Metal Oxides for Optoelectronics and Optics-Based Medical Applications 0323858244, 9780323858243

Table of contents :
Front Cover
Metal Oxides for Optoelectronics and Optics-Based Medical Applications
Copyright
Contents
Contributors
Series editor biography
Preface to the series
Preface
Section A: Technology and properties
Chapter 1: Metal oxides for optoelectronic and photonic applications: A general introduction
1. Introduction
2. Properties of MOs
2.1. Optical properties of MOs
2.2. Stability of MOs
2.3. Conductivity of MOs
2.4. Transparency of MOs
2.5. Surface properties
2.6. Other properties
3. Chemical methods of MO synthesis
3.1. Sol-gel process
3.2. Combustion synthesis
3.3. Coprecipitation process
3.4. Electrochemical deposition method
3.5. Sonochemical method
3.6. Hydrothermal process
3.7. Chemical vapor deposition (CVD)
4. Physical methods of MO synthesis
4.1. Ball milling process
4.2. Sputtering
4.3. Electron beam evaporation (EBE)
4.4. Electrospraying
4.5. Laser ablation
5. Concluding remarks/conclusions
References
Chapter 2: Recent developments in optoelectronic and photonic applications of metal oxides
1. Introduction
2. Metal oxides (MOs) for various applications
2.1. Photodetector
2.2. Photovoltaic solar cells
2.3. Photoresistors
2.4. Sensors
2.5. Phototransistors
2.6. Photocatalysts
3. Role of metal oxides for thin-film technology
4. Optoelectronic properties of metal oxides
5. Conclusion
References
Chapter 3: Metal oxide-based glasses and their physical properties
1. Introduction
2. Preparation of metal oxide glasses
2.1. Preparation methods
2.1.1. Chemical routing
2.1.2. Thermal evaporation
2.1.3. Melt quenching and heat treatment
2.1.4. Gel desiccation
2.1.5. Sputtering
2.1.6. Shockwave formation
2.1.7. Other methods
2.2. Preparation of some fundamental metal oxide glasses
2.2.1. (GeO2)1-x(PbO)x
2.2.2. (TeO2)x(ZnO)1-x and (TeO2)(PbO, PbCl2)1-x
2.2.3. Ga2O3-PbO-Bi2O3
2.2.4. Bi2O3-PbO-B2O3-GeO2
2.2.5. Glasses doped with RE3+ ions
3. Properties of metal oxide glasses
3.1. Mechanical properties
3.2. Optical properties
3.3. Electrical and dielectric properties
4. Future aspects and applications
5. Summary and conclusions
Acknowledgments
References
Chapter 4: Metal oxide-based optical fibers (preparation, composition, composition-linked properties, physical parameters ...
1. Introduction
1.1. General
1.2. Operation principle
2. Fabrication of optical fibers
2.1. Outside vapor deposition (OVD) [12-15]
2.2. Vapor axial deposition (VAD) [16]
2.3. Modified chemical vapor deposition (MCVD) [17]
2.4. Plasma-activated chemical vapor deposition (PCVD) [16]
2.5. Drawing and coating of optical fibers
3. Metal oxides in optical fibers formation
4. Applications of optical fiber
4.1. Metal oxides as electrochemical pH sensors
4.2. Metal oxide nanoparticles as gas sensors
4.3. Metal oxides in batteries
4.4. Metal oxides in antennas
4.4.1. Optical antennas for photonic applications
4.4.2. Transparent microstrip patch antennas
4.4.3. Thermal infrared detection antenna
4.5. Optoelectronics and electronics
4.6. Solar cells
4.6.1. Dye-sensitized solar cells (DSSC/DSC)
5. Conclusions
References
Section B: Optoelectronic and photonic applications
Chapter 5: Metal oxide-based LED and LASER: Theoretical concepts, conventional devices, and LED based on quantum dots
1. Introduction
2. Different metal oxide nanostructures for light-emitting diodes (LEDs) and UV detectors
2.1. Monolithic ZnO nanowire-based micro-light-emitting diode/metal oxide
2.2. Quantum dot LEDs with a solution process-based copper oxide (CuO) hole injection layer
2.2.1. Synthesis of CuO quantum dot (QD) solution for QDLED fabrication
CuO-based QDLED characterization and fabrication
2.2.2. Fabrication of oxide-metal oxide-based electrodes combined with an antireflective film to improve the performance ...
Fabrication of flexible oxide-metal oxide-based organic light-emitting diodes (OLEDs)
3. Metal oxide-based LASER diodes
3.1. Photodetecting properties of single CuO-ZnO core-shell nanowires with a p-n radial heterojunction
3.2. Fabrication of a high-performance SiO2-p-CuO/n-Si core-shell structure-based photosensitive diode for photodetection ...
4. Quantum dots used for LEDs, LASERs, and conventional devices
4.1. Optically pumped quantum dot lasing and integrated optical cavities in LEDs
4.2. Single-pot synthesis of CdTexSe(1-x) quantum dot-based LED with red light emission
4.3. Sulfur quantum dot (SQD)-based nonlinear optics and different ultrafast photonic applications
4.4. Tandem quantum dot-based light-emitting diodes (QLEDs) with individual red, green, and blue emissions
5. Conclusions
6. Recommendations
References
Chapter 6: Metal oxide-based photodetectors (from IR to UV)
1. Introduction
2. Basic mechanism of metal oxide-based photodetection
3. Fundamental parameters of metal oxide-based photodetectors
4. Metal oxide-based photodetectors: Material selection, device design, and photosensing performances
4.1. Metal oxide nanomaterials
4.2. ZnO
4.3. TiO2
4.4. CuO
4.5. NiO
4.6. Ga2O3
4.7. V2O5
4.8. SnO2
4.9. MoO3
4.10. Ternary oxides
5. Biomedical applications of metal oxide-based photodetectors
6. Conclusions and perspectives
References
Chapter 7: Optical and optoelectronic metal oxide-based sensors; (optical sensors, principle, computational modeling, and ap
1. Introduction
2. Zinc oxide nanowires
3. Deposition by using RF sputtering
4. Optical properties of the ZnO nanowires
5. Gas sensing mechanism
6. Experiment procedure
6.1. Zinc oxide nanowires on fiber optics
7. Optical H2 gas sensing setup
8. Results and discussion
9. Conclusion
Acknowledgments
References
Chapter 8: Passive optoelectronic elements
1. Introduction to the fundamentals of passive optoelectronics
2. Relevant metal oxides for passive optoelectronics and their advantages
2.1. Selective optical filters
2.2. Thin-film polarizers
2.3. Antireflective and high-reflective coatings
2.4. Transparent conducting oxides
2.5. Photochromic devices
2.6. Optical waveguides
2.7. Splitters
2.8. All-optical modulators
2.9. Connectors and couplers
3. Conclusions
References
Chapter 9: Metal oxide photonic crystals and their application (designing, properties, and applications)
1. Introduction
2. Structure
3. Synthetic strategies for PCs
3.1. One-dimensional (ID)
3.2. Two-dimensional (2-D)
3.3. Three-dimensional (3-D)
3.3.1. Opals
3.3.2. Inverse opals
3.3.3. PC beads
4. Application
4.1. Biosensors
4.1.1. Glucose detection
4.1.2. Protein detection
4.1.3. Nucleic detection
4.1.4. Cholesterol detection
4.1.5. Pathogen detection
4.1.6. Cell carriers
4.1.7. Drug delivery and screening
4.1.8. Cell scaffolds and tissue engineering
4.1.9. Label-free cell imaging
4.1.10. Monitoring biological processes
5. Conclusion
References
Chapter 10: Heavy metal oxide glasses and their optoelectronic applications (infrared transmission, luminescence, nonline ...
1. Introduction
2. Optical properties of rare-earth ion-doped bismuth- and lead-containing borate glasses
2.1. Optical properties of europium (Eu)-doped HMOs
2.2. Photophysical properties of lead (Pb)-containing glasses
2.3. Photophysical properties of bismuth (Bi)-containing glasses
2.4. Third-order susceptibility of Bi2O3-based glasses
2.5. Photophysical properties of tellurium (Te)-, germanium (Ge)-, and zinc (Zn)-containing glasses
2.6. Optical properties of Bi2O3-GeO2 glasses
2.7. Photophysical properties of bismuth in zinc borate glasses
2.8. NLO properties of bismuth tungstate and lithium tetraborate composition glasses
3. Optical properties and applications of lead (II) oxide (PbO) glasses
3.1. Photonic applications of doped PbO glasses
3.2. Optical and spectroscopic properties of lead and bismuth in borosilicate glasses
3.3. Photophysical properties of lead oxide in Sb2O3-Na2O-WO3-PbO glasses
4. Conclusions
Acknowledgment
References
Chapter 11: Integrated optoelectronics
1. Introduction
2. Electrooptical phenomena
2.1. Electrooptic sampling and photoconductive switch sampling
2.2. Electrooptic sampling
3. Integrated photonic devices
4. Waveguides including metal oxide glasses
4.1. Thin film fabrication processes
5. Light modulators
5.1. Classification of optical modulators
6. Optical switches
7. Metal oxides designed for these applications
7.1. Optoelectronic properties of metal oxides
8. Manufacturing features
9. Conclusion
References
Section C: Metal oxide-based optoelectronic devices in biomedical applications
Chapter 12: Metal oxide-based fiber technology in the pharmaceutical and medical chemistry
1. Introduction
2. Electrospinning-Cutting edge technology
2.1. Historical background of electrospinning
2.2. Conceptualization of electrospinning
2.2.1. Factors governing fiber diameter
2.2.2. Solidification
2.2.3. Deposition
2.3. Single-nozzle electrospinning
2.4. Multinozzle electrospinning
2.5. Coaxial electrospinning
3. Feed materials for electrospinning
3.1. Polymers
3.1.1. Natural polymers
3.1.2. Synthetic polymers
3.1.3. Conducting polymers
3.1.4. Small molecules
3.1.5. Colloids
3.1.6. Metal oxide nanoparticles (MONPs)
Metal/carbonaceous nanofibers
4. Applications of electrospun nanofibers in biomedical and environmental sector
4.1. Tissue engineering applications
4.2. Wound healing/dressing
4.3. Drug delivery
5. Conclusion and future outlook
Acknowledgments
References
Chapter 13: Metal oxide-involved photocatalytic technology in cosmetics and beauty products
1. Introduction
2. Overview of MONPs and their applications
2.1. Overview
2.2. Photocatalytic activity of MONPs
2.3. Engineered MONPs in cosmetics and beauty products
3. MONPs in cosmetics and beauty/personal care products
3.1. Selected MONPs as active ingredients in cosmetics and beauty care products
3.1.1. TiO2 as an active ingredient in cosmetics and beauty care products
Overview
Synthesis
Toxicity
3.1.2. ZnO as an active ingredient in cosmetics and beauty care products
Overview
Synthesis
Toxicity
3.1.3. SiO2 as an active ingredient in cosmetics and beauty care products
Overview
Synthesis
Toxicity
3.1.4. Al2O3 as an active ingredient in cosmetics and beauty care products
Overview
Synthesis
Toxicity
3.1.5. Iron oxide as an active ingredient in cosmetics and beauty care products
Overview
Synthesis
Toxicity
3.1.6. Ag2O as an active ingredient in cosmetics and beauty care products
Synthesis
Toxicity
4. Photocatalytic activity of MONPs in sunscreen products
4.1. Classification of sunscreen agents
4.2. Safety of sunscreens
5. Conclusions and future perspectives
References
Chapter 14: Illuminating metal oxides containing luminescent probes for personalized medicine
1. Introduction
1.1. Metal oxide in bioimaging and theranostics
1.2. Fluorescent probes in bioimaging
1.3. Combined effects of theranostics and antimicrobials
1.4. Metal oxides
2. Zinc oxide materials
2.1. Introduction
2.2. Zinc oxide in bioimaging and theranostics
2.3. Antimicrobial zinc oxide
2.3.1. UV-light irradiation
2.3.2. Visible light irradiation
2.3.3. Near infrared irradiation
2.4. Zinc oxide for amyloid detection and degradation against Alzheimer´s disease
3. Titanium oxide materials
3.1. Introduction
3.2. Titanium oxide in bioimaging and theranostics
3.3. Antimicrobial titanium oxide
3.3.1. Doping with non-metal elements
3.3.2. Doping with metallic elements
3.3.3. Doping with both metallic and nonmetallic elements
4. Other metal oxides
4.1. Zirconium dioxide
4.2. Yttrium oxide
4.3. Lanthanum oxide
4.4. Cerium oxide
4.5. Manganese oxide
4.6. Iron oxide
4.7. Tin oxide
5. Conclusion
References
Chapter 15: Metal oxide nanostructures and their biological applications (nonlinear photonics, plasmonic nanostructures, ...
1. Introduction
1.1. Approaches used in the study of MO-biomolecule interactions
1.2. Choice of metal (metalloid) oxide systems
1.3. Metal oxide nanoparticles: Research needs and opportunities in the biomedical field
2. Surface modification of metal oxide NPs: Biocompatibility and surface properties
2.1. Interaction of metal oxide NPs with biological systems
2.2. Interaction of nanoparticles with proteins
2.3. Interaction of nanoparticles with cells and tissues
3. Main types of metal oxide nanoparticles with potential use in biomedicine
3.1. Magnetic Iron oxide nanoparticles
3.2. Titania
3.3. Ceria
3.4. SiO2
4. Applications in biosystems
4.1. Biosensing
4.2. Bioimaging
4.3. Therapeutics
4.3.1. Drug and gene delivery
4.3.2. Cancer treatment strategies
5. Applications based on thermal effects
5.1. Photothermal therapeutics
5.2. Smart nanocarriers
5.3. Plasmonic tweezers
6. Applications of MONPs in biomedicine
6.1. Internal tissue therapy
6.2. Immunotherapy
6.3. Diagnosis
6.4. Nano-oxides in dentistry
6.5. Nano-oxides in hard tissue regeneration
6.6. Nano-oxides for wound healing
6.7. Nano-oxides used as biosensors
6.8. Antimicrobial nano-oxides
7. Nanotoxicology
8. Conclusions
References
Index
Back Cover

Citation preview

Metal Oxides for Optoelectronics and Optics-Based Medical Applications

The Metal Oxides Book Series Edited by Ghenadii Korotcenkov Forthcoming Titles l

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Palladium Oxides Material Properties, Synthesis and Processing Methods, and Applications, Alexander M. Samoylov, Vasily N. Popov, 9780128192238 Metal Oxides for Non-volatile Memory, Panagiotis Dimitrakis, Ilia Valov, Stefan Tappertzhofen, 9780128146293 Metal Oxide Nanostructured Phosphors, H. Nagabhushana, Daruka Prasad, S.C. Sharma, 9780128118528 Multifunctional Piezoelectric Oxide Nanostructures, Sang-Jae Kim, Nagamalleswara Rao Alluri, Yuvasree Purusothaman, 9780128193327 Transparent Conductive Oxides, Mirela Petruta Suchea, Petronela Pascariu, Emmanouel Koudoumas, 9780128206317 Metal Oxide-Carbon Hybrid Materials, Muhammad Akram, Rafaqat Hussain, Faheem K Butt, 9780128226940 Metal Oxide-based heterostructures, Naveen Kumar, Bernabe Mari Soucase, 9780323852418 Metal Oxides and Related Solids for Electrocatalytic Water Splitting, Junlei Qi, 9780323857352 Advances in Metal Oxides and Their Composites for Emerging Applications, Sagar Delekar, 9780323857055 Metallic Glasses and Their Oxidation, Xinyun Wang, Mao Zhang, 9780323909976 Solution Methods for Metal Oxide Nanostructures, Rajaram S. Mane, Vijaykumar Jadhav, Abdullah M. AlEnizi, 9780128243534 Metal Oxide Defects, Vijay Kumar, Sudipta Som, Vishal Sharma, Hendrik Swart, 9780323855884 Renewable Polymers and Polymer-Metal Oxide Composites, Sajjad Haider, Adnan Haider, 9780323851558 Metal Oxides for Optoelectronics and Optics-based Medical Applications, Suresh Sagadevan, Jiban Podder, Faruq Mohammad, 9780323858243

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Metal-oxides for Biomedical and Biosensor Applications, Kunal Mondal, 9780128230336 Nanostructured Zinc Oxide, Kamlendra Awasthi, 9780128189009 Metal oxide-based nanofibers and their applications, Vincenzo Esposito, Debora Marani, 9780128206294 Graphene Oxide-Metal Oxide and Other Graphene Oxide-Based Composites in Photocatalysis and Electrocatalysis, Jiaguo Yu, Liuyang Zhang, Panyong Kuang, 9780128245262 Metal Oxides in Nanocomposite-Based Electrochemical Sensors for Toxic Chemicals, Alagarsamy Pandikumar, Perumal Rameshkumar, 9780128207277 Metal Oxide-Based Nanostructured Electrocatalysts for Fuel Cells, Electrolyzers, and Metal-Air Batteries, Teko Napporn, Yaovi Holade, 9780128184967 Titanium Dioxide (TiO2) and Its Applications, Leonardo Palmisano, Francesco Parrino, 9780128199602 Solution Processed Metal Oxide Thin Films for Electronic Applications, Zheng Cui, 9780128149300 Metal Oxide Powder Technologies, Yarub Al-Douri, 9780128175057 Colloidal Metal Oxide Nanoparticles, Sabu Thomas, Anu Tresa Sunny, Prajitha V, 9780128133576 Cerium Oxide, Salvatore Scire, Leonardo Palmisano, 9780128156612 Tin Oxide Materials, Marcelo Ornaghi Orlandi, 9780128159248 Metal Oxide Glass Nanocomposites, Sanjib Bhattacharya, 9780128174586 Gas Sensors Based on Conducting Metal Oxides, Nicolae Barsan, Klaus Schierbaum, 9780128112243 Metal Oxides in Energy Technologies, Yuping Wu, 9780128111673 Metal Oxide Nanostructures, Daniela Nunes, Lidia Santos, Ana Pimentel, Pedro Barquinha, Luis Pereira, Elvira Fortunato, Rodrigo Martins, 9780128115121 Gallium Oxide, Stephen Pearton, Fan Ren, Michael Mastro, 9780128145210 Metal Oxide-Based Photocatalysis, Adriana Zaleska-Medynska, 9780128116340 Metal Oxides in Heterogeneous Catalysis, Jacques C. Vedrine, 9780128116319 Magnetic, Ferroelectric, and Multiferroic Metal Oxides, Biljana Stojanovic, 9780128111802 Iron Oxide Nanoparticles for Biomedical Applications, Sophie Laurent, Morteza Mahmoudi, 9780081019252 The Future of Semiconductor Oxides in Next-Generation Solar Cells, Monica Lira-Cantu, 9780128111659 Metal Oxide-Based Thin Film Structures, Nini Pryds, Vincenzo Esposito, 9780128111666 Metal Oxides in Supercapacitors, Deepak Dubal, Pedro Gomez-Romero, 9780128111697 Transition Metal Oxide Thin Film-Based Chromogenics and Devices, Pandurang Ashrit, 9780081018996

Metal Oxides Series

Metal Oxides for Optoelectronics and Optics-Based Medical Applications Series Editor

Ghenadii Korotcenkov Edited by

Suresh Sagadevan Nanotechnology & Catalysis Research Centre, University of Malaya, Kuala Lumpur, Malaysia

Jiban Podder Department of Physics, Bangladesh University of Engineering and Technology (BUET), Dhaka, Bangladesh

Faruq Mohammad Surfactants Research Chair, Department of Chemistry, College of Science, King Saud University, Riyadh, Saudi Arabia

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2022 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-323-85824-3 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisitions Editor: Kayla Dos Santos Editorial Project Manager: Isabella C. Silva Production Project Manager: Sojan P. Pazhayattil Cover Designer: Miles Hitchen Typeset by STRAIVE, India

Contents

Contributors Series editor biography Preface to the series Preface

Section A Technology and properties 1

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Metal oxides for optoelectronic and photonic applications: A general introduction Mehmood Shahid, Suresh Sagadevan, Waqar Ahmed, Yiqiang Zhan, and Pakorn Opaprakasit 1 Introduction 2 Properties of MOs 3 Chemical methods of MO synthesis 4 Physical methods of MO synthesis 5 Concluding remarks/conclusions References Recent developments in optoelectronic and photonic applications of metal oxides K. Tamizh Selvi and Suresh Sagadevan 1 Introduction 2 Metal oxides (MOs) for various applications 3 Role of metal oxides for thin-film technology 4 Optoelectronic properties of metal oxides 5 Conclusion References Metal oxide-based glasses and their physical properties Muhammad Nihal Naseer, Muhammad Azhar, Asad A. Zaidi, Yasmin Binti Abdul Wahab, Muhammad Asif, and Suresh Sagadevan 1 Introduction 2 Preparation of metal oxide glasses 3 Properties of metal oxide glasses

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4 Future aspects and applications 5 Summary and conclusions Acknowledgments References 4

Metal oxide-based optical fibers (preparation, composition, composition-linked properties, physical parameters, and theoretical calculations) Shahla Imteyaz 1 Introduction 2 Fabrication of optical fibers 3 Metal oxides in optical fibers formation 4 Applications of optical fiber 5 Conclusions References

Section B Optoelectronic and photonic applications 5

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Metal oxide-based LED and LASER: Theoretical concepts, conventional devices, and LED based on quantum dots Waqar Ahmed, Shahid Mehmood, Nor Azwadi Che Sidik, and Suresh Sagadevan 1 Introduction 2 Different metal oxide nanostructures for light-emitting diodes (LEDs) and UV detectors 3 Metal oxide-based LASER diodes 4 Quantum dots used for LEDs, LASERs, and conventional devices 5 Conclusions 6 Recommendations References Metal oxide-based photodetectors (from IR to UV) Zhong Ma, Jing Zhang, Hanbai Lyu, Xinyu Ping, Lijia Pan, and Yi Shi 1 Introduction 2 Basic mechanism of metal oxide-based photodetection 3 Fundamental parameters of metal oxide-based photodetectors 4 Metal oxide-based photodetectors: Material selection, device design, and photosensing performances 5 Biomedical applications of metal oxide-based photodetectors 6 Conclusions and perspectives References

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Optical and optoelectronic metal oxide-based sensors; (optical sensors, principle, computational modeling, and application-based development) Nur Amalina Syahirah Mohd Idris, Shamsu Abubakar, Ahmed Lateef Khalaf, Mohd. Hanif Yaacob, Suresh Sagadevan, and Suriati Paiman 1 Introduction 2 Zinc oxide nanowires 3 Deposition by using RF sputtering 4 Optical properties of the ZnO nanowires 5 Gas sensing mechanism 6 Experiment procedure 7 Optical H2 gas sensing setup 8 Results and discussion 9 Conclusion Acknowledgments References Passive optoelectronic elements Marlinda Ab Rahman, Noor Azrina Talik, Mohd Arif Mohd Sarjidan, and Gregory Soon How Thien 1 Introduction to the fundamentals of passive optoelectronics 2 Relevant metal oxides for passive optoelectronics and their advantages 3 Conclusions References Metal oxide photonic crystals and their application (designing, properties, and applications) Tanvir Arfin, Vinod Kumar Alam, and Pareshkumar G. Moradeeya 1 Introduction 2 Structure 3 Synthetic strategies for PCs 4 Application 5 Conclusion References Heavy metal oxide glasses and their optoelectronic applications (infrared transmission, luminescence, nonlinear optical susceptibilities, etc.) S. Mahalakshmi, J. Mayandi, Suresh Sagadevan, P. Vajeeston, and V. Venkatachalapathy 1 Introduction 2 Optical properties of rare-earth ion-doped bismuthand lead-containing borate glasses

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Optical properties and applications of lead (II) oxide (PbO) glasses 4 Conclusions Acknowledgment References

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Integrated optoelectronics Suresh Sagadevan, J. Anita Lett, Is Fatimah, Suriati Paiman, Jiban Podder, and Mohd. Rafie Johan 1 Introduction 2 Electrooptical phenomena 3 Integrated photonic devices 4 Waveguides including metal oxide glasses 5 Light modulators 6 Optical switches 7 Metal oxides designed for these applications 8 Manufacturing features 9 Conclusion References

Section C Metal oxide-based optoelectronic devices in biomedical applications 12

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Metal oxide-based fiber technology in the pharmaceutical and medical chemistry Lakshmipathy Muthukrishnan, Suresh Sagadevan, and M.A. Motalib Hossain 1 Introduction 2 Electrospinning—Cutting edge technology 3 Feed materials for electrospinning 4 Applications of electrospun nanofibers in biomedical & environmental sector 5 Conclusion and future outlook Acknowledgments References Metal oxide-involved photocatalytic technology in cosmetics and beauty products Ibrahim B. Bwatanglang, Prasanna Kumar Obulapuram, Faruq Mohammad, Aiesha N. Albalawi, Murthy Chavali, Hamad A. Al-Lohedan, and Toma Ibrahim 1 Introduction 2 Overview of MONPs and their applications 3 MONPs in cosmetics and beauty/personal care products

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4 Photocatalytic activity of MONPs in sunscreen products 5 Conclusions and future perspectives References 14

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Illuminating metal oxides containing luminescent probes for personalized medicine Estelle L eonard and Victorien Jeux 1 Introduction 2 Zinc oxide materials 3 Titanium oxide materials 4 Other metal oxides 5 Conclusion References Metal oxide nanostructures and their biological applications (nonlinear photonics, plasmonic nanostructures, etc.) Kamrun Nahar Fatema and Won-Chun Oh 1 Introduction 2 Surface modification of metal oxide NPs: Biocompatibility and surface properties 3 Main types of metal oxide nanoparticles with potential use in biomedicine 4 Applications in biosystems 5 Applications based on thermal effects 6 Applications of MONPs in biomedicine 7 Nanotoxicology 8 Conclusions References

