Functional Materials from Carbon, Inorganic, and Organic Sources: Methods and Advances 9780323857888

Table of contents :
Cover
Half Title
Functional Materials from Carbon, Inorganic, and Organic Sources: Methods and Advances
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Contents
About the Editors
Contributors
1. Exploring the world of functional materials
1.1 Introduction
1.2 General concept and fundamental properties of functional materials
1.2.1 Carbon-based functional materials
1.2.2 Polymer quantum dots and derived nanomaterials
1.2.3 Modified biochar-based functional materials
1.2.4 Carbon quantum dots as new functional materials
1.2.5 Functional materials for renewable and sustainable energy applications single and multijunction functional solar cells
1.2.6 Heterostructure functional materials
1.2.7 Ceramics as functional materials
1.2.8 Nanocomposite-based functional materials
1.2.9 Chalcogenide semiconductors as functional materials
1.2.10 Hybrid biomass-derived carbonaceous functional materials
1.3 A brief introduction to processing technology of functional materials
1.3.1 Electrodeposition
1.3.2 Chemical/physical vapor deposition
1.3.3 Epitaxial beam method
1.3.4 Ultrasonification
1.3.5 Solution phase technique
1.3.6 In situ synthesis
1.3.7 Microtechnology and flow chemistry
1.3.8 Gas-phase synthesis
1.3.9 Sol–gel route
1.3.10 Bioinspired synthesis of functional materials
1.4 Basic and advanced applications of functional materials
1.5 Concluding remarks
References
2. Preparation, characterization, and applications of graphene based quantum dots (GQDs)
2.1 Introduction
2.1.1 Graphene-based quantum dots
2.1.2 Preparation methods of graphene quantum dots
2.2 Preparation methods of graphene quantum dots
2.2.1 Top-down method
2.2.1.1 Reduced GO by modified Hummers method
2.2.1.2 Mechanical exfoliation
2.2.1.3 Hydrothermal synthesis of GQDs
2.2.1.4 Solvothermal synthesis
2.2.1.5 Acidic oxidation/oxidative cleavage/oxidative cutting/ chemical e
2.2.1.6 Ultrasonic-assisted liquid-phase exfoliation
2.2.1.7 Electrochemical synthesis
2.2.1.8 Nanolithography
2.2.2 Bottom-up methods
2.2.2.1 Carbonization/pyrolysis
2.2.2.2 Soft template method
2.2.2.3 GQDs from fullerenes
2.2.2.4 Chemical vapor deposition
2.3 Characterization of graphene QDs
2.3.1 Spectroscopic techniques
2.3.1.1 X-ray photoelectron spectroscopy
2.3.1.2 X-ray diffraction
2.3.1.3 Fourier-transform infrared spectroscopy
2.3.1.4 UV-visible
2.3.1.5 Raman spectroscopy
2.3.1.6 Dynamic light scattering
2.3.1.7 Dual polarization interferometry
2.3.1.8 Nuclear magnetic resonance
2.3.2 Microscopic methods
2.3.2.1 Scanning electron microscopy
2.3.2.2 Transmission electron microscopy
2.3.2.3 Atomic force microscopy
2.3.2.4 Scanning tunneling microscope
2.3.3 Brunauer-Emmett-Teller
2.4 Applications of graphene QDs
2.4.1 Graphene QDs in energy conversion and storage
2.4.1.1 Introduction
2.4.1.2 Graphene QDs in photoelectrochemical solar cells
2.4.1.3 Graphene QDs in fuel cells
2.4.2 Graphene QDs in biomedical applications
2.4.2.1 GQDs in drug delivery
2.4.2.2 GQDs as an antibacterial
2.4.2.3 GQDs for bioimaging
2.4.2.4 GQDs as a treatment for neurodegenerative disorders
2.4.3 Graphene QDs for sensors
2.4.3.1 Luminescence chemosensors
2.4.3.2 Electrochemical chemosensors
2.4.3.3 Biosensors
2.5 Summary
2.6 Future prospective
References
3. Synthesis and applications of carbon-polymer composites and nanocomposite functional materials
3.1 Introduction
3.2 Synthesis of graphene
3.2.1 Mechanical or micromechanical exfoliation
3.2.2 Electrochemical synthesis or exfoliation
3.2.3 Plasma discharge etching of graphite
3.2.4 Chemical vapor deposition
3.2.4.1 Thermal chemical vapor deposition
3.2.4.2 Plasma-enhanced chemical vapor deposition
3.2.5 Epitaxial growth on silicon carbide
3.2.6 Unzipping carbon nanotubes
3.2.7 Summary of graphene synthesis methods
3.3 Synthesis of functionalized graphene
3.3.1 Graphene oxide and reduced graphene oxide
3.3.2 Surface functionalization
3.3.3 Structural characteristics
3.4 Synthesis of graphene-based composites
3.4.1 Graphene-polymer composites
3.4.2 Graphene-nanoparticles composites
3.5 Graphene growth mechanism
3.6 Challenges and opportunities
3.7 Future perspectives
3.8 Summary
Acknowledgments
References
Further reading
4. Graphene and graphene oxide: Application in luminescence and solar cell
4.1 Introduction
4.2 Preparation techniques to synthesize graphene and GO
4.2.1 Mechanical cleaving (exfoliation)
4.2.2 Liquid-phase exfoliation
4.2.3 Chemical vapor deposition
4.2.4 Solvothermal synthesis
4.3 Characterization of graphene and graphene oxide
4.3.1 Electrical measurements
4.3.2 Optical measurements
4.3.3 Structural and microstructural characteristics
4.4 Fundamental information about luminescence and solar cell materials
4.4.1 Luminescent materials
4.4.2 Solar cell materials
4.5 Application of graphene and graphene oxide in field of luminescence
4.5.1 Graphene and graphene oxide as luminescent material
4.5.1.1 Graphene-based luminescence materials
4.5.1.2 GO-based luminescence materials
4.6 Application of graphene and graphene oxide in the field of solar or photovoltaics cells
4.6.1 Graphene and graphene oxide as solar cell materials
4.6.2 Traditional solar cell materials and graphene and graphene oxide
4.7 Concluding remark
References
5. Application of graphene in energy storage devices
5.1 Introduction
5.2 Types of graphene
5.2.1 Monolayer graphene
5.2.2 Multilayer graphene
5.2.3 Graphene oxide
5.2.4 Reduced graphene oxide
5.3 Application of graphene in energy storage devices
5.3.1 Graphene in lithium-ion batteries
5.3.2 Graphene in electrical double-layer capacitors
5.3.3 Graphene in dye-sensitized solar cells
5.4 Conclusions
References
6. Solar cell efficiency enhancement by modeling the downconversion and downshifting of functional materials
6.1 Introduction
6.2 Fundamental aspects of solar cell
6.3 Downconversion and downshifting for solar cell generation
6.4 Nanomaterials in downconversion process
6.5 Downconversion approach in solar cell devices
6.5.1 Downconversion in silicon solar cells
6.6.1 Luminescent downshifting applications for thin film solar cells
6.6 Functional luminescent materials for downshifting applications in solar cells
6.7 Solar cells’ functional materials with downconversion approach
6.8 Current scenario and future trends of functional luminescent materials for solar cell
6.9 Concluding remark
Acknowledgments
References
7. Exploration of UV absorbing functional materials and their advanced applications
7.1 Introduction
7.2 UV radiation absorbers
7.2.1 UV-blocking organic compounds
7.2.1.1 Avobenzone
7.2.1.2 Oxybenzone
7.2.1.3 Phenylbenzimidazole sulfonic acid
7.2.1.4 Octyl methoxycinnamate
7.2.1.5 Octyl salicylate
7.2.2 Inorganic UV-blocking compounds
7.2.2.1 TiO2 nanoparticles
7.2.2.2 ZnO nanoparticles
7.2.2.3 Other nanoparticles
7.3 Sport-specific risk factors for UV exposure
7.4 UV coatings: Materials and applications
7.5 Recent development
7.5.1 Chemistry-related developments
7.5.1.1 Solvent-borne UV coatings
7.5.1.2 Water-based UV coatings
7.5.1.3 UV powder coatings
7.6 New applications
7.6.1 Automotive applications
7.6.1.1 Suitability of UV coatings for automotive applications
7.6.1.2 UV-curable clear coats (head lamps, reflectors, Alu wheels)
7.6.1.3 UV-curable primer/sealer (eco-efficiency)
7.6.1.4 UV-curable coatings for car refinish
7.6.1.5 Scratch-resistant coatings for automotive applications
7.6.1.6 Plastic applications in automotive
7.6.2 Industrial applications
7.6.2.1 UV-curable coatings for hard topcoats on plastic
7.6.2.2 UV curing of highly flexible coatings
7.6.2.3 Coil coatings
7.6.2.4 Adhesives
7.6.2.5 UV inkjet
7.6.2.6 UV systems for dental applications
7.6.2.7 Furniture foil coatings
7.6.3 Film coating instead of painting: An innovative concept
References
8. Interface engineering in oxide heterostructures for novel magnetic and electronic properties
8.1 Magnetism in oxide materials
8.2 Exchange interaction
8.2.1 Super and double exchange
8.2.2 RKKY interaction
8.3 RKKY interaction in diluted magnetic oxide thin films
8.4 Role of nonmagnetic spacer thickness in oxide heterostructures
8.5 Spin-orbit coupling (SOC) in perovskite of 3d, 4d, and 5d transition metal oxides
8.5.1 Spin-orbit coupling
8.6 Interface-induced magnetism of perovskite oxide heterostructures: SOC role
8.6.1 Interfacial Dzyaloshinskii-Moriya interaction (iDMI)
8.6.2 Magnetic anisotropy
8.7 Surface and thickness influence on magnetic anisotropy
8.8 Interface role in determining the magnetic anisotropy
8.9 Further modification of magnetic anisotropy while competing with other physical phenomena
8.10 Summary
References
9. Composition induced dielectric and conductivity properties of rare-earth doped barium zirconium titanate ceramics
9.1 Introduction
9.2 Barium zirconium titanate (BZT)
9.3 Applications of barium zirconium titanate (BZT)
9.4 Doping of barium zirconium titanates with different rare-earth elements
9.5 Effects of rare-earth doping on different properties of BZT
9.5.1 Structural and morphological properties
9.5.2 Raman spectroscopic properties
9.5.3 Temperature and frequency dependent dielectric properties
9.5.4 Temperature and frequency dependent conductivity properties
9.5.4.1 Complex impedance spectroscopy
9.5.4.2 Modulus spectroscopy
9.5.4.3 AC conductivity
9.6 Summary
9.7 Future aspects
References
10. Nanocomposite-based functional materials: Synthesis, properties, and applications
10.1 Introduction
10.2 Nanocomposite-based functional materials: Types
10.2.1 Myxene-based nanocomposite functional materials
10.2.2 Polymer-based nanocomposites
10.2.3 Lanthanide nanocomposites
10.2.4 Inorganic material functionalized carbon nanotubes
10.3 Characterization methods
10.3.1 X-ray diffraction methods
10.3.2 Structural characterization techniques
10.3.3 Morphological studies
10.3.4 Optical properties
10.4 Novel method of synthesis of nanocomposite-based functional materials
10.4.1 Ultrasound-assisted method
10.4.2 Microwave-assisted method
10.4.3 Hydro/solvothermal method
10.4.4 Solution mixing
10.4.5 Functionalization
10.4.6 In situ polymer mixing
10.4.7 Miscellaneous novel methods
10.5 Dielectric properties of nanocomposite-based functional materials
10.6 Application of nanocomposites-based functional materials
10.7 Conclusions and outlook
References
11. Hazardness of mercury and challenges in functional materials of lighting devices
11.1 Introduction
11.1.1 Luminescence
11.1.1.1 Photoluminescence
11.1.2 Compact fluorescent lamps and linear fluorescent lamps (CFL & LFL)
11.2 Mercury
11.2.1 Ecological outcomes of Mercury
11.2.2 Mercury exposure and health impacts on humans
11.2.3 Drawbacks of Hg from lighting devices
11.2.4 Challenges in disposal of Hg-based lighting
11.3 Mercury free lighting
11.4 Conclusion
References
12. Synthesis and application of CdSe functional material
12.1 Introduction
12.2 Material properties of CdSe nanostructures
12.2.1 Size effect
12.2.2 Shape effect
12.2.3 Doping effect
12.3 Growth of CdSe nanostructures
12.3.1 Chemical methods
12.3.2 Physical methods
12.4 Applications of CdSe-based nanostructures
12.4.1 Photo-detectors
12.4.2 Field-effect transistors
12.4.3 Solar cells
12.4.4 Light-emitting diodes
12.4.5 Biological imaging
12.5 Future challenges and opportunities
References
13. Synthesis and physico-chemical characterization of ZnS-based green semiconductor: A review
13.1 Introduction
13.2 Synthesis of ZnS nanostructure semiconductor
13.3 Characterization of ZnS nanostructure semiconductor
13.3.1 Most popular morphology/forms of ZnS
13.3.2 Main properties of ZnS in micro/nanocrystalline
13.3.2.1 Optoelectronics properties of ZnS
13.3.3 ZnS semiconductor-based device fabrication methods
13.4 Physico-chemical properties of ZnS nanostructure semiconductor
13.5 Application of ZnS nanostructure semiconductor
13.5.1 Catalysis, photocatalysis, and solar cells
13.5.2 Lasers
13.5.3 Light-emitting diodes
13.5.4 Sensors
13.6 Perspectives of ZnS nanostructure semiconductor and challenges
Acknowledgments
References
Further reading
14. Multifacets of organometallic quinoline complexes
14.1 Introduction
14.2 Importance of organic complexes
14.3 Historical review on quinoline complexes
14.4 Advances in quinoline-based material
14.4.1 LED
14.4.2 OLEDs
14.4.3 Display
14.4.4 Solid state lighting
14.4.5 Solar cell
14.5 Versatile applications of quinoline-based complexes
14.6 Futures prospective of quinoline-based complexes
14.7 Conclusion
References
15. Mq2(M=Zn, Cd, Ca, and Sr) organometallic functional complexes for luminous paints
15.1 Introduction
15.2 Luminous paints
15.3 Experimental section
15.3.1 Properties of raw materials
15.3.2 Synthesis of the pigment
15.3.3 Preparation of substrate
15.3.4 Different compositions of paint
15.4 Results and discussion
15.4.1 Pigment characterization
15.4.1.1 Photoluminescence spectra
15.4.1.2 Oil adsorption capacity
15.4.1.3 Bulk density
15.4.1.4 Hiding capacity
15.5 Testing of painted panels
15.5.1 Water resistance test
15.5.2 Drying test
15.5.3 Impact test
15.5.4 Flexibility test
15.5.5 Adhesion test
15.5.6 Humidity test
15.5.7 Chemical resistance
15.5.7.1 Acid test
15.5.7.2 Alkali test
15.5.7.3 Salt spray test
15.5.8 Luminescence test
15.6 Conclusions
15.7 Future scope
References
16. Metal organic framework of Eu (dmh)3phen polymer matrices and their applications for energy-efficient solution-processed OLEDs
16.1 Introduction
16.2 Experimental
16.2.1 Preparation of blended thin films
16.3 Result and discussion
16.3.1 Characterization of blended thinfilms in solid state
16.3.1.1 UV–vis absorption spectra of blended thin films in solid state
16.3.1.2 Photoluminescence spectra of blended thin films in solid state
16.3.1.3 CIE coordinates of blended thin films in solid state
16.3.1.4 Thermal annealing effect on PL spectra
16.3.1.5 Determination of film thickness
16.3.2 Characterization of blended films in solvated state
16.3.2.1 UV–vis absorption spectra of blended thin film in various organic solvents
16.3.2.2 Absorption spectra of solvated Eu(dmh)3phen/PMMA blended thin films
16.3.2.3 Absorption spectra of solvated Eu(dmh)3phen/PS blended thin films
16.3.2.4 Determination of optical energy gap of thin films in PMMA/PS
16.3.2.5 Photoluminescence spectra of solvated Eu(dmh)3phen/ PMMA/PS thin films
16.3.2.6 Determination of relative intensity ratio (R-ratio)
16.3.2.7 CIE coordinates for blended films of PMMA and PS in solvated state
16.4 Device and industrial applications
16.5 Conclusions
References
17. Advanced functional nanomaterials of biopolymers: Structure, properties, and applications
17.1 Introduction
17.2 Biopolymers
17.3 Biopolymer-based biomaterials
17.4 Biopolymers classification and properties
17.5 Biopolymer-based nanomaterials, nanocomposites, and formulation strategies
17.6 Formulation strategies for the fabrication of biopolymeric nanocomposites
17.7 Modification and improvement in the physicochemical properties of the biopolymer composites
17.8 Thermal, mechanical, and optical properties of biopolymers and its composites
17.9 Diverse applications of biopolymers
17.9.1 Biopolymers in drug delivery
17.9.2 Biopolymers in tissue engineering
17.9.3 Biopolymer-based implants for long-term treatment
17.9.4 Biopolymers in packaging industries
17.9.5 Biopolymer-based nanocomposites as sensors
17.9.6 Biopolymer-based nanocomposites in renewable energy
17.9.7 Biopolymer-based nanocomposites in textile
17.10 Conclusion and future prospective
References
18. Synthesis and applications of biomass-derived carbonaceous materials
18.1 Introduction of biomass
18.2 Classification of biomass
18.2.1 Wood and woody biomass
18.2.2 Herbaceous biomass
18.2.3 Aquatic biomass
18.2.4 Animal and human waste biomass
18.2.5 Biomass mixture
18.3 Application of biomass feedstock
18.4 Energy from biomass
18.5 Synthesis of porous carbon from biomass
18.5.1 Rice husk–derived chemically activated carbon
18.5.2 Introduction of hybrid carbonaceous material
18.5.3 Functionalization of carbonaceous materials for catalysis applications
18.6 Application of biomass-derived hybrid carbon in organic synthesis
18.7 Conclusion
Acknowledgment
References
19. Summary, future trends, and challenges in functional materials
19.1 Introduction
19.2 Summary and highlights of the discussed chapters
19.3 Future, challenges, and scope in functional materials
References
Index

Citation preview

Functional Materials from Carbon, Inorganic, and Organic Sources

Woodhead Publishing Series in Electronic and Optical Materials

Functional Materials from Carbon, Inorganic, and Organic Sources Methods and Advances

Edited by

Sanjay J. Dhoble Amol Nande N. Thejo Kalyani Ashish Tiwari Abdul Kariem Arof

An imprint of Elsevier

Woodhead Publishing is an imprint of Elsevier 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom Copyright © 2023 Elsevier Ltd. 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. ISBN: 978-0-323-85788-8 (print) ISBN: 978-0-323-90929-7 (online) For information on all Woodhead publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisitions Editor: Stephen Jones Editorial Project Manager: Rafael Guilherme Trombaco Production Project Manager: Anitha Sivaraj Cover Designer: Mark Rogers Typeset by STRAIVE, India

Contents 1. Exploring the world of functional materials 2. Preparation, characterization, and applications of graphene-based quantum dots (GQDs) 3. Synthesis and applications of carbon-polymer composites and nanocomposite functional materials 4. Graphene and graphene oxide: Application in luminescence and solar cell 5. Application of graphene in energy storage devices 6. Solar cell efficiency enhancement by modeling the downconversion and downshifting of functional materials 7. Exploration of UV absorbing functional materials and their advanced applications 8. Interface engineering in oxide heterostructures for novel magnetic and electronic properties 9. Composition induced dielectric and conductivity properties of rareearth doped barium zirconium titanate ceramics 10. Nanocomposite-based functional materials: Synthesis, properties, and applications 11. Hazardness of mercury and challenges in functional materials of lighting devices 12. Synthesis and application of CdSe functional material 13. Synthesis and physico-chemical characterization of ZnS-based green semiconductor: A review 14. Multifacets of organometallic quinoline complexes 15. Mq2(M=Zn, Cd, Ca, and Sr) organometallic functional complexes for luminous paints 16. Metal organic framework of Eu(dmh)3phen polymer matrices and their applications for energy-efficient solution-processed OLEDs 17. Advanced functional nanomaterials of biopolymers: Structure, properties, and applications 18. Synthesis and applications of biomass-derived carbonaceous materials 19. Summary, future trends, and challenges in functional materials

About the Editors Sanjay Dhoble Prof. Sanjay J. Dhoble is presently working as a professor in the Department of Physics at R.T.M. Nagpur University, Nagpur, India. During his research career, he has worked on the synthesis and characterization of solid-state lighting materials, as well as the development of radiation dosimetry phosphors using thermoluminescence techniques and utilization of fly ash. Dr. Dhoble has more than 780 research publications in international and national peer-reviewed journals, more than 582 research papers are published in Scopus-indexed journals. Dr. Dhoble is an Editor of the journal Luminescence. Affiliations and Expertise Professor, Department of Physics, R.T.M. Nagpur University, Nagpur, India

Amol Nande Dr. Amol Nande is working as an Assistant Professor in Department of Physics, Guru Nanak College of Science, Ballarpur, India. He also leads the Department of Computer Science and Electronics, Guru Nanak College of Science, Ballarpur, India. He obtained his Ph. D. degree in Physics from the University of Canterbury, New Zealand. His Ph. D. thesis was on “Superconducting properties in percolating thin films”. He has published 17 research papers in international and national journals. He received the Marsden doctoral fellowship for completing his Ph. D. work. He also cleared several national level examinations like UGC-CSIR (all India rank 52), Graduate Aptitude Test in Engineering (GATE), Joint Entrance Screening Test (all India rank 12) and Bhabha Atomic Research Center Screening test. Affiliations and Expertise Assistant Professor, Department of Physics, Guru Nanak College of Science, Ballarpur, India

N. Thejo Kalyani N. Thejo Kalyani is Assistant Professor in the Department of Applied Physics, Laxminarayan Institute of Technology, Nagpur, India. Affiliations and Expertise Assistant Professor, Department of Applied Physics, Laxminarayan Institute of Technology, Nagpur, India

Ashish Tiwari Dr. Ashish Tiwari received his Ph. D degree from Guru Ghasidas Vishwavidhyalaya: A Central University, Bilaspur, India in 2012. The topic of his PhD dissertation was Luminescence properties of ZnS nanophosphors using different capping molecules. He is currently working as an Assistant Professor in the Department of Chemistry, in Government College Pamgarh, India. He has more than 21 research publications in International and National peer reviewed journals and authored one book (Lambert Academic Publisher) and a book chapter (Springer). He is recipient of the Young Scientist Award from Chhattisgarh Council of Science & Technology (CCOST) in 2012. His current research interest is synthesis of luminescent nanomaterials. Affiliations and Expertise Assistant Professor, Dr. Bhimrao Ambedkar Govt. College, Pamgarh, India

Abdul Arof Abdul Kariem Arof is a Retired Professor from the Department of Physics, Faculty of Science, University of Malaya in Kuala Lumpur, Malaysia. Affiliations and Expertise Retired Professor, Department of Physics, Faculty of Science, University of Malaya, Kuala Lumpur, Malaysia

Contributors

Keshaw Ram Aadil Virology Lab, Department of Microbiology, Bharat Ratna Late Shri Atal Bihari Vajpayee Memorial Govt. Medical College; Department of Botany, Govt. Digvijay Autonomous Post-Graduate College, Rajnandgaon, Chhattisgarh, India Sanu Awasthi Center for Basic Science, Pt. Ravishankar Shukla University-Raipur, Raipur, Chhattisgarh, India Tanmaya Badapanda Department of Physics, C.V. Raman Global University, Bhubaneswar, Odisha, India T.M.W.J. Bandara Department of Physics and Postgraduate Institute of Science, Faculty of Science, University of Peradeniya, Peradeniya, Sri Lanka Subhash Banerjee Department of Chemistry, Guru Ghasidas Vishwavidyalaya, Bilaspur, Chhattisgarh, India B.S.

Butola

Indian

Institute

of

Technology

Delhi,

New

Delhi,

India

Dipti Chitnis Department of Physics, Anand Niketan College, Warora, India Prachi Chopade Department of Physics, Savitribai Phule Pune University (formarly University of Pune), Pune, India Shikha Chouhan Indian Institute of Technology Delhi, New Delhi, India N.S. Dhoble Department of Chemistry, Sevadal Mahila Mahavidhyalaya, Nagpur, India S.J. Dhoble Department of Physics, R.T.M. Nagpur University, Nagpur, Maharashtra, India Ashish Dubey Center for Basic Sciences, Pt. Ravishankar Shukla University, Raipur, India Neha Dubey Department of Physics, Govt. V.Y.T.PG. Auto. College Durg, Durg, Chhattisgarh, India

xii

Contributors

Vikas Dubey Department of Physics, Bhilai Institute of Technology Raipur, Raipur, Chhattisgarh, India Sunil Dutt Department of Chemistry, Government Degree College Daulatpur Chowk, Ghanari, Una, Himachal Pradesh, India Suresh Gosavi Department of Physics, Savitribai Phule Pune University (formarly University of Pune), Pune, India Megat Muhammad Ikhsan Megat Hasnan Faculty of Engineering, Universiti Malaysia Sabah, Jalan UMS, 88400 Kota Kinabalu, Sabah, Malaysia Shweta Jagtap Department of Instrumentation Science, Savitribai Phule Pune University (formarly University of Pune), Pune, India Harit Jha Department of Biotechnology, Guru Ghasidas Vishwavidyalaya, Bilaspur, Chhattisgarh, India Abhijeet R. Kadam Department of Physics, R.T.M. Nagpur University, Nagpur, Maharashtra, India N. Thejo Kalyani Department of Applied Physics, Laxminarayan Institute of Technology, Nagpur, India Jagjeet Kaur Department of Physics, Govt. V.Y.T.PG. Auto. College Durg, Durg, Chhattisgarh, India Chitra S. Khade Department of Physics, G H Raisoni Institute of Engineering & Technology, Nagpur, India Ayush Khare Department of Physics, National Institute of Technology, Raipur, Chhattisgarh, India Raj Kumar Department of Pharmaceutical Sciences, NCRC, University of Michigan, Ann Arbor, MI, United States Savisha Mahalingam Institute of Sustainable Energy, Universiti Tenaga Nasional, Kajang, Selangor, Malaysia Abreeza Manap Institute of Sustainable Energy, Universiti Tenaga Nasional, Kajang, Selangor, Malaysia Marta Michalska-Domanska Institute of Optoelectronics, Military University of Technology, Warszawa, Poland

Contributors

xiii

G. Nag Bhargavi Department of Physics, Govt. Pt. Shyamacharan Shukla College, Raipur, Chhattisgarh, India Amol Nande Department of Physics, Guru Nanak College of Science, Ballarpur, Chandrapur, Maharashtra, India G.B.M.M.M. Nishshanke Department of Physics and Postgraduate Institute of Science, Faculty of Science, University of Peradeniya, Peradeniya, Sri Lanka Ikhwan Syafiq Mohd Noor Physics Division, Centre of Foundation Studies for Agricultural Science; Ionic Materials and Energy Devices Laboratory, Department of Physics, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia Azimah Omar Higher Institution Centre of Excellence (HICoE), UM Power Energy Dedicated Advanced Centre (UMPEDAC), Level 4, Wisma R&D, University of Malaya, Jalan Pantai Baharu, Kuala Lumpur, Malaysia Geetika Patel Department of Chemistry, Guru Ghasidas Vishwavidyalaya, Bilaspur, Chhattisgarh, India Nasrudin Abd Rahim Higher Institution Centre of Excellence (HICoE), UM Power Energy Dedicated Advanced Centre (UMPEDAC), Level 4, Wisma R&D, University of Malaya, Jalan Pantai Baharu, Kuala Lumpur, Malaysia; Renewable Energy Research Group, King Abdulaziz University, Jeddah, Saudi Arabia Swati Raut Department of Physics, R.T.M. Nagpur University, Nagpur, Maharashtra, India Janita Saji Department of Sciences and Humanities, School of Engineering and Technology, Christ (Deemed to be University), Bangalore, Karnataka, India P.G. Shende Department of Surface Coating Technology, Laxminarayan Institute of Technology, Nagpur, India R.G. Tanguturi Department of Materials Science and Engineering, Hubei University, Wuhan, Hubei, PR China T.M.A.A.B. Thennakoon Department of Physics and Postgraduate Institute of Science, Faculty of Science, University of Peradeniya, Peradeniya, Sri Lanka Ashish Tiwari Department of Chemistry, Dr. Bhimrao Ambedkar Govt. College Pamgarh, Pamgarh, Chhattisgarh, India Prajakta P. Varghe Department of Surface Coating Technology, Laxminarayan Institute of Technology, Nagpur, India

Exploring the world of functional materials

1

Amol Nandea, N. Thejo Kalyanib, Ashish Tiwaric, and S.J. Dhobled Department of Physics, Guru Nanak College of Science, Ballarpur, Chandrapur, Maharashtra, India, bDepartment of Applied Physics, Laxminarayan Institute of Technology, Nagpur, India, cDepartment of Chemistry, Dr. Bhimrao Ambedkar Govt. College Pamgarh, Pamgarh, Chhattisgarh, India, dDepartment of Physics, R.T.M. Nagpur University, Nagpur, Maharashtra, India a

1.1

Introduction

The tremendous progress of modern technology is bringing about marked changes in our society. The commercial technology and advanced research prefer to work for reducing the cost of product and improvement in synthesis process. Hence, the preferred research is oriented to a confinement process which includes the efficient production of materials in a cost-effective and definite application manner. Thus the current research is highly target motivated toward potential application and functionality [1–3]. The newly synthesized or prepared materials with potential applications are referred as functional materials. This interpretation is already implicated for several materials which include a variety of materials—carbon-based materials (such as graphene, carbon dots [CDs], and carbon nanotubes [CNTs]), biomaterials, hard materials, semiconductors, energy materials, energy storage, heterostructures, memory devices, and polymer materials [4–8]. The functional materials cover all type of materials, as every material is designed or synthesized by considering a definite chemical or physical functionality. Thus it can be said that the functional material’s properties are adopted and optimized for a particular purpose. It is further observed that the functional properties of the materials are structure, size, and morphology dependent [9–12]. This chapter encompasses the discussion about the general concept and fundamental properties of functional materials. Later, the chapter provides a brief review on selected functional material types and their application to provide the scope of the book.

1.2

General concept and fundamental properties of functional materials

The definition of functional materials represents a material’s capacity to execute a certain “function” in response to a certain stimuli [13]. Different types of functional material are engineered or morphed by changing their components. These materials Functional Materials from Carbon, Inorganic, and Organic Sources. https://doi.org/10.1016/B978-0-323-85788-8.00014-8 Copyright © 2023 Elsevier Ltd. All rights reserved.

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can be extensively categorized in various architecture viz. nanoparticles, nanorods, nanoporous materials, and other hierarchical nanostructure. A material is recognized as functional material if the changes in their properties lead to novel applications in various fields of science and technology, including optoelectronic, semiconductor devices, sensors, biomedical applications, energy storage, supercapacitor, environmental applications, magnetocaloric materials, solar harvesting functions, and so on [1,3–5,8,14]. The schematic is shown in Fig. 1.1. The advancement in technology and material innovation is advancing at an unexpected rate. The sophisticated ceramics, metals, and polymers are important classes of functional materials used in a wide variety of applications [15]. Functional materials are typically defined as those materials that have unique inherent characteristics and functions that include ceramics, metals, polymers, and organic molecules. To optimize their interaction with the environment, functional materials having a high surface-to-volume ratio are synthesized. Functional surfaces and functional particles are two common examples. This chapter mainly focuses on the development of carbon-based, inorganic, and biological functional materials.

Fig. 1.1 Schematic shows the different types of functional materials.

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1.2.1 Carbon-based functional materials Carbon-based materials include graphite, graphene, biochar, activated carbon, carbon cloth and nanotube, and maghemite and magnetite carbons [16]. CNTs, CDs, and heteroatom-doped carbon materials, and carbon-based hybrid materials have superior functionalities for potential application in diverse fields, including solar thermal fuels, advanced thermal management, and electrochemical energy storage and biological applications. The graphene has excellent properties like no band gap, hence perfect in the photovoltaic (PV) applications. Extensive studies on graphene and graphene-based materials revealed the superior properties like chemical stability, biocompatibility, low toxicity, and flexibility for surface functionalization. Graphene is the world’s thinnest ( 0.34 nm) material with no compromise in its robustness. It is transparent and stretchable crystal with stable two-dimensional crystal structures. The electrons in graphene behave as if they are mass less because of which they acquire higher velocities nearing about 300th times the velocity of light. Graphene exhibits high electrochemical efficiency, hence enhancing the catalytic application [17]. The carbon atom in the graphene structure is sp2 hybridized and have a honeycomb-like structure comprising of graphene covalent bonds. These properties allow the hydrogen to be adsorbed to the surface of the graphene by chemisorption and physisorption events, which is further escalated by functionalization and doping of graphene. These properties make it an ideal candidate for hydrogen source material. The synthesis of carbon-based functional materials from low-cost, abundant, and sustainable sources has led to its applications, in water remediation and production of electrodes for energy storage. The applications of graphene and graphene-based materials are summarized in Fig. 1.2. Graphene oxide is a two-dimensional carbon nanomaterial and generally synthesized by exfoliation of oxidized layers of graphite via chemical oxidation methods [18]. It has several functional groups, including carboxyl groups, hydroxyl, and epoxy groups, hence can easily form organic links to the host polymer. CNTs are one of the most exploited carbon species because of their high mechanical, electronic, and physical properties. CNTs, also known as bucky tubes, are cylinders of one or more layers of graphene, accordingly classified as single valve or multivalve CNTs. In other words, it is 2D graphene sheet rolled up with continuous unbroken hexagonal mesh into a cylindrical tube. They offer high electrical and thermal conductivity, lighter in weight, highly flexible, and durable. They find wide range of application in optoelectronics, biomedical, energy storage systems, flexible displays, sensors, water treatment, photoluminescence devices, drug delivery, and hydrogen storage materials. The functionalization of CNTs provides increased modification on the surface [19]. The hollow structure of the CNT is highly permeable and allow rapid mobility of liquid and gas molecules in the channel, thus are proven fillers in various membranes. Moreover, it has antifouling behavior, strength, disinfection properties, and rejections.

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Fig. 1.2 Schematic to show the applications of graphene in various fields.

1.2.2 Polymer quantum dots and derived nanomaterials Quantum dots are less than  5 nm nanoparticles showing unique and having excellent properties such as electronic, optical, luminescence, and semiconducting properties. Polymer dots are made from both conducting and nonconducting polymers with appropriate processing techniques. Polymer quantum dots can be easily surface modified and thus promising in hybrid/nanocomposite materials. They are combined with other quantum dots and nanoparticles to form advanced hybrid nanomaterials. These have important application in solar cell, supercapacitor, electronics, probes, gas sensor, biosensor, bioimaging, and drug delivery [20]. The development of high-performance polymeric materials has enabled synthesis of polymer lightemitting diodes (PLEDs) having a multilayer structure (hole injection layer [HIL]/ hole transfer layer/emission layer [EML]/electron transfer layer/electron injection layer). This can result in better electro-optical characteristics compared with PLEDs with unilayer (EML only) and bilayer structures (HIL/EML). The high charge immobility, indissolubility, and photoluminescence instability, of QDs can be effectively

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overcome by a passivating with polymers. The quantum size carrier confinement and temperature-independent emission of QDs and high technological capabilities of polymers would allow development of active waveguides based on hybrid organic (polymer)–inorganic (semiconductor quantum dots) materials. The organic polymer nanoparticles (molecular imprinted polymers) are one of the most widely used nanoparticles in analytical sciences [21]. Polymeric surfactants can effectively solubilize inorganic nanoparticles as the polymers themselves have nanometer dimensions. The polymer-coated surfaces are stabilized satirically up to a distance of nanometers and have coiled conformation, which creates large free volume inside, and can be filled by solvent. This large interaction might lead to good solubility in solvents for the polymer. Eventually, it can open the door for incorporating the fluorescent nanoparticles in semiconducting polymers and for their electrical applications.

1.2.3 Modified biochar-based functional materials Biochar is a carbon-rich material formed by thermal decomposition of biomass under limited oxygen supply. The texture and their surface chemistry vary with the source of thermal treatment, precursor (biomass), and the fabrication method. The most commonly used application of biochar is their catalytic property and as catalyst support for environmental remediation [22]. The presence of carboxylic, carbonyl, and hydroxyl groups on the biochar surface can act as the site of an electron transfer. These multifunctional groups attached on the surface can play a nucleophile or electrophile in the process of reaction. These intrinsic properties of biochar-based materials are used for the catalytic degradation and photolysis for the removal of harmful organic contaminants and heavy metals, in bioenergy and biorefinery process, in the remediation of contaminated water and soil. The biochar with its carbon richness and renewability property, surface modification, is now considerably exploited for environmental protection purposes and development of many other functionalized carbon materials for energy- and environment-associated applications. The ranges of magnetic biocomposites (BC) composites, nanometal/nanometallic oxides/hydroxide BC composites and layered nanomaterial-coated BCs, and physically/chemically activated BCs are used to remove a wide range of organic contaminants, such as organic dyes, phenols, and pharmaceutically active compounds [23].

1.2.4 Carbon quantum dots as new functional materials CDs, as a new generation of carbon-based nanomaterial, have diverse physicochemical properties, like high biocompatibility, unique optical properties, low cost, green, abundant functional groups (e.g., amino, hydroxyl, carboxyl), high stability, and electron mobility [24]. CDs are quasi-0D carbon-based material with a size below 20 nm, with intrinsic fluorescence property and were accidentally obtained from the purification of single-walled CNTs in 2004. They can be doped with N, S, P, and B heteroatoms and facile to chemical modifications [25]. CDs have excellent photo-absorption capacity with electron-accepting and transport properties, hence prominently used in

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solar cells/PV and supercapacitor/battery. carbon quantum dots (CQDs) have abundant functional groups on the surface and can be further exploited for multicomponent electrical active catalysts. These internal interactions enhance the charge transfer, thus increasing their catalytic performance. The recent significant advances include sensing, anticounterfeiting, light-emitting diodes, catalysis, PVs, supercapacitors, and also biomedical applications [26–31].

1.2.5 Functional materials for renewable and sustainable energy applications single and multijunction functional solar cells The light on exposure to a PV cell is absorbed and transfers it to negatively charged particles in the semiconducting material called electrons and this current is extracted through conductive metal contacts. The efficiency of a PV cell is simply the amount of electrical power coming out of the cell compared with the energy from the solar light falling on it and depends on the characteristics (such as intensity and wavelengths) of the light available and multiple performance characteristics of the cell. The attributes of good material for application depends on their tunable band gaps, high absorption coefficients, long charge carrier (electron–hole) diffusion lengths, and lowtemperature solution processability, morphology of each functional layer, and optimization of interfacial characteristics [32–34]. New functional materials are coming up for next-generation energy conversion and storage systems. The performances and costs of these systems are serious issue and mainly arise due to their intrinsic performance. The functional materials for sustainable energy applications includes siliconbased, thin-film, organic solar cells, Perovskite solar cells, and dye sensitized PV solar cells, thermophotovoltaic device modeling and photoelectrochemical cells [35–41]. Nowadays, multijunction solar cells, consisting of stacks of different semiconductor materials, are continuously explored [35,42]. The bandgap of each layer differently absorbs the solar spectrum, making wide range of sunlight absorption which is unachievable using the single-junction cells. Multijunction solar cells can reach record efficiency levels because the light that doesn’t get absorbed by the first semiconductor layer is captured by a layer beneath it.

1.2.6 Heterostructure functional materials Heterostructures are fabricated by incorporation of two distinctive materials and are so merged to enhance the property from its individual counterparts. Heterostructure solar cells represent a class of highly absorbing thin-film materials in which power loss owing to electron–hole recombination at the front is lesser [37,38]. Different heterostructures depict different synergistic relations between two or more building blocks leading to the multifunctionality. The integrated design and synthesis procedures of heterostructures essentially depend on the controlled growth mechanism of 1D, 2D, or 3D building blocks. Heterostructures combining graphene and h-BN are most effective van der Waals-bonded 2D hybrid crystals and used in novel optoelectronic devices. Two-dimensional (2D) materials, such as graphene, silicene, germanene, hexagonal

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boron nitride (hBN), and transition metal dichalcogenides (TMDs, such as MoS2), have gained importance owing to excellent properties and potential applications (Tareen et al., 2021). These 2D layers can be integrated into a multilayer stack (vertical 2D heterostructure), including graphene/silicene (G/Si), graphene/hexagonal boron nitride (G/hBN), silicene/HBN, silicene/GaS, TMDCs/graphene, stacked TMDCs, phosphorene/MoS2, and phosphorene/graphene for variety of applications [43–52]. The combination of heterostructures functional materials will open up new opportunities for yielding unprecedentedly high performance.

1.2.7 Ceramics as functional materials Functional ceramics are characterized by their usual functions because of their tailormade structures and properties [53,54]. Engineering ceramics can be divided into functional ceramics and high-strength structural ceramics. Metal–ceramic composites are natural candidates for these demanding applications because of the diverse and dissimilar physical properties of metals and ceramics, which give the final products attractive mechanical, electrical, thermal, and biochemical properties and property combinations [55]. Bioceramic composites are the biomaterials in distinct technological and human health-related applications [56]. These materials are revolutionizing the management of distinct pathologic conditions. Recent innovations, based on multifunctional properties, such as release of drugs/growth factors, enhanced biological functionality, and antibacterial activity, are gaining considerable importance. Porous glasses and ceramics are extensively studied in the fabrication of sensors, filters, gas absorbers, luminescent media, etc. They can be used to produce porous membranes, energy storage, and heat exchangers. Porous glasses and ceramics are of great interest for the catalysis of chemical reactors and separation applications [57–61]. Porous silica films have been shown to produce excellent results in microelectronic applications for interlayer isolation (porous low-k films) and are a promising material in nonvolatile memories. Moreover, bioactive porous glasses are attractive implant materials in medicine. Moreover, carbon-based nanomaterials, such as CNTs or graphene, are being used as reinforcements for ceramic matrices because of their small size, high aspect ratio, and exceptional mechanical properties. Their incorporation into different ceramic matrices, such as Al2O3, Si3N4, or ZrO2, enhances the mechanical behavior.

1.2.8 Nanocomposite-based functional materials The functional nanocomposite materials fall under categories of conductive functional composite materials, magnetic functional composite materials, electromagnetic absorption or permeation functional composite materials, optical functional composite materials, acoustic damping functional materials, functional materials of friction wear, medical composite materials, etc. [62]. In nanocomposites-based functional materials, the quantum size effect causes the formation of distinct energy bands (quantized energy) contrary to the continuous energy structure of bulk materials. The behavior of nanoscale materials in terms of absorbance, emission, conductivity, and so on is

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remarkably dependent on the size, shape, interparticle separation, and surface chemistry within the 100–1000 nm range. The increase in surface area/volume ratio, occurring at such small sizes, results in more and more of the atoms to expose on the surface. According to the matrix materials, nanocomposites can be broadly grouped into three categories ceramic matrix nanocomposites, metal matrix nanocomposites, and polymer matrix nanocomposites [63–66].

1.2.9 Chalcogenide semiconductors as functional materials Chalcogenides are compounds containing one or more chalcogen elements, e.g., sulfur (S), selenium (Se), and tellurium (Te). It is usually represented as (MaXb), M is an element of Group IV, Group III, Group VI, or transition metal and X: S, Se, Te. The chalcogenides are extensively used in optoelectronics, thermoelectric energy conversion. It has a broad spectrum of intrinsic properties and further confinement to lowdimensional structures has led to various advancements in fabrication of quantum structures such as quantum wells, superlattices, and quantum dots. Recent developments in transition metal chalcogenides are synthesis of nonnoble metal electrocatalysts (materials based on, e.g., MoS2, WS2, WSe2, and related TMDs) or photoabsorbers in photoelectrochemical devices (e.g., dichalcogenides, as well as complex sulfides, selenides) [67,68].

1.2.10 Hybrid biomass-derived carbonaceous functional materials The current energy demands and environmental crisis have urged material scientist to discover inexpensive and sustainable resources that are economical, green, and facile in synthesis [69]. Biomass is the most promising candidate because of its abundance and renewable nature. The hydrothermal carbonization is currently the most prominent method for synthesizing carbonaceous materials owing to its energy efficiency and wide range of applications [70]. The biopolymers like cellulose, chitin, and chitosan are biodegradable and renewable and the amino groups in chitin/chitosan can be used for synthesizing with multifunctional properties. This biomass can be applied for sustainable chemical transformations and environmental remediation. Because of its high specific surface area, abundance, and economical properties, activated carbons obtained from biomass are widely used as electrodes in supercapacitors [71]. The biomass-derived carbons because of their diverse functional groups can be used for extra charge storage in supercapacitors [72,73].

1.3

A brief introduction to processing technology of functional materials

Functional materials because of their virtues find applications in material science, mechanical engineering, manufacturing, metrology, nanotechnology, physics, chemical, biology, chemistry, civil engineering, and food science. Conventional materials as compared with functional materials often have poor mechanical properties, lower

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strength to weight ratio, and poor resistance to wear and corrosion. They lack interface bonding between the matrix and reinforcements and other technical challenges. The classification of functional materials is so wide to be precisely defined but depending on their applications they are broadly classified as biomaterials, composites, ceramics, functionally graded materials, energy materials, thin film materials, nanomaterials, nuclear materials, intermetallic, high-strength materials, structural materials, super alloys, shape memory alloys, and thermally enhanced materials. The design of complex and multifunctional materials has led to the exploration of numerous synthetic strategies that are eventually combined with shaping techniques to control their composition (single or multicomponent phase), structure, texture, and interfacial properties [74]. Therefore the synthesis of advanced functional materials requires novel techniques, which are described as follows.

1.3.1 Electrodeposition Electrodeposition techniques are used for in situ metallic coatings and usually uses an electric current on a conductive substance submerged in a solution containing a salt of the metal to be deposited. It is a surface modification method that enhances the surface characteristics and is a versatile technique for the preparation of nanomaterials. Several techniques, including pulse and pulse reverse current deposition, templateassisted deposition, and use of additives and grain refiners is generally used. It is used for manufacturing the thin films of metals, metallic alloys, and compounds, and it is low-cost, energy efficient typically carried out near room temperature [75]. It is highly cost-effective as the materials utilization in electrodeposition processes is almost 100% provided stable electrolytes with long lifetimes are used. It is extensively used in PV industry and generating mass production of copper indium gallium diselenide (CIGS) solar modules. Moreover, it is also used for manufacturing one-dimensional nanostructured catalysts having different morphological forms, including nanowires, nanotubes, and nanorods. The end products are highly uniform and their properties can be tuned by varying pH, additives, types of solvents, and temperature.

1.3.2 Chemical/physical vapor deposition Chemical vapor deposition (CVD) is a technique that exposes the substrate to one or more volatile precursors, which react and/or degrade on the substrate surface to form the desired thin film deposit. It entails chemical interactions between organometallic or halide chemicals to be deposited and other gases to generate nonvolatile solid thin films on substrates. CVD is a multidirectional deposition of material onto the substrate [15]. Several gases are introduced into the vacuum chamber via the inlet, and the newly generated chemical molecules formed by dissociation are deposited on the heated substrate. PVD does not require chemical reactions and often use evaporation and sputtering methods. In industry, CVD is frequently used to create organic and inorganic films on metals, semiconductors, and other materials. CVD has evolved into several techniques like the atmospheric-pressure chemical vapor deposition (APCVD), low-pressure chemical vapor deposition (LPCVD), plasma-enhanced

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chemical vapor deposition (PECVD), or plasma-assisted CVD (PACVD), and laserenhanced chemical vapor deposition (LECVD) [76,77].

1.3.3 Epitaxial beam method Epitaxial growth is a highly controlled technique for methodically combining different materials into artificial structures with an atomic-scale precision. Several techniques like molecular beam epitaxy, epitaxial CVD, or atomic layer epitaxy are used to push a deposited film into a high degree of crystallographic alignment with the substrate lattice (ALE) [78]. The deposition procedure must be slow enough to allow the atoms on the surface to reorganize themselves in accordance with the substrate’s lattice orientation. It can be homoepitaxy involving the epitaxial growth of a deposit on a substrate of the same material (e.g., doped Si on Si) or heteroepitaxy involving the epitaxial growth of a deposit on a substrate of a different material (Au on Ag, GaAs on Si). The mobility of the atoms and nuclei on the surface is required for epitaxial growth. It is occasionally important to specify a “epitaxial temperature” required for epitaxial growth in certain systems and under specific deposition circumstances [79,80].

1.3.4 Ultrasonification A chemical reaction can be produced by the interaction of sound waves and gas bubbles in liquids in this approach. The primary and secondary radicals, as well as the physical effects produced during sonic cavitation, are used in the synthesis of a wide range of useful materials. The sonochemistry yields various functional polymers, functional inorganic materials, functional biomaterials, and graphene-based catalytic materials, etc. [81–83].

1.3.5 Solution phase technique In the solution-phase synthesis techniques, nanostructures of the required morphology are grown in solution. Solution-phase synthesis has the advantage of mass production when compared with other methods [84].

1.3.6 In situ synthesis In this approach, there is a self-assembly of components in single-step procedure together with the shaping of materials. It results in tailored architecture and microstructure while reducing the number of manufacturing steps [85].

1.3.7 Microtechnology and flow chemistry Microtechnology and flow chemistry are the new pillars of wet chemistry because they offer great control of nanoscale synthesis [86]. It covers almost all wet-chemistry processes like emulsification, sol–gel, solvothermal, hydrothermal, photochemical,

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electrochemical, and sonochemical. However, it also allows for the study of whole new paradigms such is the utilization of supercritical fluids, multiphase flows, and reactive-gas atmospheres.

1.3.8 Gas-phase synthesis Gas-phase synthesis is one of the most scalable and cost-effective methods for creating well-controlled nanostructured materials and coatings [87]. Flames, plasmas, lasers, and wall-heated reactors provide complimentary synthesis conditions for functional nanomaterials, including the majority of the periodic table elements. These gasphase particle synthesis techniques have benefits for producing commercial quantities of nanoparticles over traditional wet-chemistry approaches because of their highthroughput production, quick processing, simple process design (manufacturing and collection), and facile synthesis (continuous solvent-free single-step processes) [88,89].

1.3.9 Sol–gel route Functional materials obtained by the sol–gel route synergistically include organic and inorganic properties. There is a range of organofunctional alkoxysilane precursors for preparing silica nanoparticles. It is effective in producing homogenous hybrid materials at low temperatures, allowing the incorporation of a wide range of chemicals. The sol–gel method, which is based on the hydrolysis and condensation of metal or silicon alkoxides, is used to produce a wide range of high-purity inorganic oxides and hybrid inorganic–organic materials that are easy to make [90].

1.3.10 Bioinspired synthesis of functional materials Biological templates, such as viral capsids, are structures that act as containers. These can serve as carriers for DNA tests and immunoassays, medicines, catalysts, and new material production. Researchers have recently exploited biological macromolecular assemblies as templates for the creation of new functional nanomaterials [91]. Protein cages are synthetic, highly symmetrical, multifunctional protein structures with three different interfaces—interior, exterior, and the interface between subunits [92,93]. These subunits may be chemically and genetically changed, resulting in a single cage with multifunctionality and can be applied in various fields such as catalytic, biomedicine, etc. [94,95].

1.4

Basic and advanced applications of functional materials

Pure materials have limited commercial applications, which lead to demand for additional functionality requirements in the inherent pure materials. Hence, there is a need for materials with specific properties to perform optimally under service conditions.

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This carved out the path toward the development of innovative functional materials. Various properties of these materials can be tailored in a regulated manner because they respond to external stimuli like temperature, light, electric field, magnetic field, etc., thereby offering wide variety of applications in diverse areas, apparently indicating broad range of technological fields [35,96–99]. The fundamental applications of the functional materials are sum-up in Fig. 1.2. However, with time, the development of new materials at substantially lower dimensions have extracted functional materials in the applications of latest technology [100–103] paving a path toward advance applications as listed in Fig. 1.3. At the nanolevel dimensions, the properties of the functional materials like surface morphology, surface area, electronic and magnetic properties, optical and luminescence properties, etc. changed markedly. This led their applications in electronics and organoelectronics, spintronics, nanomechanical devices, photocatalysis, energy saving materials, displays, and coatings. As discussed earlier in this chapter, these functional materials are found in almost every class of advanced materials such as ceramics, polymers, organic molecules, nanocomposites, nano wires/rods/dots, semiconductors for solar cells, LEDs, displays, we here discuss some of the functional materials that are in the mainstream

Fig. 1.3 Schematic for basic application of the functional materials.

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Fig. 1.4 Schematic for commercial/advanced applications of functional materials.

of technology (Fig. 1.4). In this book the synthesis processes, evolution, properties, and applications of selected functional materials are discussed.

