1999) 652
658. [22] C. Joachim, J.K. Gimzewski, A. Aviram, Electronics using hybrid-molecular and monomolecular devices, Nature 408 (6812) (2000) 541. [23] A. Nitzan, M.A. Ratner, Electron transport in molecular wire junctions, Science 300 (5624) (2003) 1384
1389. [24] L. Venkataraman, et al., Dependence of single-molecule junction conductance on molecular conformation, Nature 442 (7105) (2006) 904. [25] B. Xu, N.J. Tao, Measurement of single-molecule resistance by repeated formation of molecular junctions, Science 301 (5637) (2003) 1221
1223. [26] M. Ratner, A. Aviram, Molecular rectifiers, Chem. Phys. Lett. 29 (2) (1974) 277
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18401.

Chapter 9

Nanomaterials Tahir Iqbal Awan, Anam Ahmad, Saliha Bibi, Aqsa Tehseen and Almas Bashir

Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan

Chapter Outline 9.1 Introduction of nanomaterials 9.1.1 Dimensionality 9.2 Quantum dots 9.2.1 Applications 9.3 Nanowires 9.3.1 Synthesis 9.3.2 Properties of nanowires 9.3.3 Applications of nanowires 9.4 Nanophotonics 9.4.1 Optoelectronics and microelectronics

9.1

225 226 227 228 229 229 230 234 235 236

9.4.2 Basic principles 9.5 Magnetic nanostructures 9.5.1 Synthesis 9.5.2 Properties of magnetic nanostructures 9.5.3 Applications of magnetic nanostructures 9.6 Nano thermal devices 9.7 Nanofluidic devices 9.8 Biomimetic materials References

240 240 241 246 248 251 253 261 266

Introduction of nanomaterials

The term “nano” was derived from a Greek letter used for dwarf or aberrantly short person. For different units such as meter or second, the word nano works as a prefix and is defined as a billionth (1029) of that particular unit. Thus a nanometer (nm) is a billionth of a meter and nanomaterials are defined as materials having particle sizes lying in the range of 1
100 nm in a minimum of one dimension. Nanomaterials differs from their bulk counterparts owing to their particular properties. They can be comprised of a single material or by a combination of different materials like alloys, polymers, metals, or ceramics [1]. For example, crystals lying in the nanometer scale have low melting points, particular crystal structures, variations in electrical conductivity, reduction in lattice constants, enhanced oxidation, and greater sensitivity of sensors in comparison to that of bulk materials. Nanomaterials are classified on the basis of various features

Chemistry of Nanomaterials. DOI: https://doi.org/10.1016/B978-0-12-818908-5.00009-3 © 2020 Elsevier Inc. All rights reserved.

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such as morphology, uniformity, dimensionality, and agglomeration into 0D, 1D, 2D, and 3D nanomaterials. Due to having different properties, these nanomaterials are employed in a number of applications related to nanotechnology including for the diagnosis of several diseases and in the fabrication of different processors having integrated circuits in small sizes and with better efficiency. Owing to this diminishment, laptops are becoming lighter, cell phones are getting tinier, and an aspect of optical fiber has replaced bundles of heavy copper wire resulting in an advancement in the way of information technology [2].

9.1.1

Dimensionality

Nanomaterials are classified into different types on the basis of their different features that are described as:

9.1.1.1 Zero-dimensional nanomaterials The materials included in this category have all three dimensions negligibly small. They have distinct energy levels and so are known as artificial atoms or quantum dots. Metallic nanoparticles include silver and gold nanoparticles; however, in case of semiconductor nanoparticles, quantum dots of Cadmium selenide (CdSe) and cadmium sulphide (CdS) are included. Nanoparticles may exhibit different shapes like cubic, polygonal, spherical lying in the 1
50 nm range. One of the most common examples is that of fullerene, which is considered to be the smallest and most stable structure due to its symmetric structure. The shape of the fullerene molecule is analogous to that of a soccer ball. As a result of weak intermolecular interactions, the particles are free to rotate. Due to its 0D structure, fullerene possesses minimum surface energy [3]. 9.1.1.2 One-dimensional nanomaterials The materials included have two dimensions in the nanometer scale and one dimension is larger compared to the other two, meaning that they possess micrometer scale lengths and nanometer range diameters. Examples include nanotubes, nanofibers, and whiskers of metals or oxides, etc. They exhibit higher aspect ratios and larger surface areas, which make them useful for nanocomposites. 9.1.1.3 Two-dimensional nanomaterials These include materials with one dimension in the nanometer scale and the other two being much larger than the first in the micrometer scale. Graphene, nanofilms, nanosheets, nanoplatelets, and nanoclays are examples of 2D nanomaterials. Thin films are developed by different deposition methods and are employed in several fields such as electronics, sensor devices,

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and magneto-optical devices [4]. The coating or area of the developed nanofilms is of the order of some square centimeters while the thickness lies in the 1
100 nm range.

9.1.1.4 Three-dimensional nanomaterials All three dimensions of materials involved in this class lie outside the nanometer range. These are also mentioned as nanocrystals or equiaxed nanoparticles. They possess all dimensions greater than 100 nm. One of the best examples is nanostructured bulk materials, which have no dimensions in the nanometer scale, but are divided into equal parts being in the nanometer scale or they comprise of various arrangements of crystals in nanoscale, they are also known as bulk nanomaterials. In broader terms, bulk nanomaterials can have a diffusion of nanotubes, nanowires, or multiple nanolayers in the matrix. On the basis of morphology, nanomaterials are divided into high aspect ratio and low aspect ratio nanoparticles. High aspect ratio nanoparticles include shapes like zigzags, belts, and helices, while in the case of low aspect ratio nanoparticles, cubic, oval, and spherical-shaped nanoparticles are included [5].

9.2

Quantum dots

Crystals having particle sizes smaller than about 10 nm are called quantum dots in which the confinement of electrons occurs at largely separated energy levels. Quantum dots (QDs) are considered as a specific type of semiconductor in which it is possible to tune the electrical conductivity either by light exposure or by the variation of voltage. A lesser number of electrons (#100) are free in QDs and so are termed atomic clusters. The confinement of electrons in QDs occurs in various directions. Thus depending on these confinements, QDs are classified into three types, namely self-assembled, planar, and vertical QDs. Typically, for self-assembled QDs, pyramidal or lens-shaped structures having sizes of 10 nm are found. However, in the case of vertical and planar QDs, the dimensions are 100 nm and the sizes are 10 nm [6]. The chemical structure for QDs shows variations in the bandgap with a diminishment in dimensions. It is found that the bandgap of QDs is larger as compared to that of bulk semiconductors owing to light exposure or variations in the voltage. Dimensionality related to QDs mainly relies upon the material from which it is made. Mostly, by manipulating different methods, QDs are formed from CdSe, Zinc Sulphide (ZnS), and Cadmium Telluride (CdTe) compounds. Owing to the smaller sizes of Quantum confinement effects (QCEs) in bulk solids, the behavior of QDs differs and are categorized into three types corresponding to the semiconductor structures.

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1. When the confinement of QCEs occurs in three directions, QDs are formed that are 0D. 2. Confinement in two directions results in quantum wires that are 1D. 3. One directional confinement leads to quantum films or quantum wells that are 2D [7].

9.2.1

Applications

Quantum dots attract a lot of interest owing to their wide range of applications.

9.2.1.1 Optical applications A small filter that is made from quantum dots has been invented in order to be placed above LED or fluorescent lamps. This changes the shade of light from blueish to a more striking and attractive red shade. QDs can be held as a replacement for dyes and pigments as they are brighter and controllable compared to organic dyes. In the case of solar cells, the use of QDs enhances the efficiency of conventional semiconductors by 10% by giving out more electron
hole pairs for each photon. In portable devices, QDs could be the best choice as they produce light themselves without any need for an energy source, making them more energy efficient in devices where battery life is essential. QDs have dominancy over Liquid Crystal Display (LCD) and organic light-emitting diode (OLED) display technology due to their relatively smaller size and better resolution capabilities [8]. 9.2.1.2 Quantum computing In optical computers, the use of QDs can be manipulated same as the transistors had been used as the main component in electronic computers. The storage of binary digits occurs by individual ions, atoms, or molecules or by photons that are linked together behaving as quantum bits known as qubits [9]. One of their main advantages is that multiple tasks can be evaluated simultaneously using QDs. 9.2.1.3 Biological applications QDs have become a potential candidate for cancer treatment. For this, the dots are designed in such a way that they can accumulate in the specific part of the body where the tumor exists and then transfer anticancer drugs to that particular affected portion. They are efficient for targeted delivery as compared to conventional drugs in order to avoid the side effects resulting from the untargeted delivery of drugs (traditional chemotherapy) to the healthy potions of the body. In the biosensing field, they are used as sugar, DNA, and protein sensors. In bioimaging, they are used for single molecule tracking, in vivo animal imaging, in vitro imaging, and live cell imaging. Other different applications of QDs are shown in Fig. 9.1.

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DNA, Proteins sensors

Biosensing

Sugar sensors

Immuno-assays

In vitro imaging

Live cell imaging

Biological imaging

In vivo and animal imaging

Single molecule tracking FIGURE 9.1 Applications of quantum dots in biosensing and biological imaging applications.

9.3

Nanowires

A structure having a width or diameter in the nanometer range along with a large aspect ratio is known as a nanowire. Nanowires comprise of two dimensions in the nanoscale and a third dimension being larger than the other two. This leads to an entirely different way of electronic conduction related to nanowires as compared to that of bulk materials. In nanowires, the process of electrical conduction occurs both by bulk counterpart conduction and quantum tunneling phenomena [10]. Owing to their enhanced binging energy for excitation, large electronic state density, and large surface area to volume ratio, nanowires exhibit distinctive properties and applications in a variety of different fields.

9.3.1

Synthesis

Different approaches are used for the synthesis of nanowires. Mostly, vapor liquid solid (VLS) methods are employed for this purpose. In these processes, a liquid catalyst is used for drawing the components from the vapor phase into the solid phase. However, the chemical vapor deposition process or the laser ablation method are used for describing the synthesis method for specific experiment and this only leads to the manner owing to which production or transportation of vapor phase happens. For example, in the laser

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ablation method, the sample material is vaporized using laser light resulting in the formation of a vapor phase. Then with an upsurge in the nanowire growth, by means of a catalyst droplet, it is immersed and then deposited [11]. For the synthesis of silicon nanowires, gold nanoparticles are utilized as a coating material on the substrate. After coating, the process of heating occurs for melting the gold, resulting in the formation of small droplets. Then the vapor contained by the silicon, mostly silane, is made to pass above the substrate. The atoms of the silicon get separated from the silane and then absorbed in the nanoparticles at the vapor
silicon interface. The precipitation of silicon occurs from droplets to the substrate of gold till the saturation. The production of precipitate helps in building and lifting the catalyst apart from the substrate and leads to the growth of a vertical, wire-like structure [12]. Another method for the synthesis of nanowires is known as the template growth process. In the template growth method, the growth of nanowires occurs inside or above another structure that is in the nanometer scale. The introduction of a liquid or vapor phase occurs and then using cooling or the chemical reaction method, it is solidified with the solid phase supposing the shape and size of the surrounding material [13]. After deposition etching of substrate is frequently done to produce nanowires. The template growth process involves two commonly used methods, which are molten phase injection and electrochemical process. Apart from the VLS method and template growth processes, nanowire synthesis can also occur through lithography and solution chemistry. In most synthesis methods, the production of nanowires occurs atom by atom. This leads to distinctively defined crystalline structures along with the desired growth directions [14].

9.3.2

Properties of nanowires

Nanowires exhibit different magnetic, electrical, and optical properties that are explained as:

9.3.2.1 Magnetic properties The arrays of magnetic nanowires comprising needle shape structure have got significant importance in response to the perpendicularly lying magnetic recording. In the case of alumina templates, by adding nanowires into the pores of this template using the electrodeposition method it gives rise to the production of bit densities in the range of 100 Gbits. By applying a magnetic field in a direction that is parallel to the magnetic nanowire axis this leads to a coercive field being inversely related to the pore diameter. The squareness in hysteresis loop is also found to increase from 30% to approximately 100% with decreases in diameter of wire, that is, Dp [15].

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FIGURE 9.2 The variation of squareness of hysteresis loops of Co nanowire arrays with applied magnetic field (H) parallel (||) and perpendicular (\) to the wire axes: (A) nanowire diameter, 78 nm and (B) nanowire diameter, 18 nm. Reprinted from J. Sarkar, G.G. Khan, A. Basumallick, Nanowires: properties, applications and synthesis via porous anodic aluminium oxide template. Bull. Mater. Sci. 30 (3) (2007) 271
290, Copyright (2007), with permission from Springer Nature.

The variation in the squareness of hysteresis loop with magnetic field strength H applied in direction perpendicular and parallel to the axes of wire for variety of nanowires with different diameters in shown in Fig. 9.2A and B. By increasing the aspect ratio (length/diameter), a gradual rise in coercivity is seen. However, this change in coercivity is small when the ratio (length/diameter) is greater than a factor of 10 [17]. Furthermore, it has been recognized that on the basis of geometry, a difference lies in the local coordination number for altered sites on surfaces. This gives rise to a deviation of moment from local environment [18]. It is also found that the difference in the diameter of nanowires impacts surface energies related to different crystal planes. Through the enhancement of the diameter of nanowires, a significant difference in the surface energies of different crystallographic planes is observed. Thus the magnetic properties related to nanowires can be tuned by varying their diameter, which in turn will change their magnetization, remanence, coercivity, and squareness of hysteresis loop.

9.3.2.2 Thermoelectric properties A number of captivating electrical properties have been shown by nanowires. Their distinctive electronic band structure and electron density variation of state with diameter give rise to nanowires being considered as effective materials for different applications [16]. An enhancement in the Seebeck coefficient has been observed in metallic nanowires owing to the higher electronic state densities at the 1D edges of the sub-band perceived in the curve drawn between temperature dependence of resistance (R(T)) and temperature (T).

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FIGURE 9.3 (A) The R(T)/R (270 K) of 40 nm diameter Bi nanowires with different amounts of Te alloy concentrations and (B) carrier mobility ratio as a function of temperature of 40 nm Bi nanowires with different doped Te atom concentrations. Reprinted from J. Sarkar, G.G. Khan, A. Basumallick, Nanowires: properties, applications and synthesis via porous anodic aluminium oxide template. Bull. Mater. Sci. 30 (3) (2007) 271
290, Copyright (2007), with permission from Springer Nature.

And, it is not seen due to the lesser mobility of carriers owing to the boundary scattering effect [19]. Schemes of R(T)/T ratio calculated with respect to the temperature, T(K), for 40 nm sized Bismuth (Bi) and doped Bi nanowires with a small percentage of Tellurium (Te) are given in Fig. 9.3A. For similar nanowires, the value for mean carrier mobility ratio, μ (T)/μ (270 K), has also been calculated from R(T) along with temperature T dependent density measurements. In Fig. 9.3B, the mobility ratio, μ (270 K)/μ (T), dependent on temperature is shown, in which Nd represents the ion concentration of Te in ions/cm3. By investigations, the wave function of electron clearly found to be more localized with the decrement in nanowires diameter [20,21]. This occurrence is deliberated a significant factor that makes the nanowires preferable for exhibiting different thermoelectric properties.

9.3.2.3 Electron transport properties The electron transport properties related to nanowires are significant for both electronic and electrical applications and for interpreting the mechanism of carrier transport in 1D structures. The main parameters such as the diameter of the wire, its surface condition, and the crystal structure along with its chemical composition and orientation along the axis are found to affect the mechanism of electron transport in nanowires. Fig. 9.4A shows the current voltage (I
V) characteristic curve for Copper (Cu) nanowires at room temperature as well as at 4.2 K. The plot depicts the ohmic behavior.

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FIGURE 9.4 (A) I
V characteristics of metallic Cu nanowire at 4.2 K and at room temperature, and (B) I
V curves of the wire at 4.2 K and at room temperature recorded after the oxidation of the Cu nanowires. Reprinted from J. Sarkar, G.G. Khan, A. Basumallick, Nanowires: properties, applications and synthesis via porous anodic aluminium oxide template. Bull. Mater. Sci. 30 (3) (2007) 271
290, Copyright (2007), with permission from Springer Nature.

But its behavior differs when the Cu nanowires changes to Cu2 O by oxidation. When Cu nanowires sited in between two electrodes, two Schottky diodes are formed along with a double diode; Fig. 9.4B shows the formed I
V characteristic curve [22]. There are two types of electron transport mechanisms, one is ballistic and the other is diffusive transport phenomena, and both depend on the diameter and length of the wire used. The ballistic transport phenomenon deals with the flow of charge carriers without scattering owing to the larger mean free path as compared to the length of the wire [23]. Mostly, this type of transport mechanism is found at the contact junction along with other external circuits, wherever conductance has been quantized to an integral multiple of 2e2/h known as the universal conductance unit [24]. For metallic nanowires, the quantization of conductance occurs because the diameter of the nanowire becomes analogous to the electron Fermi wavelength. However, in the diffusive transport phenomenon, the mean free path of carriers is lesser as compared to that of the wire length. Thus the different scattering types influence the carriers of nanowires and their transport mechanism becomes the same as that of their bulk counterparts. The transport phenomena of super lattice nanowires have now gained a lot of attention due to their remarkable applications in various fields of interest. In super lattice nanowires, there exists a periodically modulated chemical composition and a crystal structure in QDs form along the axis of the wire. In between the QDs, the transport mechanism occurs by the quantum tunneling phenomenon, where regions of nanowires having a particular composition act as potential barriers.

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9.3.2.4 Optical properties By utilizing a number of different analytical techniques and optical characterizations, the optical properties of nanowires can be studied. The complex dielectric function (ε1 1 ε2) associated with nanowires that have been fixed in a host material are inferred by manipulating effective medium theories [16], which consider that both the host matrix and the nanowires work like a single material. In the case of composite materials, the absorption coefficient (k) and refractive index (n) are associated with ε1 and ε2 respectively. The complex dielectric function related to nanowires can be calculated using standard transmission and reflection measurements along with Maxwell’s equations. For nanowires, the variations in temperature corresponding to bandgap can be calculated by manipulating the measurements of the complex refractive index, which is known as a vital parameter in order to select the appropriate materials for different photonic applications. The basic information regarding plasmon frequency, the concentration of donor atoms as well as the concentration of charge carriers in nanowires can be gathered by evaluating the infrared portion of the spectrum related to nanowires [25]. Metallic nanowires show a remarkable plasmon absorption effect. It has been found that surface plasmon band energy show sensitivity to different factors including shape, particle size, interactions in between particles, and the surrounding medium [16]. The change from spherical to rod-like Ag nanostructure has seen to be resulted in two absorption bands rather than a single absorption band along with a high aspect ratio. The very first and second peaks have resulted because of transverse plasmon resonance and longitudinal plasmon resonance respectively as shown in Fig. 9.5 [26]. In Au and Ag nanowires, different surface plasmon modes depict the multipolar plasmon resonances that can be described using standing plasmon wave assumptions. In recent times, the appearance of multiple peaks has been found in plasmon absorption spectra for gold nanorods that are coated with sulfide [27]. 9.3.3

Applications of nanowires

The possible applications of nanowires in different fields include: 1. One of the most significant applications of nanowires are found in the medium of magnetic information storage. Different periodic arrays of magnetic nanowires are capable of storing a large amount of information in a small region. Their high aspect ratio contributes to increased coercivity along with the suppression of the onset of superparamagnetic limit being considered as an important factor in order to prevent the loss of information, which is recorded magnetically in between the nanowires. Nanowires are also used to fabricate the stable magnetic medium along with the packing density .1011 wires/cm2.

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FIGURE 9.5 Two plasmon absorption peaks for Ag nanospheres, nanorods, and nanowires. Republished with permission of Royal Society of Chemistry, from Oleate vesicle template route to silver nanowires, Xuchuan Jiang, Yi Xie, Jun Lu, Liying Zhu, Wei He and Yitai Qian, Copyright 2001; permission conveyed through Copyright Clearance Center, Inc.

2. The appreciable enhancement in the Seebeck coefficient related to nanowires enable them to become an important candidate for thermoelectric cooling systems as well as in energy conversion devices [16]. 3. Nanowires have a lot of potential for utilization in various electrical applications. Their efficient rectifying characteristics have been depicted by junctions corresponding to semiconductor nanowires like Gallium Arsenide (GaAs) and Gallium phosphide (GaP). The fabrication of numerous semiconducting devices including memory cells, inverters, switches, and junction diodes is done by manipulating the nanowire junctions [28]. In the case of field emission devices, nanowire arrays can be good for a dominant decrement in work function of surface electrons. Moreover, nanowire junctions are capable of performing different logical operations that make them able to be used as logic gates [29]. 4. Nanowires are considered as an effective candidate for several optical applications. The p-n junction related to nanowires has been acknowledged to be able to emit light owing to their electroluminescence or photoluminescence properties. The high conductivity and large surface area along the length related to nanowires is appropriate for organic and inorganic solar cells. Also they can be considered for laser applications. Metallic nanowires can be utilized for different barcode tags and optical read outs [16].