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Contributors

Marlinda Ab Rahman Nanotechnology & Catalysis Research Centre, University of Malaya, Kuala Lumpur, Malaysia Yasmin Binti Abdul Wahab Nanotechnology & Catalysis Research Centre, University of Malaya, Kuala Lumpur, Malaysia Shamsu Abubakar Department of Physics, Faculty of Science, Universiti Putra Malaysia, Serdang, Selangor, Malaysia Waqar Ahmed Institute for Advanced Studies, University of Malaya; Takasago i-Kohza, Malaysia-Japan International Institute of Technology, Universiti Teknologi Malaysia, Kuala Lumpur, Malaysia Vinod Kumar Alam CSIR-National Environmental Engineering Research Institute (CSIR-NEERI), Hyderabad Zonal Centre, Hyderabad, Telangana, India Aiesha N. Albalawi Haql College, University of Tabuk, Tabuk, Kingdom of Saudi Arabia Hamad A. Al-Lohedan Surfactants Research Chair, Department of Chemistry, College of Science, King Saud University, Riyadh, Saudi Arabia Tanvir Arfin CSIR-National Environmental Engineering Research Institute (CSIR-NEERI), Hyderabad Zonal Centre, Hyderabad, Telangana, India Muhammad Asif National University of Sciences and Technology (NUST), Islamabad, Pakistan Muhammad Azhar National University of Sciences and Technology (NUST), Islamabad, Pakistan Ibrahim B. Bwatanglang Department of Pure and Applied Chemistry, Faculty of Science, Adamawa State University Mubi, Mubi, Nigeria Murthy Chavali Shree Velagapudi Ramakrishna Memorial College (SVRMC-PG Studies), Guntur, Andhra Pradesh, India Kamrun Nahar Fatema Department of Advanced Materials Science & Engineering, Hanseo University, Seosan-si, Chungnam, Korea

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Contributors

Is Fatimah Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Islam Indonesia, Yogyakarta, Indonesia M.A. Motalib Hossain Nanotechnology & Catalysis Research Centre, University of Malaya, Kuala Lumpur, Malaysia Toma Ibrahim Department of Biochemistry, Faculty of Science, Adamawa State University Mubi, Mubi, Nigeria Nur Amalina Syahirah Mohd Idris Department of Physics, Faculty of Science, Universiti Putra Malaysia, Serdang, Selangor, Malaysia Shahla Imteyaz Department of Chemistry, Aligarh Muslim University, Aligarh, India Victorien Jeux Universite de technologie de Compie`gne, ESCOM, TIMR (Integrated Transformations of Renewable Matter), Centre de recherche Royallieu—CS, Compie`gne Cedex, France Mohd. Rafie Johan Nanotechnology & Catalysis Research Centre, University of Malaya, Kuala Lumpur, Malaysia Ahmed Lateef Khalaf Department of Computer Engineering Techniques, Al-Ma’moon University College, Baghdad, Iraq Estelle Leonard Universite de technologie de Compie`gne, ESCOM, TIMR (Integrated Transformations of Renewable Matter), Centre de recherche Royallieu— CS, Compie`gne Cedex, France J. Anita Lett Department of Physics, Sathyabama Institute of Science and Technology, Chennai, Tamil Nadu, India Hanbai Lyu Physics Department, Yale University, New Haven, CT, United States Zhong Ma Collaborative Innovation Center of Advanced Microstructures, School of Electronic Science and Engineering, Nanjing University, Nanjing, China S. Mahalakshmi Department of Physics, R.D. Govt. Arts College, Alagappa University, Karaikudi, Tamil Nadu, India J. Mayandi Department of Material Science, School of Chemistry, Madurai Kamaraj University, Madurai, Tamil Nadu, India Shahid Mehmood Micro Nanosystem Centre, School of Information Science and Technology, Fudan University, Shanghai, China

Contributors

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Faruq Mohammad Surfactants Research Chair, Department of Chemistry, College of Science, King Saud University, Riyadh, Saudi Arabia Mohd Arif Mohd Sarjidan Low Dimensional Materials Research Centre, Department of Physics, Faculty of Science, University of Malaya, Kuala Lumpur, Malaysia Pareshkumar G. Moradeeya CSIR-National Environmental Engineering Research Institute (CSIR-NEERI), Hyderabad Zonal Centre, Hyderabad, Telangana, India Lakshmipathy Muthukrishnan Department of Conservative Dentistry & Endodontics, Saveetha Dental College & Hospitals, Saveetha Institute of Medical and Technical Sciences, Chennai, Tamil Nadu, India Muhammad Nihal Naseer National University of Sciences and Technology (NUST), Islamabad, Pakistan; Department of Mechanical Engineering, College of Engineering, Seoul National University, Seoul, Republic of Korea Prasanna Kumar Obulapuram Wits Advanced Drug Delivery Platform Research Unit, Department of Pharmacy and Pharmacology, School of Therapeutic Sciences, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South Africa Won-Chun Oh Department of Advanced Materials Science & Engineering, Hanseo University, Seosan-si, Chungnam, Korea; Anhui International Joint Research Center for Nano Carbon-based Materials and Environmental Health, College of Materials Science and Engineering, Anhui University of Science & Technology, Huainan, PR China Pakorn Opaprakasit School of Bio-Chemical Engineering and Technology, Sirindhorn International Institute of Technology (SIIT), Thammasat University, Pathum Thani, Thailand Suriati Paiman Department of Physics, Faculty of Science; Functional Nanotechnology Devices Laboratory (FNDL), Institute of Nanoscience and Nanotechnology, Universiti Putra Malaysia, Serdang, Selangor, Malaysia Lijia Pan Collaborative Innovation Center of Advanced Microstructures, School of Electronic Science and Engineering, Nanjing University, Nanjing, China Xinyu Ping School of Arts and Sciences, University of Pennsylvania, Philadelphia, PA, United States Jiban Podder Department of Physics, Bangladesh University of Engineering and Technology (BUET), Dhaka, Bangladesh

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Contributors

Suresh Sagadevan Nanotechnology & Catalysis Research Centre, University of Malaya, Kuala Lumpur, Malaysia K. Tamizh Selvi Department of Physics, Vel Tech High Tech Dr. Rangarajan Dr. Sakunthala Engineering College, Chennai, India Mehmood Shahid Center of Micro-Nano System, School of Information Science and Technology, Fudan University, Shanghai, China; School of Bio-Chemical Engineering and Technology, Sirindhorn International Institute of Technology (SIIT), Thammasat University, Pathum Thani, Thailand Yi Shi Collaborative Innovation Center of Advanced Microstructures, School of Electronic Science and Engineering, Nanjing University, Nanjing, China Nor Azwadi Che Sidik Takasago i-Kohza, Malaysia-Japan International Institute of Technology, Universiti Teknologi Malaysia, Kuala Lumpur, Malaysia Noor Azrina Talik Low Dimensional Materials Research Centre, Department of Physics, Faculty of Science, University of Malaya, Kuala Lumpur, Malaysia Gregory Soon How Thien Low Dimensional Materials Research Centre, Department of Physics, Faculty of Science, University of Malaya, Kuala Lumpur; Centre for Advanced Devices and Systems, Faculty of Engineering, Multimedia University, Selangor, Malaysia P. Vajeeston Centre for Materials Science and Nanotechnology Chemistry, Department of Chemistry, University of Oslo, Oslo, Norway V. Venkatachalapathy Department of Physics/Centre for Materials Science and Nanotechnology, University of Oslo, Oslo, Norway; Department of Materials Science, National Research Nuclear University, Moscow, Russian Federation Mohd. Hanif Yaacob Wireless and Photonics Network Research Centre, Engineering and Technology Complex, Universiti Putra Malaysia, Serdang, Selangor, Malaysia Asad A. Zaidi Department of Mechanical Engineering, Faculty of Engineering Science and Technology, Hamdard University, Madinat al-Hikmah, Hakim Mohammad Said Road, Karachi, Pakistan Yiqiang Zhan Center of Micro-Nano System, School of Information Science and Technology, Fudan University, Shanghai, China Jing Zhang Collaborative Innovation Center of Advanced Microstructures, School of Electronic Science and Engineering, Nanjing University, Nanjing, China

Series editor biography

Ghenadii Korotcenkov received his PhD in Physics and Technology of Semiconductor Materials and Devices in 1976 and his Doctor of Science Degree (Doctor Habilitate) in Physics of Semiconductors and Dielectrics in 1990. He has more than 45 years of experience as a teacher and scientific researcher. For a long time he was a leader of the gas sensor group and manager of various national and international scientific and engineering projects carried out in the Laboratory of Micro- and Optoelectronics, Technical University of Moldova, Chisinau, Moldova. His research has received financial support from international foundations and programs such as the CRDF, the MRDA, the ICTP, the INTAS, the INCO-COPERNICUS, the COST, and NATO. From 2007 to 2008, he was an invited scientist at the Korea Institute of Energy Research (Daejeon); following which, until 2017, Dr. Korotcenkov was a research professor in the School of Materials Science and Engineering at Gwangju Institute of Science and Technology (GIST) in Korea. Currently, Dr. Korotcenkov is a chief scientific researcher at Moldova State University, Chisinau, Moldova. Specialists from the former Soviet Union know G. Korotcenkov’s research results in the field of study of Schottky barriers, MOS structures, native oxides, and photoreceivers on the basis of III–Vs compounds such as InP, GaP, AlGaAs, and InGaAs. His present scientific interests, starting from 1995, include materials science, focusing on metal oxide film deposition and characterization (In2O3, SnO2, TiO2), surface science, thermoelectric conversion, and design of physical and chemical sensors, including thin film gas sensors. Dr. Korotcenkov is the author or editor of 45 books and special issues, including the 11-volume “Chemical Sensors” series published by Momentum Press, 15-volume “Chemical Sensors” series published by Harbin Institute of Technology Press, China, three-volume “Porous Silicon: From Formation to Application” issue published by CRC Press, two-volume “Handbook of Gas Sensor Materials” published by Springer, three-volume “Handbook of Humidity Measurements” published by CRC Press, and six proceedings of international conferences published by Trans Tech Publ., Elsevier, and EDP Sciences. Currently he is a series editor of the “Metal Oxides” book series

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Series editor biography

published by Elsevier. Since its inception in 2017, more than 24 volumes have already been published within the framework of that series. Dr. Korotcenkov is author and coauthor of more than 650 scientific publications including 31 review papers, 38 book chapters, and more than 200 peer-reviewed articles published in scientific journals (h-factor ¼ 41 (Web of Science), h ¼ 43 (Scopus), and h ¼ 57 (Google scholar citation)), and is a holder of 17 patents. He has also presented more than 250 reports to national and international conferences, including 17 invited talks. G. Korotcenkov, as a cochairman or a member of program, scientific, and steering committees, has participated in the organization of more than 30 international scientific conferences. Dr. Korotcenkov is a member of editorial boards in five scientific international journals. His name and activities have been listed by many biographical publications including Who’s Who. His research activities have been honored by the Honorary Diploma of the Government of the Republic of Moldova (2020), an Award of the Academy of Sciences of Moldova (2019), an Award of the Supreme Council of Science and Advanced Technology of the Republic of Moldova (2004), the Prize of the Presidents of the Ukrainian, Belarus, and Moldovan Academies of Sciences (2003), Senior Research Excellence Award of the Technical University of Moldova (2001; 2003; 2005), and the National Youth Prize of the Republic of Moldova in the Field of Science and Technology (1980), among others. Dr. Korotcenkov has also received fellowships from the International Research Exchange Board (IREX, United States, 1998), Brain Korea 21 Program (2008– 2012), and Brain Pool Program (Korea, 2007–2008 and 2015–2017). https://www.scopus.com/authid/detail.uri?authorId¼6701490962 https://publons.com/researcher/1490013/ghenadii-korotcenkov/ https://scholar.google.com/citations?user¼XR3RNhAAAAAJ&hl https://www.researchgate.net/profile/G_Korotcenkov

Preface to the series

The field of synthesis, study, and application of metal oxides is one of the most rapidly progressing areas of science and technology. Metal oxides are one of the most ubiquitous compound groups on Earth, and have a wide variety of chemical compositions, atomic structures, and crystalline shapes. In addition, metal oxides are known to possess unique functionalities that are absent or inferior in other solid materials. In particular, metal oxides represent a varied and appealing class of materials that exhibit a full spectrum of electronic properties—from insulating to semiconducting, metallic, and superconducting. Moreover, almost all the known effects, including superconductivity, thermoelectric effects, photoelectrical effects, luminescence, and magnetism can be observed in metal oxides. Therefore, metal oxides have emerged as an important class of multifunctional materials with a rich collection of properties that have great potential for numerous device applications. Specific properties, such as the wide variety of materials with different electrophysical, optical, and chemical characteristics, their high thermal and temporal stability, and their ability to function in harsh environments, make metal oxides highly suitable materials for designing transparent electrodes, high-mobility transistors, gas sensors, actuators, acoustical transducers, photovoltaic and photonic devices, photoand heterogeneous catalysts, solid-state coolers, high-frequency and micromechanical devices, energy-harvesting and storage devices, nonvolatile memory devices, and many other applications in the electronics, energy, and health sectors. In these devices, metal oxides can be successfully used as sensing or active layers, substrates, electrodes, promoters, structure modifiers, membranes, and fibers, i.e., can be used as both active and passive components. Among other advantages of metal oxides are the low fabrication cost and robustness in practical applications. Furthermore, metal oxides can be prepared in various forms, such as ceramics, thick films, and thin films. Furthermore, thin film deposition techniques can be used for metal oxides that are compatible with standard microelectronic technology. This last factor is very important for large-scale production, because the microelectronic approach promotes low cost for mass production, offers the possibility of manufacturing devices on a chip, and guarantees good reproducibility. Various metal oxide nanostructures, including nanowires, nanotubes, nanofibers, core-shell structures, and hollow nanostructures can also be synthesized. As a consequence, the field of metal oxide nanostructured morphologies (e.g., nanowires, nanorods, nanotubes, etc.) has become one of the most active research areas within the nanoscience community. The ability to create a variety of metal oxide-based composites and the ability to synthesize various multicomponent compounds significantly expands the range of properties that metal oxide-based materials can have, making metal oxides a truly

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Preface to the series

versatile multifunctional material for widespread use. It is known that small changes in their chemical composition and atomic structure can be accompanied by spectacular variation in the properties and behavior of metal oxides; and even now, advances in synthesizing and characterizing techniques are revealing numerous new functions of metal oxides. Taking into account the importance of metal oxides for progress in microelectronics, optoelectronics, photonics, energy conversion, sensors, and catalysis, a large and varied range of books devoted to this class of materials have been published. However, one should note that some books from this list are too general, some books are collections of various original works without any generalizations, and other ones were published many years ago. Nonetheless, during the past decade great progress has been made on the synthesis as well as on the structural, physical, chemical characterization, and application of metal oxides in various devices, and a large number of papers have been published on metal oxides. In addition, until now, many important topics related to metal oxides’ study and application had not been discussed. To remedy this situation, we decided to generalize and systematize the results of research in this field and to publish a series of books devoted to metal oxides. One should note that the proposed book series “Metal Oxides” is the first one devoted solely to consideration of metal oxides. We believe that combining books on metal oxides in a series could help readers in finding required information on the subject. In particular, we intend that the books from our series, which have a clear specialization of content, will provide interdisciplinary discussion for various oxide materials with a wide range of topics, from material synthesis and deposition to characterizations, processing, and to device fabrications and applications. This series of books is prepared by a team of highly qualified experts, which guarantees it a high degree of quality. I hope that our books will be both useful and easy to use. I would also like to hope that readers will consider this “Metal Oxides” series to be like an encyclopedia of metal oxides that enables them to understand the present status of metal oxides, to estimate the role of multifunctional metal oxides in design of advanced devices, and then, based on observed knowledge, to formulate new goals for further research. The intended audience of the present series is scientists and researchers working or planning to work in the field of materials related to metal oxides, i.e., scientists and researchers whose activities are related to electronics, optoelectronics, energy, catalysis, sensors, electrical engineering, ceramics, biomedical designs, etc. I believe that this “Metal Oxides” series will also be interesting for practicing engineers or project managers in industries and national laboratories, who would like to design metal oxide-based devices, but don’t know how to do it, or how to select optimal metal oxides for specific applications. With many references to the vast resource of recently published literature on the subject, this series will serve as a significant and insightful source of valuable information, providing scientists and engineers with new insights for understanding and improving existing metal oxide-based devices and for designing new metal oxide-based materials with novel and unexpected properties. I believe that this “Metal Oxides” series of books will be very helpful for university students, post docs, and professors. The structure of these books offers a basis for

Preface to the series

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courses in the field of material sciences, chemical engineering, electronics, electrical engineering, optoelectronics, energy technologies, environmental control, and many others. Graduate students should also find the book series to be very useful in their research and in understanding features of metal oxides synthesis, study, and the application of this multifunctional material in various devices. We are sure that all of the above will find information useful for their activities. Finally, I thank all contributing authors and editors who have been involved in the creation of these books. I am thankful that they agreed to participate in this project and for their efforts in the preparation of these books. Without their participation, this project would not have been possible. I also express my gratitude to Elsevier for giving us the opportunity to publish this series. I especially thank the whole editorial team at Elsevier for their patience during the development of this project and for encouraging us during the various stages of preparation. Ghenadii Korotcenkov

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Preface

Metal oxides (MOs) are the most profuse stable materials found in the earth’s crust. When compared to conventional semiconductor materials such as silicon and other III–V group compounds, the properties offered by MOs are completely different, including the material synthesis process, electronic and charge transport mechanisms, presence of crystal defects, fabrication of thin films, and optoelectronic properties. Also, they are relatively inexpensive, robust, lightweight, long-lasting, and benefit from high material sensitivity and quick response times. Thus MOs are utilized in a variety of roles as industrial alternatives. However, the lack of basic information, including the synthesis process and the mechanism that leads to their application and efficiency control, is a major bottleneck for practicality. Therefore this book provides recent innovative information about MOs and their mechanistic roles in optoelectronic and photoluminescence applications. The book is also aimed at presenting research articles and comprehensive reviews to improve optoelectronic efficiency. Emphasis is placed on the principles and techniques from synthesis chemistry to serve fields such as integrative optoelectronics, photonic crystals, nonlinear photonics, optical fiber, bionanotechnologies, biomedicine, and others. An efficient synthesis procedure can improve MO physical properties and reflected surface chemical changes can affect performance in various devices like light-emitting diodes, luminescence materials, solar cells, biomedical probes, and others, so these topics are covered as well. Further, because of the comparative development of MOs to enhance applicability in various sectors, the challenges associated with the handling and maintenance of MO crystalline properties and n- and p-type interactions in the conduction bands are also discussed. We believe that this information will serve those looking to learn more about the manufacture of MOs as a cheaper and more effective material in the fields of electronics, photonics, biomedical, and engineering. MO-based composites are also extremely important materials from both a scientific as well as a technological viewpoint, due to the unique functionalities offered by solid-state materials. The MOs belong to an assorted and appealing class of materials in which their properties exhibit a full spectrum of electronic, structural, elastic, optical, thermal, biocompatible, and other properties, from insulating to semiconducting, metallics, and superconducting. Moreover, these materials can recognize phenomena of superconductivity, thermoelectric effects, photoelectrical effects, luminescence, and magnetism, giving them an eminent importance among multifunctional materials with their rich collection of properties having strong potential for numerous device applications. Based on the potential roles and advantages of MOs, this book is expected to serve as a vital resource for both professionals from industry and academicians working in the areas of optical devices, lighting, display and imaging

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technologies, biomedical devices, and sensors. For academicians in particular, this book can help in developing courses and research topics linked to photonics, nanosciences, luminescence, bioimaging, and optical materials. The book contains 15 chapters covering all the areas mentioned here, and the contributors are from diverse fields around the globe. The coverage of the topics is designed in such a way that the readers/subject specialists can easily find the discussions related to interdisciplinary fields of oxide materials with a wide range of topics, from material synthesis and deposition to characterizations, processing, and device fabrication and applications. This book is edited by a team of highly qualified experts, guaranteeing its high quality. We thank all the authors for their valuable contributions, as without their support, our book idea would not have become a reality. We also express gratitude to Elsevier for giving us the opportunity to work on this book series. Suresh Sagadevan Jiban Podder Faruq Mohammad

Section A Technology and properties

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Metal oxides for optoelectronic and photonic applications: A general introduction

1

Mehmood Shahid a,b, Suresh Sagadevanc, Waqar Ahmed d, Yiqiang Zhana, and Pakorn Opaprakasit b a Center of Micro-Nano System, School of Information Science and Technology, Fudan University, Shanghai, China, bSchool of Bio-Chemical Engineering and Technology, Sirindhorn International Institute of Technology (SIIT), Thammasat University, Pathum Thani, Thailand, cNanotechnology & Catalysis Research Centre, University of Malaya, Kuala Lumpur, Malaysia, dInstitute for Advanced Studies, University of Malaya, Kuala Lumpur, Malaysia

1

Introduction

Metal oxides are crystalline solids that contain a metal cation and an oxide anion. They typically react with water to form bases or with acids to form salts. Most of the oxides are formed in soil; these oxide minerals are typically composed of oxides, hydroxides, oxyhydroxides, and hydrated oxides [1]. Metal oxides (MOs) are a fascinating class of compounds with diverse properties that cover various aspects of materials science, physics, and chemistry [2,3]. MOs in bulk or in the form of thin films, or as nanostructures, behave in a highly diverse manner and exhibit a variety of functional properties, which enable them to stand out as ideal candidates for various technological applications, such as in optoelectronic devices, passive optical devices, catalysis, and solar cells, for environmental applications, and as gas sensors [1]. Metal elements can form a large variety of oxide compounds. These elements can be synthesized with diverse structural geometries and with a different electronic structure that can exhibit metallic, semiconductor, or insulator characteristics. In the field of technological applications, MOs are employed in fabricating microelectronic circuits, optoelectronics, piezoelectric devices, and catalysts. MOs behave differently at the nanoscale, and, when compared to their bulk counterparts, they exhibit different physical, chemical, optical, and electronic properties at the nanoscale due to the availability of more surface atoms than interior atoms and quantum confinement, when taking part in any reaction. Therefore, the practical applications of MOs depend upon their properties such as shape, size, surface area, crystallinity, conductivity, and photocatalytic activity [4,5]. Photo-dependent devices and applications rely on materials with a high refractive index. The refractive index (n > 3) contrast between a photonic structure and the surrounding medium enables the synthesized material to become a promising candidate Metal Oxides for Optoelectronics and Optics-Based Medical Applications. https://doi.org/10.1016/B978-0-323-85824-3.00006-3 Copyright © 2022 Elsevier Inc. All rights reserved.

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Metal Oxides for Optoelectronics and Optics-Based Medical Applications

for photonic devices. Materials with a higher refractive index have a larger crosssectional area [6] and smaller volume [7] and efficiently trap the light in solar cells and photonic crystals [8]. Vertically oriented ZnO nanowire-embedded polymeric matrices were used by Nadarajah and coworkers for making flexible LEDs [9]. Although polymers are flexible in nature, given that they are still organic materials, they have certain downsides, specifically in LED applications such as short life cycle, damages caused by humidity, screen burn-in, high-power dissipation, etc. In this perspective, as an alternative, metal oxides, which are chemically stable, robust, inexpensive, nontoxic, and can be synthesized at low temperatures, are yet to be fully integrated to accomplish inorganic flexible LEDs. It is also noteworthy that the synthesis of MOs is easier even at low temperatures, which can help in achieving better flexibility [9,10]. MOs are materials that are remarkably different from group III–V compounds and other conventional inorganic materials like silicon. Among other inorganic materials, MOs have different design concepts, electronic structure, electron/hole transportation, states of defects, processing of thin films, and optoelectronic properties. To employ MOs as thin films in high-quality electronic-grade applications such as inexpensive circuits, flexible organic light-emitting diode (OLED) displays, touch screen displyas and solar cells on plastic substrates, vapor- and solution-phase methodologies could be adopted for thin film preparation at room temperature (25 °C under air) [11,12]. The synthesis of MOs can be conducted using two major methods: (i) physical methods also known as a top-down method that includes ball milling, sputtering, laser ablation, electrospraying, electron beam evaporation, etc. and (ii) chemical methods based on a bottom-up approach such as the sol–gel method, polyol method, hydrothermal method, coprecipitation method, microemulsion technique, chemical vapor deposition, etc. In physical methods, a bulk counterpart of any material is depleted systematically for the formation of fine nanoparticles, whereas in chemical methods, the atoms or molecules are assembled to form a distribution of different sizes of NPs. The chemical methods of synthesis have the main advantage that they allow the production of particles with definite size, shape, morphology, and composition, which can be used in various applications such as catalysis, in electronic applications, for sensing, etc. As an example, the sol–gel method is a chemical method in which less energy and a low processing temperature are usually required, and it is an economical method for carrying out the synthesis [13,14]. The functional properties of MOs depend upon their crystal structure, defects, doping, composition, etc., which further express the electrical, optical, mechanical, and chemical properties of MOs. The synthesis procedures and parameters set for the synthesis determine the morphological structures and the physiochemical properties of MOs. The sizes and dimensions of oxides help control the band gaps and electronic structures of oxide materials, which eventually could be used for various potential applications depending upon their suitability [1]. To date, a number of oxides, such as TiO2, ZnO, SnO2, Co3O4, Fe3O4, CuO, and so on, have been synthesized by applying various synthesis procedures and employed either alone or as composites in various potential applications such as fuel cells [15], dye-sensitized solar cells (DSSCs) [16], photocatalysis [17], light-emitting diodes (LEDs) [18], photoelectrochemical (PEC) water splitting [19], electrochemical sensors [20], supercapacitors [21], etc.

Metal oxides for optoelectronic and photonic applications

5

Fig. 1 Strategies for the synthesis of MOs.

MO synthesis and preparation methods can be dated back to a century ago, and various new techniques and methods have been developed for the preparation of high-quality oxide materials. The block diagram in Fig. 1 shows two types of approaches used for the synthesis of nanostructured MOs. MOs with their different nanostructures have drawn great scientific interest due to their outstanding optical, physical, thermal, and electrical properties. The 2D structure of metal oxides with an ultrathin atomic layer, easy functionalization, a tunable band gap, and a large interlayer distance has enabled these MO nanomaterials to be an extraordinary candidate for the improvement of innovative optoelectronic and photonic applications [22–24]. MO nanostructures used in photonic and optoelectronic (a subbranch of photonics) applications include light-emitting diodes (LEDs), laser diodes, photodetectors, solar cells, and so on due to their unique geometry. Photonic applications include displays and imaging, integrated optics, measurement and sensing, solid-state lighting, etc. MO nanomaterials can be synthesized in different ways, as discussed earlier, by adopting top-down or bottom-up approaches and bear various interesting properties. Among semiconducting metal oxides, ZnO with a wide band gap (3.37 eV) is considered a promising candidate for optoelectronic applications. ZnO has a large exciton binding energy of 60 meV, is cost-effective, chemically stable, facile in fabrication and etching, and nontoxic for optical device applications [25]. Metal oxides are used as transparent conductive oxide thin films for optoelectronic applications. A high optical transparency >85% is among the key parameters to keep in mind for optoelectronic applications when developing display devices and other optoelectronic applications [26]. In a similar manner, carrier mobility in metal oxides with wide band gaps is an extremely important parameter, and it is known that metal oxides exhibit low electrical conductivity. Higher carrier mobility, being important,

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can be improved in MOs by the doping of carrier-rich elements for optoelectronic applications [27]. In addition, the morphology, particle size and surface properties of MOs also plays a crucial role in optoelectronic and photonic application, which are discussed in detail in the following sections.