1.5

Concluding remarks

The perceptive of this book is to understand the basic fundamental properties such as optical, electrical, and magnetic properties of the functional materials. As with better understanding of the fundamental properties, further improvement in synthesis, design, and devices of functional materials is possible. To accomplish a specific function or application, new-fangled materials that satisfy specific criteria related to the various properties can be ultimately used with all the possible potentials to fulfill maximum requirements of the changing trend. With this hope, substantial efforts are being continued globally to come up with functional materials and structures that offer better performance. Innovative perspectives of nanotechnology and the recently progressing science are highly prerequisite to develop lead-free functional materials so as to meet the global challenges in harmonization to government policies and sustainable ecofriendly environment, but they are still some way off and hence need immediate addressal at the very first place. Thus along the brief discussion of selected functional materials in this chapter, the book contains dedicated chapters to different functional materials. In those chapters the authors summarize the fundamental properties, synthesis process, characterization techniques, and recent research work and applications. The chapters included in this book possess high scientific and technology merit and are of immense interest to the national and international community. Functional materials cover a wide arrange of inorganic and organic materials. Therefore these materials endorse subjects like solidstate chemistry, solid-state physics, material science, biophysics, and engineering. Thus this book is divided into three categories (I) Carbon—based functional materials,

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(II) inorganic functional materials for renewable and sustainable energy applications, and (III) organic and biological-based functional materials. This book conveys pioneering metrologies and strategies approved in the research and development of the mentioned subject.

References [1] J.-L. Bredas, S.R. Marder, E. Reichmanis, Preface to the Chemistry of Materials Special Issue on π-Functional Materials, ACS Publications, 2011. [2] P.M. Vilarinho, Functional Materials: Properties, Processing and Applications, Dordrecht, 2005, pp. 3–33. [3] S. Banerjee, A. Tyagi, Functional materials: preparation, processing and applications, 2011. [4] H. Goesmann, C. Feldmann, Nanoparticulate functional materials, Angew. Chem. Int. Ed. 49 (2010) 1362–1395. [5] S.H. Mir, L.A. Nagahara, T. Thundat, P. Mokarian-Tabari, H. Furukawa, A. Khosla, Organic-inorganic hybrid functional materials: an integrated platform for applied technologies, J. Electrochem. Soc. 165 (2018) B3137. [6] A. Thomas, Functional materials: from hard to soft porous frameworks, Angew. Chem. Int. Ed. 49 (2010) 8328–8344. [7] F. Cheng, J. Liang, Z. Tao, J. Chen, Functional materials for rechargeable batteries, Adv. Mater. 23 (2011) 1695–1715. [8] D.D. Chung, Functional Materials: Electrical, Dielectric, Electromagnetic, Optical and Magnetic Applications:(with Companion Solution Manual), vol. 2, World Scientific, 2010. [9] R. Sch€ollhorn, Intercalation systems as nanostructured functional materials, Chem. Mater. 8 (1996) 1747–1757. [10] Q. Ye, H. Zhou, J. Xu, Cubic polyhedral oligomeric silsesquioxane based functional materials: synthesis, assembly, and applications, Chemistry–An Asian J. 11 (2016) 1322–1337. [11] M.A. Boles, M. Engel, D.V. Talapin, Self-assembly of colloidal nanocrystals: from intricate structures to functional materials, Chem. Rev. 116 (2016) 11220–11289. [12] A. Nande, S. Fostner, J. Grigg, A. Smith, K. Temst, M.J.V. Bael, et al., Quantum fluctuations in percolating superconductors: an evolution with effective dimensionality, Nanotechnology 28 (2017), 165704. [13] R.W. Cahn (Ed.), Chapter 7—Functional Materials, in: Pergamon Materials Series, vol. 5, Pergamon, 2001, pp. 253–304. [14] A.K. Vaseashta, I.N. Mihailescu, Functionalized Nanoscale Materials, Devices and Systems, Springer Science & Business Media, 2008. [15] Q. Zhang, S.-J. Liu, S.-H. Yu, Recent advances in oriented attachment growth and synthesis of functional materials: concept, evidence, mechanism, and future, J. Mater. Chem. 19 (2009) 191–207. [16] R. Paul, M. Vincent, V. Etacheri, A.K. Roy, Carbon nanotubes, graphene, porous carbon, and hybrid carbon-based materials: Synthesis, properties, and functionalization for efficient energy storage, in: Carbon Based Nanomaterials for Advanced Thermal and Electrochemical Energy Storage and Conversion, Elsevier, 2019, pp. 1–24. [17] M. Chakraborty, M.S.J. Hashmi, Graphene as a material—an overview of its properties and characteristics and development potential for practical applications, in: Reference

Exploring the world of functional materials

[18] [19]

[20]

[21]

[22]

[23]

[24]

[25]

[26] [27] [28] [29]

[30] [31]

[32] [33]

[34] [35]

15

Module in Materials Science and Materials Engineering, Elsevier, 2018. https://www. sciencedirect.com/science/article/pii/B9780128035818103194. S. Shamaila, A.K.L. Sajjad, A. Iqbal, Modifications in development of graphene oxide synthetic routes, Chem. Eng. J. 294 (2016) 458–477. L. Lavagna, R. Nistico`, S. Musso, M. Pavese, Functionalization as a way to enhance dispersion of carbon nanotubes in matrices: a review, Mater. Today Chem. 20 (2021), 100477. Z. Feng, K.H. Adolfsson, Y. Xu, H. Fang, M. Hakkarainen, M. Wu, Carbon dot/polymer nanocomposites: from green synthesis to energy, environmental and biomedical applications, Sustain. Mater. Technol. 29 (2021), e00304. M. Sobiech, P. Lulinski, P.P. Wieczorek, M. Marc, Quantum and carbon dots conjugated molecularly imprinted polymers as advanced nanomaterials for selective recognition of analytes in environmental, food and biomedical applications, TrAC Trends Anal. Chem. (2021), 116306. Z. Fang, Y. Gao, N. Bolan, S.M. Shaheen, S. Xu, X. Wu, et al., Conversion of biological solid waste to graphene-containing biochar for water remediation: a critical review, Chem. Eng. J. 390 (2020), 124611. K. Premarathna, A.U. Rajapaksha, B. Sarkar, E.E. Kwon, A. Bhatnagar, Y.S. Ok, et al., Biochar-based engineered composites for sorptive decontamination of water: a review, Chem. Eng. J. 372 (2019) 536–550. N. Tejwan, A.K. Saini, A. Sharma, T.A. Singh, N. Kumar, J. Das, Metal-doped and hybrid carbon dots: a comprehensive review on their synthesis and biomedical applications, J. Control. Release 330 (2021) 132–150. L. Ansari, S. Hallaj, T. Hallaj, M. Amjadi, Doped-carbon dots: recent advances in their biosensing, bioimaging and therapy applications, Colloids Surf. B Biointerfaces (2021), 111743. P.M. Ajayan, O.Z. Zhou, Applications of carbon nanotubes, Carbon Nanotubes (2001) 391–425. J.M. Schnorr, T.M. Swager, Emerging applications of carbon nanotubes, Chem. Mater. 23 (2011) 646–657. A. Bianco, K. Kostarelos, M. Prato, Applications of carbon nanotubes in drug delivery, Curr. Opin. Chem. Biol. 9 (2005) 674–679. M. Endo, T. Hayashi, Y. Ahm Kim, M. Terrones, M.S. Dresselhaus, Applications of carbon nanotubes in the twenty–first century, Philos. Trans. Roy. Soc. Lond. A: Math. Phys. Eng. Sci. 362 (2004) 2223–2238. M. Valcarcel, B.M. Simonet, S. Cardenas, B. Suarez, Present and future applications of carbon nanotubes to analytical science, Anal. Bioanal. Chem. 382 (2005) 1783–1790. K.P. Gopinath, D.-V.N. Vo, D.G. Prakash, A.A. Joseph, S. Viswanathan, J. Arun, Environmental applications of carbon-based materials: a review, Environ. Chem. Lett. 19 (2021) 557–582. I.M. Alarifi, Advanced selection materials in solar cell efficiency and their properties—A comprehensive review, in: Materials Today: Proceedings, 2021. A. Richter, R. M€uller, J. Benick, F. Feldmann, B. Steinhauser, C. Reichel, et al., Design rules for high-efficiency both-sides-contacted silicon solar cells with balanced charge carrier transport and recombination losses, Nat. Energy 6 (2021) 429–438. F.A. Lindholm, C.-T. Sah, Fundamental electronic mechanisms limiting the performance of solar cells, IEEE Trans. Electron Dev. 24 (1977) 299–304. P.M. Beaujuge, J.M. Frechet, Molecular design and ordering effects in π-functional materials for transistor and solar cell applications, J. Am. Chem. Soc. 133 (2011) 20009–20029.

16

Functional Materials from Carbon, Inorganic, and Organic Sources

[36] A. Nande, S. Raut, S.J. Dhoble, Chapter 9—Perovskite solar cells, in: S.J. Dhoble, N.T. Kalyani, B. Vengadaesvaran, A.K. Arof (Eds.), Energy Materials, Elsevier, 2021, pp. 249–281. [37] R. Salikhov, Y.N. Biglova, T. Salikhov, Y.M. Yumaguzin, New polymers for organic solar cell, in: Functional Materials-2013, 2013, p. 468. [38] Y. Ichikawa, T. Yoshida, T. Hama, H. Sakai, K. Harashima, Production technology for amorphous silicon-based flexible solar cells, Sol. Energy Mater. Sol. Cells 66 (2001) 107–115. [39] O. Amiri, M. Salavati-Niasari, A. Rafiei, M. Farangi, 147% Improved efficiency of dye synthesized solar cells by using CdS QDs, Au nanorods and Au nanoparticles, RSC Adv. 4 (2014) 62356–62361. [40] A. Lenert, D.M. Bierman, Y. Nam, W.R. Chan, I. Celanovic, M. Soljacic, et al., A nanophotonic solar thermophotovoltaic device, Nat. Nanotechnol. 9 (2014) 126–130. [41] H. D€oscher, J.L. Young, J.F. Geisz, J.A. Turner, T.G. Deutsch, Solar-to-hydrogen efficiency: shining light on photoelectrochemical device performance, Energ. Environ. Sci. 9 (2016) 74–80. [42] B.G. Akinoglu, B. Tuncel, V. Badescu, Beyond 3rd generation solar cells and the full spectrum project. Recent advances and new emerging solar cells, Sustain. Energy Technol. Assess. 46 (2021) 101287. [43] S. Ozdemir, Y. Yang, J. Yin, A. Mishchenko, Novel phenomena in two-dimensional semiconductors, in: 2D Semiconductor Materials and Devices, Elsevier, 2020, pp. 25–79. [44] X. Li, M. Zhu, M. Du, Z. Lv, L. Zhang, Y. Li, et al., High detectivity graphene-silicon heterojunction photodetector, Small 12 (2016) 595–601. [45] J.-Z. Wang, C. Zhong, S.-L. Chou, H.-K. Liu, Flexible free-standing graphene-silicon composite film for lithium-ion batteries, Electrochem. Commun. 12 (2010) 1467–1470. [46] Y. Fan, M. Zhao, Z. Wang, X. Zhang, H. Zhang, Tunable electronic structures of graphene/boron nitride heterobilayers, Appl. Phys. Lett. 98 (2011), 083103. [47] C.R. Dean, A.F. Young, I. Meric, C. Lee, L. Wang, S. Sorgenfrei, et al., Boron nitride substrates for high-quality graphene electronics, Nat. Nanotechnol. 5 (2010) 722–726. [48] S. Nongthombam, S. Sinha, N.A. Devi, S. Rai, R. Bhujel, W.I. Singh, et al., Charge transfer mechanism of gallium nitrite/reduced graphene oxide (GaN/rGO) nanocomposite, in: 2020 IEEE VLSI Device Circuit And System (VLSI DCS), 2020, pp. 171–175. [49] L.A. Benı´tez, J.F. Sierra, W.S. Torres, A. Arrighi, F. Bonell, M.V. Costache, et al., Strongly anisotropic spin relaxation in graphene–transition metal dichalcogenide heterostructures at room temperature, Nat. Phys. 14 (2018) 303–308. [50] M.S. Rahman, L.F. Abdulrazak, Utilization of a phosphorene-graphene/TMDC heterostructure in a surface plasmon resonance-based fiber optic biosensor, Photon. Nanostruct.—Fundamen. Appl. 35 (2019), 100711. [51] B. Cho, J. Yoon, S.K. Lim, A.R. Kim, D.-H. Kim, S.-G. Park, et al., Chemical sensing of 2D graphene/MoS2 heterostructure device, ACS Appl. Mater. Interfaces 7 (2015) 16775–16780. [52] S. Liu, Z. Li, Y. Ge, H. Wang, R. Yue, X. Jiang, et al., Graphene/phosphorene nanoheterojunction: facile synthesis, nonlinear optics, and ultrafast photonics applications with enhanced performance, Photon. Res. 5 (2017) 662–668. [53] K. Constant, Ceram. Mater. (2016). [54] R.C. Buchanan, Ceramic Materials for Electronics, CRC Press, 2018. [55] P. Hvizdosˇ, A. Vencl, Ceramic matrix composites with carbon nanophases: development, structure, mechanical and tribological properties and electrical conductivity, in: Reference Module in Materials Science and Materials Engineering, Elsevier, Amsterdam, 2021.

Exploring the world of functional materials

17

[56] M. Vallet-Regı´, A.J. Salinas, Ceramics as bone repair materials, in: Bone Repair Biomaterials, Elsevier, 2019, pp. 141–178. [57] A. Kaiser, B.R. Sudireddy, F. Akhtar, Porous ceramics for energy applications, in: Encyclopedia of Materials: Technical Ceramics and Glasses, Elsevier, 2021, pp. 380–392. [58] J. Gilchrist, M. Konczykowski, AC screening measurement for the characterization of oxide superconductors: I. Application to ceramics, Physica C: Supercond. 168 (1990) 123–130. [59] H. Iwahara, Technological challenges in the application of proton conducting ceramics, Solid State Ion. 77 (1995) 289–298. [60] C.C. Wu, M. Kahn, W. Moy, Piezoelectric ceramics with functional gradients: a new application in material design, J. Am. Ceram. Soc. 79 (1996) 809–812. [61] H. Du, C. Ma, W. Ma, H. Wang, Microstructure evolution and dielectric properties of Cedoped SrBi4Ti4O15 ceramics synthesized via glycine-nitrate process, Proc. Appl. Ceram. 12 (2018) 303–312. [62] D. Brabazon (Ed.), Front Matter Volume 1, in: Encyclopedia of Materials: Composites, Elsevier, Oxford, 2021, pp. 1–3. [63] M.M. Shameem, S. Sasikanth, R. Annamalai, R.G. Raman, A brief review on polymer nanocomposites and its applications, in: Materials Today: Proceedings, 2021. [64] J. Cho, A.R. Boccaccini, M.S. Shaffer, Ceramic matrix composites containing carbon nanotubes, J. Mater. Sci. 44 (2009) 1934–1951. [65] M. Malaki, W. Xu, A.K. Kasar, P.L. Menezes, H. Dieringa, R.S. Varma, et al., Advanced metal matrix nanocomposites, Metals 9 (2019) 330. [66] F. Hussain, M. Hojjati, M. Okamoto, R.E. Gorga, Polymer-matrix nanocomposites, processing, manufacturing, and application: an overview, J. Compos. Mater. 40 (2006) 1511–1575. [67] E.Y. Muslih, B. Munir, M.M. Khan, Advances in chalcogenides and chalcogenidesbased nanomaterials such as sulfides, selenides, and tellurides, in: Chalcogenide-Based Nanomaterials as Photocatalysts, Elsevier, 2021, pp. 7–31. [68] M.M. Khan, Introduction and fundamentals of chalcogenides and chalcogenides-based nanomaterials, in: Chalcogenide-Based Nanomaterials as Photocatalysts, Elsevier, 2021, pp. 1–6. [69] Y. Zhu, S. Gao, N. Hosmane, 2.2 Carbonaceous Materials, 2018. [70] M.M. Hassan, C.M. Carr, Biomass-derived porous carbonaceous materials and their composites as adsorbents for cationic and anionic dyes: a review, Chemosphere (2020), 129087. [71] P. Dubey, V. Shrivastav, P.H. Maheshwari, S. Sundriyal, Recent advances in biomass derived activated carbon electrodes for hybrid electrochemical capacitor applications: challenges and opportunities, Carbon 170 (2020) 1–29. [72] A. Bello, N. Manyala, F. Barzegar, A.A. Khaleed, D.Y. Momodu, J.K. Dangbegnon, Renewable pine cone biomass derived carbon materials for supercapacitor application, RSC Adv. 6 (2016) 1800–1809. [73] X.-L. Wu, T. Wen, H.-L. Guo, S. Yang, X. Wang, A.-W. Xu, Biomass-derived spongelike carbonaceous hydrogels and aerogels for supercapacitors, ACS Nano 7 (2013) 3589–3597. [74] V. Brigitte, H. Rainer, A. Dietmar, W. Petra, Functional materials and applied systems, in: Bio- and Multifunctional Polymer Architectures: Preparation, Analytical Methods, and Applications, Wiley, 2016, pp. 241–302. [75] M. Bernal Lopez, J. Ustarroz, Electrodeposition of nanostructured catalysts for electrochemical energy conversion: current trends and innovative strategies, current opinion, Electrochemistry 27 (2021), 100688.

18

Functional Materials from Carbon, Inorganic, and Organic Sources

[76] M. Ohring, Chapter 6—chemical vapor deposition, in: M. Ohring (Ed.), Materials Science of Thin Films, second ed., Academic Press, San Diego, 2002, pp. 277–355. [77] A.V. Nande, Superconductivity in Nanocluster Films, 2015. [78] H. Tanaka, 13—Epitaxial Growth of Oxide Films and Nanostructures, in: T.F. Kuech (Ed.), Handbook of Crystal Growth, second ed., North-Holland, Boston, 2015, pp. 555–604. [79] S. Nagata, T. Tanaka, Self-masking selective epitaxy by molecular-beam method, J. Appl. Phys. 48 (1977) 940–942. [80] T. Sakamoto, T. Takahashi, E. Suzuki, A. Shoji, H. Kawanami, Y. Komiya, et al., Si epitaxy by molecular beam method, Surf. Sci. 86 (1979) 102–107. [81] B. Kharisov, O. Kharissova (Eds.), Part II—Greener methods: Physical and chemical methods, in: Handbook of Greener Synthesis of Nanomaterials and Compounds, Elsevier, 2021, pp. 135–136. [82] A. Asghar, N. Iqbal, T. Noor, Ultrasonication treatment enhances MOF surface area and gas uptake capacity, Polyhedron 181 (2020) 114463. [83] M. Liu, Z. Zhang, F. Okejiri, S. Yang, S. Zhou, S. Dai, Entropy-maximized synthesis of multimetallic nanoparticle catalysts via a ultrasonication-assisted wet chemistry method under ambient conditions, Adv. Mater. Interfaces 6 (2019) 1900015. [84] K.-J. Wu, C. Edmund, C. Shang, Z. Guo, Nucleation and growth in solution synthesis of nanostructures–from fundamentals to advanced applications, Prog. Mater. Sci. (2021) 100821. [85] B. Zhang, Chapter 5—principles, methods, formation mechanisms, and structures of nanomaterials prepared via self-assembly, in: B. Zhang (Ed.), Physical Fundamentals of Nanomaterials, William Andrew Publishing, Boston, 2018, pp. 177–210. [86] D.T. McQuade, P.H. Seeberger, Applying flow chemistry: methods, materials, and multistep synthesis, J. Org. Chem. 78 (2013) 6384–6389. [87] T. H€ulser, S.M. Schnurre, H. Wiggers, C. Schulz, Gas-phase synthesis of nanoscale silicon as an economical route towards sustainable energy technology, KONA Powder Particle J. 29 (2011) 191–207. [88] C. Schulz, T. Dreier, M. Fikri, H. Wiggers, Gas-phase synthesis of functional nanomaterials: challenges to kinetics, diagnostics, and process development, Proc. Combust. Inst. 37 (2019) 83–108. [89] H. Hahn, Gas phase synthesis of nanocrystalline materials, Nanostruct. Mater. 9 (1997) 3–12. [90] R.B. Figueira, Chapter 14—Greener synthesis and applications of hybrid sol–gelprocessed materials, in: B. Kharisov, O. Kharissova (Eds.), Handbook of Greener Synthesis of Nanomaterials and Compounds, Elsevier, 2021, pp. 459–490. [91] B.A. Krajina, A.C. Proctor, A.P. Schoen, A.J. Spakowitz, S.C. Heilshorn, Biotemplated synthesis of inorganic materials: an emerging paradigm for nanomaterial synthesis inspired by nature, Prog. Mater. Sci. 91 (2018) 1–23. [92] F.C. Meldrum, V.J. Wade, D.L. Nimmo, B.R. Heywood, S. Mann, Synthesis of inorganic nanophase materials in supramolecular protein cages, Nature 349 (1991) 684–687. [93] M. Uchida, M.T. Klem, M. Allen, P. Suci, M. Flenniken, E. Gillitzer, et al., Biological containers: protein cages as multifunctional nanoplatforms, Adv. Mater. 19 (2007) 1025–1042. [94] M. Rother, M.G. Nussbaumer, K. Renggli, N. Bruns, Protein cages and synthetic polymers: a fruitful symbiosis for drug delivery applications, bionanotechnology and materials science, Chem. Soc. Rev. 45 (2016) 6213–6249.

Exploring the world of functional materials

19

[95] M. Liu, T. Yu, R. Huang, W. Qi, Z. He, R. Su, Fabrication of nanohybrids assisted by protein-based materials for catalytic applications, Cat. Sci. Technol. 10 (2020) 3515–3531. [96] J.A. Kilner, S.J. Skinner, S. Irvine, P. Edwards, Functional Materials for Sustainable Energy Applications, Elsevier, 2012. [97] J.-K. Sun, Q. Xu, Functional materials derived from open framework templates/precursors: synthesis and applications, Energ. Environ. Sci. 7 (2014) 2071–2100. [98] Z.X. Cai, Z.L. Wang, J. Kim, Y. Yamauchi, Hollow functional materials derived from metal–organic frameworks: synthetic strategies, conversion mechanisms, and electrochemical applications, Adv. Mater. 31 (2019) 1804903. [99] W.Y. Wong, Metallopolyyne polymers as new functional materials for photovoltaic and solar cell applications, Macromol. Chem. Phys. 209 (2008) 14–24. [100] W.J.M. Naber, S. Faez, W.G. van der Wiel, Organic spintronics, J. Phys. D Appl. Phys. 40 (2007) R205–R228. [101] U. Eduok, O. Faye, J. Szpunar, Recent developments and applications of protective silicone coatings: a review of PDMS functional materials, Prog. Org. Coat. 111 (2017) 124–163. [102] K. Eom, H.S. Park, D.S. Yoon, T. Kwon, Nanomechanical resonators and their applications in biological/chemical detection: Nanomechanics principles, Phys. Rep. 503 (2011) 115–163. [103] C.A. Dyke, J.M. Tour, Covalent functionalization of single-walled carbon nanotubes for materials applications, J. Phys. Chem. A 108 (2004) 11151–11159.

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T.M.W.J. Bandara, T.M.A.A.B. Thennakoon, and G.B.M.M.M. Nishshanke Department of Physics and Postgraduate Institute of Science, Faculty of Science, University of Peradeniya, Peradeniya, Sri Lanka

2.1

Introduction

Carbon makes a wide variety of materials because one carbon atom can form four chemical bonds with other atoms, including carbon. Thus carbon can create one-, two-, and three-dimensionally structured small as well as large molecules. Simply a carbon atom in a molecule can have two, three, or four branches, and it can also create cyclic structures. Graphene is a 2D carbon structure, and it is the building block of some of the carbon allotropes. Graphene is the thinnest material, just one atom thick, and it can spread over a large surface area extending its honeycomb-like lattice. Actually, Graphene is not a unique or rare substance; in fact, it has the same carbon structure as graphite. For example, in a 1-mm thick graphite sample, there are about 3 million graphene layers. Graphene, the thinnest material, has unique physical and chemical properties such as flexible structure, large surface area, high electrical and thermal conductivity, and superior chemical stability. Furthermore, electrons in Graphene have a linear relationship between energy and momentum, so its energy band structure has no energy gap [1], and a lot of research is undertaken to create a finite gap, for example, by combining with other materials. There are several studies to make graphene-based quantum dots (GQDs), which are useful in modern electronics, optoelectronics, and biomedical applications.

2.1.1 Graphene-based quantum dots Multilayer GQDs, in which size is less than 10 nm, have unique electrical and optical properties. The size of single-layer GQDs can be larger than 10 nm. The properties of materials can be modified by incorporating them with appropriate GQDs to make materials with optimized properties needed for many applications.

2.1.2 Preparation methods of graphene quantum dots The Nobel Prize for Physics in 2010 was awarded to Geim and Novoselov, who performed the first experiment on successful exfoliation of graphite into Graphene. Functional Materials from Carbon, Inorganic, and Organic Sources. https://doi.org/10.1016/B978-0-323-85788-8.00007-0 Copyright © 2023 Elsevier Ltd. All rights reserved.

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Before their study, researchers had theorized the Graphene’s existence and predicted its usefulness in modern electronics. In that first study, Geim and Novoselov used adhesive tape to remove the graphite layers, and then used the second piece of sticky tape to remove the layers of graphite from the first one. After several steps, the authors could split it into a single-atomic layer of Graphene. There are several methods to produce GQDs. Mechanical exfoliation, chemical vapor deposition (CVD), sonication, thermo-engineering, carbon dioxide reduction, cutting open carbon nanotubes (CNTs), and graphite oxide reduction and chemical exfoliation are examples. GQDs preparation methods can be divided into two groups as the top-down methods and bottom-up methods. Bottom-up approaches start from the carbon atoms and by creating bonds between two carbon atoms and step by step make graphene sheets (GSs). This method needs advanced technology and is more expensive than top-down methods. Though the top-down method is a relatively easy and low-cost method, the preparation of pure Graphene (single-layer GSs) by using top-down approaches is a challenging task. Top-down methods can be used to exfoliate graphite into GQDs by breaking van der Waals bonds between two graphene layers inside the graphite in which graphene layers are bonded. However, the isolation of single-layer Graphene is not an easy task [2,3]. A schematic diagram to show top-down and bottom-up methods tried out successfully to synthesize GQDs is illustrated in Fig. 2.1. In addition, some examples of top-down and bottom-up methods are given in Fig. 2.1.

Fig. 2.1 A schematic diagram to show top-down and bottom-up methods that are commonly used to synthesize GQDs.

Preparation, characterization, and applications of graphene-based quantum dots

2.2

23

Preparation methods of graphene quantum dots

Over the past years, a wide range of procedures has been developed to synthesize GQDs. All these methods can be categorized into two main approaches: top-down and bottom-up. In top-down synthesis, raw materials such as graphene oxide (GO), graphite, coal, and carbon fiber are cut into GQDs by using various methods. All the top-down methods, namely hydrothermal cutting, solvothermal cutting, nanolithography, electrochemical method, mechanical exfoliation and oxidative cleavage consist of multiple steps and are highly time-consuming. Moreover, it is hard to control the size and size distribution of the GQD with top-down strategies. In contrast, GQDs synthesized by bottom-up methods such as the transformation of fullerenes into Graphene that involves crystal growth are more advantageous to produce well-defined GQDs with size homogeneity.

2.2.1 Top-down method In most of the top-down strategies that are used to cut carbon materials with the aid of chemical or physical methods, GO derived from natural graphite powder by using the modified Hummers method is used as the starting material [1].

2.2.1.1 Reduced GO by modified Hummers method The modified Hummers method involves several chemical and physical processes. In the modified Hummers method, in general, graphite powder is first stirred with sodium nitrate, H2SO4, and potassium permanganate. The obtained graphite oxide is then mixed with water and followed by ultrasonication for the exfoliation. After the centrifugation of graphite oxide, the unreacted graphite is separated. Subsequently, GO is obtained by mixing the graphite oxide dispersion with water and hydrazine solution. Finally, the resulting GO is converted to reduced GO by heating in an oil bath and followed by the addition of a reducing agent like aqueous ammonia [4,5].

2.2.1.2 Mechanical exfoliation Ball-milling In a recent study [6], GQDs have been fabricated using a low-cost ball milling technique. In the study, carbon nanocapsules having the size of about 20–30 nm is used as the carbon source, whereas sodium carbonate is added as the intercalation agent for the preparation of GQDs. In the experiment, at first, the authors have mixed carbon nanocapsules, Na2CO3, and zirconia balls in a grinding jar and followed by ballmilling at 500 rpm for a 4-h period. Thereafter, deionized water was added to the mixture and followed by spontaneous settlement. Then, the supernatant is collected and it is dialyzed for 24 h through a 3500-Da dialysis bag to separate the GQDs. The researchers have discovered that 0.2 mass ratio of nanocapsules to Na2CO3 gives the optimum yield of GQDs. And the authors have managed to isolate GQDs with diameters of 1.0–4.0 nm. Hence, ball milling is a suitable method to overcome the

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limitation, such as the high preparation costs for commercial applications, of preparing bulk amounts of GQDs.

2.2.1.3 Hydrothermal synthesis of GQDs In 2010, for the first time, Pan et al. [1] introduced a hydrothermal route to cut large GSs into GQDs. The authors state that, by using hydrothermal synthesis, they overcome the drawbacks in conventional physical and chemical methods that are used to prepare GQDs. For instance, the product limitations such as direct application in nanodevices are bested with hydrothermal synthesis. In their experiment, the authors used a modified Hummers method to synthesize GO sheets from natural graphite powder. After that, micrometer-sized GSs produced by thermal deoxidization of GO sheets, in a tube furnace at 200–300°C for 2 h. The heating rate is 5°C min
1 and a nitrogen atmosphere is used inside the tube furnace. By oxidation in an acidic medium using concentrated acids H2SO4 and HNO3, the GSs are made water soluble by attaching the functional groups such as CO/COOH, OH, and CdOdC. After purifying the oxidized GSs, they disperse in deionized (DI) water and the final PH brings to 8 by adding NaOH. After transferring the resulting mixture into a poly(tetrafluoroethylene) (Teflon)-lined autoclave, the mixture anneals at 200°C for 10 h. The brown-colored filtrate (filtered solution) is then separated by filtering through a microporous membrane, after cooling down the mixture to room temperature. Finally, further dilatation of this colloidal suspension made the final product strongly fluorescent resulting in good quality GQDs. The ultrafine GQDs with strong blue emission obtained in the reported study have diameters in the range of 5–13 nm along with 1 or 2 nm topographic heights. According to the authors, as given in the schematic diagram (Fig. 2.2A), mixed epoxy chains consisted of epoxy as well as carbonyl groups that are completely broke-up along with the hydrothermal cutting. Furthermore, the study of Pan et al. [1] proposes two structural models, as shown in Fig. 2.2B because alkaline conditions lead to strong photoluminescence (PL) in the GQDs, whereas in strongly acidic conditions, the resulting PL almost disappeared. The reason for this effect is explained as the emissive triple carbene state is inactive in the acidic environment because of the protonation of free zigzag sites in GQDs where a reversible complex between the zigzag sites and H+ is formed. The alkaline environment leads to restore the PL by restoring these free zigzag sites. Fig. 2.2C illustrates two electronic transitions at 320 nm (3.86 eV) and 257 nm (4.82 eV) from HOMO to LUMO where dE is 0.96 eV, within the required value ( 12) conditions, well-crystallized GQDs in size range from 1.5 to 5 nm are produced.

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Fig. 2.2 (A) The break-up of mixed epoxy chains with the hydrothermal cutting. (B) Two structural models in alkaline conditions and strong acidic conditions. (C) Two electronic transitions of 320 nm (3.86 eV) and 257 nm (4.82 eV) from HOMO to LUMO [1].

Continuous hydrothermal flow synthesis Kellici et al. [2] reported an efficient and greener methodology to fabricate GQDs by continuous hydrothermal flow synthesis. In this study, an aqueous solution of GO and calix[4]arene tetrasulfonic acid (SCX4) was pumped to mix them with a flow of KOH at room temperature. Subsequently, GQDs are fabricated by contacting this mixture with supercritical water at 450°C and 24.1 MPa in a reactor. The procedure is illustrated systematically in Fig. 2.3. Compared with conventional hydrothermal synthesis, this process has many benefits, such as using the noncomplex approach, overcoming the limitations in using toxic reagents, and minimizing the time consumed for the experimental produce.

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Fig. 2.3 Schematics of (a) the synthesis of graphene quantum dots (GQDs) from GO and calix [4]arene tetrasulfonic acid (SCX4) by continuous hydrothermal flow synthesis [2].

2.2.1.4 Solvothermal synthesis In contrast with the hydrothermal synthesis, the solvothermal methods use organic solvents such as dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) instead of water to obtain desired reaction environment to produce GQDs. In 2011, Zhu et al. [8] synthesized GQDs from GO by applying a one-step solvothermal method using DMF as the solvent. The prepared GQDs, which had an average diameter of 5.3 nm and an average height of 1.2 nm in the study, exhibited a strong green fluorescent with an 11.4% of photoluminescent quantum yield. It has also been reported that these GQDs showed superior solubility in water and ethanol as well as in a number of polar organic solvents such as tetrahydrofuran, acetone, DMF, and DMSO. The exhibited outstanding solubility attributes to the presence of dOH, epoxy/ether, C]O, and dCOdNR2 groups in the synthesized GQDs, which originated from the decomposition of DMF. Tian et al. [9] reported a novel, eco-friendly, and cost-effective solvothermal method to prepare GQDs by using graphite as the starting material and hydrogen peroxide as the oxidant, as shown in Fig. 2.4. The preparation of GQDs in the study [9] is summarized below.

Preparation of expanded graphite from expandable graphite First expandable graphite is prepared, and this expandable graphite is placed in an alumina crucible and is kept in a muffle furnace for 10 s, which has been previously heated to 800°C for 5 h. Here, sulfuric acid and nitric acid escape from the interlayer of expanded graphite in the form of gas. The completion of this step has resulted in expanded graphite in which the resulting volume is 160 mL g
1.

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27

Fig. 2.4 Schematic depiction of GQDs prepared by solvothermal method using graphite as the starting material and hydrogen peroxide as the oxidant [9].

Preparation of GQDs from expanded graphite The experimental procedure is illustrated schematically in Fig. 2.4. As the first step, expanded graphite is mixed with DMF and followed by ultrasonication. By doing this, the air inside the layers of graphite is removed and it also led the solvent to contact with expanded graphite. Subsequently, this mixture is mixed with hydrogen peroxide and followed by stirring to obtain a homogeneous solution. The resulting mixture from this step is heated to 170°C for 5 h in an autoclave. Raw GQDs were obtained by vacuum filtration of the final solution. Thereafter, the residue is dissolved in deionized water, after evaporating the solvent. After that, the product is filtered using a slow filter paper and a 100-nm filter membrane to obtain pure GQDs with strong blue PL. GQDs fabricated by this method display an average size of 35 nm, diameters in a range of 20–40 nm and a thickness of 1–1.5 nm with 2–3 graphene layers. Hence, high purity GQDs are realized by the expansion method because this method does not use any acids such as concentrated sulfuric acid and nitric acid to treat raw material and expanded graphite. Instead, this method uses hydrogen peroxide, which had also been fully consumed during the process.

2.2.1.5 Acidic oxidation/oxidative cleavage/oxidative cutting/ chemical exfoliation Oxidative cleavage is one of the most common methods of fabricating high-quality GQDs because of the facile and low-cost preparation procedures. The carbon-carbon bonds in precursor materials such as Graphene, GO, or CNTs are cleaved using a strong oxidizing agent like H2SO4 and HNO3. In 2012, Peng et al. [10] have produced GQDs from the acid treatment of carbon fibers, as illustrated in the schematic diagram shown in Fig. 2.5. The method is as follows. At first, micrometer-sized pitch-based carbon fibers are mixed with a mixture

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Fig. 2.5 GQDs from the acid treatment of carbon fibers [10].

of concentrated H2SO4 and HNO3 and followed by sonication for 2 h. The solution is then stirred at three different temperatures of 80°C, 100°C, and 120°C for 24 h. The resulting mixture is cooled down, diluted with deionized water, and finally, Na2CO3 is added to bring the pH to 8. The product solution is dialyzed by using a 2000-Da dialysis bag for 3 days. The achieved GQDs are in the size range of 1–4 nm and in the paramount range of 0.4 and 2 nm. The synthesized GQDs had only 1–3 atomic layers. By using a comparatively similar procedure described in Peng et al. [10], Ye et al. [11] also synthesized GQDs from coal. In the reported study, the authors use three different types of coal, namely anthracite, bituminous coal, and coke, and obtained GQDs having three different morphologies. In their experiment, at first, the coal is mixed with concentrated H2SO4 and HNO3 and followed by sonication for 2 h. After stirring, the solution is heated to about 120°C for 24 h. After cooling down the solution to room temperature, it is brought to contact with ice in a beaker. Then, the pH of the solution is brought up to 7 by adding NaOH. The product solution is filtered through a 0.45-mm polytetrafluoroethylene membrane and followed by dialyzing in a 1000-Da dialysis bag for 5 days. Finally, the solid GQDs are obtained by purification and concentration by means of evaporation. It has been reported that anthracite-GQDs have an average diameter of 29  11 nm and bituminous coal-GQDs with uniformly distributed sizes and shapes have a diameter of 2.96  0.9 6 nm, whereas coke-GQDs exhibited a uniform size of 5.8  1.7 nm. It is also interesting to observe that the PL intensities of GQDs followed the same trend as the sizes of GQDs. Later, from GO, Maiti et al. [12] produced highly luminescent GQDs having a quantum yield of about 14% by using perchloric acid oxidation by replacing conventional oxidizing agents of H2SO4 and HNO3. The prepared GQDs exhibited spherical morphology with an average size of about 5.6 nm.

2.2.1.6 Ultrasonic-assisted liquid-phase exfoliation In general, liquid-phase exfoliation methods of graphite to produce Graphene is being marked as an environmentally friendly, low-cost, and industrially scalable method, which leads to producing quality products [13,14]. Conversely, many methods used to produce GQDs are time-consuming and use complicated synthesis procedures with expensive equipment, strong acids, and strong oxidants. Furthermore, these advanced processes require high cost and therefore they are only suitable for small-scale production of GQDs. In contrast, the liquid-phase exfoliation methods used to produce

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GQDs are relatively easy, less time-consuming, economical, and greener. In addition, liquid-phase exfoliation gives a relatively higher yield of GQDs. GQDs having high defects have been prepared using the ultrasonic-assisted liquidphase exfoliation method along with acetylene-black precursors [14]. For the synthesis, acetylene black mixes with N-methyl-2-pyrrolidone solvent and the resulting dispersion directs to a mild ultrasonication. Finally, GQDs are dispersed in a liquid phase together with a residual precipitate, which is later removed by centrifugation. Evaporation of product solution at 100°C leads to the removal of the excess solvent of N-methyl-2-pyrrolidone. By repeating this procedure to replace the acetylene black with nano-graphite, GQDs having low defects have been prepared successfully. The investigation concludes that the graphitic carbon precursor materials can control the size and density of defects in GQDs. By using platelet graphite nanofibers as the starting material and DMSO as the exfoliation agent, Shih et al. [15] prepared GQDs by ultrasonication-assisted liquid-phase exfoliation. In addition, recently, Zdrazil et al. [16] converted graphite flakes into expanded graphite flakes using microwave expansion, which used precursors and followed the ultrasonic-assisted liquid-phase exfoliation method described in Hassan et al. [14] to produce GQDs using platelet graphite nanofibers.

2.2.1.7 Electrochemical synthesis In general, in the electrochemical synthesis of GQDs, the carbon derivatives such as graphite and CNTs were used as electrodes, whereas platinum serves as the counter electrode. However, the GQDs synthesized by this method exhibit relatively low PL efficiency. Moreover, the product solution should undergo dialyzing, filtration, and chromatography to isolate GQDs. Blue luminescent carbon nanocrystals with a spherical shape and diameter of about 2.8 nm have been fabricated by electrochemical treatment of multiwalled carbon nanotubes (MWCNTs) by Zhou et al. [17]. The electrochemical cell has the configuration of a working electrode, a Pt wire counter electrode, and an Ag/AgClO4 reference electrode, whereas a degassed acetonitrile solution with 0.1 M tetrabutylammonium perchlorate (TBAP) is used as the electrolyte. The carbon paper, which is covered by MWCNTs, has been cut to a suitable size and placed in a Teflon jacket. The potential difference is driven between
2.0 and 2.0 V electrodes with a scan rate of 0.5 V s
1. The color of the electrolyte solution changes from colorless to yellow to dark brown. As reported, when the dark brown-colored solution is irradiated with a UV lamp, it emits blue luminescent. Hence, to obtain carbon nanocrystals, the acetonitrile is evaporated from the solution, remaining solids were dissolved in water, and followed by dialyzing through a cellulose ester membrane bag. Subsequently, in a recent study [18], GQDs were synthesized directly from GO by an electrochemical method. First, a precleaned tip of a glassy carbon electrode (GCE) is covered from GO by the drop-casting method to form a microtip. The study done by Nasibulina et al. uses a three-electrode system as already described in previous methods under electrochemical experiments. In the study, a Pt film has been used as the counter electrode, whereas a Pt wire is the quasi reference electrode. The experimental setup used in the reported study is shown in Fig. 2.6. Here, a solution of

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Functional Materials from Carbon, Inorganic, and Organic Sources

Fig. 2.6 Experimental setup for the fabrication of GQDs from GO [18].

LiClO4 in propylene carbonate, having 3 mM LiClO4 concentration, has been used as the supporting electrolyte. Oxidation of GO is done with an applied potential of +1.05 V and the oxidation is carried out for different oxidation times such as 8, 12, and 16 h. To control the size of GQDs, the potential is set to
1.05 V for 3 h, after the oxidation is completed. The purified product is obtained by dialyzing many times after collecting the samples by sonicating GCE in DI water. The GQDs prepared by this method are in size between 3 and 5 nm and in the thickness of around 2–10 nm.

2.2.1.8 Nanolithography Few studies have been reported on the use of nanolithography techniques to fabricate GQDs. In general, using nanolithography is limited because of the high cost, need of special equipment, and low yield. It has been reported that small GQDs in the size of 10 nm could be fabricated by using high-resolution electron-beam lithography. In the reported study, a polymethylmethacrylate (PMMA) mask with 30 nm thickness has been used to protect the required areas during oxygen plasma etching. Hence, Graphene could be cut into the desired geometry [19]. Thereafter, thin graphene flakes (single layer) on a Si substrate and a pattern made from exposing PMMA to standard electron beam lithography has been used to make double GQDs. To carve the unprotected areas, the oxygen reactive ion etching technique is used. As prepared, double quantum dots were observed to have two isolated central islands with a diameter of 100 nm in series [20].

2.2.2 Bottom-up methods 2.2.2.1 Carbonization/pyrolysis There are various techniques of making GQDs by carbonization, such as hydrothermal, solvothermal, and electrochemical. The detailed descriptions of making GQDs by carbonization is as follows.

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Hydrothermal synthesis of GQDs Citric acid and urea were used as the precursors to fabricate GQDs with a modified hydrothermal synthesis by Permatasari et al. [21]. Here, the carbon source citric acid (C6H8O7) and nitrogen sources urea ((NH2)2CO) are dissolved in water at room temperature under stirring. Then, a Teflon line autoclave is used to heat this solution for 180 min at 0.8–1.0 MPa under a specific temperature profile. GQDs were collected from the autoclave from elapsed reaction times of 50, 90, and 180 min during the hydrothermal synthesis. Thereby spherical-shaped GQDs having N atoms with an average diameter of 2.17 nm are obtained. Furthermore, the study reported by Permatasari et al. confirms that the PL intensity of GQDs is dependent on the CdN configurations of GQDs.

Microwave-assisted hydrothermal method Recently, carbon nanoparticles and carbon dots were prepared by using microwaveassisted methods [22,23]. However, the prepared carbon nanoparticles/carbon dots were amorphous in nature and however exhibited PL properties [22–24]. Hence, microwave-assisted methods to prepare crystalline GQDs by using appropriate additives have been developed. The main advantage is that the microwave-assisted techniques are faster methods that can reduce the reaction time to produce high yields, unlike the long reaction time in oxidative cleavage and hydrothermal and solvothermal methods. By using a microwave-assisted hydrothermal method, Tang et al. [24] produced glucose-derived GQDs having a small diameter of around 1.65 nm. The schematic diagram to illustrate the method followed is shown in Fig. 2.7. Using glucose and DI water, a series of glucose solutions are prepared. Then, 2–2.5 mL of glucose solutions are drawn into a 4-mL volume glass bottle having a tightened cover. For 1, 3, 5, 7, 9, and 11 min periods, the glass bottle is heated by using a microwave oven at the power of 280, 336, 462, 595, and 700 W. With the formation of GQDs, glucose solution changed its color from transparent to pale yellow. It is also uncovered by Tang et al. that the growth of GQDs is dependent on the microwave power,

Fig. 2.7 Microwave-assisted hydrothermal method of preparing GQD [24].

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heating time, source concentration, and solution volume. The prepared GQDs exhibit PL quantum yields of about 7%–11%. Later, Hou et al. [25] synthesized nitrogen-doped multilayer GQDs by using a microwave-assisted hydrothermal method from glucose and urea precursors. In their experiment, the authors first mix glucose and urea in Milli-Q water (Milli-Q water: water which is purified by a Milli-Q water purification having a resistance of 18 MΩ cm at 25°C). After that, this mixed solution was transferred to a glass tube and heated to 120°C for 1 min in a microwave oven. By dialyzing the product solution using 3.5-kDa dialysis membranes against Milli-Q water for 72 h, nitrogen-doped multilayer GQDs are obtained. These GQDs with oxygen-rich functional groups are reported to have a size around 3 nm, height around 2.8–25 nm with 3–10 graphene layers and 5.2% of PL quantum yield.

Doped GQDs by hydrothermal/solvothermal synthesis In most studies, GQDs dope with metal, nonmetal, or organic molecules to obtain superior optical and electronic properties [26,27]. The low quantum yield of GQDs can be overcome by doping them with appropriate substances to synthesize doped GQDs that exhibit higher luminescence responses [27,28]. By using a hydrothermal procedure, Qu et al. [27] synthesized N doped GQDs and S, N co-doped GQDs having high quantum yield. For the preparation of S, N co-doped GQDs, carbon source of citric acid is mixed with N and S source of thiourea and followed by stirring to form a clear solution. After placing this solution in a teflonlined stainless autoclave, the sealed autoclave was heated to 160°C for an additional 4 h. After adding ethanol to the resulting solution, then the mixture was centrifuged to obtain the S, N co-doped GQDs. To prepare N doped GQDs, the same procedure is repeated. However, urea is used as the N source and the time heated in the autoclave is 8 h. Both N doped GQDs and N, S co-doped GQDs showed narrow size distributions with mean diameters of around 2.69 nm and 3.10 nm, respectively. The N and N + S doped GQDs exhibit quantum yields as high as 78% and 71%, respectively. In 2018, Noor et al. [26] synthesized pyrrolic nitrogen-doped GQDs by way of a solvothermal method at an annealing temperature of 180°C along with different reaction times. In this study, precursors of citric acid are mixed with urea in the medium of DMF. The method is summarized in Fig. 2.8. The authors reported that along with the increasing reaction time, the average size of GQD also increases. Furthermore, the authors were able to synthesize multiple color-emitting GQDs of size in the range of 5–10 nm. It has been reported that the white light emission-quality of GQDs can be controlled by the concentration of pyrrolic nitrogen in the GQDs. Hence, the study concludes that pyrrolic nitrogen-doped GQDs have a good potential to fabricate white light-emitting diodes.

Electrochemical carbonization For the first time, Deng et al. [29] synthesized carbon dots by using one-pot electrochemical carbonization of low-molecular-weight alcohols. In their setup, as shown in Fig. 2.9, the authors use two Pt sheets as the working and counter electrodes, whereas

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Fig. 2.8 Method of synthesis of pyrrolic nitrogen-doped GQDs (pN-GQDs) via a solvothermal method [26].

Fig. 2.9 Setup used for electrochemical synthesis of C-dots [29].

the reference electrode is a calomel electrode. First, the authors mixed alcohol with water, and thereafter, NaOH is added under continuous stirring. The transparent solution turned into dark brown along with the reaction time of 4 h, depending on the applied potential. The current density used in the experiment is 15–100 mA cm
2. The resulting mixture was kept still overnight after salting out the NaOH in the solution by adding ethanol. After that, a yellow powder is obtained by evaporating the mixture at 80°C for 24 h. Finally, carbon dots were separated by dialyzing the product solution through a 1000 Da dialysis membrane. The carbon dots prepared by this method exhibit 15.9% quantum yield. Furthermore, the study confirms that the applied potential plays a major role in controlling the size of carbon dots as the size of carbon dots is increasing with the increase of applied potential difference. When the applied potential is 3.0, 4.5, 6.0, and 7.5 V, the diameters are centered at about 2. 1, 2.9, 3.5, and 4.3 nm, respectively. In addition, Ahirwar et al. [30] also prepared GQDs by a convenient electrochemical exfoliation method where two graphite rods were used as electrodes, whereas citric acid and alkali hydroxide in water were used as the electrolyte. In the study, GQDs were prepared by a balanced reaction of citric acid and alkali hydroxide as the

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variation of alkali hydroxide concentration led to the formation of GQDs having an average size of 2–3 nm and blue to green luminescence.

2.2.2.2 Soft template method Most of the methods, used to prepare carbon nanodots (CNDs), used water as a reaction medium or as a solvent. Therefore the synthesized CNDs consist of hydrophilic surface functional groups like carboxylic acid and aldehyde, which leads to the formation of growth sites and aggregation centers. A soft-template method has been introduced to overcome the limitations in the water-based CND synthesis strategies, especially to prepare organic-soluble oleylamine-capped CNDs [31]. As reported by Kwon et al., at first, citric acid and nitric acid were mixed with water and stirred vigorously. Then, oleylamine and octadecene are injected to the mixture with different oleylamine to octadecene ratios. A milky emulsion forms after stirring the mixture vigorously. The formed emulsion is heated to 250°C under argon for 2 h. Gradually, the color of the solution has changed to transparent to dark brown. After precipitating this dark brown-colored solution in ethanol, it is centrifuged and followed by dispersing in hexane. After repeating this final step for three times, the resultant colloidal carbon nanodots are vacuum dried to obtain the final product. The resulting CNDs exhibit 60% of quantum yield. The authors state that the size of CNDs can be easily tuned by using the soft template synthesis procedure. In addition, it introduces a method to prepare high-quality CNDs.

2.2.2.3 GQDs from fullerenes Using C60 molecules as a precursor, GQDs have been synthesized on a ruthenium surface by Lu et al. [32]. Ultrahigh-vacuum chamber is used to carry out the experiment. Sputtering of argon ion at room temperature is followed by annealing to deplete the carbon impurities on the Ru(0001) crystal. In this experiment, surface vacancies form on the ruthenium as the ruthenium surface interacts strongly with the C60 molecules. The presence of surface vacancies supports the C60 molecules to embed in the surface. Carbon clusters resulting from fragmentation of the embedded molecules at elevated temperatures undergo diffusion and aggregation to form GQDs. In this case, the shape of the resulting GQDs depends on the annealing temperature.

2.2.2.4 Chemical vapor deposition CVD is a vacuum deposition technique where the heated substrate is exposed to one or more volatile substances, which react on the substrate surface for the formation of nonvolatile thin solid films. CVD is used as a promising method to prepare Graphene regardless of the high preparation cost [33]. However, in 2013, for the first time, Fan et al. [34] synthesized GQDs having a large area and controllable size along with high dispensability by using a fast CVD method. The experiment has been carried out by means of atmospheric CVD on precleaned polycrystalline copper foils to remove the oxidized surface. First, in a mixture flow of argon (200 mL) and hydrogen (10 mL min
1), a copper foil heats up to 1000°C for about 40 min. Thereafter, the flow

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of argon (200 mL min
1) is continued for another 10 min while the hydrogen input is turned off. In this step, the residual hydrogen in the quartz tube is removed and it helps to eliminate the etching of GQDs during their growth. Thereafter, methane gas is purged to the reaction tube at the rate of 2 mL min
1 for 3 s and followed by switching off it immediately. Eventually, the copper foil is led to cool down by getting out from the heating zone in an argon environment. It was reported that the synthesized GQDs exhibited a size distribution of 5–15 nm with a few layers thick (1–3 nm) and they can also be transferred to arbitrary substrates while maintaining their pristine configuration. Fig. 2.10 shows the schematic illustration for procedures for the CVD synthesis and transfer of CGQDs. By using PMMA, CGQDs has been transferred to copper grids from copper substrates. It is mentioned that there is a loss of some CGQDs during this transformation process. In 2014, Ding et al. [35] used hexagonal boron nitride substrate to synthesize GQDs using CVD method. In this experiment, the methane flow rate and the reaction pressure are controlled along with temperatures to obtain well-ordered GQDs with better morphology. By changing the ratio of CH4:H2:Ar and maintaining a fixed reaction time, GQDs with different thicknesses were obtained. In a recent study, GQDs have been prepared by CVD method where a Misch metal-based nickel alloy (MmNi3) is the catalyst for the growth of GQDs [36]. A schematic diagram to summarize the synthesis of GQD using the method proposed by Saroja et al. is shown in Fig. 2.11.

2.3

Characterization of graphene QDs

A number of experimental methods can be used to characterize the fabricated Graphene and the intermediate products of the preparation process. In particular, X-ray-based techniques, such as X-ray diffraction (XRD), X-ray fluorescence, X-ray photo-electron spectroscopy (XPS), X-ray absorption fine structure, and X-ray absorption near edge structure can be used for the characterization of graphene as well as GQDs.