9.4

Nanophotonics

Nanophotonics is the fastest emerging field of nanotechnology in which materials are made of atoms that are arranged in a sophisticated manner at the nanolevel. An intriguing fabrication and nanoassembly of photonic structures

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at the nanoscale level is the major challenging aspect in this field. Switching speed and propagation delay between transistors are the main reasons of conversion from electronics to photonics. Photonics provide a higher resolution of mobile phone display, which requires a higher bandwidth to function. In addition to this, electromagnetic interference resistance and power consumption per unit volume both also require an appealing photonics. Thus this roadmap of nanophotonics revolutionized the technology by introducing a sufficiently large bandwidth [30]. New nanophotonic materials, being smaller, having a more rapid response, and consuming less energy, challenge the existing photonic devices. However, this resonant technology faces many difficulties such as problems related to field identity; one of which is the development of applications to commercial scale and another is the hidden role and value of this discipline. In nanophotonics, a change in the structural/chemical or physical identity of nanostructured matter occurs whenever light interacts with these nanomaterial structures at wavelength or subwavelength scales [31]. Nanophotonics covers all the phenomena of nanooptics such as absorption, emission, propagation, and scattering in the complex nanophotonic structures [32]. The most recent applications include the nano optical sensor. The discipline seems to induce disruption in numerous fields, including quantum optics, data storage, telecommunications, nanoscale imaging, medical therapies and diagnosis, nano-tagging, photonic-based molecular scale sensors, nano optical sensors, LED and solar cell applications, and for new processing techniques (including lithography and fabrication) [33].

9.4.1

Optoelectronics and microelectronics

The physics of materials and devices introduce applications that are based upon microelectronics and optoelectronics. The technical capabilities in these devices demand low power consumption and high operational speed. To meet these demands, single devices should be built as small-scale units. Nanoscale-controlled devices need special techniques for their unique fabrication and perfectness. Etching, implantation, metallization, and patterning are processes used to control the fabrication of nanodevices. The objectives require for these processes are the establishment of basic material properties, modelling the processes that occur in devices and to analyze the operational limit for these devices. There are some applications such as communication technology that are based upon both fields of microelectronics and optoelectronics. Microelectronics almost contains Si-based technology. Silicon is a semiconductor material that possesses high mechanical stability and superb thermal and electrical conductivity. Silicon wafers can be grown easily with larger area than any other semiconductor material. The oxides of silicon can resist high voltages, while its fabrication can be done by different methods. The computer (an electronic based technology) whose main part is dynamic random-access memory (DRAM), which have components such as silicon

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metal-oxide semiconductor field-effect transistors (MOSFETs). These DRAM devices are considered to be conventional that obey the laws of classical physics at 256 Mbit of chip operation till 20th century while its processing speed become larger at transition regime with 4 Gbit whose boundary meets the quantum regime in the 21st century. In the meanwhile, the ultrahigh integration of these miniature devices requires changes in the trends of lithographing techniques such as molecular beam epitaxy, ultrathin layer fabrication, super lattice fabrication, qualitative electron beam and x-ray microscopies, and lithographic microstructures. Due to advances in these lithographic techniques, quantum semiconductor heterostructures are developed. These heterostructures include 1D QDs and 2D quantum wires, etc. Lithography and etching methods are suitable for the microscale fabrication of devices. New advancements in epitaxial techniques (since the late 20th century) such as metal-organic vapor phase epitaxy, metal organic molecular-beam epitaxy, and many others are used for atomic level fabrication. In microelectronics, although there are III
V semiconductor compounds that could facilitate ultrahigh integration, the results are not better than those of silicon wafer as a unique semiconducting material [33]. The reliability of electronic devices were revealed in the mid-20th century when the first transistor was invented in 1947 and the first semiconductor laser introduced in 1962 [34]. On the other hand, optoelectronics uses the wave behavior of light in various communication technologies, and, hence, complements microelectronics. The transmission of information in optoelectronics by means of optical fiber to a high storage capacity device like a laser disk requires further advancement in this technology [33]. Optoelectronic devices consist of p- and n-type regions similar to p-n-junctions in semiconductor devices. A high conversion efficiency is obtained by optimizing the light absorption and emission. Optoelectronic devices are usually single devices such as laser diode (LD) and LED. Electronic devices are generally integrated circuits with millions of transistors interconnected. Moreover, electronic devices can be fabricated into single devices. The absorption and emission of light occurs in optoelectronic devices. Unlike electronic devices, optoelectronic devices consist of a variety of elements such as Aluminium gallium indium phosphide (AlGaInP), Gallium Indium Arsenide Antimonide (GaInAsSb), Indium Gallium nitride (InGaN), Indium gallium arsenide (InGaAs), gallium aluminum arsenide (GaAlAs), indium gallium arsenide phosphide (InGaAsP), etc., that are used in LASER diodes and Indium gallium nitride (InGaN), Aluminum nitride (AlN), Zinc selenide (ZnSe), Gallium arsenide (GaAs), Gallium phosphide (GaP), Aluminium Gallium Arsenide (AlGaAs), yttrium aluminum garnet (YAG), etc. [34]. Some optoelectronic devices, detectors, LASER, thermophotovoltaic devices are working in the mid-IR region, thus, also known as mid-IR devices. The growth of low bandgap material used for these types of devices include antimony (Sb)-based III-V alloys, binary

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alloys, quaternary alloys, and pseudo ternary alloys containing Indium phosphide (InP), Indium antimonide (InSb), Indium arsenide (InAs). All optoelectronic devices majorly consist of semiconducting materials. The most astonishing and world renowned material is the graphene (a semiconducting material) which has revolutionized the optoelectronics devices. Due to the blessing of graphene, the sensitivity, response rate, intensity, and piezoelectric property of many devices such as detectors and biosensing devices have been modified and improved. Moreover, the physical and chemical properties of semiconducting devices that are used in solar cells and biosensing have been modified [34]. Optoelectronic devices are not only used in semiconductor epitaxy, microfabrication technology, and device fabrication, but also in fiber-optics telecommunications, display panels, image recorders, optical storage, and solid-state lighting. Recently, spin optoelectronic devices have been introduced where the practical applications of spin polarized LED lights can be extended to LASER. The main difference between optoelectronic and electronic devices is that electronic devices obey Moore’s law, while there is no any universal law for optoelectronic devices. Moore’s law states that the number of transistors will increase every two years, while in optoelectronic devices such as LD, this depends upon customer demand and LEDs follow Haitz’s law. Moreover, the device material, structure, and feature size, and the complexity of electronic and optoelectronic devices are always different [34]. A list of optoelectronics and microelectronics devices introduced yearly is given in Table 9.1 The images recorded by smartphone devices or by digital camera are due to the usage of an array of silicon photodetectors or sometimes coupling with complementary metal-oxide semiconductors or with charge-coupled devices. The perovskite photodetector provides a sensitive platform to explore new avenues toward optoelectronic devices. However, the perovskite photodiode encounters the same challenges as faced by perovskite solar cells in term of competition with other technology, stability, and toxicity [35]. The photoelectronic effect is the basis of solar cells, which could be replaced by the photothermal effect. Due to photovoltaic cell, the photocurrent can be generated in graphene-metal contact. The intrinsic optoelectronics effect of graphene becomes dominant above room temperatures up to 10 K. In the advancement of the optoelectronic field, graphene played a major role due to its multiple electron
hole pair generation for single energy photon excitation and the high mobility of carriers. The only drawback faced by this technique is the low photoresistivity that arises due to low optical absorption. However, due to the addition of a new material between graphene and silicon, the photodetector responsivity of these devices increases. For a high responsivity of photodetectors, 2D crystals are introduced for future devices including Tungsten disulfide (WS2), Molybdenum disulfide (MoS2), and black phosphorous. The low photoresistivity is the prime issue

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TABLE 9.1 List of optoelectronics and microelectronics devices. Sr. #

Optoelectronics devices

Microelectronic devices

References

1

Detectors

Silicon-based technology

[33,34]

2

LASERS

Si-MOSFETs

[33,34]

3

Thermophotovoltaic

e-Beam lithography

[33,34]

4

LASER diode (LD)

Metal organic molecular beam epitaxy

[33,34]

5

Light emitting diode (LED)

[34]

6

III-V

[34]

7.

Pin-photodiodes

[34]

of graphene, which can be solved by the integration of a graphene photoconductor onto a silicon-on-insulator due to which the absorption and photocurrent increases [36]. As for electrical wiring, optical communication has replaced that; an example of which is the integration of complementary metal-oxide semiconductors (CMOS) with silicon. This is the integration of the optics and electronics fields. For this, an optically absorbing material is used, which is graphene. Graphene has proven to be an efficient photodetector. It was first observed in a back-gated transistor [36]. In addition to the characteristics of ultrawide band operation and CMOS compatibility, another benefit of using graphene is that it provides new alternative approaches for optical interconnects. Not only this, but it is also simple to use and of low cost. Moreover, it improves the devices by its operational high speed with less power consumption and enables to give small device foot print [36]. Graphene photocatalytic and photodetector proves to be very efficient close to metallic electrodes for Infrared radiation (IR) region [36]. The most recent application of optoelectronics is the optical tweezer based on the principle of the photorefractive effect. To enhance the technological potential, various techniques faced many problems. The techniques are photovoltaic or photorefractive tweezer (PVT). For the enhancement of plasmonic fluorescence, major developments have been done for the fabrication of metal Nanoparticles (NPs), photonic devices, and for the patterning of biological specie at the micro/nanolevel. Other techniques for PV field fabrication of micro and nano droplets have been introduced. Thus the developments in the PVT technique are proved to be highly promising and simple. However, the resolution of fabrication, reproducibility, and tuning of the deposition process of pattern should be at nanoscale [37].

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PART | 2 Interactions in nanomaterials

Basic principles

Optoelectronics and microelectronic devices are based upon quantum structures that follow the quantum principles of light. The electrons in semiconductor devices encounter effective mass usually less than the free electron mass. Hence the de Broglie wavelength of electrons could be found by [33]; λ5

9.5

h h m 5 pffiffiffiffiffiffiffiffiffiffiffi 5 λo   p m 2m E

Magnetic nanostructures

Nanoscience and nanotechnology have mesmerized researchers in the past few years for introducing magnetic-based nanostructure devices known as magnetic nanostructures. These nanostructure devices are available in different dimensions. The low dimensional structure, small size, and enhanced magnetic properties of nanoscale materials enable further exploration of novel devices. In nano-fabrication, the physical dimension size is reduced to different characteristic length scales such as superconducting coherence length, carrier mean free path, spin diffusion length, magnetic domain wall length, etc. The magnetic structure size and length of the physical sample plays an important role for the confinement and proximity of materials. The small size devices reveal the explosive magnetic recording density at rapid speed. This requires the pre-growth and advanced patterning of the sample using the conventional lithography technique. Hence the technology can be used in a plethora of applications such as in magnetic switching, magnetic random-access memory, and pattern recording media. Theoretical and experimental works have been done so far for the characterization, fabrication, and ordered arrangement of magnetic materials. Challenges faced in relation to ordered and unordered magnetic materials in terms of their fabrication and physical properties diverted the attention of scientists to other suitable approaches, that is,. from conventional to nonconventional approaches of fabrication and from pure forms of materials to their complex forms (from metals to metal alloys, etc.) [38]. Recently, enhanced/ suitable properties and diverse applications in the bio-catalysts and biomedicines attract the researchers. As discussed earlier, nanostructured materials display a novel set of physical characteristics that distinguish it from fabrication at the nanolevel. However, it has faced some challenges in gaining monodisperse magnetic nanomaterials. Having the dipolar interaction at nanoscale, there are many novel strategies apply to control the nucleation and magnetic nanomaterials (MNMs) growth process at the atomic level. The chemical synthesis of MNMs uses the liquid phase for fabrication. Through the liquid phase, the size, composition, and structure of the resulting nanomaterials can be controlled. As Magnetic Nanoparticles (MNPs)

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behavior is size-dependent which depends upon the direct correlation between the nucleation and growth process. 2D-shaped nanoparticles could be obtained by the employment of a surfactant. Hence the key point for high performance is that the size of metal nanoparticles (MNPs) is reduced to critical value and a single domain with maximum coercivity, which ultimately depends upon size [38]. Magnetic NPs exhibit different magnetic behaviors to bulk magnetic materials due to their magnetization cycle in hysteresis loops. MNPs (Iron (Fe), Cobalt (Co), Nickel (Ni)) are representative nanostructures, but show limited stability under ordinary conditions such as Fe. Some metal alloy nanoparticles show high chemical stability and some of them (FePd NPs) show superparamagnetic properties at room temperature. Additionally rare-metal nanoparticles with magnetic nanostructures are large and can be formed by annealing methods [38]. Metal oxides and metal carbides are used as reliable magnetic materials. Iron carbides ðFe2 C2 ; Fe2 C; Fe3 CÞ are used in biomedicine applications due to having higher magnetic saturation and stability than iron oxide. Fe5 C2 NPs are formed in the 20 n size-range. All Febased carbides hold weak ferromagnetic properties. The diverse nature of metal carbides with different phases could be obtained through the synthetic technique by tuning atomic bonding [38]. Conventional and nonconventional methods for the fabrication of these materials have been applied to get better results depending upon stability, biocompatibility, monodispersity, and low cost. Physical, chemical, and biological methods have revolutionized this field. The interaction of these devices with other materials may tune their characteristic properties and, hence, can be used in several fields. There are two categories of magnetic materials depending upon their size comparable to magnetic domain, namely single domain and superparamagnetic material [39]. Some common magnetic materials fabricated by lithography technique at nanoscale size ranging from 10 nm to 60 nm are shown in Table 9.2.

9.5.1

Synthesis

The synthesis technique involves physical, chemical, and biological techniques. A simple example of a physical method for making MNPs is the mechanical grinding method in which bulk materials are crushed to make smaller-sized particles via mixing mill, ball mill, and many other methods of grinding. But this is not beneficial as the particle sizes and shapes do not lie in the nano-dimensional range. The average particle size formed by mechanical alloying is 10 nm, whereas mechanochemical alloying obtains a particle size of about 40 nm [39]. Chemical methods include of microemulsion, hydrothermal sol
gel, and precipitation [39].

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TABLE 9.2 Common magnetic materials fabricated by lithography technique at nanoscale size ranging from 10 nm to 60 nm. Sr. #

Magnetic materials

Size (nm)

Fabrication technique

Applications

Ref.

1.

Co, Fe, Silica

10 nm

Electron beam lithography combined with direct writing

Fe, Co used as a catalyst

[40]

Silica used in biosensing

2.

Au/Co, NiFe

20 nm

Electron beam with etching process

Biosensing applications

[40]

3.

Ni

25 nm

Nanoimprinting

Catalytic reactions

[40]

4.

Fe, Co, Ni, NiCr, MnNiAl

30 nm

Lithography and lift-off

Catalytic reaction

[40]

5.

Fe, Co, Ni, NiCr, CoNi/ Pt, CoCr

40 nm

Lithography and etching

As catalysts

[40]

6.

Ni, Co, Fe, NiFe, CoFe, MoNiFe, CoPt, Fe3O4, NdFeB, FexSi1x

55 nm

Electron beam and lift-off

In the field of biomedicine/ drug delivery and magnetic separation

[39]

7.

Fe, Co, Ni, NiCr, NiFe, Co/Cu, CuO

60 nm

Step-growth

All used as catalysts in reactions, biomedical applications, and CuO in energy

[39,40]

8.

Co, NiFe

88 nm

X-ray

Catalysis

[40]

[38] [40]

There are different advanced synthesis techniques for the fabrication of optoelectronics and microelectronics. Some of these are lithography techniques and others include the step growth method, shadow mass, radiation damage, focused ion beam milling, self-assembled nanostructures, nanotemplates, sphere lithography, nanochannel glass template, etc. [40]. In the oxidation precipitation approach, Fe3 O4 NPs are formed in a spherical shape and uniform size of about 8.9
10.6 nm. CoFe2 O4 powder can be achieved through coprecipitation with an average size of 12 nm. The main advantage of this process is in it being facile and easy to produce NPs, but it

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could be limited by temperature, pH, time and concentration, which should be controlled precisely [39]. To obtain the desired size, shape, and composition of NPs, an effective approach is the hydrothermal method. It follows certain steps, including hydrothermal oxidation, hydrothermal reduction, and hydrothermal precipitation. In this process, raw materials are gained and the product formed is pure. This approach is also limited to high temperature and pressure and reaction conditions will determine the size and morphology of the product. Characterization techniques include transmission electron microscopy (TEM), X-ray diffraction (XRD), and vibrating sample magnetometer (VSM). In Lithium-ion batteries, the Li storage capability is enhanced by the αFe2 O3 synthesized by hydrothermal method [39]. The next approach is the sol
gel method, which uses specific reaction conditions (pH, temperature, reaction time, and calcination) and the product obtained can be modified with high crystallinity and purity. However, it may take a long time and uses toxic solvents. Nevertheless, by this method, Fe2 O3 photocatalysts can be obtained [39]. NPs formed by microemulsion process have excellent dispersity. The particle size can be controlled by the reaction parameters [39]. Other than physical and chemical processes of synthesis, the biological method is considered to be more reliable than any other. This is a new way to synthesis metal NPs. The method is clean to use, highly efficient, reliable, and environment-friendly as compared to others. Here bio-organisms including bacteria, viruses, yeast, fungi, and actinomycetes are used to synthesize metal NPs. These metal NPs are used in cosmetics and many other applications as well. Plants play an important role in making metal NPs as they have stability and rapid reduction efficiency as compared to those prepared by organisms (fungi/bacteria) [39]. The fabrication of magnetic nanostructures with the lithography process having different steps is shown in Fig. 9.6. The first step is the exposure to radiation of a selective area of resist using a mask. Second, the process is followed by the development of positive and negative resists. Then these resists undergo an etching process or postdeposition/electrodeposition. Etching is another fabrication process that can be done by using two approaches. One is dry etching and the other is wet etching. In dry etching, an active specie is used that reacts with the surface material. The advantage of dry etching is that it produces straight and sharp pattern edges and provides better resolution. Hence it is typically used for the patterning of ultrafine nanostructured materials. The post lithography technique involves a lift-off process where the desired deposited portion on the substrate is left behind. In this crucial process, only directional deposition will be preferred. The bottom layer of the resist is developed more efficiently than the top. Finally, electrodeposition, also known as growth technique, is used. With the help of an electrical

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PART | 2 Interactions in nanomaterials

FIGURE 9.6 The lithography process developed into positive and negative resist synchronized with (A), (B) etching (C) the lift-off process, and (D) is the postdeposition. Reprinted from J.I. Mart´ın, J. Nogue´s, Kai Liu, J.L. Vicent, I.K. Schuller, Ordered magnetic nanostructures: fabrication and properties. J. Magn. Magn. Mater. 256 (2003) 449
501, Copyright (2003), with permission from Elsevier.

current, materials can be deposited by electrolyte, which requires ambient temperature and pressure. Moreover, it is low cost, simple, and reliable, yielding better aspect ratios than the lift-off process. The lithography process is limited by the wavelength of radiation. It is crucial to indicate the source as either electron beam, optical, or X-ray lithography. UV-based lithography is more common due to its shorter wavelength. However, for magnetic systems, the mentioned techniques are not sufficient [40]. One of the most versatile fabrication techniques is electron beam lithography similar to photolithography; the only the difference is that an electron beam source is used (such as Scanning Electron Microscope (SEM)/ Transmission Electron Microscope (TEM)) instead of photons and an electron-sensitive resist is used instead of a photoresist. Moreover, this technique also involves the conjunction with general lithography techniques such as lift-off, mask technique, etching process, electrodeposition or electroplating process, and direct writing. An SEM image of a magnetic structure of an Ni-array with average dimensions of 75 nm diameter, 700 nm height, 100 nm spacing, and 9.3 aspect-ratio is shown in Fig. 9.7 [40]. It produces a

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FIGURE 9.7 An SEM image of an Ni-array of magnetic structure with dimensions of 100 nm spacing, 700 nm height, and 75 nm (average) diameter and an aspect ratio of 9.3. Reprinted from J.I. Mart´ın, J. Nogue´s, Kai Liu, J.L. Vicent, I.K. Schuller, Ordered magnetic nanostructures: fabrication and properties. J. Magn. Magn. Mater. 256 (2003) 449
501, Copyright (2003), with permission from Elsevier.

magnetic array area of about 200 3 200μm2 . The versatility of this technique allows extremely small device fabrication like quantum magnetic disks and magneto-resistive magnetic random-access memory (MRAM). X-ray lithography is similar to electron lithography, however, an X-ray source is used instead of an electron source and correspondingly an X-ray mask is used. Due to this, the mask can be applied repeatedly. There is the facility of synchrotron radiation for the exposure of a sample. A magnetic array of well-defined area of about 5 3 5mm2 can be obtained. This technique provides better sensitivity, resolution, and well-defined pattern. However, the use of synchrotron radiations makes this technique disadvantageous [40]. Another technique is holographic lithography, which is used to make more complicated patterns, also known as interference lithography. This is also similar to X-ray lithography, the only difference is that a LASER light is used as a source, which can be divided by a beam splitter into two coherent beams of incident light. These two incident light beams fall on the substrate from opposite directions and, hence, form an interference to form more complicated fabrication patterns. It covers a large area of 250 3 250mm2 with a size up to 30 nm, which make this technique advantageous. However, the main limitation in magnetic arrays is due to the interference pattern [40]. To obtain ordered magnetic array nanostructures, two different techniques, that is, atomic force microscopy (AFM) and scanning tunneling microscopy [41] are used. In these methods of fabrication, an element size of

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PART | 2 Interactions in nanomaterials

about 10 nm can be achieved, but poor reproducibility, slow process, and small fabricating area are drawbacks of these scanning probe techniques [40]. Whereas the poor reproducibility and poor contamination can be overcome by electrochemical cells. Moreover, the desired magnetic structure can be modelled by the step-growth process. Nanoimprints are used to overcome several disadvantages such as cost, slow process, small area of patterning, etc. Standard lithographic techniques produce an affect over the substrate so a shadow mask is used to overwhelm this affect. A denser array of dots and capability for direct writing is the credit that goes to focused ion beam milling, but the process is slow and costly. Other techniques involve radiation damage and pseudo-ordered structures [40].