2

Properties of MOs

It is well known that MOs are an important class of materials used in many areas of research. MOs are either used alone or in the form of composites with other materials as a major contributor to many areas of science, engineering, medicine, and biology. MOs at the nanoscale exhibit extraordinary properties as compared to those of their bulk counterparts. MOs have excellent optical, electronic, mechanical, and surface properties due to which they are considered as the chief material in various applications. The various important properties of MOs are discussed in detail in the next section.

2.1 Optical properties of MOs Metal oxides are a class of important materials that have excellent optical properties with applications in many areas of science and technology. The nature of the metal has a significant effect on the band gaps of MOs. For example, Fe2O3 has a smaller band gap due to its rich color as compared to that of ZnO because of no color. The optical properties and characteristics of metal oxides are highly required in the field of optoelectronic applications to determine the suitability of these metal oxides. The optical band gap properties of metal oxides can be calculated using an extremely common technique. Optical band gap means the threshold of a material for the absorption of light. It can be calculated by analysis of the absorption edge using the Davis–Mott relation and can be presented by Tauc plots [28]. Hydrogenated amorphous silicon as a key semiconductor material has opened new frontiers in the area of electronics, but low carrier mobility and optical opaqueness are the main challenges to future applications. To overcome these issues, MOs with excellent carrier mobilities at an amorphous even state, high optical transparency, and good compatibility with organic dielectric and photovoltaic materials are being explored [11]. MOs require an optical transparency >80% to fall under the category of TCOs to be effectively employed in optoelectronic applications. The wide range of MOs make them transparent throughout the visible region as well as in the UV region; therefore, they are suitable for optoelectronic applications [29]. As mentioned earlier, optical properties strongly depend upon the doping level. Sometimes, codopants are also introduced to enhance the optical and transition properties of MOs. The common MOs such as TiO2, SnO2, WO3, and ZnO have little visible absorption as pristine, they exhibit absorption properties due to impurities doped. These MOs play a significant role in catalysis, photocatalysis, and paint pigments. Fig. 2 shows ZnO nanoparticles and nanorods as an example. The ZnO onset absorption peaks are at around 3.6 and 3.2 eV for nanoparticles and nanorods, respectively.

Metal oxides for optoelectronic and photonic applications

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Fig. 2 UV–vis absorption spectra of ZnO quantum dots (curve a) and nanorods (curve b). Inset: transmission spectrum of ITO [30].

A blue shift is observed for nanoparticles when compared with nanorods, which suggests stronger quantum confinement in nanoparticles than in nanorods. The inset in Fig. 2 shows the transmission spectrum of ITO mostly used as a semiconductor substrate for different devices [30]. The high optical transparency of these oxides in the visible and near-IR regions of the solar spectrum is a direct consequence of them being wide-band gap semiconductors (Eg >
3.0 eV). Their fundamental absorption edge generally lies in the UV region and shifts to shorter wavelengths with increasing carrier concentration. Another example of TiO2 UV–visible spectra dispersed in ethanol with a small amount of water addition is shown in Fig. 3 [31]. Here, a red shift due to the addition of increasing water concentrations can be observed. This shift is attributed to the innervation between water and the Ti3+ center because of oxygen vacancy defects. The example of the TiO2 UV–visible spectra shows that the absorption spectrum may be sensitive to the surface characteristics of MO nanoparticles. The above discussion and examples show that the absorption spectrum of most MOs is active in near-UV and UV regions with weak or little absorption in the visible region. Still, some exceptional cases exist with strong visible absorption. The applications such as solar cells and photoelectrochemistry which need the absorption of visible light, MOs with weak visible absorption ability must be sensitive. These MOs can be sensitized using different strategies such as doping, developing hybrid/composites, dye sensitization, etc. Therefore, the study of its optical properties and characteristics is required for ascertaining the suitability of a material in optoelectronic applications and optical coatings and for assessing the nature of light–matter interactions for new materials and their arrangements.

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Metal Oxides for Optoelectronics and Optics-Based Medical Applications

Fig. 3 Absorption spectra of TiO2 nanoparticles in ethanol with different water concentrations; from bottom to top, the concentrations of water are 0%, 0.33%, 0.66%, and 1% [31].

2.2 Stability of MOs The open air stability of a material is highly desirable along with superior mobilities, in the application of optoelectronic and electronic devices [32,33]. Two-dimensional materials have recently emerged as the most attractive materials due to their novel high carrier mobilities and physical properties. However, not all 2D materials have the potential to be used in optoelectronic applications despite having high carrier mobilities (
105 and
103 cm2 V1 s1) [34,35] due to instability in the open air, such as graphene and phosphorene [36,37]. MOs are a class of materials with high carrier mobility as well as adequate stability and hence can be used in optoelectronic applications. There is a list of MOs (NiO2, InO, GeO2, SnO2, TiO2, MoO3, WO3, W2O5, and SnO) available with higher energetic stability that can be calculated with the following Eq. (1). ΔH ¼ ðEtot  n1  EM  n2  EO Þ=n

(1)

where Etot ¼ energy of the MOs; EM ¼ energy of the metal in its solid phase; EO ¼ oxygen atom in a gaseous O2 molecule; n1 and n2 ¼ numbers of atoms for each element in the unit cell; and n ¼ total number of atoms in the unit cell, for materials with energetic stability ΔH < 0 [38]. Similarly, another report has shown the proven thermal stability at high temperatures (at 600 K) by CuO for photonic applications [39]. Considering the stability of metal oxides used in various optoelectronic and photonic applications, it has been extensively proven that transition–metal oxide nanomaterials retain excellent semiconducting properties as well as nano-effects,

Metal oxides for optoelectronic and photonic applications

9

which make them an ideal material for these applications. MOs are expected to deal with several global challenges and in various areas of research, specifically in the field of portable materials and energy applications [40]. In the past, extensive efforts and time were invested in the synthesis, structures (micro, nano), and performance of MOs. A comprehensive study on a few major areas such as manufacturing MOs structures, physical properties and nano/microstructures can lead to better performance and enhancement of MOs in the field of photonics. MOs have unique components, and, as a promising functional material, they have positive metallic and negative oxygen ions and hence exhibit various structures, long-term stability, wide band gaps, and optically active electronic transitions. Transition–metal oxide (TMO) is considered to be one of the most promising functional materials because of its unique components, in which the combination of positive metallic ions and negative oxygen ions exhibits various structures and suitable physical properties such as long-term environmental stability, wide band gaps, and optically active electronic transitions. Much more attention has been paid to the use of oxide materials for photonic and electronic devices in the past few years due to their stability and optical activity, especially ZnO [41]. ZnO is used as a promising candidate in integrated photonics. It has a wide band gap of 3.37 eV at room temperature and a binding energy of 60 meV. Due to its higher binding energy, ZnO has been the focal point of light-emitting device studies for many years [42]. In a report, Choi et al. [43] further observed that ZnO single emitters show stability for
10 min during measurement and, as such, it is extremely important for future photonic applications [44].

2.3 Conductivity of MOs Materials can be categorized into different groups depending upon their ability to conduct electricity. The three main categories based on the electrical conductivity of materials are conductors, semiconductors, and insulators. Applying the definition of electrical conduction, metal oxides fall under the category of semiconductors. Semiconductors have band gaps wider than those of conductors and narrower than those of insulators. In MOs, due to a small band gap in the conduction bands as compared to insulators, electrons jump into the empty conduction bands by gaining sufficient energy. From a broader perspective, MOs being semiconductors may either be n-type or p-type depending upon the movement of their majority carriers, i.e., electrons and holes. The most important property of MOs is the change in electrical conductivity with variation in temperature. The Hall coefficient is one of the most powerful tools for the investigation of the transport properties of MOs, providing information on the mobility and the Fermi level. Besides the Hall effect, there are many other techniques available such as magnetoresistance, magneto-Seebeck effect, and magneto-Hall effect, which are used to investigate the band structure [45]. MO materials exhibit a remarkable range of electrical conductivity values. For oxide materials, at one end, we have extremely insulating behavior as characterized in the case of MnO (i.e., s ¼ 1015 W1 cm1), whereas at the other extreme, oxide materials like ReO show electrical conductivity of s ¼ 105 W1 cm1, which is almost near the conductivity of metallic copper. There exist several oxide systems that exhibit

10

Metal Oxides for Optoelectronics and Optics-Based Medical Applications

a variety of electrical properties. The conductivity between these semiconductors may involve different mechanisms, such as, in the case of Pr6O11 and Nb2O5, it involves hopping of charge carriers, on the other hand, in SrTiO3 and SnO2, it involves a mechanism of excitation from the valence band to the conduction band. Furthermore, the conduction process in oxides can be electronic or ionic, for example, lithium-doped NiO is purely an electronic conductor, whereas Ca-doped ZrO is purely an ionic conductor [46]. MOs exhibit different crystal geometries and have miscellaneous optical and electronic properties. Tuning the electrical conductivity in nanostructures and thin-film MOs can play an important role in optoelectronic applications [47]. The use of a suitable conductive material (MO) with desired properties of flexibility, optical transparency, higher stability, and cost-effectiveness is highly important. MOs as thin films or nanomaterials rather than traditional metal foils or coatings tremendously impact the cost, weight, and volume of optoelectronic and photovoltaic devices as well as their mechanical properties and, hence, can be employed by applying simple, inexpensive solution-based techniques such as spin-coating, spray coating, etc., to deposit the thin films on an insulator or on less-conducting substrates for optoelectronic applications [48]. Recently, CuCrO2 has been explored with promising results; Mg-doped CuCrO2 has the highest p-type conductivity reported for oxides to date, i.e., 220 S cm1. In related works, cation and anion substitutions and the role of Cr vacancies in this material have been studied [49,50]. High conductivity has also been shown experimentally in non-Cu metal oxides, most notably spinel-structured TCOs, such as NiCo2O4 [51,52], perovskite-structured TCOs, such as SrInxTi1-xO3 and SrxLa1-xCrO3 [53,54], and amorphous or composite oxides, such as ZnORh2O3 and In: MoO3 [55,56]. Other oxides, such as doped Cr2O3 and Ba2BiTaO6, have promise but remain to be synthesized with high conductivities.

2.4 Transparency of MOs Transparent conducting oxides are the chief material in optoelectronic and photonic devices and thus play a vital role in the construction of an efficient device either as a catalyst or as a substrate/electrode. Metal oxides such as zinc oxide (ZnO), indium tin oxide (ITO), indium oxide (In2O3), fluorine tin oxide (FTO), and tin oxide (SnO2) belong to the family of TCOs. They have a wide band gap of >3.4 eV and hence are transparent in the wavelength range of 350 to >800 nm. The thin films of these TCOs can be prepared to possess resistivities ranging from 1 to 5  104 cm. These properties enable TCOs to be well-suited electrode materials for optoelectronic applications [57]. ITO is a highly promising candidate, which offers more than 85% transparency with low resistivity in the visible range [58]. ITO can be deposited at low temperatures as compared to other metal oxides and other transparent conductive metal oxides; further, it can be etched easily. ITO thin films can be easily deposited using sputtering deposition techniques as compared to FTO and AZO [48]. AZO, on the other hand, is also a transparent material and exhibits high metallic conductivity with heavily aluminum-doped ZnO. An ample study was carried out on AZO that reported that it can be employed in ZnO-based optoelectronic applications [59].

Metal oxides for optoelectronic and photonic applications

11

AZO thin films can be deposited using techniques such as sol–gel [60], PLD [61,62], and radio frequency (RF) magnetron sputtering [63], which is not possible for other materials in the group III nitride family. GaN-based optoelectronic devices have to make a combination with either ITO or nickel–gold bilayer films, and there exists a trade-off between high optical transparency and conductivity. Using ZnO, grown by the epitaxial method, as a promising MO in optoelectronic applications, transparency can be improved by the postgrowth annealing method [64]; it can also help in the improvement of stoichiometry [65] and luminescence intensity [66,67], which is extremely important for optical applications, specifically for photonic devices.

2.5 Surface properties MOs with bulk crystal structures have been vastly explored until now, but little work has been done regarding their surface structures [68]. The physical and chemical properties of MOs mainly depend upon their surface properties, regardless of whether those MOs are being used in bulk or as nanomaterials. MOs being semiconductors containing oxygen atoms are commonly utilized in various photonic, electronic, and optoelectronic device applications due to their catalytic activities and other properties. TiO2, CuO, ZnO, and other MOs have a large variety of nanostructures, which greatly affect the surface energy and chemical properties of these materials [69]. Nanostructured metal oxides have a drastic effect on the chemical and physical properties having large surfaces because of their quantum size effect [70]. The surface of any material differs from the bulk material, for example, it allows the flow of energy and exchange of matter and energy across the interface (a boundary between two phases), the material, and the surrounding environment (liquid, solid, or gas). Thus, MOs as a catalyst can either initiate or terminate a chemical reaction. Citing the example of a bulk solid material, when it is sectioned into smaller pieces, the collective surface area is increased (compared to that of the bulk parent material), but the volume remains the same. Therefore, a term is normally used in nanoscience and nanotechnology called “surface-to-volume ratio,” which increases at the nanoscale as can be seen through Table 1 [71]. The important properties of TCO surfaces are their work functions, electron affinities, and surface band bending. These properties are relevant to electrical contact properties in organic electronics, energy band alignment, barrier formation, and sensor function. In addition, the exchange of oxygen with the environment, which is relevant, e.g., for establishing defect equilibrium during postdeposition treatments or which may affect film growth on TCO surfaces, is an important issue, which has hardly been looked into. Oxide semiconductors are materials with polar crystallographic axes along which the electrostatic potential alternates. In principle, this can give rise to surfaces with preferential cation or anion termination. However, idealized (bulk-truncated) polar surface terminations lead to electrostatic instability (sometimes referred to as polar catastrophe), which is a consequence of the electric dipoles formed by the charged crystal planes. The dipoles add up by each pair of planes, resulting in huge electrostatic energy due to the high number of planes in the crystal.

12

Metal Oxides for Optoelectronics and Optics-Based Medical Applications

Table 1 Effects of size on the surface area of a cube. S. no.

Size of the cube side

Number of cubes

Collective surface area

1 2 3 4 5

1m 0.1 m 0.01 m ¼ 1 cm 0.001 m ¼ 1 mm 109 m ¼ 1 nm

1 1000 106 ¼ 1 million 109 ¼ 1 billion 1027

6 m2 60 m2 600 m2 6000 m2 6  109 ¼ 6000 km2

Therefore, the surface morphology and lateral uniformity in coverage and structure are critical to electronic devices, preventing short circuits between conductive layers and ensuring consistent resistivity of the deposit. Thickness uniformity also ensures consistent optical absorbance across the surface, and, for flexible or curved substrates, local thickness variation may promote failure during extended flexure.

2.6 Other properties Along with the properties discussed in the above sections, MOs have other distinctive mechanical, electronic, and redox properties, due to which they are considered in almost all fields of nanotechnology. For instance, Benad et al. reported the mechanical properties of single-component MOs, Al2O3, Ga2O3, Fe2O3, and ZrO2 as well as mixed aerogels. All aerogels were produced by the epoxy method and tested using uniaxial pressure to gain Young’s modulus. They found that the pure ZrO2 aerogel showed a promising Young’s modulus of 10.7 MPa. They further examined the mixed material with 20 atm% of Zr contents with Al2O3 and recorded a record value of 125 MPa mL g1 for compressible aerogels [72]. MOs exhibit distinct redox properties and constitute the largest family of catalysts in heterogeneous catalysis. There are three key parameters that an MO should have to participate in catalysis, i.e., (i) the coordination environment of the surface atoms, (ii) the oxide should have redox properties, and (iii) the oxidation states of the surface. The first parameter can be controlled by choosing the exposed crystal plan and preparation procedure; the second parameter, however, is a matter of choice of the oxide material. Third, most of the catalysts are transition–metal oxides containing cations with variable oxidation states. These cations present redox and acid–base properties. The acid–base properties of MOs are interconnected with redox behavior [73]. Besides other important properties, the electronic properties of MOs are of equal importance. The size of MOs greatly affects their electronic properties. In any material, the nanostructure produces the so-called quantum size or confinement effects, which essentially arise from the presence of discrete, atom-like electronic states. Considering the solid-state situation, the electronic states are supposed as superpositions of the bulk-like states with an affiliated increase in oscillator strength [74]. Additional general electronic effects of quantum confinement experimentally probed on oxides are related to the energy shift of exciton levels and optical band gap [75].

Metal oxides for optoelectronic and photonic applications

3

13

Chemical methods of MO synthesis

3.1 Sol–gel process It is an outstanding process of synthesis in which nanoparticles can be synthesized by the preparation of a sol and successive gelation followed by solvent removal. The sol–gel process is typically a process of transformation of a system chemically from a liquid “sol” into a gelatinous form that is called the “gel” phase, which is treated and changed into a form of solid oxide material [76]. This method can be adopted for the synthesis of nanopowders, oxides materials, and organic and inorganic materials [77–79]. The key characteristics of the sol–gel process include high purity with an achievability of uniform nanocubes at low temperatures. It also allows the synthesis of ceramic materials with high purity and homogeneity. The scheme in Fig. 4 explains the formation of nanomaterials using the sol–gel method. This method has been used for the preparation of TiO2 nanoparticles (NPs) [80]. The size of synthesized TiO2 NPs was 10 nm with an anatase phase [14]. The sol–gel method is a potential synthesis method for optoelectronic application as reviewed by Kausar Harun and co-workers [81] in a report. They synthesized ZnO nanoparticles by the sol–gel method and studied their structural, elemental, and optical characteristics. They concluded that the sol–gel method of metal oxide synthesis shows promising results for optoelectronic applications such as LEDs [82] and third-generation solar cells [83]. In a typical sol–gel process, an MO precursor solution is prepared by dissolving metal salts in a solvent, typically H2O with a “stabilizing agent.” The resulting coordinated metal species undergo hydrolysis to form hydroxides, which slowly convert to

Fig. 4 Schematic representation of the sol–gel synthesis process [80].

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Metal Oxides for Optoelectronics and Optics-Based Medical Applications

colloidal solutions (sol) and eventually form integrated MO networks (gel) containing liquid and solid phases. For TFTs, films are cast from the sol precursors, which are then thermally annealed to condense the lattice and remove volatile organics. TFTs using MO films—including In2O3, IGZO, zinc tin oxide (ZTO), indium gallium oxide (IGO), indium zinc oxide (IZO), and ZnO semiconductors, Al2O3, ZrO2, HfO2, Ta2O5, and Y2O3 dielectrics, and ITO, In2O3, and F-doped ITO conductors—have been fabricated by sol–gel processing. Metal oxide heterostructures with two or more MO layers have recently been used in TFTs. These TFTs, as opposed to conventional single-channel-layer MO TFTs, use spin-coated In2O3–Ga2O3–ZnO–Ga2O3–In2O3 quasi-superlattices (QSLs). The performance of these transistors is not limited by the individual semiconductor bulk carrier mobility but is instead dominated by the heterointerface structural and electronic properties within the QSL. QSL TFTs processed at 200 °C show μe > 40 cm2 V1 s1, far exceeding those of single-layer devices.

3.2 Combustion synthesis Combustion synthesis, in which fuel and oxidizer pairs imbue MO precursor solutions with high internal energies, is an alternative method for lowering solution-deposited MO film processing temperatures. In contrast to conventional, highly endothermic sol–gel condensations, this results in self-generated exotherms within MO films. The self-generated, localized heat drives M–O–M lattice formation and densification after precursor deposition and minimal heating to initiate combustion. The first combustion experiments demonstrated the fabrication of In2O3, a-IZO, and a-ZTO films at temperatures as low as 200 °C [84], using redox-based urea or acetylacetone fuel and M(NO3)x as chemical precursor fuel–oxidizer pairs. To make TFTs, such combined solutions were spin-coated on doped Si–SiO2 substrates; the resulting films were annealed at 200–400 °C, and the process was repeated until 20–30 nm MO films were obtained; finally, Al source–drain contacts were deposited on top. When comparing the performance of sol–gel-derived and combustion-synthesized MO TFTs, the advantage of combustion synthesis becomes clear. To better understand the amorphous phase transport in such films, combustion-derived InXO materials for X ¼ Sc, Y, and La were prepared. The findings show that cations with radii greater than In(iii) induce amorphous character and achieve high μe at extremely low X concentrations (5% for La). Furthermore, in the trap-limited conduction regime, increased ionic radii correlate with broader tail-state trap distributions and higher potential barrier heights in the percolation regime. These findings are consistent with the local structure information derived from ab initio molecular dynamics simulations [85]. Interestingly, antiambipolar heterojunction circuits based on two TFTs and one resistor have recently been demonstrated [86] by combining combustion-processed IGZO with a sorted p-type CNT network, whereas such circuits require at least seven TFTs in conventional Si-integrated communications. A versatile new growth process that combines spray coating and combustion synthesis (SCS, spray combustion synthesis) has been recently reported [87]. Internal combustion heat combined with spray suppression of gaseous by-product trapping

Metal oxides for optoelectronic and photonic applications

15

during film growth results in ultrahigh-quality, dense (by positron annihilation spectroscopy and X-ray reflectivity) In2O3, IZO, and IGZO films in remarkably short growth times. SCS can produce 50 nm a-IGZO films (minimal thickness for commercial applications) in 30 min, with TFT performance far superior to those of spin-coated combustion films (which require 4 h growth times) and comparable mobility, defect densities, and VT shift (
1 V) to sputtered films.

3.3 Coprecipitation process The coprecipitation process is a simple and conventional method of synthesis in which the precursors of the desired MO are dissolved in a solvent. The mixture is then stirred continuously for some time to form a homogeneous solution. As represented by the name of this technique, in this method, two or more salt precursors are added to a solvent that simultaneously makes the precipitates. This process involves the nucleation, growth, and aggregation of particles in a solution. It is a useful method for the preparation of metal oxide binary and ternary nanocomposites. Yidong Shen and coworkers [88] synthesized LiNi0.5Mn1.5O4 by following the coprecipitation method using DI water as the solvent. The coprecipitation process is a suitable method for designing the material in a core–shell model, as the core–shell structure of the nanoparticles exhibits enhanced physical and chemical properties when compared to those of its single counterpart. There have been several methods adopted for the formation of core–shell nanocrystals in the past. The uniform coating of the shell leads to the enhancement of its core, thus providing protection and quality to serve its purpose well in the desired applications. In photonic applications, a shell can help protect the core from damage when highenergy laser pulses hit the nanomaterial [89]. Selvi et al. studied core (ZrO2) and shell (ZnO and SiO2) materials prepared using the coprecipitation method and their optical properties [90]. The study shows that this core–shell material can be used to tune the optical properties of the material and is a potential candidate for optical and electronic device applications. The schematic diagram in Fig. 5 shows the coprecipitation method of synthesis of MOs nanoparticles [88].

3.4 Electrochemical deposition method Electrochemical deposition, also known as electrodeposition, is a powerful method of synthesis [91]. It is a large-area deposition technique also resulting in a uniform deposition of MOs and their nanocomposites. It needs a conducting substrate material (FTO, ITO, or magnesium) that can bear an acidic environment. The process of electrodeposition takes place in an electrochemical cell in which a conducting substrate is used as a working electrode and Pt wire and Ag/AgCl or SCE serve as counter and reference electrodes, respectively. The electrochemical cell consists of the desired precursor in a solution form, and deposition takes place under the process of chemical bonding [92]. The applied voltage to the solution helps the particle form a layer that is being deposited on the substrate’s surface. It is an economical technique of synthesis in which the thickness of the deposited layer and the morphology and uniformity of the

16

Metal Oxides for Optoelectronics and Optics-Based Medical Applications

solution A

Different a amounts of ethanol

NiSO4•6H2O MnSO4•H2O

Suitable amount Na2S solution

Stirring

solution B NH4HCO3

Stirring

coprecipitation LNMO

mixing of A and B

stirring 1h and ageing

oxide product

Li2CO3

500 ºC 2 hours Ni0.25Mn0.75CO3 gradient temperature calcination

Fig. 5 Schematic representation of the coprecipitation method of synthesis [88].

film can be controlled by setting the current, voltage, and precursor used. In a report, it has been shown that the modification of the TiO2 photoanode using the electrodeposition method with a thin layer of TiO2 nanoparticle can enhance the efficiency of DSSC from 7.3% to 8.2% [93]. Hence, with many other advantages and having applicability in various technological fields, the electrodeposition method is a potential method of fabrication in the field of photonic applications. Fig. 6 shows the schematic diagram of the electrodeposition method of synthesis [94].

3.5 Sonochemical method The sonochemical method (Fig. 7) is a unique method of synthesis due to the formation of a much smaller size of MOs and a higher surface area [95]. This method has conditions distinct from other conventional synthesis techniques such as wet chemical methods, hydrothermal, photochemistry, pyrolysis, etc. in this process the high-power ultrasound waves exiting shortly generated by the tip of the instrument, which has the frequency between 20 kHz to 10 MHz has magnitude larger than the molecule size. The ultrasonic waves generated by the instrument in the sonochemical method have

Fig. 6 Schematic diagram of the electrodeposition method of synthesis [94].

Metal oxides for optoelectronic and photonic applications

17

Digital disruptor (piezoelectric ceramic)

Digital control unit Titanium horn Temperature probe

Heavy duty beaker

Fig. 7 Schematic representation of the sonochemical method of synthesis [95].

wavelengths varying from 10 cm to 100 mm. It is an extensively practiced method of synthesis with unusual characteristics, which start from the chemical effects of ultrasound arising from an acoustic cavitation, i.e., the growth, formation, and collapse of bubbles in a liquid. These ultrasounds produce hotspots that can go beyond a temperature of 5000 K, an atmospheric pressure of 1800 atm, and a cooling and heating rate of 1010 K/s [96,97]. The sonochemical method of synthesis was first led by Suslick [98], for the preparation of nanoparticles of 10–20 nm size using Fe(CCP)5 in decline solution. By utilizing the sonochemical method of synthesis, Akbari and his team synthesized Ga2O3 nanoparticles as a complementary element of the optoelectronic device. Ga2O3 has contributed considerably to the improvement of responsivity of the optoelectronic device in TiO2–Ga2O3. They further proposed that the sonochemical method has facile functionalization characteristics as well as controls the properties of nanosheets and hence can be utilized in the generation of ultrathin materials for optoelectronic applications [99].