Fig. 2.10 Schematic procedure for CVD synthesis and transfer of CGQDs [34].

Fig. 2.11 Schematic of synthesis of GQD by Misch metal-based nickel alloy (MmNi3) as the catalyst [36].

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In addition, spectroscopic methods, for example, Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), and diffuse reflectance Fourier transform infrared spectroscopy, and ultraviolet-visible spectroscopy (UV-vis), can be used to analyze the properties of Graphene and GQDs. The micrographs such as atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy are used to analyze the size, shape, and morphology. A summary of methods that are used widely to characterize GQDs is given in Fig. 2.12 for better understanding.

2.3.1 Spectroscopic techniques The spectroscopic techniques can be used to characterize graphene-based catalytic and the intermediates products of the preparation process. Therefore these techniques

Fig. 2.12 Summary of characterization methods that are commonly used to investigate GQDs.

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can be used to characterize GQDs and these can also be used to identify functional groups associated with GQDs. In this chapter, some of the main spectroscopic techniques used to characterize GQDs are discussed.

2.3.1.1 X-ray photoelectron spectroscopy XPS is based on the photoelectric effect principle, and it is a surface-sensitive and powerful quantitative spectroscopic technique. The method is also termed electron spectroscopy for chemical analysis and photoemission or photoelectron spectroscopy as well. This can be used to analyze the elemental composition of the surface and the chemical (oxidation) state or electronic state of each element. However, the width of the surface should be less than 10 nm. In XPS, the kinetic or binding energies of ejected electrons are measured by the spectrometer, and this core electron of an element has a unique binding energy that can be used as a “Fingerprint” of elements. XPS can be used to detect all elements except Hydrogen and Helium and provides a semiquantitative measure of their relative concentrations. Because GQDs are primarily made of carbon and is less than 10 nm in size, the presence of carbon can be detected using the XPS technique [10]. In addition, the elements in attached functional groups and the presence of GO can be investigated. GQDs mainly give C-1s major peak at about 284.8 eV, and if GQDs are doped by another element, then XPS will provide peaks corresponding to doped elements in such cases [1].

2.3.1.2 X-ray diffraction X-ray diffraction is a powerful, standard, and nondestructive method used to identify crystal structures. The X rays diffracted from crystal planes are analyzed based on Bragg’s law. This method gives crystallographic information based on the diffraction pattern produced by X-ray beam scattered from a sample. This technique is used as a tool to determine the crystallinity of materials. In addition, it is widely used to analyze crystallite sizes, changes in disorders, and strain, depending on peak width and position (2θ) of the diffracted rays. Hence, XRD is a commonly used technique to identify materials based on their structures, available phases, crystal plane orientations, quality, and defect in a material. Graphite gives the characteristic signature peak at 27 degrees and also GQDs (Graphene) show a broad peak near 27 degrees because of their small size [4,5].

2.3.1.3 Fourier-transform infrared spectroscopy Fourier-transform infrared spectroscopy can be used to understand the functional groups in GQDs. Here, a beam containing many frequencies in the infrared range is incident simultaneously on the sample. The results are based on the intensity of the absorbed, transmitted, or reflected radiation. Here, spectrum represents the molecular absorption and transmission of the sample that creates a fingerprint for molecules and bonds present in a molecule. Using the FTIR techniques, the presence of certain functional groups in a molecule and intermolecule interactions can be identified. FTIR

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can also be used to confirm the presence of a pure compound and detect specific impurities, bonds, and side groups [1]. Frequently, GQDs are not pure graphene particles; they contain defects, impurities, and functional groups. FTIR helps in characterizing the GQDs and functionalized GQDs successfully [35].

2.3.1.4 UV-visible UV-vis spectroscopy is a prominent method to characterize GQDs. UV-Vis spectroscopic data are analyzed based on the Beer-Lambert law. UV-Vis spectroscopy can be used to understand the absorption coefficient and absorbance. The bandgap opening of GQDs, bandgap of GQDs and transmittance of Graphene, and layers of GQDs films can be analyzed using UV-Vis spectroscopy. In addition, UV-Vis spectroscopy is used to identify the presence of other forms of graphitic samples such as GO, reduce GO, Graphene, and graphite. Furthermore, UV-Vis spectroscopy is used to determine the concentrations, sample thicknesses, and molar absorption coefficients or molar extinction coefficients of liquids or solid thin films. UV-Vis spectroscopy is an important experimental technique to characterize Quantum dots. GQDs give a peak at 270 nm wavelength and if it is functionalized, then peaks corresponding to those functional groups can be observed in the UV-Vis spectroscopy [4–6].

2.3.1.5 Raman spectroscopy Raman Spectrum is used to determine composition, crystallinity, lattice strain, defects, and crystal size. So, this is one of the best techniques to study the quality and structure of GQDs. A small fraction of monochromatic light that passes through the medium (transparent) scatters and shifts the direction with respect to incident radiation depending on the wavelength of the incident light. These shifts depend on the chemical structure and bonds in the molecule. GQDs (Graphene) show the three predominant features of the Raman spectrum: 1. D band at 1350 cm
1 2. G band at 1580 cm
1 3. 2D band at 2700 cm
1 (which is also called as G0 band and this is an overtone of the D band)

In some cases, the D0 band is also observable at 1620 cm
1, which is related to the defect-induced and double-resonance process. Furthermore, GQDs that consist of multilayer graphene show the presence of a 2D band (less intense than the G band). In addition, a negligibly small D band is indicative of GQDs. Monolayer GS shows a much intense 2D band than the G band [14,16,25].

2.3.1.6 Dynamic light scattering Dynamic light scattering, which is also called photon correlation spectroscopy, is mainly used for size characterization or measuring the hydrodynamic size of

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molecules and submicron particles. So, this method can be used to characterize GQDs, in particular suspended in liquid media [37].

2.3.1.7 Dual polarization interferometry Dual polarization interferometry helps in characterizing the surface interaction of GQDs with other molecules [38].

2.3.1.8 Nuclear magnetic resonance Nuclear magnetic resonance gives structural information (example—spent pot lining and spark plasma sintering carbon information) [39,40].

2.3.2 Microscopic methods Microscopic methods are used to investigate the growth, surface morphology, uniformity, numbers of layers, size of the layers, and the thickness of layers of GQDs. These methods can also be described as a form of photography of samples. Microscopic data give highly magnified images as outputs. The main microscopic methods used in GQDs characterization are SEM, TEM, AFM, and scanning tunneling microscopy (STM).

2.3.2.1 Scanning electron microscopy In SEM experiments, an electron beam scans over a sample to create a magnified image of the sample. Incident electrons from the electron source interact with the sample and bounce off. The reflected electron by the sample is studied using a detector and the detector transforms the data into an image. The results in images can be used to extract various information about the sample, for example, the topography (surface features of an object), surface morphology (basically the shape and size of the GQDs), the composition (the elements and compounds and crystallographic information), if there is energy-dispersive X-ray (EDX) spectroscopy attachment. SEM images produced along with the EDX elaborate the information that can be obtained from SEM. For example, the investigations can be extended to study element in the sample and analyze chemical composition qualitatively and quantitatively [11].

2.3.2.2 Transmission electron microscopy The TEM is another high-resolution electron microscopic technique and the TEM mechanism is similar to the mechanism of SEM; however, here, an electron beam is transmitted through the sample to produce an image. The thickness of the sample should be small to get a high-resolution image. Otherwise, a sufficient fraction of the electron beam will not transmit through the sample. TEM gives a clear-cut idea about the type and the size of GQDs growth and the surface morphology. This technique can also be used to study the number of layers in the GQDs sample, the atomic arrangement of the sample, crystal structure, and the kind and extent of defects associated with the sample [5,6].

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2.3.2.3 Atomic force microscopy The atomic force microscopy technique is particularly based on the interaction between sample-atoms and tip-atoms and it is capable of measuring nanoscale forces. The force could be an interaction and thus there is a different mode of analysis: contact mode or a tapping mode can be chosen. Generally, tapping mode analysis is used because more informative data can be extracted in this mode than the other modes. AFM gives information about the morphology, thickness, uniformity of the GDQs, domain growth, lateral sheet dimensions, and sheet depth to discover the number of layers in GQDs [1,4,6].

2.3.2.4 Scanning tunneling microscope The beautiful honeycomb hexagonal structure of Graphene (GQDs) can be visualized by STM analysis, which means STM can image the surface at the atomic level. STM is used to gain information on the electronic structure and mapping the local density of states as a function of energy [41]. In STM, a metal tip is placed above the sample in a high vacuum, and a bias potential difference is applied across the interface between the tip and sample, which creates a tunneling current into or out of the sample. The current is proportional to the convolution of the density of states of the tip and the sample. The small size of the tip permits STM to collect data with an atomic level resolution.

2.3.3 Brunauer-Emmett-Teller This is not a microscopic technique; however, the BET method is used to determine the specific surface area (SSA) of powder samples. This is the “bulk” technique that can compare materials and calculate through the adsorption of gas molecules (typically nitrogen) on a solid surface. So, by using this BET technique, the SSA of GQDs can be calculated [42].

2.4

Applications of graphene QDs

Graphene QDs are attractive materials in many applications and are thus used in various fields. Graphene QDs are used in tissue engineering, polymerase chain reaction, drug delivery, testing toxicity in medicinal applications, transistors, electron-conducting electrodes, hall effect sensors, conductive ink as an electronics applications, optical modulator, photodetector, infrared light detection, pressure sensors, body motion sensors, biosensors, magnetic sensors, water filtration, permeation barrier, contaminant removal and reference materials, redox, radio wave absorption, lubricant, coolant additive, etc. At present, graphene QDs are used in photonics, energy conversion and storage devices, medicine, spintronic, organic electronics, and suggested material to improve the properties of many other composite materials used in various other applications. This chapter focuses on explaining the applications of graphene QDs in devices and GQDs synthesis and optimization along with desired properties. GQDs play a crucial

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role in enhancing the performance of various applications. The applications of GQDs are mainly in the following fields. 1. Graphene QDs in energy conversion and storage 2. Graphene QDs in biomedical applications 3. Graphene QDs for sensors

2.4.1 Graphene QDs in energy conversion and storage 2.4.1.1 Introduction In the modern world, to sustain the modern lifestyle, many devices, equipment, and tools are extensively used to make our life more comfortable. Almost all of these devices are run on energy and thus energy in the appropriate form should be supplied to power these devices and equipment. Therefore the demand for energy is continuously increasing, and as a result, our energy resources are overexploited. In particular, most of our present energy demands are supplied by nonrenewable energy resources such as fossil fuels and nuclear energy. As reported by the International Energy Agency, 85% of our energy demands are supplied, and 4% is from nuclear energy in 2018. The total carbon dioxide emission is 33.3 billion metric tons because of fuel combustion as in 2019. The widely used energy resources today are neither reliable nor appropriate to fulfill future energy demands because they are nonrenewable and their utilization is environmentally harmful [43]. Therefore it is imperative to improve the methods that convert the energy in renewable energy resources into useful forms of energy in eco-friendly means. Solar cells are ideal for the purpose because they convert a renewable energy resource (sunlight) to electricity (one of the most useful forms of energy) with minimum environmental pollution. In the recent past, extensive research works have been conducted to improve the performance of solar cells. When considering the applications of GQDs, one of the potential use of GQDs is in the field of solar cells. In general, light-harvesting of a solar cell is done by the semiconductor or photo-sensitizer such as dyes and/or quantum dot attached to the semiconductor. As already explained, Graphene has unique optical and electrical properties, and subsequently, GQDs have extraordinary optical and electrical properties resulting in quantum confinement. The optical and electrical properties of GQDs can further be tuned by varying the size, shape, and type of the edge [44]. The important properties, such as light-harvesting efficiency and charge carrier transport of solar cell components, can be modified appropriately, incorporating suitable GQDs into respective components in solar cells [45]. GQDs are perceptive material for improving solar cells because of their versatility in enhancing light-harvesting efficiency and electrons/holes transport and transfer within the material and between contacts. The GQDs contribute to improving solar cell efficiency through three mechanisms. (1) Tuner the bandgap and modify the light-harvesting efficiency. (2) Work as electron-hole transfer material [46]

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(3) Suppress the carrier recombination (4) Convert the solar spectrum (work as downconverters and upconverters) [47,48]

The role of GQDs in a solar cell depends on the type of the solar cell and the solar cell component that the GQDs are incorporated. Therefore in this section, application of GQDs in different conversion devices will be discussed. Apart from solar cells, the fuel cell that converts energy in chemical fuels such as H2, Methanol, ethanol to electricity without combustion is also an efficient energy conversion device. The fuel cell is a beneficial energy conversion device because its environmental impact is less compared to that of fossil fuel combustion devices. Section 2.4.1.3 is devoted to explaining the application of GQDs in fuel cells. The energy converted to useful forms needs to be stored. Energy storage is vital to store energy to consume it whenever needed. For example, secondary batteries convert electrical energy to chemical energy (charging process) and store it. Whenever required, we can get electrical energy out of the battery (discharging process). Batteries, supercapacitors, and H2 storage are popular energy storage methods.

Graphene QDs in PV cells Introduction to PV cells Photovoltaic PV solar cells are dependable devices to fulfill world energy demands. PV cells obtain energy from the sunlight and convert it to electricity. Annually, the Earth receives 3  1024 J, which is thousands of times more than the present total energy consumption [43,49]. For instance, the total annual total energy consumption in 2014 was about 3  1020 J. Solar energy is a renewable energy resource, and the output of PV solar cells is electricity. Electricity is the most useful and usable form of energy because it can be converted to other forms of energy, such as mechanical, heat, light, sound, and chemical energy that we use every day. Furthermore, electricity is easily storable and transportable. For example, electrical energy can simply be stored by charging a battery and transported by using a metal cable. Clearly, PV cells are environmentally friendly energy conversion devices, unlike the devices that generate power from the combustion of fossil fuel and nuclear reactions. In general, fossil fuel combustion releases a large amount of CO2 and other toxic gases such as nitrogen oxides (NOx), sulfur dioxide (SO2), and particulate matter like lead (Pd) and soot, which cause serious environmental issues and health problems. Conversely, PV cells are zero-emission devices and thus they are a green energy source. More importantly, there is not even sound pollution from solar cells. The solar cells are suitable to install in urban or crowded areas because of zero-emission, silent functioning, safety, and free and abundant supply of sunlight. Another main advantage of the utilization of solar cells to fulfill power needs is that it is safe and less hazardous compared with many other main energy conversion methods. Solar cells do not produce any harmful waste products, whereas waste disposal is the main problem associated with nuclear power. Therefore PV solar cells are eco-friendly devices to fulfill world energy demands. The first- generation and second-generation solar cells are solid-state PV devices. Several solar cells are stacked to produce solar panels or modules. Large or small area solar panels of PV cells are constructed depending on the power requirements. Small

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43

area modules (panels) are prepared to power small portable devices, sometimes along with a battery storage facility. To fulfill large power requirements such as feeding the national grid, solar PV panel arrays that cover a large area are prepared. For example, the solar PV farm in Tengger Desert Solar Park in China reports solar power conversion of about 1547 MW. At the end of 2018, PV systems having a total power of more than 500 GW have already been installed [50]. Solar PVs are presently a fast-growing and popular energy conversion method in the world. Solar PV cells are stable devices, along with excellent chemical and physical stability. They can function for more than 30 years without faults. Because there are no moving or mechanical parts in PV devices, no wear and tear, and hence, the maintenance cost is negligible. The only disadvantage is a small efficiency drop of about 5% per 10 years exhibited by PV cells. Working principle of PV cells The photovoltaic effect or generation of a potential difference between electrode and electrolyte junction under the irradiation of sunlight was observed by Becquerel in 1839 [51]. The photovoltaic effect in silicon p-n junction type devices was observed by Oh1 in 1940 [52]. So far, PV cells have undergone so many developments. The first-generation and second-generation solar cells are PV devices. The first-generation solar cells are crystalline Si thin films-based devices. Their energy conversion efficiency is relatively high compared with other types of solar cells. The production cost of the second-generation solar cells is less than that of the first generation. However, the disadvantage is their efficiency is also lower than the first generation [43]. Amorphous silicon, cadmium telluride, cadmium telluride/cadmium sulfide, Gallium arsenide, copper indium selenide, and copper indium gallium diselenide thin films are commonly used to prepare the second-generation PV cells. Working principle of solar cell The photovoltaic effect is defined as the generation of a potential difference between two connections of a device leading to an electric current flow through an external circuit upon irradiation of light. To observe PV performance, basically, three processes should be completed. 1. Charge carrier generation upon irradiation of light, 2. Charge carrier separation or photocurrent generation mechanism, 3. Transport of carriers through the external load. (1) Charge carrier generation upon irradiation of light The most straightforward PV device at least consists of a semiconductor film sandwiched between two conducting layers. To illuminate the semiconductor layer, at least one contact (the front contact) should be transparent to visible light or should be a metal grid, as illustrated in Fig. 2.13. When the light energy (photons) is incident on the semiconductor layer in the PV cell, if the energy of photons is higher than the bandgap energy of the semiconductor, it can create electron-hole pair, that is, conduction band electron and valence band electron as illustrated in Fig. 2.14. Semiconductors should be exposed to sunlight and the bandgap energy of the semiconductor should be less than the energy of the incident light photon. These are the two conditions that lead to achieve the charge generation mechanism.

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Fig. 2.13 A schematic diagram to illustrate the cell configurations needed to achieve charge generation. (A) Metal grid front contact and (B) transport conductive layer front contact. Fig. 2.14 An illustration to show electron excitation upon absorption of light photons.

Graphene QDs are capable of absorbing visible light photons because of their smaller size, and thus they can be used to assist the charge generation mechanism in a PV cell. It is reported that high transparency and conductivity of graphene make it a promising material for front contact. (2) Charge carrier separation (photocurrent generation) mechanism However, to observe the PV effect, the created electrons and holes in the charge generation process should be transported to the external circuit before recombination. Therefore the charge separation mechanism in the device is important for the PV effect. The charge separation can be achieved by selecting appropriate conductors as front and back contacts. Here, one contact should be a Schottky-barrier and the other contact should be an Ohmic contact. Therefore the material selection for PV devices depends on the Fermi levels of the semiconductor, front and back contacts. A built-in-potential barrier in the cell acts as the driving force on photogenerated conduction band electrons and valance band holes to produce a voltage (the photo-voltage). This photo-voltage drives a current through the circuit. Fig. 2.15 illustrates the mechanism of a simple Schottky-barrier type PV device based on n-type semiconductor. The cell performance can be enhanced by using a p-n junction

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Fig. 2.15 Illustration of energy bands in Schottky-barrier (n-type semiconductor-conductor junction) type conventional photovoltaic solar cell.

or p-i-n junction to achieve an efficient charge separation mechanism. The driving force for electrons to travel through the external load is the charge separation process. The excited (or conduction band) electrons in the semiconductor move to the one contact through the Ohmic contact. The valence band electrons move to the other contact through the Schottky-barrier. The potential step in n-p junction type devices provides efficient charge separation than Schottky-barrier (Fig. 2.16). Consequently, n-i-p junction type devices are more efficient because of the built-in potential gradient (Fig. 2.17). (3) Transport of carriers through the external load The completion of an external circuit between front and back contacts through the external load provides the charge transport mechanism. The built-in potential difference between front and back contacts upon illumination, when the external load is infinite, is the opencircuit voltage (Voc) of a solar cell. Current through the external circuit of an operating solar cell, when the load resistance is zero, is the short circuit current (Isc) of the solar cell. By dividing the Isc from the illuminated cell area, the short circuit current density (Jsc) can be obtained. The conventional method to characterize a solar cell is by its current density vs. voltage (J-V) curve. J-V curves enables the determination of Jsc, Voc, Pm. Jm, Vm, and the power conversion efficiency (PCE) of a PV device.

Fig. 2.16 Illustration of energy bands in n-p junction type conventional photovoltaic solar cell.

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Fig. 2.17 Illustration of energy bands in a p-i-n junction type conventional photovoltaic solar cell. PV performance and characterization V-I characteristics of a photovoltaic cell

A schematic diagram of a J-V is shown in Fig. 2.18. The performance of a solar cell is generally described by the PCE and the photocurrent quantum efficiency. However, higher quantum efficiency does not guarantee high overall energy conversion as it corresponds to a specific wavelength of the incident light (monochromatic). Hence, J-V characteristics obtained under standard irradiation, air mass 1.5 global (AM 1.5 ¼ 1000 W m
2), is important to compare the performance of the cells such as PCE, short-circuit current density (JSC), and open-circuit voltage (VOC). In addition, the photocurrent and photovoltage at the maximum power point (Pmax) is also an indicator of the performance of a cell. The total energy conversion efficiency of the solar cell is defined as; PCE ¼

P max  100% Total incident power density of light

(2.1)

where the maximum output power density of the device, Pmax, is given by the product JmVm, where Jm is the current density and Vm is the voltage at the peak power. That is, PCE ¼

Im V m  100% Total incident power density of light

(2.2)

The values of Jm and Vm can be obtained from the J-V characteristics curve of the solar cell. The short-circuit current density, JSC, of a cell is defined as the current density of

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Fig. 2.18 Typical J-V characteristics of a solar cell under illumination.

the cell when the applied voltage is zero. The open-circuit voltage, VOC, is the voltage when the current through the cell is zero, as shown in Fig. 2.18. The PCE expression in Eq. (2.2) can also be written as in Eq. (2.3). η¼

J SC V OC FF  100% Total incident power density

(2.3)

where FF is called the fill factor of the cell, which can be determined by Eq. (2.4). FF ¼

Jm Vm J SC V OC

(2.4)

The monochromatic PCE of a cell at the wavelength λ can be written as: η ðλÞ ¼

J SC ðλÞ V OC ðλÞ FF ðλÞ  100% Total incident power density ðλÞ

(2.5)

The incident photon to current conversion efficiency (IPCE%) or quantum efficiency is also an important parameter to characterize PEC solar cells. The IPCE is determined for a wavelength (λ), by getting the ratio of the number of photo-electrons produced by the cell under the short circuit condition per unit time and area (nelectron,λ) to the number of photons incident on the cell per unit time and area nphoton,λ (see Eq. (2.6)). IPCE ¼

nelectron,λ  100% nphoton,λ

(2.6)

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The number of photons incident per unit time and per unit area of the cell nphoton is given by the equation: nphoton,λ ¼

I ðλÞ ðW m
2 Þ hν ðJÞ

(2.7)

where I(λ) is the intensity of monochromatic light, h Planck’s constant, ν the frequency of the photon. The υ in the equation can be replaced by c/λ, where c is speed of light. nphoton,λ ¼

Iλ λ hc

s
1 m
2




(2.8)

The number of electrons contributing to the photocurrent under short circuit conditions per unit time per unit area of the cell is given by: nelectron,λ ¼

J SC,λ
1
2
s m e

(2.9)

According to Eqs. (2.7) and (2.9), the IPCE is given by equation: IPCE ¼



1 J SC hc 1240 JC nm J SC ðmA cm
2 Þ  100% ¼ I ðλÞ eλ I ðmW cm
2 Þ λ ðnmÞ

(2.10)

Graphene quantum dots in crystalline Si solar cells GQDs are more advantageous to be used in applications because of high abundance and nontoxicity. The UV-vis light absorption properties of GQDs can be tuned as a result of quantum confinement and edge effects [53,54]. The first-generation Si solar cells are successful as potential commercial photovoltaics because of the high availability, high efficiency, and mature Si solar cell preparation technologies. Furthermore, it has been predicted that GQDs in silicon-based solar cells could effectively overcome the Shockley-Queisser efficiency limit [55,56]. The tunable bandgap of GQDs makes it a suitable light-harvesting material in photovoltaic devices [57,58]. Gao Peng et al. [59] introduced a heterojunction PV cell based on crystalline silicon and GQDs-based active layer. The study reveals that the charge separation of the device is supported by the band structure of GQDs, providing a large junction gap between GQDs and n-Si. In addition, the GQDs acted as an electron blocking layer minimizing the carrier recombination at the anode. The Voc of the PV cell increases whereas the Jsc drops when the GQD size is decreased. This can be understood because the decreasing size of the QDs leads to an increase in the heterojunction barrier. The PCE of 6.63% has been achieved by optimizing the GQDs size and layer thickness. This optimum device performance is shown by the GQDs of the size of 2–6 nm.

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Diao Senlin et al. [60] also reconfirmed the role of GQDs in n-type silicon heterojunction solar cells as a hole transport layer that facilitates the carrier separation and a layer that suppresses the carrier recombination. In addition, in the study, Graphene has been used as the transparent top electrode, ensuring an efficient light absorption and carrier collection and thus achieved PCE of 12.35% under AM 1.5G irradiation. The main difference between heterojunction PV cells based on crystalline silicon and GQDs reported by Gao Peng et al. [59] and Diao Senlin et al. [60] is the top-contact. The reported efficiency with thick Au top electrode 6.63% [59] is almost doubled when it is replaced by Graphene [60]. In the study, the simple drop-casting method is used to prepare QDs/Si heterojunction (SHJ) PV cells. The preparation of highly efficient and stable GQDs/SHJ solar cells by means of low-cost methods is the significance of the above-mentioned two studies. Subsequently, in 2015, Tsai et al. reported a PCE of 16.55% for an n-type SHJ solar cell improved by using GQDs [48]. The efficiency enhancement attributes to the photon downconversion phenomenon of GQDs that makes more photons absorbed in the depletion region for active carrier separation. The downconversion phenomenon that alters the effective spectrum is illustrated in Fig. 2.19. According to the study of Tsai et al., the incorporation of GQDs to n-type SHJ solar cells led to an increase in Jsc from 35.31 to 37.47 mA cm
2, exhibiting 6.1% enhancement. The FF of the device increases from 70.29% to 72.51%. The added GQDs improve the PCE from 14.77% to 16.55%, highlighting an efficiency enhancement of about 10.75%. In the study, GQDs are spin-coated on top of SHJ devices at 800 rpm for 90 s. The optimum efficiency is given for 0.3% GQDs of concentration. The study shows that GQDs are appropriate material to enhance efficiency in commercial SHJ devices. It has already been proved that GQDs improve the efficiency of SHJ PV devices, enhancing the carrier transport, carrier separation, and photon harvesting efficiency. Many researchers are working to strengthen the efficiency of the solar cell through the efficient and effective separation of photogenerated electron-hole pairs and to suppress the recombination of electron-hole pairs. However, more enchantment of efficiency can be expected from the nano-structural modifications. In particular, the enhancement of spectral absorption through downconversion, upconversion, and downshifting would open the path to achieving highly efficient Si solar cells.

Fig. 2.19 Energy diagrams showing photon adsorption and subsequent a) downshift, b) downconversion, and c) upconversion.

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Functional Materials from Carbon, Inorganic, and Organic Sources

Fig. 2.20 P3HT and PCBM are commonly used, electron donors and acceptors, to prepare active layer in OPVs.

Organic photovoltaic PV cells OPVs attracted attention among researchers because of their advantages, such as the possibility of fabrication of large-area, low-cost, and flexible devices. In general, OPVs are bulk heterojunction devices composed of p-type and n-type materials such as polymers and fullerene derivatives [61]. Poly(3-hexylthiophene) (P3HT) and the fullerene derivative like (6,6)-phenyl-C61 butyric acid methyl ester (PCBM) are the popular electron donor and acceptor materials that are frequently used to fabricate organic PVs (Fig. 2.20). A fundamental issue in these devices is the mutual tradeoff between optical absorption and charge separation. Therefore it is changing to challenging to improve PCE in OPV. Various materials have been investigated to prepare active-layers that offer higher PCE. For example, diverse research is carried out to improve OPVs using metal nanoparticles, numerous quantum dots, and CNTs [43]. However, to enhance efficiency in OPVs, GQDs are more attractive because they offer (a) the possibility of controlling the energy gap, thus giving a possibility to improve spectral absorbance, (b) good chemical and long-term stability, (c) proper blending with organic active layer materials, (d) facile functional group attachment, (d) nontoxicity, and (e) efficient electron-hole pair separation compared with other types of QDs. GSs-blended conjugated polymers were, conventionally, used to prepare OPVs However, GQDs can be used to boost the cell performance. In 2011, Gupta et al. reported that organic PV made with an active layer of GQDs-blended P3HT exhibits a significant enhancement of OPV performance compared with that of the GSs incorporated device [62]. In the study, for the solar cell preparation, GQDs have been functionalized with aniline (ANI). The average size of GQDs is 9 nm, and the sizes distribute in the range of 5–15 nm. The OPV configuration is ITO/PEDOT:PSS/ P3HT:ANI-GQDs/LiF/Al and the top contact is vacuum deposited LiF and Al (Fig. 2.21). The maximum values of Voc, Jsc, and FF and PCE resulted together with the addition of 1 wt% of ANI-GQD to P3HT is given in Table 2.1.

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Fig. 2.21 A schematic diagram to show the configuration of ITO/PEDOT:PSS/ P3HT:ANI-GQDs/LiF/Al OPV prepared by Gupta et al. [62].

Table 2.1 Voc, Jsc, FF, and PCE of the OPVs that contain P3HT with ANI-GQDs and P3HT with ANI-GSs under AM1.5G 100 mW irradiation [62]. Type of graphene

Graphene wt. (%)

Voc (V)

Jsc (mA cm22)

Fill factor (FF)

Efficiency (PCE) (%)

ANI-GQDs ANI-GSs

1 1

0.61 0.72

3.51 0.19

0.53 0.22

1.14 0.03

To understand the influence of blended GQDs, respective cell performance parameters of the OPV based on a single active layer that contained 1 wt.% of ANI-GSs on P3HT is also given in the table. Gupta et al. have achieved a marked improvement in efficiency (from 0.03% to 1.14%) in OPV resulting from the blended GQDs (Table 2.1). In 2013, polymer solar cells improved by adding GQDs derived from doublewalled CNTs were reported by Li et al. [61]. The wet synthesized GQDs, which show uniform size distribution, have displayed bright blue emission upon excitation from UV light. In the study, light-emitting GQDs made from double-walled CNTs have been used along with P3HT and PCBM to assemble OPV that exhibits an appreciable PCE. For the fabrication of the OPV based on GQDs, a precursor solution that contained P3HT (10 mg mL
1) and variable amount of PCBM (weight ratio 0.5, 0.6, 0.8, 1, 1.25, 1.67, and 2 ¼ PCBM: P3HT) have been prepared in chlorobenzene by Li et al. [61]. The GQDs concentration is 0.05 mg mL
1. The precursor solutions have been spin-coated on ITO substrates. Subsequently, coated with poly(ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) conductive polymer as the bottom contact. The top contact LiF and Al layers have been prepared by the thermal evaporation method. The OPV configuration is ITO/PEDOT:PSS/ P3HT:PCBM:GQDs/LiF/Al, and it is illustrated in Fig. 2.22. The PCE of the cell improves to 5.24% by optimizing the PCBM composition in the active layer. This ternary system active layer, based on a blend of P3HT, PCBM, and GQDs, introduced a new approach to enhancing the efficiency of OPVs. In 2016, Novak et al. reported OPVs system that contains P3HT blended with functionalized GQDs having an average PCE of 4.1% [63]. In the study, P3HT serves as a donor, PCBM serves as an acceptor, and GQDs functionalized by polyethylene glycol (PEG) with different molar weights (or chain lengths) serve as performance

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Fig. 2.22 A schematic diagram to show the configuration of ITO/PEDOT:PSS/ P3HT:PCBM:GQDs/LiF/Al OPV prepared by Li et al. [61].

Fig. 2.23 The basic diagram design of the OPV prepared by Novak et al. [63].

enhancers. Fig. 2.23 shows the diagram of the basic design of the OPV prepared by Novak et al. The study reveals that the shorter PEG chains (MW 200) give a more substantial electronic effect in the active layer to fast exciton dissociation from P3HT. The authors have blended P3HT and PCBM along with PEG-functionalized GQDs to prepare the active layer of the OPV cell. As a result, 36% efficiency improvement in the solar cell is achieved compared with general P3HT- and PCBM-based solar cells. The efficiency enhancement is attributed to the improved active layer absorption and dissociation of exciton in P3HT in conjunction with the incorporation of appropriately functionalized GQDS. OPVs that contained functionalized GQDs having longer PEG chain lengths show poor PCE, which can be due to chain wrapping of the longer PEG chains around the GQD. Peak performance has been achieved for the shortest chain length studied (200 MW). The PCE of the best device that contains PEG-GQDs is 4.24%, whereas that of the reference cell (cell without GQDs) is 3.05%. The results give some indications for selecting suitable functional groups for GQDs and the proper active layer configuration to develop future OPVs. In addition, the study reveals the importance of studying the nature of functional groups, namely the type and size attached to the GQDs on the performance of OPVs based on GQDs. The optical and electrical properties of GQDs can be engineered by modifying the lateral size and the edge structure. Recently, Wu et al. [64] have investigated the performance of inverted OPV using GQDs of different lateral sizes and using them as the second electron acceptor. The OPVs are based on P3HT:PCBM and have been prepared along with blue, green, and orange GQDs. For this purpose, different GDS are prepared by a photon-Fenton reaction. The blue, green, and orange GQDs are separated through gel column chromatography. Binary and ternary active layer OPVs with configurations of (ITO/ZnO/P3HT:GQDs/MoO3/Ag) and (ITO/ZnO/P3HT:PCBM:GQDs/MoO3/Ag) have been fabricated with three different GQDs sizes (Fig. 2.24).

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Fig. 2.24 The configuration cell (ITO/ZnO/Active layer/MoO3/Ag) as reported by Wu et al. [64].

Table 2.2 Performance details (Voc, Jsc, and PCE) of ternary solar cells (P3HT: PCBM:GQDs) based on different size GQDs. Type of GQDs

GDS size (nm)

Jsc (mA cm22)

Voc (V)

PCE (%)

None GQDs GQDs-blue GQDs-green GQDs-orange

– 5.6 11.2 17.6

9.57 10.33 13.34 11.19

0.60 0.60 0.60 0.60

3.06 3.54 4.43 3.73

OPV performances, namely Voc, Jsc, FF, and PCE of solar cells based on an active ternary layer based on a blend of (P3HT:PCBM:GQDs) are given in Table 2.2 along with GQDs size, as reported by Wu et al. [64]. The PCE of the P3HT:PCBM cells enhance significantly with the inclusion of the GQDs as the second electron acceptor. The PCE of OPVs containing 0.8% of GQDs shows the highest PCE. The interesting observation made by Wu et al. is that green GQDs (dot size 11.2 nm) gives the highest PCE. The Voc of the cell remains fixed with the change of GQDs size. The study reveals that the addition of GQDs slightly affects the light absorption intensity of the active layer of the OPV. However, it modifies the exciton separation and carrier transport. Performance parameters of ternary solar cells prepared using different amounts of GQDs-green are given in Table 2.3. We et al. compared the performance of binary and ternary OPVs by investigating two systems having the configurations (ITO/ZnO/P3HT:GQDs/MoO3/Ag) and (ITO/ ZnO/P3HT:PCBM:GQDs/MoO3/Ag) [65]. In the study, the GQDs prepared from a photon-Fenton reaction are used in hybrid-binary and hybrid-ternary active layerbased OPVs. The binary-hybrid solar cell exhibits the highest PCE of 0.25%. Table 2.3 Performance details (Voc, Jsc, FF, and PCE) of ternary solar cells based on GQDs-green with different ratios. Contents of GQDs-green

Jsc (mA cm22)

Voc (V)

FF (%)

PCE (%)

GQDs-0.2 wt% GQDs-0.4 wt% GQDs-0.8 wt% GQDs-1.6 wt% GQDs-3.2 wt%

10.91 12.70 13.44 11.67 10.31

0.61 0.59 0.60 0.61 0.60

53.90 54.96 55.77 53.23 50.93

3.56 4.12 4.43 3.80 3.16

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However, as uncounted by We et al., the hybrid-ternary OPVs show a PCE of 4.13%. The exhibited enhancement of cell performance with the added PCBM GQDs is markedly high. The value of the presence of GDS in the active layer is highlighted by the exhibited 40% PCE enhancement for the ternary hybrid OPV that contains (P3HT: PCBM:GQDs) active layer compared with (P3HT:PCBM) active layer. As revealed by Wu et al. [65], the PCE enhancement is contributed by exciton separation and charge transport processes improved along with added GQDs.

2.4.1.2 Graphene QDs in photoelectrochemical solar cells Graphene QDs in DSCs Jahantigh et al. [66] synthesized hybrid dye-sensitized solar cells (DSCs) based on single-layer graphene quantum dots (SLGQDs) by adding SLGQDs into N719/ TiO2 nanoparticles photoanode as co-sensitizer. It has been reported that the incorporation of carbon-based nanomaterials in the photoanode leads to reduce the charge recombination and it also enhances the charge collection efficiency. Excellent optical properties and low resistance of GQDs can improve the performance of DSC when incorporated with TiO2 nanostructures [14,15]. In this study, TiO2 nanoparticle films are immersed in a solution of SLGQDs and followed by immersing in the N19 dye to sensitize the photoanodes. To compare the effect of SLGQDs-N719 on the efficiency of the cells, the measurements are taken in DSCs prepared by photoanodes based on SLGQDs, N719, and SLGQDs-N719 separately. As reported, the bandgap energy of TiO2 and TiO2/SLGQDs are about 3.1 and 2.9 eV, respectively. The authors suggested that this decrease in the bandgap of TiO2 by SLGQDs can be used for higher absorption over the visible light. From the CV measurements, it has been concluded that the addition of SLGQDs onto TiO2 nanoparticles does not affect the charge transfer at Pt/electrolyte interface. Photovoltaic values of DSCs fabricated using TiO2, SLGQDs/TiO2, N719/ TiO2, and (SLGQDs-N719)/ TiO2 as photoanodes are given in Table 2.4. As given in Table 2.4, it can be seen that Jsc of the solar cells improved in the order of (N719-SLGQDs)/ TiO2 > N719/ TiO2 > SLGQDs/TiO2 > TiO2. More importantly, the (N719-SLGQDs)/TiO2 co-sensitized solar cell has exhibited the highest efficiency of 8.92% among all other fabricated solar cells.

Table 2.4 Jsc and efficiency of DSCs fabricated using bare TiO2 nanoparticles and sensitized TiO2 nanoparticles as photoanodes. Sample

JSC (mA cm22)

Efficiency (PCE) (%)

TiO2 SLGQDs/TiO2 N719/TiO2 (SLGQDs-N719)/TiO2

0.93  0.07 1.36  0.09 14.47  0.38 20.03  0.49

0.12  0.01 0.33  0.03 6.57  0.32 8.92  0.49

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2.4.1.3 Graphene QDs in fuel cells In a recent study [67], GQDs/functionalized multiwalled carbon nanotubes (f-MWCNTs) composite has been used to fabricate a direct-methanol fuel cell. First, GCE has been cleaned. Then, GQDs, f-MWCNTs, and GQDs/f-MWCNTs dispersions have been dropped onto the clean GCE surfaces to fabricate GQDs/GCE, f-MWCNTs/ GCE, and GQDs/f-MWCNTs/GCE, respectively. The active electrochemical areas of bare GCE, f-MWCNTs/GCE, GQDs/GCE, and GQDs/f-MWCNTs/GCE were reported as 17.00, 41.90, 50.70, and 119.00 m2/g, respectively. The highest current density has been exhibited by GQDs/f-MWCNTs/GCE according to the chronoamperometry measurements. Hence, the authors conclude that GQDs/f-MWCNTs composite having a large active surface area, high electro-oxidative activity, and superior CO tolerance will be an effective electrocatalyst in the future for using direct-methanol fuel cell applications.

2.4.2 Graphene QDs in biomedical applications Recently, GQDs were used in many applications such as a nanocarrier in drug delivery [68], biosensing agent [69,70], bioimaging [71], and as an antibacterial [72,73]. Furthermore, GQDs play a major role in cancer therapy as they are used in photodynamic therapy (PDT) [74,75] and photothermal therapy [76,77]. GQDs gain much attention as they are currently used as a treatment for neurodegenerative disorders such as Parkinson’s and Alzheimer’s diseases [78,79].

2.4.2.1 GQDs in drug delivery Today, nanomaterials play a key role in the development of targeted drug delivery systems because of their properties such as improving the solubility of hydrophobic drugs, achieving targeted delivery as well as their potential for drug delivery across the blood-brain barrier and tumor-targeted drug delivery [80,81]. When considering the applications in GQDs in drug delivery, most researchers used density functional theory calculations [82] and molecular dynamics simulations [68,83] to theoretically study the properties of GQDs.

As a nanocarrier In 2017, Iannazzo et al. [84] used GQDs for the development of a biocompatible and cell traceable drug delivery system to deliver the DNA intercalating drug doxorubicin (DOX) to cancer cells. Here, GQDs have been covalently linked to the tumor-targeting module biotin (BTN) and loaded with DOX. Then, this developed system has been tested for the ability to cross cancer cell membranes and release the DNA intercalating drug into the nucleus. Cytofluorimetric analysis has been performed by using A549 cell suspensions treated with GQD-BTN-DOX, the GQD loaded with the drug and lacking the biotin recognition molecule, and with the free drug at the same doses loaded in GQD. The higher emission value in GQD-BTN-DOX-treated cells in comparison to free DOX and GQD-DOX-treated cells underpinned biotin

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Fig. 2.25 Illustration of GQDs as nanocarriers (receptor-mediated endocytosis of targeting ligand-conjugated GQDs along with an anticancer drug to a tumor cell and drug release inside the cell, as reported by Wu et al. [81]).

functionalization to enhance the cancer cell uptake of the drug-loaded GQD. It is also reported that the acidic environment of cancer cells causes the drug detachment from the nanosystem, which leads to a delay of nuclear internalization. Fig. 2.25 illustrated the processes of GQDs as nanocarriers.

Photodynamic therapy Ge et al. [74] fabricated a novel GQD-based PDT agent that can produce 1O2 by way of a multistate sensitization process. This novel PDT agent exhibited a quantum yield of 1.3, which is among the highest reported for PDT agents. In vitro imaging and PDT: Laser-scanning confocal microscopy is used to understand the morphology changes of HeLa cells in the presence of GQDs. The shrinkage of cells and the formation of many blebs have been observed, which show that irradiation leads to cell shrinkages and numerous blebs formation. In contrast, obvious morphological variations have not been identified in the control experiments done without GQDs. Moreover, PDT efficiency and cytotoxicity of the GQDs compared with the classic photosensitizer PpIX has been investigated by using a standard 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium hydrobromide (MTT) assay. It is found that HeLa cell viability is decreasing with increasing GQD concentration upon irradiation. Cell viability of 60 and 20% is observed with 0.036 μM GQDs and 1.8 μM GQDs solution, respectively. However, a significant change in cell viability has not been observed with the increasing PpIX concentration. In vivo imaging and PDT: GQD aqueous solution has been injected into the back of a nude mouse to investigate the in vivo fluorescence imaging capability of GQDs. It was reported that fluorescence intensity is not decayed at the injection sites and there

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has been an absence of diffusion in the injected GQDs 1 week after injection. One of the mice group (PDT group) is intratumorally injected with GQDs and followed by irradiation on the first and seventh days twice for 10 min with white light (400– 800 nm) at a power density of 80 mW cm
2. In this PDT group, at first, the tumor size has been slightly increased and then tumors began to decompose and destroy after 9 days and 17 days, respectively. More importantly, for a 50-day time period, tumor regrowth has not been observed in the PDT group. In conclusion, it is obvious that GQDs can be applied as an effective PDT agent because of simultaneous imaging and enabling highly efficient treatment of cancer, as confirmed by in vitro and in vivo studies.

Photothermal therapy Wang et al. [77] synthesized ultra-small size ( 5 nm) dual-doped GQDs with both nitrogen and boron (N-B-GQDs). These N-B-GQDs act as a second near-infrared window (NIR-II) florescent agent by exhibiting a peak fluorescent emission at 1000 nm and high photostability. With the irradiation of an external NIR source, N-B-GQDs are capable of efficiently absorbing and converting NIR light into heat. It has been observed that the photothermal therapeutic effect of N-B-GQDs destroys cancer cells in vitro and completely terminate tumor growth in a glioma xenograft mouse model.

2.4.2.2 GQDs as an antibacterial In a recent study [85], a composite material that has a core-shell structure was fabricated by coating GQDs on the surfaces of silver nanoparticles (AgNPs). To determine the antibacterial properties, the antibacterial ring characterization was carried out for GQDs, AgNPs, and GQDs@AgNPs against Escherichia coli and Staphylococcus aureus. It has been found that antibacterial ring diameters of GQDs, AgNPs, and GQDs@AgNPs against E. coli are 8.3 mm, 12.6 mm, and 15.8 mm, whereas the antibacterial rings of GQDs, AgNPs, and GQDs@AgNPs against S. aureus are 7.1 mm, 11.4 mm, and 13.9 mm, respectively. Hence, it has been concluded that the synergistic antibacterial effect between AgNPs and GQDs results in excellent antibacterial properties in GQDs@AgNPs composite material. It has also been reported that GQD-based band-aids having a low dose of H2O2 have been used for wound disinfection by Sun et al. [73]. This study shows that the peroxidase-like activity of GQDs leads to catalyze H2O2 decomposition into OH. Therefore using GQDs enables the use of low concentrations of H2O2 as OH with higher antibacterial activity improves the antibacterial performance of H2O2.

2.4.2.3 GQDs for bioimaging Kuo et al. [86] successfully synthesized nitrogen-doped GQDs (N-GQDs), which are effective in antimicrobial therapy as well as in bioimaging. In this study, PDT experiments against E. coli were carried out with a 670-nm laser at 3 min exposure time to determine the antimicrobial performance. Improved antimicrobial properties were

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exhibited by N-GQDs with an N 1s—C 1s ratio of approximately 5.1%, when compared with nitrogen-free GQDs according to the PDT results. It has been reported that N-GQDs (5.1%) have a relative PL QY of around 0.253 and PL spectrum of the N-GQDs (5.1%) strongly emitted at 728 nm in the NIR region. Hence, it is obvious that N-GQDs could potentially serve as a promising contrast probe to track and localize nanomaterial-treated bacteria and provide additional data on the irradiated bacterial status following laser exposure. More importantly, the photoexcited N-GQDs (5.1%) serve as a prominent photosensitizer as it showed higher photostability than conventional photosensitizers. As given in Fig. 2.26A, there is no emitted luminescence in laser-treated bare E. coli without any treatment, whereas the N-GQD (5.1%)-AbLPS-treated (antilipopolysaccharide (LPS) antibody(Ab): AbLPS) on the bacterial surface started to emit luminescence when irradiation is introduced, resulting in a clear image and localization of bacteria (Fig. 2.26B).

2.4.2.4 GQDs as a treatment for neurodegenerative disorders Alzheimer’s disease or AD is among the most common neurodegenerative diseases, which is caused by misfolding and abnormal aggregation of b-amyloid peptides. As a treatment for AD, it is crucial to inhibit the Aβ peptide aggregation. One of the applications of GQDs is inhibiting the Aβ peptide aggregation because of their biocompatibility and low cytotoxicity [78,87]. In one study, thioflavin T (ThT) fluorescence assay has been used to measure the effectiveness of GQDs on the assembly of Aβ1–42 peptides. It has been found that the accumulation rate of Aβ1–42 peptides slowed down observably in the presence of GQDs as the formation of amyloid fibrils is suppressed by GQDs [87]. A recent study also used ThT assay for monitoring amyloid aggregation to observe the aggregation kinetic process of Aβ1–42 peptide in the presence of GQDs. This study also confirmed that there is a significant influence on the aggregation of Aβ1–42 peptide in the presence of GQDs compared with the free

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Fig. 2.26 Luminescence images of (A) bare Escherichia coli without any treatment, and (B, C) N-GQD (5.1%)-AbLPS-treated E. coli for 3 h of incubation and photoexcited nanomaterial treated bacteria with a 670-nm laser exposure from 0 to 3 min. Reproduced with permission from W.S. Kuo, H.H. Chen, S.Y. Chen, C.Y. Chang, P.C. Chen, Y. I. Hou, … J.Y. Wang, Graphene quantum dots with nitrogen-doped content dependence for highly efficient dual-modality photodynamic antimicrobial therapy and bioimaging, Biomaterials 120 (2017) 185–194.

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Fig. 2.27 (A) TEM images of Ab1
42:GQDs ¼ 100:1 mixture and (B) TEM images of the Ab1–42: GQDs ¼ 1:1 mixture. From Y Liu, L.P. Xu, W. Dai, H. Dong, Y. Wen, X. Zhang, Graphene quantum dots for the inhibition of β amyloid aggregation, Nanoscale 7(45) (2015) 19060–19065.

Aβ1–42 peptide. The TEM results indicate that there is a distinct typical fibrils structure in the mixture in the presence of low content of GQDs (Ab1–42: GQDs ¼ 100: 1), whereas the typical fibrils disappeared with the high content of GQDs (Ab1–42: GQDs ¼ 1: 1) as shown in Fig. 2.27A and B.

2.4.3 Graphene QDs for sensors A subset of carbon quantum dots (CQDs) normally derived from graphene or GO [9] are GQDs [88]. Because of their chemical and physical properties of high chemical stability, luminescence, and high conductivity, GQDs have great interest. Some of the properties of CQDs vary because of the dominance of the edge effect with CQDs and quantum confinement, even if GQDs exhibit the same chemical and physical characteristics as graphene [89,90]. CQDs and GQDs show substantial photostability against blinking and photobleaching, low toxicity, and therefore potentially greater biocompatibility compared with the other semiconducting quantum dots (CdSe, WO3-x 15, and CdS quantum dots) [91,92]. These properties are used for sensing applications. The sensors based on GQDs and CQDs can be split into three main categories: luminescence chemosensors, biosensors, and electrochemical chemosensors [93].

2.4.3.1 Luminescence chemosensors This is one of the primary GQDs/CQDs applications. This can be broken down into PL chemosensors, chemiluminescence (CL) chemosensors, and electrochemiluminescence (ECL) chemosensors [94].

Photoluminescence chemosensors CQDs and GQDs have been used as fluorescent probes to detect pesticides and heavy metal ions. Therefore sensors for a series of metal ions, including Fe3+ [95], Hg2+ [96], Zn2+ [97], Cd2+, Cu2+ [98], Au3+ [99], Co2+ [100], Ni2+ [101], Pd2+ [102], Pb2+ [103],

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Mn2+ [104], Bi3+ [105], Al3+ [106], K+ [96], Be2+, Sn2+ [107], Cr6+ [108], and Ag+ [109] have been developed. Usually, the studies do not involve any specific surface modification; for example, it was found by Zhang et al. [110] that Hg2+ quenched the fluorescence of N-doped CQDs, which was attributed to the change of the surface states of N-CQDs influenced by the Hg2+. With a well-controlled fluorescence turn-on process based on PET between the GO and the GQDs, Qian and co-workers [103] recorded a GO and GQDs hybrid sensor for Pb2+. With a wide linear range of up to 400.0 nM, a fast response time, and a low detection limit of 0.6 nM, it could detect Pb2+. Furthermore, it could distinguish Pb2+ from other ions with good reproducibility and high sensitivity.

Chemiluminescence chemosensors With the benefits of simple instrumentation, high sensitivity, wide linear range, no interference from background scattering light, CL chemosensor has seen its popularity increase in sensing. Amjadi et al. [111] recorded a CQDs-based sensor where the transduction is accomplished using CL to detect Cu2+.

Electrochemiluminescence chemosensors ECL has gradually become an important and effective analytical method in many fields, such as pharmaceutical research, environmental contamination determination, and immunoassay because of its high sensitivity, low context, and good controllability [112]. CQDs-based composites have been reported for the detection of microRNA46, sophoridine [113] chlorinated phenols [114], chlorpromazine [115], and pentachlorophenol143 using ECL by integration with other materials.

2.4.3.2 Electrochemical chemosensors CQDs and GQDs have also been used in electrochemical sensors [116]. For example, a hydrogen peroxide sensor was developed by Xi et al. [117] using Pd nanoparticles (PdNPs)-functionalized N-GQDs@N-doped carbon hollow nanospheres. In this method, the authors proposed the application of N-GQDs and PdNPs in electrochemical sensing as both signal-amplifying probes and efficient electrocatalysts. A variety of electrochemical studies have also examined the use of GQDs for the identification of organic molecules under differential pulse voltammetry and CV measurements in the electrochemical sensors, GQDs act as multivalent redox species [116]. For example, the Liu group [118] detected glucose using a uniform threedimensional graphene nanodots-encaged porous gold electrode with a broad linear range from 0.05 to 100 mM and a detection limit of 30 μM. CQDs have been used more extensively in the electrochemical study of organic molecules compared with GQDs [119]. In 2016, Nguyen et al. [116] successfully developed an electrochemical sensing device for etoposide detection through differential pulse voltammetry by means of the modification of a GCE with CQDs.

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2.4.3.3 Biosensors When using CQDs and GQDs for biosensing, queries are avoided as these systems include modification of enzymes, DNA, RNA antibodies, antigens, cells, or tissues of microorganisms [118]. The number of biosensors that use CQDs or GQDs is small, which was reported by Ge et al. [120].