9.5.2

Properties of magnetic nanostructures

As magnetic signals are small, it is difficult to analyze them. For this purpose, researchers devised a technique called the averaging method in order to analyze the magnetic properties of a large array of “identical’’ nanoparticles. Different averaging techniques for characterization include vibrating sample magnetometry (VSM), superconducting quantum interface device (SQUID), alternating gradient magnetometry (AGM), Ferromagnetic resonance (FMR), scanning magneto-resistance (SMR), scanning hall microscopy, torque magnetometry, and magneto-optical Kerr effect or Brillouin light scattering (BLS). However, other techniques are involved in the study of single nanoparticles such as micro-SQUID (μ-SQUID), Hall bars, magnetic force microscopy (MFM), Lorentz microscopy, electron holography, spin polarized STM, magnetostriction AFM, magnetic transmission X-ray microscopy, focused Magneto-Optical Kerr Effect (MOKE), The different parameters to study an array of magnetic nanoparticles for characterization are [40]: 1. In a polycrystalline structure, small-sized atoms show that the system consists of a finite number of grains. Thus the average of the grain properties is not possible due to regular variations in the finite size grains and orientation among elements. 2. Random defects and various types of roughness of edge shows that elements undergo small differences might be due to the different grain structure or lithographic techniques. 3. These random defects are primarily important for magnetization switching as shown in Fig. 9.8. 4. The magnetic dipole (Hd ) created among the elements of magnetic nanostructures is given by Hd B

2mr 2m  2 2 goes to 3 ðr .. lÞ r l=2 

½r 2 2

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FIGURE 9.8 The distribution of magnetic coercivity in macroscopic and microscopic magnetization as the different dots (symbolically similar) show the magnetic reversal due to an applied field. Reprinted from J.I. Mart´ın, J. Nogue´s, Kai Liu, J.L. Vicent, I.K. Schuller, Ordered magnetic nanostructures: fabrication and properties, J. Magn. Magn. Mater. 256 (2003) 449
501, Copyright (2003), with permission from Elsevier.

Where Hd is the field dipole, m is the moment, r is the distance, and l is the length. Single dots resulting from arrays have magnetic properties, which depend upon the balance of exchange energy, magnetic anisotropy and demagnetizing field. Some anisotropic magnetic nanomaterials exhibit anisotropies such as crystalline, shape, surface, interface, and other induced anisotropies. Examples are bulk Fe, hexagonal close-packed (HCP) Cobalt, etc. Demagnetization in magnetic materials, proportional to the property of magnetization, depends upon the geometry of the magnetic material. While the geometry of the material includes size, shape, aspect ratio, and thickness. However, the elliptical shape of magnetic materials shows homogeneous demagnetization fields. As a micromagnetic study shows, all dot magnetic nanostructures have inhomogeneous demagnetization. But by approximation, homogeneity in demagnetization can be achieved. Shape anisotropy is considerably more important for the investigation of magnetic structure, especially in polycrystalline structures [40]. Ferromagnetic materials possess magnetic domains that reduce demagnetization, and gain magnetostatic energy. Magnetostatic energy is a property of the magnetic domain forms due to decreased magnetizing fields. This energy is always greater than the energy used during the alignment of magnetic domains. It is noteworthy that the magnetic domains align due to a balance of exchange energy and magnetization energy. Magnetization energy is basically dependent upon the magnetic moment and shape. While the alignment of magnetic domains in magnetic nanostructures mainly focuses on two

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PART | 2 Interactions in nanomaterials

critical length scales; one is the critical single domain radius (RSD ) and other is the domain wall thickness (δ). Magnetic nanostructures with sizes smaller than the δ will always remain a single domain. If the single dots in volume V having anisotropic energy comparable to the thermal energy then they become superparamagnetic materials [40]. The effects of interaction among single dots should never be ignored as they are due to changes in the coercivity, which dominates the shape anisotropy and small crystallines. Moreover, the magnetization state of the single dots remains, even at zero external flux at remanence [40]. The stray fields of the single domain observed by electron beam are much larger. Stray fields rely on the magnetization state of the dots. In square and hexagonal arrays, the dipole interaction is isotropic, but the square array of perm alloy shows the different case by exhibiting the anisotropic in plane coupling. The spin arrangement and size of nanostructures ensure the minimum energy of the system. The magnetic reversal state is measured by Kerr microscopy, and can be obtained by the precession of magnetization, while the precession state depends upon the symmetry of the underlying equilibrium spin configuration. The theoretical approach used to probe the magnetic reversal of a nanostructure is governed by micro-magnetics. The other theoretical approach than the micro magnetics are used to mention arrays of sub microns, the properties of single domain element array coupled magnetically, to analyze the geometry of the coupling array and to predict the magnetization and magnetic susceptibility of the interacting system. There is simulation-based software such as Mote Carlo simulation used to investigate the magnetization switching of dots. Essential factors to magnetic NPs ranging in size from 1 nm to 100 nm are surface effect, quantum size, small-size effect, and microscopic quantum tunneling. The small-size effect limits the single domain structure and superparamagnetic limit. While for bigger magnetic nanoparticles, it limits the multidomain structure [39].

9.5.3

Applications of magnetic nanostructures

The unique magnetic properties (by tuning the size and shape) of NPs enable them to show rapid development in the fields of synthetic chemistry, nanotechnology, and surface chemistry and modification as catalysts used in biotechnology, biomedicine, and many other thermoelectric materials as shown in Fig. 9.9 [39]. The real-time analysis of MNMs used in biomedicine, gene, drug delivery, etc., shows unique magnetic properties under the applied field with unlimited penetration. Several biomolecular hydrophilic interactions reveal the accessibility, stability, and biocompatibility of MNMs before being applied to the clinic-level. After modification, MNMs could detect/target

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FIGURE 9.9 Shows the novel applications of magnetic nanomaterials in various fields. Reprinted with permission from K. Zhu, et al., Magnetic nanomaterials: chemical design, synthesis, and potential applications. Acc. Chem. Res. 51 (2) (2018) 404
413, Copyright (2018) American Chemical Society.

biological substrates. The imaging-guided photothermal therapy/drug delivery and Magnetic resonance imaging (MRI) are the prominent applications of MNMs in research laboratory which play a leading role due to the irreplaceable advantages. In MRI, several nanoplatforms could serve to kill tumor cells based upon radiotherapy/photothermal therapy or chemotherapy, and hence, lead toward cancer therapy [38]. These MNPs are used in biosensing devices. There are different categories of biosensors including immune-sensors, electrochemical sensors, and DNA/RNA sensors. The performance of the different biosensing devices depends upon the size, shape, and morphology of the NPs, which can be tunable [39]. This provides low cost and high sensitivity to biosensors. In order to monitor anatomical details or tissue morphology, high resolution MRI is used. There is an advanced clinical option to replace the contrast agents used in MRI probes with MNMs (with higher magnetization). For this purpose, Fe5 C2 NPs are used, which are tunable and provide suitable colloidal stability and, hence, prove to be excellent contrast agents for MRI [38]. In order to monitor anatomical details or tissue morphology, high resolution MRI is used. There is an advanced clinical option to replace the contrast agents used in MRI probes with MNMs (with higher magnetization). For this purpose,

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PART | 2 Interactions in nanomaterials

Fe5 C2 NPs are used, which are tunable and provide suitable colloidal stability and, hence, prove to be excellent contrast agents for MRI [38]. Real-time monitoring shows that MNPs also assists in drug delivery to specific target sites and are used in imaging guided drug delivery. Fe5 C2 NPs provide a reliable nanoplatform for theranostics. Radiotherapy/photothermal therapy induce efficient heat generation and are used in cancer treatment. This heat generation increases the temperature of the tumor cells without damaging neighboring cells, hence, killing the tumor cells. Fe5 C2 NPs with carbon shells are also used for this purpose. Fig. 9.10 shows images of mice bearing tumors before and after injection with Fe5 C2 2 ZHER2:342 and Fe5 C2 2 PEG. Au along with Fe are also used as optical components and when incorporated with iron carbide result in Janus NPs (JNPs) used for imaging-guided photothermal therapy. These NPs provide a multifunctional nanoplatform. Moreover, MNMs are also used as catalysts due to having high surface to volume ratios and susceptibility to applied magnetic fields; MNMs also act as catalyst supporters. Some examples include Pt-CNPs, transition metal oxides and carbides in electrochemical process, and FePt for efficient electrochemical processes with low cost. Fabrication of methanal with noblemetals species such as Pt-based concave nanotubes. Graphite carbon nitride (GCN) nanocomposites are used as renewable energy applications. Also, Febased carbides are used instead of petroleum than motor fuel [38]. The availability of bacteria, microalgae, and toxic ions in water cause environment pollution. MNMs are, hence, are used for the purification of

FIGURE 9.10 MRI of mice bearing tumors after intravenous injection of Fe5 C2 2 ZHER2:342 and Fe5 C2 2 PEG at different time points. Reprinted with permission from K. Zhu, et al., Magnetic nanomaterials: chemical design, synthesis, and potential applications. Acc. Chem. Res. 51 (2) (2018) 404
413, Copyright (2018) American Chemical Society.

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water, to kill bacteria, degrade dyes, etc. [38]. The primary concept to understand for nanomaterials is the phenomenon of quantum tunneling, which shows the penetration of a particle from the barrier with a particle energy lesser than the barrier energy. Macroscopic tunneling emerges due to the magnetization of microparticles. This quantum mechanical property of magnetic particles, thus, shows wave
particle duality. This property makes them suitable for use in transistors, microwave oscillators, and other electronic devices and total harmonic resonant tunneling devices. These particles, hence, become the building blocks of modern miniaturized electronic devices and the magnetic storage capacity of disks/tapes also increases in this way [39]. Other advantages of MNMs are that they can be used as catalysts, are readily available, and can be reused after reaction. Catalysts are normally covered by any magnetic NPs to from composite materials. Examples of some catalyst carriers are Cobalt Nanoparticles (Co NPs), Platinum-Cobalt (Pt-Co) catalysts, Cobalt tetraoxide (Co3O4), Cobalt oxide (Co-O), cobaltboron (Co-B) catalysts, ferrous oxide (FeO NPs), and silicon dioxide NPs used to raise the catalytic efficiency [39]. Magnetic NP composites can also be used in energy production. Due to global warming, researchers are moving toward materials that should be environment-friendly by using renewable energy resources. The thermoelectric property of thermoelectric composites can be increased by the addition of NPs, which improve thermal conductivity due to the quantum size effect. There are many thermoelectric nanomaterials including skutterudite (CoSb3), Lead telluride (PbTe) and Bismuth telluride (Bi2Te3). The performance of Li-ion batteries depend upon the electrode used, which could be replaced by nanomagnetic materials/hybrid materials with carbon that induce less toxicity in the environment [39].

9.6

Nano thermal devices

According to Moore’s law, thermal properties and transportation phenomena in nanostructured devices become dominant when the structure squeezes to nano-size. These devices have two contemporary requirements, one is high thermal conductivity for solving thermal heating of the sub-100 nm devices and the other is low thermal conductivity to deal with thermoelectric properties/applications. There are two types of carriers for thermal conductivity, namely electrons and phonons (packets of thermal energy), the latter has the dominant characteristics in thermal conductivity. Phonon group velocity, heat capacity, and phonon scattering are the properties studied in nano thermal devices. The phonon boundary scattering effect reduces the overall thermal conductivity process. As a device’s dimensions become smaller, the more complications in the study of its thermal properties arise. The geometry of devices tells us about the mean free path, which is comparable to the object size. When the object size is larger than the mean free path (L . λ),

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PART | 2 Interactions in nanomaterials

then the thermal effects are not significant and the thermal conductivity is limited by Umklapp scattering. When L . λ, Moore’s law is valid. When the object size, L, is smaller than or equal to the mean free path, then the thermal effects are dominant, Moore’s law is no longer valid, and the continuous energy model (used for bulk materials) is no longer used; such devices are called nano thermal devices. In nanothermal devices, the object size L is smaller than the mean free path λ (the distance between two consecutive particles). The basic theme of Moore’s law is that the number of transistors double every 18 months [42]. But present trends indicate that Moore’s law no longer exists. As size decreases, the electron flow path or mean free path decreases, therefore, more hindrance is created and, thus, resistance increases. Smaller transistor sizes show that resistance increases so thermal effects dominate and heat losses occur due to which the overall efficiency decreases. The size of a particle at nanoscale indicates the nano thermal effects. The nano thermal devices are looking for the thermal interface materials that could work against thermal resistance appear between the backside of electronic device at moderate temperature and in conventional environment [43]. Recent development for high performance integrated circuit electronic device such as solid-state LASER, flip flop circuits and power amplifier etc., there must be an increase in the densities and power level of these type of devices. As the size of devices reduces, heat dissipation becomes a challenge generated by hundreds of millions of transistors, hence, demanding a more efficient substantial thermal control system. Thermal interface materials such as carbon nanotube (CNTs) arrays, nanoparticles such as silver nanoparticles and nano silicon carbide particles are introduced to high conductivity in nano thermal devices [42]. Different analysis techniques are used such as optical microscopy and SEM for the analysis of morphology, thermogravimetric analysis (TGA) to characterize the manufacturing of nanoparticles, while softness/melting or degrading are considered by differential scanning calorimetric [42,44]. The development of nano thermal sensors that sense temperature in water entails high accuracy and precision. Various fabrication methods were used. The tungsten temperature sensor proves to be highly efficient due to its speed and modified fabrication based on FIB-etching and FIB-CVD techniques [45]. Some examples include CNT-based ultrasmall thermal sensors, temperature sensors based upon atomic force microscopy (AFM) cantilever, indium tin oxide, and fluorescent dye-based microchips. A 3D nanotool was built by FIB and chemical vapor deposition (CVD) that can be used as a temperature sensor in different environments. But it still faces some challenges. Bulk CNTs are also used for sensing temperature, but are limited to ultrasmall sensing sites [45]. The carbon nano thermometer is a liquid-filled thermometer used in a vacuum environment. Another approach is the AFM cantilever scanning

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thermal microscope temperature sensor. Resistant thermometry is used to sense changes in temperature in a single cell. Indium tin oxide (ITO) electrodes are limited by scanning temperature distribution technology [45]. The Thermal interface material (TIM) thermal properties can be evaluated by flash method with Infrared (IR) detector. Pure carbon metals show high thermal conductivity and are, thus, used to make structured carbon TIMs with high thermal conductivity, but they require a harsh environment for fabrication such as a temperature of greater than 700 C and a long time with greater than 5 min. CNTs can also be used to fabricate with Cu [45]. Over past 20 years, there has been an intense usage of scanning probe microscopy (SPM) in nanotechnology. Particularly, scanning thermal microscopy (SThM) shows outstanding measurement for arbitrary selective point with resolution in nanometers ranges. Pd-based resolution probes are also used as efficient temperature sensors that also work in vacuum, air, or liquid, but it still faces some ambient conditions in such a way that they enabled to exhibit thermal transport in a limited environment [46].

9.7

Nanofluidic devices

Commonly, the most accepted definition of nanofluidics is the study of the transport of fluids with dissolved species (ions/molecules) confined within the geometrical structure with at least one dimension in the 1
100 nm range. It not only involves the transport of mass, but also the chemical reaction of ions and molecules confined within the nanostructure device. There is a close relationship between the dimensions and size of nanofluidics structures, which give rise to many other unique phenomena. Water and aqueous solutions are the main focuses of the study of these devices. Due to its high polarizability and the complex nature of interactions with other molecules to form hydrated ions, these unusual properties make water critical to many process such as DNA hybridization and protein folding [47]. With the help of nanofluidic devices, various physical and chemical properties have become easy to analyze and investigate, which were previously difficult to study at the microlevel [48]. Although nanofluidics got its inspiration from microfluidics, nanofluidics possesses some unique characteristic properties including scaling of nanochannels/pores, which should be comparable to the range of interfacial forces, and most importantly the size of biomolecule. Hence nanofluidics gives us a platform where such novel transportation of molecules occurs only in nanoscale devices. It also provides ultrahigh surface to volume ratios, which induces negative pressure in water during capillary action, limitation in diffusion reactions, and governs smooth ion transportation. Moreover it ensures selective biomolecular transportation due to its size being comparable to its dimensions [49]. As the term nanofluidic gets its scope due to the nanoscale fabrication, which is remarkably different from the conventional nanofluidic devices such as membrane/any

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mesoporous media. It also has some revolutionary history regarding its 0
2 dimensional nanosized fabrication [49]. Reynolds number (Re) is the dimensionless figure used for flow in nanostructured materials, which is the ratio of two characteristic forces acting on the same volume. When the rate of flow reaches the limit of 10 m/s, the Re becomes unity. To get that high speed limit in nanochannel structure is difficult to adopt as it requires infinite pressure gradient that could not be achieved by using conventional pumping system. Interface chemistry also plays a vital role to make a flow in a nanochannel due to hydrophobic surface interactions [50]. The interest shifted from microfluidics to nanofluidics, due to its simple control of geometrical parameters for fabricating devices and its glamorous applications. The geometry and applications of nanofluidic devices make a clear contrast between microfluidic fluid transport and nanofluidic transportation. Various nanoscale devices are classified according to their different aspect ratios. Microfluidic devices use standard photolithography, while nanofluidic devices use the well-defined film deposition and etching processes. High aspect ratio nanochannel structures are more challenging, but give a higher production rate and coordination level than the low aspect ratio structures. The low to high aspect ratio of 1D and 2D nanochannel devices with geometrical parameters are shown in Fig. 9.11 [51].

FIGURE 9.11 The 1D and 2D of planar, square, and high aspect ratio nanochannels. Reprinted with permission from P. Abgrall, N.T. Nguyen, Nanofluidic devices and their applications. Anal. Chem. 80 (7) (2008) 2326
2341, Copyright (2008) American Chemical Society.

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To shrink the microchannel device from 20
30 μm down to 400 nm by heating and pulling the thermoplastic using wet oxidation process. Nanochannel collapsing is an issue. The main difference between nanochannel and the microchannel is due to electro kinesis and electroosmosis. Electroosmosis is a well-defined method. The nanochannel has smallest dimension comparable to λd . Electroosmosis is observed in nanofluidic devices. A huge mass transport occurs through CNTs, which plays an important role for future nanoscale devices. Recent nanofluidic chip, slab gel electrophoresis are also used nowadays by biologists which is the most prominent application. The dimension of nanochannels and size of biomolecules make nanofluidics more powerful for genomic systems [51]. Some conventional approaches for the fabrication of nanofluidic devices majorly include the microelectromechanical system (MEMS) technique based upon fabrication and various methods that involve different nanomaterials and nanolithography. The incident light wavelength limits the resolution of nanolithography; therefore, it is devised to fabricate nanofluidic devices using lithography techniques other than conventional nanolithography with nanometer-sized dimensions and resolution. To solve the diffraction limit problem in standard photolithography, several nanolithography techniques are used including FIB, EBL, sphere lithography, nanoimprint lithography, and interferometric lithography. The former two (FIB and EBL) are used to fabricate single nanofluidic devices at small scale with direct writing, whereas the other methods are used to fabricate nanochannel/pores on a large scale [49]. EBL-based fabrication is similar to photolithography; however, the patterning in the resist occurs with direct writing instead of through the use of a mask (which is used in conventional lithography). This technique involves two patterning approaches; one is the use of a patterned nanostructure and patterned substrate (this could make less hydrophobic nanostructures), while the other is to construct the nanochannel directly on the substrate. Nevertheless, a nanochannel polymer with a defined channel width can be produced by these methods. Furthermore, the integration of EBL with other classical microfabrication processes may yield more complexity in micro/ nanofluidic devices [49]. Next is the FIB milling technique, which does not require a photoresist or mask and that can make a platform on a hard substrate at nanoscale. The standard microfabrication process makes a thin membrane that is used in the FIB milling process to fabricate nanopore structures. The surface charge effect can be minimized by electron beam radiation. FIB fabrication involves several steps, namely (1) deposition of thin film, (2) FIB milling process, and (3) to shrink the nanopores using isotropic deposition, then nanofabrication starts where FIB scanning and etching of the sacrificial layer occurs, which helps to remove ridges during FIB scanning. This is shown in Fig. 9.12 [49].

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FIGURE 9.12 The fabrication of nanostructures for nanofluidic devices; for nanopore fabrication and nanochannel fabrication. Reprinted from C. Duan, W. Wang, Q. Xie, Fabrication of nanofluidic devices. Biomicrofluidics 7 (2) (2013) 026501, with the permission of AIP Publishing.