3.6 Hydrothermal process A hydrothermal method is a process that refers to the heterogeneous synthesis of inorganic materials in an aqueous solution above room temperature and under pressure conditions. In this method, the precursors of the desired nanoparticles are mixed with a suitable solvent (water) and transferred to a sealed stainless steel autoclave and heated (in a heating oven) above the boiling point of water. The pressure continuously increases with heating and reaches above the atmospheric pressure dramatically. This

18

Metal Oxides for Optoelectronics and Optics-Based Medical Applications

synergistic effect of high temperature and pressure helps in the production of a highly crystalline material without any annealing treatment after the synthesis in just a onestep process. Hydrothermal synthesis technologies have been developed for the formation of a broad range of nanoparticles, which also include magnetic nanoparticles. The reaction parameters have a great impact on the product synthesized; these parameters include precursor type, concentration, solvent, stabilizing agents, reaction time, and temperature [100]. This method can be used to form highly crystalline magnetic materials as compared to the low-temperature coprecipitation method discussed above. Highly crystalline magnetic nanomaterials can be obtained by the hydrothermal method due to higher temperatures and high pressure conditions. This method is used to obtain high surface areas of well-aligned ZnO nanorod arrays for visible light-emitting devices, display devices, and other optoelectronic applications. Plasma-treated ZnO samples are observed with the enhancement of the PL intensity as compared to untreated ZnO samples in an orange/red region [101]. Fig. 8 shows the hydrothermal synthesis of TiO2 nanoparticles [102].

3.7 Chemical vapor deposition (CVD) CVD is a complex process in which solid materials are deposited on the substrate at a high temperature as a result of a chemical reaction [103]. The schematic diagram in Fig. 9 shows the CVD process of nanomaterial synthesis. It is a widely used technology of material processing in which a thin film is formed on a heated substrate with the aid of a chemical reaction of gas-phase precursors [104]. These chemical reactions take place on or in the vicinity of a heated substrate. The obtained materials by CVD consist of thin-film powder or a single crystal [105]. There are different experimental conditions such as substrate temperature, composition of the gas mixture, pressure, and gas flow, which play a crucial role in the formation of thin solid films with a wide range of physical, chemical, and tribological properties. CVD offers a characteristic feature of excellent throwing power, which enables the production of im

me

rse

d

2Fe3++Co2+/pomelo peels

transferred

pomelo peels

one-step hydrothermal method

2Fe3++Co2+/H2O

CoFe2O4/graphene-like carbons Co2+

Fe3+

Fig. 8 Schematic representation of the hydrothermal synthesis of TiO2 nanoparticles on a cellulose fiber [102].

Metal oxides for optoelectronic and photonic applications

19

Fig. 9 Schematic diagram of a typical CVD setup [104].

a uniform film with a controlled thickness and low porosity on a substrate of even complicated shape [106,107]. There are different variants of the CVD system available, which have been developed and used for the formation of thin solid films such as atmospheric pressure chemical vapor deposition (APCVD), metal–organic chemical vapor deposition (MOCVD), low-pressure chemical vapor deposition (LPCVD), laser chemical vapor deposition (LCVD), photochemical vapor deposition (PCVD), and aerosol-assisted chemical vapor deposition (AACVD) [108]. The CVD method is often used to manufacture optoelectronic devices like solid-state diode lasers, fiber optics cables, and light-emitting diodes to achieve the desired coating and the refractive index profile to construct a device with enhanced performances [109].

4

Physical methods of MO synthesis

4.1 Ball milling process Ball milling is the process of grinding and blending bulk material into nanosized particles/QDs with the help of different sizes of balls. It is a physical technique that is used to crush and grind the material into fine particles, containing a cylindrical shell that rotates about its axis. The balls are made of steel, stainless steel, ceramic, or rubber, which are filled into the cylinder containing the material to be milled. Ball milling has a simple working principle that slow destruction of the size of the bulk material takes place when the ball drops from near the top of a rotating hollow cylindrical shell [110,111]. The size of nanostructures can be varied using different sizes of balls, several balls, rotation speed, and choice of the material to be milled. Ball mills are commonly used for crushing and grinding materials into an extremely fine form. A ball mill contains a hollow cylindrical shell that rotates about its axis. There are different types of ball mills available such as attritor, horizontal, planetary, high energy, or shaker [112] as can be seen in Fig. 10. A few key advantages of this process are low cost, green, easy operation, reproducibility, applicable in dry and wet conditions, etc. Bhuyan et al., in their report, proposed high-energy ball milling as a potential method for the synthesis of Mg2TiO4 (MTO). The PL measurements at room

20

Metal Oxides for Optoelectronics and Optics-Based Medical Applications

Fig. 10 Schematic representation of a ball mill [111].

temperature show that the bands belong to the near band edge of 537 nm. They thoroughly characterized the synthesized material by the mechanical alloying method and observed that the MTO nanoparticles exhibit promising optical properties, which are suitable for optoelectronic applications. Furthermore, it has been studied that MTO is an excellent material with a wide band gap and a higher refractive index for various optical and electronic devices such as infrared optical sensors, waveguides, and electro-optical switch application [113].

4.2 Sputtering Sputtering (Fig. 11) is a process of momentum transfer that depends upon the energy of the incident particles, angle of incidence, mass of the colliding particles, and binding energy of the surface atoms [114,115]. In this method, thin films are deposited with the aid of controlled gases, such as argon gas (103 to 101 Torr), into vacuum chambers and an electrically energized cathode for the establishment of selfsustaining plasma. The cathode side has an exposed surface, which is called a target, a slab of material coated on a substrate. The material may be pure metals, nonmetals, or other molecules such as oxides or nitrides. For the deposition of thin films on a substrate, the gas present inside the vacuum chamber loses electrons, which travel and strike the target with sufficient kinetic energy above a certain threshold (10–30 eV) by dislodging the atoms or molecules of the target material. The electrons lost by the gas in the vacuum chamber involved with the plasma become positively charged ions. The material is thus sputtered and establishes a stream of vapors that pass through the chamber and strike the substrate by forming a layer of thin film [114,116]. For good adhesion of the film, it is necessary to have a clean surface of the substrate. Before sputtering the film, the chamber should be cleaned properly by employing the necessary steps. In this method of synthesis, one can have the liberty of formation of variable thickness, uniform and adhesive film formation, different morphology and grain size, and high yield with optical and electrical properties. This method allows

Metal oxides for optoelectronic and photonic applications

21

Fig. 11 Schematic diagram of the standard sputtering synthesis method [114].

the deposition of compounds and mixtures with a low substrate temperature [115]. The radiofrequency (RF) spurring method was used by researchers in a report for the deposition of ZnO thin film on glass, quartz, and silica-on-silicon (SiO2/Si) substrates to study the characteristics for optoelectronic applications [117]. All these samples were thoroughly characterized, and it was revealed through UV–vis measurements that the film deposited on the quartz substrate had higher transparency with above 84% of optical transmittance with an optical band gap of 3.23eV. It was concluded that the film deposited by RF sputtering on a quartz substrate has the potential to be used in applications needing enhanced luminescent properties.

4.3 Electron beam evaporation (EBE) This is a synthetic route of preparing the nanomaterial under the physical method of synthesis, where current is passed through the tungsten filament, which is responsible for electron emission (Fig. 12) [118]. The electron beam is highly focused on and directed toward the crucible, which contains the source material to be deposited. The electron beam is focused on the applied magnetic field. A 100 kV DC voltage source is used to accelerate the electrons at a temperature of around 3000 °C. It is an extremely useful process for the deposition of tungsten and tantalum, a material having a higher melting point. When the heated electrons strike the source, the kinetic energy is converted into thermal energy and, thus, increasing the temperature of the source material causes its evaporation. These vapors coat the surface of the substrate in the form of a thin film. If a nonmetallic film is to be deposited using EBE, gasses such as oxygen, nitrogen, etc., are used inside the chambers during the evaporation [115,118]. EBE is an effective technique used for the deposition of thin films, specifically for optoelectronic applications. A thin transparent film could be obtained by employing the EBE technique with higher transparency as reported [119]. According to this study, an aluminum-doped zinc oxide thin film was deposited on a glass

22

Metal Oxides for Optoelectronics and Optics-Based Medical Applications

Rotation

QCM deposition monitor

Wafer

Vapor Magnetic field

Shutter

E-beam Source Vacuum pump

Filament

Accelerator

Fig. 12 Schematic diagram of the EBE process [118].

substrate, and its structural, optical, and electrical properties were studied for optoelectronic applications after annealing the deposited thin film by EBE. The EBEdeposited film showed very strongly enhanced properties with a transmittance of 84% in the visible region. It further showed 97% of transmittance values in the near-infrared (NIR) region. An electrical resistivity of 4.6  103 cm was recorded at room temperature for the EBE-deposited thin film. The obtained results confirmed that the AZO thin film manufactured by EBE is the potential electrode for optoelectronic applications with enriched characteristics.

4.4 Electrospraying Electrospraying, also known as electrodynamics, is a similar technique as electrospinning using a similar technology for the production of nanostructures. This technique is used for producing minuscule droplets with a submicron size using an electric field as shown in the schematic diagram in Fig. 13 [120]. It is the most efficient field for producing nanoparticles/nanospheres. The experimental setup of this technique consists of a syringe pump containing polymer solution and a high-voltage power supply (tens of kV) consisting of the capillary (needle). The power source is applied to the needle, and a collector opposite to the needle functions as a ground electrode. The high-voltage power source (tens of kV) is applied to the needle containing the polymer solution at its tip, which evaporates during passage to the collector [121]. The morphology of the structure depends upon the physical properties of the polymer and the adjusted parameters of the electrospraying device such as needle gauge diameter, applied voltage, flow rate, and the distance between the needle and collector. The

Metal oxides for optoelectronic and photonic applications

23

Fig. 13 Schematic diagram of the electrospraying synthesis method of nanoparticles [120].

key advantages of these techniques include reproducibility, scalable synthesis, and higher encapsulation efficiency [122]. Jiang et al. [123] used the electrospraying technique for the fabrication of perovskite solar cells (PSCs) by printing all three electrodes. They managed to deposit a pinhole-free, even, and flat perovskite film, thus confirming that electrospraying is a powerful technique to coat a homogeneous film for PSCs in the subrange of 100–500 nm. The photoconversion efficiency (PCE) of electrospraying PSCs was 15%. This technique offers roll-to-roll compatibility with zero waste for optoelectronic applications.

4.5 Laser ablation Laser ablation is a process in which a laser beam is used as the main tool for the ablation of the target material. A laser, as a higher concentrated energy source, is centered at a specific place of the target material for the evaporation of light-absorbing materials. The term “ablation” means the removal of surface atoms by a multiphoton excitation process, i.e., thermal evaporation. Briefly, when the laser beam is centered on a target material in an ambient (liquid or gas) medium, it increases the temperature of the irradiated spot and evaporates it. The evaporated species and the surrounding molecules thus collide with each other, which results in the excitation of the electron state coupled with light emission and generation of ions and electrons, thus forming a laserinduced plasma plume. The structure of the plasma depends upon the target material, ambient medium (liquid or gas), ambient pressure, and laser used for ablation [124,125]. Laser ablation can be employed for the generation of higher-purity nanoparticles that depends upon the purity of the ambient medium (liquid or gas) and target material. Using laser ablation, it is hard to prevent the particles from agglomeration, controlled size distribution, and formation of crystal structures

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Metal Oxides for Optoelectronics and Optics-Based Medical Applications

because the nanoparticles are formed by following the Brownian motion of molecules. Mohamed et al. used the laser ablation method for the manufacturing of Cu2O nanoparticles for optoelectronic applications. They immersed a Cu plate into a liquid medium and irradiated it with pulses to control the particle size. As the pulse rate increases, the particle size tends to decrease. They characterized the prepared colloidal nanoparticle suspension by X-rays, UV–vis, and laser-induced breakdown spectroscopy, and optical linearity was also tested using Z-scan techniques to further study the different parameters. It was concluded that the Cu2O material synthesized by laser ablation is the potential material for optoelectronic applications [126]. Similarly, SnO2 nanoparticles are prepared by laser ablation by irradiation of tin in an aqueous solution of methanol and also in NaCl by applying nanosecond laser for photodetector applications [127]. XRD, TEM, and SEM studies of SnO2were analyzed for the formation of SnO2 material, size, and shape of the nanoparticle, respectively. The particle size of SnO2 nanoparticles was 40 nm with an optical band gap of 3.8 eV in methanol and 25 nm in NaCl with an optical band gap of 3.95 eV. The responsivity of the SnO2 photodetector prepared in methanol solution was 0.43A/W, and, in NaCl solution, it was 0.53 A/W at 410 nm. The schematic diagram in Fig. 14 shows the laser ablation process of nanoparticle synthesis [128].

5

Concluding remarks/conclusions

This book chapter has highlighted the importance of MOs in the context of current and future industrial applications. The basic knowledge and detailed synthesis of MOs have been presented by physical and chemical methods in detail. Various important properties of MOs have also been considered and explained in depth to understand their applications in various fields of optoelectronics and photonics. From the literature, it has been observed that most of the researchers preferred the chemical methods of preparation of MOs. Most of the synthesis procedures were an improvement, a combination, or a variant of the already existing methods in industries. There still exist

Fiber laser

Mirror

Lens

Electric agitator

Dispersed Ni nanoparticles in water

Fig. 14 Schematic diagram of the experimental setup for the production of Ni nanoparticles using laser ablation (liquid phase) [128].

Metal oxides for optoelectronic and photonic applications

25

challenges in the synthesis of MOs with high yield, controllable parameters, and low costs that need to be considered in the current ongoing synthesis procedures. Importantly, the aim of this book chapter was to elucidate the use of these synthesized MOs via various physical and chemical methods for optoelectronic and photonic applications.

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Recent developments in optoelectronic and photonic applications of metal oxides

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K. Tamizh Selvia and Suresh Sagadevanb a Department of Physics, Vel Tech High Tech Dr. Rangarajan Dr. Sakunthala Engineering College, Chennai, India, bNanotechnology & Catalysis Research Centre, University of Malaya, Kuala Lumpur, Malaysia

1

Introduction

Metallic oxides (MOs) are among the most promising functional materials for a variety of real-world applications. Because of the high electronegativity of oxygen, the combination of positive metallic and negative oxygen ions results in the long-term stability of these compounds. The presence of oxygen atoms combined with metal atoms in the crystal lattice causes the formation of a bandgap in the range of 1.10 eV [1], allowing these materials to be classified as semiconductors or insulators. These values may vary depending on the crystallographic form of the specific oxide. Last but not least, because of the abundance of mineral resources, a majority of them are not very expensive and can be an environmental-friendly alternative to a variety of toxic or rare materials. They are more dependable and resistant to degradation due to humidity or harsh climatic conditions than organic substances. Considering all of these benefits, it is not surprising that MOs already play an important role in our daily lives, as well as being a common subject of research being led in many different scientific fields. MOs are currently used as pigments in the chemical and food industries, which is a common application. They can be met in the form of high-opacity powders. TiO2 and ZnO, in particular, are widely used as “perfect whites” in paints, cosmetics, and food coloring. They are also used to form a UV-shielding layer, e.g., in sunprotective creams, optical filters, and anticorrosive coatings, due to their bandgap that corresponds to UV light [2]. Due to the high thermal conductivity of ZnO, which depends also on its form [3,4], it is a common additive to rubber which enhance the heat dissipation rate [5]. The most popular industrial application in optoelectronics is Indium Tin Oxide (ITO), used in touch screens, LCDs, solar cells, etc. [6]. The range of applications under research is also very wide. As in the group of MOs, one can find those which can be either intrinsically, or by doping, p-type or n-type semiconductors, they are used in the formation of various complex architectures in electronics. Many of the oxides have been used in the fabrication of thin-film transistors [7], as well as charge-injecting and charge-transporting layers [8,9]. Light-

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Metal Oxides for Optoelectronics and Optics-Based Medical Applications

emitting diodes [10], laser diodes, photodetectors [11], and solar cells [12–14] are being built using MOs in bulk or thin films. Transparent conducting oxides (TCOs) like Sn-doped In2O3 (indium tin oxide, ITO), Al-doped ZnO, and Sb-doped SnO2 are commonly used as transparent electrodes in liquid crystal displays (LCDs), organic light-emitting diodes (OLEDs), and solar cells. Furthermore, TCOs are applicable to transparent optoelectronics due to their unique properties of optical transparency in the visible region and controllable electrical conductivity ranging from nearly insulating to degenerate semiconducting (104 S/cm). Novel functions may be integrated into the materials since oxides have a variety of elements and crystal structures, providing great potential for realizing a diverse range of active functions. However, the application of TCOs has been restricted to transparent metals, although TCOs are n-type semiconductors. Almost no applications based on the electron activity of semiconductors have been realized as far as we know. The primary reason is the lack of ptype TCOs, because many of the active functions in semiconductors originate from the nature of the pn-junction. In recent years, the generic optical and electrical behavior of metal oxides has the potential for optoelectronic and photoelectronic devices. The novel features of metal oxides such as high physical and chemical stability, absorption coefficient, physical defects, crystalline structure, and charge transportation make it suitable for optoelectronic devices. Optoelectronics is the study and application of electronic devices which use electromagnetic radiation (light). Optoelectronic devices mean the components used to detect (light-emitting devices) or emit (light detecting devices) electromagnetic (EM) radiation, generally in the visible and near-infrared (NIR) regions of the electromagnetic spectrum (Fig. 1). The working

Radiation Type Wavelength (m)

Radio 103

Microwave 10−2

Infrared 10−5

Visible 0.5 ×10−6

Ultraviolet 10−8

X-ray 10−10

Gamma ray 10−12

Approximate Scale of Wavelength

Buildings

Humans

Butterflies Needle Point Protozoans Molecules

Atoms

Atomic Nuclei

Frequency (Hz) 104

108

Temperature of objects at which this radiation is the most intense wavelength emitted

Fig. 1 Electromagnetic spectrum.

1012

1K −272°C

100 K −173°C

1015

10,000 K 9,727°C

1016

1018

10,000,000 K ~10,000,000°C

1020

Recent developments in optoelectronic and photonic applications

35

principle of these devices is based on the interaction of light photons with matter. Photons are the fundamental units of the EM spectrum. To gain the knowledge on working of these devices it is essential to know the light-matter interaction. Consider a light photon of wavelength λ, incident on metal with bandgap Eg, the energy of the emitted light radiation E is given by, E ¼ hc/λ, where c is the velocity of light. Flexible electronic displays are of tremendous interest for future applications, including mobile phone displays, automotive stereo panels, smart identity cards, and wearable displays, because of the greater freedom that they afford in their design. Among the various display modes, including liquid crystal displays (LCDs) and electronic paper (E-paper), the organic light-emitting diode (OLED) is considered the ultimate choice for flexible displays, because it offers several advantages such as a vivid moving picture, low power consumption, no cell-gap problem, and good form factor for ultra-slimness. Flexible active-matrix (AM) OLED displays can be achieved by using a bendable substrate, such as a metal foil and plastic sheet, instead of a conventional glass substrate. So far, flexible AMOLED displays have been demonstrated on both metal foil and plastic substrates, using polycrystalline silicon (poly-Si), amorphous silicon (a-Si) or organic transistors as the active elements. Researchers focus on the extraordinary properties of high physically stable metal oxides for optical devices by tuning their preparation processes and operating conditions. Metal oxides are zinc oxide (ZnO), titanium oxide (TiO2), gallium oxide (Ga2O3), tungsten oxide (WO3), silicon oxide (SiO2), tin oxide (SnO2), magnesium oxide (MgO), indium oxide (In2O3), cupric oxide (CuO), cuprous oxide (Cu2O), cadmium oxide (CdO), vanadium oxide (V2O5), nickel oxide (NiO), aluminum oxide (Al2O3), molybdenum oxide (MoO2), etc., which show the multifunctional behavior including optical, electrical, magnetic and optoelectronic. These materials play a vital role in different fields such as paint pigments, medical diagnostics, pharmaceuticals, cosmetics, catalysis and supports, magnetic and optical devices, flat panel displays, batteries and fuel cells, electronic and magnetic devices, implantation materials, protective coatings, etc. In particular, these materials exhibit great potential in optoelectronic and photoelectronic devices such as optical sensors [15,16], solar cells [17,18], p-n junction diodes [19], random access memory devices [20], transistors [21], organic photovoltaics [22–25], photochromic and photo electrochromic system for digital display [26,27], fiber-optic communication [28], photoresistors [29], etc. Many of them are important photocatalysts for various organic transformations and photodegradation reactions as shown in Fig. 2. MOs continue to offer clear advantages for optoelectronic applications due to their combination of high carrier mobility, good optical transparency, simple synthetic access, large-area electrical uniformity, and mechanical flexibility. Therefore, in this chapter, we have discussed some of the applications of metal oxides.

36

Metal Oxides for Optoelectronics and Optics-Based Medical Applications

Fig. 2 Metal oxides for various applications.

2

Metal oxides (MOs) for various applications

2.1 Photodetector The photodetector is a light detecting optoelectronic device that converts optical energy into electrical energy known as photocurrent. When the light photons are incident on the surface of the photodiode, electron-hole pairs are generated. These generated electron-hole pairs are drifted toward their opposite polarity by the application of electric field as a result the photocurrent is produced. The photocurrent is proportional to the intensity of the light that falls on it which is the square of the light intensity. Hence the photodiode is also known as a square-law detector. The photodiode finds its applications in many fields such as light-wave communication systems, sensors, and solar cells, territory intrusions, imaging technology, optocoupler, and optoelectronic circuits. A highly efficient photodetector should possess high sensitivity, high spectral selectivity, and high signal-to-noise ratio, high speed, and stability.

Recent developments in optoelectronic and photonic applications

37

The parameters such as responsivity, quantum efficiency, and dark current (leakage current) of a photodiode also decide the quality of the photodiode. Aluminum doped zinc oxide (AZO) supported photodiode was fabricated by Najla et al. [30]. Huang et al. [31] were fabricated the ZnO photodiode by depositing the Al–N-codoped p-type ZnO film on the Al-doped ZnO film by reactive magnetron sputtering method. The photodiode exhibited the distinct rectifying I–V characteristics with a turn-on voltage of about 2.0 V and the cut-off wavelength of the photodiode was measured by Spectral photoresponse which is about 375 nm. Liu et al. [32] were fabricated and studied the high photoresponse, stable and selfpowered performance metal oxide semiconductor (MOS) photodetector based on Gallium Oxide (Ga2O3) material for the applications of the solar-blind photodetector (SBPD). They constructed the Ni/SiO2/β-Ga2O3 MOS-structured photodiode that attained high responsivity (R) of 189.89 AW1 (3.96 AW1) and a high external quantum efficiency (EQE) of 92,879% (1936%) at 10 V (10 V), responding to a small UV light signal with the intensity of 1.1 μW cm2, suggesting a highresolution and sensitive detection responding to the small signal. This MOS device shows a larger rectifying ratio owing to its lower reverse leakage current and higher forward saturation current compared to the Metal Semiconductor device. Similar studies were carried out by Qin et al. [33] using Metal-SemiconductorMetal (MSM) ε-Ga2O3 for the applications of SBPD and the obtained results are shown in Table 1. SBPD is an emerging technology for forest fire, ozone hole monitoring, deep space exploration, satellite, and security communication [42,44,45]. Li et al. [36] were fabricated the poly(3,4-ethylene dioxythiophene): polystyrene sulfonate (PEDOT: PSS)/Ga2O3 heterojunction organic-inorganic hybrid self-powered solar-blind photodetector with large open-circuit voltage (Voc  0.9 V). UV photodetectors (PDs) have a wide application in the field of optical switching, fire detection, chemical/biological analysis, astronomy, space technology, and so on. Nowadays Researchers focus on wide bandgap metal oxides for the fabrication of UV photodetectors. The β-Ga2O3 photodiode can detect deep-UV light at wavelengths below 260 nm. It shows that the metal oxide β-Ga2O3 can act as UV PDs [46]. Several studies were reported pure and doped Zinc Oxide (ZnO) material for UV PDs. Since ZnO is an important transition metal oxide (TMO) with a wide and direct bandgap of 3.37 eV with high exciton binding energy (60 meV) that makes it a promising material for optoelectronic devices. It has been a pioneering material in the optical and electrical fields for many years. ZnO is often called an II–IV group compound. The bonding between Zinc and Oxygen is an ionic character which explains its strong piezoelectric property. Since ZnO has both semiconducting and piezoelectric properties; therefore, it has enormous potential in energy harvesting applications. Singh [47] has been investigated the Al-doped ZnO-based metal–insulator–semiconductor– insulator–metal (MISIM) PDs with a thin SiO2 insulating layer. Three MISIM devices with three different electrode spacing “s” 10, 20, and 50 μm were fabricated using palladium (Pd) metal. The value of contrast-ratio for the three Pd/SiO2/AZO/SiO2/Pd

Table 1 Comparison of photoresponse behavior of different metal oxides. Metal oxide

Preparation method

UV light (nm)

Responsivity (R) A/W

Specific detectivity (D*) Jones

Rejection ratio

leakage current

external quantum efficiency (EQE) %

Decay time

References

MSM-εGa2O3 Ni/SiO2/ β-Ga2O3 MSM-βGa2O3 β-Ga2O3

MOCVD

254

230

1.2  1015

1.7  105

23.5 pA

1.13  105 at 6 V

24 ms

[33]

MOCVD

254

189.89/3.96

1013

2  103

1.05 pA

92,879 (1936) at 10 V (10 V)

–

[32]

CVD

254

29.8

1.45  1012

9.4  103

Radio frequency magnetron sputtering MOCVD and spin coating

254

96.13

–

>103

1.4 pA

4.768  104 at 5 V

78 ms

[35]

254

37.4  103

9.2  1012

7  103

3.7 pA

18.3 at 0 V

71.2 μs

[36]

Laser molecular beam epitaxy Radio frequency magnetron sputtering Radio frequency magnetron sputtering Thermal oxidation

254

0.763

127

0.272 s

[37]

254

0.35

14.8

0.29 nA

1.7  102

–

[38]

254

43.31

20

0.42 nA

2.1  104 at (10 V)

210 ms

[39]

266

0.01 (0 V) 2.9 (50 V)

38

10 pA (30 V)

64 μs

[40]

PLD

254

54.43  103

152

0.08 s

[41]