Optical biosensors Shi et al. [121] used GQDs in combination with AuNPs to establish what the authors refer to as a biosensor for fluorescence resonance energy transfer for the identification of a gene sequence unique to S. aureus. The GQDs have been modified by activating surface carboxylic moieties on the GQDs with classical carbodiimide chemistry with aminated ssDNA. The AuNPs with thiolated DNA probes were also updated. If the S. aureus sequence is present, it can bridge between the GQDs probe sequences and the AuNPs to bring the two together sufficiently close to the occurrence of energy transfer. Another well-defined modification of GQDs has been used in a detection biosensor in which the GQDs were modified with a fluorescence biosensor based on pyrene-functionalized molecular beacons (py-MBs) [104]. In the development of immunosensors, CQDs and GQDs can be used [113,122]. For example, Zhang et al. [113] prepared an immunosensor for 8-hydroxy-20 -deoxyguanosine (8-OHdG) where CQD-coated Au/SiO2 core-shell NPs were immobilized onto a platinum electrode.

Electrochemical biosensors In the construction of biosensors, GQDs have been used as a platform for enzyme immobilization [123]. Shan et al. have recently developed a biosensor used for epinephrine focused on immobilizing laccases and GQDs on GCEs [124]. Biosensors that detect nucleic acids have also started to use GQDs. Li et al. [125] constructed GQDsbased electrochemical DNA sensor where pyrolytic graphite electrode was modified with GQDs.

Photoelectrochemical biosensors The optical properties of CQDs and GQDs provide opportunities for these nanomaterials to be used only under photo-irradiation in a photoelectrochemical biosensor where electron transfer occurs at an electrode [126]. An aptamer-based photoelectrochemical biosensor has been developed by Yin group [126] for zeatin detection based on the GQDs/graphite-like C3N4(g-C3N4) nanocomposite framework.

2.5

Summary

GQDs are zero-dimensional graphene derivatives and they consist of one to few layers of GSs. Generally, the size of GQDs is in the range of 1–20 nm. Researches synthesized GQDs by using many different techniques, which can be categorized into two main approaches, top-down and bottom-up. Recently, GQDs have gained much attention because of their unique electronic, optical, chemical, and mechanical properties.

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Moreover, GQDs are used in a wide range of applications because of their small size, tunable fluorescence, high quantum efficiency/quantum confinement, chemical inertness, edge effect, biocompatibility, low toxicity, photostability, and water solubility. These excellent properties lead to many GQD-based applications in the fields of optoelectronics, energy conversion, and storage devices like OPVs, DSCs, batteries, and supercapacitors, biomedical applications such as a nanocarrier in drug delivery, cancer therapy, antibacterial, and bioimaging and the fabrication of various types of sensors such as luminescence chemosensors, electrochemical chemosensors, and biosensors.

2.6

Future prospective

GQDs are novel nanoparticles based on carbon and give unique optical, electronic, and physicochemical properties. They show great potential to be used in several research fields such as optoelectronics, energy conversion and storage, sensor technology, and biomedical applications. The possibility of making zero-, one-, and twodimensional architectures with quantum-confined particles makes them a wonderful material for future optoelectronic devices. As described in this chapter, GQDs synthesized by various techniques are used in a wide range of applications. However, it is challenging to synthesize high-quality and monodispersed GQDs using low-cost and high-yield synthesized techniques. Therefore the invention of prominent low-cost and high-yield production methods will make GQDs a versatile application in many fields, particularly in optoelectronics, biomedical engineering, and energy harvesting. Photophysical applications based on GQDs have not yet been improved to a satisfactory level. Hence, there is an urgent need to develop such devices using relevant functional properties of QDs to optimize the device performance. In particular, bandgap opening and engineering, upconversion and downconversion capabilities of GQDs will record the efficiencies beyond some theoretically predicted performance limitations of photophysical devices.

References [1] D. Pan, J. Zhang, Z. Li, M. Wu, Hydrothermal route for cutting graphene sheets into blue-luminescent graphene quantum dots, Adv. Mater. 22 (6) (2010) 734–738. [2] S. Kellici, J. Acord, N.P. Power, D.J. Morgan, P. Coppo, T. Heil, B. Saha, Rapid synthesis of graphene quantum dots using a continuous hydrothermal flow synthesis approach, RSC Adv. 7 (24) (2017) 14716–14720. [3] T.M.W.J. Bandara, T.M.A.A.B. Thennakoon, G.G.D.M.G. Gamachchi, L.R.A.K. Bandara, B.M.K. Pemasiri, U. Dahanayake, An electrochemical route to exfoliate vein graphite into graphene with black tea, Mater. Chem. Phys. 289 (2022) 126450, https:// doi.org/10.1016/j.matchemphys.2022.126450. [4] M. Dutta, S. Sarkar, T. Ghosh, D. Basak, ZnO/graphene quantum dot solid-state solar cell, J. Phys. Chem. C 116 (38) (2012) 20127–20131. [5] Z.S. Wu, W.C. Ren, L.B. Gao, J.P. Zhao, Z.P. Chen, B.L. Liu, D.M. Tang, B. Yu, C.B. Jiang, H.M. Cheng, ACS Nano 3 (2009) 411–417.

Preparation, characterization, and applications of graphene-based quantum dots

63

[6] M.J. Youh, M.Y. Jiang, M.C. Chung, H.C. Tai, Y.Y. Li, Formation of graphene quantum dots by ball-milling technique using carbon nanocapsules and sodium carbonate, Inorg. Chem. Commun. 119 (2020), 108061. [7] D. Pan, L. Guo, J. Zhang, C. Xi, Q. Xue, H. Huang, M. Wu, Cutting sp 2 clusters in graphene sheets into colloidal graphene quantum dots with strong green fluorescence, J. Mater. Chem. 22 (8) (2012) 3314–3318. [8] S. Zhu, J. Zhang, C. Qiao, S. Tang, Y. Li, W. Yuan, B. Yang, Strongly greenphotoluminescent graphene quantum dots for bioimaging applications, Chem. Commun. 47 (24) (2011) 6858–6860. [9] R. Tian, S. Zhong, J. Wu, W. Jiang, Y. Shen, T. Wang, Solvothermal method to prepare graphene quantum dots by hydrogen peroxide, Opt. Mater. 60 (2016) 204–208. [10] J. Peng, W. Gao, B.K. Gupta, Z. Liu, R. Romero-Aburto, L. Ge, P.M. Ajayan, Graphene quantum dots derived from carbon fibers, Nano Lett. 12 (2) (2012) 844–849. [11] R. Ye, C. Xiang, J. Lin, Z. Peng, K. Huang, Z. Yan, J.M. Tour, Coal as an abundant source of graphene quantum dots, Nat. Commun. 4 (1) (2013) 1–7. [12] S. Maiti, S. Kundu, C.N. Roy, T.K. Das, A. Saha, Synthesis of excitation independent highly luminescent graphene quantum dots through perchloric acid oxidation, Langmuir 33 (51) (2017) 14634–14642. [13] L. Lu, Y. Zhu, C. Shi, Y.T. Pei, Large-scale synthesis of defect-selective graphene quantum dots by ultrasonic-assisted liquid-phase exfoliation, Carbon 109 (2016) 373–383. [14] M. Hassan, K.R. Reddy, E. Haque, A.I. Minett, V.G. Gomes, High-yield aqueous phase exfoliation of graphene for facile nanocomposite synthesis via emulsion polymerization, J. Colloid Interface Sci. 410 (2013) 43–51. [15] Y.W. Shih, G.W. Tseng, C.Y. Hsieh, Y.Y. Li, A. Sakoda, Graphene quantum dots derived from platelet graphite nanofibers by liquid-phase exfoliation, Acta Mater. 78 (2014) 314–319. [16] L. Zdrazil, R. Zahradnicek, R. Mohan, P. Sedlacek, L. Nejdl, V. Schmiedova, J. Hubalek, Preparation of graphene quantum dots through liquid phase exfoliation method, J. Lumin. 204 (2018) 203–208. [17] F. Li, C.J. Liu, F. Tian, J. Yang, Z.H. Li, Fabrication of strong photoluminescent carbon nanodots and its preliminary application in cell imaging, in: Advanced Materials Research, vol. 800, Trans Tech Publications Ltd, 2013, pp. 312–316. [18] L.I. Nasibulina, T.S. Koltsova, T. Joentakanen, A.G. Nasibulin, O.V. Tolochko, J.E. Malm, E.I. Kauppinen, Direct synthesis of carbon nanofibers on the surface of copper powder, Carbon 48 (15) (2010) 4559–4562. [19] L.A. Ponomarenko, F. Schedin, M.I. Katsnelson, R. Yang, E.W. Hill, K.S. Novoselov, A. K. Geim, Chaotic Dirac billiard in graphene quantum dots, Science 320 (5874) (2008) 356–358. [20] L.J. Wang, H.O. Li, T. Tu, G. Cao, C. Zhou, X.J. Hao, G.P. Guo, Controllable tunnel coupling and molecular states in a graphene double quantum dot, Appl. Phys. Lett. 100 (2) (2012), 022106. [21] F.A. Permatasari, A.H. Aimon, F. Iskandar, T. Ogi, K. Okuyama, Role of C–N configurations in the photoluminescence of graphene quantum dots synthesized by a hydrothermal route, Sci. Rep. 6 (1) (2016) 1–8. [22] H. Zhu, X. Wang, Y. Li, Z. Wang, F. Yang, X. Yang, Microwave synthesis of fluorescent carbon nanoparticles with electrochemiluminescence properties, Chem. Commun. 34 (2009) 5118–5120. [23] X. Zhai, P. Zhang, C. Liu, T. Bai, W. Li, L. Dai, W. Liu, Highly luminescent carbon nanodots by microwave-assisted pyrolysis, Chem. Commun. 48 (64) (2012) 7955–7957.

64

Functional Materials from Carbon, Inorganic, and Organic Sources

[24] L. Tang, R. Ji, X. Cao, J. Lin, H. Jiang, X. Li, S.P. Lau, Deep ultraviolet photoluminescence of water-soluble self-passivated graphene quantum dots, ACS Nano 6 (6) (2012) 5102–5110. [25] X. Hou, Y. Li, C. Zhao, Microwave-assisted synthesis of nitrogen-doped multi-layer graphene quantum dots with oxygen-rich functional groups, Aust. J. Chem. 69 (3) (2016) 357–360. [26] N.F.A.M. Noor, M.A.S. Badri, M.M. Salleh, A.A. Umar, Synthesis of white fluorescent pyrrolic nitrogen-doped graphene quantum dots, Opt. Mater. 83 (2018) 306–314. [27] D. Qu, M. Zheng, P. Du, Y. Zhou, L. Zhang, D. Li, Z. Sun, Highly luminescent S, N codoped graphene quantum dots with broad visible absorption bands for visible light photocatalysts, Nanoscale 5 (24) (2013) 12272–12277. [28] Y.P. Sun, B. Zhou, Y. Lin, W. Wang, K.S. Fernando, P. Pathak, S.Y. Xie, Quantum-sized carbon dots for bright and colorful photoluminescence, J. Am. Chem. Soc. 128 (24) (2006) 7756–7757. [29] J. Deng, Q. Lu, N. Mi, H. Li, M. Liu, M. Xu, S. Yao, Electrochemical synthesis of carbon nanodots directly from alcohols. Chemistry–a, Euro. J. 20 (17) (2014) 4993–4999. [30] S. Ahirwar, S. Mallick, D. Bahadur, Electrochemical method to prepare graphene quantum dots and graphene oxide quantum dots, ACS omega 2 (11) (2017) 8343–8353. [31] W. Kwon, G. Lee, S. Do, T. Joo, S.W. Rhee, Size-controlled soft-template synthesis of carbon nanodots toward versatile photoactive materials, Small 10 (3) (2014) 506–513. [32] J. Lu, P.S.E. Yeo, C.K. Gan, P. Wu, K.P. Loh, Transforming C 60 molecules into graphene quantum dots, Nat. Nanotechnol. 6 (4) (2011) 247–252. [33] Z. Eresˇ, S. Hrabar, Low-cost synthesis of high-quality graphene in do-it-yourself CVD reactor, Automatika 59 (3–4) (2018) 254–260. [34] L. Fan, M. Zhu, X. Lee, R. Zhang, K. Wang, J. Wei, H. Zhu, Direct synthesis of graphene quantum dots by chemical vapor deposition, Part. Part. Syst. Charact. 30 (9) (2013) 764–769. [35] X. Ding, Direct synthesis of graphene quantum dots on hexagonal boron nitride substrate, J. Mater. Chem. C 2 (19) (2014) 3717–3722. [36] A.P.V.K. Saroja, M.S. Garapati, R. ShyiamalaDevi, M. Kamaraj, S. Ramaprabhu, Facile synthesis of heteroatom doped and undoped graphene quantum dots as active materials for reversible lithium and sodium ions storage, Appl. Surf. Sci. 504 (2020), 144430. [37] S. Javanbakht, H. Namazi, Solid state photoluminescence thermoplastic starch film containing graphene quantum dots, Carbohydr. Polym. 176 (2017) 220–226. [38] H. Shu, Q. Deng, M. Jin, Y. Tao, X. Wang, Z. Zhou, Efficient graphene phase modulator based on a polarization multiplexing optical circuit, in: Optical Fiber Communication Conference (pp. W2A-9). Optical Society of America, 2019. [39] T. Borkar, H. Mohseni, J. Hwang, T. Scharf, J. Tiley, S.H. Hong, R. Banerjee, Spark plasma sintering (SPS) of carbon nanotube (CNT)/graphene nanoplatelet (GNP)-nickel nanocomposites: Structure property analysis, in: Advanced Composites for Aerospace, Marine, and Land Applications II, Springer, Cham, 2015, pp. 53–79. [40] R. Sekiya, Y. Uemura, H. Murakami, T. Haino, White-light-emitting edge-functionalized graphene quantum dots, Angew. Chem. 126 (22) (2014) 5725–5729. [41] L. Tapaszto´, G. Dobrik, P. Lambin, L.P. Biro, Tailoring the atomic structure of graphene nanoribbons by scanning tunnelling microscope lithography, Nat. Nanotechnol. 3 (7) (2008) 397–401. [42] Y.C. Nie, F. Yu, L.C. Wang, Q.J. Xing, X. Liu, Y. Pei, S.L. Suib, Photocatalytic degradation of organic pollutants coupled with simultaneous photocatalytic H2 evolution over

Preparation, characterization, and applications of graphene-based quantum dots

[43]

[44] [45] [46]

[47] [48]

[49]

[50] [51]

[52] [53]

[54] [55]

[56] [57]

[58] [59] [60]

65

graphene quantum dots/Mn-N-TiO2/g-C3N4 composite catalysts: performance and mechanism, Appl. Catal. B Environ. 227 (2018) 312–321. T.M.W.J. Bandara, J.L. Ratnasekera, Polymer electrolytes for quantum dot-sensitized solar cells (QDSSCs) and challenges, in: Polymer Electrolytes: Characterization Techniques and Energy Applications, 2020, pp. 299–337. Z.Z. Zhang, K. Chang, F.M. Peeters, Tuning of energy levels and optical properties of graphene quantum dots, Phys. Rev. B 77 (23) (2008), 235411. S. Paulo, E. Palomares, E. Martinez-Ferrero, Graphene and carbon quantum dot-based materials in photovoltaic devices: from synthesis to applications, Nano 6 (9) (2016) 157. M. Li, W. Ni, B. Kan, X. Wan, L. Zhang, Q. Zhang, Graphene quantum dots as the hole transport layer material for high-performance organic solar cells, Phys. Chem. Chem. Phys. 15 (2013) 18973–18978. T. Trupke, M.A. Green, P. W€urfel, Improving solar cell efficiencies by down-conversion of high-energy photons, J. Appl. Phys. 92 (3) (2002) 1668–1674. M.L. Tsai, W.C. Tu, L. Tang, T.C. Wei, W.R. Wei, S.P. Lau, J.H. He, Efficiency enhancement of silicon heterojunction solar cells via photon management using graphene quantum dot as downconverters, Nano Lett. 16 (1) (2016) 309–313. T.M.W.J. Bandara, L.A. DeSilva, J.L. Ratnasekera, K.H. Hettiarachchi, A.P. Wijerathna, Thakurdesai, B.E. Mellander, High efficiency dye-sensitized solar cell based on a novel gel polymer electrolyte containing RbI and tetrahexylammonium iodide (Hex4NI) salts and multi-layered photoelectrodes of TiO2 nanoparticles, Renew. Sust. Energ. Rev. 103 (2019) 282–290. A. Louwen, W. van Sark, Photovoltaic solar energy, in: Technological Learning in the Transition to a Low-Carbon Energy System, Academic Press, 2020, pp. 65–86. M.E. Becquerel, Memoire sur les effets electriques produits sous l’influence des rayons solaires, Comptes rendus hebdomadaires des seances de l’Academie des Sciences 9 (1839) 561–567. R.S. Ohl, U.S. Patent No. 2,402,662. Washington, DC: U.S. Patent and Trademark Office, 1946. R. Zhang, S. Qi, J. Jia, B. Torre, H. Zeng, H. Wu, X. Xu, Size and refinement edge-shape effects of graphene quantum dots on UV–visible absorption, J. Alloys Compd. 623 (2015) 186–191. Z. Zhang, J. Zhang, N. Chen, L. Qu, Graphene quantum dots: an emerging material for energy-related applications and beyond, Energy Environ. Sci. 5 (10) (2012) 8869–8890. N. Li, W. Yan, W. Zhang, Z. Wang, J. Chen, Photoinduced formation of Cu@Cu2O@C plasmonic nanostructures with efficient interfacial charge transfer for hydrogen evolution, J. Mater. Chem. A 7 (33) (2019) 19324–19331. C.A. Nelson, N.R. Monahan, X.Y. Zhu, Exceeding the Shockley–Queisser limit in solar energy conversion, Energy Environ. Sci. 6 (12) (2013) 3508–3519. Y. Li, Y. Hu, Y. Zhao, G. Shi, L. Deng, Y. Hou, L. Qu, An electrochemical avenue to green-luminescent graphene quantum dots as potential electron-acceptors for photovoltaics, Adv. Mater. 23 (6) (2011) 776–780. C.X. Guo, H.B. Yang, Z.M. Sheng, Z.S. Lu, Q.L. Song, C.M. Li, Layered graphene/quantum dots for photovoltaic devices, Angew. Chem. Int. Ed. 49 (17) (2010) 3014–3017. P. Gao, K. Ding, Y. Wang, K. Ruan, S. Diao, Q. Zhang, J. Jie, Crystalline Si/graphene quantum dots heterojunction solar cells, J. Phys. Chem. C 118 (10) (2014) 5164–5171. S. Diao, X. Zhang, Z. Shao, K. Ding, J. Jie, X. Zhang, 12.35% efficient graphene quantum dots/silicon heterojunction solar cells using graphene transparent electrode, Nano Energy 31 (2017) 359–366.

66

Functional Materials from Carbon, Inorganic, and Organic Sources

[61] F. Li, L. Kou, W. Chen, C. Wu, T. Guo, Enhancing the short-circuit current and power conversion efficiency of polymer solar cells with graphene quantum dots derived from double-walled carbon nanotubes, NPG Asia Mater. 5 (8) (2013) e60. [62] V. Gupta, N. Chaudhary, R. Srivastava, G.D. Sharma, R. Bhardwaj, S. Chand, Luminscent graphene quantum dots for organic photovoltaic devices, J. Am. Chem. Soc. 133 (26) (2011) 9960–9963. [63] T.G. Novak, J. Kim, S.H. Song, G.H. Jun, H. Kim, M.S. Jeong, S. Jeon, Fast P3HT exciton dissociation and absorption enhancement of organic solar cells by PEG-functionalized graphene quantum dots, Small 12 (8) (2016) 994–999. [64] W. Wu, J. Zhang, W. Shen, M. Zhong, S. Guo, Graphene quantum dots band structure tuned by size for efficient organic solar cells, Phys. Status Solidi A 216 (24) (2019) 1900657. [65] W. Wu, H. Wu, M. Zhong, S. Guo, Dual role of graphene quantum dots in active layer of inverted bulk heterojunction organic photovoltaic devices, ACS Omega 4 (14) (2019) 16159–16165. [66] F. Jahantigh, S.B. Ghorashi, A. Bayat, Hybrid dye sensitized solar cell based on single layer graphene quantum dots, Dyes Pigments 175 (2020), 108118. [67] B. Gizem G€unes¸ tekin, H. Medetalibeyoglu, N. Atar, M. L€ utfi Yola, Efficient direct-ethanol fuel cell based on graphene quantum dots/multi-walled carbon nanotubes composite, Electroanalysis 32 (9) (2020) 1977–1982. [68] Z. Xue, Q. Sun, L. Zhang, Z. Kang, L. Liang, Q. Wang, J.W. Shen, Graphene quantum dot assisted translocation of drugs into a cell membrane, Nanoscale 11 (10) (2019) 4503–4514. [69] X.K. Li, W.J. Ji, J. Zhao, S.J. Wang, C.T. Au, Ammonia decomposition over Ru and Ni catalysts supported on fumed SiO2, MCM-41, and SBA-15, J. Catal. 236 (2) (2005) 181–189. [70] R. Guo, S. Zhou, Y. Li, X. Li, L. Fan, N.H. Voelcker, Rhodamine-functionalized graphene quantum dots for detection of Fe3 + in cancer stem cells, ACS Appl. Mater. Interfaces 7 (43) (2015) 23958–23966. [71] S. Qiu, Z. Lin, Y. Zhou, D. Wang, L. Yuan, Y. Wei, G. Chen, Highly selective colorimetric bacteria sensing based on protein-capped nanoparticles, Analyst 140 (4) (2015) 1149–1154. [72] B.Z. Ristic, M.M. Milenkovic, I.R. Dakic, B.M. Todorovic-Markovic, M.S. Milosavljevic, M.D. Budimir, V.S. Trajkovic, Photodynamic antibacterial effect of graphene quantum dots, Biomaterials 35 (15) (2014) 4428–4435. [73] H. Sun, N. Gao, K. Dong, J. Ren, X. Qu, Graphene quantum dots-band-aids used for wound disinfection, ACS Nano 8 (6) (2014) 6202–6210. [74] J. Ge, M. Lan, B. Zhou, W. Liu, L. Guo, H. Wang, X. Han, A graphene quantum dot photodynamic therapy agent with high singlet oxygen generation, Nat. Commun. 5 (1) (2014) 1–8. [75] G. Yapar, B. Şenel, N. Demir, M. Yıldız, Synthesis and characterization of 2aminoethylphosphonic acid-functionalized graphene quantum dots: biological activity, antioxidant activity and cell viability, vol. 59A, NISCAIR-CSIR, India, 2020, pp. 317–323. [76] J. Fang, Y. Liu, Y. Chen, D. Ouyang, G. Yang, T. Yu, Graphene quantum dots-gated hollow mesoporous carbon nanoplatform for targeting drug delivery and synergistic chemo-photothermal therapy, Int. J. Nanomedicine 13 (2018) 5991. [77] H. Wang, Q. Mu, K. Wang, R.A. Revia, C. Yen, X. Gu, M. Zhang, Nitrogen and boron dual-doped graphene quantum dots for near-infrared second window imaging and photothermal therapy, Appl. Mater. Today 14 (2019) 108–117.

Preparation, characterization, and applications of graphene-based quantum dots

67

[78] C. Liu, H. Huang, L. Ma, X. Fang, C. Wang, Y. Yang, Modulation of β-amyloid aggregation by graphene quantum dots, R. Soc. Open Sci. 6 (6) (2019), 190271. [79] D. Kim, J.M. Yoo, H. Hwang, J. Lee, S.H. Lee, S.P. Yun, H.S. Ko, Graphene quantum dots prevent α-synucleinopathy in Parkinson’s disease, Nat. Nanotechnol. 13 (9) (2018) 812–818. [80] N. Maurer, D.B. Fenske, P.R. Cullis, Developments in liposomal drug delivery systems, Expert. Opin. Biol. Ther. 1 (6) (2001) 923–947. [81] T.K. Henna, K. Pramod, Graphene quantum dots redefine nanobiomedicine, Mater. Sci. Eng. C 110 (2020), 110651. [82] M. Vatanparast, Z. Shariatinia, Computational studies on the doped graphene quantum dots as potential carriers in drug delivery systems for isoniazid drug, Struct. Chem. 29 (5) (2018) 1427–1448. [83] M. Vatanparast, Z. Shariatinia, Revealing the role of different nitrogen functionalities in the drug delivery performance of graphene quantum dots: a combined density functional theory and molecular dynamics approach, J. Mater. Chem. B 7 (40) (2019) 6156–6171. [84] D. Iannazzo, A. Pistone, M. Salamo`, S. Galvagno, R. Romeo, S.V. Giofre, A. Di Pietro, Graphene quantum dots for cancer targeted drug delivery, Int. J. Pharm. 518 (1–2) (2017) 185–192. [85] L. Zhang, L. Liu, J. Wang, M. Niu, C. Zhang, S. Yu, Y. Yang, Functionalized silver nanoparticles with graphene quantum dots shell layer for effective antibacterial action, J. Nanopart. Res. 22 (2020) 1–12. [86] W.S. Kuo, H.H. Chen, S.Y. Chen, C.Y. Chang, P.C. Chen, Y.I. Hou, J.Y. Wang, Graphene quantum dots with nitrogen-doped content dependence for highly efficient dual-modality photodynamic antimicrobial therapy and bioimaging, Biomaterials 120 (2017) 185–194. [87] Y. Liu, L.P. Xu, W. Dai, H. Dong, Y. Wen, X. Zhang, Graphene quantum dots for the inhibition of β amyloid aggregation, Nanoscale 7 (45) (2015) 19060–19065. [88] Y. Xu, J. Liu, C. Gao, E. Wang, Applications of carbon quantum dots in electrochemiluminescence: a mini review, Electrochem. Commun. 48 (2014) 151–154. [89] S. Zhu, Y. Song, J. Wang, H. Wan, Y. Zhang, Y. Ning, B. Yang, Photoluminescence mechanism in graphene quantum dots: quantum confinement effect and surface/edge state, Nano Today 13 (2017) 10–14. [90] R. Ye, Z. Peng, A. Metzger, J. Lin, J.A. Mann, K. Huang, J.M. Tour, Bandgap engineering of coal-derived graphene quantum dots, ACS Appl. Mater. Interfaces 7 (12) (2015) 7041–7048. [91] D. Zhang, L. Wen, R. Huang, H. Wang, X. Hu, D. Xing, Mitochondrial specific photodynamic therapy by rare-earth nanoparticles mediated near-infrared graphene quantum dots, Biomaterials 153 (2018) 14–26. [92] C. Sahub, T. Tuntulani, T. Nhujak, B. Tomapatanaget, Effective biosensor based on graphene quantum dots via enzymatic reaction for directly photoluminescence detection of organophosphate pesticide, Sensors Actuators B Chem. 258 (2018) 88–97. [93] W.L. Zhang, F. Qu, Y. Tian, H.J. Choi, L. Deng, J. Liu, H. Liu, Elegant surface of CoNi alloys toward efficient magnetorheological performances realized with carbon quantum dots, Adv. Mater. Interfaces 5 (15) (2018) 1800164. [94] J. Zhang, S.H. Yu, Carbon dots: large-scale synthesis, sensing and bioimaging, Mater. Today 19 (7) (2016) 382–393. [95] R. Wang, X. Wang, Y. Sun, One-step synthesis of self-doped carbon dots with highly photoluminescence as multifunctional biosensors for detection of iron ions and pH, Sensors Actuators B Chem. 241 (2017) 73–79.

68

Functional Materials from Carbon, Inorganic, and Organic Sources

[96] J. He, H. Zhang, J. Zou, Y. Liu, J. Zhuang, Y. Xiao, B. Lei, Carbon dots-based fluorescent probe for “off-on” sensing of Hg (II) and I
, Biosens. Bioelectron. 79 (2016) 531–535. [97] Z. Zhang, Y. Shi, Y. Pan, X. Cheng, L. Zhang, J. Chen, C. Yi, Quinoline derivativefunctionalized carbon dots as a fluorescent nanosensor for sensing and intracellular imaging of Zn 2 +, J. Mater. Chem. B 2 (31) (2014) 5020–5027. [98] A. Salinas-Castillo, D.P. Morales, A. Lapresta-Ferna´ndez, M. Ariza-Avidad, E. Castillo, A. Martı´nez-Olmos, L.F. Capitan-Vallvey, Evaluation of a reconfigurable portable instrument for copper determination based on luminescent carbon dots, Anal. Bioanal. Chem. 408 (11) (2016) 3013–3020. [99] J. Gu, D. Hu, W. Wang, Q. Zhang, Z. Meng, X. Jia, K. Xi, Carbon dot cluster as an efficient “off–on” fluorescent probe to detect Au (III) and glutathione, Biosens. Bioelectron. 68 (2015) 27–33. [100] J. Shi, C. Lu, D. Yan, L. Ma, High selectivity sensing of cobalt in HepG2 cells based on necklace model microenvironment-modulated carbon dot-improved chemiluminescence in Fenton-like system, Biosens. Bioelectron. 45 (2013) 58–64. [101] X.W. Tan, A.N.B. Romainor, S.F. Chin, S.M. Ng, Carbon dots production via pyrolysis of sago waste as potential probe for metal ions sensing, J. Anal. Appl. Pyrolysis 105 (2014) 157–165. [102] V. Sharma, A.K. Saini, S.M. Mobin, Correction: multicolour fluorescent carbon nanoparticle probes for live cell imaging and dual palladium and mercury sensors, J. Mater. Chem. B 4 (36) (2016) 6154. [103] Z.S. Qian, X.Y. Shan, L.J. Chai, J.R. Chen, H. Feng, A fluorescent nanosensor based on graphene quantum dots–aptamer probe and graphene oxide platform for detection of lead (II) ion, Biosens. Bioelectron. 68 (2015) 225–231. [104] L. Chen, C.X. Guo, Q. Zhang, Y. Lei, J. Xie, S. Ee, C.M. Li, Graphene quantum-dotdoped polypyrrole counter electrode for high-performance dye-sensitized solar cells, ACS Appl. Mater. Interfaces 5 (6) (2013) 2047–2052. [105] F. Qu, S. Wang, D. Liu, J. You, Differentiation of multi-metal ions based on fluorescent dual-emission carbon nanodots, RSC Adv. 5 (100) (2015) 82570–82575. [106] L. Feng, L. Tan, H. Li, Z. Xu, G. Shen, Y. Tang, Selective fluorescent sensing of αamanitin in serum using carbon quantum dots-embedded specificity determinant imprinted polymers, Biosens. Bioelectron. 69 (2015) 265–271. [107] S.N.A.M. Yazid, S.F. Chin, S.C. Pang, S.M. Ng, Detection of Sn (II) ions via quenching of the fluorescence of carbon nanodots, Microchim. Acta 180 (1–2) (2013) 137–143. [108] L. Fang, L. Zhang, Z. Chen, C. Zhu, J. Liu, J. Zheng, Ammonium citrate derived carbon quantum dot as on-off-on fluorescent sensor for detection of chromium (VI) and sulfites, Mater. Lett. 191 (2017) 1–4. [109] W. Zhu, J. Zhang, Z. Jiang, W. Wang, X. Liu, High-quality carbon dots: synthesis, peroxidase-like activity and their application in the detection of H 2 O 2, Ag+ and Fe 3 +, RSC Adv. 4 (33) (2014) 17387–17392. [110] R. Zhang, W. Chen, Nitrogen-doped carbon quantum dots: facile synthesis and application as a “turn-off” fluorescent probe for detection of Hg2+ ions, Biosens. Bioelectron. 55 (2014) 83–90. [111] M. Amjadi, J.L. Manzoori, T. Hallaj, N. Azizi, Sulfur and nitrogen co-doped carbon quantum dots as the chemiluminescence probe for detection of Cu2 + ions, J. Lumin. 182 (2017) 246–251. [112] Q. Zhai, J. Li, E. Wang, Recent advances based on nanomaterials as electrochemiluminescence probes for the fabrication of sensors, ChemElectroChem 4 (7) (2017) 1639–1650.

Preparation, characterization, and applications of graphene-based quantum dots

69

[113] T.T. Zhang, H.M. Zhao, X.F. Fan, S. Chen, X. Quan, Electrochemiluminescence immunosensor for highly sensitive detection of 8-hydroxy-20 -deoxyguanosine based on carbon quantum dot coated Au/SiO2 core–shell nanoparticles, Talanta 131 (2015) 379–385. [114] S. Yang, J. Liang, S. Luo, C. Liu, Y. Tang, Supersensitive detection of chlorinated phenols by multiple amplification electrochemiluminescence sensing based on carbon quantum dots/graphene, Anal. Chem. 85 (16) (2013) 7720–7725. [115] Z. Liu, F. Zhang, L. Cui, K. Wang, H. Zhan, Fabrication of a highly sensitive electrochemiluminescence chlorpromazine sensor using a Ru (bpy) 32 + incorporated carbon quantum dot–gelatin composite film, Anal. Methods 9 (6) (2017) 1011–1017. [116] H.V. Nguyen, L. Richtera, A. Moulick, K. Xhaxhiu, J. Kudr, N. Cernei, R. Kizek, Electrochemical sensing of etoposide using carbon quantum dot modified glassy carbon electrode, Analyst 141 (9) (2016) 2665–2675. [117] J. Xi, C. Xie, Y. Zhang, L. Wang, J. Xiao, X. Duan, S. Wang, Pd nanoparticles decorated N-doped graphene quantum dots@ N-doped carbon hollow nanospheres with high electrochemical sensing performance in cancer detection, ACS Appl. Mater. Interfaces 8 (34) (2016) 22563–22573. [118] J. Wang, H. Zhu, Y. Xu, W. Yang, A. Liu, F. Shan, J. Liu, Graphene nanodots encaged 3-D gold substrate as enzyme loading platform for the fabrication of high performance biosensors, Sensors Actuators B Chem. 220 (2015) 1186–1195. [119] Y. Li, Y. Zhong, Y. Zhang, W. Weng, S. Li, Carbon quantum dots/octahedral Cu2O nanocomposites for non-enzymatic glucose and hydrogen peroxide amperometric sensor, Sensors Actuators B Chem. 206 (2015) 735–743. [120] S. Ge, J. He, C. Ma, J. Liu, F. Xi, X. Dong, One-step synthesis of boron-doped graphene quantum dots for fluorescent sensors and biosensor, Talanta 199 (2019) 581–589. [121] J. Shi, C. Chan, Y. Pang, W. Ye, F. Tian, J. Lyu, M. Yang, A fluorescence resonance energy transfer (FRET) biosensor based on graphene quantum dots (GQDs) and gold nanoparticles (AuNPs) for the detection of mecA gene sequence of Staphylococcus aureus, Biosens. Bioelectron. 67 (2015) 595–600. [122] N.L. Li, L.P. Jia, R.N. Ma, W.L. Jia, Y.Y. Lu, S.S. Shi, H.S. Wang, A novel sandwiched electrochemiluminescence immunosensor for the detection of carcinoembryonic antigen based on carbon quantum dots and signal amplification, Biosens. Bioelectron. 89 (2017) 453–460. [123] Y. Li, W. Zhang, L. Zhang, J. Li, Z. Su, G. Wei, Sequence-designed peptide nanofibers bridged conjugation of graphene quantum dots with graphene oxide for high performance electrochemical hydrogen peroxide biosensor, Adv. Mater. Interfaces 4 (3) (2017) 1600895. [124] C. Shan, H. Yang, D. Han, Q. Zhang, A. Ivaska, L. Niu, Graphene/AuNPs/chitosan nanocomposites film for glucose biosensing, Biosens. Bioelectron. 25 (5) (2010) 1070–1074. [125] J. Zhao, G. Chen, L. Zhu, G. Li, Graphene quantum dots-based platform for the fabrication of electrochemical biosensors, Electrochem. Commun. 13 (1) (2011) 31–33. [126] Y. Wang, Y. Zhou, L. Xu, Z. Han, H. Yin, S. Ai, Photoelectrochemical apta-biosensor for zeatin detection based on graphene quantum dots improved photoactivity of graphite-like carbon nitride and streptavidin induced signal inhibition, Sensors Actuators B Chem. 257 (2018) 237–244.

Synthesis and applications of carbon-polymer composites and nanocomposite functional materials

3

Savisha Mahalingama, Azimah Omarb, Abreeza Manapa, and Nasrudin Abd Rahimb,c a Institute of Sustainable Energy, Universiti Tenaga Nasional, Kajang, Selangor, Malaysia, b Higher Institution Centre of Excellence (HICoE), UM Power Energy Dedicated Advanced Centre (UMPEDAC), Level 4, Wisma R&D, University of Malaya, Jalan Pantai Baharu, Kuala Lumpur, Malaysia, cRenewable Energy Research Group, King Abdulaziz University, Jeddah, Saudi Arabia

3.1

Introduction

Materials play a main key role in various technologies in our daily life. They are being upgraded as new materials with improved properties to fulfill the demand, to meet the necessity, and to create novel materials. Graphene is the simplest form of carbon structure and the thinnest material ever produced. It is categorized as a two-dimensional (2D) material composed of a single layer of sp2-bonded carbon atoms packed in a honeycomb crystal lattice [1]. The term “graphene” was first used by Boehm et al., while describing single-layer carbon foils. It was derived from graphite and the suffix “-ene.” Graphite flakes are basically a stack of graphene layers with an interplanar ˚ and a CdC bond length of about 0.142 A ˚ [2]. With sp2-hybridized spacing of 3.5 A carbon atoms, graphene exists as hexagonal, Bernal, or rhombohedral stacking and partially filled π orbitals in the above and below the graphene planes [3]. In general, a graphitic layer is known as single-layer graphene (SLG); 2 and 3 graphitic layers stacked together are termed bilayer graphene and tri-layer graphene, respectively; 5–10 graphitic layers are generally referred to as a few-layer graphene (FLG); and a layered assembly of about 20–30 graphitic layers is known as multilayer graphene. SLG usually exists in rippled form and does not demonstrate any stacking, whereas FLG may have several stacking arrangements, including Bernal (ABAB), rhombohedral (ABCABC), and AAA stacking. In the FLG with no discernible stacking or turbostratic, the interlayer space (>0.342 nm) is larger than that of crystalline graphene (0.335 nm). Moreover, the electronic and magnetic properties of graphene can be significantly affected by the presence of edges and steps with zigzag motifs. Moreover, the properties of graphene are controlled by the number and thickness of the graphene layers and the density of defects [3]. Functional Materials from Carbon, Inorganic, and Organic Sources. https://doi.org/10.1016/B978-0-323-85788-8.00020-3 Copyright © 2023 Elsevier Ltd. All rights reserved.

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Since Geim et al. reported single atomic layer graphene in 2004, graphene has drawn great interest in both the scientific and industrial worlds. It has an outstanding material with intrinsically superior electrical conductivity (106 Ω
1 cm
1), high optical transmittance (97.7% for monolayer graphene), high intrinsic mobility (200,000 cm
2V
1s
1), high thermal conductivity (5000–5300 W m
1 K
1), high Young’s modulus (1.1 TPa), high fracture or tensile strength (125 GPa), and large theoretical specific surface area (2630 m2 g
1) and is almost transparent. Table 3.1 summarizes the physical and chemical properties of graphene [3–6]. These properties make this forthcoming material great potential applications in many different fields of electronics, semiconductors, photonics, biomedicines, reinforced composites, ballistic transistors, field emitter, components of integrated circuits, sensors, catalysis, energy conversion and storage devices, photovoltaic, filters, and composite materials [3,6,7]. Moreover, graphene with high electron (or hole) mobility and low Johnson noise (electronic noise generated by the thermal distress of the charge carriers inside an electrical conductor at equilibrium and at any applied voltage) allows them to be utilized as the channel in a field-effect transistor (FET). A combination of excellent electrical properties and low noise also make graphene an excellent material for sensors. The 2D structure and entire volume expose to the surrounding making it very efficient to detect adsorbed molecules. In addition, high electrical conductivity and high optical transparency promote graphene as a suitable material for transparent conducting electrodes, touch-screens applications, liquid crystal displays, organic photovoltaic cells, and organic light-emitting diodes (OLEDs). And most of the applications require the growth of SLG on a suitable substrate that is very difficult to control and achieve. Therefore numerous reports are available on graphene synthesis methods. However, the focus is more on mechanical exfoliation, thermal graphitization of an SiC surface, and chemical vapor deposition (CVD) [7].

Table 3.1 Physical and chemical properties of graphene. Property

Value

Reference

Electrical conductivity Optical transmittance Intrinsic (electron) mobility Thermal conductivity Young’s modulus Tensile strength Surface area Oxidation temperature Permeability

106 Ω
1 cm
1 97.7% 200,000 cm
2V
1s
1

[4] [5] [5,6]

5000–5300 W m
1 K
1 1.1 TPa 125 GPa 2630 m2 g
1 450°C Impermeable to liquid/gases; permeable to protons

[4] [3,5,6] [3,6] [3,5,6] [3] [3]

Synthesis and applications of carbon-polymer composites

3.2

73

Synthesis of graphene

The synthesis of monolayer graphite was first tried in 1975, when Lang showed the formation of mono- and multilayered graphite by thermal decomposition of carbon on single-crystal Pt substrates. However, due to a lack of consistency between properties of such sheets developed on different crystal planes of Pt and failure to recognize the potential applications of the product, the process was not studied extensively, at that period. After a long gap, scattered attempts to produce graphene were reported again from 1999. However, Novoselov et al. have been credited for the discovery of graphene in 2004. They have first shown repeatable synthesis of graphene through exfoliation. The technique has been and is being followed since then, along with efforts to develop new processing routes for the efficient synthesis of large-scale graphene [7]. The synthesis routes of graphene can be categorized into two different categories: (1) top-down and (2) bottom-up approaches. In the top-down process, graphitic microstructures, such as graphite (graphite oxide and graphite fluoride), carbon fibers, and carbon nanotubes (CNTs), are the starting materials and individual graphene layers are extracted or peeled either by mechanical, electrochemical, or chemical methods. The bottom-up approach implements carbon atoms as building blocks whereas epitaxial growth of graphite on silicon carbide (SiC) surfaces and CVD are the most popular methods [8]. However, not all of them can produce good quality graphene efficiently. Three common methods, that is, mechanical exfoliation, graphene oxide reduction, and CVD, are considered promising methods to synthesize graphene for future large-scale graphene production [9]. Nevertheless, it was soon realized that these methods offer a low production revenue and were time consuming that delayed the effective and full exploitation of graphene-based materials. In the following section, several common methods to synthesize graphene are discussed, which include mechanical or micromechanical exfoliation, electrochemical synthesis, plasma discharge etching of graphite, CVD, epitaxial growth on SiC, and unzipping CNTs.

3.2.1 Mechanical or micromechanical exfoliation Graphene sheets of different thicknesses can be obtained through a top-down technique of mechanical exfoliation or by peeling off layers from graphitic materials, such as highly ordered pyrolytic graphite (HOPG), single-crystal graphite, or natural graphite. The first graphene sheet was synthesized by this technique whereby pretreated graphite was fixed onto a photoresist and graphene layers were peeled off by a scotch tape or an adhesive tape to obtain the graphene sheets [9,10]. A complete process can be conducted as follows [5]: 1. 2. 3. 4. 5.

After peeling off the graphite, multiple-layer graphene (MLG) remains on the tape. The peeling process is repeated a few times so that MLG is split into several flakes of FLG. Subsequently the tape is fastened to the acetone substrate to separate the tape. Finally, the last peeling with an unused tape is performed. The obtained flakes vary significantly in their size and thickness, where the sizes range from nm to several tens of μm for SLG.

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Functional Materials from Carbon, Inorganic, and Organic Sources

Fig. 3.1 Scotch tape method of graphene synthesis from HOPG block [11].

Fig. 3.1 shows the scotch tape method of mechanical exfoliation from the HOPG block [11]. However, this method produced uneven film thickness, is an inefficient technique, and is not suitable for large-scale production of graphene [2,9,10]. Therefore a few other methods have been conducted to obtain high-quality graphene on a larger scale, such as using ultrasonic devices, solvent and surfactants [9], electric field, and transfer printing technique [5]. The solvents and the surfactants can be intercalated into the atomic layers of graphite to form graphite intercalation compounds. This approach is performed to prevent agglomeration and support the separation of SLG. The influence of ultrasonication’s power, time, and solvent used on the volume of graphite intercalation compounds was explored. However, this approach requires high solvent cost and high solvent boiling point, which may have difficulties in depositing the graphene [9]. Viculis et al. used potassium metal to intercalate a pure graphite sheet and then exfoliate it with ethanol to form a dispersion of C sheets. During the sonication process, the exfoliated nano-carbon sheets formed nanoscrolls [7]. Although the exfoliation and reduction of graphite oxide appear to be a more efficient approach for the mass production of graphene sheets, the sheets usually have residual oxygen functionalities and network defects [10]. In general, graphene flakes obtained by mechanical exfoliation are characterized by optical microscopy, Raman spectroscopy, transmission electron microscopy (TEM), and atomic force microscopy (AFM) analysis. Whereby optical microscopy is usually used to discover SLG and AFM analysis is carried out to measure the graphene thickness and number of layers. Raman spectroscopy is the quickest and most accurate method to identify the thickness of graphene flakes and its crystallinity [5].

3.2.2 Electrochemical synthesis or exfoliation The electrochemical exfoliation method is a simple method to produce graphene materials for large-scale production. The electrochemical devices include a graphite working electrode, a graphene auxiliary electrode, electrolyte solution (aqueous

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75

Pristine graphene

Power supply

–

Covalently functionalized graphene materials

+ Graphene oxide Electrochemical exfoilation

Heteroatom doped graphene

Graphene based hybrid/composites

Graphite d = 0.34 nm

Fig. 3.2 A schematic illustration of electrochemical exfoliation of graphite to produce various graphene materials [13].

solution, organic solvent, and ionic liquid), and a DC power stabilizer [12]. The graphite working electrode is in various geometries, such as graphite powders, foils, rods, flakes, or plates [13]. Fig. 3.2 shows a schematic illustration of the electrochemical exfoliation of graphite to produce various graphene materials. When the power supply with a DC voltage is applied across the electrodes, an electrical field is formed. Under the electric forces, the electrolyte will be separated into cations and anions and moved to the cathode (applying a negative bias) and anode (applying a positive bias), respectively. These ions will be implanted into the graphite layers whereby leads to the expansion and deformation of graphite. Then, graphene layers will be peeled off from the graphite electrode to form graphene [12,13]. In addition, Fig. 3.3 illustrates a schematic overview of the cathodic and anodic exfoliation of graphite. A positively charged ion in the electrolyte (e.g., Li+) would be attracted to the graphite electrodes in cathodic exfoliation. And negatively charged ions (e.g., SO2
4 ) may be attracted to the electrodes in anodic exfoliation. Ions, electrolyte molecules, or cointercalating species in electrolytes are first intercalated among the graphene layers of the graphite. The electrochemical reactions also offer a driving force to break the van der Waals forces and therefore lead to the structural expansion of graphite. Moreover, by controlling the process parameters, such as applied electrical potentials, currents, processing time, and the composition of electrolytes, the graphene materials of different defect densities, O contents, graphene layer numbers, and lateral sizes can be obtained. Later, chemical reactions with functionalizing agents can simultaneously take place during the electrochemical exfoliation to achieve in-situ chemical doping (functionalization) of graphene materials to produce various graphene-based composite materials [13]. In summary, the electrochemical synthesis or exfoliation can be categorized into anodic oxidation, cathodic reduction, and electrochemical reduction. Table 3.2 highlights the features, advantages, disadvantages, and examples of each category [12].

3.2.3 Plasma discharge etching of graphite The plasma technique is a promising method to synthesize graphene. The method involves surface modification since the activated species, such as atoms, ions, and

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Power supply

After mechanical exfoliation/sonication or spontaneous exfoliation

e–

Graphite Cathode = Positively charged ions (e.g. Li+) Electrolyte or co= intercalating species (e.g. propylene carbonate)

Intercalation or co-intercalation into graphite lattice

Treatment with functionalizing agent ( ) Power supply

e–

Single and few-layered graphene

Functionalized graphene

Graphite Anode =

Negatively charged ions (e.g. SO42–)

=

Electrolyte or cointercalating species

Intercalation or co-intercalation into graphite lattice

Single and few-layered graphene

Fig. 3.3 A schematic overview of cathodic and anodic exfoliation of graphite [13].

radicals, in the plasma phase can directly act on the graphene surface. In the plasma discharging process, oxygen plasma can transform the top layer of graphene sheets or establish structural defects in the graphene sheets. Zhao et al. reported a simple and environmentally friendly plasma technique for the fabrication of few-layered graphene using H2O2 plasma etching of graphite. H2O2 is the most abundant reactive oxygen whereby it can be excited and form strong oxidation free radicals of OH, O, HO2, etc. The strong oxidation free radicals, such as OH, are useful to unzip graphite and form a few-layered graphene. Fig. 3.4 shows the proposed mechanism of H2O2 plasma etching of graphite to form few-layered graphene, whereby the following steps take place [14]: 1. H2O2 is first excited and forms strong oxidation free radicals (OH, O, HO2, etc.). 2. For example, OH can oxidize the sp2-hybridized graphite-like carbon (-C ¼ C-) and the sp3-hybridized carbon (-C-C-) on graphite surfaces, that is, introduce the oxygen-containing functional groups onto graphite surfaces and therefore induces the etching of graphite. 3. The etching depth of graphite increases with the discharging processing of H2O2 plasma etching. 4. Finally, a few-layered graphene can be removed from graphite surfaces.

Table 3.2 Features, advantages, disadvantages, and examples for each electrochemical method. Electrochemical method

Features

Advantage

Disadvantage

Example

Anodic oxidation

▪ Separated anion embedded into the graphite anode layers by controlling the applied potential (electric force) to produce graphene ▪ Oxygen promotes the expansion of anode, introduces anions into graphite layers, and later exfoliates the graphite ▪ Electrolytes include H2SO4, a mixed solution of H2SO4 and KOH, and aqueous ammonium hydroxide solution

▪ Low cost, simple process, and large-scale production

▪ Limited by experimental conditions (voltage, type of electrolytes, and other parameters)

▪ Wang et al. used poly(sodium-4styrenesulfonate) as the electrolyte. It electrolyzed the graphite anode under a constant voltage of 5 V to obtain graphene sheets [12] ▪ Coros¸ et al. proposed the electrochemical exfoliation of graphite rods using mixed (H2SO4:HNO3) electrolytes. Different electrochemical parameters (such as applied bias and electrolyte concentration) were studied in detail. Their experimental work demonstrated that the graphene flakes’ size and the exfoliation/oxidation level can be controlled [10]

Cathodic reduction

▪ Uses electric forces to exfoliate the graphite. ▪ Hydrogen facilitates the expansion of cathode and cations are introduced into graphite layers. ▪ Mainly happens in the cathode. ▪ Influence by the electrolyte and surfactants

▪ High production and high purity of graphene prevent the production of GO

–

▪ Wang et al. used Li+ and propylene carbonate mixed solution as the electrolyte and obtained well-dispersed graphene in the cathode without going through other processes, such as oxidation and strong acid treatment [12] ▪ Li+ promoted the polymerization of propylene carbonate and destroyed the graphite intercalation force [12]. Continued

Table 3.2 Continued Electrochemical method Electrochemical reduction

Features

Advantage

Disadvantage

▪ Directly transforms GO made from Hummers’ method into graphene (does not involve the process of cation embedded into the graphite)

▪ Green method and large-scale production.

–

Example

H-O-O-H

* HO2*

* H-O-O-H

H-O* O*

Fig. 3.4 Schematic diagram of the proposed H2O2 plasma etching of graphite to form synthesized graphenes as indicated in (A) SEM image and (B) AFM image [14].

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This etching process can be conducted in different positions and directions. It also forms a small quantity of high-quality graphene in the small-scale plasma device. Whereby a larger plasma device may synthesize a large amount of few-layered graphene. Fig. 3.4A shows the obtained scanning electron microscope (SEM) images of single-layered or bilayered graphenes with half-transparent or partially transparent images. Moreover, the AFM characterization shown in Fig. 3.4B also indicates the formation of single-layered and bilayered graphenes with a height of 0.9 and 2 nm. From XPS analysis, high resolution of C 1 s and O 1 s peaks can be deconvoluted as shown in Fig. 3.4 with various components of sp2-hybridized graphite-like carbon (C ¼ C), sp3-hybridized carbon (CdC), hydroxyl groups (C-OH), carbonyl groups (C ¼ O), and carboxyl groups (COO-) attached on graphene surfaces. The results are highlighted in the table in Fig. 3.4 [14].

3.2.4 Chemical vapor deposition CVD is one of the most promising processes for the large-scale production of monolayer graphene or FLG films. CVD appears to be the most desired technique among all the other graphene syntheses to produce high-quality graphene covering a large area with a controllable number of layers [4,15–21]. Two types of CVD approaches have been introduced, which are thermal CVD and plasma CVD. Thermal CVD includes the deposition of the desired product from the thermally decomposed precursors onto a substrate surface at high temperatures. Since some applications require a lowtemperature environment, plasma-assisted decomposition and reaction may lower the process temperature. Fig. 3.5 illustrates the schematic of thermal and plasmaenhanced CVDs.