Multifunctional micro/nanofluidic devices based upon Si-wafers can be fabricated by FIB. On combining several fabricating techniques, namely photolithography, etching, lift-off, FIB deposition, and direct milling, the desired modification in the structure of the nanochannel can be obtained. These techniques are not only used for their fabrication on large scales, but also for their low cost [49]. In addition to FIB and EBL, nanoimprint lithography (NIL) is a conventional approach for the formation of 1D and 2D nanochannels in nanofluidic devices. It has the capability to nullify the diffraction limit that requires the critical adjustment of temperature and pressure. Just like other techniques mentioned, this approach is followed by oxygen plasma etching. The imprint modes (generally silicon and quartz) used in NIL are made by other nanofabrication techniques such as EBL, interferometric lithography, and deep UV lithography, which then follow the reaction ion etching process. All these techniques (EBL, FIB, and NIL) follow a feature size of B10 nm and NIL uses the B10 nm feature size at a relatively lower cost than FIB and EBL. Moreover, for the fabrication of micro/nanofluidic devices, NIL has a great compatibility in microfabrication processes. Although this approach only takes a single step for the fabrication, this approach is limited by two features; one is the template, which is limited to use, and the other is the high cost issue, which make NIL not so promising for use in nanofluidic devices [49]. Another mask-less process of fabrication is known as interferometric lithography also known as holographic lithography, where the width of the pattern formed on the photoresist depends upon the development time and the angle and wavelength of incident light, while the nanopattern is achieved

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by etching process, about 200 nm that could be reduced to 100 nm by thermal oxidation. The fabrication of nanochannels on a large scale is done by this process. This technique is limited to only fabricating single nanochannels at small scales [49]. Colloidal lithography or sphere lithography is obtained at low cost and makes 2D ordered nanostructures, etching masks, and high-quality monolayers of nanoparticles. However, it still faces some challenges due to the special instrumentation required and tricky techniques. A monolayer of nanoparticles can be achieved by spin coating as well as by sphere lithography that is appropriate for micro/nanofluidic devices [49]. The MEMS-based nanofabrication process is also popular due to its low cost, high production mode, and efficient processing ability. The process is suitable for 2D planar (with low aspect ratio) and vertical nanochannels (with high aspect ratio). MEMS-based nanofabrication includes sacrificial layer releasing, etching and bonding, etching and deposition, edge lithography, and spacer technology. Fabrication based upon nanomaterials includes ion selective polymers, nanoporous materials, nanoparticle crystals, nanowires, and nanotubes. Sometimes nonconventional technologies are not reliable for the fabrication of alternative nanofluidic devices [49]. In microfluidics, thermoplastics such as polyethylene terephthalates (PET) are used. Thermoplastic has fascinated the globe due to its diverse nature in fabrication, give more production for low cost in hot embossing/ injection molding technique. Similarly, thermoplastics (possessing such a physiochemical property) are used in nanofluidic intended applications for high mode production at low cost including in NIL and compression injection molding. Early nanofluidic devices were fabricated to nanopores in order to analyze mass transport in nanostructures with depths comparable to their diameters. The modern nanofluidic devices are more flexible with 2D nanostructures and tunable size and shape [48]. Etching is a fabricating technique used for etching tiny channels on silicon/glass wafers [52]. Silicon, glass, and silica are commonly used as inorganic substrates in the fabrication of nanochannels for providing good insulation, favorable wetting conditions, and efficient top-down fabrication techniques [48]. The theoretical based fabrication of mass transport systems or fluid dynamics/molecular dynamics in nanochannels can be studied by various computational softwares [48]. For the fabrication of a nanofluidic device, the first step is the selection of a substrate for the nanochannel or nano slit device. The substrate may be of organic, inorganic, or a desired nano-dimensional structure. Organic substrates include thermoplastics and elastomers, while inorganic substrates include fused silica, silicon, and glass. These inorganic substrates have good optical properties and surface chemistry and are used in well-established topdown fabrication techniques including EBL combined with etching, FIB, and direct writing. These techniques have been widely used for the investigation of mass transport or biomolecule transport analysis in nanochannels [48].

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Other techniques have been established i.e., the direct writing of 10 nm structures in Silicon/Helium (Si/He) ion beam writing, conventional machining system by etching, nanowires usage (as a sacrificial template), and UV LASER pulse for the closing of nanochannels. In order to form small dimensional nanostructure channels, there is a need of ions of low atomic mass such as He, which is used instead of Gallium (Ga) ions, providing lesser scattering then the later [48]. For the fabrication of organic substrates, generally thermoplastics are used for this purpose due to having better physiochemical properties and higher molecular weights and Young’s moduli than elastomers. They are efficiently used for microfluidic devices, however, their significant properties such as their glass transition temperature (Tg ) make them more reliable than glass (inorganic substrate), thus, providing a high production mode at low cost. Moreover, thermoplastic copolymers can also be used as substrates during fabrication processes. The first use of NIL was reported in the 1990s, for the fabrication of thermoplastic substrates for nanochannels resulting in sub10 nm structures [48]. The NIL technique is used to make nanochannels based on organic substrate, that is, thermoplastics. The most prominent advantage of this technique is that, in a single imprint step, NIL has the potential to form a multiscale pattern [48]. In addition to the mentioned techniques, there are others including hot embossing, compression of microchannels, direct proton beam writing, sidewall lithography, UV lithography, oxygen plasma etching, the use of silica nanowires as a template, and the thermomechanical deformation technique, which can also be used for the fabrication of nanochannels in thermoplastic substrate [48]. Researchers also developed a chip-based upon the junction of micro/ nanofluidics, in order to illustrate electro-kinetics enrichment ratio, its integration manufacturing and enrichment mechanism. The micro/nanofluidic chip was successfully developed using the UV lithography (photoresist free) and double lithography techniques [53]. A more recent nanofluidic chip based upon organic substrate, that is, elastomers, for the detection of DNA, its analysis, and manipulation, has been cultivated. For the first time, the elastomer-based nanofluidic device was fabricated using FIB lithography and some PDMS-based optimizers that could control or adjust the geometry of the nanostructure device [54]. These nanolithography techniques exhibit high resolution and excellent compatibility with classical microfabrication processes. To determine the most suitable fabrication approach for a nanofluidic device, one must consider its application, choice of materials, its dimensions, channel uniformity, and feature size. Moreover, material selection also plays an important role for the proper functioning of a device and should be of comparably low cost. A feature size of about 5 nm is the present choice of the nanofluidic devices to explore and enhance fluid transport in nanochannels [49].

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The size of the mass transport should be comparable to the size of the nanochannel device. Due to this unique property, these devices have a number of applications in the field of chemical analysis with limited mass, single molecule analysis, DNA electrophoresis, desalination, biomolecule probing in real-time, DNA separation, to control molecular transport, ion transport, and electrophoretic separation, etc. [48]. Also they can potentially be used in DNA sequencing (nanogap detector as shown in Fig. 9.13), drug delivery, epigenetic analysis, gene-therapy, and toxicity analyses. Analyzing single DNA strands, nano flow speed, and DNA translocation with improvements in femto-sized quantity and analyzing micro-RNA, Messenger Ribonucleic acid (mRNA), and polymerase chain reaction (PCR) products can be done by this process [52]. The thermoplastic-based nanochannels fabrication is used for the analysis of DNA elongation/linearization and genomic mapping/ nanoelectrophoresis. A nanopore device (also known as a nanofluidic separator) has been generated for DNA sequencing using electrophoresis and LASER tweezer techniques. DNA stretching not only gives us the information about the thermal fluctuation in nanochannels, but also about the imperfections in these nanochannels as well. Similarly, thermoplastic fabrication was also used to evaluate reaction kinetics in enzyme molecules, for electrochemical detection of tiny molecules, and to identify single nucleotide polymorphism (SNP). Moreover, commercialized thermoplastic-based nanofluidics (CTBN), Bio nano Genomics (Irys system), Irys Next generation mapping

FIGURE 9.13 Shows the nanogap detector in nanofluidic channel for the detection of DNA. (A) Describing a 9 nm gap detector on a 45 nm nanochannel on a silicon wafer. (B) Single DNA molecule detection with generating electrical signal. Reprinted from Elsevier Books, M.E. Ali, M.M. Rahman, T.S. Dhahi, M. Kashif, M.S. Sarkar, W.J. Basirun, S.B.A. Hamid, S.K. Bhargava, Reference Module in Materials Science and Materials Engineering, Copyright (2016), with permission from Elsevier.

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(NGM) chip that can handle 1 kb DNA with a minimum cost of $1000/chip [48]. All the properties, thus, make nanofluidics more valuable in biosensing and nanofluidic-based electronics. Based upon the principle of nanofluidics, solutions were devised toward energy storage/conversion and water purification, which are currently major challenges for the whole world [49]. A dual NIL approach was also used for the detection of in situ DNA transportation [49]. A micro/nanofluidic chip has been introduced to purify protein and to analyze enzymatic reaction kinetics [53]. The most versatile elastomer-based nanofluidic nanopores are manipulated for DNA stretching and probing. Moreover, a combination of this technique with the resistive pulse sensing technique gives information about the DNA translocation process and its motion inside the nanochannels. It may, thus, allow the labelling of free sensing, which will become crucial to biosensing in the medical field. The DNA speed inside the nanochannel gives a clue toward the emerging DNA barcoding technology as well [54]. Nanofluidics solved many physiochemical mysteries that could not be solved by microfluidics. However, nanofluidic devices also have some problems that should be discussed for future fabrication. Inorganic-based nanofluidic devices have been the trend for many years such as glass/silicon wafers used for the fabrication of these devices, but their sophisticated and expensive structures are now becoming a challenge for scientific society [48]. The ion selectivity for pores in the manmade process is different to the biological selectivity of ions. It remains a tough challenge for scientific society to develop artificial nanofluidic devices by molecular dynamic simulation. Advances in nanomaterial devices are organized to the theoretical and experimental technique for the selection of ion transport by nanopores or by nanochannels [47]. The use of thermoplastic substrates for the fabrication of nanofluidic devices has encountered challenges during the fabrication of molecular assay assemblies as channel deformation and collapse occur. This problem arises due to insufficient understanding of charged surface mass transport in thermoplastic nanochannels. Thermoplastic-based nanofluidic devices are more promising than glass, but there is still room for development in the study of theoretical-based physical phenomena occurring in microchannels (As we know that nanoscale fabrication for thermoplastics is not so different in nature than in microfluidics fabrication), as well as in experimentation for the production of accurate nanoscale devices, especially in term of mass transport in nanochannels. For this purpose, density functional theory (DFT) may be a helpful technique. Electrical mass flow cannot be handled by thermoplastic fabrication and hybrid thermal bonding is prevented as it may affect the performance of a device [48]. For the fabrication of nanofluidic channels based upon inorganic substrates, there are techniques such as FIB, EBL, and UV lithography that are used that may be expensive as they require individual-based fabrication. So there arises some challenges for inorganic-substrate fabrication that requires an alternative approach for

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fabrication [48]. EBL, which is used for small-scale fabrication, is generally not suitable for relatively high production rates with effective cost and scanning speeds [49]. FIB can make nanoscale structures and is compatible with other techniques, however, its efficiency is lower than that of EBL due to the need for direct milling. Moreover, costly equipment also remains a challenge for the FIB milling process [49].

9.8

Biomimetic materials

Biomimetic materials are engineered artificially by making replicas of biomaterials the occur in nature. These biomaterials work where the original materials cease to perform their functions properly or are used to maintain the environment in tissue culture, cell growth, biotechnological manufacturing, etc. Biomimetic materials, which are manufactured or have designs derived from nature, are used to make several peptides and proteins. Variants of elastin have been modelled artificially by Urry et al. Several polymers have been engineered to induce better mechanical properties and strength [55]. In a biomimetic approach, the initial step is to identify the performance of biomaterials occurring in natural systems and then to understand the mechanism of these performances, which can be done by scanning probe microscopy. In order to illustrate the mentioned steps, mollusk shells (from the Latin word “mollis” meaning “soft”) is a remarkable example in this field of studying a natural system for the establishment of a duplicate artificial system. The shell Mollusk comprised 95% of minerals and tougher for efficient protection. A study on 20 different species of mollusk revealed that its shell provides an excellent model for artificial model armor designs because of its light-weight, strong, tough, and reliable mechanical properties. Another natural model is the nacre (the strongest nacreous structure ever). The nacre is the best example for the investigation of such highly sophisticated structures in nanoscale and microscale (using SEM). These features make it possible for manmade designs. Being ductile and having growth lines make this material an optimal armor system and crack deflector. Artificial shells, hence, can be made possible for the intricate fabrication of materials at the microscale. The MEMS technique has been used for the crossed lamellar microstructure of queen conch (Strombus gigas) [56]. Repairing, reproducibility, the ability to transduce energy, assembly, and connectivity to functional structures such as cilia, antenna, flagella, etc., are the main focuses of bioscientists [56]. Some biomimetic material may be used in some injured part of the human body. There are four distinctive strategies involved in the investigation of the behavior of biomimetic materials, namely the insertion of bioactive peptides into a given biomaterial, to understand the functionalization of these peptides with the biomaterial, to evaluate the interaction compatibility and cell response, and the development of an innovative strategy for the incorporation of extracellular matrix (ECM)

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(decellularized) into the biomaterial [57]. Scientists move toward novel approaches for the complete study of the behavior of cell responses to intraor extracellular signals for the exact mimicry of the natural systems in regenerative medicines [58]. There is a plethora of applications of biomimetic materials and further modification and research development are required in this field. Formerly, the first generation of biomimetic materials used the off-theshell approach where the synthetic material should be inert. The example quoted as the poly(methyl methacrylate) (PMMA) are used in the manufacturing of lenses [59]. Biomimetic materials are not totally inert so when exposed to blood, urine, body plasma, or tears, they must be masked by a biofilm. Therefore researchers moved toward new techniques to control the biochemical responses of bioactive materials, the adsorption of protein process, and the tissue regeneration and integration processes; this approach was known as the second-generation approach. A better performance of implanted materials and their ability to interact with surrounding systems were the basis of the third-generation approach in the 1990s. Musculoskeletal repair mechanism, mechanically/electrically stimulated drug delivery, and ophthalmic devices are under the consideration of medical applications [60]. Aerospace and automotive applications (AM additive manufacturing) applications are also major focuses of biomimetic engineering [61]. Through the blessing of genetic engineering, many identical copies of genetically engineered microorganisms have been created so far. Similarly, biomaterials derived from peptides and proteins can be obtained by molecular biology or by fermentation. Using repeating units of oligometric peptides, protein polymers are formed, which can be controlled by producing bacteria with inserted genetic information [60]. We know that natural enzymes are highly specific to substrates, have selectivity properties and efficient catalytic activity, and operate normally under optimum conditions such as moderate temperature, pressure, and optimum pH. Any change in the local environment may lead to the denaturization of enzymes and they, hence, cease to perform accurately. Therefore this problem can be overcome by biomimetic catalysts based on molecular imprinting polymers (MIP). Biomimetic catalysts use three basic approaches for their design preparation. These are the design approach, the transitionstate analogue approach, and the catalytic activity selection approach. A biomimetic enzyme must have a high binding affinity, catalytic activity, and be substrate specific, which are the basic requirements for the design approach. A host molecule of desired reaction is been prepared that could serve as a biomimetic catalyst. Catalytic antibodies have emerged as an amazing success of the transition-state analogue approach. For a unique chemical reaction, a catalyst can be selected from a large number of host compounds that could be obtained from a combinatorial library. This catalytic selection approach proved to be a useful technique for the innovative creation of

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enzyme mimics. The mentioned MIP technology can be extensively used in elimination reactions, hydrolysis reactions, C
C bond formation reactions. The study shows that the advances in the MIP approach could provide new routes to biomimetic materials, however, the process needs more homogeneous binding sites before MIP-based enzymatic materials [62]. In myocardial infraction, the removal of cardiac tissues occurs in order to avoid cardiac dysfunction. Hence the bioengineering of biomimetic materials plays an important role for the support, protection, and facilitation of the regeneration of cardiac tissues. This biomimetic material guides and helps in the regeneration of injured cardiac tissues. Nanotechnology-based biomimetic materials have been introduced in the field of cardiovascular science (in cardiac and peripheral bypass-surgery) and cellular transplantation. In bypass surgery, native damaged tissues are replaced by synthetic cardiovascular tissues. However due to synthetic grafting, there arises some complications in patients. So during grafting, there is a need for mechanical stability, off-the-shell availability, infection resistance, and biocompatibility of the synthetic biomimetic material before implantation. Endothelial cell (ECs) seeding is another approach for the seeding of original cells into their duplicate copies using the tissue culture technique. In this method, a grafting of a patient’s own body cells can be done before implantation instead of the insertion of foreign cells as they may evoke an immune system response. However, this technique faces some detachment challenges that can be solved by the addition of biomimicry for fibrillar surface modification of vascular grafts. For the development of polymeric fibers at the submicron or nanolevel, three methods have emerged, namely the selfassembly method, phase separation, and electrospinning. Nowadays, fibronectin with arginine-glycine-aspartic acid (RGD) containing peptides is introduced and provides adhesion to ECs. The synthetic material duplicate of fibronectin, that is, polyurethanes with RGD containing sequences, proves to be an excellent attachment for ECs. Moreover, extracellular matrix (ECM) protein production has been modified and increased, which provide mechanical stability, tissue viability, and regeneration of vascular grafting. Tissue engineering is now modified to 3D biomimetic material of which metal has gained prominent properties due to its biocompatibility and remarkable mechanical strength. Various metal nanoparticles including Zn, Sr, Zr, B, Ti, Mg, Ag, Cu have now become attractions of the biomedical field (tissue culturing/drug delivery). The antimicrobial study of nanoparticles shows that they possess antimicrobial activities. They also help in stem cell differentiation, osteoblasty, osteoconductivity, osseointegration, orthopedic surgery, dental implantation, bone substitution, and in the regeneration and formation of bones [63]. Ceramic designs play an important role in dental and orthopedic surgery. Fibrin glue is used as a suitable adhesive gel in surgical treatment. Hydroxyapatite (HA) osteointegration helps in place of bone mineral formation and, hence, proves to be an efficient discovery of

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FIGURE 9.14 The biodegradable materials used in clinical surgery fabrication based upon soybeans. Reprinted from Wiley Books, G.J. Phillips, M. Santin, Biomimetic, Bioresponsive, and Bioactive Materials: An Introduction to Integrating Materials with Tissues, Copyright (2012), with permission from John Wiley and Sons.

excellent integration with the natural environment. Metal and ceramic-based components used in clinical surgery are shown in Fig. 9.14 [59]. The antifungicidal study of metal nanoparticles shows that they help in the sterilization and surgical instrument cleaning process. Moreover, the drug delivery process has now become easy. Through surface modification, bacterial growth on the surface of implant material can be reduced [63]. The cell membrane (also called plasmalemma) helps in the regulatory mechanism with the surrounding environment. Bio mimics of the cell membrane can be done by a polymer phosphatidylcholine [59]. Silk secreted from silkworms is now used in suturing materials. Similarly, from soybean extractions, fibers, glue, and other material items can be fabricated. Biodegradable materials can be fabricated using soy as shown in Fig. 9.14 [59]. Thin film introduction in nanomaterials is also one of the proof study of biomimetic material synthesis [64]. Biomimetic peptides can be introduced into future smart devices that may detect the selective phenotype [57]. The biomimicry of nature encounters intensive challenges due to its manufacturing strategy, variation in the mechanical properties of natural systems, and catalytic activity in synthetic chemistry [55]. Initially, inertness with biocompatibility was a major issue for scientists, whereas biocompatibility was defined as “In specific applications, the ability of a material to act properly with a host response,” in 1986 by the Consensus conference of European Society for Biomaterials [59]. Innovative methods for the fabrication of artificial devices are emerging in the current era including modification in in situ microscopy experiments. Further principles regarding nature’s nanomotors should be explored [56]. Biomimetic engineering can be done on the micro as well as on the nanoscale [65]. Natural tissues are also fabricated in the nanoscale, having collagen fibrils and proteins of .500 nm

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FIGURE 9.15 Shows the synthetic biomedical components used in orthopedic surgery; (A) is the femur head, (B) used in place of hip socket, (C) stent (made of titanium coated with porous titanium foam), (D) stent (made of titanium coating with hydroxyapatite), (E) used as knee implantation component, (F) vascular grafting component, (G) coronary stent, (H) ureteral stent, (I) intrauterine component. Reprinted from Wiley Books, G.J. Phillips, M. Santin, Biomimetic, Bioresponsive, and Bioactive Materials: An Introduction to Integrating Materials with Tissues, Copyright (2012), with permission from John Wiley and Sons.

dimensions. Although metal nanoparticles prove to be an excellent substitute for natural systems, they may induce cytotoxicity and even genotoxicity as well. Some metal alloys such as Cobalt chrome molybdenum (CoCrMo) and Titanium Aluminum Vanadium (TiAlV) result in genotoxicity when they undergo medical implantation. Other metals like Al, Cu, Zn, Mg, and Ag help in enzyme activity, hormonal activity, and aid in DNA synthesis, but heavy doses may lead to cytotoxicity. A study shows that extra doses of metal composite nanoparticles may cause DNA damage and oxidative lesions as well [63]. For instance, ceramic devices for bone generation face difficulty due to the mimicry of their mechanical properties and processing techniques (Fig. 9.15). The mechanical properties of bone such as its fracture toughness, composition, compression, tensile strength, elastic modulus, and flexibility vary with age, gender, and type of bone as well. Highly complex structures need a well-developed processing technique such as computeraided design or computer-aided manufacturing. A 3D printing image of femur bone formation is shown in Fig. 9.16. If mechanical could be improved, then other remains a challenge and vice versa. If processing technique is availed then resolution would be a challenge [66].

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FIGURE 9.16 (1) The 3D theoretical modelling of bone that mimic the natural structure of bone; (2) the printed image of 3D model of bone structure; (3) is the micrograph illustration of bone integrated with ceramic coating delamination upon implantation. (A) Lithographic-based ceramic bone (frozen structure); (B) the computed tomography image of dense bone model; (C) femoral bone model (frozen); (D) femoral bone model (CT-image). Reprinted from Wiley Books, Gary J. Phillips Matteo Santin, Biomimetic, Bioresponsive, and Bioactive Materials: An Introduction to Integrating Materials with Tissues, Copyright (2012), with permission from John Wiley and Sons. Reprinted from Elsevier Books, D. Marchat, E. Champion, Advances in ceramic biomaterials, 279
311 Copyright (2017), with permission from Elsevier.

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311.

Chapter 10

Nanobiotechnology Sumera Afsheen1, Muhammad Irfan2, Tahir Iqbal Awan3, Almas Bashir3 and Mohsin Ijaz3 1

Department of Zoology, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan, 2Department of Biochemistry and Biotechnology, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan, 3Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan

Chapter Outline 10.1 Introduction to nanobiotechnology 10.2 DNA microarrays 10.2.1 Principle 10.2.2 Applications 10.3 DNA assembly of nanoparticles 10.3.1 Uses

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10.4 Protein and DNA assembly 10.4.1 Protein assembly 10.4.2 DNA assembly 10.5 Digital cells 10.6 Genetic circuits 10.7 DNA computing References

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10.1 Introduction to Nanobiotechnology Nanotechnology is an important, but immature, technology of the 21st century. This field is the result of interaction of three technological domains including materials science, information technology, and biotechnology, hence, it gives a vivid insight into the chemical, physical, and biological information pertaining to the matter under consideration. Consequently, this new emerging field of nanobiotechnology will substitute the costly traditional manufacturing process with cheaper and eco-friendly products that will be of durable architecture while having flexibility and precision. This technology could be utilized for designing durable but light materials such as drug delivery systems, sensors, micro robots, surgical instruments, electronic devices, and circuits. Nanospheres coated with fluorescent polymers can be found in nanobiotechnological research as another example. Scientists are working on finding suitable designs of polymers that can quench the fluorescent light when they interact with some specific type of molecules. In other words there are a

Chemistry of Nanomaterials. DOI: https://doi.org/10.1016/B978-0-12-818908-5.00010-X © 2020 Elsevier Inc. All rights reserved.