MOCVD

250

160  106

3.1 eV) used as a transparent metal electrode which allows for applications in the visible and near UV spectral range. TMOs such as Molybdenum oxide (MoO3), Zirconium oxide (ZrO2), Hafnium oxide (HfO2) and Tantalum oxide (Ta2O5) are all transparent in the visible region and nominally insulating [61,62]. Among the different phases of Tungsten oxide, WO3 is the most stable phase whose Eg value is 2.7 eV with a monoclinic structure [63]. Nickel Oxide is another commonly using TMO with a bandgap of 3.8 eV is a chemically stable and transparent material to visible light. P-type Cuprous Oxide (Cu2O) (Eg  2 eV) is one of the low-cost MO used as photovoltaic absorbers [64]. The fusible metal oxides are zinc oxide (ZnO) (3.2 eV), cadmium oxide (CdO) (2.2 eV), gallium oxide (Ga2O3) (4.6 eV), indium oxide (In2O3) (3.2 eV), and tin oxide (SnO2) (3.5 eV) possess a wide bandgap finds a widespread application in PV devices. Oxides of the semimetals such boron (B), aluminum (Al), carbon (c), silicon (Si), and germanium (Ge) are all generally wideband gap and ionic. Aluminum oxide (Al2O3) is a relatively wide bandgap, high dielectric oxide, and transparent to visible light. Because of its stability and chemical inertness, it is used as a coating material for mechanical protection and barrier layer in PV devices [65]. Multinary oxides (oxygen combined with two or more distinct cation elements), for example, CdIn2O4 [66,67], Cd2SnO4 [67], ZnGa2O4, and In2ZnO4 [68] are also used for PV device fabrication to improve the efficiency of solar cells. Tseng et al. [51] were deposited the Cu2ZnSnSe4/CH3NH3PbI3/ZnS/IZO/Ag/FTO nanostructure on Mo/FTO (fluorine-doped tin oxide) bilayer glass substrates. The results show that the addition of IZO could further increase the PV cell parameters of Ag/ZnS/MAPbI3/Cu2ZnSnSe4/Mo/FTO nanostructured solar cells. That is, the open-circuit voltage was enhanced from 0.98 to 1.10 V, short-circuit current was enhanced from 20.4 to 20.8 mA/cm2, the fill factor (FF) was increased from 71.3% to 76.3%, the PCE value was enhanced from 14.3% to 17.4%, and the device output power Pmax value was improved into 1.74 mW from 1.43 mW. The substrate FTO used for the deposition could notably decrease the photon absorption at the back electrode. Kingsley et al. [52] were fabricated the NiO/TiO2 PdN heterojunction solar cell on ITO coated glass and soda-lime glass substrate. The p-n junction is formed when P-type (NiO) and N-type (TiO2) semiconducting materials are placed in contact with each other. They achieved the PCE value of 2.3% and the open-circuit voltage, short-

Recent developments in optoelectronic and photonic applications

43

circuit current, and fill factor values were 350 mV, 16.8 mA, and 0.39 under 100 mW/cm2 illuminations, respectively. The heterostructure of CdS nanoparticles encapsulated ZnO nanorods without the use of a capping agent was fabricated for the applications of solar cells using successive ionic layer adsorption and reaction (SILAR). The PCE value obtained for this heterostructure was 0.123% under standard illumination of simulated sunlight [53]. Tadatsugu et al. [54] were fabricated the Cu2O based solar cells using Al2O3–Ga2O3–MgO–ZnO (AGMZO) multicomponent oxide thin films and achieved the efficiency of 4.82% and open-circuit voltage (VOC) of 0.98 V. In addition, an enhanced PCE of 6.25% and 5.4% were achieved by AZO (n-type) multicomponent oxide/Cu2O (p-type) heterojunction solar cells fabricated using Na-doped Cu2O (Cu2O:Na) sheets and MgF2/AZO/n-AGMZO/p-Cu2O:Na heterojunction solar cell, respectively. In this device, the Cu2O sheets have functioned as an active layer as well as the substrate. MgO coated TiO2/CH3NH3PbI3 perovskite solar cells were designed by Han et al. [55] and the results show that the inclusion of MgO nanolayer on TiO2 nanofilm increases the PCE from 11.4% to 12.7%, than the pure TiO2 based perovskite solar cell, which demonstrates that the addition of MgO layer will ultimately retard the charge recombination interface in solar cells. Dye-sensitized solar cells (DSSCs) are considered to be a better alternative to conventional solid-state solar cells because of their high photovoltaic performance and costeffectiveness. Quantum dot-sensitized solar cells (QDSSCs) are one form of the new generation solar cell, in which the configuration and functions are similar to the DSSCs. TiO2/CdS QDSSC was fabricated by the cost-effective SILAR method on FTO substrate which yields a PCE of 1.75% under 100 mW/cm2 illuminations [56]. Alavi et al. [69] were fabricated several types of CdS/TiO2 based QDSSCs, studied their improvement step by step, and the high PCE of 4.62%, VOC of 0.72 V, was obtained for GO/N-doped TiO2/CdS/10 MnZnS/Zn-porphyrin. Gaikwad et al. [70] were designed several ZnO nanorods photoelectrode for DSSC application through a simple SILAR method using Ruthenium (Ru) as a dye material by varying the dye loading time. They achieved the PCE of 0.25%, 0.36%, 0.18%, 0.58%, 0.70%, 0.44%, 0.38%, and 0.55%, respectively. The highest PCE of 0.7% was obtained by maintaining the dye loading time of 18 h for the ZnO-based DSSC prepared. Suleiman et al. [57] were fabricated the FTO-ZnO/Cu2O-Cu and FTO-TiO2/Cu2O-Cu Solar cells using the spray pyrolysis method. The PCE and VOC of fabricated solar cells are 0.0364 V and 0.0076%, and 0.03 V and 0.037%, respectively. Rokhmat et al. [58] fabricated TiO2/CuO and TiO2/CuO/Cu solar cells using fix current electroplating and spray method with different electroplating current (0.1–100 mA) for 10 s. The I–V characteristics were studied under a photo intensity of 120 W/m2. The circuit current, VOC, fill factor (FF), and PCE of TiO2/CuO solar cell were 0.8 mA, 0.62 V, 0.33%, and 0.14%, respectively. These values are enhanced to 0.12 mA, 0.61 V, 0.35%, and 0.21%, respectively, after adding copper (Cu). Further, the efficiency of the solar cell was improved to 0.8% by changing the electroplating current (10 mA). In 2019, Chawalit et al. [59] were studied photovoltaic parameters of MAPbI3 perovskite solar cell adopting Al-doped ZnO (AZO) as an electron transport material (ETM) for the perovskite solar cell. The highest PCE of 1.83% was obtained for 2.5 at.% of Al doping.

44

Metal Oxides for Optoelectronics and Optics-Based Medical Applications

2.3 Photoresistors Photoresistors are light-controlled or light-sensitive resistor and it is also known as light-dependent resistors (LDRs) or photo-conductive cell. The resistance of the photoresistor decreases with the increase of the intensity of light that falls on it. When a photoresistor is placed in a dark condition, its resistance is very high (1 MΩ). But when it is exposed to light, the resistance drops dramatically, even down to a few ohms, depending on the intensity of incident light. Photoresistors are often used as light sensors. Other important applications of LDR are night light and photographic light meter, fire or smoke alarms, combustion process monitoring, and solar emission monitoring, chemical and biological analysis and also used to detect nuclear radiation. There will be several advantages of using metal oxide in photoresistors than the conventional metals due to being cost-effective, can operate at high temperature, high reliability and low noise. Witkowski et al. [71] studied the ZnO rods-based photoresistors and the high resistive p-type Si-substrate used as a substrate. They recorded the changes of resistance with respect to the light of different wavelengths (400– 1100 nm) exposed to the ZnO nanorods and the different gaseous environments and found that the sample is highly sensitive and enables usage as an optical switch. Kin et al. [72] investigated the photosensitive behavior of ZnO nanorods/CdS film grown on the glass substrate. The photoresponsivity of ZnO nanorod at 500, 350, and 200 nm, and non-ZnO/CdS film were found to be 12.86, 3.83, 0.91, and 0.75 A/W, respectively with an incident wavelength of 350 nm and 5 V applied bias. The photoresponsivity of the ZnO nanorods on the CdS film was greater than the pure CdS film and also increases with the increase in ZnO nanorod length. The photoresponsivity of amorphous-TiO2/SrRuO3 (SRO) and epitaxial TiO2/ SrRuO3 heterostructure was compared by Liu et al. [73]. The result shows that the amorphous-TiO2/SRO achieves a photoresponsivity of 6.56 A/W at 1 V. Such a performance is hardly obtained in typical oxide-based photoresistors. SRO strongly absorbs visible light accompanied by small negative photosensitivity, indicating that the hot carriers become excited under illumination [74,75]. These photo-excited hot carriers are injected spontaneously from SRO to TiO2 due to the built-in potential resulting from the Schottky barrier at the interface.

2.4 Sensors Metal oxide sensors are highly suitable for detecting combustible, reducing, or oxidizing gases by conductive measurements. The variation in the resistance of the metal oxides due to the adsorption of gases is used to detect the various types of gases. Metal oxides selected for the gas sensors can be determined from their electronic structure. The metal oxides were divided into two main categories because the range of electronic structures of oxides is so wide. They are transition metal oxides (Fe2O3, NiO, Cr2O3, etc.) and nontransition metal oxides, which include (a) pretransition metal oxides (MgO, Al2O3, etc.) and (b) posttransition metal oxides (ZnO, SnO2, etc.). Transition metal oxides (TMOs) are more sensitive than pre-TMOs to the environment because the pre-TMOs have difficulties in electrical conductivity measurements; wide bandgap, structure instability and nonoptimality of other parameters limit

Recent developments in optoelectronic and photonic applications

45

their field of applications. The energy difference between a cation dn configuration and either a dn+1 or dn-1 configuration is often small. Only transition-metal oxides with d0 and d10 electronic configurations find their real gas sensor application. The d0 configuration is found in binary transition metal oxides such as TiO2, V2O5, and WO3. d10 configuration is found in posttransition metal oxides, such as ZnO, SnO2. Metal oxide sensors are widely used to monitor physical, chemical, biological, and environmental parameters. There are different types of sensors such as temperature sensors, proximity sensors, IR sensors, pressure sensors, light sensors, smoke, alcohol and gas sensors, touch sensors, color sensors, humidity sensors, tilt sensors, flow and level sensors, etc., are used to measure physical properties like temperature, resistance, pressure, displacement, capacitance, etc. A good sensor should possess important characteristics such as short response time and high response. Sensitivity of NO2 and H2 gas for pure vanadium pentoxide (V2O5) thin films and different doping ratios of Cerium (Ce) prepared by spray pyrolysis technique were studied by Jassim et al. [76]. Ali et al. [77] were prepared the pure and Neodymium (Nd) doped V2O5 using the spray pyrolysis technique. The maximum sensitivity to be observed for the V2O5 thin film doped with 7% Nd was 55% at room temperature to the NO2 gas and 66% to the H2 gas at room temperature with a small response and recovery times. Titanium dioxide (TiO2) is a chemically stable and electrically semiconductive material that has extended the application to sense materials [78,79]. The CO2 gas sensing behavior of TiO2/AgNP (silver nanoparticle) heterostructure thin films deposited on soda-lime glass substrate were investigated by Raza et al. [80]. Tin dioxide (SnO2) is one of the attractive gas sensing n-type semiconducting metal oxides. Because of its wider bandgap, thermal and chemical stability, and wide operation temperature (200–600 °C), it can be used for the detection of various gases [81– 83]. Tsymbalov et al. [84] were fabricated the Ammonia (NH3) sensors based on Tin Oxide prepared using RF magnetron sputtering, which shows the short response times. Kim et al. [85] have fabricated the hybrid gas sensors, such as SnO2 nanoparticles and TiO2 nanotubes using MEMS (micro-electro-mechanical systems) process and studied the sensing behavior of gas sensors for the gases CO and CH4. The sensitivities were observed to be 61–462 mV for CO and 413–1462 mV for CH4, respectively. Gas sensing behavior of silver (Ag) doped TiO2 thin film deposited on quartz substrate manufactured by sol–gel spin coating method was studied by Nataraj et al. [86]. The synthesized Ag-doped TiO2 thin films showed the highest response of 1.247 for 300 °C annealing temperature, 0.025 Silver dopants and 5 ppm CO gas exposure. Magnesium Oxide is an important dielectric metal oxide used as a buffer layer in magnetic tunnel junction (MTJ) sensors and radiators in thermophotovoltaic (TPV) devices [87,88]. Chen et al. [89] prepared the MgO barrier MTJ for flexible and wearable spintronic devices stacks with layer sequences Ta5/Ru30/Ta5/Ni81Fe19 (NiFe) 5/ Ir22Mn78 (IrMn) 10/Co90Fe10 2.5/Ru 0.9/Co20Fe60B20 (CoFeB) 3/MgO2.4/CoFeB3/ Ta5/Ru5 (thicknesses in nanometers) were prepared at room temperature in a modified three-chamber Shamrock sputtering tool. MgO was grown by RF sputtering using a target-facing-target gun in a different chamber. Dohmeier et al. [90] were investigated the CoFeB/MgO/CoFeB tunnel junctions were deposited at an argon pressure of 1:2  103 mbar prepared via DC sputtering except MgO barrier, which is deposited via RF sputtering at a pressure of 2:2  102 mbar. A yolk-shell Fe3O4@PA-Ni@Pd/

46

Metal Oxides for Optoelectronics and Optics-Based Medical Applications

Chitosan nanocomposite is used for the sensitive determination of fluconazole (2,4-difluoro-α,α1-bis(1H-1,2,4-triazol-1-ylmethyl)) [91]. Fluconazole is commonly used to diagnose the immune deficiency diseases such as AIDS [92].

2.5 Phototransistors Phototransistors are a tri-terminal (emitter, base, and collector) current amplification component which has a light-sensitive base region. When light falls on the junction, reverse current flows between the collector and the emitter region. It works on a principle similar to LDR but it can produce both current and voltage while photoresistors are only capable of producing current due to change in resistance. It is used to amplify the photocurrent and suppresses the dark current under dynamic photostimulus. Thus, this device significantly provides a large gain, high photosensitivity, a large dynamic range, and low noise levels for next-generation highly integrated photosensor systems. The phototransistors find their applications in object detection, encoder sensing, security systems, punch-card readers, relays, automatic electric control systems such as in light detectors, computer logic circuitry, counting systems, smoke detectors, Laser-ranging finding devices, optical remote controls, CD players, astronomy, night vision systems, Infrared receivers, printers and copiers, cameras as shutter controllers, level comparators, etc. The output of the phototransistors depends on (i) the wavelength of the incident light, (ii) the area of the light-exposed collector-base junction, and (iii) the DC gain of the transistor. Cho et al. [93] were fabricated the high-performance ZnO field effect transistor (FET) through a spray pyrolysis technique in an ambient atmosphere which exhibits the mobility of 14.7 cm2/V s1, an on/off ratio of 109, and a sub-threshold slope (SS) of 0.49 V/decade. The gate bias-induced instabilities were analyzed under prolonged positive (+20 V) and negative (20 V) gate voltage for 20 ks which induces threshold voltage shifting with the bias stress which was expected that defect creation and charges trapping at or near the interface between semiconductor (channel) and gate insulator, respectively. Cho et al. [94] were produced the metal oxide/chalcogenide hybrid structure–based visible-light phototransistor with multistacked functional layers. Herein, the multistacked structure contains the zinc tin oxide (a-ZTO bottom), a high efficiency coarsened crystalline cadmium sulfide (CdS) visible-light absorber, defective surface passivation (a-ZTO top) to boost photosensitivity. The multistacked a-ZTO/cc-CdS/a-ZTO phototransistor achieved repeatable/reproducible real-time full-color photo-sensing with high photosensitivity and fast photo transient speeds under the dynamic stimulus of primary colors.

2.6 Photocatalysts A photocatalyst is a material that converts light energy into chemical energy by absorbing photons. Commonly, photocatalysis is a process that involves the absorption of light by a catalyst to initiate and boosting up chemical reactions [95]. Photocatalysts are the materials that decompose the adverse substances under the sunlight containing UV rays. Applications of photocatalysts include water and air treatment, hydrocarbon generation, hydrocarbon production, the transformation of specific compounds, photocatalytic water splitting and self-cleaning coatings as well as drug

Recent developments in optoelectronic and photonic applications

47

delivery. Semiconducting metal oxide such as TiO2, SnO2, Fe2O3, ZnO, CeO2, ZrO2, WO3, V2O5, etc., was extensively used as a photocatalyst for several decades. The biocompatibility, exceptional stability, favorable combination of electronic structure, their ability to generate charge carriers when stimulated with required quantity of light energy, charge transport behavior and excited lifetimes of metal oxides has made it possible material for their applications as photocatalyst [96–99]. Jayaraj et al. [100] have been studied the photocatalytic activity of hydrothermally prepared orthorhombic structured V2O5 nanorods. The photodegradation of Rhodamine 6G (Rh-6G), methyl orange (MO), and methylene blue (MB) occurs under visible-light irradiation. The degradation efficiency was maximum for Rh-6G (85%), 48% for MO, and 24% for MB, respectively, and it was faster in Rh-6G. Han et al. [101] were investigated the degradation of methyl green and formaldehyde solution using pure SnO2, SnO2–ZnO, B-doped SnO2–ZnO, B/Ag didoped SnO2–ZnO B/Ag/F, and tridoped SnO2–ZnO composite film synthesized by sol–gel method. The result shows that the tridoped SnO2–ZnO composite film has shown the highest photocatalytic activity. The composite photocatalyst has potential applications in industrial wastewater treatment. The photodegradation of Rhodamine B under visible light using V2O5– TiO2 nanocomposite was studied by Mandal et al. [102]. This nanocomposite showed the enhanced photocatalytic activity of 89% with the addition of 1 mL of H2O2 to the aqueous solution in comparison with both nanocrystalline TiO2 and V2O5. Muntadher et al. [103] were investigated the photocatalytic activity of different concentrations of iron oxide (Fe2O3)-doped Zinc Oxide (ZnO) nanostructure for methyl blue (MB) dye degradation under visible-light radiation. They found that the enhanced degradation percentages of 80.6%, 83.8%, 85.5%, and 92% for doped ZnO, respectively, as compared to 73.8% for undoped ZnO.

3

Role of metal oxides for thin-film technology

The discovery of p-type transparent conducting oxides (TCOs) has led this class of material to the frontier of transparent oxide semiconductors (TOSs). It thus becomes possible to fabricate pn-junctions using an appropriate combination of p- and n-type TOSs similar to group III–V semiconductors such as GaAs and GaN. This enables the creation of a variety of transparent electronic devices. Another reason for their limited application is the scarcity of high-quality epitaxial or single-crystalline TOS films. Although many reports of the fabrication of transparent optoelectronic devices such as pn-junction diodes and transistors using TOSs have been published, their performance is significantly inferior to that of III–V semiconductor devices because the thin films are polycrystalline, i.e., defects and grain boundaries in the active layer degrade device performance. Optoelectronic properties, and therefore, device performances are expected to improve drastically by using single-crystalline or high-quality epitaxial film in place of nonoriented polycrystalline films. The researchers have explored the use of metal oxides (MOs), which, compared with crystalline silicon and other III–V semiconductors, exhibit unique properties, including excellent carrier mobilities even in the amorphous state, mechanical stress tolerance, compatibility with organic dielectric and photoactive materials, and high optical transparency. Furthermore, high-quality

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Metal Oxides for Optoelectronics and Optics-Based Medical Applications

electronic-grade MO thin films are accessible using vapor- and solution-phase methodologies in near-ambient conditions (25 °C under air), widening their applicability to high-value products such as inexpensive circuits, and to flexible organic lightemitting diode (OLED) displays and solar cells on plastic substrates. The primary role of a buffer layer in heterojunction devices is to form a junction with the absorber layer while receiving a maximum amount of light to the junction region and absorber layer. Liu et al. [104] have been studied the structural and optical properties of aluminum-doped zinc oxide (AZO) with Al2O3 buffer layer, films were deposited by radiofrequency magnetron sputtering on the polyethylene terephthalate (PET) substrates and the results were compared with AZO thin films without Al2O3 buffer layer. The results showed that the inclusion of Al2O3 buffer layer enhanced the crystal quality of AZO thin films and provides a reference for the applications of the AZO/Al2O3 films on flexible optoelectronics. Magnesium oxide (MgO) is an exceptionally important dielectric material; it has been proposed to replace current dielectric material (SiO2) [105] due to its higher thermal stability and chemical inertness. Choi et al. [106] have been synthesized the inverted top-emitting organic light-emitting diodes (ITOLEDs) and investigated the performance of the MgO buffer layer on the enhancement of electron injection ITOLEDs. The luminescence value increased from 560 to 1000 cd/m2 ITOLEDs using the MgO buffer layer. Synchrotron radiation photoelectron spectroscopy (SRPES) revealed that the surface of Al cathode modified to Al oxide due to the deposition of MgO and the results showed that the work function of ITOLEDs increased to 0.8 eV in comparison with the Al cathode. Zinc oxide is an important metal oxide with a wide and direct bandgap of 3.37 eV with high exciton binding energy (60 meV) that makes it promising material for photonic and optoelectronic devices. The optical and electrical quality of ZnO films grown on c-sapphire with MgO buffer layer was studied by Setiawan et al. [107]. The photoluminescence spectra revealed that the intensity of free exciton emission is maximum and twice higher for the sample with MgO buffer annealing than without annealing. The improved electron mobility of ZnO films grown with MgO buffer annealing is due to a decrease in dislocation density. Zhou et al. [108] studied the performance of organic light-emitting diodes (OLED) by using Al2O3 as a buffer layer. The power efficiency and current efficiency of the OLED device increased by 12.5% and 23.4%, respectively, with an optimal thickness of 1.4 nm highquality Al2O3 buffer layer due to lower energy loss during the holes injection process and better-balanced charge injection. Transparent thin-film transistors (TFTs) using transparent oxide semiconductors (TOSs) as the channel layer have several merits compared with conventional Si-TFTs when applied to flat panel displays. These include the efficient use of backlight in LCDs or emitted light in OLEDs, as well as device performance insensitivity to visible-light illumination. Furthermore, oxide TFTs may outperform semiconductorbased TFTs in terms of high voltage, temperature, and radiation tolerance. However, practical TOS-TFTs have yet to be realized due to their low on-to-off current ratio and poor reproducibility, which is presumably due to a lack of high-quality, singlecrystalline TOS thin films. To date, conventional TOSs such as SnO2 and Zn have been used to fabricate TTFTs with insufficient performance [109,110]. For example, their on-to-off current ratios and field-effect mobilities, are on the order of 103 and as low as 5 cm2/Vs, respectively, and the devices are “normally-on.” Despite the fact that

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a polycrystalline ZnO-TTFT has been found to have “normally-off” characteristics with an on-to-off current ratio of 107, its field-effect mobility remains 3 cm2/Vs. Potential grain-boundary barriers also limit performance [111]. The large off-current and normally-on characteristics may be due to the fact that these conventional TOSs contain many carriers in the as-prepared state (due to the somewhat large nonstoichiometry in the chemical composition), making it difficult to reduce the carrier density to less than 1017 cm3 without the use of counter-doping acceptors (which leads to the reduction of the field-effect mobility). As a result, it is critical to select a material capable of reducing carrier concentration to the intrinsic level and to devise a method for producing high-quality, single-crystalline films. Transparent oxide semiconductors (TOSs) with an atomically flat surface offer a novel opportunity as a promising electrode material for realizing the optoelectronic functions of organic molecules in molecular electronics. Organic transparent thin-film transistors (TFTs) with greatly increased mobility have been obtained for epitaxially grown organic semiconductor molecules on atomically flat ITO. Another key issue is the development of an “amorphous transparent oxide semiconductor” (a-TOS) to realize flexible optoelectronic devices, as amorphous thin films can be fabricated on plastics at room temperature.

4

Optoelectronic properties of metal oxides

Carrier mobility is a key metric of semiconductor performance. Traditional band theory descriptions of transport in semiconductors, including wide-gap ionic MOs, represent the electronic structure in reciprocal space, with the conduction band minimum (CBM) and the valence band maximum (VBM) curvatures and dispersions determining the electron and hole effective masses, respectively. Smaller effective masses mean greater CBM and VBM hybridization, affording larger carrier mobilities, all other factors being equal. Note that transport in MOs is very different from that in Si semiconductors, where hybridized sp3 σ-bonding and sp3 σ*-antibonding states define the VBM and CBM, respectively. In the oxides of interest here, the VB is typically composed of occupied 2p O antibonding states and the CB primarily of unoccupied ns metal bonding states. Tuning the bandgap of metal oxides by incorporating a foreign element (doping) is the core for current research and optoelectronic applications such as photocatalysis, gas sensing, photoluminescence, energy harvesting, etc. By incorporating impurities into TCOs introduces intermediate energy levels between the bandgap. Rosario et al. [112] studied the significant improvement in photocatalytic activity of Fe doped ZnO nanoparticles by increasing the doping concentration due to the change in the bandgap. Fe ions doped in ZnO nanoparticles lowers the average crystallite size and narrow the energy gap. Yongchun [113] synthesized the codoped ZnO nanorods for the photocatalytic activity of photodegradation of organic dye alizarin red (AR) under the visible-light irradiation (λ ¼ 420 nm) which is comparatively better than pristine ZnO. The visible-light photocatalytic activity of the ZnO:Co rod is due to the energy gap absorption onset was extended into the visible-light region (λ ¼ 400–700 nm) for all codoped ZnO samples. Several types of research [114–

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120] have been studied for the reduction of the bandgap of doped and undoped TiO2 nanoparticles for photocatalytic applications. Khan et al. [121] were produced the modified light gray TiO2 (g-TiO2) by treating white TiO2 (w-TiO2) in the cathode chamber of a Microbial Fuel Cell (MFC), and perform different studies to understand the decrease of the bandgap of g-TiO2 (Eg ¼ 2.8 eV) compared with a bandgap of wTiO2 (Eg ¼ 3.1 eV). The X-ray photoelectron study (XPS) revealed that the variations in the surface states, composition, oxygen vacancies, Ti4+ and Ti3+ ratio in the g-TiO2. The result shows that the Ti3+ and oxygen vacancy induced improves the visible-light photocatalytic activity of g-TiO2 and it was confirmed by degrading different model dyes. Borse et al. [122] were synthesized Lead (Pb) substituted BaSnO3 by a modified citrate complex method by variation of the Pb/(Sn + Pb) mole ratios 0.1–0.8 mol/mol. The bandgap significantly decreases with the increase in Pb concentration, which suggests that the contribution of Pb 6s orbitals synchronously evolves near the bottom of the conduction band. In the extreme of complete substitution of Sn with Pb, the valence and CB overlap around the Fermi level, yielding a semimetallic band structure. The photocatalytic activity for the photo-oxidation of water can be attributed to the suitable band structure of Pb substituted BaSnO3 that yields an active semiconductor with optimal BG for significant absorption of visible-light (λ ¼ 420 nm) irradiation.