3.2.4.1 Thermal chemical vapor deposition Historically, the first monolayer graphitic materials on Pt were first fabricated in 1975 by Lang [22]. Later in 1979 the CVD process evolved by doping single crystal Ni (111) with carbon forming a graphite layer on Ni. The evolution found a great deal MFC

Furnace Quartz Tube

H2 C2H2

l

Sample Heater

TC

metal Ga

MFC MFC

sample holder substrate with catalyst

Pressure Controller

Electrode

V

Exhaust

Mech pump

Sample rotation

Pump

Chamber

NH3 H2

Exhaust

Thermal CVD

Plasma CVD

Fig. 3.5 Schematic representation of thermal and plasma-enhanced chemical vapor depositions.

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owing to the suitable carbon phase segregation on Ni (111) [23]. However, due to the lack of applications for thin graphite materials, this area was not investigated further for almost two decades. After the discovery of graphene in the 21st century, it opened a new era of graphene-based electronics and attracted immense attention from the scientific and industrial fields [24–26]. Somani et al. have successfully synthesized Ni foil using camphor (terpenoid, C10H16O) as the precursor material [27]. They found 35 layers of single graphene sheets with an interlayer distance of 0.34 nm via TEM. This has opened a new path to the scientific community toward layers and thickness controllable large-scale graphene growth using thermal CVD. Since then, many efforts have been explored to resolve graphene synthesis issues, such as controlling the number of layers and minimizing the folding of graphene [21,28–33]. Nevertheless, Obraztsov et al. also produced mono and multiple layers of graphene via thermal CVD and the final product was 1- to 2n-nm-thick graphene bilayer covered with surface ridges due to the thermal expansion coefficient mismatch between graphene and Ni [34]. However, the same process failed to produce well-ordered graphene on the Si surface except for amorphous carbon. Kim et al. reported an interesting graphene growth as stretchable transparent electrodes on silicone-based organic polymer [35]. The process includes growth over e-beam evaporated Ni under 1000°C via the CVD technique. They further deposited the resultant product over a flexible, stretchable polydimethylsiloxane (PDMS) exhibiting around 280 Ω/square of sheet resistance, 3750 cm2V
1 s
1 of carrier mobility, and 5  1012/cm
2 [35]. Almost similar, Reina et al. demonstrated mono- to bilayer graphene on polycrystalline Ni. They employed a wet chemical method to transfer the CVD graphene onto the SiO2/Si substrate under 900–1000°C [36]. More interestingly, wang et al. came up with a low-cost unique process using a substrate-free and gram-scale yield of graphene [37]. The process involves thermal CVD at 1000°C using a C-supported MgO catalyst. However, the synthesized graphene suffered from poor structure and morphology with crumples and random aggregation. Large-scale graphene production took a new look in 2009 when the Ruoff uncovered the catalytic graphene deposition on Cu substrate at elevated temperatures [21]. They demonstrated the growth of single, double, and triple layers of uniform graphene on Cu foil on a large scale. The method includes heating the quartz furnace at 1000°C and a graphene transfer using solution etching of Cu. The Ruoff group further added that the deposition of graphene on a Cu surface is due to the surface catalyzed related to the limited solubility of carbon in copper [21]. It was concluded that graphene deposition on Cu differs from the Ni deposition, because Cu deposition involves a segregation process or a surface adsorption process. Bae and coworkers made their first breakthrough by rolling up the Cu foil and placed in a quartz furnace for graphene growth at 1000°C and further transferring the CVD graphene onto any flexible polymer substrate [38]. They reported a large 30-in. wide graphene films. The monolayer film has a low sheet resistance of around 125 Ω/square with 97.4% optical transmittance. While, the bilayer films exhibited lower sheet resistance around 30 Ω/square with 90% optical transmittance, denoting immense prospects to replace commercial transparent electrodes, such as transparent conducting oxides.

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Graphene growth by substitutional doping by introducing other gases, such as NH3 is a fascinating characteristic of the CVD approach [39–41]. They are doped in three different forms, such as “pyridinic,” “graphitic,” and “pyrrolic” forms. Qu et al. have demonstrated nitrogen-doped graphene (N-graphene) and the resultant electrode showed a catalytic current of 3 times higher than that of Pt-electrode [40]. They employed the N-graphene for reduction of oxygen in fuel cells and achieved long-term stability and Poisson’s effect over the Pt/C. The nitrogen-doped electrodes also demonstrated interesting properties in a lithium-ion battery application. For example, the resultant surface defects upon nitrogen doping caused a high reverse discharge capacitance, which was around two times greater than the nitrogen-free graphene [41]. Ismach et el. utilized a novel approach by directly growing graphene dielectric substrates in 2010 [42]. The process involves direct deposition of Cu thin film under thermal CVD at 1000°C and with ambient pressure of 100–500 mT. The direct deposition of graphene on dielectric substrates takes place by the surface catalyzed, de-wetting, and evaporation processes of Cu. Therefore the direct synthesis of graphene eliminates surface defects and contamination in the final product. Nonetheless, the direct synthesis route of graphene on insulators needs to be investigated further to synthesize defect-free, well-ordered, and large-scale graphene for electronic applications.

3.2.4.2 Plasma-enhanced chemical vapor deposition Plasma-enhanced chemical vapor deposition (PECVD) can be carried out at a relatively low temperature than thermal CVD. This route is more feasible for industrial-scale applications. The first reported PECVD mono- and bilayer graphene sheets were in 2004 [43,44]. The process involves a gas mixture of 5%–100% CH4 in H2 via PECVD at 680°C. Since then much progress has been made to obtain graphene layers with controllable thickness [45–47]. The advantages of plasma over thermal CVD are a lower deposition time of 5 min and a lower growth temperature of 650°C (1000°C for thermal CVD). PECVD was further explored by system coupling with RF and microwave technologies. Zhu et al. developed the growth of CNT and vertical free-standing graphene on a variety of crystal-free substrates via the RF PECVD system [48]. The growth of graphene was induced due to the increasing concentration of hydrocarbon and hydrogen gases in the feed gas mixture, which caused a higher accumulation of activated carbon species. Microwave-assisted PECVD was carried out by several other researchers. Vertically oriented graphene was synthesized only via PECVD, which was not reported in other synthesis routes. The PECVD method offers high-purity and high-crystalline graphene. However, PECVD needs to be further explored to produce uniform large-area and SLG to gain better control over morphology and growth rate.

3.2.5 Epitaxial growth on silicon carbide Epitaxial growth on a single crystalline SiC surface is one of the most commended methods of graphene synthesis. The term “epitaxy” derives from the Greek where the prefix epi means “over” or “upon” and taxis means “order” or “arrangement.”

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Whereby the epitaxial growth involves a deposition (simple heating and cooling down) of a single high-crystalline film on a single-crystalline SiC substrate and produces an epitaxial film. There are two general epitaxial growth processes depending on the substrate types; (1) homo-epitaxial growth (the deposited film on the substrate is of the same material) and (2) hetero-epitaxial growth (the deposited film and substrate are of different materials) [5,49]. The growth rate of graphene on SiC depends on the specific polar SiC crystal face and graphene forms much faster on the C-face than on the Si-face. On the C-face, the growth produces larger domains (200 nm) of multilayered, rotationally disordered graphene. And on the Si-face, the ultra-high vacuum (UHV) annealing leads to small domains (30–100 nm) with SLG or bilayer graphene [5,49]. In addition, the obtained graphene is highly dependent on the parameters used, such as temperature, heating rate, or pressure. If the temperature and pressure are set too high the growth of CNTs occurs instead of graphene [49]. When Si (0001) and C (000–1 terminated) are annealed at high temperature (T > 1000°C) under UHV, they become graphitized due to the evaporation of Si. Graphene films prepared by thermal decomposition of SiC (above 1000°C) grow on a C-rich 6 √ 3  6 √ 3R30° with respect to the SiC surface [5]. The surface of SiC affects the thickness, mobility, and carrier density of the graphene whereby the obtained graphene tends to have weak antilocalization [2]. The graphitization of hexagonal SiC crystals during high temperatures annealing in vacuo was firstly reported by Badami in 1961. Under this condition, the top layers of SiC crystals undergo thermal decomposition, Si atoms desorb and the remaining carbon atoms on the surface rearrange and re-bond to form epitaxial graphene layers. The reactor pressure and the type of gas atmosphere influence the kinetics of graphene formation and the resulting graphene structure and properties. Moreover, the growth on the Si-face of hexagonal SiC wafers, that is, h-SiC (0001), under appropriate conditions exhibits controllable growth kinetics compare to the C-face growth and therefore can control the number of graphene layers. On the other hand, the azimuthal orientation of epitaxial graphene on the Si-face provided by the crystal structure of the substrate offers a pathway toward uniform coverage and structural coherence at the wafer scale [50]. In general, graphene formation starts at the top surface layers of SiC and proceeds inwardly. Whereby three SidC bilayers decay (ca. 0.75 nm) to form one graphene layer (ca. 0.34 nm). The C-rich 6 √ 3  6 √ 3R30° surface reconstruction (named as “buffer layer”), involves the isostructurally arrangement of C atoms to graphene without sp2 structure and covalent bonds to the underlying Si atoms. This process forms an insulating layer and does not exhibit the electronic properties of graphene. Then, a new buffer layer forms below the original one that is simultaneously converted to graphene. The buffer layer is responsible for the n-type doping of pristine graphene on SiC (0001). A second graphene layer also grows in the same way. However, the rate of graphene formation is vividly slowed down after the second layer due to the inhibition of Si removal from the buried SiC decomposition front. Thus Si atoms must diffuse to a defect in graphene (a pinhole or grain boundary), SiC terrace edge, or sample edge, to escape. And more graphene layers form when graphene defectivity

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increases. In a high vacuum, graphetization starts at relatively low temperatures (1100–1200°C), where C atoms are not sufficiently mobile and defective graphene films with a variable thickness up to 6 layers are formed on a heavily rough SiC surface. In an inert gas atmosphere (e.g., Ar) at pressures up to 1 bar, the sublimation rate of Si is reduced dramatically, and graphenization starts at temperatures higher than 1450–1500°C, where C atoms are more mobile and form higher-quality graphene films with a thickness limited to only 1–2 layers. The lower thickness is likely the result of the small concentration of graphene defects, which otherwise would have acted as escape routes for Si atoms [50]. In summary, the epitaxial graphene growth on SiC is a promising method for largescale production and commercialization of graphene in high-frequency electronics applications, light-emitting devices, and radiation hard devices. The devices that have been fabricated from graphene on SiC on a wafer scale include top gated transistors and 100 GHz high-frequency transistors [5].

3.2.6 Unzipping carbon nanotubes Unzipping CNTs offers another route of graphene synthesis by the chemical and plasma-etched methods. The route produces a thin elongated strip of graphene with straight edges, called a graphene nanoribbon (GNR). An interesting feature of the unzipping approach to synthesize graphene is the electronic state transformation from semimetal to semiconductor [51]. Further studies are still in progress to explore the electronic properties of thin strip GNRs [52,53]. The produced graphene layers whether single- or multilayer depends strongly on the source of carbon used, single-walled CNT and multiwalled CNT (MWCNT), respectively. The longitudinal unzipping of CNTs was performed by intercalation of lithium and ammonia followed by oxidation in 2009 [54]. The intercalation of CVD-grown MWCNT was allowed by adding Li in a ratio of 10:1 (Li:C) with subsequent HCl mixing to assist in complete exfoliation. The unzipping of the MWCNT mechanism took place under two processes: first, the exothermic reaction of HCl and Li and, second, the neutralization of NH3 which causes further unzipping of the nanotubes. Unzipping of CNTs can also be performed by a different chemical process via an oxidation process. Li et al. unzipped armchair CNT along the tube direction upon oxidation to form GNRs [55]. They found that the oxygen atoms have a preference for the adsorption sites of the corresponding carbon with a large density of states. The second oxygen atom further adsorbs at the parallel site of the carbon leading to a long epoxy chain along the tube. They also considered a zigzag CNT saturated by hydrogen atoms where the adsorption site preference was at the edge site and leading to poor formation of the epoxy chain. Thus the formation of nanographene occurs under oxidative cutting. Therefore to synthesize high-quality GNRs it is recommended to use armchair CNTs. Almost similar, the Tour research group performed oxidation cutting by a step-by-step process using H2SO4, KmnO4, and H2O2 [56]. They found out that a high concentration of KmnO4 acting as the oxidizing agent caused a larger opening of the MWCNT layer. However, the high amount of KmnO4 resulted in oxidized GNR, which needs to undergo a reduction process to restore its electrical properties.

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Shimizu and coworkers demonstrated the synthesis of GNR through oxidation and longitudinal unzipping of MWCTs in H2SO4, followed by KmnO4 treatment [57]. The authors were able to completely remove the oxygen from the bulk and edges of the nanoribbons. The process includes three main stages. The first stage involves annealing at 800°C to remove the bulk of oxygenation. The second stage involves electron-beam lithography to produce edge termination and charge carrier doping. The final stage was carried out at 300°C to clean the surface. The resultant graphene suggests to be a good quality of the obtained GNRs but also shows some surface defects, which in turn affect the electrical properties. Although the GNRs synthesized through various unzipping methods have shown superior electrical properties to the mechanically peeled graphene sheets, these pioneer studies have paved the way leading to large-scale production of GNRs. Future research will focus on industrially applicable GNRs to produce highly ordered, defect-free, and high-quality graphene sheets.

3.2.7 Summary of graphene synthesis methods Table 3.3 summarizes the main synthesis methods for top-down and bottom-up approaches, their advantages and disadvantages, and their application.

3.3

Synthesis of functionalized graphene

Graphene functionalization has aroused a great deal of research interest because of the boom in the chemical modification in graphene. Functionalization in graphene started to be investigated in detail when certain applications require some characteristics that even graphene, with all its properties, cannot provide. Besides, some methods in graphene synthesis require high cost; thus the chemical modification in pristine graphite is a promising economical alternative to graphene. Chemical conversion of graphite into GO is the most common and widely used method [58–60]. Besides it, surface functionalization with other functional groups offers another potential route for graphene functionalization.

3.3.1 Graphene oxide and reduced graphene oxide GO is an oxygen-functionalized graphene with several functional groups, such as hydroxyl (OH), carbonyl (C¼ O), and carboxyl (COOH) groups. Thus GO is hydrophilic in nature and provides a bandgap (3.2 eV [61]), which serves high importance for electronic and energy-harvesting applications. Most of the oxygen-functionalized graphene is obtained either from Brodie, Staudenmaier, or Hummers [62–64]. The oxidation process involves strong acid attacks with H2SO4, HNO3, H2O2, KmnO4, or a combination of KclO3 with HNO3. Yanwu et al. synthesized GO using graphite salts (graphite with H2SO4, HNO3, and KclO3) [65]. The molecular structure of GO and rGO is shown in Fig. 3.6.

Table 3.3 A summary of the graphene synthesis method. TOP-DOWN Method

Advantage

Disadvantage

Application

Reference

Micromechanical exfoliation

▪ High quality ▪ Large size and unmodified GS (5–10 μm)

▪ Research purpose

[2,5,11]

Electrochemical synthesis/exfoliation

▪ Single-step functionalization and exfoliation ▪ High electrical conductivity of functionalized graphene

▪ High cost ▪ Uneven films thickness ▪ Very small-scale production ▪ High cost of ionic liquids

[5]

BOTTOM-UP Plasma discharge etching of graphite

▪ Can produce 10 g/h of graphene

CVD (on Ni, Cu, Co)

▪ Large size ▪ High quality

Epitaxial growth on SiC

▪ Very large area of pure graphene ▪ High quality

Unzipping carbon nanotubes

▪ Size controlled by selection of the starting nanotubes

▪ Low yield of graphene ▪ Carbonaceous impurities ▪ Small production scale ▪ High cost ▪ High process temperature (>1000°C) ▪ Very small scale ▪ High cost ▪ High process temperature (1500°C) ▪ Very expensive substrate

▪ Expensive starting material ▪ Oxidized graphene

[5]

▪ Touch screens ▪ Smart windows ▪ Flexible LCDs & OLEDs ▪ Solar cells ▪ Top gated transistors circuits ▪ Interconnects memory ▪ Semiconductors devices ▪ 100 GHz highfrequency transistors ▪ FETs interconnects ▪ NEMs composites

[5,11]

[5,11]

[5,11]

Synthesis and applications of carbon-polymer composites

87 Reduction

Oxidation

carbonyl O

hydroxyl

O OH

OH

OH

O OH

hydroxyl OH

O

O OH

carboxyl Graphite layer

HO

O

O

O

O

epoxy

O

HO

O

OH

HO

OH HO

HO

carboxyl

Graphene oxide

Reduced-Graphene oxide

Fig. 3.6 Oxidation of graphite to GO and reduction of GO to rGO.

The figure clearly shows that the GO contains a mix of sp2 (COOH group) and sp (C]O group) hybridization of carbon. The hydrophilic nature of GO eases the process of exfoliation in aqueous solution. The resultant exfoliated GO has similar chemical properties as that of the graphite oxide (bright yellow). To obtain GO (brown-reddish) from the resultant graphite oxide, further multistep washing with DI water in a centrifuge and exfoliation in the ultrasonic bath is required. The produced GO has π➔π⁎ and n➔π⁎ transitions for CdC and C]O bonding, respectively [61]. However, GO, which is extremely hydrophilic, suffers from several limitations, such as insolubility in organic solvents (oDCB, toluene, and chloroform) and the functional groups of OdH, and epoxy distorts the electronic structure. Nevertheless, the bandgap of GO is too high, and a common strategy used to modify the bandgap is chemical reduction via reducing agents, such as hydrazine. The GO that has been reduced chemically by removing the oxygen atoms is called reduced graphene oxide (rGO) [66–68]. rGO has smaller bandgaps than GO with 2.7– 0.1 eV and partially restores its conductivity from the electronic structure distortion of GO [69]. Besides hydrazine [70–72], the chemical reduction of GO can be performed using various reducing agents, including sodium borohydride [68,73,74] and hydroquinone [75]. Among all the reagents explored, hydrazine provides the best quality of graphene with a thin layer, and most importantly, it does not react with water. However, the hydrophobic characteristic of rGO causes agglomeration issues unless stabilized by the selected surfactants. Athanasios and coworkers used NaB4 as an alternative to rGO to effectively reduce GO [68]. Surprisingly, NaB4 performed better than hydrazine with lower sheet resistance (59 kΩ/square), and a high C:O ratio (13.4:1). Unfortunately, NaB4 still lacks a low rate of hydrolyzation with water. However, due to toxicity researchers began to use harmless reducing agents, such as fructose, glucose, and ascorbic acid, and obtained bandgap values of 2.7–1.1 eV [61,76]. Ascorbic acid was referred to as the best green-reducing agent due to its good recovery of sp2 domains and elimination capability of epoxide and hydroxyl groups [61]. Besides chemical reduction, the thermal reduction was studied by a few researchers to remove the oxygen-containing functional groups by heat treatment. A high-temperature treatment was performed by Allister et al. at 1050°C [77]. They found that thermal reduction induces vacancies and structural defects that may affect the mechanical and electrical properties of the produced rGO. Meanwhile, Dubin et al. 3

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proposed low-temperature thermal reduction of GO at 200°C. The process involves dispersion in organic solvents, such as N-methyl-2-pyrrolidinone. However, overall GO resulting from rGO continues to demonstrate low dispersibility in organic solvents; thus a necessity to functionalize them with long-chain hydrocarbon molecules is vital. Surface functionalization by means of controlling the exfoliation behavior of GO and rGO opens a new path to widen their potential applications in industrial communities.

3.3.2 Surface functionalization The surface-modified GO has good stability and assists in dispersion improvement in any polymer matrix. It also holds the key to the gate toward various applications. The mechanism of surface functionalization covers covalent and noncovalent functionalization. Covalent functionalization takes place with a direct chemical reaction with the edge-COOH groups and epoxy/hydroxyl-basal plane, and the corresponding solvents. For instance, the edge-COOH groups are activated with solvents, such as thionyl chloride [78], 1-ethyl-3-carbodiimide [79], and N,N-di cyclohexyl carbodiimide [80]. The dispersion difficulties of GO in nonpolar solvents due to the hydrophilic nature were solved by Paredes et al. [81]. They suggested that utilizing solvents with high electrical dipole moment, including DMF, THF, and ethylene glycol, would be the best solution for this endeavor. The formation of amides or esters via the covalent attachment between nucleophilic species from the solvents, such as amines or alcohols, and the functional groups forms amine-functionalized GO. Amine functionalized GO has been used in different applications of optoelectronics [78,82,83], drug-delivery materials [84], and energy harvesting [85]. The amine-functionalities improved the dispersibility of the modified GO in organic solvents [86], while porphyrin-functionalized primary amines and fullerene-functionalized secondary amines demonstrated attractive optical properties [78,83]. The amine and OH groups on the basal plane of GO can be further functionalized to attach polymers to form nanocomposites. To initiate polymerization, an atom transfer radical polymerization (ATRP) usually α-bbromoiobutyrylbromide attaches to the graphene surfaces [87,88]. Moreover, epoxy groups on GO covalently attach to different amine functional groups, including octadecylamine [89] and ionic liquid 1-(3-aminopropyl)-3-methylimidazolium bromide [78]. Organic isocyanates treatment reduces the hydrophilicity of GO to give several chemically functionalized GO. Isocyanates form covalent functionalization with COOH and OH groups forming amide and carbamate, respectively. Thus isocyanate-functionalized GO treats the dispersion issues of GO forming a stable dispersion in polar aprotic solvents producing a single graphene sheet with a thickness of 1 nm [90]. The resultant GO dispersion was further functionalized with matrix polymers due to the well mixing between both compounds. This route provides a novel synthesis method for producing graphene-polymer nanocomposites. Besides, the surface-functionalized GO in the suspension could be chemically reduced further to render electrical conductivity in the nanocomposite [90].

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89

The weak π–π, Van der Waals, and electrostatic interactions between the GO and the functionalizing agent can be used to functionalize GO in noncovalent functionalization. These types of functionalization are suitable for applications with critical optical or electronic properties. The noncovalent functionalization occurs during the chemical reduction of GO between the aromatic molecules and the conjugated polymers (poly(3-hexylthiophene) [91], 7,7,8,8-tetracyanoquinodimethane anion [92], cellulose derivatives [93], conjugated polyelectrolyte [94], porphyrin [92,95], pyrene and perylenediimide decorated with water-soluble moieties [67], poly(sodium 4-styrenesulfonate) [96], sulfonated polyaniline [97], and tetrasulfonate salt of copper phthalocyanine (TSCuPc) [98]) and rGO nanosheets via the π–π interaction. The large aromatic planes of the aromatic compounds offer high stability to the rGO. For instance, Chunder et al. found that the large sulfonate groups on TSCuPc anchors onto the rGO surface with some negative charges produce TSCuPc functionalized rGO for various applications [98]. While the nonfunctionalized rGO caused irreversible aggregation and precipitation of graphitic sheets. However, the extreme stability between them causes difficulty in removing the dispersant agent from the graphene surface [99,100].

3.3.3 Structural characteristics Exploring the properties of graphene provides a wide range of potential applications. The structural defects and functional groups strongly influence the optical and electrical properties of rGO. To obtain rGO with the desired optical and electrical properties, it is necessary to understand the molecular structure evolution of the GO structure during chemical reduction. Structural characteristics of rGO have been explored both theoretically [101] and experimentally [102–106]. Under the theoretical approach, a detailed study has been carried out to investigate the atomic structure evolution from GO to rGO [101]. These theoretical findings were corroborated by the experimental results (FTIR spectroscopy and X-ray photoelectron spectroscopy). The authors reported that the thermal annealing did not affect the graphene basal plane when the OH groups on GO desorb at low temperatures. However, the more stable epoxy groups attached to the GO surface via noncovalent functionalization disrupted the graphene basal plane. The short distance between the initial OH and epoxy groups tends to remove more carbon from the graphene plane leading to structural defects. The thermal annealing reaction led to the formation of thermodynamically stable C]O and ether groups. Mattevi et al. investigated the structural properties of rGO synthesized via thermal treatment in UHV [104]. Raman spectroscopy was conducted to reveal the structural evolution from the area ratio of the D and G bands using the Tuinstra-Koeing relation [105]. ID/IG ratio around 2.5 nm indicated the lateral dimension of sp2 ring clusters in a network of sp3 and sp2 bonded carbon. The TEM results revealed that the sp2 sites are isolated by disordered domains. Almost similar, the structure of rGO was studied by Gomez-Navarro et al. using high-resolution transmission electron microscopy (HRTEM) [107]. They reported a visible well-crystallized graphene from 3 to 6 nm, covering 60% of the surface with hexagonal lattice. They also found larger

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holes that were caused by the electron irradiation from HRTEM. The defects found in rGO have a larger number of topological defects compared to the mechanically exfoliated graphene. The TEM investigations suggest that oxidation reaction produces isolated highly oxidized areas leaving the major graphene surface uninterrupted. While the reduction reaction restores the oxidized areas to the sp2 domain leaving imperfect crystallinity of intact graphene. The resultant rGO leads to strain and in-plane and outof-plane deformations due to the isolated topological defects [107]. Plenty of efforts have been made so far to remove oxygen-containing functional groups. It should be noted that both chemical and thermal reduction methods can partially restore electrical conductivity but introduces structural defects that decrease the electrical properties than pristine graphene. Besides revealing the structural defects through Raman spectroscopy, it measures the compressive and tensile strains in graphene. Ni et al. reported that the presence of controlled strain is possible to tune the bandgap since strain may change the electric band structure [107]. In addition to that, the defect edge scattering and isotropic doping in graphene sheets can affect the electrical conductivity of graphene [108]. Therefore more research needs to be done in unfolding the structural capabilities of graphene and its derivatives that influence the electrical, optical, and also mechanical properties.

3.4

Synthesis of graphene-based composites

Several reports demonstrated genotoxicity via induction of reactive oxygen species [109]. Graphene-based composites introduce different polymers and nanoparticles for tuning the special properties of graphene over the unwrapping of CNTs. It opens a vast potential exposure to the environment and living systems. The large-scale production of GO has offered great opportunities to explore graphene with polymer and nanoparticles in the composite.

3.4.1 Graphene-polymer composites Graphene-polymer composites have shown tremendous potential in various applications, such as the food and beverages industry, medicine, and gas sensors, due to the low permeability of gas molecules, such as N2, O2, moisture, and CO2 [110–112]. As we have discussed in earlier sections, the synthesis of high-yield mono-layer graphene from the chemical and thermal reduction of GO disturbs the conjugated electrical structure and lowers the electrical conductivity. Besides, dispersion of GO and rGO in different solvents need to be improved further. Although several studies have been explored to enhance dispersion by surface functionalization with various functional groups [66,84], the resultant surface defects on GO lead to irreversible agglomeration in aqueous medium. This issue can be solved by stabilizing the GO with polymers and surfactants [67]. Good compatibility between the polymer matrix and GO is essential to obtain well dispersion in different organic solvents [81]. Covalent and noncovalent functionalization mechanisms are involved in the synthesis of graphene–polymer composites. The noncovalent polymer functionalization between

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the polymer matrix (1-pyrenebutyrate [113] and 7,7,8,8-tetracyanoquinodimethane [91]) and graphite structure provides strong affinity via the weak π-π interactions to produce desirable properties for potential applications. The covalent functionalization at the surfaces and edges of GO with amine groups was reported by Xu et al. [80]. They coupled amine-functionalized porphyrin (TPP-NH2) at the edges of COOH-GO. Another approach to polymer functionalization is by introducing ATRP initiator for good dispersion of graphene sheets in polymer matrix [114,115]. The ATRP initiator covalently immobilizes onto the GO surface allowing controlled polymerization of the molecular structure of the grafted polymer. There are various synthesis methods of graphene-based polymer composites, such as solution blending, melt mixing, and in situ polymerization. Salavagione and coworkers used a solution blending strategy to dissolve PVA in DMSO via esterification to produce a PVA-GO composite [116]. Other polymer matrices, such as PVC [117] and PVDF [118], were also synthesized through the solution blending method. However, this method causes severe agglomeration issues leading to the inhomogeneous distribution of sheets in the polymer matrix. The melt mixing method is more environmentally friendly ad this process is free from the usage of toxic solvents. The graphene–polymer composites were formed under high temperatures with shear forces to allow effective dispersion of GO and rGO sheets. This method is more practically preferred for thermoplastic polymers including, poly(methyl methacrylate) (PMMA) [119], polypropylene (PP), (ethylene-2,6-naphthalate) [110], and polycarbonates [111]. In situ polymerization takes place with monomer dispersion and the monomers are further polymerized. Various condensation reactions are involved in this technique allow covalent functionalization between the functional groups of the sheets and polymer matrix. Noncovalent functionalization of the GO with PMMA [120] and PP [121] also uses this method. Besides, in situ polymerization increases the interlayer spacing of graphite upon intercalation with the polymer matrix. However, the commercialization of graphene-based polymer is still in progress as it faces a few challenges, such as GO dispersion in the polymer matrix and the mass production of high-purity graphene synthesis.

3.4.2 Graphene-nanoparticles composites Graphene-nanoparticles composite has been developed to realize the exceptional properties and potential for various applications. The excited state of semiconducting nanoparticles is incorporated into graphene to tune the optoelectronic properties over a wider range of the spectrum. These photogenerated excitons in semiconductor nanoparticles, such as CdS [122], CdSe [123], ZnO [124], TiO2 [125], and In2O3 [126], show amazing potential in solar cells. The rGO/CdSe nanoparticles demonstrated improved photo response due to the efficient transfer from the CdSe photoinduced carriers to the rGO [127]. Three main strategies are employed to incorporate nanoparticles in graphene, which are pregraphitization, postgraphitization, and syn-graphitization. One of the most popular methods used to integrate nanoparticles is via in situ growing of graphene-nanoparticles. In regard to this, the functional groups and defects caused

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by synthesis help in nucleation and provide the controllable size of the nanoparticles. Wang et al. mentioned that the functional groups (COOH, OH, and epoxy) attached to the surface of GO provide preferred nucleation sites to control the size, morphology, and crystallinity of the nanoparticles [128]. They reported strong interactions between the anchoring functional groups of Ni(OH)2 nanocrystal. Meanwhile, the rGO with less functional groups leads to the formation of large crystals as these groups favor diffusion and recrystallization. Almost the same, hybrid electrodes (Co3O4 and Mn3O4/graphene) were developed for lithium-ion batteries [128]. The produced electrode enhances the electrochemical performance. The solvothermal method used to reduce GO under high temperature and pressure is a one-pot reaction to synthesize nanoparticles decorated graphene composites. Cao et al. fabricated rGO/CdS and improved the aggregation and obtained a high yield of SLG [122]. This method produces higher electrical conductivity than those formed from the hydrazine reaction. Although the functional groups assist in nucleation sites, the produced graphene suffers from several issues including poor dispersity of GO in organic solvents and lack of control of the functional groups. Another well-known method is the sol–gel technique. In the beginning, sol–gel was conducted to prepare graphene/silica composite to thick films for transparent conductors and was further reduced by hydrazine vapors from rGO [129]. Few studies have modified the sol–gel method for TiO2/GO was synthesized using the blending of GO sheets with titanium hydroxide-based ionic salt [130]. Moreover, GO was reduced by UV-assisted photocatalytic for composite formation. A high photocatalytic reduction was observed under ultraviolet (UV) light [131]. In addition, better dispersion in TiO2-rGO suspension was also observed. Williams et al. found that TiO2 acts as a scavenger and separates the electron–hole pair under UV radiation [132]. The electrons on TiO2 surface quickly reduce the oxygencontaining groups of GO. However, this method is more preferred for nanoparticles, which are sensitive to UV radiation. On the other hand, the ex situ synthesis is conducted on the presynthesized nanoparticles. They are mixed with the GO and followed by reducing the hybrid composites via chemical/thermal reactions. In contrast to the in situ synthesis, this technique does not cause any interference from the reduction reactions and promotes precise control of the size of nanoparticles. However, ex situ synthesis may alter the surface properties and distort graphene lattice due to the chemical/thermal reduction process. Therefore future research is needed to control the surface properties of the graphene-based nanoparticles for high-quality control of mono and a few layers of graphene.

3.5

Graphene growth mechanism

With the advent of synthesis methods of graphene that were explored in this review, there has been tremendous growth in the technological sphere. Among all the synthesis methods, CVD is the most utilized fabrication technology. PECVD technique is preferred over the conventional thermal CVD for low-temperature applications. However, both share common parameters, such as time, pressure, and gas flow, which

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Atomic % Carbon

1

10–1 0.6

Solubility 0.49 eV In X = –0.2 – k Tp

Segregation 0.55 eV In X = –0.17 – k Ts

0.7

0.8

1.0 0.9 103/T (K)

1.1

1.2

1.3

Fig. 3.7 Carbon monolayer phase condensation on Ni [23].

contribute to graphene production. Thus the graphene growth mechanism is heavily influenced by the major parameters involved in the synthesis of graphene. In general, graphene is deposited on other metal surfaces during CVD under the surface catalyzed process. The most common metal substrates that were used so far are Co [133], Cu [20], Ru [134], Rh [135], Pd [136], Ir [137], and Pt [22]. Ni has been used as the common medium to deposit graphene. The important parameter of the postdeposition cooling rate controls the segregation of graphene on Ni and consequently affects the morphology and the final properties of graphene. To understand the involvement of these parameters, a detailed thermodynamic study is vital. Eizenberg and Blakely have studied the solubility between C and Ni and the segregation of graphene on Ni [23]. Fig. 3.7 shows the carbon monolayer phase condensation on Ni as reported in [23]. They reported that as the atomic percentage of carbon increases, the temperature decreases. The solubility and segregation are calculated based on the following equations [23]. Solubility : ln x ¼
0:2


0:49eV kT p

Segregation : ln x ¼
0:17


0:55eV kT s

(3.1)

(3.2)

where k is the Boltzmann constant, x is the atomic percentage of carbon, and Ts and Tp are high and low temperatures, respectively. According to the curves observed in Fig. 3.7, the slope and intercept at 1/T ¼ 0 are the values of partial atomic heat of segregation (△ Hseg) and entropy of segregation (△ Sseg). They found that the partial atomic heat of segregation was lower than that of thick graphite and the entropy of

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segregation did not show many variations; hence, the authors made an inference that both monolayer and bilk graphite have the same degree of disorder. In this regard, Ni is used as the metal catalyst substrate for CVD to decompose hydrocarbon under ambient pressure and produces ultrathin graphite film condensation via the segregation mechanism described above. The segregation mechanism involves two processes, which are carbon phase dissolution and segregation on Ni (111) plane. Epitaxial growth of carbon atoms on Ni takes place because both carbon and nickel unit cells have the same dimensions [23]. Zhang et al. reported graphene growth on single crystalline and polycrystalline Ni surfaces [138]. They found that mono-layer and bilayer graphene grows more preferably on single-crystal Ni whereas multilayer graphene is preferred on polycrystalline Ni surfaces. Lahiri et al. proposed that graphene formation on Ni has the influence of complex carbide growth via the Ni2C phase [139]. Besides Ni, Cu is one of the popular transition metals utilized as a substrate for graphene growth. Unlike Ni, graphene is deposited on Cu’s surface via a surface adsorption mechanism. The first reported work on graphene precipitation on Cu substrate was conducted by Ruoff and coworkers by the surface-catalyzed technique [19–21]. In 2009 Li et al. proposed graphene deposition on Ni and Cu by carbon isotope labeling as shown in Fig. 3.8 [20]. Graphene growth on Ni catalyst involves (1) decomposition of methane, (2) Dissolution of carbon phase, (3) segregation on Ni surface, and (4) precipitation of graphene. Fig. 3.8 also illustrates the formation of graphene layers by the surface adsorption process on the Cu surface. The starting material used was also methane and graphene formation occurred under methane decomposition followed by domain growth upon surface nucleation throughout the entire surface. The nucleation stops when the Cu surface is fully deposited with graphene.

(a) 13

CH4

(b) 13

CH4

Dissolution

Surface segregation

12

CH4

Surface adsorption 12

Precipitation

CH4

Fig. 3.8 (A) Graphene formation mechanism by surface segregation and precipitation, and (B) mechanism of surface adsorption [20].

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Table 3.4 Carbon adsorption energy on Cu and Ni [141]. Adsorption site

Eads (in eV) on Cu

Eads (in eV) on Ni

100 110 111 111 111


6.42
5.57
4.88
4.89
4.88


8.48
7.74
7.09
7.14

(H) (H) hcp fcp bridge

Graphene growth of Cu and Ni can be compared by refereeing to the binary phase diagram of NidC and CudC. It shows that Ni is more soluble in carbon than Cu. Thus more dissolution of carbon can be observed from Ni than from Cu. However, this large amount of excess carbon leads to thick graphene growth. Yu et al. proposed a faster cooling rate [140] or using a thin Ni [36] substrate to control the precipitation of the excess carbon. The first thing we need to look into is the atomic and crystal structure of Cu and Ni when we compare the graphene growth on both substrates. Ni and Cu have the same crystal structure but different electronic structures. Hu et al. reported the carbon adsorption energy on Cu and Ni on three different sites namely (100), (110), and (111) as tabulated in Table 3.4 [141]. They demonstrated that Ni and Cu with (100) plane are the most stable adsorption sites of carbon. While (111) may easily diffuse the adsorbed carbon atom. They concluded that the graphene growth on the Ni surface has a higher binding energy of carbon atoms.

3.6

Challenges and opportunities

The unique properties of graphene have brought outstanding contributions in a wide range of applications based on graphene synthesized products. However, there are several challenges that need to be addressed before further advancements can take place in the synthesis of graphene, especially for the large-scale production of high-quality graphene sheets. Thus the production of high-quality graphene should be produced with well-controlled properties, cost-effective, and environmentally friendly. According to Wu et al., the challenges involve [3]: 1. Understanding a relationship between sp3 carbon content and transformation from sp3 carbon to good-quality graphene. 2. Detailed study on carbon–metal interactions encountered with different processes (e.g., deposition and phase transformation). 3. More research studies on identifying the actual role of the substrate material, carbon layer thickness, stacking configuration, and postprocess (annealing or irradiation) parameters on the growth process of various types of graphene. 4. Investigation of nucleation and growth mechanisms and structure property of graphene. 5. Improvement in the efficiency of microanalysis techniques currently being used to analyze the nanostructure, composition, physical properties, and quality of graphene.

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In parallel to the above challenges, there are various opportunities that can be explored by the researchers in this field especially the implementation methods for large-scale production of graphene. For example, the bottom-up approach of the CVD technique requires some modifications to obtain excellent graphene thickness and quality suitable for industrial purposes. The epitaxial graphene growth on SiC is also a high potential method for largescale production and commercialization of graphene in high-frequency electronics applications. Moreover, the implementation of graphene-based materials (GO and rGO thin films) in memory devices also improved due to simple processing, and the presence of functional group-induced charge controlling properties. In photovoltaic applications, graphene-based materials can function as transparent electrodes, electron acceptors, and light absorbers. In addition, thin films made of graphene come with various opportunities, such as cheap material, flexible, and durable supplements, compared to conventional transparent ITO electrodes in optoelectronic devices. However, a great challenge still exists in tuning the thin film’s bandgap via chemical routes even though the electrical and optical properties of GO/rGO can be controlled by varying their size, surface functionality, and degree of reduction [142].

3.7

Future perspectives

The attractiveness of graphene is evident from the exceptional physical, optical, thermal, mechanical, and chemical properties that can be applied to future various engineering applications. Graphene is a potential material to even replace the most popular silicon material in many electronics, energy storage, and energy conversion devices. Graphene modification has also opened a new path with tunable bandgap for largescale production of FET. For future enhanced engineering developments, graphene needs to be further developed in terms of sophisticated characterization and synthesis techniques. The current most common graphene-synthesis technique, CVD, produces mono-, bi-, and multilayers of graphene growth and offers exciting potential in semiconductor applications. For a brighter future in the CVD field, further extending the CVD process may exhibit high quality and cleaner transfer process. A breakthrough happened when thermal and chemical reduction offered a cost-effective large-scale production of graphene. However, this process reduced the electrical and mechanical properties leading to restricting controllable functionalization by other functional groups. Besides, this large-scale production leads to health risks due to inhaling and handling toxic reducing chemicals. Since modified graphene offers extraordinary properties for potential applications in various filed, this health risk associated with toxicity and biocompatibility needs to be further investigated and tailored accordingly. Therefore it is suggested to conduct graphene modifications in controlled environments. Graphene synthesis routes have a great potential to replace the common reinforcements used for metal matric composites. It is evident that reinforcement of aluminum (Al) matrix in graphene enhances the properties of the base material. Shin et al. reported an increment of 71.8% in tensile strength using graphene layer as reinforced

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material on Al matrix in [143]. On the other hand, breaking down graphene into nanoribbon provide great promise for FET logic applications. The major disadvantages in conventional FET applications are advised to be tackled via tunnel FETs and bilayer pseudospin FETs. These proposals have been conducted under simulation study and need to be conducted practically through experiments. Thus significant research interest has been triggered for atomic-scale systematical exploration and geometrical microscopic studies for bandgap tuning through the graphene modification process. Therefore graphene and graphene-based synthesis routes have opened a wide opportunity for future practical electronic and optoelectronic devices.

3.8

Summary

In this chapter, various graphene and graphene-based synthesis techniques were explored. The CVD method offers an exceptional potential for industrial graphene growth with several improvements. The epitaxial growth of graphene on SiC produces graphene with a unique azimuthal orientation over a whole wafer scale suitable for high-end applications. The main challenge in graphene synthesis is controllability of defect levels in the produced graphene. More research and developments are essential to have a deep understanding of graphene formation and its’ performance in various applications. Large-scale production can evolve as an economical alternative for graphene in the industry under controllable reduction conditions and environments. The synthesized graphene via different approaches opens a new path toward potential applications, such as energy-related (photodetectors, electrochemical sensors, solar cells), energy storage (supercapacitors), packaging for food and beverages, medicine, and electronics. Since graphene exploration has been ongoing for more than a few decades, graphene is summarized as the future material unfolding interesting new properties and applications.

Acknowledgments The authors thank the technical and financial assistance of UM Power Energy Dedicated Advanced Centre (UMPEDAC) and the Higher Institution Centre of Excellence (HICoE) Program Research Grant, UMPEDAC—2020 (MOHE HICOE—UMPEDAC), Ministry of Education Malaysia, TOP100UMPEDAC, RU003-2020, Universiti Malaya. The authors would also like to thank Universiti Tenaga Nasional (UNITEN) for the funding under the UNITEN BOLD of Grant No. J510050002/2021091 and Telkom University-UNITEN International Collaboration Grant of Grant No. 2020101TELCO.

References [1] H.C. Lee, W.-W. Liu, S.-P. Chai, A.R. Mohamed, C.W. Lai, C.-S. Khe, C.H. Voon, U. Hashim, N.M.S. Hidayah, Synthesis of single-layer graphene: a review of recent development, Procedia Chem. 19 (2016) 916–921. [2] S.S. Shams, R. Zhang, J. Zhu, Graphene synthesis: a review, Mater. Sci.-Pol. 33 (3) (2015) 566–578.

98

Functional Materials from Carbon, Inorganic, and Organic Sources

[3] Y. Wu, S. Wang, K. Komvopoulos, A review of graphene synthesis by indirect and direct deposition methods, J. Mater. Res. 35 (1) (2020) 76–89. [4] X.S. Li, Y.W. Zhu, W.W. Cai, M. Borysiak, B.Y. Han, D. Chen, R.D. Piner, L. Colombo, R.S. Ruoff, Transfer of large-area graphene films for high-performance transparent conductive electrodes, Nano Lett. 9 (12) (2009) 4359–4363. [5] M.S.A. Bhuyan, M.N. Uddin, M.M. Islam, F.A. Bipasha, S.S. Hossain, Synthesis of graphene, international, Nano Lett. 6 (2016) 65–83. [6] L. Kui, Z.G. Xia, W.X. Ke, A brief review of graphene-based material synthesis and its application in environmental pollution management, Chin. Sci. Bull. 57 (11) (2012) 1223–1234. [7] W. Choi, I. Lahiri, R. Seelaboyina, Y.S. Kang, Synthesis of graphene and its applications: a review, Crit. Rev. Solid State Mater. Sci. 35 (1) (2010) 52–71. [8] B. G€ur€unl€u, C¸.T. Y€ucedag, M.R. Bayramoglu, Green synthesis of graphene from graphite in molten salt medium, J. Nanomater. 2020 (2020), 7029601. p. 12. [9] Y. Tong, S. Bohm, M. Song, Graphene based materials and their composites as coatings, Austin J. Nanomed. Nanotechnol. 1 (1) (2013) 1003–1019. [10] M. Coros¸ , F. Poga˘cean, M.-C. Ros¸ u, C. Socaci, G. Borodi, L. Magerus¸ an, A.R. Biris¸ , S. Pruneanu, Simple and cost-effective synthesis of graphene by electrochemical exfoliation of graphite rods, RSC Adv. 6 (2016) 2651–2661. [11] G. Mittal, V. Dhand, K. Rhee, S.-J. Park, W. Lee, A review on carbon nanotubes and graphene as fillers in reinforced polymer nanocomposites, J. Ind. Eng. Chem. 21 (2015) 11–25. [12] M.Q. Jian, H.H. Xie, K.L. Xia, Y.Y. Zhang, Chapter 15 - Challenge and opportunities of carbon nanotubes, in: H. Peng, Q. Li, T. Chen (Eds.), Micro and Nano Technologies, Industrial Applications of Carbon Nanotubes, Elsevier Inc, 2017, pp. 433–479. [13] F. Liu, C. Wang, X. Sui, M.A. Riaz, M. Xu, L. Wei, Y. Chen, Synthesis of graphene materials by electrochemical exfoliation: recent progress and future potential, Carbon Energy 1 (2019) 173–199. [14] G. Zhao, D. Shao, C. Chen, X. Wang, Synthesis of few-layered graphene by H2O2 plasma etching of graphite, Appl. Phys. Lett. 98 (18) (2011), 183114. [15] P.W. Sutter, J.I. Flege, E.A. Sutter, Epitaxial graphene on ruthenium, Nat. Mater. 7 (5) (2008) 406–411. [16] A.T. N’Diaye, J. Coraux, T.N. Plasa, C. Busse, T. Michely, Structure of epitaxial graphene on Ir(111), New J. Phys. 10 (2008), 043033. [17] S.Y. Kwon, C.V. Ciobanu, V. Petrova, V.B. Shenoy, J. Bareno, V. Gambin, et al., Growth of semiconducting graphene on palladium, Nano Lett. 9 (12) (2009) 3985–3990. [18] A. Reina, S. Thiele, X.T. Jia, S. Bhaviripudi, M.S. Dresselhaus, J.A. Schaefer, J. Kong, Growth of large-area single- and bi-layer graphene by controlled carbon precipitation on polycrystalline Ni surfaces, Nano Res. 2 (6) (2009) 509–516. [19] X.S. Li, C.W. Magnuson, A. Venugopal, J.H. An, J.W. Suk, B.Y. Han, et al., Graphene films with large domain size by a two-step chemical vapor deposition process, Nano Lett. 10 (11) (2010) 4328–4334. [20] X.S. Li, W.W. Cai, L. Colombo, R.S. Ruoff, Evolution of graphene growth on Ni and cu by carbon isotope labeling, Nano Lett. 9 (12) (2009) 4268–4272. [21] X.S. Li, W.W. Cai, J.H. An, S. Kim, J. Nah, D.X. Yang, et al., Large-area synthesis of high-quality and uniform graphene films on copper foils, Science 324 (5932) (2009) 1312–1314. [22] B. Lang, A LEED study of the deposition of carbon on platinum crystal surfaces, Surf. Sci. 53 (1) (1975) 317–329.

Synthesis and applications of carbon-polymer composites

99

[23] M. Eizenberg, J.M. Blakely, Carbon monolayer phase condensation on Ni(111), Surf. Sci. 82 (1) (1979) 228–236. [24] K.S. Novoselov, A.K. Geim, S.V. Morozov, et al., Electric field effect in atomically thin carbon films, Science 306 (5696) (2004) 666–669. [25] M.I. Katsnelson, Graphene: carbon in two dimensions, Mater. Today 10 (1–2) (2007) 20–27. [26] A.K. Geim, P. Kim, Carbon wonderland, Sci. Am. 298 (4) (2008) 90–97. [27] P.R. Somani, S.P. Somani, M. Umeno, Planar nano-graphenes from camphor by CVD, Chem. Phys. Lett. 430 (1–3) (2006) 56–59. [28] V. Singh, D. Joung, L. Zhai, S. Das, S.I. Khondaker, S. Seal, Graphene based materials: past, present and future, Prog. Mater. Sci. 56 (8) (2011) 1178–1271. [29] T.E. Thompson, E.R. Falardeau, L.R. Hanlon, The electrical conductivity and optical reflectance of graphite–SbF5 compounds, Carbon 15 (1977) 39. [30] H. Cao, Q. Yu, R. Colby, D. Pandey, C.S. Park, J. Lian, et al., Large-scale graphitic thin films synthesized on Ni and transferred to insulators: structural and electronic properties, J. Appl. Phys. 107 (2010), 044310. [31] Y. Lee, S. Bae, H. Jang, S. Jang, S.-E. Zhu, S.H. Sim, et al., Wafer-scale synthesis and transfer of graphene films, Nano Lett. 10 (2010) 409. [32] S. Bhaviripudi, X. Jia, M.S. Dresselhaus, J. Kong, Role of kinetic factors in chemical vapor deposition synthesis of uniform large area graphene using copper catalyst, Nano Lett. 10 (2010) 4128. [33] S.J. Chae, F. Gunes, K.K. Kim, E.S. Kim, G.H. Han, S.M. Kim, et al., Synthesis of largearea graphene layers on poly-nickel substrate by chemical vapor deposition: wrinkle formation, Adv. Mater. 21 (2009) 2328. [34] A.N. Obraztsov, E.A. Obraztsova, A.V. Tyurnina, A.A. Zolotukhin, Chemical vapor deposition of thin graphite films of nanometer thickness, Carbon 45 (10) (2007) 2017–2021. [35] K.S. Kim, Y. Zhao, H. Jang, et al., Large-scale pattern growth of graphene films for stretchable transparent electrodes, Nature 457 (7230) (2009) 706–710. [36] A. Reina, X. Jia, J. Ho, et al., Large area, few-layer graphene films on arbitrary substrates by chemical vapor deposition, Nano Lett. 9 (1) (2008) 30–35. [37] X.B. Wang, H.J. You, F.M. Liu, et al., Large-scale synthesis of few-layered graphene using CVD, Chem. Vap. Depos. 15 (1–3) (2009) 53–56. [38] S. Bae, H. Kim, Y. Lee, et al., Roll-to-roll production of 30-inch graphene films for transparent electrodes, Nat. Nanotechnol. 5 (8) (2010) 574–578. [39] D. Wei, Y. Liu, Y. Wang, H. Zhang, L. Huang, G. Yu, Synthesis of N-doped graphene by chemical vapor deposition and its electrical properties, Nano Lett. 9 (2009) 1752. [40] L. Qu, Y. Liu, J.-B. Baek, L. Dai, Nitrogen-doped graphene as efficient metal-free electrocatalyst for oxygen reduction in fuel cells, ACS Nano 4 (2010) 1321. [41] A.L.M. Reddy, A. Srivastava, S.R. Gowda, H. Gullapalli, M. Dubey, P.M. Ajayan, Synthesis of nitrogen-doped graphene films for lithium battery applications, ACS Nano 4 (2010) 6337. [42] A. Ismach, C. Druzgalski, S. Penwell, et al., Direct chemical vapor deposition of graphene on dielectric surfaces, Nano Lett. 10 (5) (2010) 1542–1548. [43] J.J. Wang, M.Y. Zhu, R.A. Outlaw, X. Zhao, D.M. Manos, B.C. Holoway, Free-standing subnanometer graphite sheets, Appl. Phys. Lett. 85 (2004) 1265. [44] J.J. Wang, M.Y. Zhu, R.A. Outlaw, X. Zhao, D.M. Manos, B.C. Holoway, Synthesis of carbon nanosheets by inductively coupled radio-frequency plasma enhanced chemical vapor deposition, Carbon 42 (2004) 2867.