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variety of polymers that can detect different molecules, hence these could have potential applications in new biological assays leading to a new dimension of diagnosis of human metabolites concerned with cancers and other fatal diseases. The treatment of nanobacteria, ranging from 25 nm size to 200 nm, has been successfully done by NanoBiotech Pharma, which is the example of nanoscopic level evaluation and therapy. The future seems to be of nanobiology because several methods pertaining to diagnosis and treatment rely on it, besides the nanobiology is in its infancy now a days. Living organisms possess nanoscale biological systems. Biology and nanosciences could collectively converge to yield molecular machines that could synthesize biomacromolecules resembling natural molecules. Although there is a challenge to be faced for converging nanotechnology disciplines, while mimicking and controlling the machines using molecules. Humans and all other all other living things are the nanofoundries. With the passage of billions of years, nanobiology has attained an optimized natural form with the natural process of evolution. Now, in 21st century, humans have reached a point where they have developed systems that are capable of providing insight into artificial nanobiology which could be phrased as “organic merging with synthetic.” A single biochip could have several colonies of living neurons. Similarly, nanotubes that can self-assemble could be utilized in the design of structural systems, for instance nanotubes with rhodopsin could enhance the optical computing system for the storage of biological materials. An important example of bionanotechnology is DNA nanotechnology. The promising area of this modern technology is the usage of the inherent characteristics of nucleic acids (e.g., DNA) for the synthesis of useful materials. Synthetic membrane production could also be done using this technology. Programmable nanomaterials can be produced at a large scale, which is the property of self-assembling proteins and could provide functional materials. Another example includes the synthesis of amyloids that are present in biofilms found in bacteria. These biofilms are genetically programmable nanomaterials for attaining altered properties. A third aspect of the research includes the folding study of proteins, although there is an insufficient degree of accuracy for the prediction of the folding properties of proteins. As the knowledge of folding patterns is of vital importance, nanobiotechnology, in the future, could give vivid insight into the folding patterns of proteins. Lipid nanotechnology is another important field of bionanotechnology. In this field, physicochemical properties of lipids are studied, namely selfassembly, antifouling, and others, and are exploited for the production of devices that could be utilized in engineering and medicine.

10.2 DNA microarrays A DNA microarray is a collection of tiny DNA spots that are attached to a solid surface. This is also termed a DNA chip or biochip. This technique can

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FIGURE 10.1 Hybridization of the target to the probe. Adapted with permission under the terms of CC0 1.0 Universal (CC0 1.0) (https://creativecommons.org/publicdomain/zero/1.0/).

be applied to measure the expression of large number of genes at a time. To determine the genotype of several regions of DNA, DNA microarrays are also used. This technique involves the use of picomoles (10212 moles) of the DNA sequence as a probe (also known as oligos or reporters). Under high stringency conditions, these probes are for binding to the complementary regions of cDNA or cRNA (antisense RNA) targets in the sample under study [1]. The detection of hybridization of target and probe is done and quantified using different markers including fluorophore, silver, or chemiluminescence labeled targets. This gives an estimation of the relative abundance and quantitation of specific nucleic acid sequence found in the sample under study (Fig. 10.1) [2].

10.2.1 Principle Hybridization is the main principle of DNA microarray technology and takes place between probe and the complementary regions of target DNA strands of nucleic acids by establishing hydrogen bonds. If there is tight bonding between both strands, it depicts the elevated number of complementary base pairs of nucleotides. After hybridization, a washing step is performed that retains the tightly bound sequences within the solution while nonspecific strands are eluted and washed off. At a later stage, fluorescent labeled target sequence is used that binds to the probe sequence and produces signals under prescribed conditions. The strength of the signal from a specific spot is directly proportional to the amount of target DNA. In microarray technology, quantitation is done using the intensity of a feature compared to the intensity of another feature obtained from a specific spot that is identified by its position [3]. There are different types of microarrays and the main distinction feature is the spatial arrangement either on the surface or on coded beads. mRNA is

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mostly extracted and cDNA is synthesized by a process called reverse transcription. This mRNA is usually taken from two or more sources for the sake of comparison of expression (e.g., diseased and normal). cDNA is labeled with distinct fluorophore (e.g., green-emitting fluorescent for normal and red for diseased) (Fig. 10.2) [3,4]. These are then mixed and hybridized into a single chip. The probe, having different color detection for green and red fluorescent, detects and quantifies the relative quantity of diseased and normal genes [5,6]. This is sometimes used as a reverse pattern and hybridized to another microarray. A sample fluorescent image of the microarrays of two different genes (DNA parts of nucleotides) is illustrated in Fig. 10.3.

FIGURE 10.2 Diseased (red) and normal (green) labeling. Adapted with permission under the terms of CC0 1.0 Universal (CC0 1.0) (https://creativecommons.org/publicdomain/zero/1.0/).

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FIGURE 10.3 Sample fluorescence image of a microarray. Adapted with permission under the terms of CC0 1.0 Universal (CC0 1.0) (https://creativecommons.org/publicdomain/zero/1.0/).

10.2.2 Applications DNA microarray technology has vast applications in the field of research, diagnosis and treatment of diseases, mutation, methylation, alteration, and insertion or deletion of DNA or RNA fragments [3,7 9]. G

G

Detection of DNA or RNA is possible with microarray technology and it is capable of indicating the RNA whose translational stage is not being processed and its target proteins are not being synthesized. This is termed as expression profiling or expression analysis. Another major application of microarrays is to identify those genes whose response is altered by changing their environment. Change in biological condition could also be revealed by microarray technology and is determined by comparing expression ratios.

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Coexpression is also identified using microarrays. This illustrates the genes, which give product under prescribed conditions. The use of different software enables the further clustering of genes. Microarray technology allows us to measure the amount of DNA methylation, the blocked segment of DNA [10]. DNA damage can also be studied using this technology [11].

10.3 DNA assembly of nanoparticles The molecules produced from DNA are complex enough to be explored for their applications. This is possible if these structures are converted into superstructures of colloidal form when DNA is used for controlling the delivery resulting in the removal of inorganic nanoparticles. The building block of whole superstructure-designed molecules are composed of nanoparticles that exhibit the properties of the superstructure, namely surface chemistry, size, and assembly architecture. The whole structure is capable of interacting with organelles of cells according to their built-in design and later on are catabolized into building blocks for exocytosis. This results in time reduction of intracellular residing of nanoparticles hence improving the in vivo accumulation of a tumor as well as elimination of the whole body. The superstructures could be utilized for imaging or as a therapeutic agent carriage to save the enzymes from degradation. There should be some strategies for engineering nanostructures that may yield molecules that have the capability of biodegradability along with nanomedicines performing multiple functions [12 14].

10.3.1 Uses Nanoparticles are being used for the following: G

G G G

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Nanoparticles have multiple applications in daily life, including sports, cosmetics, dyes and veneers, nutrition, and catalysis functions [15]. Nanotechnology is used in medicine [16]. Nanoparticles are utilized as antimicrobial and anticancer agents [17]. These agents are used for research purposes, e.g., therapeutic advantages in atherosclerosis [18]. Internal body parts imaging could be done using nanoparticles especially for the central nervous system (brain and spinal cord) [19].

10.4 Protein and DNA assembly 10.4.1 Protein assembly Life has been rewarded with a diverse variety of protein assemblies. Protein assembly occurs through transcription and translation steps; where translation is the point of protein synthesis and assembly. In transcription information of

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DNA is copied in the form of messenger RNA, while translation is the step of decoding this information of mRNA into proteins with the help of decoding machinery called ribosomes. The translation occurs in three major steps, initiation, elongation, and termination. Initiation and termination are done by reading the start and stop codes. Elongation is the actual part for synthesis and assembly of the proteins [20]. Researchers are interested in artificially assembling nanostructures with the inspiration of natural phenomenon of protein synthesis. Performing protein assembly artificially helps not only in understanding the natural procedure, but also enables to synthesize advanced nanomaterials. Over the past few decades there has been tremendous advancement in the protein assembly domain resulting in several strategies pertaining to biotechnology, namely chemical, biotechnological, and the combination of these strategies with end products of desired nanostructures of three-dimensional shape capable of self-assembling [21,22]. A huge study of assemblies of biomolecules is required prior to predicting the protein protein assembly and this requires a lengthy time span for precise prediction [23,24] besides these studies are in their infancy. However, the computational power could not be deployed to the prediction model for a routine task of protein assembly prediction, although there are huge developments in computing algorithms, ad-hoc hardware, and computing resources. For example, long simulations have already exploited and explored the characteristic properties of different protein receptors (for binding and assembly) with membrane coupling [25], and the interaction of two building blocks to form dimers [26]. The assembly of proteins rely on the presence of amino acids, the building blocks of proteins, that determine the specific final three-dimensional structure of the proteins. Proteins attain different structures, namely nanowires, nanorings, nanobranched structures, monolayers, tetrahedron, cubic nanostructures, protein nanotubes, and protein nanocages (Fig. 10.4) [27 29].

10.4.2 DNA assembly The phenomenon of self-assembling of biological molecules greatly depends on the repulsive and attractive forces found among the constitutive atoms of the molecules. These forces are the result of an electric charge at different sides of the particles. These forces are at peak when the atoms are suspended in a colloidal solution that is naturally found in the cells of living organisms. DNA, RNA, and proteins are colloidal particles; hence, these could selfassemble in the solution forming a three-dimensional structure. The selfassembly is facilitated with patches, with differently charged sides on the particles [31].

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FIGURE 10.4 Protein assembly patterns. Reprinted with permission from Q. Luo, et al., Protein assembly: versatile approaches to construct highly ordered nanostructures. Chem. Rev. 116 (22) (2016) 13571 13632 [30]. Copyright (2016) American Chemical Society.

Synthetic biology relies greatly on DNA assembly and several DNA molecules could be synthesized in single-step assembly. Very large spaces could be tested and applied to optimize the production through biochemical pathways. For performing DNA assembly three parameters are required: a lot of DNA that has the information for specific gene, the method for combining DNA pieces precisely, and the methodology to introduce the recombinant DNA’s into host. There are few variations for producing these recombinant DNA, so, polymerase chain reaction (PCR) is usually used at the initial stage followed by plasmid preparations and synthesis. The recombinant DNA is usually in a small circular form called plasmid (Fig. 10.5) that is introduced into hosts. There are different DNA assemblies available on the market. Generally, the DNA assembly consists of less than 10 DNA parts at s time with sizes ranging from 0.4 to 2.5 kilobases with a total size of less than 12 kilobases. If the size exceeds from 12 kilobases, the efficiency gradually drops with the increase in size.

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FIGURE 10.5 A sample plasmid. Adapted with permission under the terms of CC0 1.0 Universal (CC0 1.0) (https://creativecommons.org/publicdomain/zero/1.0/).

10.5 Digital cells Digital cells are also known as in silico biology that depends on the central dogma of life, that is transcription of secret information of DNA in the form of RNA and translation of RNA into proteins. This part of DNA is the genes which are distinct from others in all genes of an organism. Genes are transcribed into proteins that perform their specific function in the cytoplasm that are involved in several pathways, the ways of carrying out metabolic tasks. If the cellular function is termed in computer language, the genes can change with others but these must have the capability of collaboration, synchronization, and messaging. The genes are metabolic digital libraries that are the part of an operating system. The proteins must boot with start of operating system to regulate all functions of the digital cell including metabolome, signaling, and so forth. Similar to the decision making of an operating system, digital cells also have the system to take an appropriate decision to take action [32]. For example, a phage (a bacterial attacking virus) has two options of its life cycle, a lytic cycle and lysogenic cycle. The phage has to decide which pathway to adopt (Fig. 10.6). A decision has to be taken on when the gene is to be transcribed, and where is it to be transcribed. How much and who are to be transcribed? For this, gene circuits include regulatory or inhibition, promotors, and coexpression. Digital cells depend on genetic algorithms (Fig. 10.7) that give the

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FIGURE 10.6 Regulatory interaction between the DNA region and its gene product of phage. Reprinted under the licensed creative commons, R.I. Joh, J.S. Weitz, To lyse or not to lyse: transient-mediated stochastic fate determination in cells infected by bacteriophages (2011).

FIGURE 10.7 Binary string for potential solution.

optimal pathway for the solution of the problem. Just like the binary code in computer (0,1) biological systems contain different codes, namely A, T, C, G, or U. Before a gene sequence starts, there are several segments lying before it such as the promotor, operator, and so forth. There are different genes that have specific proteins to be bound with promotors or operators, or with both, to start transcription of the genes. If one is lacking, no protein results are

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FIGURE 10.8 Digital cell. Reprinted with permission from B. Saltepe, et al., Cellular biosensors with engineered genetic circuits. ACS Sens. 3 (1) (2017) 13 26 [36]. Copyright (2018) American Chemical Society.

possible, although there are several genes that could be transcribed with only one protein attached to promotor (i.e., no need of the other to perform normal transcription process) [33 35]. Similarly, some genes work in collaboration of each other in such a way that the resulting protein of two genes works as an activator for the resultant protein production of a third gene (Fig. 10.8).

10.6 Genetic circuits A genetic circuit is the synthesis of DNA segments that encode specific RNA molecules or proteins for controlling the production amount of protein or RNA. This science is the local and bottom-up approach to synthetic biology and can be distinguished from top-down global genomic engineering. In other words, the genetic circuit is the collaboration and interaction of the RNA or protein encoding parts that makes the individual cells capable of performing logical functions. Many cellular regulation processes involve the use of genetic circuits. Promotors, repressors, oscillators, and so forth control many cellular mechanisms. Intrinsic and extrinsic signals control and regulate the transcription and translation of the cells to keep check on the

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production of the specific genes. Custom-made pathways are being adopted to obtain many industrial compounds of interest. Many researchers have studied methods to alter these biological pathways and some researchers have introduced new pathways into existing microbes. All this was done to obtain the optimum amount of products [37 39]. Designing functional parts pertaining to biological systems for the production of biochemicals is the basic aim of researchers along with the production of energy. To address environmental pollution, which is highly undesirable and causing unpleasant effects on life, digital circuits are very important as they play a pivotal role and key part in the construction of biosensors. Biosensors are highly cost-effective and site-directed devices to monitor environmental pollution in time and consequently take appropriate necessary actions on the bases of detection done by biosensors, making effective bioremediation possible. The traditional approaches for monitoring the environment can be replaced with biosensors as they provide field analytical methods as well as laboratory-based methods simultaneously [40]. Genetic circuits are based on the principle of using two repressors (A and B) controlled by two inducers (A and B). The inducers are the molecules that activate the expression of the gene. RNA regulators are better as they are capable of blocking, activating, or terminating the transcription process [41,42]. The key molecule in genetic circuit designing is RNA, either in form of inducer or in form of repressors depending on the function of a toggle switch, the switch that can change the expression of one gene to the expression of other gene. Toehold, for example, is the switch that is based on the strand displacement mechanism. It triggers the expression of desired gene as per needs. Another regulatory system is the CRISPER/Cas system, controlling the transcription of the cell [43]. A sample genetic circuit is given the Fig. 10.9. Gene regulatory networks greatly depend on positive and negative feedback loops, these act as control switches or devices that can be excited, and oscillators. In future, these could prove vital devices for regulation modulators.

10.7 DNA computing The branch of computing that utilizes biochemistry, DNA, and molecular biology hardware rather than silicon-based traditional computing technologies is termed DNA computing. DNA computing is a computer based on the uses of DNA to store information as well as perform complex calculations. DNA computers have the advantage that they could solve complex problems by generating their response during processing, which is termed parallel processing [44,45]. DNA computers use enzymes as programs for calculation purposes on taking DNA segments as input. These computers are very convenient as they

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FIGURE 10.9 A sample genetic circuit. Reprinted with permission from B. Saltepe, et al., Cellular biosensors with engineered genetic circuits. ACS Sens. 3 (1) (2017) 13 26. Copyright (2018) American Chemical Society.

can hold huge amount of data from a few milligrams of DNA. It is estimated that 453.592 grams of DNA bears the data more than all the computers built up till now. Binary computers use only 0 and 1 for coding purposes while DNA computers use nucleotide bases (A, T, C, G) for coding the information [46,47]. The first DNA computer was invented by Leonard Adleman, whose computer could analyze genes (Fig. 10.10). Adleman performed an experiment by following the Hamilton Path Problem [48]. It was based on the inspection of the shortest path to reach the destination by comparing all possible ways that lead to the destination. Similarly, DNA computing also works on finding all possible solutions and then analyzing the most convenient response to the given situation [49,50]. Typically, DNA computing depends on the working of gates (AND, OR, NOR) which have associations with logic gate functioning. There are different bases of DNA computing, such as deoxy oligonucleotides, DNA enzymes, toehold exchange, enzymes, and so forth [51]. DNA computing allows to solve difficult tasks with ease. The traditional computing system faces difficulty in handling complex data calculations and takes a longer time. Therefore a strategic assignment problem is also proposed to tackle such hurdles. This depends on DNA strand manipulation, which is far better than conventional silicon-based computing systems [52] To conclude, it can be stated that DNA computing has a brilliant future although it is in its infancy currently.

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FIGURE 10.10 Leonard Adleman, first DNA computer inventor. Used with permission under the terms of CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/).

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Chapter 11

Nanotechnology: the road ahead Muhammad Bilal Tahir1, Muhammad Abrar2, Aqsa Tehseen1, Tahir Iqbal Awan1, Almas Bashir1 and Ghulam Nabi1 1

Department of Physics, Faculty of Science, University of Gujrat, Hafiz Hayat Campus, Gujrat, Pakistan, 2Department of Physics, Hazara University, Mansehra, Pakistan

Chapter Outline 11.1 Nanostructures 11.1.1 Nanoscaled biomolecules 11.2 Structure of carbon nanotubes 11.3 Quantum dots (QDs) 11.3.1 Properties of quantum dots 11.3.2 Fabrication of quantum dots 11.4 Energy harvesting and storage

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11.4.1 Piezoelectric nanogenerators 296 11.4.2 Solar cells 297 11.4.3 Electrochemical energy storage 299 11.5 Quantum informatics 301 11.5.1 Nanostructures in quantum informatics 302 References 305

11.1 Nanostructures In order to fabricate traditional devices and materials an assembly of small building blocks with defined arrangements are used. These arrangements are carved out from larger blocks using physical method. Designing and manufacturing objects and materials using this technique is very useful and tremendously influential in the market. This methodology to manufacture microscale objects is successful with greater reproducibility. The recent developments in science and technology has increased the challenge to produce target materials at nanoscale. Designing and manufacturing of nanosized materials with subnanometer precision is a challenging task [1]. Nanotechnology is mainly concerned with nanostructure fabrication, design, and application, along with essential understanding of the relations among material dimensions and physical properties. Nanostructural materials are categorized as 0-D, 1-D, 2-D, and special nanostructure materials with Chemistry of Nanomaterials. DOI: https://doi.org/10.1016/B978-0-12-818908-5.00011-1 © 2020 Elsevier Inc. All rights reserved.

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FIGURE 11.1 The typical range of dimensions of nanostructure materials. Reprinted by permission from Springer Nature, L.H. Madkour, Introduction to nanotechnology (NT) and nanomaterials (NMs), in: L.H. Madkour (Ed.), Nanoelectronic Materials: Fundamentals and Applications, Springer International Publishing, Cham, 2019. pp. 1 47 [2], Copyright (2019).

typical ranges. Fig. 11.1 provides a list of the different ranges of nanostructure materials.

11.1.1 Nanoscaled biomolecules Nature build nanostructures of remarkable degree of structural control with well-defined shapes, functions and properties. In contrast with physical topdown approaches, nature follows a bottom-up process in which small parts are joined to form large-scale objects. In nanoscale biomolecules, we can see beautiful and indigenous examples of molecule-based building blocks.

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Nanoscaled and nanostructured molecules have fundamental roles at macromolecules in biological processes such as in nucleic acid, protein, and others. Nucleic acids are the source of expression and transmission of genetic information. Such molecules are present in linear polymers joining nucleotide replicating units and are shown in Fig. 11.2A, whereas every single nucleotide carries four heterocycle bases as shown in Fig. 11.2B. The nanoscale characteristics of deoxyribonucleic acid (DNA) have made it a promising featured molecule for the creation of nanostructured materials and nanodevices [3]. In the DNA, synergism of H-bonds with [π


π] assembly corresponding to polynucleotide strands in attractive dual helical super molecules with exact structural control at nanoscale is shown in Fig. 11.2A C [1,3].

FIGURE 11.2 A polynucleotide strand: (A) nucleotide repeating units, (B) alternating phosphate and sugar residues, and (C) the formation of nanoscale double helix. Reprinted by permission from Springer Nature: Springer eBook, B. Bhushan, Springer Handbook of Nanotechnology, Springer Berlin Heidelberg, 2010, Nanomaterials Synthesis and Applications: Molecule-Based Devices, Franc¸isco M. Raymo, Copyright (2010).