5

Conclusion

This chapter reveals the development of metal oxides by capitalizing its unique electrical and optical properties for numerous applications in optoelectronic and photoelectronic devices with many notorious advances in recent years. Metal oxides provide a novel opportunity as a gifted material for flat panel displays, solar cells, p-n junction, sensors, etc. Bandgap modulation of metal oxides by different methods such as composition, doping, strain, and quantum confinement makes remarkable enhancement in optical behavior. Overall, this chapter demonstrates that metal oxides are successfully paved the way for the next generation of photonic devices and other related applications. Furthermore, metal oxide TFT technology has recently gained considerable attention, due to its high mobility, low-temperature capability, good transparency to visible light, and relatively low fabrication cost. The high fabrication cost of poly-Si TFTs aforementioned can be alleviated by replacing them with metal oxide TFTs, because oxide TFTs can be fabricated without crystallization and intentional doping process.

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Metal oxide-based glasses and their physical properties

3

Muhammad Nihal Naseera,b, Muhammad Azhara, Asad A. Zaidic, Yasmin Binti Abdul Wahabd, Muhammad Asifa, and Suresh Sagadevand a National University of Sciences and Technology (NUST), Islamabad, Pakistan, bDepartment of Mechanical Engineering, College of Engineering, Seoul National University, Seoul, Republic of Korea, cDepartment of Mechanical Engineering, Faculty of Engineering Science and Technology, Hamdard University, Madinat al-Hikmah, Hakim Mohammad Said Road, Karachi, Pakistan, dNanotechnology & Catalysis Research Centre, University of Malaya, Kuala Lumpur, Malaysia

1

Introduction

Glass is the most used material in the history of humankind, from mirrors to advanced electronics, optics, and many other applications [1,2]. According to Varshneya [3], the definition of glass is “In order not to be overly restrictive, we are left to define glass as a solid with liquid-like structure, a noncrystalline solid or simply an amorphous solid.” Traditionally glasses are defined as “the rigid metastable solid produced by quenching a liquid form rapidly enough to prevent crystallization” [4]. In various contexts, glass is classified in different ways, a common classification is depicted in Fig. 1. The glass transition is a phenomenon of slow and reversible transition that especially happens in amorphous materials which causes the change from the glassy state into a sticky liquid with an increase in the temperature [6] (see Fig. 2). The amorphous material which shows this type of phenomenon is called glass. The temperature at which the glass transition phenomenon occurs is called transition temperature. The transition temperature is most frequently used in glass manufacturing and is considered a crucial factor in glass-forming techniques [8]. The reverse transition of the above phenomenon also exists that is caused by supercooling a viscous liquid, known as vitrification [9]. Vitrification is a very important process that is mostly applied in all glass forming techniques. Composite materials are made up of two or more phases of different materials and if one of those phases has a dimension in the order of a nanometer (10 9), those composites are called nanocomposites [10]. There are various nanocomposites of glasses available in the literature (Fig. 3) and are considered of great importance in fields of nanotechnology and electronics [12–14]. The concept of extraction from metal waste has also got much attention in this field [15,16]. Metal oxide glasses are one of the nanocomposites of glass; they have larger refraction indices and smaller phonon energies as compared to other glasses. They show transparency in the middle infrared spectrum and, offer excellent infrared transmission, large nonlinear susceptibilities, Metal Oxides for Optoelectronics and Optics-Based Medical Applications. https://doi.org/10.1016/B978-0-323-85824-3.00014-2 Copyright © 2022 Elsevier Inc. All rights reserved.

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Fig. 1 Classification of glasses [5]. From S. Bhattacharya, Metal oxide glass nanocomposites, Metal Oxide Glass Nanocomposites, Elsevier, 2020, pp. 27–35

Fig. 2 Graph of temperature vs volume showing freezing point and glass transition temperature [7]. From S. Bhattacharya, Fundamentals of glasses, Metal Oxide Glass Nanocomposites, Elsevier, 2020, pp. 3–25.

and better chemical durability [17]. Therefore, metal oxide glasses are very valuable and important materials for fields of photonics, optics, and optoelectronics [12]. Metal oxide glasses can be used as active and passive optical fibers, in sensor systems, in advanced solar cells, and optical devices [18].

2

Preparation of metal oxide glasses

Glasses are the most used materials world widely through the centuries and there are many methods for the preparation of the glasses depending upon their types [19]. The widely used methods for the preparation of metal oxide glasses will be discussed and the preparation of some fundamental oxide glasses will also be elaborated in this session.

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Fig. 3 Structures of glass composites [11]. From A. Sengupta, Electrodes, Metal Oxide Glass Nanocomposites, Elsevier, 2020, pp. 249–257.

2.1 Preparation methods Following are the methods which are usually used to prepare the metal oxide glasses. Some are older and traditional methods and others are newly discovered methods that are being used in the advanced nanotechnology field to prepare nanocomposites and complex semiconductors using various materials like metal oxide glasses.

2.1.1 Chemical routing This is a novel method for glass preparation that was proposed by Wang and Yang [20]. In this method, various precursor chemicals are passed through the heat treatment with different conditions then pressure injection techniques are applied for the synthesis of required composites. Many nanowires, semiconductors, and nanocomposites including glasses are prepared through this method [20,21].

2.1.2 Thermal evaporation As the name suggests a very high amount of heat is required and used for making glass in the thermal evaporation method (Fig. 4). The thermal evaporation technique is the most widely used method for the preparation of various amorphous materials of very small size including metal oxide glasses [23,24]. This technique was first proposed by Faraday in the 1950s. This technique works on basis of vapor deposition principle which is a vacuum deposition method used to produce high quality and highperformance solid materials [24]. In this process, the material is vaporized first after that it is collected on the substrate, the first form of the material is in powder. Vacuum creation is required for this process as it is performed in vacuum conditions. The material is heated using high voltage direct current or by any other source like using heat from combustion reaction, bombarding of high energy beam of electrons, or use of the LASER gun. There are various factors on which thermal evaporation depends; one of those factors is the temperature of the substrate if this temperature is too high the material crystallizes instantly.

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Metal Oxides for Optoelectronics and Optics-Based Medical Applications Vacuum chamber Substrate holder

Substrate

Vapor flux

Crucible containing target material

To pumping system

Fig. 4 Thermal evaporation [22]. From R.J. Martı´n-Palma, A. Lakhtakia, Chapter 15—Vapor-deposition techniques, in: A. Lakhtakia, R.J. Martı´n-Palma (Eds.), Engineered Biomimicry, Elsevier, Boston, 2013, pp. 383–398, https://doi.org/10.1016/B978-0-12-415995-2.00015-5.

2.1.3 Melt quenching and heat treatment The fact is slow cooling of the liquid phase of any material causes to form crystalline structure and glass is an amorphous material; therefore, supercooling is applied to material so that atoms do not have sufficient time to make a crystalline structure and an amorphous solid is formed by this method [25]. Hence the crucial condition for making glass, i.e., the cooling rate must be fast enough to prevent the crystallization of the material, is met. In this technique, other techniques’ knowledge can also be integrated and applied for the preparation of glass. This method is one of the oldest and most popular techniques in glassmaking and preparation of other amorphous materials (see Fig. 5).

2.1.4 Gel desiccation The sol–gel process has a considerable contribution to the preparation of the glasses. This method is mostly used for materials having high viscous melts and materials in which homogeneous mixing is difficult to achieve. The gel is an elastic solid product that is prepared from the viscous liquid by the process of polymerization. The gel is mixed with silica to form appropriate aqueous solutions and then it is heated which causes densification and removal of volatile components after that process fusion is applied to produce the amorphous solid. pH, size of particle, temperature, and pressure are factors that can affect the gelling process. We can convert gel into an

Metal oxide-based glasses and their physical properties

Raw materials (selection, calculation, weighing, and mixing)

Batch

63

Fig. 5 Glass preparation using melt-quenching [26]. From S. Bhattacharya, Features of metal oxide glass composite synthesis, Metal Oxide Glass Nanocomposites, Elsevier, 2020, pp. 37–49.

(Process: melting at the desired temperature in a suitable crucible with intermittent stirring)

Glass melt (Process: casting in a suitable mold and annealing)

Annealed as-prepared glass (Process: cutting, grinding, and polishing)

Sized and shaped glasses (characterization by different techniques)

amorphous solid by heating at a temperature lower than transition temperature, by sintering at a temperature greater than transition temperature [27].

2.1.5 Sputtering Sputtering is based on the principle of deposition by using plasma. In this technique, energetic ions are bombarded using low-pressure plasma which causes erosion of material, hence, deposition of material on substrate takes place. Deposition of the material is the effect that is caused by the bombardment of ions that makes an amorphous structure. The sputtering rate depends upon the source of plasma used in the process [28].

2.1.6 Shockwave formation This technique offers a high heating rate, rapid quenching, the high cooling rate of compacts, and nonuniform warm-up of materials. Shock waves are produced using laser-induced plasma which generates the stress effect; these stress effects are then used for making amorphous materials like glass [29] (see Fig. 6).

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Shock wave High power and short pulse

High pressure plasma

Laser beam

Constrain layer (water or optical glass) Workpiece

Workpiece Absorbing protective layer (black paint, black tape or aluminum foil)

Constrain layer(water or optical glass) Absorbing protective layer (black paint, black tape or aluminum foil)

Fig. 6 Laser shock processing [30].

2.1.7 Other methods There are some other methods also available for metal oxide glass formation. These include glow discharge decomposition, template-assisted growth, electrolytic deposition, chemical vapor deposition, and irradiation. The discharge decomposition technique was first proposed by Chittick et al. [31]. In this method, amorphous materials are formed using deposition caused by radio-frequency discharge deposition [31]. In template-assisted growth technique, ultrahigh vacuum (UHV) and chemical vapor deposition (CHV) is used to create amorphous solids like glass. Electroless deposition and bottom print are two methods of template-assisted growth. In the electrolytic deposition method, amorphous solids are made using deposition which is caused by passing an electric current through a solution containing salts. In the chemical vapor deposition technique, gas (precursor) is dispersed on the material which causes deposition hence forming the amorphous structure of the material [32]. In irradiation technique, materials are melted quickly using radiations in microwave oven and then supercooled to form glasses [33].

2.2 Preparation of some fundamental metal oxide glasses There are various metal oxide glasses and their preparations methods are almost the same but different conditions are required depending upon the nature of glass being prepared, properties required, and required form of the final product. Preparations of some fundamental metal oxide glasses given below are elaborated in detail.

2.2.1 (GeO2)1-x(PbO)x The glasses with x  (0, 0.5) are prepared by first melting the material in many crucibles like alumina, Pt, Au, and quartz under the atmosphere of atomic chlorine. The atmosphere of atomic chlorine is obtained by thermally decomposing the CCl4. Before the start of the process, the starting oxides are dried in the atmospheric reactions. The procedure of preparation starts with heating N2 and O2 (100–300 °C) to remove the physical bound water from the material then a reactive Cl atmosphere is used to

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65

remove chemically bound OH groups. Melting in the O2 atmosphere is then used and applied to remove the black point. After that mixture of the materials is melted for 1–2 h at a temperature of 900–1000 °C, then the liquid mixture is annealed and cooled to room temperature. In this way, metal oxide glasses are produced in many forms [18].

2.2.2 (TeO2)x(ZnO)1-x and (TeO2)(PbO, PbCl2)1-x The glasses with x  (0.3, 0.9) are prepared using the same technique that is used in the preparation of (GeO2)1-x (PbO) x which is described above. The material is melted in the alumina, Pt, Au, and quartz crucibles for 30min at about 800 °C. Few other preparation processes are then performed to reduce the OH content from the glass. Drying in a vacuum with a reactive atmosphere, substitution of PbCl2 by PbF2, and addition of NH4NO3, CaCl2, and AlF3 during melting are few procedures that can help in reducing OH from the glasses. At last, liquids are poured into respective molds for shaping purposes [18].

2.2.3 Ga2O3-PbO-Bi2O3 This class of glasses is also prepared using the procedure as described above but, in this class, reactive halogen’s presence affects the final product quality and glassforming ability. Therefore, the use of reactive halogens is avoided and pure oxygen is used only to get the good quality of the glass. Preparation starts with melting of material in Pt and alumina crucibles for 0.5–2 h at a temperature of about 1000 °C. Liquids are then poured into respective molds depending upon the shape of glass required and annealed at a temperature of 350 °C. Starting materials are dried in oxygen and nitrogen atmospheres and also heated in a vacuum to reduce the concentration of the OH groups [18].

2.2.4 Bi2O3-PbO-B2O3-GeO2 The preparation of this class of glass is almost the same as the preparation of the other glasses. Reagent grade bismuth oxide, extra pure germanium dioxide, lead oxide, and boric acid are first mixed and then melted in a crucible of pure alumina. Super Khanthal electrical resistance furnace is used for melting at a temperature of 1000 °C for 30 min. The liquid is then poured on the stainless steel plate after that it is annealed for 5 h at a temperature of 3500 °C [34].

2.2.5 Glasses doped with RE3+ ions Some glasses are stable during the devitrification process and others are not. Almost all those glasses whose composition is stable against the devitrification are processed through this method. In this preparation, ions of various rare earth metals like Pr, Er, and Nd are doped on the materials with stable composition. Doping is done by melting the compounds together with doping materials, doping materials are added in form of oxide, sulfide, or any other metal. Conditions like temperature, melting time, cooling

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rate, and annealing are also the same for the mixed doping substances. The range of concentrations of RE ions is 500–2000 wt ppm [18].

3

Properties of metal oxide glasses

Metal oxide glasses are characterized by various properties, some of those are highlighted in this session. They have a lower density than metals, their elastic modulus is comparable to diamond, they have greater strength than steel and their plastic deformation is different from metals and carbon fiber. The properties of metal oxide categories are divided into three categories:

3.1 Mechanical properties Mechanical properties are considered very important in materials as they show the nature and behavior of the material. There are various mechanical properties of the metal oxide glasses on basis of these properties’ materials are recognized and are utilized in different fields. Some mechanical properties are determined from the geometry of basic units of glass; mechanical properties are also determined by performing lab experiments on the materials like Knoop and Vickers hardness tests. Load-depth curve and indentation tests are also used for determining various mechanical properties of the glass-ceramics. Mechanical properties of metal oxide glasses are as following [35–38]: l

l

l

l

l

The addition of other materials (impurities) can cause changes in the mechanical properties of metal oxide glasses. The addition of nanoparticles can improve the mechanical properties of host metal oxide glasses. There is a very large improvement in mechanical properties even if 1%–5% filler materials are added. Generally, the stiffness and strength of nanocomposites tend to increase with the increase in the filler volume fraction. It is noticed that in some special nanocomposites if the volume fraction of filler material is constant then the strength and stiffness of that nanocomposite with a decrease in particle size.

3.2 Optical properties Metal oxide glass is much used in the field of optics, optoelectronics, and nanotechnology because of its superior optical properties. Following are the prominent properties of these glasses. Metal oxide glasses are transparent in the visible and infrared spectrum. They also have not characteristic of optical anisotropy as it is a property of crystals and glass is an amorphous solid. Metal oxide glasses have electronic transitions in interior molecules which cause them to absorb light energy in the ultraviolet region. They also absorb light energy in the infrared region because of molecular vibrations. The addition of alkali oxide is directly proportional to optical density in the ultraviolet region if the wavelength is longer than the absorption edge. The “n”

Metal oxide-based glasses and their physical properties

67

is the refractive index and “k” is the extinction coefficient and values of both “n” and “k” are constant with different values for different glasses. Propagation of electromagnetic waves through glasses depends upon the two constants of glass “n” and “k.” The Refractive index affects the phase of the propagating electromagnetic wave and the extinction coefficient affects the amplitude of the incoming wave. The refractive index and transmission range are affected by the addition of the chemical additives. Electronic absorption and vibrational spectra are dependent on the structure of metal oxide glasses. The reciprocal of relative dispersion of glass gives Abbe numbers; highly refractive glasses have very small Abbe numbers. The average electron donor power of all the oxide atoms in the medium is called Optical Basicity. The optical properties of metal oxide glasses can be changed by the strong electric field and intense radiation fields [39–42].

3.3 Electrical and dielectric properties Dielectric and electrical properties of metal oxide glasses have a lot of scope in the future; therefore, they are getting very much attention in electronics [12]. Metal oxide glasses are much used for dielectric purposes in advanced electronics, nanodevices, semiconductors, and capacitors [13,14]. In this session, the electric and dielectric properties of glasses like electric modulus and relaxation time are elaborated. These glasses can be used as electric capacitors and electric insulators in various electronic circuits [43]. Data of ionic and electronic transport mechanisms in amorphous materials is provided by frequency-dependent conductivity and dielectric constant, which makes metal oxide glasses a promising technology [44]. Almost alldielectric properties of metal oxide glasses are determined with dielectric constant, dielectric permittivity, dielectric conductivity, and electric modulus.

4

Future aspects and applications

In the 21st century, the world is moving away from traditional energy resources like fossil fuels and renewable energy resources are being adopted [45,46]. Therefore, the scientific community is focusing on low-carbon energy devices and harvesting unexploited energy from the environment to power small electronic devices. Now researchers are working on the nanodevices that harvest energy or are used as powerful computers because these devices consume very low power, have very high sensitivity, very small in size, and can be used as multifunctional devices. Metal oxide glasses have a lot of applications regarding nanodevices and they are building blocks of nanotechnology in advanced electronics (Fig. 7). The development of nanosystems to harvest energy and to consume that energy for utilization of semiconductor devices is a great idea that has got a lot of attraction in this century. As the research is going on, the new preparation methods for the metal oxide glasses are being developed and their properties are being enhanced to complete the requirements of the world. With time both preparation methods and properties of the metal oxide glasses are changing to develop more advanced and speedy devices or systems [33].

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Metal Oxides for Optoelectronics and Optics-Based Medical Applications

Al

+ V Ag2O-CdO-MoO3 (∼45–50 nm) p-Silicon

–

Al

Fig. 7 Schematic of device prepared using metal oxide glass nanocomposite [26]. From S. Bhattacharya, Features of metal oxide glass composite synthesis, Metal Oxide Glass Nanocomposites, Elsevier, 2020, pp. 37–49.

5

Summary and conclusions

In the literature glass is often described as “In order not to be overly restrictive, we are left to define glass as Solid with liquid-like structure, a noncrystalline solid or simply an amorphous solid.” The phenomenon of slow and reversible transition that especially happens in amorphous materials causes the change from the glassy state into a sticky liquid with an increase in the temperature, known as called Glass transition. The temperature at which glass transition occurs is called transition temperature. Composite materials are made up of two or more phases of different materials. If one or more phases of composite materials have dimensions in the order of nano (10 9) then they are called nanocomposites. Metal oxide glasses are one of the many nanocomposites of glass; they have larger refraction indices and smaller phonon energies as compared to other glasses. There is a various method for glass preparation as discussed in this chapter. In the chemical routing method, metal oxides are prepared by passing chemicals through heat treatment then pressure injectors are used to get the required composites. Material is first vaporized then vacuum deposition is applied in the thermal evaporation method. In the melt quenching and heat treatment method, the material is melted then supercooled to form an amorphous structure like glass. Gel desiccation is used for materials having high viscous melts, sol-gel process is applied for the preparation of glasses. Sputtering, shockwave formation, chemical vapor deposition, glow discharge decomposition, template-assisted growth, and electrolytic deposition are methods that are used for the preparation of various glasses. In all these methods, deposition of atoms is applied using different sources to prepare amorphous structures.

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Moreover, melting of material using microwaves then supercooling is done in the method of irradiation. (GeO2)1-x(PbO)x, Ga2O3-PbO-Bi2O3, (TeO2)x(ZnO)1-x and (TeO2)(PbO, PbCl2)1-x, and Bi2O3-PbO-B2O3-GeO2 glasses doped with RE3+ ions are prepared by first melting the starting material and then mixed with crucibles depending upon the material being prepared. After that different methods for different glasses are applied for the removal of OH groups or any other compounds. Mechanical properties of the metal oxide glasses are determined by lab tests, indentation tests, load-depth curves, and geometry of the glass. The addition of different filler materials in metal oxide glasses can change the mechanical properties. Usually, filler materials are added to enhance the mechanical properties. Metal oxide glasses are transparent in the visible and infrared spectrum, and they do not possess the property of anisotropy. The refractive index and extinction coefficient are two important coefficients that help in the determination of the optical properties of metal oxide glasses. The dielectric properties of metal oxide glasses are determined with dielectric constant, dielectric permittivity, dielectric conductivity, and electric modulus. Metal oxide glasses have a lot of applications regarding nanotechnology and research on the development of nanosystems to harvest energy and to consume that energy for utilization of semiconductor devices is the main focus of the research community.

Acknowledgments The authors extend their appreciation to University of Malaya for funding this work under grant number ST030-2019.

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Metal oxide-based optical fibers (preparation, composition, composition-linked properties, physical parameters, and theoretical calculations)

4

Shahla Imteyaz Department of Chemistry, Aligarh Muslim University, Aligarh, India

1

Introduction

1.1 General The use of optical fibers has gained a lot of research interest in last decades in the field of data transmission as it can operate well in a wider transmission range of 0.1– 100 GHz with a minimal attenuation of 0.15–5 dB/km [1]. However, the efficient transmission of light through these optic fibers at an operational wavelength(s) is a prerequisite thing for its large-scale applications in the areas of communications, optical fiber lasers, sensors, optical modulators [2,3], surgical and biomedical applications [4,5], electrochemical sensors [6–8], and gas sensors [9]. Optical fibers are hair-fine flexible and transparent filament that serves as a waveguide for light transmission between its two ends. Optical fibers have tendency to transmit data at a speed of 10 GB/sec over very long distances. Optical fiber consists of four components as shown in Fig. 1: (a) core, (b) cladding, (c) buffer, (d) and jacket. The transparent core is generally made from oxide glasses, the most common of which is silica (SiO2) and is the light carrying component of the fiber. The core is surrounded by another transparent layer known as cladding, which is made up of glass or plastic and have a lower refractive index than the core section. Buffer is an upper protective layer made of plastic that surrounds the cladding and prevents it from physical damage, moisture absorption, and scattering losses. The outermost layer is the jacket layer which characterizes the fiber’s type.

1.2 Operation principle Optical fiber operates on the principle of total internal reflection at the core–cladding interface along the length of the fiber (Fig. 2). Let us suppose that there are two mediums with different refractive indices namely as n1 and n2. It is imperative that, for a total internal reflection back into the medium Metal Oxides for Optoelectronics and Optics-Based Medical Applications. https://doi.org/10.1016/B978-0-323-85824-3.00016-6 Copyright © 2022 Elsevier Inc. All rights reserved.

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Metal Oxides for Optoelectronics and Optics-Based Medical Applications

Fig. 1 Pictorial representation of structure of optic fiber with its components.

Fig. 2 Mechanism of total internal reflection: (A) in two mediums with different refractive indices n1 and n2 and (B) in fiber structure.

with n1, the light source entering into the fiber (with refractive index n1) must have the angle of incidence (θ) to be greater than the critical angle (θc). However, during the propagation of light through the length of optic fibers, the intensity of light reduces as it propagates, termed as attenuation which further results in the degradation of signal strengths, thereby, making it less viable for efficient economic use of optical fibers. All attenuation mechanisms can be traced back to the multilength scale structure of the

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glass itself or from the uniformity and interfacial structure of the core–clad interface [4]. Thus, chemical compounds that are used as raw materials, the fabrication technologies (i.e., preform, coating, and drawing) are the most important factors that must be taken into consideration while fabricating optical fibers for data and signal transmission.

2

Fabrication of optical fibers

There are two main methods for the fabrication of optical fibers. First is the (a) direct melt method and the other is (b) vapor phase oxidation method. The direct melt method is a single-step method in which silica powder is melted in the crucible to produce multimode fibers for short-distance transmission of signals. Though, being a simple and economical method, it has disadvantages to obtain homogenous and high purity glass fibers. To overcome this limitation and to obtain fibers of high purity, three-step processes are being used: where solid cylinder of core and cladding material is prepared known as a preform, by various vapor phase oxidation methods to produce single-mode fibers for long-distance transmission of signals. These preforms are then extracted into optical fibers by vertical fiber drawing system, where the preforms are heated to a temperature of about 2000 °C followed by pulling off a tiny strand of glass at one end to control the diameter of core and cladding [10]. Finally, coating is done to the formed optical fibers to provide mechanical protection. The vapor phase oxidation method has been further explored with different modifications to obtain optical fibers with (a) low loss in signal strength [11], (b) low in OH concentration, (c) low in metallic-ion contamination, (d) low cost, (e) high homogeneity, and (f) reproducible. To obtain optical fibers, the raw materials are processed with the two main steps: Manufacturing of the pure glass rod, i.e., preform, and drawing of the preform. The different techniques that are used to manufacture the preform are outside vapor deposition, vapor axial deposition, modified chemical vapor deposition, and plasma activated chemical vapor deposition [1] which will be discussed below in detail.

2.1 Outside vapor deposition (OVD) [12–15] This process is also called as “soot process.” The starting materials such as SiCl4, GeCl4, BCl3, and O2 are reacted in hot flame to produce small particles of very high purity glass of the desired composition. The mixture of hot gases is passed in between the target rod and heat source where the rod is rotating and traversing speedily on the lathe (Fig. 3). The reaction of hot gases with the starting materials produces soot which gets stick to the rod in a partially sintered state and its deposition took place layer by layer resulting in the formation of preform. After adequate layering of the soot, the target rod is removed and subsequent collapsing of the residual preform soot into a solid rod follows. Thereafter, the fiber of desired length is pulled off from the drawing

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Metal Oxides for Optoelectronics and Optics-Based Medical Applications

Soot Preform Rotating

Flame

Target Rod

Gases Heat Source Moving Back and Forth

Outside vapor deposition (OVD).

Fig. 3 Diagram of outside vapor deposition method for preform fabrication. From http://www.fiberoptics4sale.com/wordpress/the-manufacturing-of-optical-fiber/ Schematic.

tower. So, by properly controlling and sequencing the metal halide vapor stream composition during the soot deposition process, the porous glass with desired properties (composition, refractive index) could be obtained for both the core and cladding regions [1].

2.2 Vapor axial deposition (VAD) [16] This deposition is similar to outside vapor deposition method for the fabrication of SiO2 soot preform, except, in VAD the deposition takes place at the end of a glass rod which is attached in the upright position to the motor. Here, the deposition of soot takes place in an axial position as the rod is rotating in upward direction. This method is environmentally clean as it is done inside a closed chamber. One of the foremost advantages of this method is that no hole is created during preform formation, whereas in OVD central hole is created which results in the mismatching of core and cladding composition. Since most of the silicate glasses require high softening temperature (1950–2250 °C) for sintering the deposited material. Both OVD and VAD results in the fabrication of very large preforms and the layer deposition of the soot is performed outside the mandrel by flame hydrolysis technique.