100

Functional Materials from Carbon, Inorganic, and Organic Sources

[45] A. Malesevic, R. Vitchev, K. Schouteden, A. Volodin, L. Zhang, G.V. Tendeloo, et al., Synthesis of few-layer graphene via microwave plasma-enhanced chemical vapour deposition, Nano 19 (2008), 305604. [46] R. Vitchev, A. Malesevic, R.H. Petrov, R. Kemps, M. Mertens, A. Vanhulsel, et al., Initial stages of few-layer graphene growth by microwave plasma-enhanced chemical vapour deposition, Nanotechnology 21 (2010), 095602. [47] M. Zhu, J. Wang, B.C. Holoway, R.A. Outlaw, X. Zhao, K. Hou, et al., A mechanism for carbon nanosheet formation, Carbon 45 (2007) 2229. [48] J.W. Zhu, G.Y. Zeng, F.D. Nie, et al., Decorating graphene oxide with CuO nanoparticles in a water-isopropanol system, Nanoscale 2 (6) (2010) 988–994. [49] N. Krane, Preparation of graphene, Selected Topics in Physics: Physics of Nanoscale Carbon (2011). [50] P. Avouris, C. Dimitrakopoulos, Graphene: synthesis and applications, Mater. Today 15 (3) (2012) 86–97. [51] Z.H. Chen, Y.M. Lin, M.J. Rooks, P. Avouris, Graphene nano-ribbon electronics, Phys. E-Low-Dimen. Syst. Nanostruct. 40 (2) (2007) 228–232. [52] L.Y. Jiao, L. Zhang, X.R. Wang, G. Diankov, H.J. Dai, Narrow graphene nanoribbons from carbon nanotubes, Nature 458 (7240) (2009) 877–880. [53] L.Y. Jiao, X.R. Wang, G. Diankov, H.L. Wang, H.J. Dai, Facile synthesis of high quality graphene nanoribbons, Nat. Nanotechnol. 5 (5) (2010) 321–325. [54] A.G. Cano-Marquez, F.J. Rodriguez-Macias, J. Campos-Delgado, et al., Ex-MWNTs: graphene sheets and ribbons produced by lithium intercalation and exfoliation of carbon nanotubes, Nano Lett. 9 (4) (2009) 1527–1533. [55] A. Hashimoto, K. Suenaga, A. Gloter, K. Urita, S. Iijima, Direct evidence for atomic defects in graphene layers, Nature 430 (7002) (2004) 870–873. [56] D.V. Kosynkin, A.L. Higginbotham, A. Sinitskii, et al., Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons, Nature 458 (7240) (2009) 872–876. [57] T. Shimizu, J. Haruyama, D.C. Marcano, D.V. Kosinkin, J.M. Tour, K. Hirose, et al., Large intrinsic energy bandgaps in annealed nanotube-derived graphene nanoribbons, Nat. Nanotechnol. 6 (2010) 45. [58] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, et al., Graphene-based composite materials, Nature 442 (2006) 282. [59] H.C. Schniepp, J.L. Li, M.J. McAllister, H. Sai, M. Herrera-Alonso, D.H. Adamson, et al., Functionalized single graphene sheets derived from splitting graphite oxide, J. Phys. Chem. B 110 (2006) 8535. [60] C. Gomez-Navarro, R.T. Weitz, A.M. Bittner, M. Scolari, A. Mews, M. Burghard, et al., Electronic transport properties of individual chemically reduced graphene oxide sheets, Nano Lett. 7 (2007) 3499. [61] M.A. Velasco-Soto, et al., Selective band gap manipulation of graphene oxide by its reduction with mild reagents, Carbon 93 (2015) 967–973. [62] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (6) (1958) 1339. [63] B.C. Brodie, On the atomic weight of graphite, Philos. Trans. R. Soc. Lond. 149 (1859) 249–259. [64] L. Staudenmaier, Verfahren zur darstellung der graphits€aure, Ber. Dtsch. Chem. Ges. 31 (2) (1898) 1481–1487. [65] Z. Yanwu, M. Shanthi, C. Weiwei, L. Xuesong, S. Ji Won, R. Potts Jeffrey, et al., Graphene and graphene oxide: synthesis, properties and applications, Adv. Mater. 22 (2010) 3906e24.

Synthesis and applications of carbon-polymer composites

101

[66] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, et al., Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide, Carbon 45 (2007) 1558. [67] S. Stankovich, R.D. Piner, X.Q. Chen, N.Q. Wu, S.T. Nguyen, R.S. Ruoff, Stable aqueous dispersions of graphitic nanoplatelets via the reduction of exfoliated graphite oxide in the presence of poly(sodium 4-styrenesulfonate), J. Mater. Chem. 16 (2006) 155. [68] B. Athanasios, D.G. Bourlinos, P. Dimitrios, S. Tamas, S. Anna, D. Imre, Graphite oxide: chemical reduction to graphite and surface modification with aliphatic amines and amino acids, Langmuir 19 (2003) 6050. ´ . Soto, L.L. Jimenez, J.A ´ . Quintana, S.A. Garcı´a, Graphene deriva[69] U.A. Romero, M.A tives: controlled properties, nanocomposites, and energy harvesting applications, in: Graphene Materials: Structure, Properties and Modifications, InTech, 2017, p. 77. [70] H.A. Becerril, J. Man, Z. Liu, R.M. Stoltenberg, Z. Bao, Y. Chen, Evaluation of solutionprocessed reduced graphene oxide films as transparent conductors, ACS Nano 2 (2008) 463. [71] C.-G. Lee, S. Park, R.S. Ruoff, A. Dodabalapur, Integration of reduced graphene oxide into organic field-effect transistors as conducting electrodes and as a metal modification layer, Appl. Phys. Lett. 95 (2009), 023304. [72] G. Eda, G. Fanchini, M. Chhowalla, Large-area ultrathin films of reduced graphene oxide as a transparent and flexible electronic material, Nat. Nanotechnol. 3 (2008) 270. [73] H.-J. Shin, K.K. Kim, A. Benayad, S.-M. Yoon, H.K. Park, I.-S. Jung, et al., Efficient reduction of graphite oxide by sodium borohydride and its effect on electrical conductance, Adv. Funct. Mater. 19 (2009) 1987. [74] S. Yongchao, T. Edward, Synthesis of water soluble graphene, Nano Lett. 8 (6) (2008), 1679e82. [75] W. Guoxiu, Y. Juan, P. Jinsoo, G. Xinglong, W. Bei, L. Hao, et al., Facile synthesis and characterization of graphene nanosheets, J. Phys. Chem. 112 (2008) 8192. [76] Y. Shen, et al., Evolution of the band-gap and optical properties of graphene oxide with controllable reduction level, Carbon 62 (2013) 157–164. [77] M.J. McAllister, J.-L. Li, D.H. Adamson, H.C. Schniepp, A.A. Abdala, J. Liu, et al., Single sheet functionalized graphene by oxidation and thermal expansion of graphite, Chem. Mater. 19 (2007) 4396. [78] Z. Liu, J.T. Robinson, X. Sun, H. Dai, PEGylated nanographene oxide for delivery of water-insoluble cancer drugs, J. Am. Chem. Soc. 130 (2008) 10876. [79] L.M. Veca, F. Lu, M.J. Meziani, L. Cao, P. Zhang, G. Qi, et al., Chem. Commun. (2009) 2565. [80] Y. Xu, Z. Liu, X. Zhang, Y. Wang, J. Tian, Y. Huang, et al., A graphene hybrid material covalently functionalized with porphyrin:synthesis and optical limiting property, Adv. Mater. 21 (2009) 1275. [81] J.I. Paredes, et al., Graphene oxide dispersions in organic solvents, Langmuir 24 (19) (2008) 10560–10564. [82] S. Giyogi, E. Bekyarova, M.E. Itkis, J.L. McWilliams, M.A. Hamon, R.C. Haddon, J. Am. Chem. Soc. 128 (2006) 7720. [83] Z.-B. Liu, Y.-F. Xu, X.-Y. Zhang, X.-L. Zhang, Y.-S. Chen, J.-G. Tian, J. Phys. Chem. B 113 (2009) 9681. [84] H. Yang, C. Shan, F. Li, D. Han, Q. Zhang, L. Niu, Covalent functionalization of polydisperse chemically-converted graphene sheets with amine-terminated ionic liquid, Chem. Commun. 3880 (2009).

102

Functional Materials from Carbon, Inorganic, and Organic Sources

[85] E. Kowsari, M.R. Chirani, High efficiency dye-sensitized solar cells with tetra alkyl ammonium cation-based ionic liquid functionalized graphene oxide as a novel additive in nanocomposite electrolyte, Carbon 118 (2017) 384–392. [86] X. Fan, W. Peng, Y. Li, X. Li, S. Wang, G. Zhang, et al., Deoxygenation of exfoliated graphite oxide under alkaline conditions: a green route to graphene preparation, Adv. Mater. 20 (2008) 4490. [87] M. Fang, K. Wang, H. Lu, Y. Yang, S. Nutt, Covalent polymer functionalization of graphene nanosheets and mechanical properties of composites, J. Mater. Chem. 19 (2009) 7098. [88] S. Wang, P.J. Chia, L.L. Chua, L.H. Zhao, R.Q. Png, S. Sivaramakrishnan, et al., Bandlike transport in surface-functionalized highly solution-processable graphene nanosheets, Adv. Mater. 20 (2008) 3440. [89] S. Stankovich, R.D. Piner, S.T. Nguyen, R.S. Ruoff, Synthesis and exfoliation of isocyanate-treated graphene oxide nanoplatelets, Carbon 44 (2006) 3342. [90] A. Chunder, J. Liu, L. Zhai, Reduced graphene oxide/poly(3-hexylthiophene) supramolecular composites, Macromol. Rapid Commun. 31 (2010) 380. [91] R. Hao, W. Qian, L. Zhang, Y. Hou, Aqueous dispersions of TCNQ-anion-stabilized graphene sheets, Chem. Commun. 6576 (2008). [92] Q. Yang, X. Pan, F. Huang, K. Li, Fabrication of high-concentration and stable aqueous suspensions of graphene nanosheets by noncovalent functionalization with lignin and cellulose derivatives, J. Phys. Chem. C 114 (2010) 3811. [93] X. Qi, K.-Y. Pu, X. Zhou, H. Li, B. Liu, F. Boey, et al., Conjugated-polyelectrolytefunctionalized reduced graphene oxide with excellent solubility and stability in polar solvents, Small 6 (2010) 663. [94] J. Geng, H.-T. Jung, Porphyrin functionalized graphene sheets in aqueous suspensions: from the preparation of graphene sheets to highly conductive graphene films, J. Phys. Chem. C 114 (2010) 8227. [95] A. Wojcik, P.V. Kamat, Reduced graphene oxide and porphyrin. an interactive affair in 2-D, ACS Nano 4 (2010) 6697. [96] H. Bai, Y. Xu, L. Zhao, Y. Hou, Non-covalent functionalization of graphene sheets by sulfonated polyaniline, Chem. Commun. 1667 (2009). [97] A. Chunder, T. Pal, S.I. Khondaker, L. Zhai, Reduced graphene oxide/copper phthalocyanine composite and its optoelectrical properties, J. Phys. Chem. C 114 (2010) 15129. [98] S. Qu, et al., Noncovalent functionalization of graphene attaching [6,6]-phenyl-C61butyric acid methyl ester (PCBM) and application as electron extraction layer of polymer solar cells, ACS Nano 7 (5) (2013) 4070–4081. [99] S. Bai, et al., Reversible phase transfer of graphene oxide and its use in the synthesis of graphene-based hybrid materials, Carbon 49 (13) (2011) 4563–4570. [100] A. Bagri, C. Mattevi, M. Acik, Y.J. Chaba, M. Chhowalla, V.B. Shenoy, Structural evolution during the reduction of chemically derived graphene oxide, Nat. Chem. 2 (2010) 581. [101] W. Gao, L.B. Alemany, L. Ci, P.M. Ajayan, New insights into the structure and reduction of graphite oxide, Nat. Chem. 1 (2009) 403. [102] M. Acik, G. Lee, C. Mattevi, M. Chhowalla, K. Cho, Y.J. Chabal, Unusual infraredabsorption mechanism in thermally reduced graphene oxide, Nat. Mater. 9 (2010) 840. [103] C. Mattevi, G. Eda, S. Agnoli, S. Miller, K.A. Mkhoyan, O. Celik, et al., Evolution of electrical, chemical, and structural properties of transparent and conducting chemically derived graphene thin films, Adv. Funct. Mater. 19 (2009) 2577.

Synthesis and applications of carbon-polymer composites

103

[104] K. Erickson, R. Erni, Z. Lee, N. Alem, W. Gannett, A. Zettl, Determination of the local chemical structure of graphene oxide and reduced graphene oxide, Adv. Mater. 22 (2010) 4467. [105] C.G. Mavarro, J.C. Meyer, R.S. Sundaram, A. Chuvilin, S. Kurasch, M. Burghard, et al., Atomic structure of reduced graphene oxide, Nano Lett. 10 (2010) 1144. [106] F. Tuinstra, J.L. Koenig, Raman spectrum of graphite, J. Chem. Phys. 53 (1970) 1126. [107] Z.H. Ni, T. Yu, Y.H. Lu, Y.Y. Wang, Y.P. Feng, Z.X. Shen, Uniaxial strain on graphene: Raman spectroscopy study and band-gap opening, ACS Nano 2 (2008) 2301. [108] J. Lee, D. Yoon, S.W. Kim Lee, H. Cheong, Thermal conductivity of suspended pristine graphene measured by Raman spectroscopy, Phys. Rev. B 83 (8) (2011). [109] Y. Zhang, S.F. Ali, E. Dervishi, Y. Xu, Z. Li, D. Casciano, et al., Cytotoxicity effects of graphene and single-wall carbon nanotubes in neural phaeochromocytoma-derived PC12 cells, ACS Nano 4 (2010) 3181. [110] H. Kim, C.W. Macosko, Morphology and properties of polyester/exfoliated graphite nanocomposites, Macromolecules 41 (2008) 3317. [111] H. Kim, C.W. Macosko, Processing-property relationships of polycarbonate/graphene composites, Polymer 50 (2009) 3797. [112] H. Kim, Y. Miura, C.W. Macosko, Graphene/polyurethane nanocomposites for improved gas barrier and electrical conductivity, Chem. Mater. 22 (2010) 3441. [113] Y.X. Xu, H. Bai, G.W. Lu, C. Li, G.Q. Shi, Flexible graphene films via the filtration of water-soluble noncovalent functionalized graphene sheets, J. Am. Chem. Soc. 130 (2008) 5856. [114] Y. Yang, J. Wang, J. Zhang, J. Liu, X. Yang, H. Zhao, Exfoliated graphite oxide decorated by PDMAEMA chains and polymer particles, Langmuir 25 (2009) 11808. [115] G. Goncalves, P. Marques, A. Barros-Timmons, I. Bdkin, M.K. Singh, N. Emami, et al., Graphene oxide modified with PMMA via ATRP as a reinforcement filler, J. Mater. Chem. 20 (2010) 9927. [116] H.J. Salavagione, M.A. Gomez, Martinez G. Polymeric modification of graphene through esterification of graphite oxide and poly(vinyl alcohol), Macromolecules 42 (2009) 6331. [117] S. Vadukumpully, J. Paul, N. Mahanta, S. Valiyaveettil, Flexible conductive graphene/ poly(vinyl chloride) composite thin films with high mechanical strength and thermal stability, Carbon 49 (2011) 198. [118] X. Zhao, Q.H. Zhang, Y.P. Hao, Y.Z. Li, Y. Fang, D.J. Chen, Alternate multilayer films of poly(vinyl alcohol) and exfoliated graphene oxide fabricated via a facial layer-bylayer assembly, Macromolecules 43 (2010) 9411. [119] K. Kalaitzidou, H. Fukushima, L.T. Drzal, A new compounding method for exfoliated graphite–polypropylene nanocomposites with enhanced flexural properties and lower percolation threshold, Compos. Sci. Technol. 67 (2007) 2045. [120] J.Y. Jang, M.S. Kim, H.M. Jeong, C.M. Shin, Graphite oxide/poly(methyl methacrylate) nanocomposites prepared by a novel method utilizing macroazoinitiator, Compos. Sci. Technol. 69 (2009) 186. [121] Y. Huang, Y. Qin, Y. Zhou, H. Niu, Z.-Z. Yu, J.-Y. Dong, Polypropylene/graphene oxide nanocomposites prepared by in situ Ziegler–Natta polymerization, Chem. Mater. 22 (2010) 4096. [122] A. Cao, Z. Liu, S. Chu, M. Wu, Z. Ye, Z. Cai, et al., A facile one-step method to produce graphene–CdS quantum dot nanocomposites as promising optoelectronic materials, Adv. Mater. 22 (2010) 103.

104

Functional Materials from Carbon, Inorganic, and Organic Sources

[123] X. Geng, L. Niu, Z. Xing, R. Song, G. Liu, M. Sun, et al., Aqueous-processable noncovalent chemically converted graphene – quantum dot composites for flexible and transparent optoelectronic films, Adv. Mater. 22 (2010) 638. [124] Y. Zhang, H. Li, L. Pan, T. Lu, Z. Sun, Capacitive behavior of graphene–ZnO composite film for supercapacitors, J. Electroanal. Chem. 634 (2009) 68. [125] S.R. Kim, M.K. Parvez, M. Chhowalla, UV-reduction of graphene oxide and its application as an interfacial layer to reduce the back-transport reactions in dye-sensitized solar cells, Chem. Phys. Lett. 483 (2009) 124. [126] S. Mahalingam, H. Abdullah, Electron transport study of indium oxide as photoanode in DSSCs: a review, Renew. Sustain. Energy Rev. 63 (2016) 245–255. [127] Y. Lin, K. Zhang, W. Chen, Y. Liu, Z. Geng, J. Zeng, et al., Dramatically enhanced photoresponse of reduced graphene oxide with linker-free anchored CdSe nanoparticles, ACS Nano 4 (2010) 3033. [128] H. Wang, J.T. Robinson, G. Diankov, H. Dai, Nanocrystal growth on graphene with various degrees of oxidation, J. Am. Chem. Soc. 132 (2010) 3270. [129] S. Watcharotone, D.A. Dikin, S. Stankovich, R. Piner, I. Jung, G.H.B. Dommett, et al., Graphene–silica composite thin films as transparent conductors, Nano Lett. 7 (2007) 1888. [130] K.K. Manga, S. Wang, M. Jaiswal, Q. Bao, K.P. Loh, High-gain graphene–titanium oxide photoconductor made from inkjet printable ionic solution, Adv. Mater. 22 (2010) 5265. [131] G. Williams, B. Seger, P.V. Kamat, TiO2–graphene nanocomposites. UV-assisted photocatalytic reduction of graphene oxide, ACS Nano 2 (2008) 1487. [132] G. Williams, P.V. Kamat, Graphene–semiconductor nanocomposites: excited-state interactions between ZnO nanoparticles and graphene oxide, Langmuir 25 (2009) 13869. [133] J.C. Hamilton, J.M. Blakely, Carbon segregation to single-crystal surfaces of PT, PD, and CO, Surf. Sci. 91 (1) (1980) 199–217. [134] P. Sutter, M.S. Hybertsen, J.T. Sadowski, E. Sutter, Electronic structure of few layer epitaxial graphene on Ru(0001), Nano Lett. 9 (7) (2009) 2654–2660. [135] D.G. Castner, B.A. Sexton, G.A. Somorjai, LEED and thermal desorption studies of small molecules (H2, O2, CO, CO2, NO, C2H4, C2H2, AND C) chemisorbed on rhodium (111) and (100) surfaces, Surf. Sci. 71 (3) (1978) 519–540. [136] L.G. De Arco, Y. Zhang, C.W. Schlenker, K. Ryu, M.E. Thompson, C.W. Zhou, Continuous, highly flexible, and transparent graphene films by chemical vapor deposition for organic photovoltaics, ACS Nano 4 (5) (2010) 2865–2873. [137] J. Coraux, A.T. N’Diaye, M. Engler, et al., Growth of graphene on Ir(111), New J. Phys. 11 (2009) 22. [138] Y. Zhang, L. Gomez, F.N. Ishikawa, et al., Comparison of graphene growth on single crystalline and polycrystalline Ni by chemical vapor deposition, J. Phys. Chem. Lett. 1 (20) (2010) 3101–3107. [139] J. Lahiri, T. Miller, L. Adamska, I.I. Oleynik, M. Batzill, Graphene growth on Ni(111) by transformation of a surface carbide, Nano Lett. 11 (2) (2011) 518–522. [140] Q.K. Yu, J. Lian, S. Siriponglert, H. Li, Y.P. Chen, S.S. Pei, Graphene segregated on Ni surfaces and transferred to insulators, Appl. Phys. Lett. 93 (11) (2008). [141] T. Hu, Q.M. Zhang, J.C. Wells, X.G. Gong, Z.Y. Zhang, A comparative first-principles study of the adsorption of a carbon atom on copper and nickel surfaces, Phys. Lett. A 374 (44) (2010) 4563–4567. [142] X. Huang, Z. Yin, S. Wu, X. Qi, Q. He, Q. Zhang, Q. Yan, F. Boey, H. Zhang, Graphenebased materials: synthesis, characterization, properties, and applications, Small 7 (14) (2011) 1876–1902.

Synthesis and applications of carbon-polymer composites

105

[143] S.E. Shin, H.J. Choi, J.H. Shin, D.H. Bae, Strengthening behavior of few-layered graphene/aluminum composites, Carbon 82 (2015) 143e51.

Further reading M. Herrera-Alonso, A.A. Abdala, M.J. McAllister, I.A. Aksay, R.K. Prud’homme, Intercalation and stitching of graphite oxide with diaminoalkanes, Langmuir 23 (2007) 10644.

Graphene and graphene oxide: Application in luminescence and solar cell

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Amol Nandea, Ashish Tiwarib, Swati Rautc, and S.J. Dhoblec a Department of Physics, Guru Nanak College of Science, Ballarpur, Chandrapur, Maharashtra, India, bDepartment of Chemistry, Dr. Bhimrao Ambedkar Govt. College Pamgarh, Pamgarh, Chhattisgarh, India, cDepartment of Physics, R.T.M. Nagpur University, Nagpur, Maharashtra, India

4.1

Introduction

Graphene has been experimentally studied for more than 50 years [1–9], but graphene oxide (GO) has been prepared a long time back—more than 150 years back and has been synthesized on large scale with cost-effective methods [10]. Graphene is an atom-thick layer, having a single layer of carbon atoms, which is a perfect nanoscale material. However, for graphene’s outstanding properties, the graphene won a noble prize and pushed researchers to work in this field. Graphene and GO are received enormous attention in many applications due to their numerous ranges of supreme properties. Graphene has a two-dimensional honeycomb lattice structure of carbon atoms. It has outstanding electrical conductivity, optical transmittance, intrinsic mobility, and thermal stability. It has unique electronic properties showing high transparency of atomic monolayer. The monolayer of graphene has transparency in the visible region and transmittance decreases with an increasing number of layers. Graphene and GO also show application for flexible membrane [11]. Moreover, nanostructure graphene and GO show variable optical and electronic band gaps depending on their size and dimensions. Variable band gap makes them a stronger candidate for optical and electronic device applications. The optical band gap of GO varies from 3.5 to 1.0 eV. These nanostructures are eco-friendly, inexpensive, water soluble, stable, photo and chemical, and biocompatible [12]. In this chapter, we discuss recent development in the field of graphene and GO. This chapter discusses the preparation and characterization of graphene and graphene oxide. Later, to make awareness about the luminescence and photovoltaic properties, fundamental properties of luminescent and solar cell materials will be discussed. Furthermore, in this chapter, applications of graphene and GO in the field of luminescence and solar cells will be discussed. This chapter also discusses the conclusion and future scope of the field.

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Preparation techniques to synthesize graphene and GO

The extraordinary physical characteristics of graphene, as well as their futuristic applications, have piqued the interest of researchers. Although scientists were aware of the existence of one-atom-thick, two-dimensional crystal graphene, the technique to extract it from graphite was developed by Geim and Novoselov in 2004. Thin flakes of graphene were initially exfoliated from their bulk equivalents in 2004 [13]. Graphite fluoride was converted into soluble graphene layers in a single process by the functionalization of the basal plane. It leads to the easy solution processing of 2D-graphene layers [14]. By ultrasonic treatment, a few graphenes were produced from commercially available exfoliated graphite. Droplets from the suspension were distributed over a silicon wafer and successively oxidized in air. Thermal oxidation destroyed two to three graphenes from the platelets, leaving just single graphene layer [15]. The extent of thermal expansion is determined by the source of graphite utilized and the intercalation process. However, the graphite nanoplatelets produced by this method are generally made up of hundreds of stacked graphene layers. Stankovich et al. utilized a repeatable method to create graphene sheets from graphite. They utilized graphite oxide (GO) as the material and exfoliated GO into individual GO sheets and then reduced them in situ to generate individual graphene-like sheets [16]. In recent years various techniques are evolving in the graphene synthesis. Some of the important techniques are highlighted.

4.2.1 Mechanical cleaving (exfoliation) The production of graphene by electrochemical exfoliation of graphite or graphite oxide in the electrolyte solution is often utilized for the synthesis of high-quality graphene nanosheets on a large scale. Najafabadi and Gyenge described the electrochemical exfoliation of graphite assisted by ionic liquids (ILs). It is a high-throughput, environmentally friendly, and scalable graphene manufacturing method. They employed an IL/acetonitrile electrolyte with significantly lower IL loading (1:50 IL/acetonitrile vol. ratio). Because of the low IL content, it was cost-effective, had extended electrochemical stability in a nonaqueous electrolyte, and a high exfoliation yield due to effective anionic intercalation inside the graphitic layers. They used an isomolded graphite rod as the anode and obtained up to 86% of exfoliation, resulting in primarily the graphene flakes and low quantity of carbonaceous particles and rolled sheets [17]. Chen and Xue prepared the colloidal graphene in quantity via the anodic exfoliation of graphite in (NH4)2SO4 aqueous solution. It resulted in the mass production of high-quality graphene in a short reaction time. The suggested electrochemical exfoliation process demonstrated that SO4 2
and H2O may be intercalated into those graphite sheets, and monolayer and few-layer graphene were produced by forming gaseous SO2 and O2 within graphite sheets [18]. For in situ exfoliation and functionalization through the electrochemical method, many techniques have been used.

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The electrochemical procedures (such as graphite type, electrolyte, voltage, and so on), electrode preparation, and suggested mechanisms for one-step exfoliation and functionalization of graphite are extensively reviewed by Aghamohammadi et al. [19]. Very recently a defect-free graphene with one to three layers (very-few-layer graphene) was successfully manufactured by Amri et al., utilizing two-step shear exfoliation technique in a rotating blade mixer (kitchen blender) and a high shear rotor–stator mixer. They used a low-cost and safe anionic surfactant of sodium lauryl sulfate [20].

4.2.2 Liquid-phase exfoliation The liquid-phase exfoliation (LPE) of graphene through sonication in 2008 was reported and since then enormous progress has been achieved in the last decade. Graphene is produced in a variety of media, including organic solvents, ILs, water/polymer or surfactant solutions, and several other green dispersants. LPE methods, such as sonication, high-shear mixing, and microfluidization, are used. LPE is a novel top-down technique that produces a stable dispersion of monolayer or few-layer defect-free graphene by exfoliating natural graphite with high-shear mixing or sonication. Consequently, LPE is more viable as compared to other techniques, and graphene LPE will become an important technology and is a green process [21]. The sonication is mostly employed; however, the inhomogeneous acoustic field and low-solution processability limit its industrial application. The usage of colloidal suspensions is used in direct LPE, and one stage involves the physical transition of graphite into graphene. As exfoliation media, different organic solvents, ILs, and water/surfactant have been used, with varying exfoliation efficiency [22]. The graphene synthesis depends on several factors, such as exfoliation techniques, including ultrasonication, shear force and supercritical fluid, the dispersion of graphene in water and organic solvents, surfactants, polymers, and other stabilizers. The dispersion efficiency regulates the yield, fraction of monolayers, and graphene concentration [23]. Performing LPE under modest pressure can provide high-shear rates and/or decreases cavitating flows in conjunction with a low-pressure input. Consequently, lab-on-a-chip (LOC) technology is a great approach because of its ability to rapidly explore many types of flow regimes (laminar, turbulent, single liquid phase, or cavitating flows) with high shear rates. The size of the initial pure graphite particles, which must not block the channel, as it may limit the size reduction. Qiu et al. recently used a low-pressure cavitating LOC to exfoliate surfactant stabilized graphene from natural graphite powder. The bubbles collapsing and the high shear rates are thought to exfoliate graphitic particles [24]. Very recently Gomez et al. discussed the role of solvent type, graphite amount, sonication time, centrifugation time, and centrifugation speed by employing dimethylformamide (DMF) and ethanol in the exfoliation process. The study revealed that the short-time exfoliation process was optimized by dispersing graphene in DMF. Exfoliating graphene in ethanol resulted in edge-type defects [25].

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4.2.3 Chemical vapor deposition Chemical vapor deposition (CVD) is a viable route to synthesize high-quality largearea single-crystal monolayer (1L) graphene on copper (Cu) substrates. The growth of graphene by CVD on metal foils is a promising technique to deliver large-area films with high electron mobility. The CVD of hydrocarbons, other carbon precursors on copper, and other metal substrates have evolved. Lisi et al. used ethanol as a safe and inexpensive precursor to grow graphene on copper in the short duration of time [26]. Graphene with a controllable thickness and quality can be synthesized using liquid catalysts. Various molten metals and alloys, including indium, tin, tin-nickle, and tin-copper, have been reported for graphene synthesis. Ding et al. reported the graphene formation Ga surface, with the growth characteristics depending on the temperature, having a time-dependent thickness control and a decrease in quality with increasing hydrogen flow [27]. Trinsoutrot et al. discovered that increasing the annealing pretreatment period increased the average size of copper grains, resulting in bigger graphene flakes with greater crystallinity and a smaller number of layers. Changing the methane content and run duration also resulted in a decrease in graphene quality and an increase in the number of graphene layers [28]. Bouhafs et al. recently described a low-pressure CVD (LPCVD) to grow Rhombohedral-stacked few-layer graphene crystals on Cu substrate. The studies revealed that it consists of alternating domains with ABA and ABC stacking. The domains of ABC-FLG have a large size up to 4-μm width and 90-μm long, which is higher than previous reports [29]. Plasma-enhanced chemical vapor deposition is an emerging technology to synthesize graphene at lower temperatures as compared to conventional CVD process. It has a better control over the deposition parameters and is cost-effective, scalable, singlestep synthesis of a high-quality vertically aligned graphene at low temperatures. Kulczyk-Malecka et al. recently used microwave (MW)-assisted CVD reactor, involving the decomposition of CH4 and H2 gas mixtures in Ar as a carrier gas, and the graphene was deposited directly onto Si wafer substrates (no additional heating required). The results suggested that higher synthesis temperature increases defects content in graphene, prolonged synthesis time at increased temperature leads to sp2 to sp3 transition, also appropriate CH4:H2 gas ratio allows for etching amorphous carbon and growing graphene [30].

4.2.4 Solvothermal synthesis Bottom-up chemical production of carbon nanosheets in gram-scale quantities usingordinary laboratory chemicals ethanol and sodium was described by Choucair et al. The intermediate solid formed was subsequently pyrolized, giving a fused array of graphene sheets that are dispersed by mild sonication [31]. Li et al. reduced hexachloro-1,3-butadiene (C4Cl6) by metallic sodium (Na) in polyethylene glycol600 (PEG-600) at 300°C solvothermally and synthesized graphene nanosheets [32]. Wang et al. described that under hydrothermal circumstances, hydrophilic graphene nanosheets can be produced by reacting GO nanosheets with poly(sodium 4-styrene sulfonate) and concurrently by reducing hydrazine hydrate. Similarly, by interacting

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with octadecylamine and reducing with hydroquinone in a reflux process, organophilic graphene nanosheets were synthesized [33].

4.3

Characterization of graphene and graphene oxide

4.3.1 Electrical measurements Graphene is a zero-overlap semimetal with extremely high electrical conductivity as both holes and electrons are its charge carriers. Each carbon atom in graphene is linked to three other carbon atoms on the two-dimensional plane, leaving one electron free in the third dimension for electrical conduction [34]. The electronic characteristics of graphene are fundamentally determined by the bonding and antibonding (valance and conduction bands) of the pi orbitals. Electronic conductivity is really relatively low due to the zero density of states near the Dirac points. However, the Fermi level may be altered by doping (with electrons or holes) for conducting electricity. A significant band gap expansion occurs in graphene nanoribbons (GNRs), which open up possibilities for graphene-based transistor applications. The computational prediction of electrical structures in ideal GNRs suggest that are strongly linked to edge configurations The effects of edge disorder and charge impurity on Anderson localization and Coulomb blockade have an influence on transport characteristics [35]. The functionalization of graphene is required to interface it with other moieties and therefore increase electrical/electronic applications. Chemical functionalization and/or molecular interactions with graphene significantly modify its electrical characteristics. There are several factors altering the electrical characteristics, such as transformation of carbon atoms’ hybridization state, formation of a barrier at the functionalization site and within the electron-potential continuum, 2D planar lattice distortion, molecular dipole-induced doping, graphene’s high quantum capacitance leading to high density of states, molecular orbital hybridization with graphene, graphene lattice inclusion and/or defects, introduction of new energy levels, and introduction of strong edge states and quantum confinement via structural restrictions, such as nanoribbons, quantum dots, and nanomesh [36]. The method of synthesis also affects the electrical conductivity. Leandro et al. applied an alternative nitric acid method powder and used river basin coal and obtained graphene, which is comparable to Hummers’ method. The electrical conductivity of powder coal char reduced GO via this method is increased to 4800 S m
1 at 2500°C [37]. Huang et al. found that in graphene/carbon nanofiber composites, the conductivity increased with graphene content and for the sample containing 5 wt% of graphene, it was 0.39 S/cm as compared to 0.19 S/cm for the pure fiber. The conductivity also increases with carbonization temperature [38]. GO, no matter the synthesis method, usually has less electrical conductivity. It is generally reduced to improve its electrical conductivity. Ashery et al. find that raising the temperature to about 400°C during preparation the obtained GO has high electrical conductivity for Au/GO/ SiO2/p-Si/Al composites. Riahi et al. investigated the electric and dielectric behaviors of GO using modified Hummer’s method. They found that the electric behavior

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varied with the temperature (electrical conductivity is thermally deactivated) and frequency [39].

4.3.2 Optical measurements Graphene has unusual optical transitions and can absorb light at a variety of frequencies due to its band structure, absence of the band gap, and interaction between electromagnetic radiation and the Dirac fermions in the graphene sheet. Each of these optical reactions is distinct; in the visible-near-infrared light region, it is generated through the intraband transitions and while for far-infrared absorption free carrier absorption processes is also possible. As the graphene does not have defined energy band levels like other materials, it can absorb this radiation regardless of its frequency. Johari and Shenoy studied theoretically the optical properties of GO via controlled deoxidation, focusing on the role of the individual functional group. The ab initio density functional studies revealed that the functional groups, such as epoxides, hydroxyls, and carbonyls, significantly govern the electron energy loss spectra (EELS). It conformed well to the experimental data and the EELS of GO suggested that the absorption coming originating from the π electron transition of the carbonyl groups has a lower energy than that of epoxy and hydroxyl groups. The increase in the hole-width (attributed to the carbonyl groups) significantly decreases the optical gap and opens the band gap [40]. Shen et al. found that the band gap of reduced GO can be varied from 2 to 0.02 eV depending on its reduction level. It is believed to originate from different inter- and intra-band transitions [41]. The optical absorption and emission properties of GO varied with the oxygen functional groups as studied by Maiti et al. The photochemical reduction by IR radiation revealed a yellow-red emission ( 610 nm) originating from the defect-assisted localized states in GO due to epoxy/hydroxyl (CdO/dOH) functional groups and a blue emission ( 500 nm) attributed to the carbonyl (C]O)assisted localized electronic states is prominent. The intensity of the yellow-red and the blue emission depended on the reduction time and power density of IR radiation [42]. Kumar et al. synthesized the GO powders using Tour’s method and using different concentration of H2O2. The study suggested that the H2O2 concentration strongly affects the degree of oxidation level. It can control the surface and molecular structures of GO, resulting in the tunable, optical properties [43]. Recently Naghdi et al. doped GO with alkali metal followed by the thermally reduction. The optical properties of GO varied effectively via the different doping agents and followed by thermal reduction and there was a shift in the work function of GO [44]. Rasheed et al. performed the optical studies of GO thin films using acetone, water, and pure ethanol. The transmittance increases with increasing wavelength, while the reflectivity decreases with increasing wavelength. The UV-visible spectra revealed an absorption peak at 300 nm suggesting the n-π* transition attributed to the carbonyl group in GO. The absorption peak wavelength at 230 nm indicated the electronic transmission π-π*, due to double carbon bond (C]C) in GO. Similarly, the extinction coefficient was found to increase with increasing the wavelength and the refractive

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index decreases with increasing the wavelength as the reflectance decreases at this range of wavelengths [45].

4.3.3 Structural and microstructural characteristics Various synthetic techniques significantly affect the morphological and structural properties graphene and GO. The method employed, use of intercalating agent, oxidizing reagent for introducing the oxygen functionalities, and other factors have been studied by various research groups. Burresi et al. found that the sonication time and the washing procedure affect the structure and the morphology of the GO flakes. The use of KOH as alkaline agent in a chemical reducing treatment of the GO powder before sonication alters drastically the native structure of GO [46]. Zhao et al. reported that in ZnO/rGO composites, the morphology was a function of aging time, heating mode, and rGO mass concentration. The optical properties, the light absorption and photo-luminescence emission were morphology dependent [47]. Mao et al. systematically investigated the influence of micromorphology on lubrication properties. The reduced GO sheets with different micro-morphologies, viz. regular edges (RG), irregular edges (ir-RG), and both irregular edges and wrinkles (ir-RWG), were obtained and the study suggested that the morphological regularity of the graphene sheets increases their lubricant properties [48]. Tsou et al. demonstrated that the different techniques, such as pressure-, vacuum-, and evaporation-assisted selfassembly techniques, for fabricated composite GO/mPAN has affected the morphology and induced different GO assembly layer microstructures as revealed by the ˚ to 11.5 A ˚ [49]. Zou et al. suggested that XRD changes in the d-spacing from 8.3 A altered morphology can have novel application of the GO thin films. They fabricated highly wrinkled GO films by vacuum filtration of a GO suspension through a prestrained filter and found it effective against different bacterial species [50]. Parviz et al. studied the graphene aerogels and suggested that the ammonia content and the parent nanosheet morphology (crumpled vs flat) strongly affect the cross-linking. The GO nanosheet morphology was altered by spray drying and crumpled GO nanosheets were formed [51]. Chen et al. altered the morphology by transforming it into hybrid siloxane-epoxy coating reinforced by worm-like GO. The mechanistic formation of GO nanoscrolls involves comprehensive effect of the sonication process and inorganic nanoparticle-induced self-assembly. Besides, the nanoscrolls were not enclosed boundary, which had siloxane-epoxy resin embedded in it. GO sheets were initially functionalized with TEOS through a sol-gel technique before being integrated into the SE resin using a wet transfer approach. FGO sheets with active dSidOH groups, enhanced surface wrinkles, and reduced layers were produced after chemical, structural, and morphological studies. SidOH directed the transformation of FGO into tubular structure during the transfer procedure. Ultrasonication provided the required activation for scrolling and the vacuum cavity offered the deformation space. The necessary activation for scrolling was given by ultrasonication, while the deformation space was provided by the vacuum cavity [52]. The structure and other properties of the graphene depend on oxidation conditions, and reduction process; also on the type of graphite from which the graphene

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(rGO) has been synthesized. Sieradzka studied the effect of graphite grain sizes on the morphology and structure of the reduced GO. The oxidation and reduction processes slightly alter the lateral size for rGO samples obtained from graphite with sizes 10,000 mm/min, whereas the nano-indentation covers velocities from 0.1 μm/min to 100 mm/min. Since it is

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known from stress-strain measurements of polymers that higher draw speeds increase the brittleness of the polymer, an exact simulation of the conditions in car wash units in terms of velocity cannot be provided with the indentation setup so far. Now, is it possible to correlate the mechanical properties of coatings with scratch test results? Comparison of UV coatings to standard 1K and 2K clears in order to get an overview of the scratch resistance of several different clear coatings, Poppe and Kutschera [82] compared the scratch resistance of thermal and UV-cured coatings described in Table 10.5. They performed field tests of the coatings in a car wash unit and correlated these results with scratch results of laboratory scratch tests and with mechanical response properties determined by nanoindentation measurements The field tests were performed on coated small panels, which were mounted on a car roof. By frequently changing the positions of the panels on the car roof, differences in the influence of the washing brush due to the positioning were eliminated. The car wash testing was performed once per week in a public car wash facility, as shown in Fig. 7.25. After 30 weeks, field tests were finished, and the panels finally evaluated. All panels were visually inspected and ranked (Table 7.7). In parallel, the scratch resistance was evaluated by microscopic investigation (Fig. 7.26) and gloss measurements. The gloss of the damaged panels was measured at 20 degrees angle and compared with the initial gloss values. From the gloss data it can be seen that only the 1K conventional clear coat and the 2K conventional clear coat show a rather poor performance. All other clear coats exhibited the same residual gloss values within the tolerance of the measurement. Thus, a clear ranking of the higher scratch-resistant coatings on the basis of gloss measurements only was not possible. From visual inspection, however, huge differences could be detected, which yielded the same ranking as was found with optical microscope inspection. These eight clear coats where also subjected to laboratory scratch tests and nanoindentation measurements in order to determine the mechanical response data of the coatings. To find the decisive mechanical material properties for extremely scratch resistant surfaces, AMTEC and Crockmeter results can be plotted versus the mechanical response data of nanoindentation investigations. The results of the AMTEC and Crockmeter loadings (dry scratch resistance employing an Atlas

Fig. 7.25 Car wash field test procedure.

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Table 7.7 Field test results after 30 cycles field test.

Clear coat UV monocure 2K evolution 2K variant Water-borne clear coat 2K UV dual cure Powder clear coat 2K conventional 1K conventional

Ranking/ visual inspection

Ranking/ microscopic investigation

Gloss loss (%)

Ranking/ CSEM nanoscratch test

1 2 3 4

1 2 3 4

4 3 3 3

1 2 3 5

5 6

5 6

3 4

4 6

7 8

7 8

7 10

7 8

Fig. 7.26 Microscopic investigation of panels after 30 cycles field test (ranking was done by evaluating the number and area covered by scratches).

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Fig. 7.27 Correlation of AMTEC residual gloss vs. elasticity (left) and crockmeter residual gloss vs. elasticity (right).

AATCC Crockmeter CM 5 according to EN ISO 105-X12) are shown in Fig. 7.27, plotted exemplarily against the elastic response values from the nano-indentation evaluations. It can be seen that the best ranked coatings in the field test feature a high elastic response, but looking only at the elastic response does not reflect the same ranking of the coatings observed in the visual inspection. However, correlations were found between residual gloss, elastic, and fracture response, as well as plastic deformation. Corresponding data are plotted in Fig. 7.28. The remaining gloss in the AMTEC test increases with increasing elasticity and decreasing plastic as well as

Fig. 7.28 Comparison and correlation of nano-scaled, single contact indent results with AMTEC remaining gloss level.

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Fig. 7.29 Dynamic hardness and recovery against normal force.

fracture response of the coatings. Similar to the described elastic response, the ability of a coating to return to its original shape has also been defined by the “Recovery” (R ¼ (penetration depth  residual depth)/penetration depth; R ¼ 1 corresponds to no scratch) as shown in Fig. 7.29 [83]. The analysis of the nanoscratch indentation data of a similar series of clear coats reported by Fey [84], comparing the scratch resistance of different clear coats with special focus on UV-dual cure coatings, gave the same ranking in terms of recovery as in the field tests. It has been shown that the recovery increases [85] with decreasing tan δ (increasing storage modulus) and coatings with high recovery exhibited high scratch resistance. As is well known from the literature, UV technology [86] offers one promising way to enhance the scratch resistance of clear coats. The excellent performance of the UV-cured clear coats can also be seen in the laboratory scratch test results. The UV (mono)-cured coating showed by far the best performance, with residual gloss values up to 94% in the AMTEC and 96% in the Crockmeter-test. However, it was found that the relative ranking of the different clear coats was not the same in AMTEC and Crockmeter compared with the car wash field tests. In car wash field tests, the total damage is by far not as severe as for AMTEC testing (Fig. 7.30, bottom). Therefore, Fey [84] proposed a modified AMTEC test, relying on low numbers of AMTEC double runs (2 DR), instead of the DIN conditions (10–50 double runs). He showed that the ranking of different highly mar resistant coatings, UV monocure, UV-dual cure coatings, and standard 2K polyurethanes, is the same in the modified (2 DR) AMTEC, in Crockmeter, in car wash, and in

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Fig. 7.30 Benchmarking of UV and conventional clears: AMTEC results by gloss retention and brightness measurements in comparison to field test car wash.

single-scratch indenter tests. Furthermore, he also pointed out that the impressions of the visual inspection are correlated much more with optical measurement of the brightness (dL 15 degrees) than with gloss measurements. Since coatings exhibiting more scratches appear gray on black panels, the dL 15 degrees values proved to be a more sensitive tool to describe the scratch results of highly scratch resistant coatings. The dL 15 degrees results of the (2DR) modified AMTEC corresponded very well with the ranking of the field test car wash runs (Fig. 7.30). It was obviously shown that clear coats exhibiting high levels of residual gloss after scratching in the AMTEC test are mainly characterized by an elastic response pattern. Elasticity has to be interpreted in this context in terms of rubber-like elasticity and not flexibility. Moreover, as a result of microscopic investigations, there seem to be more abrasive fracture than plastic deformation scratches in the real car wash machines than in the AMTEC Kistler apparatus. Thus, the description of the pure elastic/plastic material properties as measured by indentation is not sufficient to characterize the scratch performance properly. Instead, single scratch tests have to be done to estimate the field performance of the surfaces more reliable. In those tests, in addition to the elastic properties, the susceptibility of the surfaces toward fracture is described more accurately. At the end, a combination of enhanced elasticity and resistance against abrasion-type scratches seems to be decisive for extremely scratch-resistant surfaces. Therefore, the clear coats evaluated by Poppe and Kutschera [82] were also subjected to nano-indentation measurements in order to determine the fracture load at first crack (abrasive behavior) and the residual depth pattern after application (elastic response or

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Fig. 7.31 Classification of the results of nanoscratch experiments of different clear coats (corresponds to elastic versus fracture response).

recovery behavior) of a constant load (5 mN load). With this method developed by Lin et al. [81], it is possible to evaluate different clear coats regardless of their curing conditions with respect to their scratch performance. This classification is shown in Fig. 7.31 for the eight clear coats under investigation. The UV coatings are by far the best in this classification. As a general result, there was no clear correlation found for the individual mechanical properties, such as E-modulus, creep, elasticity, plastic deformation, or fracture load. However, the ranking obtained from Fig. 7.31 coincides with that of the field test results indicating that one single parameter alone is not sufficient to describe the field test scratch performance properly. Field and AMTEC test studies have demonstrated that both, extreme elasticity and strong resistance against fracture-like scratches are the most important requirements extremely scratch-resistant clear coats have to fulfill. For optimizing the elasticity, the corresponding networks have to be highly cross-linked, and since the rubber elasticity is an entropic effect, minimizing the interactions of the. chains segments between cross-links increases the probability of conformational changes. Such high recovery or elastic responses [87] can be obtained with highly cross-linked or low cross-linked elastomeric formulations. However, with the low cross-linked elastomeric formulations, the chemical resistance of the coating is worse. Due to the fact that extremely elastic, scratch-resistant surfaces are realized through high cross-link densities, internal tensions may appear in those coatings. UV-cured systems have demonstrated the ability to exhibit

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extremely high scratch resistance, however, in view of the overall requirements, OEM clear coats have to fulfill, there are certain limitations in the development regarding scratch resistance. The most important one is a loss in the ability of stress relaxation and consequently an increased susceptibility toward cracking. Both phenomena can be explained by an excess cross-link density. Because extremely scratch-resistant clear coats have to meet a number of other contrary requirements, at the end a reasonable compromise has to be achieved, for which UV-curable coatings are promising candidates. The potential has already been demonstrated with several different clear coats subjected to scratch tests before and after Jacksonville aging, where a UV-cured top coat, used as sealer in a twolayer top clear coat, exhibited the best performance by far after aging [88] (Fig. 7.32, left). This behavior is extremely encouraging for the further development of UV-curable coatings, since the second best coating in this test series, a highly cross-linked polyurethane coating used as sealer, can also exhibit high scratch resistance, but suffered from lower etch resistance (Fig. 7.32, right). The reasons for the lower etch resistance are not discussed in the publication, but may be due to the increased hydrophilic urethane group content. If this assumption holds true, the diametrically opposed relationship of scratch resistance and etch resistance may be overcome by UV-curable coatings, where more hydrophobic cross-linkers can be employed. Furthermore, UV technology allows faster cure and requires less space. The comparison to standard thermal OEM coating processes reveals that a combined thermal-UV dual cure process will already reduce the space requirements by 50% at much faster cycle times (Fig. 7.33).

Fig. 7.32 Scratch resistance of different clear coats before and after Jacksonville test (left) and diametrically opposed relationship of scratch and acid etch resistance (right).

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Fig. 7.33 Scheme of standard and UV-dual cure processing for automotive top coat application.

7.6.1.6 Plastic applications in automotive Several applications of UV-curable coatings on plastics for automotive parts are already realized, such as headlamp lenses and head lamp reflector housings. Other plastic applications are in implementation or development, such as tail lens assemblies, exterior plastic trims, body side moldings, and interior coatings. The major reasons for using the UV process are reduction of parts-in-process, lower energy costs, and reduced floor space. Due to a superior abrasion resistance compared with PU and acrylic thermal clear coats and a deeper “wet” look, UV-curable clear coats for acrylic tail lenses evolved in the decorative application process. The UV clear coat had to exhibit enough flexibility to match the thermal expansion of the underlying basecoats during heating and cooling cycles. Exterior plastic trim coatings with UV systems are in development for plastic wheel covers. Painting plastic parts with conventional basecoat/clearcoat combinations can account for up to 50% of the costs of such parts. Cheaper alternatives are molded-in-color (MIC) plastic parts; however, they suffer from a cheap and poorquality image. High gloss UV-curable clear coats are an option to improve the design and optics as well as the performance, such as scratch resistance, gloss, and exterior stability, of such low-cost parts. An example of a target application for interior automotive parts is vinyl applique decorated plastic parts. The major challenge for UV coatings to replace 2K polyurethanes is adhesion and the low gloss requirement. Therefore, water-based UV-curable clearcoats have been developed, which exhibit excellent scratch and chemical resistance at very low gloss; however, flexibility of up to 300% elongation is critical [68].

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7.6.2 Industrial applications 7.6.2.1 UV-curable coatings for hard topcoats on plastic The main applications of UV coatings for plastics are mobile phones, disc players, electronic devices, TVs, cameras, Pacs, audio players, cosmetic packaging [89]. In order to protect these devices against mechanical damage, hard topcoats are applied. Besides hardness, abrasion, and scratch resistance as well as certain flexibility, further key requirements are good adhesion, no yellowing and high gloss without haze. Preliminary formulations have been developed with proprietary resin types for hard topcoats for mobile phone coatings, disk players, cameras, and small electronic devices and topcoats for vacuum metallization coatings and cosmetic packaging.

7.6.2.2 UV curing of highly flexible coatings Recently, the question was raised, if “UV-cured and elastomeric coatings” are a contradiction? [90] UV-cured coatings are usually attributed with high cross-link density, high hardness, and limited flexibility. Most of the applications realized today with UV coatings do not ask for flexibilities above 50% elongation. Since urethane chemistry is known to provide high flexibility in textile or leather coatings, Weikard [90] and coworkers have evaluated the potential of UV-cured urethane acrylates in order to provide high flexible coatings. The approach is based on the use of polymeric diols in the molecular weight range of about 2000 g/mol, which are subjected to polyaddition reactions with different diisocyanates and terminated with hydroxyalkyl acrylates. The resulting molecular weights of these resins were in the range of 16,000 g/mol. These resins were then diluted with monofunctional monomers and UV-cured. The viscosities were ranging from 6 to 100 Pa s at 60°C, influenced by the polymeric diol, the diisocyanate used, and the urethane content (hydrogen bridging). The elongation at break could be extended to values from 150% up to 750%; the latter, however, at viscosities of about 36 Pa s at 60 °C (Fig. 7.34). Such coatings could be used on leather or textiles, if they are optimized to also exhibit good resistance and adhesion properties.

7.6.2.3 Coil coatings Coil coating is a technology where “prefinished” sheets of galvanized steel or aluminum are produced, which are stamped, deep drawn, bent, or otherwise formed into the final shape by the processing industry. The coating process (Fig. 7.35) and with it the environmental regulations are shifted to the coil coaters. Four major market segments are served by coil-coated materials. Each market segment is mainly using one or two specific resins types. The market segments are construction (roofing, panels, garage doors), transportation (truck and bus body panels, automotive exterior trim components, gas tanks, engine components), consumer products (appliance “white goods,” such as refrigerators or washing machines, office furniture, shelving, computer components, signs, industrial equipment), and packaging (cans, containers, crowns, barrels, drums). The biggest market in terms of coating usage is the construction sector,

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Fig. 7.34 Structure scheme of elastomeric UV-cured coatings.

Fig. 7.35 Coil coating process.