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11.2 Structure of carbon nanotubes Sumio Iijima discovered carbon nanotubes (CNTs) in 1991 [4], and just after that discovery, the structure of CNTs became a hot topic for researchers. Research has revealed surprising properties of CNTs and their various types. Naturally, in various crystalline forms, elemental carbon occurs or it can be seen as amorphous carbon on Earth. Diamonds and graphite are the crystalline forms of carbon. The sp3 hybridized carbon atoms are bonded with other similar atoms. Graphite occurs in the form of various layers that are piled up over each other. Van der Waals forces are responsible to interlink these layers with each other and can slide over each other as these forces are very weak. Every layer is a graphene sheet. In this sheet, the sp2 hybridized carbon atom is connected to three other carbon atoms present with in same sheet. The hexagonal crystal lattices are formed by linking atoms where CNTs are made up of a single building unit of a graphene sheet. CNTs are formed by rolling up these graphene sheets as shown in Fig. 11.3. The diameter of these tubes is of the order of nanometers so these are also called nanotubes. There are different types of CNTs, that is, single-walled, double-walled, and multiwalled. In single-walled CNTs, the rolling of the graphene sheet is done only once whereas in double-walled CNTs, the rolling of graphene sheet is done twice. In multiwalled CNTs, the rolling of the graphene sheet is done in various layers to make concentric tubes of ascending diameter [5]. There are astonishing properties of CNTs due to their nanoscale structure and exceptional chemical bonding. In CNTs every carbon atom is sp2 hybridized [6]. These bonds of CNTs are stronger compared to diamond sp3 bonds. CNTs have remarkable mechanical strength due to the powerful bonding mechanism between the atoms. The value of their Young’s modulus is about 1000 Gpa which is five times of the modulus of steel. The tensile strength is as high as 63 GPa, which is 50 times the strength of steel. This makes them

FIGURE 11.3 Rolling up of graphene sheets to form carbon nanotubes. Reprinted from S. Sheshmani, A. Ashori, M.A. Fashapoyeh, Wood plastic composite using graphene nanoplatelets. Int. J. Biol. Macromol. (2013) 1 6, Copyright (2013), with permission from Elsevier.

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very strong mechanically. Diamond is the hardest of all materials. Carbon is diamond-hybridized sp3, while sp2 is hybridized in CNTs. Chemical binding dependent on the orbital sp2 hybridized is better than that of the orbital sp3 hybrid. The bulk CNT modulus is 462 546 GPa, which exceeds the diamond cost of 420 GPa. CNTs are found to be even harder than diamonds based on their binding power. CNTs have a curved hollow internal structure that is not sufficiently strong under the stress of compression.

11.3 Quantum dots (QDs) The theory of QDs has been developed in solid (glass) and liquid conditions [7]. The Optical Society of America credited Brus, Efros, and Ekimov with the discovery of QDs in the early 1980s [8,9]. The term “quantum” refers to a discrete unit with a particular property of any physical object. The name comes from the fact that quantum mechanics determines the optical properties of nanocrystals (NC), that is, “dots” [10]. This miniature structure is known as an inorganic crystal semiconductor shaped as a latest generation nanometer. Semiconducting nanoparticles, usually referred to as quantum dots, have different optoelectronic properties depending on their size and shape. Technically, they are known as small crystals possessing a variable number of electrons occupying well-defined, distinct quantum states, and having intermediate electronic properties between large and discrete basic particles. It has the same atom structure as in the corresponding bulk material, but due to three-dimensional notation, there are many more atoms on the surface [11]. This miniature entity has a size range between 2 nm and 10 nm in diameter with a physical dimension lower than exciton Bohr radius. Structural QDs comprise of a semiconductor core, coated by a shell, and a cap that allows aqueous buffer solubility. The inorganic core is responsible for its basic optical and semiconductive properties.

11.3.1 Properties of quantum dots A semiconductor nanocrystal is characterized by its band-gap energy, the amount of energy needed to excite an electron at a higher energy level from one electron band to another, which eventually creates an electron-hole pair called exciton. Upon deexcitation to their ground states, a photon liberates with a specific energy and the phenomenon known as fluorescence is observed [7]. The particle acquires special electrical and optical properties when the semiconducting nanocrystal volume is close to that of the bulk Bohr exciton radius with a characteristic length in the range of 2 10 nm. There is an inverse relationship between nanocrystal size and energy band gap, a decrease in wavelength of excitation or emission spectra as the bandgap increases, as shown in Fig. 11.4. This is known as the quantum size effect and the particle is represented as a quantum box [8,9].

FIGURE 11.4 Schematic illustration of quantum dots (QDs). (A) General approach of synthesis of QDs classified into two broad categories as top-down and bottom-up methods. (B) Size-tunable optical properties of QDs representing a decrease in band gap (decrease in emission wavelength) with an increase in quantum size.

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FIGURE 11.5 Application of quantum dots (QDs) in various fields such as quantum computing, optical, biological, and catalysis.

It is possible to observe a change from red to blue light during this process. Owing to the well-known phenomenon of the quantum-confinement effect (QCE), these QDs exhibit size-tunable optical and electrical properties that makes them extremely desirable for applications ranging from optoelectronics, microelectronics mechanical system (MEMs), to biotechnology [10,12,13]. Similarly, due their versatile properties, QDs are extensively used in various other applications such as solar cells, ultrafast switching, sensors, photocatalysis, quantum computing, and LED devices, among others, as shown in Fig. 11.5 [14 16].

11.3.2 Fabrication of quantum dots Quantum dot production can be divided into two main categories, namely bottom-up and top-down processes. Various techniques are used in the bottom-up method, such as chemical vapor deposition, ion implantation, and sputtering. As the latter uses atomic or molecular precursors for QD formation, so the bottom-up approach varies from the top-down approach. Compared to the top-down method this technique results in higher atomic utilization, structural and size controllability, and tunable morphology [12]. Solution-based methods for monodispersed QDs (colloidal) have mainly employed solution-phase colloidal chemistry with effective nucleation and growth steps are controlled to get monodispersed crystals. However, Brichkin et al. [17], further classified the bottom-up approach into physical (where there is no chemical transformation of matter takes place) and chemical ones (where chemical reactions occurs for the formation of these

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nanosized entities). In 1990 the development of the first high-precision method (physical) called molecular beam epitaxy (MBE) was introduced for the preparation of quantum dots. This is most valuable physical method involves the nucleation and growth steps in vapor phase or sometimes in solution phase. In which deposition of molecular or atomic size monolayers of quantum dots arrays on crystalline surface is deposited using ultra high vacuum [18]. When the thickness of the monolayer reaches a critical level, it spontaneously breaks down to repeating rectangular-pyramid structure QDs on the crystalline substrate, but this method is expensive and complicated [19]. Later, in 1993, another highly efficient method called high temperature colloidal synthesis was introduced and proved to be excellent approach toward large-scale chemical synthesis of structurally perfect colloidal QDs [20]. Various other methods of chemical synthesis of QDs have been introduced, such as (1) micellar synthesis, (2) refluxing, (3) hydrothermal, (4) microreactor synthesis, and (5) microwave-assisted methods [17]. Every method has limitations and advantages and depends on the end use of QDs. Although the micellar method has the advantage of easy variation in size of QDs by altering the water and surfactant concentration, the drawback is that there is less yield. Regarding synthesis of semiconductor QDs hydrothermal [21] and microwave-assisted [22] methods have been used extensively, which has an advantage of getting high crystallinity and high yield in less time. Similarly, emphasis on the posttreatment of synthesized QDs is necessary as the freshly synthesized QDs are often characterized by low-quantum yield of luminescence. The top-down approach requires the deposition and exfoliation of relatively inexpensive bulk material through chemical, electrochemical, and physical methods. This technique includes (1) hydrothermal (solvothermal), (2) electrochemical exfoliation, (3) acid etching, and (4) ultrasonication exfoliation. However, controlling the size distribution and morphology of QDs is a challenging issue.

11.4 Energy harvesting and storage 11.4.1 Piezoelectric nanogenerators Generation of electricity by the piezoelectric effect was discovered in the 17th century when a primitive form of friction was converted in to electricity. The energy through piezoelectric effect is generated from the different sources of waste mechanical energy from the environment. Generally, nanogenerators are used to produce energy at a small scale. Piezoelectric nanogenerators (PENG) revolutionized the modern era due to their various applications ranging from energy harvesting, sensors, and carefree systems. They can convert mechanical energy of human motion such breathing, running, and heartbeat into electric energy. The first effective PENG was

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FIGURE 11.6 Different structures of piezoelectric nanogenerators (PENGs). Adapted with permission under the terms of the CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

fabricated in 2006 using ZnO nanowires which was able to deliver an efficiency of 17% 30% [23]. The structure of PENG consists of two electrodes of balanced Fermi levels fabricated on piezoelectric material. External stress is applied on the electrodes that produces a difference between the internal and external Fermi levels of electrodes. In order to balance the Fermi level, charge carriers start moving through external circuit. Conversely, if an electric field is applied on a piezoelectric material, it produces mechanical stain [24]. Based on the working phenomenon, there are two types of PENGs. In first kind of the PENGs, strain is exerted in perpendicular direction of the nanowires or nanorods which results in the production of electric field as shown in Fig. 11.6. However, in the other case, strain is applied parallel to the direction of nanowires or nanorods [24]. PENGs are a highly sustainable source of small amounts of energy that makes them suitable for various kinds of applications, especially for selfpowered sensors, nanoelectronics, and other elastic and wearable biomedical smart electronics applications. Since a PENG is able to produce 20-fold higher energy compared to other bulk cantilever type energy harvesters, it is widely used as a flexible sensor for the human body, and wearable and implantable devices. So far different materials with good piezoelectric potential were studied and includes GaN, ZnS, InN, CdSe, InAs, and 2D MoS2 [23]. The performance of PENGs can be improved further by using state-ofthe-art nanomaterials with tailored nanostructures. Additionally, stability, flexibility, and efficiency can be improved further by optimizing the structure, design, and integration of PENGs.

11.4.2 Solar cells A solar cell is a device that converts incident light into electric energy by electrochemical reaction (photovoltaic effect). Photovoltaic effect was first

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demonstrated in 1839 by French scientist Edmond Becquerel. Later, in 1883, Charles Fritts fabricated the first photovoltaic cell by making a thin film gold electrode on a selenium semiconductor. The beginning of the 20th century opened a new paradigm of solar-energy harvesting when Einstein proposed his quantum interpretation of the photoelectric effect. After the discovery of the p-n junction in 1941, the first photovoltaic cell was invented in Bell laboratories [25]. Overall, solar or photovoltaic technology can be classified into three generations based on fabrication techniques. First-generation solar cells are based on the most effective and commercially viable silicon. Today, 80% of solar cell production is based on silicon-based solar cells of the first generation. Solar cells of the first generation consist of solar cells of monocrystalline, polycrystalline, amorphous, and hybrid silicon. Monocrystalline silicon solar cells are made by single crystal of silicon that is grown under extremely controlled conditions. These solar panels can deliver photoconversion efficiency (light to electric energy conversion efficiency) of 24%, but their production cost and poor performance at elevated temperatures are the major issues. Polycrystalline silicon solar cells are developed by growing different interlocking crystals of silicon, which makes it cheaper compared to monocrystalline, and are easy to prepare (as they do not need highly controlled fabrication conditions) [26]. However, photoconversion efficiency is sacrificed over the preparation cost. One of the first-generation solar cells are the amorphous silicon solar cells that are used to harvest energy on a small scale, such as for portable and self-powered electronic devices. Unlike monocrystalline and polycrystalline solar cells (where silicon crystals are grown), in amorphous silicon solar cells, a thin film of silicon is deposited on metallic, glass, or plastic substrates. Due to the thin film, this solar cell is highly flexible, but its efficiency is much less (10%) and it does not work at low-light intensities. In order to combine the advantages of amorphous and crystalline silicon, hybrid silicon solar cells have been developed. In this kind of solar cell, amorphous silicon is deposited over a crystalline silicon wafer. The photoconversion efficiency of hybrid solar cell in diffuse light is higher compared to its counterparts. Moreover, the performance is also enhanced at elevated temperatures due to the presence of amorphous silicon. Second-generation solar cells are thin film (few microns) based. The structure of the solar cell is layered, and the amount of material used is much less compared to crystalline silicon solar cells. Additionally, the fabrication cost is also much less, which leads highly economical production of solar cells. These solar cells include amorphous solar cells and two nonsilicon-based solar cells (cadmium telluride and copper indium gallium diselenide). The maximum efficiency recorded for a thin film solar cell was 24.7%. However, the use of cadmium is a serious environmental concern [27].

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The latest technology of solar cells is third-generation solar cells. Largescale research is dedicated to the development of third-generation solar cells. The focus of the research is to enhance the efficiency by using innovative nanomaterials such as silicon nanowires, nanotubes, nonsilicon materials, organic dies, and conducting polymers. The maximum efficiency recorded for a third-generation solar cell is 44 %. But most the third-generation research has not yet been commercialized [28].

11.4.3 Electrochemical energy storage The rapid development of renewable energy resources and state-of-the-art technical equipment has prompted researchers to develop high-capacity, lowcost, and efficient electrical energy storage devices. Work on electrochemical energy storage systems has increased dramatically. Generally, electrochemical energy storage devices can be categorized into three types based on their energy and power density [29]. The trend of energy density and power density is summarized in Fig. 11.7.

11.4.3.1 Rechargeable batteries Rechargeable batteries are energy storage devices that store electrical energy at its electrode by reversible redox reactions. There are different types of

FIGURE 11.7 Range plot of electrochemical energy devices (energy density vs. power density). Reprinted from A.L. Zhu, D.P. Wilkinson, Xinge Zhang, Yalan Xing, A.G. Rozhin, S.A. Kulinich, Zinc regeneration in rechargeable zinc-air fuel cells—a review. J Energy Storage (2016) 35 50, Copyright (2016), with permission from Elsevier.

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electrolyte-based rechargeable batteries such as nickel cadmium, nickel metal hydroxide, lithium ion, lead acid, sodium ion, and magnesium ion batteries. Each type has advantages and inconveniences. Nickel cadmium rechargeable batteries that were marketed and sold in the early 1990s for domestic applications. Nickel hydroxide was used as the positive electrode while cadmium and potassium hydroxide were used as the negative electrode and electrolyte, respectively. Due to their highly conductive nature, nickel cadmium batteries were able to charge and discharge quickly. Moreover, they could run efficiently at very low temperatures, even as low as 220 C. Their energy density was around 45 50 Wh/kg. The self-discharging nature of these batteries as well as the use of a toxic metal (cadmium) are the disadvantages of nickel cadmium batteries [30]. Nickel hydroxide batteries comprise of nickel hydroxide (charge state) as a positive electrode while a metal alloy (MH) with potassium hydroxide as an electrolyte is contained in the negative electrode. The energy density of nickel metal hydroxide battery was around 60 120 Wh/kg, which is comparatively higher than nickel cadmium batteries and the toxicity of this battery is less but the life cycle was also comparatively less. These batteries were mostly used in portable electronics such as mobile phones and laptops, among others [31]. Lithium ion batteries are a fast-growing technology due to their promising features of being light weight, having high energy density (110 160 Wh/kg), and a wider operating potential window. The anode and cathode act as a source for lithium ions when charging the discharge cycle in lithium ion batteries. Lithium ions pass from cathode to anode during charging and are intercalated at the anode, while the lithium ion reverse movement occurs during discharge. One of the shortcomings of these batteries is safety, as an additional circuits are required [32]. Lead acid batteries were invented in 1859 and the first rechargeable batteries to be commercially available. The energy density from lead acid batteries is in the range of 30 50 W/kg and they are mostly used as uninterrupted power sources in automobiles, fork lifts, and so forth. Although rechargeable batteries are extensively used there are a lot of challenges and issues that need to be addressed. Short-life cycle, low-power, and low-energy densities as well as safety issues are the major concerns. Future research must focus on performance enhancement, especially the power density of the battery in order to meet the latest technological demands [32].

11.4.3.2 Supercapacitors Supercapacitors or ultracapacitors differ from conventional capacitors due to their fast charge discharge rates, longer life cycle, high power, and high energy density [33]. There are two types of supercapacitors depending on the charge storage. The first type is a double-layer electrical capacitor (EDLC) that

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stores electrical energy by intercalating charges at the electrode electrolyte interface forming the double layer of charges. The charges are physically deposited by electrostatic attraction, resulting in rapid charge discharge kinetics, high-power density, and long-life cycle (as no chemical reaction is involved) [34]. Carbonaceous materials (graphene, carbon nanotubes, activated carbon, graphite, etc.) are mostly used as the electrode material to store charges. The second type of supercapacitor is known as a pseudocapacitor as it uses faradaic reactions to store electric energy. RuO and MnO are well-known pseudocapacitive materials with specific capacitance closer to their theoretical limit [35]. Since electrochemical signature of pseudocapacitor is like EDLC, it is called a pseudocapacitor. The energy density of this supercapacitor is higher compared to EDLC, but the involvement of chemical reactions decreases its power density as well as life cycle. In electrochemical energy storage device, both type of capacitor materials are combined in a single device to harvest of the advantages of both capacitive materials. A hybrid energy storage device can deliver high energy and power density compared to EDLC and pseudocapacitive material alone. Overall, supercapacitors are capable to deliver high-power density compared to batteries, but their energy density is still far behind batteries [36].

11.4.3.3 Fuel cell Fuel cells are another electrochemical energy storage system that transform the fuels’ chemical energy through redox reactions into electrical energy. They consists of two electrodes and a predominantly hydrogen fuel electrolyte [37]. Unlike batteries, fuel cells need continued fuel and oxygen supply to generate electricity at their electrodes and produce water as a by-product. The main fuel cell types are solid oxide fuel cell, alkaline fuel cell, fuel cell of phosphoric acid, carbonate molten, and fuel cell of the membrane exchange of protons. The fuel cells’ energy density is greater than batteries and supercapacitors, but have a very low power output. Platinum and its alloys are mostly used as the catalyst in the fuel cell, which makes it expensive. Alternative material with good catalytic properties are being researched [38].

11.5 Quantum informatics The development in quantum information processing has led to the creation of many physical quantum computing devices. Quantum computation deserves more attention than quantum gravity computing and warm-hole computing, among others [39]. To realize quantum computation, we need a scalable physical system that has well characterized quantum bits [40]. Semiconductor quantum dots are often used in the quantum information field as physical carriers for quantum bits. Electrons that are present in

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quantum dots have information coded in them. The information is coded in the orbital and spin degree of freedom of excitons [41]. Quantum dots are a very important component in a quantum network such as for terahertz lasers. There has been a lot of development in the fields of fabrication and application of optical systems. Many methods have been developed to integrate quantum dots in these structures. One being the interaction between two quantum dots via phonon mode. Such quantum dots depend on their size, type, material, and photonic structure [42]. Thus the search for optimal ways to achieve these parameters for quantum dots are in study for many years. Our main focus in this section is the use of quantum dots or photonic molecules in new systems with the use of nanostructures and nanotechnologies [39].

11.5.1 Nanostructures in quantum informatics In the next topic, we will explore the application and use of various nanostructures and nanostructures in different fields of quantum computing and quantum information processing.

11.5.1.1 Semiconductor nanostructures in quantum computation Quantum informatics such as quantum computation and quantum information processing has gained increased interest in various fields from optics to condensed matter physics due to the widespread interest of physicists in quantum informatics [37,43]. For a working quantum computer, an algorithm should be designed that can factorize very large numbers way faster than any classical algorithm available [44]. The increase in speed in a quantum algorithm is due to the evolution of quantum mechanics and its parallelism with the superposition principle. A computer that is made of qubits and works on the principle of quantum mechanics will allow quicker factorization of large numbers that is generally very time consuming in classical computers. This type of quantum computer will revolutionize the field of quantum information theory and quantum cryptography. Shor’s factorizing algorithm led to the fact, we can perform error corrections on a quantum system which implies that quantum computer doesn’t need to be extremely perfect [45]. These two inventions led to the fields of quantum computation and quantum information. The main candidate in quantum computation or quantum computer is qubits, and many propositions have been made for qubits such the use of electrons or spin of semiconductors [46] as qubits. Electron spin of silicon quantum dots [39,47] and nuclear spin in semiconductors [48,49] have been proposed to be used as qubits. Donor electrons can also be used as qubits. Quantum computers based on donor electrons are particularly useful because

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of the wave functions of a donor electron in the semiconductors are same. In addition, quantum computers form connections between quantum devices and microelectronic devices. Doping has played a big role in microelectronic technology over past 50 years. As we are reducing the size of integrated circuits (ICs) and transistors the configuration of impurities used for doping is becoming more and more sensitive [50]. This led Kane [46] to propose a donor base semiconductor quantum computer. In this semiconductor quantum computer, the P impurities that are monovalent in semiconductors act as quantum bits. We know that orbital and spin degree of freedom can be used as quantum bits in semiconductor nanostructures. In semiconductor quantum dots the orbital spin or degree of freedom of electron is quantized on atomic level with a 10 nm radius [51]. One of two such quantum dots can form a quantum bit. There is a great advantage of using such orbital qubits because of their simple specific measurements. Charge being experimentally measured via SET [52]. One of the disadvantages of such qubit is that it is highly susceptible to the environment and its decoherence time is way too short for quantum error corrections to be useful. Large-range dipolar coupling for charge qubits is the same as inter qubit coupling. This coupling is necessary for quantum computation due to two qubit gate operations necessary for computing [53,54]. This introduces the difficulty of increasing decoherence because of scaling up. We cannot scale up the quantum computer architecture because that would lead to more qubit coupling via dipolar coupling. Decoupling techniques have to be developed to control decoherence and select qubit gate operations. The main useful aspect of solid state quantum computer architecture is its scalability, but orbital qubit quantum computation via semiconductor nanostructures is very useful because of its entanglement characteristics and unscalable dechoerence [55]. Compared to quantized orbital charge bits, spin qubits in semiconductor nanostructures also have pros and cons. One of the disadvantages of spin qubit is measurement of spin of single electron. One of the big advantages of spin qubit is large spin coherence time at low temperature in semiconductors. The large coherence of spin qubits has led to electron spin qubits to be used as quantum dots and as P donors in semiconductors for quantum computer architecture. Besides the large spin qubits of coherence, there is also an exchange window. Exchange gate is responsible for coupling inter qubit and is short. This short range makes it possible to manipulate two qubit gates. In this quantum computer architecture, there is no question of scaling up due to coupling of only two close qubits, regardless of the number of qubits. The experimental work in semiconductor-based quantum computer architecture has been slow over past five years, although current computers are based on semiconductors ICs with a smaller size. Therefore semiconductor nanostructure-based quantum computers should be scalable given already existing microelectronic

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technology. In semiconductor nanostructures the ultimate dechoerence mechanism for electron spin is spectral diffusion because it cannot be removed from a given sample due to its extremely small nuclear spin scale. The coherence time in GaAs nanostructures is smaller than in semiconductor nanostructures [56]. Quantum computation is a very versatile field. The use of semiconductor nanostructures indicate how nanotechnology will help us improve the world of technology. The use of nanostructures as quantum dots or qubits in the quantum computation field of quantum informatics is a very significant example of how much the advancement have been scaled up.