2.3 Modified chemical vapor deposition (MCVD) [17] This technique was firstly reported by Bell Laboratories in 1974 and is most widely used for the fabrication of active and passive optical fiber preform [15]. Halide compounds such as SiCl4, GeCl4, POCl3, BBr3, BCl3, SiF4, SF6 are allowed to pass through the inside of tube as mixture of gases either by passing a carrier gas (oxygen) through liquid dopant sources or by using gaseous dopants as shown in Fig. 4 [11]. A hollow glass tube is placed either vertically or horizontally inside the mandrel and is allowed to rotate swiftly. A very high temperature source of about 1800 °C could be used for the heating of tube from outside which causes the vaporization of SiO2 to

Metal oxide-based optical fibers

O2 SiCl4

GeCl4

77

Chemical Vapours

Rotating Glass Tube Echaust

Fused Silica

Unfused Silica

Burner POCl3

Fig. 4 Schematic diagram for outside vapor deposition method for preform fabrication. From S. Addanki, I.S. Amiri, P. Yupapin, Review of optical fibers-introduction and applications in fiber lasers, Res. Phys. 10 (2018) 743–750. doi:10.1016/j.rinp.2018.07.028.

form soot that can be deposited on the inner side of the glass tube just ahead of the burner. To obtain the optical fibers of desired refractive index profile, dopants and their concentration can be varied as per the required fabrication of step-index and graded-index optical fibers. Finally, the tube is collapsed at a very high temp (above 2000 °C) and the preform is drawn from the drawing tower of the desired length.

2.4 Plasma-activated chemical vapor deposition (PCVD) [16] This process for the fabrication of preform was developed by scientists at Philips Research in 1975. Here, the microwave frequency range of 2.45 GHz is employed for the formation of ionized gas plasma inside the silica tube instead of soot formation by flame in MCVD. The plasma increases the temperature (to about 1000–1200 °C) for the reaction to proceed. This results in very thin layers being deposited inside the tube. Despite being useful for the deposition of layers at low temperatures, the rate of deposition is slow as compared to other methods. Herein, the fiber drawn is approximate 30 km in length.

2.5 Drawing and coating of optical fibers After drawing of optical glass fibers coating is done to (i) protect the fibers from moisture and to preserve their mechanical strength, (ii) protect the optical properties, and (iii) electrical or thermal conductivity or transmission of mechanical stress parameters from embedding media in the fiber [18,19].

3

Metal oxides in optical fibers formation

One of the foremost criteria for the selection of optical materials is their transparency for efficient transmission of light to optical frequencies. Other criterion includes: (i) the compatibility between fiber and cladding materials so that the phase diagram

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Metal Oxides for Optoelectronics and Optics-Based Medical Applications

Table 1 Types of dopants used in core and cladding formation of optic fibers. Core

Cladding

GeO2-B2O3-SiO2 GeO2-P2O3-SiO2

B2O3-P2O5-SiO2 B2O3-P2O5-SiO2 F-P2O5-SiO2 SiO2 F-P2O5-SiO2 SiO2 P2O5-SiO2 F-SiO2 F-P2O5-SiO2 F-SiO2

SeO2-SiO2 GeO2-SiO2

SiO2

remains stable and (ii) that the material can be drawn into thin long fibers. The most abundant, widely prevalent and inexpensive material used for fabrication of optical fibers is silica, i.e., SiO2 which is made up of sand and glass and has a refractive index of 1.458 at 850 nm. It has highly desirable properties like resistance to deformation even at high temperatures, i.e., 1000 °C, and has good chemical durability. However, the insertion of various oxides as dopants either enhances or reduces the refractive indices of glass. Therefore, care must be taken while selecting the material composition for optical fibers as they might results either in the refractive loss or in unstable phase diagram. Dopants used in the fabrication of optic fiber materials either increases (Ge, P, Se) or decrease (F, B) its refractive indices. Some of the composition of core–cladding materials is given in Table 1 [1].

4

Applications of optical fiber

4.1 Metal oxides as electrochemical pH sensors The biological systems at macroscopic and cellular levels function well under a muchregulated system of chemicals and their interactions. Most of these systems maintain equilibrium to sustain life by altering the pH balance in the body. Even cytoplasm, and circulatory systems function well within a certain range of buffer systems. Any deviation in the pH value would result in the improper functioning of biological systems and can be seen as disease or anomalies. Thus, in order to monitor the pH changes in the biological systems, sensors have gained a lot of attention due to their small size and less power consumption. Materials used for pH sensors include ZnO, PtO2, PbO2, IrO2, Sb2O3, RuO2, TiO2, Ta2O5, WO3, RhO2, OsO2, PdO, CuO, and SnO2 [20–27]. A very high sensitivity of
59 mV/pH at room temperature could be achieved by IrO2 pH sensors as it can work well under high pressures. Besides these, IrO2 sensors are stable over wide pH ranges, temperatures, and media. They have significant properties like biocompatibility,

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minimal potential drift, excellent chemical selectivity, durability, no requirement for pretreatment [20,23,28,29]. ZnO is one of the most prevalent metal oxides used in pH sensing and it can be modified into different forms and structures such as nanoflakes [30], nanowires [31–34], nanohelices, and its properties can be altered by using various dopants for sensitivity based applications [35]. ZnO with nanorod structure shows a sensitivity of
59 mV/pH at room temperature within a pH range of 1–14 due to their high surface-to-volume ratio [36]. Conducting polymers are also widely used as pH sensing material (Table 2) because of their good ion–exchange properties [37]. Polyaniline (PANI), besides being a highly conducting polymer and easy to synthesize [38], also does not induce skin irritation or provoke any sensitization [39].

4.2 Metal oxide nanoparticles as gas sensors Metal oxides play a vital role in gas sensing applications since the 1960s due to their high sensitive response and low fabrication cost. However, when we compare the coarse bulk material to that of nanoparticle material of metal oxides with the same composition, the latter show much better results than the former due to its unique properties such as: nonlinear optical properties, higher ductility, cold welding properties, superparamagnetic behavior, and unique catalytic activity [44]. So, the research has been shifted to nanoparticle metal oxides for these applications as they show (i) low melting point [45] and (ii) unusual absorptive properties and fast diffusivities [46], than that of bulk materials. Metal oxides nanoparticle gas sensors usually consist of three regions: (a) a heating layer to have an optimal temperature, (b) conducting electrodes, and (c) and changing film which changes its resistance upon exposure [44]. The gas sensing is based on the number of reactive surface sites which further increases upon adsorption of oxygen species [47]. The oxygen vacancies present at the surface of metal oxide increases its conductivity, whereas absorbed ions decrease it [44]. Upon adsorption of molecules such as O2 or NO2 at vacancy sites of the oxide, the electrons are flowed out from the conduction band and conductivity decreases, whereas CO or H2 in the oxygen-containing atmosphere react with adsorbed O2 releasing electrons and increasing conductance [44]. From Table 3, it can be elucidated that the particle size of the material, their morphology and dopants into metal oxides clearly affect the sensitivity of the metal oxides. Doping enhances the catalytic properties of the oxides by increasing the density of the functional group [48]. Thus, not only the composition of metal oxides but also the morphology, chemical nature, size should be taken into consideration while developing sensors.

4.3 Metal oxides in batteries Lithium ion batteries represents a class of rechargeable batteries showing various properties like high energy density, low self-discharge, and small memory effect [73]. So, the search for better electrode materials with improved properties for these batteries is

Table 2 Polymeric pH sensors with sensitivity response at different pH ranges. Sensing material

Setup

Sensitivity

PANI nanopillar array PANI, PVB (as reference electrode) Graphene-PANI

Open circuit potential (OCP) OCP

Linear Nernstian response of
60.3 mV/pH Linear Nernstian response of
58 mV/pH

Amperometric

PANI

OCP

50.14 μA/pHcm2 in pH 1–5, and 139.2 μA/pH cm2 in pH 7–11 Average slope of
62.5 mV/pH

Response time

pH range

References

2, an optical transmission in the range of 350–5000 nm, and a melting temperature at about 700 °C. Additionally, such glasses have a high-density medium with good attenuating features to be utilized for radiation-shielding applications.

Acknowledgment The authors are thankful to Dr. V. Ragavendran for his support in preparing the figures.

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Integrated optoelectronics

11

Suresh Sagadevana, J. Anita Lettb, Is Fatimahc, Suriati Paimand, Jiban Poddere, and Mohd. Rafie Johana a Nanotechnology & Catalysis Research Centre, University of Malaya, Kuala Lumpur, Malaysia, bDepartment of Physics, Sathyabama Institute of Science and Technology, Chennai, Tamil Nadu, India, cDepartment of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Islam Indonesia, Yogyakarta, Indonesia, dDepartment of Physics, Faculty of Science, Universiti Putra Malaysia, Serdang, Selangor, Malaysia, eDepartment of Physics, Bangladesh University of Engineering and Technology (BUET), Dhaka, Bangladesh

1

Introduction

Microwave signals with high-frequency, high-spectral-purity, and frequency tunability, as well as microwave generators that are lightweight, low-cost, compact, and power-efficient, demands high due to the fast growth of modern communication systems, radar, and wireless networks. Integrated microwave photonics (IMWP), which combines microwave and photonic circuits on a single chip, seems to be a promising way to meet these demands. Flexible, agile, and compact devices are needed for transmitting the signals between wireless and wired signals in applications, optical fiber network segments, and interfacing radiofrequency (RF) signals [1–4]. Some of these examples are Global wireless fifth-generation (5G) communications [5], radar-based civil surveillance [6], fast analog-to-digital converters [7], and clock recovery systems. Also, signal generation and processing areas [2,5,7] need high complexity RF systems, a high tuning range of frequency, and wide bandwidth [4,8]. Microwave photonics (MWP) aims to develop and incorporate hybrid optoelectronic systems which would achieve the above goals [2,9–11]. Communication networks are in desperate need of these technologies in the short/medium term, as they face the 5G revolution, aiming for the easy access of the users [12,13]. Due to scarcity of the spectrum at microwave frequencies, 5G networks must operate in and cover the maximum band of the millimeter-wave (mm-Wave) spectrum in the Radio Frequency bandwidth [14]. The mm-Wave sources producing high purity of the spectrum, tunable frequency range, dramatically small size of the coverage cell area, and reduction base station measurements are all the expected characteristics of 5G networks [5]. These needs are fulfilled by the requirements such as low power consumption, a compact design, and a small footprint. Furthermore, Integrated microwave photonics (IMWP) were also introduced with the above criteria by integrated microwave technology and photonics technology [3]. The optoelectronic oscillator (OEO) is observed as an ideal candidate as it can produce high-frequency signals together with ultralow phase noise for an mm-Wave generation [1,15]. From the first OEO proposal in 1996, many

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schemes have been published to understand and have improved with the further development of the scheme [16,17]. Multiloop designs [18], stimulated Brillouin scattering effects [15], fiber Bragg gratings [19], injection-locked [20], and coupled cavity designs [21] are just some of the examples. Therefore, this chapter provides an overview of integrated optoelectronics (waveguides, including metal oxide glasses), electrooptical phenomena, light modulators, optical switches, and integrated photonic devices as well as these glasses’ basic properties.

2

Electrooptical phenomena

Electrooptic modulation (EOM) is an important optical manipulation used in on-chip photonics, optical communication, and optical sensing. Integrated electrooptic modulation and broadband electrooptic modulation are becoming more significant due to the rise of its demand for reliable, and high-performance. For communication or modulation, optical information may be coded/modulated by changing the amplitude or phase of the field optics. Using advanced micro/nanofabrication techniques, electrooptics crystal can be diminished into the appropriate shape/volume precisely, suitable in the integrated modulator, waveguide, or meta-surface for nanophotonic applications paving the path for impending nanophotonic devices. Advanced electrooptics modulators with large electrical modulation bandwidth (over 100 GHz), ultralow optical insertion loss (>1 GHz), scalable size, low half-wave drive voltage, ultrafine signal quality, mass-producible fabrication capability, with the ease of integration on the various platforms of insulator are required and used in the applications of microwave photonics, optical communication networks, photonic quantum computing, and optical sensing [22–24]. Electrooptic modulators (EOMs) plays a vital role in current telecommunication networks and microwave-photonic systems with the basis of nonlinear optical materials such as lithium niobate (LiNbO3), lithium tantalite (LiTaO3), potassium titanium oxygenic phosphate (KTiOP4), liquid crystal on silicon (LCOS), or 2D layered material modulators were frequently used in electronic signals to the optical domain conversion, through the generation of optical phase carriers or harmonics of higher order [23,25]. The optical data can be coded/manipulated, by the redistribution of frequency characters and stimulating the known phase carrier through EOMs to a particular optical field. Then the optical data were obtained from photodetectors or spectrometers and modified via algorithms [26–31]. High-driven voltage (>100 V) for relatively low-frequency band (DC to 1 GHz) in bulky size is often used in the practical optoelectronic applications, while resonant LiNbO3 makes a radiofrequency slewing rate at low driven voltages (>50 V) which is well suited in CMOS devices. The electrical waveform applied externally may have different waveforms such as sinusoidal, triangle, saw-tooth, trapezoid, or other artificial waveforms, along with constant direct voltage. For enhancement of the bulky lithium niobate crystal, many methods are currently well-developed; the actual challenge now are: (1) EOM microminiaturization and mass integration on different substrates [32–35]; (2) how to achieve significant phase

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or amplitude variations in small EOMs, where a short optical path inside the EOM aids in reducing optical propagation loss [36–38]; and (3) to increase the efficiencies of electrooptical, matching of group velocity, and the threshold for optical damage [38]. In the network of optical communication, nonreciprocal optics, and photonics of microwave and quantum, solving the above challenges is highly regarded [39]. The advancement in the technology of micro/nanofabrication offers the source to incorporate EOMs with high-power ultrafast laser processing, artificially fabricates the stiff, transparent crystal with ultrafine spatial resolution on the substrate through high instantaneous power nanosecond or picosecond laser, showing interest in the wide applications of today’s world. The upcoming period of photonic applications especially in the on-chip photonic devices through integrated EOMs will pay attention in the field around optics, microwave, telecommunication, and large-volume memory, bringing with it a plethora of new photonic applications.

2.1 Electrooptic sampling and photoconductive switch sampling Electrooptic (EO) sampling is a field interaction-based noncontact probe method. EO has the widest bandwidth which could work with the broad variety of optical wavelengths. This approach relies on the EO effect (induced birefringence), electrical coupling to such devices are indirect, which leads to an inefficient sample of the signal of interest [40]. PCS sampling is a contacting (though high-impedance) probe method that offers the highest sensitivity. It can directly calculate the electrical signal’s voltage and be programmed to produce a picosecond electrical pulse [41]. It’s easy to put together and uses popular semiconductor materials. Due to the poor optical to electrical performance, it could necessitate the use of a high-power laser.

2.2 Electrooptic sampling If an RF/microwave electric field (E) is applied to a birefringent crystal (a crystal with different indices of refraction along different crystal axes), the electric field differentially changes the refractive indices in different directions. It results in the polarization change of an optical beam propagating through this crystal; hence, data can be transferred from the RF/microwave signal to the optical beam, which is known as EO sampling, or the Pockels effect [42]. Electrooptic sampling remains the highest time-domain resolution owing to the shorter length of the laser pulse, such as 100 fs. As a series of ultrafast laser pulses, the optical beam appeared is delayed variably against the measured signal. A controlled optical polarization circuit usually transforms this polarization change into a change in intensity, as shown below: I ¼ I 0 Sin2




Γ0+ ΔΓ 2

 (1)

where I0 is the input intensity, Γ0 is the static birefringence, and ΔΓ is phase retardation that occurred due to the EO effect and the electrical field [42]. The change in

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optical intensity due to the measured sinusoidal electric field at a given time can be derived below: " !# E0z sin ðωm tÞ I0 I¼ 1 + sin π 2 Eπ

(2)

where I0 is the input intensity, E0z and ωm denotes the electric field amplitude and frequency which are measured, respectively, Eπ represents the half-wave electric field, and time is represented as “t” [42]. Fig. 1 depicts the schematic representation of the EO sampling method. It’s a laserbased device with optical analysis circuitry (delay rail, polarization optics, and acoustic-optic modulator), PCS, and an EO sampling head. A beam splitter divides the laser pulse into two beams: an excitation beam for producing a high-speed electrical pulse and a probe beam for receiving the signal reflected from the DUT. The excitation beam reaches the EO sampling head after beam splitting. A PC switch, optical fiber for the excitation beam, EO crystal (LiTaO3) connected to the optical fiber for the probe beam, a 110 GHz high-frequency probe, a 110 GHz high-speed connector, and DC bias circuit are all assembled in an aluminum housing piece for the EO sampling head. The PC switch, based on LT-GaAs, is a coplanar stripe line with a 14-m opening on one trace. The excitation beam shines on the opening, generating an electrical pulse travelling into the DUT through the high-frequency probe (110 GHz), which reflected by an open or short, and then picked up by the EO crystal and probe beam. The resolution of this device is its distinguishing feature. The term “resolution” refers to the ability to distinguish appreciable changes in reflected signals on a gold coplanar waveguide (CPW) line from an open-end when the CPW pushed a certain distance, such as 50 μm. The measurement results showed that by translating the CPW relative to the EO probe, this EO sampling device would resolve 50 μm on gold CPW [43]. Fig. 1 (A) Schematic of the EO sampling system and (B) schematic of EO sampling head.

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Integrated photonic devices

Integrated photonics has evolved into a powerful method for processing both data of classical and quantum. Integrated photonics is widely used for optical interconnects [44], on-chip signaling [45], and on-chip photonic processing [46] in the duration of massive data. Integrated photonics are used to process quantum information that shows great promise [47,48], with proven benefits from long-distance quantum communications [49] to on-chip quantum simulation [50,51]. Integrated photonics modules and circuits, on the other hand, continue to be a big bottleneck [52]. Computational tractability and the requirement of qualified researchers are complicating in the existing design flows [53]. Photonic integrated circuits, different from their electronic counterparts, need computationally intensive simulation routines to find their optical response functions precisely. To assemble, integrated photonic devices takes a longer time than manufacturing and testing them. Integrated photonics are awaited to play a progressively significant part in optical communications, imaging, computing, and sensing, lowering the cost and weight of these devices significantly. The skill to design optical components with compact and reliable and to integrate them on a common substrate is crucial to the potential development of this technology. Functional components can be constructed and also easily developed into photonic circuits which are made up of the materials of planar gradient index, for emerging transformation optics technique. It permits the development of a huge quantity of new devices, including a light source collimator, waveguide adapters, and a waveguide crossing, with a wide range of implementations in integrated photonic chips and are adaptable with recent fabrication technology. A schematic of a photonic integrated circuit made up of various Transformation optics (TO) components is shown in Fig. 2 [54].

4

Waveguides including metal oxide glasses

In the applications of photonics, the widespread and utilization of optical glasses, especially integrated optics (IO), has been lead to the ascendancy of optical fibers in the light wave communications that use silica (i.e., amorphous SiO2) as the substrate. The field of IO was established in 1969 [55] to supplement the passive transmissive medium (i.e., optical fibers) with integrated chips that could perform both active (signal generation or amplification) and passive (signal addressing and processing) functions through directed light propagation. Since then, a lot of work has gone into developing what we now call as photonic integrated circuits (PICs); an important summary was reported a few years back by Kaminow, a member of the Bell Labs community that pioneered in this field [56]. Though various kinds of optical materials are now used for producing PICs, glass holds to be the most significant. Glasses and fabrication processes have been used to meet the increasing demand, and glass IO has evolved into a field of photonics having the ability to provide advancement and to figure out the solutions for a wide range of problems using relatively simple technologies. Miller’s groundbreaking paper [55] used the term

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Fig. 2 A schematic of a photonic integrated circuit composed of various transformation optics (TO) p components.

“integration” as a keyword. Although the target of complete monolithic integration is feasible in semiconductor materials, it is not always the best option, while hybrid integration also allows for the desirability of the complementary properties with various groups of materials required in IO (namely, semiconductors, ferroelectrics, polymers, and glass). Glassy materials retain several recommendable properties, including affordable price, outstanding clarity, a high threshold for optical harm, a selective broad range of refractive indexes, and finally ease through which active, functional atoms and oxides can be doped. Glass tends to be a critical component for the two major PIC technical platforms, silica-on-silicon (SOS) and silicon-on-insulator (SOI).

4.1 Thin film fabrication processes The glass used as wave guides (WG) has an excellent properties because of the simple technology, minimum loss of propagations, and ease of pairing with glass fibers. WG fabrication methods are significant due to the availability of low-loss glasses as well as affordable and dependability. The first and foremost important things for fabrication technologies are high yielding capacity, replicability within distinct tolerances, and intrinsically low price for designing. Glass integration optics (IO) can be categorized according to the scheme in Fig. 3. The most essential criterion for optical confinement is that the guiding layer has a higher refractive index (RI) than the surrounding media, there are two key factors: (a) deposition of a thin film and (b) local alteration of the bulk material. Radiofrequency (RF) and magnetron sputtering, chemical vapor deposition (CVD, and in particular plasma-enhanced chemical vapor deposition (PECVD),

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Fig. 3 Glass optical waveguides (WGs) fabrication techniques can be divided into two categories: thin film deposition techniques, which can be physical or chemical, and processes for local modulation of the refractive index of the glass, which can be done in bulk or thin film format.

flame hydrolysis deposition (FHD), spray pyrolysis (SP) deposition, pulsed laser deposition (PLD), and sol-gel coating are examples of the former approach; the latter approach, appropriate for the unmediated description of a channel WG, involves in the processes such as ion exchange, ion implantation, UV irradiation, and femtosecond laser writing. Other methods, such as annealing, photolithography, and etching, can then be used to follow or supplement the above. It is noted that the fabrication techniques shown in Fig. 3 are not specific to glass IO: which were commonly used in many fields, including thin-film coatings, microelectronics, material processing, and glass strengthening. However, there methods, such as resistive evaporation, which are hardly used for the fabrication of WGs, as a common method in the production of thin metal films and, as such, primarily helpful to form the metal layers used in the patterning of IO circuits and electrical contacts. Even though evaporation with an electron gun would be more productive, but it has not been commonly utilized for optical WGs. In the method of SP, a thin film is coated by spraying a solvent on a heated surface, which reacts and forms a chemical compound [57]. The atomized precursor solution transported resulting aerosol, and the precursor decomposed on the substrate, with the last layer containing sintering of the solid particles, are the three main steps. SP may be used to make porous and dense oxide films, ceramic coatings, and powders. The main advisability of spray pyrolysis is that it does not require high-quality substrates or chemicals. Solar cells, sensors, antireflection coatings, thermal coatings, solid oxide fuel cells, and other devices have all used SP-prepared films; for example, rare earth-doped Al2O3 films prepared by the SP have shown effective blue-green luminescence and white light emission [58].

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Light modulators

A device that modulates a beam of light is known as an optical modulator. The beam can travel through a vacuum or via an optical waveguide (optical fibers). Modulators are categorized as follows, amplitude modulators, phase modulators, polarization modulators, and so on, depending on the parameter of an optical beam that is controlled. Modulating the current driving through a light source, such as a laser diode, is also the simplest thing for modulating the light beam intensity. In contrast to the external modulation done by a light modulator, this type of modulation is known as direct modulation. As a result, light modulators are referred to as external light modulators in applications such as fiber-optic communications. Direct modulation is avoided using laser diodes as a narrow line width because of the high bandwidth “chirping” effect when adding and removing current to the laser.

5.1 Classification of optical modulators Modulators are divided into two methods based on the properties of the material for modulating the optical beam: absorptive modulators and refractive modulators. The absorption coefficient of the material is varied in absorptive modulators, while the refractive index of the material is changed in refractive modulators. The Franz–Keldysh effect, the quantum-confined Stark effect, excitonic absorption, Fermi level changes, and changes in free carrier concentration can all affect the modulator’s absorption coefficient. When some of these effects occur at the same time, the modulator is known as an electroabsorptive modulator. Electrooptic effects are commonly seen in refractive modulators. Some modulators make use of the acousto-optic or magneto-optic effects, as well as liquid crystal polarization changes. Electrooptic modulators, acousto-optic modulators, and so on are the different types of refractive modulators. The effect of each of the above types of refractive modulators is to adjust the phase of a light beam. With the use of an interferometer or a directional coupler, phase modulation can be transformed into amplitude modulation. Spatial light modulators are different types of modulators (SLMs), which aims to vary the twodimensional amplitude and/or phase distribution of an optical wave.

6

Optical switches

Optical methods for signal transmission have proven to be extremely competitive, and they have rapidly become the foundation for a maturing industry. For example, Wideband transmission has been considered as implying a potential required for wideband switching; however, the production of optical switches was found to be difficult. Likely, the idea of optical computing has a lot of potentials, but it’s still in its infancy. Electronic technology has also proven to be extremely adapt at adjusting to everincreasing speeds. Electronic circuits are used almost universally for signal generations transmitted over wideband optical links and their subsequent use. For the near future, it is fair to say that most wideband equipment will be designed using a mixture

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of electronic and optical methods. Optical signal processing in electronic devices, such as computers, is now generating a lot of interest. The existence of both the signals of optical and electronic in a single, hybrid device allows the optoelectronic interface to be used in itself. This method appears to be very realistic in the application of wideband switching. To prevent misunderstandings, it’s a good idea to define the terminology in a right away. “Photonic” switches are those that use light signals as a part of the switching mechanism in some way. There are two types of subcategories. Optical switches steer a light signal in one of two directions. Optoelectronic switches are hybrids that accept optically and produce electronic outputs (or the other way around) switch using a conversion input signals mechanism. Fig. 4 shows a layout of a standard optical communications link with a repeater. Electrical signal paths are displayed as broken lines, while optical signal paths are shown as solid lines. According to the types of inputs and outputs, there are four possible positions for matrix switches in such a device. Optoelectronic switches are described by two of these positions. Wideband switching applications have already been demonstrated to benefit from optoelectronic space–division matrices. An optoelectronic switch, by design, receives an optical signal producing an electronic signal (the reverse is less attractive due to practical reasons). Switching occurs as a result of a mechanism that takes place during the transfer of a signal from one type to another. As a result, the system incorporates a part of an optical repeater stage and forces signal conversion between optical and electronic form. As compared to optical switches, an optical signal can pass through unaltered in theory but it is not practically applicable, this function is often seen as a drawback. However, at switching devices, certain realistic circumstances require intelligence access for a signal, such as multiplexing or even billing data collection. This capability would necessitate the conversion of signals in an optical switching network in optoelectronics. There is no use of an optical switch if the necessary wideband switching features obtained from the optoelectronic conversion directly.

Fig. 4 Optical communication system employing optoelectronic switching.