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followed by consumer products and transportation. The coatings used are either thermoplastic or thermosetting. With existing technology the line speeds are in the range of > 200 m/min, the curing times in the range of 10–60 s, in which the coating must have good leveling properties and exhibit good adhesion to the metal substrate as it is bent, punched, or drawn during fabrication. The coatings are almost all solvent-based. Up to now, water-based, powder, and UV-curable systems have not succeeded to gather a significant market share for a variety of reasons. The water-based systems are difficult to clean when switching from one color to another and increased use of gas to fuel the incinerators reduces economy, whereas powder coatings suffer from the difficulty to obtain the smooth, low coating thicknesses commonly demanded. UV-curable coatings show poor adhesion due to high volume shrinkage. The resin types used shifted in their market prominence from alkyd, vinyl, and epoxies to polyesters. For exterior metal buildings the polyesters were not durable enough, whereby siliconized polyesters and fluorocarbons are rapidly emerging. The basic resin systems, polyesters (60% share), PVC plastisols (30%), PVDF (5%), and others, as well as their performance profile regarding a few key requirements, such as flexibility, surface hardness, metal adhesion, corrosion protection, weathering resistance, and heat resistance have been compared [91]. The high curing speed of the coil coating process and the two-dimensional application should be well suited for UV-curable systems; however, the technical problems, mainly adhesion, flexibility, and through cure issues in pigmented coatings, have to be solved. Despite the fact that a complete shift from the high-temperature curing (up to 230°C) to the low-temperature UV curing should be an important economic driver for change, the use of UV-curable coatings is developed in order to substitute one layer (primer) or introduce a third layer in the conventional two-layer (primer, topcoat) coating assembly. Earlier developments are cationically curing systems based on hydroxyl terminated polyesters and epoxide cross-linkers (Cyracure UVR 6110, Dow). A recent study on the adhesion and flexibility in radiation cured coil coatings evaluated the differences of conventional and radiation curable systems and point out ways to better understand and improve UV-curable formulations for coil coating. It has been the common perception in conventional coil coatings that the solvent helps to improve adhesion due to good wetting characteristics and the high curing temperature helps to relieve stresses introduced by the curing process, both factors being absent in UV-curable systems. However, it has been shown that these factors were not valid in the case of the investigated systems, for instance, cationically UV-cured coating. The authors have identified segregation of components, in both UV curing and thermal curing coatings, which contributed to the level of adhesion. In the cationic system they identified the sulfonium salt photoinitiator to migrate to the metal interface, limiting the adhesion potential of this system. In the thermal curing coating the amino melamine cross-linker was identified at a much higher level at the interface than in the bulk, resulting in this case in the enhancement of the adhesion properties. Thus, the classical adhesion tests are useful to classify on pass/fail criteria, but are improper in defining either the mode of failure or its cause. A closer insight into the chemistry present and acting at the interface is necessary in order to design UV coatings, which

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Fig. 7.36 Coil coating line. Courtesy of Fusion UV.

can overcome the performance problems and ensure durability without suffering adhesion failures. The design of such a UV-curable line (Fig. 7.36) demonstrates that the low space requirements allow such a line to be placed between or in addition to conventional thermal curing ovens in order to introduce another coating layer or replace, for instance, the thermal primer by a UV coating. Further evaluations of UV-curable systems for direct metal use [92–95] and the through cure of pigmented UV-curable coatings [96] have been reported.

7.6.2.4 Adhesives Adhesives can be classified by many criteria, by function (structural, nonstructural), by chemical composition (thermoplastic, thermosetting, elastomeric), by mode of application or reaction (hardening by drying, by cooling from melt, or by chemical reaction, such as moisture, radiation or heat), or by physical form (liquid, paste, powder, film). Chemical (covalent bonds), physical (hydrogen bonding, dipoles, van-der Waals forces), and mechanical (jams, surface fissure) forces are involved in adhesion. Adhesive failure can be caused by different modes, depending on the type of force interacting or by cohesive failure of the adhesive layer. In adhesives, generally, the tack, peel strength, and cohesion cannot be varied independently. With increasing molecular weight, the cohesive strength of the adhesive layer increases and the tack decreases, due to an increase of the glass transition temperature. The peel strength, however, passes through a maximum. At low molecular weights, the tack is high enough to get good adhesion to the substrates, but the cohesive strength is low since the chain links can slide easily from each other and facilitate easy pull apart of the substrates. At high molecular weights, the cohesive strength is

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high; however, the high Tg or even brittleness reduces the adhesion to the substrate significantly (Fig. 7.36). Therefore, the molecular weight of the specific adhesives plays a decisive role, if one likes an adhesive that sticks permanently or one that can be removed again easily. Hence, for every application, the molecular weight has to be controlled carefully. Therefore, adhesives consist of long-chain polymers, often containing functional groups for interaction with the surface. In order to obtain high-molecular-weight systems preformed polymers, such as thermoplastics can be used. However, these systems need solvents to reduce the application viscosity, or thermoset polymers are applied, which start from lower-molecular-weight compounds, and lower viscosity, and are reacted after the application. Due to environmental concerns other alternatives are emerging and UV-curable systems have been focused on (Fig. 7.37). Radiation or UV-curing adhesives have been developed to take advantage of their easy application possibilities, low viscosity, high curing speed, while simultaneously eliminating mixing, heat curing, as well as solvents [97]. The use of UV-curable adhesives is limited to applications where only the surface of a substrate has to be coated

Fig. 7.37 Principle of the cross-linking reaction of AcResin adhesives and effect on adhesive and cohesive forces.

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with the adhesive or where transparent materials are used when the adhesive has to be cured between substrates. Examples of applications where only one side is coated are labels. Here pressuresensitive adhesives (PSAs) are often used, with which the substrates can be joined with very little pressure. PSAs can be applied from solvent solutions, dispersions, or hot melts. They exhibit good flexibility, permanent tackiness, and peel strength. UV-curing has already been used in these applications for 10 years [98]. UV-curable systems can be applied at low viscosities without solvents; however, the conventional UV systems react very fast to very high-molecular-weight cross-linked networks and the molecular weight control is difficult. However, by functionalizing relatively low-molecular-weight polymer molecules with hydrogen abstraction type photoinitiators, such as benzophenone, the viscosity remains at a relatively low level and the molecular weight build-up can be easily controlled by the UV energy density (Fig. 7.38). When these commercially available acResins (BASF) are exposed to large amounts of UV light, large numbers of the photo-reactive groups become active and a large number of cross-linking bonds are created between the polymers. This makes the glue more internally cohesive and thus less adhesive to other surfaces, which means that a label treated in this way is easier to remove without leaving any residues. By contrast, less ultraviolet exposure leads to less bonding between the polymers. If the label is now removed, the polymer chains tend to separate more easily than the bonds between the label and the surface to which it is adhered. The label remains firmly stuck until it tears. Adhesive residues or even large areas of the label remain behind, enabling retailers, for example, to detect when a label has been tampered with.

Fig. 7.38 Chemistry of the cross-linking reaction of AcResin adhesives.

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The application of such UV acrylates for adhesive tapes has been adapted to suitable production equipment in order to achieve coating line speeds of at least 350 m/ min on PET with coating thicknesses between 20 and 80 μm. Recently, water-dispersible UV-curable polyurethane PSAs based on a similar concept containing benzophenone in the backbone have also been described [99]. Structural adhesives have taken over a lot of bonding applications that had formerly been dominated by metal-joining techniques, because they distribute loads over wider surface areas and join assemblies made from dissimilar materials such as plastic, aluminum, or steel. The chemistry used for these adhesives is based on epoxies, silicones, cyano-acrylates, and acrylics. Silicones have the lowest bond strength, but it remains constant over a wide temperature range constant. The most common structural acrylics are acrylated urethanes and also acrylated elastomers. They can be cured with heat activators or with UV light. The advantages of these systems lie in the instantaneous fixture, the bond-on-demand characteristics, induced by the UV cure. They exhibit high peel strength, high toughness, and compared with cyano-acrylates, better moisture and heat resistance. Furthermore, they allow long open times and do not suffer from the handling problems of moisture curable cyano-acrylates. Assembled parts can be handled as soon as the light has been switched off. One of the fastest growing applications for UV-curable adhesives is optical disks and displays. Digital Versatile Disk or Digital Video Disk (DVD) proved to be a massively popular data storage medium, since its introduction in 1997. DVDs are made of one or two sandwiched polycarbonate disks of 8 or 12 cm diameter, like a CD. Storage capacities range from 4.7 GB (DVD-5) to 17 GB (DVD-18), depending on disk type. DVD-9 is a current successful format for recording and replaying films and other video entertainment. After metallizing, the substrates are bonded together with the metallized surfaces on the inside of the sandwich, normally using a UV-cured lacquer. The adhesive bonding layer must be optically transparent, uniform, and of the correct thickness with no bubbles or other defects. The UV curing presents no problem for bonding DVD-5 and DVD-9, since up to DVD-9 one disk is unmetallized and the curing can be done through this disk. Polycarbonate is highly absorbing from 180 to 300 nm, but becoming transparent above 300 nm with transmissions up to 60%. This is sufficient for conventional UV curing. However, DVD-10 and DVD-18 contain two metallized disks (aluminum or silicon) with thicknesses in the range of 50 nm (Fig. 7.39). Only about 0.1% of the UV light passes the metal layer, thus high energy densities have to be applied in order to cure the adhesive. The use of high-energy exposure systems may, however, cause problems with heating and destroying the disk. Special lamps operating with only reflected and filtered light are reported to optimally match the absorption spectrum of the photoinitiators in the transparent region of the polymer and produce minimal heating. In DVD-R (recordable), the recording layer contains a dye. According to strong requirements for cost reduction, pure silver has been introduced as the semitransparent layer instead of silver-gold alloys. Thus, the adhesive must be compatible with various recording dyes, exhibit excellent heat and corrosion resistance of pure silver, and nonyellowing behavior. The corrosion of silver is caused by oxidizing chemicals as well as water. Thus, improved UV adhesives, often urethane acrylates, achieve reduced

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Fig. 7.39 Scheme of DVD disks with different storage capacities based on Al or Si data layers.

permeability of chemicals and water. Free-radical polymerizing UV-curable adhesive are available to meet these demands (e.g., Data Shield 6-005, Borden Chemicals). While DVD (R) uses a red laser to read and write data, a new format uses a blueviolet laser instead, named Blu-ray disk. The benefit of using a blue-violet laser (405 nm) is that it has a shorter wavelength than a red laser (650 nm), which makes it possible to focus the laser spot with even greater precision resulting in data storage capacities of about 25 GB/50 GB for the same disk space. In contrast to a 1.2 mm thick substrate for CD or two 0.6 mm thick substrates (bonded together) for DVD, the Blueray disk will be molded as a 1.1 mm thick substrate. This substrate will then be coated with a 0.1 mm thick protective layer, which must be clean and optically pure. This protective layer will most likely be applied as a scratch-resistant UV-cured lacquer in a spin-coating process (Fig. 7.40), but it could also be applied as a plastic film that is laminated to the Blue-ray substrate.

7.6.2.5 UV inkjet UV-curable inkjet inks are the focus of recent developments in order to achieve faster drying behavior and better durability. The main requirements of inkjet printing are related to a low viscosity, substrate wettability, and absence of solvents (VOC). Liquid UV-curable coatings with solid contents of 100% are a good choice, if the low viscosity of 10–20 m Pa s can be obtained, which is required by the ink formulation to be ejected by Piezo driven inkjet equipment. Low-viscosity resins, such as hyperbranched or dendritic acrylates, are therefore screened in combination with appropriate monomers. Furthermore, water-based UV-curable coatings are an option,

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Fig. 7.40 Scheme of Blue-ray disk with UV protective coating.

since there the viscosity is not a function of the resin molecular weight, but of the solids content. Water would be absorbed by many substrates, such as paper or textiles, and would not need any drying step prior to UV curing. The cure rate of the UV inkjet formulation also has to be adjusted in order to comply with the speed and latitude of printing. Key factors for UV curing of inkjet printing, especially with relevance to the UV exposure process, the determination of the cure “window” without losing key physical properties, such as adhesion, solvent, and scratch resistance have been evaluated recently [100]. Furthermore, evaluating the influences of optimized additives, such as photoinitiators, and pigment selection on the performance of UV inkjet inks has resulted in the development of new raw materials [101] and optimized UV-curable inkjet inks [102].

7.6.2.6 UV systems for dental applications In dental applications for fillings and restorative work, metal alloys have been dominant in the past; however, due to the toxicity of these mercury-containing systems replacements are desired. UV-curable glassy materials mainly based on polymethacrylates have been introduced for such dental applications [103]. The main issue of improvement of such polymer materials is related to the shrinkage behavior and the long-term abrasive stability. By the use of oligomeric methacrylates as model polymers for dental applications, a relatively low volume shrinkage of less than 5% has been obtained, however, due to vitrification the overall conversions of monomers did not exceed 93% and therefore residual monomers may be left. In order to be used in dental applications, the polymerization conditions have to be optimized [104]. In

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order to reduce shrinkage formulations for dental fillings therefore often contain already polymerized PMMA, which is dissolved in dimethacrylates. Furthermore, in order to reduce the abrasion resistance inorganic particles are added.

7.6.2.7 Furniture foil coatings Furniture foil (laminated surface) is a printed and coated paper that is glued onto chip board as decorative surface. Coating of furniture foils is conventionally done with an acid catalyzed water-borne system of melamine resin/emulsion combination. Major drawbacks are high energy consumption for the curing process and emission of formaldehyde. With a radiation curable coating the process is much simplified and more economical (Fig. 7.41).

7.6.3 Film coating instead of painting: An innovative concept In the Daimler Chrysler High Tech Report 2005 [105], it has been published that the film coating technology is on the way to readiness for series production. The roof module of the Mercedes-R-Class has been tested for qualification purposes under Kalahari climate conditions at temperatures up to 90°C. What is film coating and what was special about that roof module? It was the type of the roof module applied. Normally such modules consist of shaped steel or plastic body parts, which are spray-coated and thermally cured. With the new roof module, the production process is reversed. A thin (engineering) plastic substrate film is coated with an uncured, but solid paint film and shielded from contamination with a protective foil. This compound is then molded into the shape of the body part, subsequently the top coat cured with UV radiation and the substrate film reinforced with the same engineering plastic material used for the substrate (Fig. 7.42).

Fig. 7.41 Scheme of furniture foil production by thermal curing (AC) and UV/EB cured coatings.

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Fig. 7.42 Scheme of the foil coating technology.

Fig. 7.43 Foil coating process.

The production of this “painted film” is outlined in Fig. 7.43. It has been done in a sophisticated coating plant at the Daimler Chrysler Research Center in Ulm, where a 65cm-wide thermoplastic film runs from a roll into a 16-m-long machine, where it is coated subsequently with the base coat that provides color and then with a “thermoplastic” UV clear coat, which is not cured yet, but physically dried to a solid film, and finally

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covered with a transparent protective foil. Afterward, the “painted film” can be cut into sheets, stacked and processed further elsewhere. On the way to becoming a body part, the sheets of the painted film are shaped in a deep-drawing tool, where the film is heated and deep drawn with the help of vacuum, like the processes well known in the plastics industry, for example, yogurt cups are produced in the same way. This step gives it the shape of the desired component that of a roof element, for instance, or a vehicle bumper or piece of side rail trims. In the next step, the top coat of the film is hardened using ultraviolet light (UV light), resulting in a top performance clear coat with a highly durable and scratch-resistant surface. Since the 3D shape that results already exhibits the contours of the component to be produced, it is not stiff enough to be used as a body part. That is why the molded and cured paint films then go into special injection, compression, or foam molds made of steel, where they are reinforced with engineering plastic by back injection or stabilized with (polyurethane) foams. Foam backing is a process well suited to flat parts such as roof modules and hatch doors, because the elements produced in this fashion are heat-resistant. Back injection is a typical process for robust addon parts such as side rail trim or bumpers. At this stage, the plastic parts coated with paint films can be delivered to the appropriate conveyor lines of the automotive plants for immediate assembly and without further postprocessing. The advantages of this technology open up new manufacturing processes at the automakers and suppliers, offering enormous economic and ecological advantages. Ecological advantages of paint films are primarily due to the fact that they dispense with the usual liquid painting, since no “overspray” occurs, that must be recycled or disposed in environmentally sound ways. Liquid painting is furthermore costly. Painting lines are among the most expensive components at plants where cars and automotive parts are manufactured. The result is that the painting costs of a plastic part can make up 30%–50% of its total cost. Aside from the cost savings and the variety of components that will be possible, the paint films offer also other potential advantage, for instance, the integration of further functions, such as embedding electrical lines, antennas, and even sensors into the paint film elements during component production, which would not be possible for metal substrates. The paint films can be used for even the difficult metallic hues of the body color palette, that there are not any discernible differences between these and components painted in the conventional manner. The color matching to the spray-coated body parts has also been achieved by means of a specially developed paint application technique during the film-coating process, and many tests and improvements have meanwhile led to approval for the paint film system. The key to the success of this film technology is the use of UV-curable formulations as top clear coat, which can be applied as a solid thermoplastic film, to be molded into the desired shape, and cured to a highly cross-linked coating, exhibiting the desired performance requirements, such as high scratch and chemical resistance.

References [1] K. Chaochanchaikul, V. Rosarpitak, N. Sombatsompop, Photodegradation profiles of PVC compound and wood/PVC composites under UV weathering, Express Polym. Lett. 7 (2012) 146–160, https://doi.org/10.3144/expresspolymlett.2013.14.

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[2] A. Piegari, P. Polato, Multilayer coatings on glass for painting protection and optimized color rendering, Appl. Optics 41 (2002) 3319–3326, https://doi.org/10.1364/ AO.41.003319. [3] A. Piegari, P. Polato, Wideband optical coatings for protecting artwork from ultraviolet and infrared radiation damage, J. Opt. A Pure Appl. Opt. 5 (2003) S152–S156, https://doi. org/10.1088/1464-4258/5/5/357. [4] G. Eggset, G. Volden, H. Krokan, UV-induced DNA damage and its repair in human skin in vivo studied by sensitive immunohistochemical methods, Carcinogenesis 4 (1983) 745–750, https://doi.org/10.1093/carcin/4.6.745. [5] A. Kricker, B.K. Armstrong, D.R. English, Sun exposure and nonmelanolytic skin cancer, Cancer Causes Control 5 (1994) 367–392, https://doi.org/10.1007/BF01804988. [6] R. Rodil, M. Moeder, R. Altenburger, M. Schmitt-Jansen, Photostability and phytotoxicity of selected sunscreen agents and their degradation mixtures in water, Anal. Bioanal. Chem. 395 (2009) 1513–1524, https://doi.org/10.1007/s00216-009-3113-1. [7] J.L. Gerlock, W. Tang, M.A. Dearth, T.J. Korniski, Reaction of benzotriazole ultraviolet light absorbers with free radicals, Polym. Degrad. Stab. 48 (1995) 121–130, https://doi. org/10.1016/0141-3910(95)00027-J. [8] J.E. Pickett, Permanence of UV absorbers in plastics and coatings, in: J.W. Martin, D.R. Bauer (Eds.), Service Life Prediction: Methodology and Metrologies, ACS, Washington, 2001, pp. 250–265, https://doi.org/10.1021/bk-2002-0805.ch012. [9] E. Chatelain, B. Gabard, Photostabilization of Butyl methoxydibenzoylmethane (Avobenzone) and Ethylhexyl methoxycinnamate by Bis-ethylhexyloxyphenol methoxyphenyl triazine (Tinosorb S), a New UV Broadband Filter, Photochem. Photobiol. 74 (3) (2001) 401–406. [10] R.K. Chaudhuri, et al., Design of a photostabilizer having built-in antioxidant functionality and its utility in obtaining broad-spectrum sunscreen formulations, Photochem. Photobiol. 82 (3) (2006) 823–828. [11] K.M. Hanson, E. Gratton, C.J. Bardeen, Sunscreen enhancement of UV-induced reactive oxygen species in the skin, Free Radic. Biol. Med. 41 (8) (2006) 1205–1212. [12] M.E. Burnett, S.Q. Wang, Current sunscreen controversies: a critical review, Photodermatol. Photoimmunol. Photomed. 27 (2011) 67. [13] http://www.uspdqi.org/pubs/monographs/sunscreen-agents.pdf. [14] S.B. Levy, Sunscreens and photoprotection, Dermatol. Ther. 4 (1997) 59–71. [15] http://www.merck.de/servlet/PB/menu/1254590/index.html. [16] The Skin Cancer Foundation’s Guide to Sunscreens, Skin Cancer Foundation, 2011. [17] Z.L. Wang, Zinc oxide nanostructures: growth, properties and applications, J. Phys. Condens. Matter 16 (25) (2004) R829. [18] J.H. Xin, W.A. Daoud, Y.Y. Kong, A new approach to UV blocking treatment for cotton fabrics, Text. Res. J. 74 (2004) 97–100. [19] W.A. Daoud, J.H. Xin, Low temperature sol-gel processed photocatalytic titania coating, J. Sol-Gel Sci. Technol. 29 (1) (2004) 25–29. [20] B. Fei, Z. Deng, J.H. Xin, Y. Zhang, G. Pang, Room temperature synthesis of rutile nanorods and their applications on cloth, Nanotechnology 17 (2006) 1927–1931. [21] W.A. Daoud, J.H. Xin, Y.H. Zhang, K. Qi, Surface characterization of thin titania films prepared at low temperatures, J. Non Cryst. Solids 351 (2005) 1486–1490. [22] R.H. Wang, J.H. Xin, X.M. Tao, UV-blocking property of dumbbell-shaped ZnO crystallites on cotton fabrics, Inorg. Chem. 44 (2005) 3926–3930. [23] Q. Yu, A. Shen, Anti-ultraviolet treatment for cotton fabrics by dyeing and finishing in one bath and two steps, J. Fiber Bioeng. Inform. 1 (1) (2008) 65–72.

240

Functional Materials from Carbon, Inorganic, and Organic Sources

[24] J. So´jka-Ledakowicz, J. Lewartowska, M. Kudzin, M. Leonowicz, T. Jesionowski, K. Siwi
nska-Stefa
nska, A. Krysztafkiewicz, Functionalization of textile materials by alkoxysilane-grafted titanium dioxide, J. Mater. Sci. 44 (2009) 3852–3860. [25] J. Sawaji, T. Yoshikawa, Quantitative evaluation of antifungal activity of metallic oxide powders (MgO, CaO and ZnO) by an indirect conductimetric assay, J. Appl. Microbiol. 96 (4) (2004) 803–809. [26] F. Zhang, J. Yang, Preparation of nano-ZnO and its application to the textile on antistatic finishing, Int. J. Chem. 1 (1) (2009) 18–22. [27] R.H. Wang, J.H. Xin, X.M. Tao, W.A. Daoud, ZnO nanorods grown on cotton fabrics at low temperature, Chem. Phys. Lett. 398 (2004) 250–255. [28] A. Yadav, V. Peasad, A.A. Kathe, S. Raj, D. Yadav, C. Sundaramoorthy, N. Vigneshwaran, Functional finishing in cotton fabrics using zinc oxide nanoparticles, Bull. Mater. Sci. 29 (6) (2006) 641–645. [29] N. Vigneshwaran, S. Kumar, A.A. Kathe, P.V. Varadarajan, V. Prasad, Functional finishing of cotton fabrics using zinc oxide–soluble starch nanocomposites, Nanotechnology 17 (2006) 5087–5095. [30] H. Lu, B. Fei, J.H. Xin, R. Wang, L. Li, Fabrication of UV-blocking nanohybrid coating via miniemulsion polymerization, J. Colloid Interface Sci. 300 (2006) 111–116. [31] A. Becheri, M. D€urr, P. Lo Nostro, P. Baglioni, Synthesis and characterization of zinc oxide nanoparticles: application to textiles as UV-absorbers, J. Nanopart. Res. 10 (2008) 679–689. [32] Z. Mao, Q. Shi, L. Zhang, H. Cao, The formation and UV-blocking property of needleshaped ZnO nanorod on cotton fabric, Thin Solid Films 517 (2009) 2681–2686. [33] A.M. Grancari
c, A. Tarbuk, I. Kovacek, Nanoparticles of activated natural zeolite on textiles for protection and therapy, Chem. Ind. Chem. Eng. Q. 15 (2009) (in press). [34] M. Moehrle, Outdoor sports and skin cancer, Clin. Dermatol. 26 (1) (2008) 12–15. [35] M.L. Williams, R. Pennella, Melanoma, melanocytic nevi, and other melanoma risk factors in children, J. Pediatr. 124 (6) (1994) 833–845. [36] A. Wysong, H. Gladstone, D. Kim, et al., Sunscreen use in NCAA collegiate athletes: identifying targets for interventions and barriers to use, Prev. Med. 55 (5) (2012) 493–496. [37] A.J. Bruce, D.G. Brodland, Overview of skin cancer detection and prevention for the primary care physician, Mayo Clin. Proc. 75 (5) (2000) 491–500. [38] M. Moehrle, W. Koehle, K. Dietz, et al., Reduction of minimal erythema dose by sweating, Photodermatol. Photoimmunol. Photomed. 16 (6) (2000) 260–262. [39] J. Boer, A.A. Schothorst, B. Boom, et al., Influence of water and salt solutions on UVB irradiation of normal skin and psoriasis, Arch. Dermatol. Res. 273 (3–4) (1982) 247–259. [40] D.S. Rigel, E.G. Rigel, A.C. Rigel, Effects of altitude and latitude on ambient UVB radiation, J. Am. Acad. Dermatol. 40 (1) (1999) 114–116. [41] M. Blumthaler, Solar UV measurements, in: M. Tevini (Ed.), UV-B Radiation and Ozone Depletion. Effects on Humans, Animals, Plants, Microorganisms, and Materials, Lewis, Boca Raton, 1993, pp. 71–94. [42] R. Chadysiene, A. Girgzdys, Ultraviolet radiation albedo of natural surfaces, J. Environ. Eng. Landsc. Manag. 16 (2) (2008) 83–88. [43] E.G. Rigel, M.G. Lebwohl, A.C. Rigel, et al., Ultraviolet radiation in alpine skiing: magnitude of exposure and importance of regular protection, Arch. Dermatol. 139 (1) (2003) 60–62. [44] M. Moehrle, B. Dennenmoser, C. Garbe, Continuous long-term monitoring of UV radiation in professional mountain guides reveals extremely high exposure, Int. J. Cancer 103 (6) (2003) 775–778.

Exploration of UV absorbing functional materials and their advanced applications

241

[45] M.W. Wright, S.T. Wright, R.F. Wagner, Mechanisms of sunscreen failure, J. Am. Acad. Dermatol. 44 (5) (2001) 781–784. [46] L.E. Rhodes, B.L. Diffey, Quantitative assessment of sunscreen application technique by in vivo fluorescence spectroscopy, J. Soc. Cosmet. Chem. 12 (1998) 54–61. [47] A.M. Dattilo, F. Decembrini, L. Bracchini, et al., Penetration of solar radiation into the waters of Messina Strait (Italy), Ann. Chim. 95 (3–4) (2005) 177–184. [48] E.S. Hamant, B.B. Adams, Sunscreen use among collegiate athletes, J. Am. Acad. Dermatol. 53 (2) (2005) 237–241. [49] R.M. Ellis, M.R. Mohr, S.H. Indika, et al., Sunscreen use in student athletes, J. Am. Acad. Dermatol. 67 (1) (2012) 159–160. [50] J.A. Laffargue, J. Merediz, M.M. Bujan, et al., Sun protection questionnaire in Buenos Aires adolescent athletes, Arch. Argent. Pediatr. 109 (1) (2011) 30–35. [51] C.M. Ambros-Rudolph, R. Hofmann-Wellenhof, E. Richtig, et al., Malignant melanoma in marathon runners, Arch. Dermatol. 142 (11) (2006) 1471–1474. [52] N.C. Berndt, D.L. O’Riordan, E. Winkler, et al., Social cognitive correlates of young adult sport competitors’ sunscreen use, Health Educ. Behav. 38 (1) (2011) 6–14. [53] D.B. Buller, P.A. Andersen, B.J. Walkosz, et al., Compliance with sunscreen advice in a survey of adults engaged in outdoor winter recreation at high-elevation ski areas, J. Am. Acad. Dermatol. 66 (1) (2012) 63–70. [54] P.H. Cohen, H. Tsai, J.C. Puffer, Sun-protective behavior among high-school and collegiate athletes in Los Angeles, CA, Clin. J. Sport Med. 16 (3) (2006) 253–260. [55] World Triathlon Corporation, Ironman Triathlon World Championship: Contestant Information Guide, HI, Kailua-Kona, 1999, p. 3. [56] S. Lawler, K. Spathonis, E. Eakin, et al., Sun exposure and sun protection behaviours among young adult sport competitors, Aust. N. Z. J. Public Health 31 (3) (2007) 230–234. [57] B. von Blankenhagen, D. Tonova, J. Ullman, Appl. Optics V41 (16) (2002) 3137. [58] A. Tauber, T. Scherzer, R. Mehnert, UV curing of aqueous polyurethane acrylate dispersions. A comparative study by real-time FTIR spectroscopy and pilot scale curing, J. Coat. Technol. 72 (911) (2000) 51–60, https://doi.org/10.1007/BF02720525. [59] The table of essential elements for the New Millenium, Chem. Eng. News 79 (45) (2001), https://doi.org/10.1021/cen-v079n045.p006. Cover Story. [60] J. Vandendriessche, B. Bontinck, K. Buysens, Surf. Coat. Int. A (2004/2005) 226–229. [61] D. Harbournc, Overview and update of the radiation curing market and technology in North America, in: RadTech Asia 2005, Proceedings, 2005, pp. 3–22. [62] V. Luo, Perspectives on radiation curing industry in Asia, in: RadTech Asia 2005, Proceedings, 2005, pp. 24–25. [63] P. Snowwhite, Performance Requirements Overview for Automotive Coatings, RadTech Report, RadTech North America, 2001, pp. 43–45. [64] O. Starzmann, UV applications in the automotive industry, in: Proceedings, RadTech Asia 2005, Shanghai, China, 2005, pp. 271–282. [65] U. Poth, Topcoats for the automotive industry, in: G. Fettis (Ed.), Automotive Paints and Coatings, VCH Verlagsgesellschaft, Weinheim, 1995, pp. 119–147. Chapter 5. [66] T. Fey, J. Muhle, UV-DualCure systems for automotive applications, in: RadTech Europe2003, Conference Proceedings, vol. II, 2003, pp. 739–748. [67] F.-L. Siever, 21st International Conference on Automobile Body Finishing, SURCAR Conference Book, 26–27, Cannes, 2003. [68] K. Joesel, Current and Future Automotive Applications for UV Curing Technology. http://www.radtech.org/industry/Automotives/current_future.pdf. [69] D. Braun, Shortening the gap between automotive refinish and OEM coatings, in: RadTech Europe 2003, Conference Proceedings, vol. II, 2003, pp. 709–712.

242

Functional Materials from Carbon, Inorganic, and Organic Sources

[70] C.A. Wall, B.M. Richards, C. Bradlee, The ecological and economical benefits of UV curing technology, in: RadTech (USA) Report, 2004, pp. 25–29. [71] P. Saling, A. Kicherer, B. Dittrich-Kraemer, R. Wittlinger, W. Zombik, I. Schmidt, W. Schrott, S. Schmidt, Int. J. Life Cycle Assess. 7 (4) (2002) 203. [72] The methodology was created in partnership with an external consultant, and has been further developed by BASF. BASF’s eco-efficiency group has conducted over 220 analyses. [73] (a) J. Muhle, T. Fey, M. Wulf, Farbe Lack 10 (2003) 18. (b) K. Maag, H. L€ offler, W. Lenhard, Eur. Patent Appl. EP 1032474;. (c) K. Maag, H. L€ offler, W. Lenhard, Progr. Org. Coat. 40 (2000) 93. [74] G. Meichsner, K. Vogg, Mehr Aufmerksamkeit f€ ur einen Strahlertyp: Die UV-Blitzlampe Untersuchungen zur Lackh€artung, Farbe Lack 105 (8) (1999) 8–9. [75] B. Richards, RadTech Europe, Conference Proceedings, 2003, pp. 773–778. [76] H. Bach, C. Gambino, L. Galeza, M.J. Dvorchak, T. F€acke, C. Detrembleur, M. Ehlers, H. Mundstock, J. Schmitz, J. Weikard, UV refinish primer and clearcoat, in: RadTech Europe, Conference Proceedings, 2003, pp. 731–738. [77] (a) N. Dogan, H. Klinkenberg, L. Reinerie, D. Ruigrok, P. Wijnands, K. Dietliker, K. Misteli, T. Jung, K. Studer, P. Contich, J. Benkhoff, E. Sitzmann, Fast UV-A curable clearcoat, in: RadTech Europe 2005, Conference Proceedings, vol. I, 2005, pp. 203–214. (b) Studer, K., Jung, T., Dietliker, K., Benkhoff, J., Sitzmann, E. and Dogan, N., RadTech 2006, Conference Proceedings, Chikago, IL. 2006, on CD. [78] K. Dietliker, K. Misteli, T. Jung, K. Studer, P. Contich, J. Benkhoff, E. Sitzmann, A novel photo latent base catalyst for UV-A clearcoat applications, in: RadTech Europe 2005, Conference Proceedings, vol. II, 2005, pp. 473–478. [79] Y. Hara, T. Mori, T. Fujitani, Relationship between viscoelasticity and scratch morphology of coating films, Prog. Org. Coat. 40 (1–4) (2000) 39–47. [80] W. Shen, et al., Use of a scanning probe microscope to measure marring mechanisms and microhardness of crosslinked coatings, J. Coat. Technol. 69 (873) (1997) 123–135. [81] L. Lin, G.S. Blackman, R.R. Matheson, A new approach to characterize scratch and mar resistance of automotive coatings, Prog. Org. Coat. 40 (1–4) (2000) 85–91. [82] (a) A. Poppe, M. Kutschera, COSI Conference, 2005. (b) M. Kutschera, R. Sander, P. Hermann, U. Weckenmann, A. Poppe, J. Coat. Technol. Res. 3 (2) (2006). [83] R. Nothhelfer-Richter, E. Klinke, C.D. Eisenbach, Evaluation of the scratch resistance with nano-and multiple scratching methods, Macromol. Symp. 187 (1) (2002) 853– 860. Weinheim: WILEY-VCH Verlag. [84] T. Fey, UV dualcure systems for automotive applications, in: RadTech Europe 2003, Conference Proceedings, vol. II, 2003, pp. 741–747. [85] R. Nothhelfer-Richter, E. Klinke, C.D. Eisenbach, Best€andig gegen Kratzer: NanoScratch-Test unter trockenen und nassen Bedingungen, Farbe Lack 111 (12) (2005) 42–46. [86] R. Schwalm, UV-curable automobiles paints draw neares. Effect of crosslinking of UVcurable coatings on scratch resistance, hardness, and flexibility, Farbe Lack 106 (4) (2000) 58–69. [87] L.R.G. Treloar, The Physics of Rubber Elasticity, Clarendon Press, Oxford, 1940. [88] E. Frigge, Scratch resistant finishes for cars [Laques resistant a la rayure pour automobiles], Farbe Lack 106 (7) (2000) 78–80. [89] E. Muzeau, P. Yan, New developments of UV resins and formulations for plastic hard topcoats, in: Proceedings, RadTech Asia 2005, Shanghai, China, 2005, pp. 331–338.

Exploration of UV absorbing functional materials and their advanced applications

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[90] J. Weikard, W. Fischer, E. L€uhmann, D. Rappen, UV cured and elastomeric coatings—a contradiction? in: RadTech USA, e/5 2004, Technical Proceedings, 2004. on CD. [91] M. Schmitthenner, Basic resins for coil coatings, Eur. Coat. J. 9 (1998) 618–625. [92] M. Heylen, New developments in UV resins for metal coatings, in: RadTech Europe 2005, Conference Proceedings, vol. I, 2005, pp. 181–184. [93] J. Weikard, Urethane acrylates on metal substrates, in: RadTech Europe’05, Conference Proceedings, vol. I, 2005, pp. 187–192. [94] J. Amigo, Innovative developments in UV pigmented low viscosity (100% solids) for the automotive and metal coatings industry, in: RadTech Europe 2005, Conference Proceedings, vol. I, 2005, pp. 195–200. [95] N. Pietschmann, UV curable metal coatings—special possibilities and problems, in: RadTech Europe 2005, Conference Proceedings, vol. I, 2005, pp. 523–530. [96] T. Frey, UV meets colour: a numerical simulation of the through-cure of pigmented UV coatings, in: RadTech Europe 2005, Conference Proceedings, vol. I, 2005, pp. 515–520. [97] C. Bachmann, Expanding capabilities with UV/visible light curing adhesives, Adhes. Age 38 (1995) 14. [98] R. M€uller, Ten years of UV crosslinking of hotmelt PSA—a success story, in: RadTech Europe 2005, Conference Proceedings, vol. I, 2005, pp. 597–605. [99] Z. Czech, M. Kocmierowska, Synthesis of novel photoreactive water dispersible polyurethane pressure-sensitive adhesives. Part I: introduction and background, Coating 38 (11) (2005) 475–478. [100] R.W. Stowe, Key factors of UV curing of ink jet printing, IS&T’s NIP19, in: International Conference on Digital Printing Technologies, Final Programm and Proceedings, New Orleans, LA, US, Sept. 28–Oct. 3, 2003, 2003, pp. 186–189. [101] A. Fuchs, M. Richert, S. Biry, S. Villeneuve, T. Bolle, IS&T’s NIP19: International Conference on Digital Printing Technologies, Final Programm and Proceedings, New Orleans, LA, US, Sept. 28–Oct. 3, 2003, 2003, pp. 268–271. [102] S. Edison, Optimization of UV curable inkjet ink properties for jet stability, in: RadTech 2006, Conference Proceedings, Chicago, IL, 2006. on CD. [103] K. Miyazaki, T. Horibe, Polymerization of multifunctional methacrylates and acrylates Biomed, J. Mater. Res. 22 (1988) 1011–1022, https://doi.org/10.1002/jbm.820221105. [104] M.H. Bland, N.A. Peppas, Photopolymerized multifunctional (meth) acrylates as model polymers for dental applications, Biomaterials 17 (11) (1996) 1109–1114. [105] DaimlerChrysler, “Am Anfang ist der Lack”, DaimlerChrysler HighTech Report, vol. 1, 2005, pp. 10–15.

Interface engineering in oxide heterostructures for novel magnetic and electronic properties

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R.G. Tanguturia and Amol Nandeb Department of Materials Science and Engineering, Hubei University, Wuhan, Hubei, PR China, bDepartment of Physics, Guru Nanak College of Science, Ballarpur, Chandrapur, Maharashtra, India

a

8.1

Magnetism in oxide materials

Magnetism in materials is a diverse research field in both aspects of fundamental scientific discovery as well as inventing cutting-edge technology. Magnetic materials exhibit solitary magnetic phenomena like ferromagnetism, antiferromagnetism, and exchange bias. These phenomena arise from the interaction between spin sites and also accompanying long-range ordering owing to the competition between itinerant charge entities. Though the itinerant electrons favor the cause of ferromagnetism in 3d transition metals, the electrons are treated as localized magnetic moments and fixed firmly in the reference frame of the magnetic structure [1]. Pauli’s exclusion principle and Stoner’s criterion for spin band structures rule out the magnetism caused by the motion of electrons. Compared to 3d transition metals (TMs), transition metal oxide samples are shown to have a fundamentally distinct magnetic response from the metallic systems and other systems. In the oxides, the d-orbitals of TM are encountered by a large crystal field produced by neighboring ligands so that the angular momentum of d-orbitals quenches. Subsequently, a strong intertwining of 3d orbitals and 2p-states of ligand atoms share the electrons to form covalent bonds. Therefore, the electron transfer has taken place through nonmagnetic ligand (oxygen) ions in which the ligands are responsible for unveiling an indirect exchange phenomenon like super-exchange, double exchange, RKKY interaction, etc. These magnetic phenomena have been stimulated by the short-range interaction between the degrees of freedom, however, they alter the functionalities at interfaces.

8.2

Exchange interaction

8.2.1 Super and double exchange The notion of an indirect exchange mechanism via nonmagnetic anions was theoretically developed by Anderson in the year 1950 that explicitly explained the magnetic phenomenon in oxide transition metals [2]. Super exchange is a magnetic Functional Materials from Carbon, Inorganic, and Organic Sources. https://doi.org/10.1016/B978-0-323-85788-8.00005-7 Copyright © 2023 Elsevier Ltd. All rights reserved.

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phenomenon observed in the oxide samples where the 3d-orbitals of transition metals (TMs) are strongly hybridized with the 2p-orbital of nonmagnetic ions (oxygen). Such hybridization creates a network chain of TM-O-TM, which assists to transfer the electrons simultaneously. As a result, the exchange of valence states between the TM can unveil an antiferromagnetic ordering in the oxides. In the successive year, Zener confirmed the magneto-conductive properties of these mixed-valence oxides and the hopping mechanism of electrons by the double exchange phenomenon [3]. Thus, high energetic states of identical cations emerge ferromagnetic ordering without flipping of spins while the electron hopping process takes place.

8.2.2 RKKY interaction The aforementioned exchange phenomena do not explain the underlying magnetic mechanism of rare earth elements with partially filled 4f-shell and magnetic multilayers separated by a nonmagnetic spacer. Indeed, another indirect magnetic mechanism, namely RKKY interaction, prevails over the earlier exchange mechanisms, which suggests the coupling strength between magnetic moments has been raised by the electrons’ motion rather than the electron hopping. The exchange energy between two magnetic ions via a conducting electron cloud can be represented by an effective Hamiltonian of Heisenberg-like interaction: Hij ¼ Jij(R) Si Sj. The overall energy and the magnetic interaction are governed by the coupling constant which is the average distance between two magnetic ions. The interactive term (Jij) of two either magnetic nucleus ions of f-elements or magnetic layers separated by nonmagnetic metallic spacer can be expressed as follows [4]: " # j cos 2RkF sin 2RkF  Jij ðRÞ ¼ 9π EF ½2kF R3 ½2kF R4 Here, R corresponds to the site separation from the centric distance of nonoverlapping atomic nuclei, kF and EF are attributed to the wave vector and energy of the Fermi surface, respectively. The value of R goes high therein long range of RKKY interaction can be established in rare earth metals. On the other hand, the coupling constant of multilayers is also wave vector dependent which is comparable to the nonmagnetic layer thickness. The magnitude of the wave vector is the order of spacer thickness, and oscillatory coupling constants are found within well-defined values while altering the sign. In contrast, the values are diminished with an irregular oscillation behavior once the thickness exceeds the limit. The interaction is either ferromagnetic or antiferromagnetic depending upon the sign of the magnetic coupling constant. The first experimental evidence for existing the RKKY interaction was given by Yafet in a cohesive superlattice of magnetized Gd layers separated by Y spacer. The primary importance of the interaction is to explore the magnetic response in magnetically diluted alloys like CuMn, ZnMn, CuFe, etc. The magnetic coupling constant of these systems is completely dependent on the strength of magnetic impurities. The strength (v0) of magnetic impurities (like Mn) is interacting with the conducting

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non-magnetic elements (like Cu, Zn) given by the impurity potential v ¼ v0 cosr32kF r . Moreover, the typical concentration (n) of magnetic impurities in the system decides the global magnetic response. Furthermore, the characteristic energy possessed by the solitary impurity is kBT, here T represents the contribution from low temperature, due to the contribution of Kondo lattice. At very low concentrations, the magnetic impurity acts as an individual magnetic entity, whereas the RKKY interaction is more dominant among the impurities at high concentrations. Once the condition, i.e., kBT  nv0 satisfies, both characteristics of the characteristic energy and the overall magnetic potential of impurities will be the measurable quantities through the experiments. The nominal concentration and potential strength of magnetic impurities are determined experimentally by measuring the ratio of residual resistance and magnetization at high magnetic fields. For instance, more than four decades before, Smith implemented the experimental approach to correlate the theoretical potential strength of the RKKY interaction for CuMn alloy [5]. To determine v0, they followed the Larkin method for quantifying the saturation magnetization from M-H1 curves while establishing for high magnetic field condition: μBH ≫ kBT and nv0. First, the magnetic measurements helped to determine the value of n from the Curie constant C ¼ ng2μ2BS(S + 1)/3kB and the saturation magnetization Msat ¼ ngμBS. Further, the value of v0was estimated for the approach to saturation magnetization (Msat) from magnetization M ¼ ngμBS[1  2(2S + 1)nv0/ 3gμBH]. Therefore, the potential strength of Mn in Cu was determined v0 ¼ 7.5  1037 erg cm3. Based on these formulas, the potential strength of other diluted magnetic alloys like AuMn, AgMn, and ZnMn was estimated to be 2.4, 3.5, and 20  1037 erg cm3, respectively [6]. In the case of multilayer heterostructures like Co/Cu/Ni/Cu (001) multilayers, the magnetic coupling between Co and Ni is governed by the Cu spacer thickness. Thus, the thickness of the spacer layer mediates the two magnetic ordering systems with an oscillatory magnetic coupling, which signifies RKKY-type interaction in multilayers.

8.3

RKKY interaction in diluted magnetic oxide thin films

Unlike diluted magnetic alloys, some of the oxide thin films reveal RKKY interaction even at a low level of magnetic impurity concentrations. The active concentration of impurities calculated at 10 K for the Zn0.98Mn0.02O thin films was 6.31 1019 cm3, which was much lower than that of the bulk counterpart (8.4  1020 cm3) [7]. Moreover, the estimated RKKY potential was 9  1037 erg cm3. The overall thermal energy of Zn0.98Mn0.02O thin films determined at 10 K was 0.863 meV, much higher than nv0 ¼ 0.02 meV. This signifies that the availability of an adequate impurity concentration allowed the RKKY-FM interaction even at low temperatures. Such ferromagnetic behavior in oxides could be expected due to either Jij(R) < 0 when kFR ! 0 or low carrier density (nc) relevant to magnetic impurity concentration (ni), i.e., nc ≪ ni [8]. Thus, the role of carrier density (nc) in FM ordering needs to be addressed in terms of Curie temperature (TC). Also, the TC of this ferromagnetic behavior is utterly dependent upon the response of magnetic impurities interacting with the carrier

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density. In non-degeneracy carrier states (a simple classical approach to find the Brillouin function), the conducting spins encounter an effective magnetic field induced by the localized moments and vice versa. Then the resultant effective magnetic field triggered by impurity spin is close to the magnetic transition associated with a critical temperature. Therefore, the magnetization is dominated by localized moments rather than conductive spins. Recent developments in diluted magnetic oxides and their thin films, Sharma et al. reported the first experimental observation of ferromagnetism above room temperature in both bulk and thin films of Mn (60 mTorr, below 40 mTorr, SRO films exhibit tetragonal structure in out-of-plane orientation [36]. Such structural phases are responsible to change in the magnetic moments of films from in-plane to out-of-plane. Like other oxide films, the magnetic sensitivity of SRO films is also dependent on the growth conditions and low thicknesses. Similar structural phase transition has appeared within the ferromagnetic region of SRO/STO superlattices. SRO/ STO revealed a concomitant structural phase transition that has taken place from a highly symmetric phase of P4/mbm to the reduced symmetric phase of P21/c in accordance with the report given by Ref. [37]. SRO/STO SLs exhibit a magnetic transition from ferromagnetic metal to antiferromagnetic insulator while varying the thickness of the SRO layer [37]. Besides, there is a discrepancy found in the occupancy around the vicinity of structural transition, nevertheless, the occupation of the dzx and dyz states of Ru ions are the same in the relaxed strain case. The electronic structure of Ru-t2g orbitals for the SLs of 1 and 2 u.c. thicknesses disclosed a bandgap between dxy and dzx/yz originated by crystal field splitting (CFS), but the gap was apparently closed with spin polarized states for SL of 3 u.c. [35]. Such dimensionality-dependent electronics cause the electron’s motion to restrain in highly localized states rather than mobilized states. On the other side, by a means of DFT calculations, Liebsch et al. proposed an effective approach to explaining metal-insulator transition with the electron occupancy of d-orbitals by considering the relativistic coulomb energy [38]. In the absence of CFS, the low value of columbic energy encourages to transfer the charges from dzx/yz states to the dxy states. As a consequence, both the occupancy of dxy state and gain of the columbic energy have enhanced with reducing of intra t2g screening of U. Moreover, the electronic states of Ru-t2g orbitals are involved in the occupation of the electrons at first in the levels of dxy and further the subsequent filling of dzx/yz states [38].

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However, the occupancy of the Ru-t2g orbitals is independent on band shape but effectively filled the states of dxy due to lowering energy than the dzx/yz states. In addition to the magnetic transition, DFT calculations for the SLs support the dimensionaldependent magnetic transition (from AFMI to FMM) along with a distinguishable electronic phase transition. SrRuO3/SrIrO3: Unlike 3d and 4d transition metals, 5d transition metals exhibit strong spin-orbit coupling those favors enhancing the perpendicular magnetic anisotropy and Dzyalonshinskii-Moriya interaction (DMI). SOC is an essential feature found in 5d transition metals, which is insignificant in 3d transition metals, and plays an important contribution in determining novel physical phenomena such as superconductivity, weyl semimetals, quantum Hall effect, etc. [39]. A typical example of such physical phenomena found in 5d transition metals is iridates where the nonnegligible spin-orbit coupling has often been considered in the competition of multiple physical interactions. Strontium iridate is indeed one of the potential compounds that preserves strong SOC to modulate the electron spin degrees and proper carrier conduction for oxide spintronic applications. However, unlike other iridates, SrIrO3 (SIO) is a paramagnetic semimetal with a larger lattice constant than that of ferromagnetic insulator Sr2IrO4. Thus, several engineering processes are employed to construct the artificial structures of ultra-thin film, superlattice, etc., to induce the magnetic response in SIO [40–42]. In the superlattice of [(SIO)m/(STO)], magnetically ordering parameters such as ferromagnetic transition, Hall coefficient are stimulated by varying the thicknesses of SIO nearly unit cell dimensions where the individual bulk counterparts are in nonmagnetic behavior [43]. Moreover, these magnetic characteristics result from weak ferromagnetic moments identified in the direction of IrO2 plane which could be originated from the in-plane rotations of IrO6 octahedra accompanying DMI. Matsuno et al. [44] investigated ferromagnetic layer thickness-dependent magnetic interaction of (SRO)m/(SIO)2 bilayer system, where m is the thickness of SRO. In such a system, the magnetic response is triggered by interfacial DMI to form the skyrmion phase owing to strong SOC and broken symmetry in the SIO layer. In contrast, the magnetic behavior of SRO/SIO superlattice showed a spin glass behavior associated with ferromagnetic ordering, which is decreased with an increase in SIO layer thickness [45]. Further, spin-glass behavior in SRO/SIO SLs could be developed by interfacial DMI, which arises from chiral spin ordering texture at the interfaces. Equal-periodic repetitions of (SRO)m/(SIO)m well-engineered superlattice structure have revealed retrieving of ferromagnetic ordering (m  3) with enhanced transition temperature associated with a strong perpendicular magnetic anisotropy [46].

8.6.1 Interfacial Dzyaloshinskii-Moriya interaction (iDMI) Symmetry breaking in a crystal structure has shown clear evidence of existing SOC in various 5d elements and their derived oxides [47]. Particularly, the lack of inversion symmetry in bulk crystals produces spin chirality accompanying the orbital degeneracy. The spin chirality in a crystal shows asymmetry when the crystal structure is inverted. Subsequent magnetic moments are in orthogonal crystal planes satisfying

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asymmetric spin chirality which involves in developing weak ferromagnetism. This weak ferromagnetism is named as Dzyaloshinskii-Moriya interaction (DMI). The interaction of two orthogonally spin sites of Si and Sj yields a nonzero energy term due to the breaking the inversion symmetry of system. Such a system follows the rela! ! ! tion: H DM ¼ Dij  Si  Sj , here Dij is an interactive vector of DzyaloshinskiiMoriya and it is always along the direction of normal to the plane of spin sites [48]. Dzyaloshinskii first introduced the asymmetric interactive term to explain the weak ferromagnetic behavior of α-Fe2O3 [49] and Moriya extracted the term from the analytical derivation of spin-orbit Hamiltonian of an electron in relativistic motion [50]. However, in the case of layered films, a strong SOC contribution has been involved in breaking the inversion symmetry due to the presence of the SOC layer. Further, the magnetic moment drives the interaction along the plane of the interface. As a consequence, the interface significantly modifies the bulk properties and helps to induce novel magnetic features in nonmagnetic materials. DMI is a successful mechanism to describe weak ferromagnetism (wFM) of oxide materials like BiFeO3 and LaFeO3 in which the wFM is one of the intriguing multifunctional characteristics induced by the interaction of noncollinear spin states [51]. Such interaction mainly originates from spins arrangement in a noncentrosymmetric lattice structure. In contrast, the interfacial DMI is observed due to the breaking inversion symmetry at interfaces of multilayer thin films. The SOC is responsible for determining DMI; the strength of DMI is directly related to the SOC strength, which is commonly observed in heavy materials like Pt and Ir [52]. Hence the heavy metals in a heterostructure are the possible source of creating spin textures, stabilization, and their motion. Moreover, the interfaces coupled with SOC compounds can modify the magnetic response of heterostructures. In this section, we have highlighted some of the recently developed techniques involved in determining the features of iDM interaction in different oxide multilayered systems. Breaking of inversion symmetry can be achieved in the oxide heterostructures by introducing the strain, which causes multiple structural distortions in the oxide structures. Strain-engineering is one of the important factors that can control the structural asymmetry originated by rotation, tilt of octahedral, modify the magnetic structure, and the interactions of magnetic moments. Recently, a report of results has described a correlation between the magnetization of SIO/STO superlattice and structural distortion [53]. Fig. 8.2 illustrates the spin arrangement in structurally distorted octahedral of IrO6 with possible rotations and tilts. In the preliminary case, the magnetic response of the SL could be evolved by varying the rotation or tilting angles of the IrO6 octahedral, as shown in Fig. 8.2A, whereas the octahedral of TiO6 is fixed. In the case of a straight bond formation of IrdOdIr chain, the spin interactions of Ir are homogenously distributed in all directions through the chain. Such spin interactions arise due to a Heisenberg type-antiferromagnetic ordering where both DMI and symmetric anisotropy are absent. By considering the rotation of IrO6 without tilting angle (see in Fig. 8.2B), an in-plane canted moment of Ir instigate DM interaction. As shown in Fig. 8.2C, the tilt of octahedral from its normal yields out-of-plane spin canting, and subsequent interaction of spin moments will results in nonzero DM

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Functional Materials from Carbon, Inorganic, and Organic Sources

(A)

a = 0°, E = 0°

Centro-symmetric

z y y x

Ly = 0.00 Ly = 0.28 Lz = 0.00

Sx = 0.00 Sy = 0.16 Sz = 0.00

(B)

a = 14°, E = 0°

Rotation

(C) a = 0°

E

Tilting

E = 2°

z a

xy

Sx = 0.03 Sy = 0.08 Sz = 0.00

Lx = 0.08 Ly = 0.30 Lz = 0.00

Sx = 0.00 Sy = 0.11 Sz = 0.01

Lx = 0.00 Ly = 0.30 Lz = 0.01

Fig. 8.2 Structural distortion of IrO6 along with the magnetic moments shows (A) the octahedral neither rotates nor tilts, only considering the rotation (B) and tilting (C) [53].

term. Moreover, the magnetic anisotropy of the system is sensitive to epitaxial strain and is governed by the amount of octahedral tilt. DM interaction and nontrivial spin textures are discovered in oxide heterostructures while probing them electrically using the topological Hall effect (THE) technique [42,54]. Electrical detection of anomaly in the transverse resistivity confirms the presence of THE that is a gesture of formation spin chirality observed only by a fictitious magnetic field derived from the real-space Berry phase. Thus, it is believed that the spin textures in the heterostructures are developed by DM interaction. Matsuno et al. [44] considered magneto-transport measurements to investigate the transport property coupled interface DMI and subsequent magnetic skyrmions formation in the oxide interface. In addition, the magneto-transport measurements at low temperatures have revealed Hall resistivity maxima  200% for SRO (10 u.c.)/SIO (2 u.c.) bilayers which are larger than anomalous resistivity at high fields [55]. Such Hall resistivity maximum is attributed to the THE due to the presence of skyrmions.