11.5.1.2 Nanostructures for quantum information processing The main component of quantum information is the qubit. The extensive research in the field of quantum information processing over the past ten years assures that qubits can be used for quantum computation and communication [57]. For practical implementation of quantum information processing the qubit needs to be isolated so they can be initially pre´cised and measured in a controlled environment [58]. The interaction between two quantum bits should be tuned. Universal quantum operations need to be made possible for any quantum gate operation to occur. The quantum system should be scalable up to a few quantum bits. Scalability is a very important factor in quantum information processing. Quantum information processing realized via massive particles such as atoms, ions, and photons are the clearest of all [59]. Atoms and ions are coupled strongly or weakly with photons with well-defined coupling and decaying channels and thus this setting provides an ideal experimental setup for quantum computation and communication. Trapped ions are also considered for complex quantum information processing architecture [60,61]. For large quantum processors quantum information processing can be realized via the solid-state system. A lot of solid-state systems have been proposed in recent years that can be used for quantum computation [62 66]. All the proposals require the same coherent quantum system in complex setting, and they all face the same problem of decoherence. Solid-state systems have a short dechoerence time and their structure is small. It is possible to transfer or manipulate quantum information at a time that is less than the decoherence time in such systems. Solid-state systems that make this communication of quantum information are called nanostructures. The advance in the nanofabrication of solid-state systems such as artificial and naturally existing nanostructures and quantum dots is becoming an active candidate for quantum information processing [67,68]. Quantum dots that possess atom-like properties and have small sizes compared with atoms are considered localized and can be used for storing quantum information.

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Quantum information can be stored in such nanostructures called quantum dots via various two-level systems such as electron spin qubits or charged exciton bits. Coupling of two quantum dots results in a new molecule that can also serve as a qubit, and can be used for storing quantum information. Quantum dots can be integrated over nanocavities for quantum electrodynamics applications [47,69]. The existence of an ultrafast laser that has very large pulse energy, width, repetition rate, and wavelength enhances control of the coherence in semiconductor nanostructures before transient regime [70]. For the implementation of quantum communication and computation, semiconductor nanostructures that are equipped with ultrafast optical technology can serve as solid-state alternative [71].

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Glossary Atomic force microscopy (AFM) or scanning probe microscopy (SPM) a highresolution device used to map topography or other functional properties of surface atoms at atomic resolution capabilities Atomic manipulation atom-by-atom modification of surface structure or chemistry made possible by advanced techniques like atomic force microscopy and scanning tunneling microscopy Band gap energy gap between the valence band and conduction band in a solid in which all electronic energy states are empty Biocompatibility capability of a material in contact with a biological system to perform its intended function without causing deleterious changes Bottom-up a strategy for synthesizing nanomaterials from atomic scale fundamental units where the fundamental units link up to form nanoparticles or nanostructures Carbon nanotube (CNT) an allotrope of carbon with a cylindrical nanostructure and having high-aspect ratios; their unusual electronic and magnetic properties and wide applications Chemical vapor deposition (CVD) a technique for depositing thin films on a substrate using gaseous reactants DNA chip a sensor based on a semiconductor microchip used to identify mutations or alterations in genes Electron microscope a microscope that focusses a collimated accelerated electron beam on the specimen to produce a magnified image at atomic resolution Epitaxy growth of a secondary phase maintaining a perfect crystallographic registry (coherency) with the underlying substrate Fuel cell an electrochemical cell capable of producing electrical energy with fuel or reactant being used up from an external source Molecular electronics the study and application of molecules for electronic device applications Moore’s law a long-term trend in computing hardware suggesting that the number of transistors built in a unit area of the device approximately doubles every 18 months Multilayers thin films of differing chemistry or structure deposited one over the other Nano Greek prefix meaning dwarf or something very small; depicts one billionth (1029) of a unit Nanobots a robot (semi- or fully automated intelligent machine) consisting of components of a few hundred nanometer-dimensions; they are also referred to as nanorobots, nanites, nanomachines, or nanomites Nanofiber fiber with a diameter less than 100 nm Nanofluid colloidal suspension of nanoparticles of metals, ceramic, carbon nanotubes, and so forth.

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Glossary

Nanolithography a nanofabrication technique for patterning nanoscale features; used extensively in the fabrication of Integrated Circuits (ICs) and Nano-Electro-Mechanical Switches (NEMS) Nanomaterial class of materials in which at least one of the dimensions is on the nanoscale (,100 nm) Nanorods 3D nanostructures with an aspect ratio typically in the range of 3 5; all their dimensions are in the range 1 100 nm Nanoshells a thin coating over a core object a few tens of nanometers in diameter Nanotechnology study of manipulating matter on an atomic and molecular scale; generally deals with structures sized between 1 and 100 nanometers in at least one dimension, and involves developing materials or devices possessing at least one dimension within that size Nanowires 1D nanostructures with width of nanometric dimensions and exhibiting aspect ratios of 1000 or more Physical vapor deposition (PVD) a variety of vacuum deposition techniques involving vaporization of atoms from the target material to produce a thin film on a substrate Quantum dots 0D nanostructures in which electron energy states are confined in all three spatial dimensions; their electronic properties are between that of clusters and bulk semiconductors Qubit a quantum-computing equivalent to a bit; with an additional dimension of quantum properties of atoms Resonant tunneling devices (RTD) 2D quantum devices that consist of a long and narrow semiconductor island, with electron confinement only in two directions Scanning near-field optical microscopy (SNOM) illuminates a specimen through an aperture of a size smaller than the wavelength of light used and with the specimen positioned within the near-field regime of the source; by scanning the aperture across the sample through a conventional objective, an image can be formed Self-assembly process in which the components interact within themselves to form aligned or organized structures without any external force Single electron transistor (SET) devices that can detect very small variations in the charge of the gate; charge differences of even one electron can cause the on-and-off switching function of SET Scanning tunneling microscope (STM) an instrument used for imaging surfaces at the atomic level; it works on the principle of quantum tunneling Top-down involves fragmentation of a microcrystalline material to yield a nanocrystalline material; all solid-state synthesis routes of nanostructures fall into this category

Index Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively.

A Accidental NPs, 14 Acid etching, 296 Active nanostructures, 10 Additive manufacturing (AM), 262 AFM. See Atomic force microscope (AFM) AGM. See Alternating gradient magnetometry (AGM) ALD. See Atomic layer deposition (ALD) Alternating gradient magnetometry (AGM), 246 247 Aluminum (Al), 104 AM. See Additive manufacturing (AM) Angular magnification, 122 123 Anisotropies, 247 Anthropogenic nanoparticles, 14 15 Antifungicidal study of metal nanoparticles, 264 Antimony-based III-V alloys (Sb-based III-V alloys), 237 238 APCVD. See Atmospheric pressure-CVD (APCVD) Arc welding, 74 75 Arginine-glycine-aspartic acid (RGD), 263 Artificial crystals, 75 Artificial nanomaterials, 17 Artificial shells, 261 262 Atmospheric pressure-CVD (APCVD), 76 Atom probe instrument, 146 150, 148f comparison with tunneling electron microscope and SIMS, 149 150 construction, 147 limitations of, 149 mathematical analysis, 149 working of, 147 148 Atomic force microscope (AFM), 6 7, 137 144, 137f, 138f, 138t, 139f, 141f, 245 246, 252 advantages and disadvantages, 143 144 applications, 144

construction, 139 140 cantilever, 140 feedback electronics, 140 laser, 139 140 photodetector, 140 sample, 140 scanner, 140 modes of operation, 142 143 contact mode, 142f contact/repulsive mode, 142 noncontact mode, 143f noncontact/attractive mode, 142 tapping/intermittent mode, 142 143 tapping mode during scanning in, 143f working principle of, 141 Atomic layer deposition (ALD), 65 67, 77 Atomic wave function, 194 Atoms, quantum information processing, 304 Automobile industries, nanotechnology in, 22 Averaging method, 246 247

B Band gaps, 197 Band structure, 194 197 band gaps, 197 energetic bands, 196 197, 197f Binding energy, 181 182 Bio nano Genomics, 259 260 Biocompatibility, 264 265 Biomimetic engineering, 264 265 enzyme, 262 263 materials, 261 265 patterning, 58 peptides, 264 Bloch wave vector, 194 Bloch’s theorem, 193 194 implications of, 194 BLS. See Brillouin light scattering (BLS) Bohr’s radius, 11 12

311

312

Index

Bottom-up approach, 15, 153 154, 295 296 Bragg reflected waves, 195 196 Bragg’s law, 37 Bright-field microscopy, 125 Brillouin light scattering (BLS), 246 247

C Cadmium (Cd), 298 Cantilever, 140 Capillarity, 52 Carbon, 292 293 carbon-based materials, 17 nano thermometer, 252 253 Carbon nanotubes (CNTs), 292 arrays, 252 structure, 292 293 Carbonaceous materials, 300 301 Catalytic antibodies, 262 263 cDNA, 276 Cell membrane, 264 Ceramic designs, 263 264 Charge state logics, 201 Charge transport in weakly interacting molecular solids, 217, 217f, 218f Chemical bonds, 90 97, 180 covalent bonding, 92 energy levels of metal nanoparticles, 96f ionic bonds, 91 92 metallic bonds, 92 94 Van der Waals interactions, 95 97 Chemical etching, 57 58 Chemical grafting, 62 63 Chemical surface modification, 61 63 Chemical vapor deposition (CVD), 61, 76 78. See also Physical vapor deposition (PVD) ALD, 77 APCVD, 76 electroplating, 77 78 LPCVD, 76 MOCVD, 76 PECVD, 76 77 Cladding, 69 72 explosion, 71 72 laser, 69 71 CMOS. See Complementary metal-oxide semiconductors (CMOS) CNTs. See Carbon nanotubes (CNTs) Coarse grained materials (CG materials) CTE, 111t Cu, 97 98 features of metals, 111t

nanocrystalline, 109 CoCrMo, 264 265 Coefficient of thermal expansion (CTE), 109, 111t Colloidal lithography, 257 Colloids, 10 Communication technology, 236 237 Complementary metal-oxide semiconductors (CMOS), 239 Complex dielectric function, 234 Compound microscope, 123 124, 124f Compton effect, 29 30 Computer-aided design, 264 265 Computer-aided manufacturing, 264 265 Conducting polymers, 299 Contact printing, 158 Contact/repulsive mode, 142 Cosmetics, nanotechnology in, 23 Covalent bonding, 92, 93f, 181 Cross-conjugation, 216 CTE. See Coefficient of thermal expansion (CTE) CVD. See Chemical vapor deposition (CVD) Cybex nanometer, 6 7

D D-B-A systems. See Donor-bridge-acceptor systems (D-B-A systems) Dark-field microscopy, 125 Davisson Germer experiment, 36 37, 36f de Broglie hypothesis, 32 35 derivation, 32 35 implications, 35 de Broglie wavelength of electrons, 240 Debye temperature, 112 113 Decoupling techniques, 303 Dendrimers, 17 Density functional theory (DFT), 218, 260 261 Density of states (DOS), 39 Deoxyribonucleic acid (DNA), 291 assembly, 279 281 of nanoparticles, 278 uses, 278 computing, 284 286, 286f hybridization, 253 254 microarrays, 275 278 applications, 277 278 principle, 275 277 technology, 277 278 stretching, 259 260 Deposition techniques, 67 78

Index chemical vapor deposition, 76 78 PVD, 67 75, 67f DFT. See Density functional theory (DFT) Diamonds, 292 293 Diblock copolymers, 58 59 Differential interference contrast microscopy, 127 Digital camera, 238 Digital cells, 281 283, 283f Dimensionality of nanomaterials, 226 227 one-dimensional nanomaterials, 226 three-dimensional nanomaterials, 227 two-dimensional nanomaterials, 226 227 zero-dimensional nanomaterials, 226 5-Dimethyl-4-(9-anthracenyl)-julolidine (DMJ-An), 215 Dimethyl-sulfoxide (DMSO), 63 Dip-pin lithography, 60 61 Dipolar coupling, 303 Dirac notation, 194 195 Dissipative self-assembled systems, 81 DMSO. See Dimethyl-sulfoxide (DMSO) DNA. See Deoxyribonucleic acid (DNA) Donor acceptor properties, 215, 216f Donor-bridge-acceptor systems (D-B-A systems), 207, 215 Doping, 302 303 DOS. See Density of states (DOS) Double-walled CNTs, 292 DRAM. See Dynamic random-access memory (DRAM) Drude Sommerfeld model, 190 Drug delivery assistance by porous surface, 64 65 Dry etching process, 243 DSA system. See Dynamic self-assembly system (DSA system) Dynamic random-access memory (DRAM), 236 237 Dynamic self-assembly system (DSA system), 78 79, 79f

E EBL. See Electron beam lithography (EBL) ECM. See Extracellular matrix (ECM) EDLC. See Electrical double-layer capacitor (EDLC) Effective surface area (ESA), 55 Einstein’s mass energy relation, 32 Elastic modulus of nanomaterials, 102 103 Elastin, 261 262 Electric field, 147 148

313

Electric permittivity, 30 Electrical charge separation, 53 Electrical conductivity, 97 98, 181 182, 185 Electrical double-layer capacitor (EDLC), 300 301 Electrochemical energy storage, 299 301, 299f fuel cells, 301 rechargeable batteries, 299 300 supercapacitors, 300 301 Electrochemical exfoliation, 296 Electrodeposition, 243 244 Electrolysis process, 24 25 Electromagnetic radiations, 30 31 Electromagnetic (EM) waves, 30, 30f, 35 Electron beam lithography (EBL), 159 160, 244 245 creation of patterning by electron beam, 161f EBL-based fabrication, 255 procedure, 159 160 aggressive solvent mixture, 159 160 coating by resist, 159 deposition of metallic layer, 159 Electron microscope (EM), 127 129 characteristics of, 128t interaction of primary electrons, 130f with material sample, 127 129 SEM, 129 132 TEM, 132 134 working of, 129 Electron(s), 90, 179 180, 183f beam, 133 134 evaporation, 68 69, 70f in bonding, 180 181 emission of, 181 185, 184f field emission, 183 184 photoelectric emission, 184 secondary electron emission, 184 185 thermionic emission, 182 183 evidence for wave nature, 35 37 Davisson Germer experiment, 36 37, 36f G. P. Thomson’s experiment, 37, 37f holography, 246 247 multiplier, 171 in nanostructures, 187 190. See also Nanostructure fabrication band structure, 194 197 Bloch’s theorem, 193 194 emission of electrons, 181 185 free electron model, 190 193

314

Index

Electron(s) (Continued) 1D potential energy model, 202f quantum effects, 188 190 resonant tunneling, 202 204 SET, 198 202 oscillations across, 115 transfer between molecules, 216 transparency, 129 Electronic(s) conductivity, 99 devices, 237 238 nanotechnology in, 21 properties of materials, 185 187 electrical properties, 185 186 optical properties, 186 187 Electroosmosis, 255 Electroplating technique, 77 78, 78f Elemental carbon, 292 EM. See Electron microscope (EM) EM waves. See also Electromagnetic waves (EM waves) Endothelial cell (ECs) seeding, 263 Endothelial nitric-oxide-synthase (eNOS), 56 Energetic bands, 196 197, 197f Energy harvesting and storage, 296 301 electrochemical energy storage, 299 301 PENG, 296 297, 297f solar cells, 297 299 nanotechnology in energy production, 22, 22f quanta, 30 31 transfer, 220 221 Engineered NPs, 15 eNOS. See Endothelial nitric-oxide-synthase (eNOS) Epitaxial techniques, 236 237 Equilibrium self-assembly, 81 ESA. See Effective surface area (ESA) Etching method, 159, 236 237, 243, 257 Exciton, 293 Explosion cladding, 71 72, 72f Explosive welding. See Explosion cladding Extracellular matrix (ECM), 263

F Feedback electronics, 140 Fermi energy, 189, 191 192 Fermi liquid model, 190, 190f Fermi Dirac statistics, 190 Ferromagnetic materials, 247 248 Ferromagnetic resonance (FMR), 246 247

FIB. See Focused ion beam (FIB) Fibrin glue, 263 264 Fibronectin, 263 Field emission, 183 184 First-generation solar cells, 297 298 2,7-Fluorenone (FNn), 215 Fluorescence, 293 method, 144 145 energy level diagram, 145f singlet and triplet excited states of molecules, 145f FMR. See Ferromagnetic resonance (FMR) Focused ion beam (FIB), 154 155, 167 173, 168f, 170f, 172f applications of, 172 173 construction of, 167 172 dual platform, 171 172 gas source usage or deposition, 171 imaging detectors, 171 ion column, 170 171 LMIS, 168 170 sample stage, 171 vacuum system, 168 milling technique, 255 principle, 172 Food industry, nanotechnology in, 25 Free electron model, 190 193, 195f Fuel cells, 301

G G. P. Thomson’s experiment, 37, 37f Gas metal arc welding (GMAW), 74 75, 74f Gas tungsten arc welding (GTAW), 74 75 GB. See Grain boundary (GB) GCN. See Graphite carbon nitride (GCN) Genetic circuits, 283 284, 285f Glass, 257 GMAW. See Gas metal arc welding (GMAW) Grafting chemical, 62 63 copolymerization technique, 62 63 method, 61 polyethylene glycol, 62 63 synthetic, 263 Grain boundary (GB), 101 Graphene, 238 239 sheet, 292, 292f Graphite, 292 Graphite carbon nitride (GCN), 250 Green nanotechnology, 8 Green’s function, 218 219 Growth technique. See Electrodeposition

Index GTAW. See Gas tungsten arc welding (GTAW)

J

H

K

Heat capacity of nanomaterials, 103 104 Heisenberg’s uncertainty principle, 37 39 Heteroepitaxy, 162 High temperature colloidal synthesis, 295 296 Holographic lithography. See Interferometric lithography Home appliances, nanotechnology in, 24 25 Homoepitaxy, 162 Houston-based Nanospectra Biosciences, 23 24 Hybrid silicon solar cells, 297 298 Hybridization, 275 276, 275f of atomic orbitals, 213 215, 214f, 215f Hydrothermal synthesis, 296 Hydroxyapatite (HA) osteointegration, 263 264

I ICs. See Integrated circuits (ICs) Imaging-guided photothermal therapy, 250 Impact angle, 71 72 Incidental NPs, 14 Infrared radiation detection, 201 Innovation, 9 Inorganic crystal semiconductor, 293 Inorganic NPs, 13 Inorganic-based nanofluidic devices, 260 261 Integrated circuits (ICs), 39 40 Interaction volume, 129 Interfaces, 51 chemical composition at metal air interface, 52f chemistry, 253 254 surface and, 54 55 Interference lithography. See Holographic lithography Interferometric lithography, 245, 256 257 Interlocking crystals of silicon, 297 298 Intermolecular interactions, 81 Ion(s), 304 column, 170 171 implementation method, 61 ionic bonds, 91 92, 91f Irys Next generation mapping chip (Irys NGM chip), 259 260 Irys NGM chip. See Irys Next generation mapping chip (Irys NGM chip)

315

Janus NPs (JNPs), 250

Kepler ocular, 120 Kerr microscopy, 248 Kinetic energy, 36

L Langmuir Blodgett films (LB films), 63 64 Laser, 139 140, 245 cladding, 69 71, 70f Lateral force microscopy (LFM), 164 165 LB films. See Langmuir Blodgett films (LB films) LCD. See Liquid crystal displays (LCD) Lead acid batteries, 300 Lewis structures, 209 212 limitations, 210 212 LFM. See Lateral force microscopy (LFM) Li-ion batteries. See Lithium ion batteries (Liion batteries) Light, 29 30 propagation, 30 31 quanta, 30 31 Linear equations, 213 Liquid crystal displays (LCD), 207 Liquid-metal ion source (LMIS), 167 170, 169f Lithium ion batteries (Li-ion batteries), 251, 300 Lithography, 59 60, 155 160, 155f, 236 237, 242t, 243, 244f EBL, 159 160 photolithography, 156 159 LM. See Longitudinal mode (LM) LMIS. See Liquid-metal ion source (LMIS) Longitudinal mode (LM), 115 Lorentz microscopy, 246 247 Low-pressure CVD (LPCVD), 76

M Macromolecules, 91 92 Magnetic magnetic-based nanostructure devices, 240 NPs, 248, 251 tapes, 111 transmission X-ray microscopy, 246 247 Magnetic force microscopy (MFM), 246 247 Magnetic nanomaterials (MNMs), 240 241 Magnetic nanostructures, 240 251 applications, 248 251, 249f

316

Index

Magnetic nanostructures (Continued) MRI of mice bearing tumors, 250f magnetic materials fabrication by lithography technique, 242t properties, 246 248 distribution of magnetic coercivity, 247f synthesis, 241 246 Magnetization energy, 247 248 Magneto-optical Kerr effect, 246 247 Magneto-resistive magnetic random-access memory (MRAM), 244 245 Magnetostatic energy, 247 248 Magnetostriction AFM, 246 247 Magnetron sputtering, 73 74 Materials, 226 selection, 258 waves, 35 Mathessiens’s rule, 185 186 MBE. See Molecular beam epitaxy (MBE) Medicine, nanotechnology in, 23 24 Melting point, 104 109, 104f, 106f enthalpy and, 105f MEMS. See Microelectromechanical system (MEMS) MEMs. See Microelectronics mechanical system (MEMs) Messenger Ribonucleic acid (mRNA), 259 260, 276 Metal inert gas welding (MIGW), 74 75 Metal nanoparticles (MNPs), 13, 241, 263 264 Metal organic-CVD (MOCVD), 76 Metal-based nanostructures, 17 Metallic bonds, 92 94, 96f Metallic nanoparticles, 226 MFM. See Magnetic force microscopy (MFM) Micellar synthesis, 296 Micro-SQUID (µ-SQUID), 246 247 Micro-transfer molding, 59 60 Micro/nanofluidic chip, 258 Microelectromechanical system (MEMS), 255, 261 262 MEMS-based nanofabrication process, 257 Microelectronics, 236 239, 239t Microelectronics mechanical system (MEMs), 295 Microfluidic devices, 253 254 Microreactor synthesis, 296 Microscopy, 120 127, 126t. See also Optical microscopy concept of, 120 122 inborn spherical aberration, 121f ray diagram of simple microscope, 122f

Microstructural defects, 99 Microwave-assisted methods, 296 MIGW. See Metal inert gas welding (MIGW) MIP. See Molecular imprinting polymers (MIP) MNMs. See Magnetic nanomaterials (MNMs) MnO, 300 301 MNPs. See Metal nanoparticles (MNPs) MOCVD. See Metal organic-CVD (MOCVD) Molecular beam epitaxy (MBE), 161 164, 163f, 295 296 advantages and disadvantages, 164 epitaxy growth of crystalline layers on substrate, 162 features, 163 164 heteroepitaxy, 162 homoepitaxy, 162 layout, 162 163 in situ growth monitoring techniques, 164 working principle, 162 Molecular electronics, 207 208 charge transport in weakly interacting molecular solids, 217 donor acceptor properties, 215 electron transfer between molecules, 216 hybridization of atomic orbitals, 213 215 room-temperature conductivity value, 213f indicator of evolution, 209f Lewis structures, 209 212 single molecule electronics, 217 222 variational approach to calculate molecular orbitals, 212 213 Molecular engineering, 80 Molecular imprinting polymers (MIP), 262 263 Molecular nanosystems, 10 Molecular self-assembly (MSA), 80 idea of, 80 systems, 79 80 Mollusk shells, 261 262 Monocrystalline silicon solar cells, 297 298 Moore’s law, 39 41, 238, 251 252 limits, 41 second law, 40 Mote Carlo simulation, 248 MRAM. See Magneto-resistive magnetic random-access memory (MRAM) MSA. See Molecular self-assembly (MSA) Multifunctional micro/nanofluidic devices, 256 Multiwalled CNTs, 292

Index

N Nacre, 261 262 Nano thermal devices, 251 253 Nano-sized building blocks (NBBs), 166 Nanobiotechnology, 4, 273. See also Nanotechnology digital cells, 281 283 sample plasmid, 281f DNA assembly of nanoparticles, 278 diseased and normal labeling, 276f sample fluorescence image of microarray, 277f DNA computing, 284 286 DNA microarrays, 275 278 genetic circuits, 283 284 binary string for potential solution, 282f regulatory interaction, 282f protein and DNA assembly, 279 281 Nanoceramics, 20 21 Nanochannel collapsing, 255 Nanocoatings, 16 17 Nanocomposites, 16 17 Nanocrystalline (NC), 97 98 coarse-grained, 109 Nanocrystals (NC), 293 Nanoelectrodes, 154 155 Nanofluidic devices, 253 261 fabrication of nanostructures for, 256f 1D and 2D of planar, square, and high aspect ratio nanochannels, 254f Nanofluidic separator. See Nanopore device Nanoimprint lithography (NIL), 256, 258 Nanoimprints, 245 246 Nanomaterials, 4 6, 15 20, 89 91, 97 115, 153 154, 225 227 biomimetic materials, 261 265 dimensionality, 226 227 elastic modulus of, 102 103 electrical properties, 97 99 magnetic nanostructures, 240 251 magnetic properties, 109 113 mechanical properties elastic modulus, 102 103 graph of thin films, 100f Hall Petch plot, 102f hardness, 100 102 nano thermal devices, 251 253 nanofluidic devices, 253 261 nanophotonics, 235 240 nanowires, 229 235 one-dimensional, 18 19, 226 optical properties, 113 115

317

QDs, 227 228 relation between molecular geometry, 94f subatomic particles, 89 90 subatomic physics to chemical systems, 90 97 thermal properties coefficient of thermal expansion, 109 grain-size dependence, 110f heat capacity, 103 104 melting point, 104 109 size-dependent melting behavior, 107t three-dimensional, 20, 227 two-dimensional (2D), 19 20, 226 227 types, 16t, 17 20, 18f zero-dimensional, 17 18, 226 Nanometer, 9 Nanometer scale, 10 13 special at nanoscale, 10 13 quantum effects, 11 12 surface area-to-volume ratio, 12 13 Nanooptics, 236 Nanoparticles (NPs), 5, 13 17, 14f anthropogenic nanoparticles, 14 15 natural nanoparticles, 14 Nanophotonics, 235 240 basic principles, 240 optoelectronics and microelectronics, 236 239 Nanopillar array, 58 Nanoplates, 16 17 Nanopore device, 259 260 Nanorobots, 23 24 Nanoscale, 10 fabrication, 253 254 materials, 89 Nanoscaled biomolecules, 290 291 Nanoscience, 3 6, 4f, 240 from nanoscience to nanotechnologies, 6 in nature, 5 Nanosensors, 23 24 Nanostructural materials, 289 290 typical range of dimensions, 290f Nanostructure fabrication, 153 154, 154f. See also Electron(s)—in nanostructures FIB, 167 173 lithography, 155 160 MBE, 161 164 self-assembled masks, 164 166 stamp technology stamping, 173 174 Nanostructure technology, 89, 90f Nanostructured magnetic materials, 111

318

Index

Nanostructured metals (NS metals), 112 113, 112t Nanostructures, 39, 304 in quantum informatics, 302 305 semiconductor nanostructures in quantum computation, 302 304 for quantum information processing, 304 305 Nanotech-based sunscreens, 9 Nanotechnology, 3 10, 4f, 240, 289 290. See also Nanobiotechnology applications, 20 25 in automobile industries, 22 in cosmetics, 23 in electronics, 21 in energy production, 22 in food industry, 25 in home appliances, 24 25 in medicine, 23 24 nanotechnology in sports equipment, 25 in space technology, 23 in textile industry, 24 challenges in, 25 emergence, 9 10 first generation, 9 fourth generation, 10 second generation, 10 third generation, 10 energy harvesting and storage, 296 301 Feynman talks on small structures, 7 8 nanoscaled biomolecules, 290 291 from nanoscience to, 6 nanotechnology-based biomimetic materials, 263 QDs, 293 296 quantum informatics, 301 305 structure of CNTs, 292 293 Nanotubes, 16 17, 292, 299 Nanowires, 229 235 applications of, 234 235 plasmon absorption peaks, 235f properties of, 230 234 electron transport, 232 233, 232f magnetic, 230 231 optical, 233f, 234 thermoelectric, 231 232 variation of squareness, 231f synthesis, 229 230 Nanoworld, 4 Naphthalene-1,8:4,5-bis (dicarboximide) (NI), 215 Natural enzymes, 262 263

Natural nanomaterials, 17 Natural nanoparticles, 14 NBBs. See Nano-sized building blocks (NBBs) NC. See Nanocrystalline (NC); Nanocrystals (NC) NESS. See Nonequilibrium steady states (NESS) Newton’s second law, 191 Nickel cadmium rechargeable batteries, 299 300 Nickel hydroxide batteries, 300 NIL. See Nanoimprint lithography (NIL) Noncontact/attractive mode, 142 Noncovalent interactions, 80 Nonequilibrium self-assembly, 81 Nonequilibrium steady states (NESS), 81 Nonsilicon materials, 299 NPs. See Nanoparticles (NPs) NS metals. See Nanostructured metals (NS metals) Nuclear fusion, 46 47 Nucleic acids, 291

O Off-the-shell approach, 262 Ohm’s law, 188, 191 Oligophenylene (Phn), 215 One-dimension (1D) confinement, 39 nanomaterials, 15, 18 19, 226 One-dimensional nanomaterials, 18 19, 226 Optical communication, 239 Optical microscopy, 122 125 advantages and disadvantages, 125 characteristics, 128t compound microscope, 123 124 magnification of, 123 124 limitations, 124 125 microscopic techniques, 125 127 bright-field microscopy, 125 dark-field microscopy, 125 differential interference contrast microscopy, 127 phase contrast microscopy, 125 127 simple microscope, 122 123 magnification of, 122 123 Optoelectronics, 236 239, 239t Organic dies, 299 Organic NPs, 13 Organic photovoltaic (PV) devices, 207 Oxidized wafer, 157

Index

P Packaging systems, 25 Palladium (Pd), 98 99 Passive nanostructures, 9 Patterning, 58 59 PBN. See Pyrolytic boron nitride (PBN) PCR. See Polymerase chain reaction (PCR) PEB. See Pulsed electron beam (PEB) PECVD. See Plasma-enhanced chemical vapor deposition (PECVD) PENG. See Piezoelectric nanogenerators (PENG) PET. See Polyethylene terephthalates (PET) Pharmaceutical attachment to surfaces, 64 Phase contrast microscopy, 125 127 p-phenylethynylene (PEnP), 215 Photoconversion efficiency, 297 298 Photodetector, 140 Photoelectric effect, 29 30 Photoelectric emission, 184, 185f Photoelectronic effect, 238 239 Photografting of polymers, 62 Photolithography, 59 60, 64 65, 156 159, 160f etching, 159 exposing to UV light, 158 contact printing, 158 primary exposure, 158f projection printing, 158 proximity printing, 158 masking, 157 oxidation, 157 steps of, 156 159, 156f surface preparation, 156 157 types of, 157 Photonics, 235 236 Photons, 304 Photooxidation (PO), 62 Photorefractive tweezer, 239 Photovoltaic cell, 238 239 Photovoltaic effect, 297 298 Photovoltaic tweezer (PVT), 239 Physical vapor deposition (PVD), 67 75, 67f. See also Chemical vapor deposition (CVD) arc welding, 74 75 cladding, 69 72 sputtering, 72 74 thermal spraying, 75 vacuum deposition, 68 69 Piezoelectric effect, 296 297

319

Piezoelectric nanogenerators (PENG), 296 297, 297f Planck relation, 30 31 Planck’s quantum theory, 29 30 Plasma deposition method, 64 Plasma vapor deposition (PVD), 61 62 Plasma-based etching, 57 58 Plasma-enhanced chemical vapor deposition (PECVD), 76 77 Plasma-related CVD methodologies, 61 Plasmalemma. See Cell membrane PLD. See Pulsed laser deposition (PLD) PMMA. See Poly methyl methacrylate (PMMA) p-n junction, 297 298 PO. See Photooxidation (PO) Poly methyl methacrylate (PMMA), 157, 262 Polycrystalline silicon solar cells, 297 298 Polyethylene glycol (PEG) grafting, 62 63 Polyethylene terephthalates (PET), 257 Polymerase chain reaction (PCR), 280 Polymers, 56 57, 59 60 films, 63 64 polymer-derived stents, 55 56 Polynucleotide strand, 291, 291f Post lithography technique, 243 244 Power radiation, 146 Primary electrons, 127, 130f Programmable SET logic, 201 202 Projection printing, 158 Protein(s), 59 60 assembly, 279 281, 280f folding, 253 254 Proximity printing, 158 Pseudocapacitor, 300 301 Pt-based concave nanotubes, 250 Pulsed electron beam (PEB), 75 Pulsed electron deposition, 75 Pulsed laser deposition (PLD), 75 PVD. See Physical vapor deposition (PVD); Plasma vapor deposition (PVD) PVT. See Photovoltaic tweezer (PVT) Pyrolytic boron nitride (PBN), 163 164

Q QCEs. See Quantum confinement effects (QCEs) QDs. See Quantum dots (QDs) Quadruple mass spectrometer (QMS), 164 Quantum, 188 computers, 302 303 computing, 228

320

Index

Quantum (Continued) confinement, 39, 188 phenomena, 4 principles of light, 240 size effect, 293 Quantum computation, semiconductor nanostructures in, 302 304 Quantum confinement effects (QCEs), 11 12, 39, 295 Quantum dots (QDs), 39, 90, 199 200, 227 228, 229f, 293 296, 294f, 295f, 301 302, 304 305 applications, 228 biological, 228 optical, 228 quantum computing, 228 chemical synthesis, 296 fabrication, 295 296 properties, 293 295 Quantum effects, 11 12 de Broglie hypothesis, 32 35 electromagnetic waves, 30 energy quanta, 30 31 evidence for wave nature of electrons, 35 37 exercise, 47 Heisenberg’s uncertainty principle, 37 39 Moore’s law, 39 41 QDs, 39 quantum tunneling, 41 47 wave particle duality, 29 30 Quantum informatics, 301 305 nanostructures in, 302 305 Quantum information processing, nanostructures for, 304 305 Quantum tunneling, 41 47, 250 251 applications, 46 47 through single potential barrier, 43 46 Qubits, 302 304 coupling, 303 Queen conch (Strombus gigas), 261 262

R Radio-frequency/direct current battery (RF/ DC battery), 72 73 Radioactive decay process, 46 47 Radiotherapy/photothermal therapy, 250 Reaction diffusion phenomena, 81 Rechargeable batteries, 299 300 Reflection high-energy electron diffraction (RHEED), 162 164 Reflective back cantilever, 140 Refluxing, 296

Resistant thermometry, 252 253 Resolving power (RP), 120 121 Resonant tunneling, 202 204 Reynolds number (Re), 253 254 RF/DC battery. See Radio-frequency/direct current battery (RF/DC battery) RGD. See Arginine-glycine-aspartic acid (RGD) RHEED. See Reflection high-energy electron diffraction (RHEED) Richardson-Dushman equation, 183 Rock’s law. See also Moore’s second law RP. See Resolving power (RP) RuO, 300 301

S Sb-based III-V alloys. See Antimony-based III-V alloys (Sb-based III-V alloys) Scalability, 304 Scanner, 140 Scanning electron microscope (SEM), 6, 129 132, 131f, 138t, 167 advantages of, 131 132 disadvantages of, 132 dissimilarities between, 135 limitations, 132 working of, 130 131 Scanning hall microscopy, 246 247 Scanning magneto-resistance (SMR), 246 247 Scanning probe microscopy (SPM), 253 Scanning thermal microscopy (SThM), 253 Scanning tunneling microscope (STM), 6 7, 135 137, 135f, 245 246 components and workings, 136 137 features, 136 137 Schrodinger wave equation, 43 SE. See Secondary electrons (SE) Second-generation approach of biomimetic materials, 262 solar cells, 298 Secondary electrons (SE), 127 emission, 184 185 Secondary ion mass spectrometry (SIMS), 149 150 Self-assembled masks, 164 166, 165f building blocks, 166 distinctive features, 165 examples, 166 interactions, 165 166 macroscopic scale, 166 order, 165 properties, 166

Index Self-assembled monolayers (SAMs), 61, 63, 78 79 Self-assembly, 78 81 equilibrium and nonequilibrium selfassembly, 81 MSA systems, 79 80 SEM. See Scanning electron microscope (SEM) Semiconductor laser, 236 237 nanocrystal, 293 nanostructures in quantum computation, 302 304 quantum dots, 301 302, 304 305 SET. See Single electron transistor (SET) Severe plastic torsion straining (SPTS), 112 113 Shape memory polymers (SMPs), 58 61, 63 64 Shor’s factorizing algorithm, 302 Silica, 257 nanowires, 258 Silicon, 236 237, 257 nanowires, 299 Silver nanoparticles (AgNPs), 24 25 SIMS. See Secondary ion mass spectrometry (SIMS) Single electron spectroscopy, 201 Single electron transistor (SET), 198 202, 198f, 199f applications, 200 202, 200f charge state logics, 201 detection of infrared radiation, 201 plots of, 201f programmable SET logic, 201 202 single electron spectroscopy, 201 supersensitive electrometer, 200 operation of, 199 200 Single molecule electronics, 217 222 acceptor-bridge-donor system, 221f DNA double-helix, 222f examples, 220 222 theoretical background, 218 220, 220f terminal single molecule device, 219f Single nucleotide polymorphism (SNP), 259 260 Single-walled CNTs, 292 Slab gel electrophoresis, 255 Slack interactions, 165 166 SM. See Surface modification (SM) Small crystals, 293 Smartphone devices, 238 SMPs. See Shape memory polymers (SMPs)

321

SMR. See Scanning magneto-resistance (SMR) SNP. See Single nucleotide polymorphism (SNP) Solar cells, 297 299 Sol gel method, 242 243 Solid-state systems, 304 Solution-based methods, 295 296 Space technology, nanotechnology in, 23 Speed of light, 30 Sphere lithography, 257 Spin coating, 157 Spin polarized STM, 246 247 Spin qubit, 303 SPM. See Scanning probe microscopy (SPM) Sports equipment, nanotechnology in, 25 SPR. See Surface plasmon resonance (SPR) SPTS. See Severe plastic torsion straining (SPTS) Sputtering process, 72 74, 73f magnetron sputtering, 73 74 SQUID. See Superconducting quantum interface device (SQUID) SSA system. See Static self-assembly system (SSA system) Stain-proof clothes, 24, 24f Stamp technology stamping, 173 174 industrial applications, 174 operations, 173 174 stamping lubricant, 174 Standoff distance, 71 72 Static self-assembly system (SSA system), 78 79, 79f Stencil-assisted printing, 60 61 SThM. See Scanning thermal microscopy (SThM) STM. See Scanning tunneling microscope (STM) Strombus gigas. See Queen conch (Strombus gigas) Structural QDs, 293 Supercapacitors, 300 301 Superconducting quantum interface device (SQUID), 246 247 Supersensitive electrometer, 200 Supramolecular chemistry, 81 Surface, 51 and interface, 54 55 patterning, 58 61 physics and chemistry, 53 54 scratching/roughening, 56 58 tension, 55 Surface area-to-volume ratio, 12 13

322

Index

Surface modification (SM), 55 65 methods, 55 65 chemical surface modification, 61 63 drug delivery assistance by porous surface, 64 65 pharmaceutical attachment to surfaces, 64 surface patterning, 58 61 surface scratching/roughening, 56 58 thin films and surface coatings, 63 64 Surface plasmon resonance (SPR), 114 115 Synchrotron radiation, 145 146, 146f Synthetic grafting, 263 System of nanosystems, 10

T Tapping/intermittent mode, 142 143 TCNQ. See Tetracyanoquinodimethane (TCNQ) TEM. See Transmission electron microscope (TEM) Tensile strength, 292 293 Tetracyanoquinodimethane (TCNQ), 220 221 Tetrathiafulvalene (TTF), 220 221 Textile industry, nanotechnology in, 24 TGA. See Thermogravimetric analysis (TGA) Thermal evaporation, 68 Thermal spraying technique, 75, 75f Thermionic emission, 182 183 dependence of, 183 Thermogravimetric analysis (TGA), 252 Thermomechanical deformation technique, 258 Thermoplastics, 257 258 Thin films deposition, 65 78, 65f deposition techniques, 67 78 properties and typical applications, 66t and surface coatings, 63 64 Third-generation solar cells, 299 Three-dimensional nanomaterial (3D nanomaterials), 20, 227 Three-dimensional nanomaterials, 20, 227 TiAlV, 264 265 Time-independent Schrodinger equation, 212 Tissue engineering, 20 21, 263 264 Titanium aluminum (TiAl), 101 TM. See Transverse mode (TM) Top-down approach, 15, 154 155, 295 296 Torque magnetometry, 246 247 Transmission electron microscope (TEM), 132 134, 133f, 138t advantages of, 134

applications of, 134 disadvantages of, 134 dissimilarities between, 135 working of, 133 134 Transverse mode (TM), 115 Trigonometric ratios, 123 TTF. See Tetrathiafulvalene (TTF) Tungsten inert gas (TIG). See Gas tungsten arc welding (GTAW) Tunneling current, 136 quantum, 41 47, 250 251 resonant, 202 204 Two-dimension (2D) nanomaterials, 19 20, 226 227 nanoparticles, 240 241 structures, 39 Two-dimensional (2D) nanomaterials, 19 20, 226 227

U Ultra-high vacuum (UHV), 163 164 Ultrasonication exfoliation, 296 Ultraviolet (UV), 121

V Vacuum deposition, 68 69 electron beam evaporation, 68 69, 70f thermal evaporation, 68, 69f Vacuum system, 168 Van der Waals forces, 292 Van der Waals interactions, 95 97 Vapor liquid solid (VLS), 229 230 Vibrating sample magnetometry (VSM), 243, 246 247 VLS. See Vapor liquid solid (VLS) VSM. See Vibrating sample magnetometry (VSM)

W Wave particle duality, 29 30

X X-ray lithography, 245

Y Young’s modulus, 113, 292 293

Z Zero dimension (0D) structures, 39 Zero-dimensional nanomaterials, 17 18, 226 Zinc oxide NPs, 23