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From the THE maximum, the rough estimation of skyrmion size could be found around 6.2 nm in diameter at 5 K, and for 90 K, the diameter was estimated of 2.4 nm. On the other hand, the typical diameter of magnetic skyrmion was found to be 9–20 nm in SRO thin films [56]. Further, Gu et al. [56] considered the advantage of the high dielectric constant of SrTiO3 substrate at low temperatures to manipulate the chemical potential as well as tuning the THE of 5 u.c. thick SRO film by applying gate voltage. Under applied magnetic field on [(SIO)m/(STO)] superlattices, a weak ferromagnetic ordering is detected along the in-plane direction of IrO2 plane where the saturated magnetic moments are determined  0.02μB/Ir [43]. Such weak ferromagnetic moments could be obtained by the DM interaction accompanying the rotation of IrO6 octahedral. Recently, the construction of two electromagnetically coupled layers has suggested to overcome the microstructural defects and stabilize the chiral spin textures in a thick ferromagnetic layer [57]. These chiral spin textures are further studied along with their stabilization by considering the advantage of ferroelectricity and magnetism in multiferroic compounds. The spin textures in SRO/BiFeO3 heterostructure are enhanced with increasing AFM/FE thickness, whereas these features are suppressed by increasing the thickness of the FM layer [57]. SrRuO3/BaTiO3 bilayers are induced iDM interaction due to the presence of strong SOC strength (0.1–0.15 eV) and breaking the inversion symmetry of SRO layer at the interface [58]. Like BiFeO3, BaTiO3 is also a ferroelectric material but magnetically inactive. The inherent property of ferroelectric (FE) distortion in BTO layer is supported to deform the SRO layer while inducing the FE-like distortion in SRO that favored to compensate the depolarized field in BTO [58,59]. As a consequence, DMI is induced in SrRuO3/BaTiO3 bilayers along with a developed robust magnetic skyrmions size in the range of 50–100 nm. Further, the ferroelectric-driven skyrmion density can be controlled by the electrical field because BTO has spontaneous polarization with the switching of polarity through electrical fields. Moreover, these spin textures can be stabilized over a temperature range of 5–80 K and in the magnetic field (4 to +4 T).

8.6.2 Magnetic anisotropy Magnetic anisotropy is another key ingredient to study the magnetic response of magnetic materials that plays a vital role in developing the magnetic storage devices. Due to the emergence of spintronic-based magnetic recording media, magnetic anisotropy has been examined and exploited to construct highly efficient nonvolatile magnetic memory storage devices. The source of magnetic anisotropy is in the microscopic level with high degrees of asymmetry of spin arrangement and the degeneracy of electronic states. Thus, the prerequisite energy involved in the magnetization is varied with any crystallographic directions which leads to the cause of the magnetic anisotropy. In several oxide materials, the magnetization of spins shows spontaneous alignment in one crystalline axis (easy-axis) and in other directions (hand-axis), it is difficult to achieve the spontaneous magnetization. For example, BiFeO3 and LaFeO3 are the compounds that showed an easy axis anisotropy, i.e., spin alignment, along the

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axis of structural distortion [51]. Even in the absence of all magnetic anisotropies, the orientation of magnetic moments is controlled in the ultra-thin films along a plane or axis. Consequently, the effective magnetic anisotropic axis is dependent on the axialvector associated with the surface of the plane. The magnetic anisotropy energy of local spin environments in the uniaxial system P is represented as: H 1 ¼  K i ðSi ni Þ2. Here Si is the spin state of index i along the unit i

vector of n. MnO and NiO are typical examples of the crystal systems with AFM behavior of negligible crystalline anisotropy [60]. Magnetic anisotropy energy found in a perfect crystal symmetry like cubic structure is very few orders in magnitude ( 103–106 eV), so that the spins are aligned easily in any preferred direction of the crystal frame. On the other hand, the magnetic anisotropy energy in ferromagnetic thin films is in the typical order of three (103 eV) which is higher than that of the bulk counterpart. Since the magnetic anisotropy is rotational dependent property of spin arrangement, the magnetic anisotropy induced by the interfaces of heterostructures also shows a directional dependent spin alignment along the perpendicular to the interface. Thus, the anisotropy sign of the surface energy contribution will be always positive which favors to the evolution of the perpendicular magnetization where the anisotropy is unidirectional, i.e., perpendicular to the surface. Magnetic anisotropy is very sensitive to the strain induced by the substrate. The easy axis magnetization of a complex oxide thin film is preferable to the crystal structure in which the growing orientation of the film is governed by the terminated surface orientation of the substrate [61]. In the case of LSMO films, the substrate-induced strain dramatically changed the occupancy of an electron in the eg orbitals, which favors a drift in the sensitivity of magnetic anisotropy of the films. An in-plane tensile strain supports to occupy the electrons in the dx2 y2 orbital where an in-plane magnetic anisotropy has been found. In contrast, compressive strain in the LSMO favors to occupy the d3z2 r2 orbital that leads to a perpendicular magnetic anisotropy [62]. The uniaxial anisotropy is purely dependent upon the easy axis direction relative to the crystal direction of substrates. For instance, torque magnetometry measurements of La0.7Ca0.3MnO3 (LCMO) films confirmed that the films experienced crystal axes directional dependent biaxial magnetic anisotropy up to  104 J/m3 and LCMO films with a uniaxial anisotropy were independent on crystal axes [63]. In the case of SrRuO3 films, the magnetic anisotropy field of 12T was estimated at low temperatures which requires large magnetic anisotropy energy to the saturated magnetization of 1.59μB/u.c. [34,64]. Besides, there are several magnetic anisotropies observed in the oxide thin films and their derived heterostructures by changing the characteristics of growth conditions such as substrates induced strain, thickness, surface roughness, etc.

8.7

Surface and thickness influence on magnetic anisotropy

Magnetic anisotropy is modified by the film surface where the surface features are in steps of the terrace rather than a flat surface. The miscut angle (α) and miscut direction

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of vicinal substrates provide the step formation that can show a significant impact on the film growth, domain structure, and surface morphology accompanying the magnetic properties of oxide thin films [65,66]. These steps are provided an additional contribution to inducing the uniaxial anisotropy. Oxide films grown on vicinal substrates are shown an easy magnetization direction either in the direction of the edge of the step or perpendicular to the step edge. Several oxide perovskite thin films unveil a uniaxial anisotropy along the plane of the film where the step like features is appeared by the broken rotational symmetry of the film surface. The step direction of La0.7Sr0.3MnO3 thin films grown on STO substrate with a low vicinal angle ( 0.13°) was found to be  133 5° along with the easy-axis direction of 130° [67]. The LSMO film on STO with the vicinal angle of 10°, large value  7.29  104 erg/cm3 of magnetic anisotropy was detected, whereas the anisotropy was found to be 5.69  104 erg/cm3 for the film on flat STO substrate [66]. Perna et al. [68] reported the evolution of magnetization rotation of LSMO/STO from the easy [100] to hard axis [110] of substrate orientation along with low surface roughness (0.45 nm) to high surface roughness (1.25 nm). Furthermore, the uniaxial anisotropy was found in the direction of substrate steps where a large value magnetic anisotropy fields was determined at higher thicknesses [68]. Surface roughness is a considerable parameter that can contribute to the dipolar anisotropy (Kdip). Magnetic dipoles are present at the edges of uneven surface, i.e., terrace and dip that the dipoles are induced stray fields along the plane of the film. The dipolar anisotropy can be expressed in terms of the surface roughness (σ) to the thickness (d) ratio as: Kdip ¼ (3/8)σμ0M2s [1  f(2πσ/d)], here, f(2πσ/d) is a function estimated by Bruno [69]. This relation succeeded in determining the surface anisotropy of the transition metal multilayers [70]. By considering the asymmetric environment of the surface, the magneto-crystalline surface anisotropy is more contribute part from the reduced surface roughness, and the anisotropy diminishes by an amount ΔK/ K ¼  2σ/d. Neel proposed magnetic anisotropy for the reduced symmetry of surface spins rather differently from the bulk counterpart. The surface anisotropy is significant in low dimensional atomic layers with few nanometers in thickness, even though there are structural and morphological properties conserved at the surface. Thus, effective magnetic anisotropy (Keff) of the thin film thickness of t can be expressed in terms of volume (Kv) and surface (Ks) contributions of anisotropy as follows: Keff ¼ Kv + 2Ks

 t

Here, the prefactor of Ks corresponds to the contribution of anisotropy from either side of the surface and this is independent of thickness. The aforementioned equation is quite useful to estimate the volume as well as surface anisotropy of the films. For instance, thickness-dependent magnetic anisotropy study of La0.6Sr0.4MnO3 thin films on MgO and SrTiO3 substrates measured by FMR technique was found +2.6  105 and 3.6  105 erg/cm3 for volume and 0.66 and 0.89 erg/cm2 for surface contributions, respectively [71]. Here the opposite sign in the volume contribution signifies that the easy uniaxial axis lies parallel to the film normal for the positive sign, whereas the negative sign is assigned to an in-plane easy axis. On the other hand, the positive

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surface contribution is perpendicular to the film anisotropy. However, Ghosh et al. calculated the surface anisotropy value of LSMO as 9.57  104 J/m2 in the trilayer of SrRuO3/LaNiO3/LSMO [72]. Besides, in the aforementioned equation, the demagnetization field of thin films also comes into account of effective anisotropy in which the thickness of the film is in the order of few monolayers thick [73]. Mostly, demagnetization is the resultant of stray fields induced by the magnetic moments in the absence of an external magnetic field.

8.8

Interface role in determining the magnetic anisotropy

In the oxide heterostructures, the interface is one of the prominent tunable parameters to manipulate both the direction and magnitude of the magnetic anisotropy. In general, structural, electronic, and valence states are either deviated or discontinued at the interfaces that ultimately imply the modified local environments in the complex oxide heterostructures. In particular, magnetically inactive behavior has been recognized at around few unit cell thicknesses of ferromagnetic layer grown on bare substrate. Such an absent magnetic response retrieves in those low dimensional ferromagnetic layers by introducing a spacer layer between film and substrate. For instance, a large magnetic anisotropy of 2  105 erg/cm3 is found in one of the strain-relaxed ferromagnetic films (LSMO) due to the spacer layer (La0.7Sr0.3CoO3) thickness of two unit cells [74]. In contrast, an unusual ferromagnetic enhancement has been found in LaMnO3 (LMO) thin films when the LMO films were capped by few unit cell thickness of LaAlO3 rather than a buffer layer [75]. On the other hand, the coherent growth of La0.7Sr0.3CoO3/La0.7Sr0.3MnO3/La0.7Sr0.3CoO3(LSCO) trilayer system showed that ferromagnetic behavior in LSMO layer was re-established an easy axis along the plane of the film rather than the common perpendicular anisotropy behavior of LSMO films [76]. This orientation of magnetic anisotropy could be observed while the deviation of Mn ions alignment toward oxygen octahedral at the interface of LSCO/LSMO. As mentioned earlier, the magnetic anisotropy of thin films can be controlled by varying the thickness, the substrate-induced strain, altering the growth mechanism, etc. [77]. Predominantly, the dimensionality-controlled magnetic response is a prominent approach to revealing the hidden magnetic phases in oxide heterostructures [78]. Adopting the engineering of dimensionality as a controlled parameter, the magnetic phenomena in the oxide heterostructures have been modulated by the interfaces. Chen et al. proposed three steps interface-engineering method to address the systematic variation of magnetic anisotropy in complex heterostructures [79]. The preliminary approach is to vary the thickness (18–64 u.c.) of LSMO film on bare substrate NdGaO3 (NGO). The magnetic anisotropy energy (MAE) is improved from 4.7  103 erg/cm3 to 6.8  104 erg/cm3 with an invariant easy axis. In general, an inplane MAE axis was found in ultra-thin LSMO films [80]. In the second approach, the MAE is further modified by introducing a buffer layer of SrTiO3 with a thickness range of 0–12 u.c., whereas the LSMO thickness is fixed at 18 u.c., that leads to suppress the oxygen octahedral rotation at the interface of LSMO/NGO. In the final approach, a superlattice of 18 u.c. thick LSMO divided into N + 1 sublayers by

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inserting N-sublayers of 12 u.c. of thin STO improves the MAE of 8.9  104 and 7.0  104 erg/cm3 for N ¼ 1, 2, respectively. In addition, the interface number also plays a major role in controlling the magnetic anisotropy energy, for instance, by varying the stacking periodicity of [La0.67Ca0.33MnO3/SrRuO3]n superlattice the easy axis magnetic anisotropy is switched from [010] to [100] direction with an increase in the number (n) from 1 to 15 [79]. The earlier discussion on magnetic anisotropies pronounced that the magnetic moments of complex oxide heterostructures were easily aligned toward the plane of the films. Such anisotropy systems are lacking from the fabrication of conventional memory read heads. Therefore, the interface has potential importance in controlling the PMA of TMO heterostructures. Though in a few cases perpendicular magnetic anisotropy (PMA) is noticed that can be controllable in the magnitude [73] and encourages to furtherance study of the PMA with different controlled parameters. Walter et al. demonstrated the production of PMA of magnitude up to 6.0  106 erg/cm3 in the compressive strained La1 xSrxCoO3 δ films while varying the oxygen vacancies as a controlled parameter [73]. In contrast, the magnetic moments of La0.67Sr0.33MnO3 (LSMO) film sandwiched between two La1 xSrxCoO3 layers are switched in the in-plane direction even though the interface of trilayers experienced compressive strain by the substrate [76]. In addition, the magnetic anisotropy of [La0.7Sr0.3MnO3/SrIrO3] superlattice has revealed a high value of 4.0  106 erg/cm3 perpendicular magnetic anisotropy which is one order higher than that of strained LSMO [81]. The interface of SrRuO3/ SrIrO3 heterostructures with a short periodicity (i.e., n ¼ 1–5) assisted to retrieve the ferromagnetic properties for the periodicity of 1 SL, where the enhanced PMA is found at around 1.6  106 erg/cm3 [46]. A recent report on La0.67Sr0.33MnO3/ LaCoO2.5 (perovskite/brownmillerite) heterostructures showed that strong orbital reconstruction is developed across the interface by altering the stacking arrangement due to the presence of symmetry discontinuity at the interface [82]. The authors mainly highlighted their study on perovskite/brownmillerite that the perpendicular magnetic anisotropy (PMA) found at the interface, which is not observed in regular perovskite/perovskite oxide interfaces. Moreover, the maximal effective PMA quantified as 5.4  106 erg/cm3 at 10 K is gradually decreased with the increase in temperature and finally lost the features above 220 K [82]. DFT calculations supported the unusual PMA in the perovskite/brownmillerite system appeared due to the symmetry break and unique lattice distortion. Such interfacial effects could be the possible sources to develop the magneto-crystalline anisotropy. The in-plane anisotropy or PMA is ruled by the choice of electron occupancy in the eg orbitals. Such electron occupancy can be controlled by monitoring the substrateinduced strain and the periodicity of SL. In the case of the oxide films with the tensile strain, the electrons are preferred to occupy the x2-y2 orbital to achieve an in-plane magnetic anisotropy [62], whereas in the compressive strained films, a PMA is found for electron occupancy in the 3z2-r2 orbital [83]. Such strain mediates electron occupation is shown in Fig. 8.3. Fig. 8.3A and B show the strain influence on the orbital arrangement of transition metals, i.e., Mn and Ru in La0.67Ca0.33MnO3 (LCMO) and SrRuO3 (SRO), respectively, on NdGaO3 (NGO) substrate. The LCMO films are

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Fig. 8.3 The easy axis dependent stacking periodicity (N) of LCMO/SRO superlattice related to the orbital configuration of LCMO (A and D) SRO (B and E) for N ¼ 1 and 15 [79].

elongated in the b-direction and compressed in the a-direction that can lead preferred to occupy electrons in the d x2 y2 orbital of Mn ions. A large amount of experimental evidence on preferred occupation of electrons in Mn ions has been confirmed by using soft X-ray magnetic spectroscopy and SQUID measurements [84]. On the other hand, unlike LCMO films, SRO film has four electrons in the t2g states (dxy, dyz, and dxz orbitals) and they are spatially extended compared to Mn eg states. Thus, the possible mechanism of (both in-plane and perpendicular) magnetic anisotropy in the LCMO/ SRO SLs may be originated by the hybridization of Ru t2g states and Mn eg states. For the periodicity of SL is one (Fig. 8.3C), the hybridization of Ru and Mn is enforced to occupy the electrons in Ru dxy orbital while suppressing the out-of-plane dyz/dxz orbitals due to the domination of two-dimensional features. Thus, the magnetic preferred axis is found in [010] direction like LCMO/NGO (001) films. With increasing the periodicity of SLs (Fig. 8.3F), the preferential occupation of Ru dyz/dxz orbitals satisfy the strong hybridization of Ru dyz/dxz orbitals and Mn d x2 y2 orbital that switches magnetic easy-axis from [010] to [100]. First-principles calculations of SrRuO3/SrTiO3 heterostructures demonstrated the enhancement of magnetic anisotropy with PMA features that show an oscillatory with increasing atomic layer thickness from 3 to 9 [85]. Like the above strain case, the preferred occupancy of the d-orbitals governs magnetic anisotropy energy that can be

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changed by the layer number. Such oscillatory characteristics attribute to the reconstruction of in-plane dx2 y2 and out-of-plane dyz + dxz + dz2 orbitals near Fermi level.

8.9

Further modification of magnetic anisotropy while competing with other physical phenomena

Immense electrical transport phenomena such as metal-insulator transition, electronelectron interaction, low dimension electron gas, ferroelectricity, etc. are found in 3d transition metal oxide heterostructures, which are strongly inherent behaviors of 3d TMOs accompanying the magnetic response. These properties are strongly connected with the magnetic behavior and structural modification. Take, for instance, multiferroic properties of perovskite thin films that can be modulated by the structural control via octahedral distortion. A distorted TiO6 octahedral caused to induce ferroelectric polarization in EuTiO3 thin films which favors the ferromagnetic state whereas the bulk counterpart is in paraelectric polarization with antiferromagnetic states [86]. Thus, the preferred direction of polarization is parallel to octahedral distortion but is perpendicular to the easy magnetization axis. Such results indicate the presence of magnetoelectric coupling that can enable the possibility of multiferroic behavior, i.e., magnetization controlled by the electric field. Electric field modulation is an effective approach to confine the electrons which create distinct magnetic phases. Particularly, by applying the electric fields through gate electrodes the oxide heterostructure forms or annihilates the oxygen vacancies that can reduce the operating temperatures in the solid fuel cells. As a result, a sequence of magnetic phase transitions can be operated by applying the wide range of electric fields with altering its sign [87]. In the bilayer of La0.8Sr0.2CoO3/La0.67Sr0.33MnO3, for example, change in valence state of Co ions in La0.8Sr0.2CoO3 using a top ionic liquid electrode could be switched the magnetic easy axis of La0.67Sr0.33MnO3 between in-plane and out-of-plane directions [88]. The charge transfer between Mn-Co ions is the main interfacial mechanism for exchanging anisotropy which is associated with exchange coupling and orbital occupancy. On the other hand, crystal field splitting is weak in 4d and 5d TMOs where strong SOC contribution is preserved. Like in DMI, SOC is the common source to the dawn of magnetic anisotropy in thin films and multilayers. Large magnetic anisotropy can commonly appear in the heterostructures comprising heavy nonmagnetic metal/ magnetic transition metal in which the heavy metals possess large SOC [89,90]. Such large overvalued SOC strength is found because of neglecting the screening effect of outermost electrons. Moreover, the asymmetry of spin chirality associated with SOC can lead to the magnetic anisotropy induced by crystal structure in bulk systems. The reduced size of a material, surface, and the interface of two dissimilar adjoined layers are also accountable for the anisotropy in thin films and heterostructures. Charge gathered across the interface by employing the electric field leads to chemically imbalanced potentials, the unequal density of spin states, and local change in the occupancy which alter the interfacial magnetization. The tunable magnetization via

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an applied electric field is the most promising method for the switching of magnetic in magnetic memories.

8.10

Summary

This chapter has summarized the overall discussion of various common magnetic phenomena complex oxide heterostructures associated with distinct magnetic responses which are developed by the interfaces. Particularly, spin, electron, and orbital degrees of freedom significantly contribute to the essential effects observed in magnetism such as polar metallic states, Dzyaloshinskii-Moriya interaction, perpendicular magnetic anisotropy, spin-polarization, etc. The magnetic phenomena of oxide heterostructures are extensively investigated by a means of different interfacial engineering approaches such as thickness, substrate-induced strain, oxygen vacancies, etc., to address the fast growth mechanisms of magnetic devices. In the heterostructures, inversion symmetry breaks at the interface that can produce high mobile charge concentration, hidden magnetic phase instability, and spin textures. The nonlinear spin states induce weak ferromagnetism with static spin textures and the spin textures are further explored in terms of their stability, configuration, and dynamics in various heterostructures by considering the design and applying external fields as tuning parameters. The electronic state degeneracy and the preferred occupancy of electrons in the eg orbitals are favored to vary the sensitivity of magnetic anisotropy from inplane to the perpendicular direction. All these intriguing magnetic phenomena are directly connected to the electronic structure of transition metal. Therefore, it is necessary to understand the design and significant contribution of interfaces of the heterostructures with different composed oxide perovskites that commensurate the degeneracy and occupancy of d-orbitals.

References [1] N.W. Ashcroft, N. David Mermin, Magnetic Ordering, Solid State Physics, Harcourt, USA, 1976, pp. 694–699. [2] P.W. Anderson, Antiferromagnetism. Theory of superexchange interaction, Phys. Rev. 79 (1950) 350–356. [3] Zener Clarence, IInteraction between the d-shells in the transition metals. II. Ferromagnetic compounds of manganese with perovskite structure, Phys. Rev. 82 (151) 403–405 In press. [4] V.A. Stephanovich, M.D. Glinchuk, R. Blinc, Magnetoelectric effect in mixed-valency oxides mediated by charge carriers, Europhys. Lett. 83 (2008) 37004. [5] F.W. Smith, Strength of the Ruderman-Kittel-Kasuya-Yosida interaction in dilute CuMn alloys, Phys. Rev. B 14 (1976) 241–243. [6] F.W. Smith, Magnetization of a dilute magnetic alloy with magnetic interactions: ZnMn, Phys. Rev. B 10 (1974) 2980–2986. [7] D. Mukherjee, P. Mukherjee, H. Srikanth, S. Witanachchi, Carrier-mediated interaction of magnetic moments in oxygen vacancy–controlled epitaxial Mn-doped ZnO thin films, J. Appl. Phys. 111 (2012) 07C318.

Interface engineering in oxide heterostructures

265

[8] P. Sharma, A. Gupta, K.V. Rao, F.J. Owens, R. Sharma, R. Ahuja, J.M. Guillen, B. Johansson, G.A. Gehring, Ferromagnetism above room temperature in bulk and transparent thin films of Mn-doped ZnO, Nat. Mater. 2 (2003) 673–677. [9] K.R. Nikolaev, A.Yu. Dobin, I.N. Krivorotov, W.K. Cooley, A. Bhattacharya, A.L. Kobrinskii, L.I. Glazman, R.M. Wentzovitch, E. Dan Dahlberg, A.M. Goldman, Oscillatory exchange coupling and positive magnetoresistance in epitaxial oxide heterostructures, Phys. Rev. Lett. 85 (2000) 3728–3731. [10] P. Padhan, R.C. Budhani, R.P.S.M. Lobo, Overdamped interlayer exchange coupling and disorder-dominated magnetoresistance in La0.7Ca0.3MnO3/LaNiO3 superlattices, Europhys. Lett. 63 (2003) 771–777. [11] T. Kim, I.H. Cha, Y.J. Kim, G.W. Kim, A. Stashkevich, Y. Roussign
e, M. Belmeguenai, S. M. Ch
erif, A.S. Samardak, Y.K. Kim, Ruderman–Kittel–Kasuya–Yosida-type interfacial Dzyaloshinskii–Moriya interaction in heavy metal/ferromagnet heterostructures, Nat. Commun. 12 (2021) 3280. [12] H. Wei, J.L. Barzola-Quiquia, C. Yang, C. Patzig, T. H€ oche, P. Esquinazi, M. Grundmann, M. Lorenz, Charge transfer-induced magnetic exchange bias and electron localization in (111)- and (001)-oriented LaNiO3/LaMnO3 superlattices, Appl. Phys. Lett. 110 (2017) 102403. [13] H. Bhatt, Y. Kumar, C.L. Prajapat, C.J. Kinane, A. Caruana, S. Langridge, S. Basu, S. Singh, Emergent interfacial ferromagnetism and exchange bias effect in paramagnetic/ferromagnetic oxide heterostructures, Adv. Mater. Interfaces 7 (2020) 2001172. [14] J.D. Hoffman, B.J. Kirby, J. Kwon, G. Fabbris, D. Meyers, J.W. Freeland, I. Martin, O.G. Heinonen, P. Steadman, H. Zhou, C.M. Schlep€utz, M.P.M. Dean, S.G.E. te Velthuis, J.-M. Zuo, A. Bhattacharya, Oscillatory noncollinear magnetism induced by interfacial charge transfer in superlattices composed of metallic oxides, Phys. Rev. X 6 (2016) 041038. [15] S. Thota, S. Ghosh, S. Nayak, D.C. Joshi, P. Pramanik, K. Roychowdhury, S. Das, Finitesize scaling and exchange-bias in SrRuO3/LaNiO3/SrRuO3 trilayers, J. Appl. Phys. 122 (2017) 124304. [16] Y. Tokura, N. Nagaosa, Orbital physics in transition-metal oxides, Science 288 (2000) 462–468. [17] M.K. Crawford, M.A. Subramanian, R.L. Harlow, J.A. Fernandez-Baca, Z.R. Wang, D.C. Johnston, Structural and magnetic studies of Sr2IrO4, Phys. Rev. B 49 (1994) 9198–9201. [18] J.A. Sulpizio, S. Ilani, P. Irvin, J. Levy, Nanoscale phenomena in oxide heterostructures, Annu. Rev. Mater. Res. 44 (2014) 117–149. [19] Z. Zhong, A. To´th, K. Held, Theory of spin-orbit coupling at LaAlO3/SrTiO3 interfaces and SrTiO3 surfaces, Phys. Rev. B 87 (2013). [20] H. Nakamura, T. Kimura, Electric field tuning of spin-orbit coupling in KTaO3 field-effect transistors, Phys. Rev. B 80 (2009) 121308. [21] T. Neumann, G. Borstel, C. Scharfschwerdt, M. Neumann, Electronic structure of KNbO3 and KTaO3, Phys. Rev. B 46 (1992) 10623–10628. [22] S. Koseki, D.G. Fedorov, M.W. Schmidt, M.S. Gordon, Spin orbit splittings in the thirdrow transition elements: comparison of effective nuclear charge and full Breit Pauli calculations, J. Phys. Chem. A 105 (2001) 8262–8268. [23] W. Lin, L. Li, F. Dogan, C. Li, H. Rotella, X. Yu, B. Zhang, Y. Li, W.S. Lew, S. Wang, W. Prellier, S.J. Pennycook, J. Chen, Z. Zhong, A. Manchon, T. Wu, Interface-based tuning of Rashba spin-orbit interaction in asymmetric oxide heterostructures with 3d electrons, Nat. Commun. 10 (2019). [24] G. Herranz, M. Basletic, M. Bibes, C. Carretero, E. Tafra, E. Jacquet, K. Bouzehouane, C. Deranlot, A. Hamzic, J.M. Broto, A. Barthelemy, A. Fert, High mobility in LaAlO3/

266

[25] [26] [27]

[28] [29]

[30] [31]

[32]

[33]

[34]

[35]

[36]

[37]

[38] [39] [40]

[41]

Functional Materials from Carbon, Inorganic, and Organic Sources

SrTiO3 heterostructures: origin, dimensionality, and perspectives, Phys. Rev. Lett. 98 (2007) 216803. N. Nakagawa, H.Y. Hwang, D.A. Muller, Why some interfaces cannot be sharp, Nat. Mater. 5 (2006) 204–209. A. Ohtomo, H.Y. Hwang, A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface, Nature 427 (2004) 423–426. B. Kalisky, J.A. Bert, B.B. Klopfer, C. Bell, H.K. Sato, M. Hosoda, Y. Hikita, H.Y. Hwang, K.A. Moler, Critical thickness for ferromagnetism in LaAlO(3)/SrTiO(3) heterostructures, Nat. Commun. 3 (2012) 922. J.S. Lee, Y.W. Xie, H.K. Sato, C. Bell, Y. Hikita, H.Y. Hwang, C.C. Kao, Titanium dx y ferromagnetism at the LaAlO3/SrTiO3 interface, Nat. Mater. 12 (2013) 703–706. H. Ikeno, F.M.F. de Groot, E. Stavitski, I. Tanaka, Multiplet calculations of L2,3x-ray absorption near-edge structures for 3d transition-metal compounds, J. Phys. Condens. Matter 21 (2009) 104208. S. Stemmer, S. James Allen, Two-dimensional electron gases at complex oxide interfaces, Annu. Rev. Mater. Res. 44 (2014) 151–171. Y. Du, G.Z. Xu, X.M. Zhang, Z.Y. Liu, S.Y. Yu, E.K. Liu, W.H. Wang, G.H. Wu, Crossover of magnetoresistance in the zero-gap half-metallic Heusler alloy Fe2CoSi, Europhys. Lett. 103 (2013) 37011. A. Joshua, J. Ruhman, S. Pecker, E. Altman, S. Ilani, Gate-tunable polarized phase of twodimensional electrons at the LaAlO3/SrTiO3 interface, Proc. Natl. Acad. Sci. 110 (2013) 9633. B. Kim, S. Khmelevskyi, C. Franchini, I.I. Mazin, K. Kim, SrRuO3  SrTiO3 heterostructure as a possible platform for studying unconventional superconductivity in Sr2RuO4, Phys. Rev. B 101 (2020). G. Koster, L. Klein, W. Siemons, G. Rijnders, J.S. Dodge, C.-B. Eom, D.H.A. Blank, M.R. Beasley, Structure, physical properties, and applications of SrRuO3 thin films, Rev. Mod. Phys. 84 (2012) 253–298. S.G. Jeong, T. Min, S. Woo, J. Kim, Y.Q. Zhang, S.W. Cho, J. Son, Y.M. Kim, J.H. Han, S. Park, H.Y. Jeong, H. Ohta, S. Lee, T.W. Noh, J. Lee, W.S. Choi, Phase instability amid dimensional crossover in artificial oxide crystal, Phys. Rev. Lett. 124 (2020) 026401. W. Lu, W. Dong Song, K. He, J. Chai, C.-J. Sun, G.-M. Chow, J.-S. Chen, The role of octahedral tilting in the structural phase transition and magnetic anisotropy in SrRuO3 thin film, J. Appl. Phys. 113 (2013) 063901. M. Gu, Q. Xie, X. Shen, R. Xie, J. Wang, G. Tang, D. Wu, G.P. Zhang, X.S. Wu, Magnetic ordering and structural phase transitions in a strained ultrathin SrRuO3/SrTiO3 superlattice, Phys. Rev. Lett. 109 (2012) 157003. A. Liebsch, H. Ishida, Subband filling and Mott transition in Ca2-xSrxRuO4, Phys. Rev. Lett. 98 (2007) 216403. J. Bertinshaw, Y.K. Kim, G. Khaliullin, B.J. Kim, Square Lattice Iridates, Annu. Rev. Condens. Matter Phys. 10 (2019) 315–336. D. Lee, S. Roh, J. Hwang, J. Park, T.-Y. Koo, J.-H. Park, J. Song, Engineering electrical property of Dirac semimetal perovskite SrIrO3 thin films by subtle changes in lattice structure, Appl. Phys. Express 13 (2020) 015510. K. Huang, L. Wu, M. Wang, N. Swain, M. Motapothula, Y. Luo, K. Han, M. Chen, C. Ye, A.J. Yang, H. Xu, D.-c. Qi, A.T. N’Diaye, C. Panagopoulos, D. Primetzhofer, L. Shen, P. Sengupta, J. Ma, Z. Feng, C.-W. Nan, X. Renshaw Wang, Tailoring magnetic order via atomically stacking 3d/5d electrons to achieve high-performance spintronic devices, Appl. Phys. Rev. 7 (2020) 011401.

Interface engineering in oxide heterostructures

267

[42] N. Mohanta, E. Dagotto, S. Okamoto, Topological Hall effect and emergent skyrmion crystal at manganite-iridate oxide interfaces, Phys. Rev. B 100 (2019). [43] J. Matsuno, K. Ihara, S. Yamamura, H. Wadati, K. Ishii, V.V. Shankar, H.Y. Kee, H. Takagi, Engineering a spin-orbital magnetic insulator by tailoring superlattices, Phys. Rev. Lett. 114 (2015) 247209. [44] J. Matsuno, N. Ogawa, K. Yasuda, F. Kagawa, W. Koshibae, N. Nagaosa, Y. Tokura, M. Kawasaki, Interface-driven topological Hall effect in SrRuO3-SrIrO3 bilayer, Sci. Adv. 2 (2016) e1600304. [45] B. Pang, L. Zhang, Y.B. Chen, J. Zhou, S. Yao, S. Zhang, Y. Chen, Spin-glass-like behavior and topological Hall effect in SrRuO3/SrIrO3 superlattices for oxide spintronics applications, ACS Appl. Mater. Interfaces 9 (2017) 3201–3207. [46] Z. Zeng, J. Feng, X. Zheng, C. Wang, J. Liu, Z. Lu, F.-X. Jiang, X.-H. Xu, Z. Wang, R.-W. Li, Emergent ferromagnetism with tunable perpendicular magnetic anisotropy in shortperiodic SrIrO3/SrRuO3 superlattices, Appl. Phys. Lett. 116 (2020) 142401. [47] K.V. Shanavas, Z.S. Popovi
c, S. Satpathy, Theoretical model for Rashba spin-orbit interaction indelectrons, Phys. Rev. B 90 (2014). [48] K. Zakeri, Probing of the interfacial Heisenberg and Dzyaloshinskii-Moriya exchange interaction by magnon spectroscopy, J. Phys. Condens. Matter 29 (2017) 013001. [49] I. Dzyaloshinsky, A thermodynamic theory of “weak” ferromagnetism of antiferromagnetics, J. Phys. Chem. Solids 4 (1958) 241–255. [50] T. Moriya, Anisotropic superexchange interaction and weak ferromagnetism, Phys. Rev. 120 (1960) 91–98. [51] C. Weingart, N. Spaldin, E. Bousquet, Noncollinear magnetism and single-ion anisotropy in multiferroic perovskites, Phys. Rev. B 86 (2012) 094413. [52] H. Yang, A. Thiaville, S. Rohart, A. Fert, M. Chshiev, Anatomy of Dzyaloshinskii-Moriya interaction at Co/Pt interfaces, Phys. Rev. Lett. 115 (2015) 267210. [53] T.R. Dasa, L. Hao, J. Yang, J. Liu, H. Xu, Strain effects on structural and magnetic properties of SrIrO3/SrTiO3 superlattice, Mater. Today Phys. 4 (2018) 43–49. [54] C. Wang, C.H. Chang, A. Herklotz, C. Chen, F. Ganss, U. Kentsch, D. Chen, X. Gao, Y.J. Zeng, O. Hellwig, M. Helm, S. Gemming, Y.H. Chu, S. Zhou, Topological Hall effect in single thick SrRuO3 layers induced by defect engineering, Adv. Electron. Mater. 6 (2020) 2000184. [55] K.Y. Meng, A.S. Ahmed, M. Bacani, A.O. Mandru, X. Zhao, N. Bagues, B.D. Esser, J. Flores, D.W. McComb, H.J. Hug, F. Yang, Observation of nanoscale skyrmions in SrIrO3/SrRuO3 bilayers, Nano Lett. 19 (2019) 3169–3175. [56] Y. Gu, Y.-W. Wei, K. Xu, H. Zhang, F. Wang, F. Li, M.S. Saleem, C.-Z. Chang, J. Sun, C. Song, Interfacial oxygen-octahedral-tilting-driven electrically tunable topological Hall effect in ultrathin SrRuO3 films, J. Phys. D. Appl. Phys. 52 (2019) 404001. [57] H. Wang, Y. Dai, Z. Liu, Q. Xie, C. Liu, W. Lin, L. Liu, P. Yang, J. Wang, T.V. Venkatesan, G.M. Chow, H. Tian, Z. Zhang, J. Chen, Overcoming the limits of the interfacial Dzyaloshinskii-Moriya interaction by antiferromagnetic order in multiferroic heterostructures, Adv. Mater. 32 (2020) e1904415. [58] L. Wang, Q. Feng, Y. Kim, R. Kim, K.H. Lee, S.D. Pollard, Y.J. Shin, H. Zhou, W. Peng, D. Lee, W. Meng, H. Yang, J.H. Han, M. Kim, Q. Lu, T.W. Noh, Ferroelectrically tunable magnetic skyrmions in ultrathin oxide heterostructures, Nat. Mater. 17 (2018) 1087–1094. [59] G. Gerra, A.K. Tagantsev, N. Setter, K. Parlinski, Ionic polarizability of conductive metal oxides and critical thickness for ferroelectricity in BaTiO3, Phys. Rev. Lett. 96 (2006) 107603.

268

Functional Materials from Carbon, Inorganic, and Organic Sources

[60] D. Rodbell, I. Jacobs, J. Owen, E. Harris, Biquadratic exchange and the behavior of some antiferromagnetic substances, Phys. Rev. Lett. 11 (1963) 10. [61] H. Boschker, M. Mathews, E.P. Houwman, H. Nishikawa, A. Vailionis, G. Koster, G. Rijnders, D.H. Blank, Strong uniaxial in-plane magnetic anisotropy of (001)-and (011)-oriented La0.67 Sr0.33MnO3 thin films on NdGaO3 substrates, Phys. Rev. B 79 (2009) 214425. [62] D. Pesquera, G. Herranz, A. Barla, E. Pellegrin, F. Bondino, E. Magnano, F. Sa´nchez, J. Fontcuberta, Surface symmetry-breaking and strain effects on orbital occupancy in transition metal perovskite epitaxial films, Nat. Commun. 3 (2012) 1189. [63] K. Steenbeck, R. Hiergeist, Magnetic anisotropy of ferromagnetic La0.7(Sr,Ca)0.3MnO3 epitaxial films, Appl. Phys. Lett. 75 (1999) 1778–1780. [64] D.J. Singh, Electronic and magnetic properties of the 4 d itinerant ferromagnet SrRuO3, J. Appl. Phys. 79 (1996) 4818–4820. [65] Q. Gan, R.A. Rao, C.B. Eom, Control of the growth and domain structure of epitaxial SrRuO3 thin films by vicinal (001) SrTiO3 substrates, Appl. Phys. Lett. 70 (1997) 1962–1964. [66] Z.-H. Wang, G. Cristiani, H.U. Habermeier, Uniaxial magnetic anisotropy and magnetic switching in La0.67Sr0.33MnO3 thin films grown on vicinal SrTiO3(100), Appl. Phys. Lett. 82 (2003) 3731–3733. [67] M. Mathews, F.M. Postma, J.C. Lodder, R. Jansen, G. Rijnders, D.H.A. Blank, Stepinduced uniaxial magnetic anisotropy of La0:67Sr0:33MnO3 thin films, Appl. Phys. Lett. 87 (2005) 242507. [68] P. Perna, C. Rodrigo, E. Jim
enez, F.J. Teran, N. Mikuszeit, L. M
echin, J. Camarero, R. Miranda, Tailoring magnetic anisotropy in epitaxial half metallic La0.7Sr0.3MnO3 thin films, J. Appl. Phys. 110 (2011) 013919. [69] P. Bruno, J.P. Renard, Magnetic surface anisotropy of transition metal ultrathin films, Appl. Phys. A Solids Surfaces 49 (1989) 499–506. [70] M. Arora, R. H€ubner, D. Suess, B. Heinrich, E. Girt, Origin of perpendicular magnetic anisotropy in Co/Ni multilayers, Phys. Rev. B 96 (2017) 024401. [71] L.B. Steren, M. Sirena, J. Guimpel, Substrate effect on the magnetic behavior of manganite films, J. Appl. Phys. 87 (2000) 6755–6757. [72] S. Ghosh, R.G. Tanguturi, P. Pramanik, D.C. Joshi, P.K. Mishra, S. Das, S. Thota, Lowtemperature anomalous spin correlations and Kondo effect in ferromagnetic SrRuO3/ LaNiO3/La0.7Sr0.3MnO3 trilayers, Phys. Rev. B 99 (2019) 115135. [73] J. Walter, S. Bose, M. Cabero, G. Yu, M. Greven, M. Varela, C. Leighton, Perpendicular magnetic anisotropy via strain-engineered oxygen vacancy ordering in epitaxial La1xSrxCoO3δ, Phys. Rev. Mater. 2 (2018) 111404. [74] S. Koohfar, Y. Ozbek, H. Bland, Z. Zhang, D.P. Kumah, Interface-driven magnetic anisotropy in relaxed La0.7Sr0.3CrO3/La0.7Sr0.3MnO3 heterostructures on MgO, J. Appl. Phys. 129 (2021) 055301. [75] L. Wu, C. Li, M. Chen, Y. Zhang, K. Han, S. Zeng, X. Liu, J. Ma, C. Liu, J. Chen, Interface-induced enhancement of ferromagnetism in insulating LaMnO3 ultrathin films, ACS Appl. Mater. Interfaces 9 (2017) 44931–44937. [76] J. Zhang, X. Chen, Q. Zhang, F. Han, J. Zhang, H. Zhang, H. Zhang, H. Huang, S. Qi, X. Yan, Magnetic anisotropy controlled by distinct interfacial lattice distortions at the La1–xSrxCoO3/La2/3Sr1/3MnO3 interfaces, ACS Appl. Mater. Interfaces 10 (2018) 40951–40957. [77] S.K. Chaluvadi, F. Ajejas, P. Orgiani, S. Lebargy, A. Minj, S. Flament, J. Camarero, P. Perna, L. M
echin, Epitaxial strain and thickness dependent structural, electrical and

Interface engineering in oxide heterostructures

[78]

[79]

[80]

[81]

[82]

[83] [84]

[85]

[86]

[87]

[88]

[89]

[90]

269

magnetic properties of La0.67Sr0.33MnO3 films, J. Phys. D. Appl. Phys. 53 (2020) 375005. C.M. Xiong, J.R. Sun, B.G. Shen, Dependence of magnetic anisotropy of the La0.67Ca0.33MnO3 films on substrate and film thickness, Solid State Commun. 134 (2005) 465–469. L. Qu, D. Lan, K. Zhang, E. Hua, B. Ge, L. Xu, F. Jin, G. Gao, L. Wang, W. Wu, Continuous tunable lateral magnetic anisotropy in La0.67Ca0.33MnO3/SrRuO3 superlattices by stacking period-modulation, AIP Adv. 11 (2021) 075001. Z. Liao, M. Huijben, Z. Zhong, N. Gauquelin, S. Macke, R. Green, S. Van Aert, J. Verbeeck, G. Van Tendeloo, K. Held, Controlled lateral anisotropy in correlated manganite heterostructures by interface-engineered oxygen octahedral coupling, Nat. Mater. 15 (2016) 425–431. D. Yi, C.L. Flint, P.P. Balakrishnan, K. Mahalingam, B. Urwin, A. Vailionis, A.T. N’Diaye, P. Shafer, E. Arenholz, Y. Choi, K.H. Stone, J.H. Chu, B.M. Howe, J. Liu, I. R. Fisher, Y. Suzuki, Tuning perpendicular magnetic anisotropy by oxygen octahedral rotations in (La1-xSrxMnO3)/(SrIrO3) Superlattices, Phys. Rev. Lett. 119 (2017) 077201. J. Zhang, Z. Zhong, X. Guan, X. Shen, J. Zhang, F. Han, H. Zhang, H. Zhang, X. Yan, Q. Zhang, L. Gu, F. Hu, R. Yu, B. Shen, J. Sun, Symmetry mismatch-driven perpendicular magnetic anisotropy for perovskite/brownmillerite heterostructures, Nat. Commun. 9 (2018) 1923. J. Dho, Y.N. Kim, Y.S. Hwang, J.C. Kim, N.H. Hur, Strain-induced magnetic stripe domains in La0.7Sr0.3MnO3 thin films, Appl. Phys. Lett. 82 (2003) 1434–1436. S. Koohfar, A.B. Georgescu, I. Hallsteinsen, R. Sachan, M.A. Roldan, E. Arenholz, D.P. Kumah, Effect of strain on magnetic and orbital ordering of LaSrCrO3/ LaSrMnO3 heterostructures, Phys. Rev. B 101 (2020) 064420. Z. Li, X. Liu, J. Jiang, W. Mi, H. Bai, Atomic layer and interfacial oxygen defect tailored magnetic anisotropy and Dzyaloshinskii–Moriya interaction in perovskite SrRuO3/ SrTiO3 heterostructures, ACS Appl. Electron. Mater. 2 (2020) 2591–2600. X. Ke, T. Birol, R. Misra, J.H. Lee, B.J. Kirby, D.G. Schlom, C.J. Fennie, J.W. Freeland, Structural control of magnetic anisotropy in a strain-driven multiferroic EuTiO3 thin film, Phys. Rev. B 88 (2013). B. Cui, C. Song, F. Li, X.Y. Zhong, Z.C. Wang, P. Werner, Y.D. Gu, H.Q. Wu, M.S. Saleem, S.S.P. Parkin, F. Pan, Electric-field control of oxygen vacancies and magnetic phase transition in a cobaltite/manganite bilayer, Phys. Rev. Appl. 8 (2017) 044007. J. Song, Y. Chen, X. Chen, T. Khan, F. Han, J. Zhang, H. Huang, H. Zhang, W. Shi, S. Qi, F. Hu, B. Shen, J. Sun, Electric tuning of magnetic anisotropy and exchange bias of La0.8Sr0.2CoO3/La0.67Sr0.33MnO3 bilayer films, Phys. Rev. Appl. 14 (2020). Z. Guo, D. Lan, L. Qu, K. Zhang, F. Jin, B. Chen, S. Jin, G. Gao, F. Chen, L. Wang, W. Wu, Control of ferromagnetism and magnetic anisotropy via tunable electron correlation and spin-orbital coupling in La0.67Ca0.33MnO3/Ca(Ir,Ru)O3 superlattices, Appl. Phys. Lett. 113 (2018) 231601. B. Dieny, M. Chshiev, Perpendicular magnetic anisotropy at transition metal/oxide interfaces and applications, Rev. Mod. Phys. 89 (2017) 025008.

Composition induced dielectric and conductivity properties of rare-earth doped barium zirconium titanate ceramics

9

G. Nag Bhargavia, Tanmaya Badapandab, and Ayush Kharec a Department of Physics, Govt. Pt. Shyamacharan Shukla College, Raipur, Chhattisgarh, India, bDepartment of Physics, C.V. Raman Global University, Bhubaneswar, Odisha, India, c Department of Physics, National Institute of Technology, Raipur, Chhattisgarh, India

9.1

Introduction

The Perovskite structured (ABO3) ferroelectric materials; particularly polycrystalline ceramics have drawn the attention of many research groups owing to their considerable applications in various areas. These applications include high dielectric constant multilayer ceramic capacitors [1–8], ferroelectric random-access memories [9–15], actuators [16–18], pyroelectric sensors [19], opto-electronic devices [20–23], piezoelectric transducers [24,25], positive temperature coefficient thermistors [26,27], energy storing devices [28–32], etc. Perovskites (ferroelectric materials) give their best performance due to the presence of permanent dipole moment even in the absence of an external electric field. BaTiO3 (BT) and PbTiO3 (PT) are two most fascinating and commonly known ferroelectric materials since their discovery [33–41]. The basis of ferroelectricity in these materials is the displacement of centers of positive charges with respect to the centers of negative charge. Ultimately, it can be said that the origin of this property lies in its structure. Ferroelectric materials are good piezoelectrics and pyroelectrics as well [6]. When the temperature increases, these materials undergo displacive structural phase transitions from centrosymmetric to ferroelectric phase of lower symmetry. In the present scenario, the ferroelectrics with high dielectric constant and low dielectric loss are attractive and relevant in the field of materials research. Also, there is a search for ecofriendly Perovskite materials as well. In condensed matter research, the oxide perovskites have been studied much because they have the tendency to put up most of the metal ions in the periodic table. Another benefit of Oxide perovskite is that they allow considerable substitutions at one or both the cation sites (A and/or B). In the common structure of Perovskites (Fig. 9.1), “A” and “B” are cations of different ionic sizes bonded with oxygen (anion), “A” being bigger than “B.” Atom “A” has 12-fold co-ordination positioned at the center of unit cell while “B” has 6-fold co-ordination positioned at corners of unit cell. A generalized Perovskite structure is a three-dimensional network of Functional Materials from Carbon, Inorganic, and Organic Sources. https://doi.org/10.1016/B978-0-323-85788-8.00017-3 Copyright © 2023 Elsevier Ltd. All rights reserved.

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Fig. 9.1 The unit cell of ideal perovskite structure.

BO6 octahedral. BO6 octahedral plays a key role in ferroelectricity and ferromagnetism in perovskites. In this journey, Lead (Pb) based perovskites have been the most widely used piezoelectric ceramics for electromechanical applications [42–49]. But due to toxicity and hazardous impacts of Pb, in 2002, the European Union has regulated directives for not including Pb and other toxic elements in commercial products. This triggered an increase in research related to Pb free piezoelectric materials [30,50–53]. Since the discovery of ferroelectricity in BaTiO3 ceramics, it has been one of the most extensively studied perovskite materials. BaTiO3 exhibits tetragonal symmetry at room temperature with spontaneous polarization along c-axis [54]. The spontaneous polarization in BT is related to the shift of Ti4+ and O2
ions relative to the Ba2+ ion. The Curie temperature (Tc) for BT falls near 125°C. BT undergoes several structural phase 125°C ðTc Þ

5°C ðT1 Þ

transitions (Fig. 9.2): Cubic (Pm3m)





! tetragonal (P4mm)



! ortho
90°C ðT2 Þ

rhombic (Amm2)





! rhombohedral (R3m) [55]. These phase transitions are accompanied by some specific anomalies in thermal, mechanical and piezoelectric properties that are used in various device applications. Fortunately, BT ceramic offers two main disadvantages: firstly, it has tetragonal to orthorhombic phase transition below room temperature (i.e., 5°C) and secondly the resonant frequency rises rapidly with temperature in the tetragonal phase. In order to attain desired properties, the original BT structure needs some modifications, and this can be done by adding suitable dopants. The electrostatic charge compensation of ions within the structure is one 5°C