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Metal oxides designed for these applications

Metal oxides (MOs) are the common substances found on the surface of the crust used to produce traditional ceramics. MO semiconductors vary significantly from traditional inorganic semiconductors like silicon and III–V compounds in terms of the principles of designing the materials, electronic structure, charge transport mechanisms, defect states, thin-film processing, and optoelectronic properties, allowing for traditional as well as novel functions. Recent achievements in MO semiconductors for electronics include the spotting and instrumentation of new transparent conducting oxides, the actualization of p-type MO semiconductors for transistors, p–n junctions, and complementary circuits, formulations for printing MO electronics, and, most notably, the commercialization of amorphous oxide semiconductors for flat panel displays. The ascertain of hydrogenated amorphous silicon (a-Si:H) in vast area electronics opened up new possibilities. Although this semiconductor is an essential component of thin-film transistors (TFTs) employed in display backplane electronics, its low carrier mobility (0.5–1.0 cm2 V1 s1), optical opacity, low current-carrying ability, along with restricted mechanical versatility pose for outstanding potential applications. Researchers have surveyed into the avail of metal oxides (MOs), which have unique properties in comparison with the crystalline silicon and other III–V semiconductors, such as estimable mobilities carriers also in the amorphous state, mechanical stress resistance, compatibility with organic dielectric, and photoactive materials, and high optical transparency. Furthermore, high-quality electronic-grade MO thin films can be made at room temperature conditions (25 °C under air) using vapor- and solution-phase methodologies, allowing them to be used in high-value products like inexpensive circuits, as well as flexible organic light-emitting diode (OLED) displays and solar cells on plastic substrates.

7.1 Optoelectronic properties of metal oxides Semiconductor efficiency is measured using the important metric, carrier mobility. Traditional band theory definitions of transport in semiconductors, including widegap ionic MOs, represent the electronic structure in reciprocal space, with the electron and hole effective masses determined by the conduction band minimum (CBM) and valence band maximum (VBM) curvatures and dispersion respectively. All other factors being equivalent, smaller effective masses mean greater CBM and VBM hybridization, allowing for larger carrier mobilities. Transport in MOs should be noted as it differs significantly from that in Si semiconductors, as the VBM and CBM are described by hybridized sp3σ-bonding and sp3σ*-antibonding states, respectively [8]. The VB in the oxides of interest is usually made up of occupied 2p O antibonding states, while the CB is mainly made up of unoccupied ns metal bonding states [59]. As the spherically symmetrical metal s-orbital overlap is minimally influenced by lattice distortions, the spatially extended, spherically symmetrical ns-orbital CBM affords small electron effective masses and efficient electron transport even in the amorphous state. In the amorphous (a-Si) state, carrier mobilities in the spatially directional

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Fig. 5 Electronic structures of Si and metal oxides. Comparison of subgap DOS in a-Si:H and a-IGZO (different DOS govern n- or p-type TFT behaviors, donor levels and Hall effect parameters; the barrier is the energy distribution when working in percolation conduction). VB, valence band; CB, conduction band [60].

Si sp3 σ states are drastically decreased. As a result, the electron mobilities of AOSs are frequently comparable to their of the respective single crystals, and the subgap distributions of the density of states (DOS) between amorphous covalent (such as a-Si) and amorphous oxide (such as a-IGZO) semiconductors vary significantly (Fig. 5) [60]. In a-IGZO, the subgap DOS is 10–100 times lower than in a-Si:H [61,62]. Most of the MO (semi)conductors have energy gaps greater than 3 eV, and hence they are visible-transparent. While high conductivity and optical clarity appear to be incompatible [63], strong interactions among the O 2p- and metal ns-orbitals produce a band structure specified by high free-electron mobility (e) because of the low effective mass and low optical absorption due to the wide bandgap (Burstein–Moss shift) in several (semi) conducting MOs and the low DOS in the CB [63–65].

8

Manufacturing features

Multiple functionalities, high accuracy, stability, streamlined packaging with low price are crucial in fabricating photonic integrated circuits (PICs) and optoelectronic integrated circuits (OEICs) for the growth of optical communication systems in the following years. As long as from the first practical description in the early 1980s, Quantum-wells (QWs) and quantum-dots (QDs) which are quantum-confined heterostructures of the postgrowth disorder are globally and highly researched due to their simplicity, flexibility, and low-price gained for PICs/OEICs. Integrated circuits on silicon chips along with the billions of transistors concluded with the incredible advantages in computing and information technology as a result of the integration of electronic devices. Because of the extended wavelength of photons in comparison to electrons and the lack of a single material framework to meet the multifunctionality requirement in the system of photonics, integration of devices such as photonic/optoelectronic has progressed at a much slower rate. Integrated passive photonic devices are usually made of silicon or silica, while active photonic devices are made of

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compound semiconductors. Hybrid integration has been attempted, but monolithic integration remains ideal for elevated performance integrated products. Selective area epitaxy (SAE) and disordering/intermixing of quantum heterostructures are the two key techniques used in monolithic integration. Photonic integrated circuits (PICs) and optoelectronic integrated circuits (OEICs) has been evolved as high-bit-rate information is transmitted between active devices via low-loss waveguides with the benefits of device miniaturization, multiple functionalities, improved reliability and robustness, and simplified packaging, and affordable price in the realization for future optical communication systems. One of the provocations of PICs/OEICs is transition from bulk three-dimensional (3D) optics functions including image creation by deviating rays in higher index media in lenses, separating different wavelengths in gratings or prisms, reflecting radiation with mirrors, and radiation guiding in optical fibers to similar functions accomplished in a planar configuration, which is uttered by the primary 2D growth robotics for optoelectronic materials. In addition to the purely optical functions, a PIC must support a wide range of optoelectronic mixed functions, such as highly efficient radiation generation via carrier injection in semiconductor laser diodes (LDs), high-speed radiation modulation via changes in absorption in electroabsorption modulators (EAMs), radiation amplification in SOAs, and radiation detection in photodiodes (PDs), etc. A single-mode planar dielectric waveguide, also known as a passive waveguide, is the building block of a PIC. Its purpose is to guide radiation in the plane of a 2D structure, as shown in Fig. 6. Miller suggested this idea in the September 1969 issue of the Bell System Technical Journal, which was inspired by metal waveguides used in millimeter-wave components (1969). Metals supplanted by dielectrics are widely used in the current telecommunication applications for shorter wavelengths. In dielectric waveguides, radiation was done by using a higherrefractive-index center enclosed by two lower-refractive-index media [66]. This is accomplished in the vertical direction by accumulating planar layers with distinct refractive indices, and in the lateral direction by etching the 2D structure after deposition, as shown in Fig. 6.

Fig. 6 Planar dielectric waveguide.

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Conclusion

In the communications, computer, and consumer industries, integrated optoelectronics is becoming increasingly relevant. It is used in devices with the broad region, at cheap cost, reliable optical components in consumer electronics to high-speed broadband information networks capable of supporting video and multimedia conferencing. The need to manufacture low-cost, high-reliability components for use in these modern systems has posed a technological challenge. By incorporating both optical and electronic components in a highly functional chip, integrated optoelectronics promises to meet the performance and cost objectives of these applications.

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[53] W. Bogaerts, L. Chrostowski, Silicon photonics circuit design: methods, tools and challenges, Laser Photonics Rev. 12 (4) (2018) 1700237. [54] W. Qi, J.P. Turpin, D.H. Werner, Integrated photonic systems based on transformation optics enabled gradient index devices, Light: Sci. Appl. 1 (2012), https://doi.org/ 10.1038/lsa.2012.38, e38. published online 23 November 2012. [55] S.E. Miller, Integrated optics: an introduction, Bell Syst. Tech. J. 48 (1) (1969) 20592069, https://doi.org/10.1002/bltj.1969.48.issue-7BSTJAN 0005-8580. [56] I.P. Kaminow, Optical integrated circuits: a personal perspective, J. Lightwave Technol. 26 (9) (2008) 994–1004, https://doi.org/10.1109/JLT.2008.922149JLTEDG 0733-8724. [57] P.S. Patil, Versatility of chemical spray pyrolysis technique, Mater. Chem. Phys. 59 (3) (1999) 185–198, https://doi.org/10.1016/S0254-0584(99)00049-8MCHPDR 0254-0584. [58] R. Martinez-Martinez, et al., Blue-yellow photoluminescence from Ce3 +!Dy3 + Ce3 +!Dy3+ energy transfer in HfO2 ∶ Ce3 +∶Dy3 + HfO2 ∶Ce3 +∶Dy3 + films deposited by ultrasonic spray pyrolysis, J. Alloys Compd. 509 (6) (2011) 3160–3165. [59] K. Nomura, et al., Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors, Nature 432 (2004) 488–492. [60] T. Kamiya, K. Nomura, H. Hosono, Origins of high mobility and low operation voltage of amorphous oxide TFTs: electronic structure, electron transport, defects and doping, J. Disp. Technol. 5 (2009) 273–288. [61] M. Kimura, et al., Extraction of trap densities in ZnO thin-film transistors and dependence on oxygen partial pressure during sputtering of ZnO films, IEEE Trans. Electron. Devices 58 (2011) 3018–3024. [62] H.-H. Hsieh, T. Kamiya, K. Nomura, H. Hosono, C.-C. Wu, Modeling of amorphous InGaZnO4 thin film transistors and their subgap density of states, Appl. Phys. Lett. 92 (2008), 133503. [63] A. Facchetti, T.J. Marks, Transparent Electronics: From Synthesis to Application, Wiley, 2010. [64] E. Fortunato, P. Barquinha, R. Martins, Oxide semiconductor thin-film transistors: a review of recent advances, Adv. Mater. 24 (2012) 2945–2986. [65] H. Yanagi, H. Kawazoe, A. Kudo, M. Yasukawa, H. Hosono, Chemical design and thin film preparation of p-type conductive transparent oxides, J. Electroceram. 4 (2000) 407–414. [66] X. Li, J. Yu, Generation and heterodyne detection of > 100-Gb/s Q-band PDM-64QAM mm-wave signal, IEEE Photon. Technol. Lett. 29 (1) (2017) 27–30.

Section C Metal oxide-based optoelectronic devices in biomedical applications

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Metal oxide-based fiber technology in the pharmaceutical and medical chemistry

12

Lakshmipathy Muthukrishnana, Suresh Sagadevanb, and M.A. Motalib Hossainb a Department of Conservative Dentistry & Endodontics, Saveetha Dental College & Hospitals, Saveetha Institute of Medical and Technical Sciences, Chennai, Tamil Nadu, India, bNanotechnology & Catalysis Research Centre, University of Malaya, Kuala Lumpur, Malaysia

1

Introduction

Nanotechnology and its implications has provided novel strategies in maneuvering materials at nanoscale conferring splendid physical, chemical, and optochemical properties. Such fine tweaking in the properties has given rise to materials of zerodimensional characteristics such as nanoparticles or quantum dots; one-dimensional nanowires, nanorods, nanofibers, and nanotubes; two-dimensional nanosheets and nanofilms; and three-dimensional forms mostly comprising bundles or dispersions of several nanomaterials [1–4]. Among various nanomaterials, nanofibers have gained popularity as far its wide application is concerned. The outstanding features such as high surface area to volume ratio, flexibility, mechanical strength and high porosity make them stand out from other nanomaterials.. The materials with such characteristics are preferred as a robust candidate for much advanced applications pertaining to tissue engineering, drug delivery and sensors [5,6]. Moreover, they have also been widely employed in textile industries and in aerospace engineering for reinforced clothing. It is the fiber diameter that determines the performance characteristics, processability, and practicality of the fibrous structures destined to various applications. These were the results of the outstanding contributions of Professor Darrell Reneker for the introduction, rediscovery and popularizing the technique of electrospinning [7]. By way of this technique, various materials such as polymers (synthetic and natural), carbon-based materials, semiconducting materials and even nanocomposites have been subjected to electrospinning to create nanoscale fibers with superior characteristics [8–12]. Besides, several other alternatives in the fabrication and generation of nanofibers, electrospinning is the much preferred technique owing to its simplicity, versatility and viability [13]. Most importantly, there has been a rapid progress in the fabrication and functional applications of nanofibers in biomedical, clinical and healthcare settings. Here in this chapter, a detailed information on the evolution, and emerging development of nanofiber technology in biomedical field and tissue engineering has been Metal Oxides for Optoelectronics and Optics-Based Medical Applications. https://doi.org/10.1016/B978-0-323-85824-3.00005-1 Copyright © 2022 Elsevier Inc. All rights reserved.

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elaborated. As electrospinning is the most sought after technique for synthesizing specific nanofiber pertaining to electronics, photonics, environmental treatment, energy generation and storage, a clear visionary about the timelines, fabrication techniques and their broad spectrum application in areas encompassing biomedical engineering and healthcare industry have been briefly described.. Moreover, the current status and future directions augmenting commercialization and implementation have also been discussed.

2

Electrospinning—Cutting edge technology

It is one of the most widely employed and a conventional technique used for the generation of specific fibers at different scales [14]. Indeed, it is a biomimetic process which had been greatly inspired from Nature where spiders [15] and silkworms [16] produced composite fibers with surprising powers of strength, durability, and elasticity. Interestingly, man has already mastered the art of weaving during 5000 BC as evidenced from the fragments of cotton articles unearthed during archaeological excavation. Notably, silkworm cultivation began only during 2700 BC and around 1300 BC spindles were invented progressing into production of fabrics and clothes from wool and cotton leading to the establishment of textile industry in 1880s. Plant based materials namely cotton or wood cellulose fibers were efficiently used by man to create the first synthetic product, Rayon in 1891 [17]. Notably with the introduction of commercially viable synthetic fiber nylon by DuPont in 1938 by the integration of chemistry and polymeric science, the scope for fiber-based technology saw a great expanse in wide applications [18]. There were many methods developed for producing artificial fibers using polymers under the influence of various physical, chemical and mechanical processes where the resultant fibers were found to have limited stretchability and viability. It was Charles V. Boys who in 1887 have reported the synthesis of fibers using a viscoelastic fluid (beeswax and collodion) under the influence of external electric field [19]. From then on, it was termed electrospinning that facilitated the synthesis of ultrathin fibers having diameters in the nanometer scale.

2.1 Historical background of electrospinning The conception of electrospinning gained meaning and popularity by the notable contribution of William Gilbert (1600). His study reported a cone-shaped water droplet when an external electrical field was applied. Simultaneously, Stephan Gray had reported on the atomization of the water droplet and the generation of very fine stream. There was a continuous progress and improvisation in the electrospinning technique that augmented its industrial application targeting broad range of applications (Table 1).

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Table 1 Historical background of Electrospinning and the outstanding contributions Timeline

Contributors/ Inventors

1600

William Gilbert

1700

Stephen Gray

1747

Abb
e Nollet

1887

Charles Boys

1902

John Cooley and William Morton –

1938

1964– 1969 1980s

Geoffrey Taylor

2000s

Donaldson Company Inc. (USA) Darrell Reneker and Gregory Rutledge –

2010s

–

1990s

Achievements/Contributions

References

Reported the formation of cone-shaped water droplet under the external electric field Electrohydrodynamic atomization of water droplet Performed electrospraying where water was used as an aerosol Generated fibers from viscoelastic fluid using electrospinning Filed two patents on the prototype for setting up electrospinning apparatus

[20]

Nanofibers synthesized using electrospinning were first introduced in air filters to capture aerosols Reported the formation of Taylor cone

–

Commercialization of electrospun fibers as fibers for air filtration

–

Reinvented the electrospinning technique. Various organic polymers were electrospun into nanofibers Generation of composite, ceramic, core-sheath, hollow, aligned, and continuous yarns of fibers using electrospinning Industry based production of multiple–needle and needleless electrospinning

[28,29]

[21] [22] [19] [23,24]

[25–27]

[30,31]

–

2.2 Conceptualization of electrospinning This typically involves the electrohydrodynamic generation of ultrafine fibers under the influence of an external electric field. In this process, a liquid drop (polymer/emulsion) is placed under high electric field to generate a jet followed by a sequential stretching and elongation (bending instabilities) resulting in a hyperstretching of the jet called fiber(s) [32,33]. A schematic representation of the electrospinning process is illustrated in Fig. 1.

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4

1

Emulsion

Taylor cone Spinneret

2 3

Syringe pump

Flying jet

Collector

High voltage

Fig. 1 Schematic representation of the Electrospinning process. (1) Formation of Taylor cone by the electrification of liquid droplet; (2) stretching of the charged jet (flying jet); (3) thinning of the jet into finer diameters owing to bending instability; and (4) solidification and collection of fine-tuned jet into fiber(s).

At the first instance, the liquid is extruded from the spinneret by a pumping mechanism and the droplet is formed as a result of surface tension. Once the electric field is applied, the totality of surface charges present on the droplet undergoes electrostatic repulsion thereby deforming the droplet into a Taylor cone responsible to eject the charged jet. The jet initially travels in a straight line path and then exhibits vigorous lashing movements owing to bending instabilities (Whipping instability). This mechanism was well explained by Yarin et al. [34] where they have calculated the shape of the envelope cone surrounding the bending loops during the whipping of the droplet. Once the jet is stretched to a critical point and into finer diameters, they are solidified quickly and deposited on a grounded collector.

2.2.1 Factors governing fiber diameter The key parameters that influence the reproducibility and consistency of fibers include electric field strength, concentration & viscosity of the polymer solution and the spinning distance. Efforts were taken to understand the significant role of factors affecting the spinnability. In particular the diameter of the fibers and troubleshooting the technological bottlenecks limiting its wide use. The diameter of the fibers generated is regulated according to the following Eq. [35]: 

Q2 2 d ¼ γε 2 π ð 2 ln χ  3Þ I

1=3

where d represents the diameter of the fiber, γ.

(1)

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With typical understanding of the process and control over the flow rate, conductivity and the spinneret diameter, the fiber diameter can be adjusted to improve the functional characteristics pertaining to various applications. Studies reported that the diameter of the fiber could be minimized by increasing the electric field strength and the conductivity potential with the introduction of conductive fillers such as carbon black, carbon nanotube (CNTs), metals and polymer mixture. For instance, by increasing the electrical conductivity of the fiber by 32 times, a 10-fold decrease in the diameter could be achieved. Furthermore, reducing the diameter of the spinneret nozzle had subsequently reduced the diameter of the fiber. Besides many other factors influencing the fiber generation via electrospinning process, Fridrikh Model tried to emphasize fiber diameter as a function of some of the independent parameters. However, the model does not consider the solution properties as an important parameter while proposing. As a result, a dimensionless index Berry’s number was proposed by Ko et al. [36]. It is defined as the product of intrinsic viscosity and the concentration of polymer present in the solution. This Berry number had significantly brought a correlation between the electrospinnability and the fiber diameter [37]. The diameter of the fiber can be estimated using the equation d ¼ aBc

(2)

where d is the fiber diameter, a represents Mark–Houwink constant, B, the Berry number and c corresponds to an experimentally determined value related to but not dependent on the crystallinity of the polymer. For instance, the Berry’s number (B) for polyacrylonitrile (PAN) polymer is divided into four levels. Level 1 ¼ B < 1; level 2 ¼ B (1–2.7); level 3 ¼ B (2.7–3.6); level 4 ¼ B > 3.6. It typically infers that when the B of a polymer is at the first level, it represents droplet form rather than the fiber. From the experimental results, the smallest size (diameter) fiber could be generated at level 2.

2.2.2 Solidification The sequential elongation and whipping of the jet generates fiber. Upon elongation, the jet (polymeric emulsion) tends to form fibers either by the evaporation of the solvent or by the cooling effect of the melt. However, slower the solidification process, longer would be the elongation of the charged jet into fiber with smaller core diameter. Shin et al. [29] have claimed that the dry fiber generated had a cross sectional radius 1.3  103 times that of the initial jet. This might be as a result of the elongation and solidification of the initial jet and evaporation of the solvent. Notably, the instabilities on the surface of the dried fibers cease with the entrapment of the surface charges.

2.2.3 Deposition Deposition represents the final step in the electrospinning process in which the flying jet with whipping instabilities is deposited onto a grounded collector. It is this step that determines the fiber morphology accompanied by the nature of bending instabilities.

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For instance, nonwoven fibers could be collected in the loop region of the first bending instability on a collector. Most importantly, fibers with coiled morphology could be obtained when the jet is collected during second and third whipping instabilities. Once deposited successfully, most of the surface charges carried by the fibers are neutralized, but a traceable amount of charge is appreciated on the surface of the fibers owing to its low conductivity [38,39]. Such residual charges on the fibers accumulate and tend to repel the jet carrying similar charges resulting in a pendulum-like motion [40]. This event tends to restrict the diameter of the fibers and fine tune them in the range of 0.5–1 mm.

2.3 Single-nozzle electrospinning It is the conventional method of electrospinning that involves electrification of a polymer solution or melt flow mediated through a single orifice. This approach facilitates fabrication of composite fibers integrating multiple polymer/solvent system. For instance, sodium alginate-polyethylene oxide (PEO) and chitosan-PEO blends were successfully electrospun for tissue regeneration applications. Compatibility in the desired properties of the polymers decides the success in electrospinning of polymer blends. Notably particles possessing biological activity can also be integrated with the polymeric matrix and electrospun into nanofibers. For instance, antimicrobial silver nanoparticles can be introduced into the nanofibers possessing antimicrobial properties by dispersing them into solutions containing biocompatible polymer [41,42]. In addition, water-in-oil emulsion of amphiphilic PEG-PLLA (poly(L-lactic acid)) copolymers was successfully woven into electrsopun fibers encapsulating the water soluble anticancer agent, doxorubicin hydrochloride (DOX) [43]. Similarly, Chen et al. [44] have conducted studies using PLLA electrospun nanofibers encapsulating titanocene dichloride for effective delivery against lung tumor cells. There are instances where the insoluble part of the drug was made soluble small droplets by dispersing and continuously agitating them with the aid of surfactant sodium dodecyl sulfate (SDS). Besides drug delivery and protein release approach, Kim and coworkers [45] have fabricated electrospun PCL nanofibers serving as a biomedical scaffold owing to its remarkable mechanical properties.

2.4 Multinozzle electrospinning Single-nozzle electrospinning offers the advantage of generating composite nanofiber using compatible polymers blend. On the other hand, multinozzle electrospinning offers much more advantage in assembling composite nanofibers generated from two or more immiscible polymeric solutions. In this approach, the nozzles can be placed either side-by-side or in opposite directions of the grounded collector. While placing the nozzles on the opposite sides of the collector, each nozzle must be equipped with two power supplies and this may induce charge fluctuations due to the variations in the charge supplied to each nozzle. However, this may be overcome by placing the nozzles side-by-side where a uniform charge distribution is supplied using a common metallic bar. This ensures effective assembly of uniform composite

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fibers by the dispersion of two or more polymeric solutions [46] and electrospun from separate orifices. Multinozzle electrospinning is extensively studied in recent years to enhance the productivity, to fabricate core-shell fibers, composite fibers and other special featured fibers [47].

2.5 Coaxial electrospinning This is the modified version of the multinozzle electrospinning, where the spinneret with one nozzle is placed inside a larger nozzle. Such engineering would facilitate generation of core-shell nanofibers, hollow fibers, fibers from nonelectrospinnable materials for drug delivery and protein translocation applications. Alongside, unique properties in the product could be materialized toward to clinical applications. Typical set-up of coaxial electrospinning fed with two different solutions to generate coreshell fibers at the capillary tip is illustrated in Fig. 2. Here in the core-shell configuration, the unique properties of compatible polymers are retained even after blending of materials where the core material is completely surrounded by the shell (sheath) material. The feasibility in fabricating such architecture was initially reported by Sun et al. [48]. In this line, Zhang et al. [49] have successfully demonstrated the preparation of core-shell nanofibers using PCL as shell encapsulating gelatin core for drug delivery and tissue engineering applications. They have also demonstrated that the size of the core material can be modified by varying the concentration of the polymer. This indeed improved the mechanical properties of the polymers by their varied degrading and delivery efficiencies exhibiting its Fig. 2 Working of coaxial electrospinning fed with two different polymer solutions generating core-shell fibers.

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potential in drug delivery applications [50]. Similarly, Jiang et al. [51] have developed bovine serum albumin (BSA) dispersed dextran encapsulated PCL core-shell nanofiber. The use of PCL confers advantages in sustained release of the drugs owing to its optimal porosity and reduction in burst release. Such remarkable properties have made it the potential candidate for delivering drugs at much a slower pace. The applications are not limited to the delivery of drugs but carbon, ceramic and metal nanoparticles with multifunctions could be impregnated to the polymer matrix for applications in energy storage, luminescence, catalysis and filtration [52]. Applied voltage remains to be one of the critical factors influencing the generation of fibers in coaxial electrospinning. Li and Xia [53] have reported the correlation between the applied voltage and the diameter of the fiber. Upon increasing the electric field strength, the outer and the inner diameters of the hollow fiber decreased. On the contrary most of the studies have reported on only one value of the applied voltage resulting in the formation of stabilized compound Taylor cone and jet.

3

Feed materials for electrospinning

Electrospinning involves the utilization of mostly organic polymer materials in the form of a solution or a melt to generate nanofibers. Besides, small molecules are also efficiently electrospun into fibers provided they exhibit the property of self-assembly and chain entanglement. Moreover, the involvement of sol-gel chemistry has taken electrospinning to a different level in generating fibers from composite materials. Notably, materials constituting different dimensions/morphologies, viz., nanoparticles, nanowires, nanotubes etc., can also be directly electrospun into fibers.

3.1 Polymers Organic polymers are the most facile materials that can be directly utilized for electrospinning process provided their dissolution into appropriate solvent system and the use of high molecular weight polymers that determine the success of electrospinning (Fig. 3). There are two methods namely solution and melt electrospinning that utilizes organic polymers as feed. Solution electrospinning is the most common method where a jet of organic polymer solution is stretched, elongated and thinned by innumerous bending (whipping) instabilities. This is followed by the evaporation of the solvent, solidification of the jet and deposition of fibers in the form of nonwoven mat onto the grounded collector [54]. On the other hand, melt electrospinning involves direct generation of nanofibers from the melts of the solvent insoluble polymers such as polyethylene and polypropylene [55]. Although electrospinning process generates nanofibers with thinner diameter and varied architecture, the choice of the feed materials plays a pivotal role in deciding the functional aspects for biomedical applications (Table 2).

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Solubility

Viscosity

Reconcilability

Chief Characteristics Electrical conductivity

Stability

(A)

High Molecular weight polymer

Spinnability

Appropriate Solvent

SUCCESSFUL ELECTRO SPINNING

(B) Fig. 3 (A) Essential parameters influencing the electrospinning process and (B) polymeric parameters for successful electrospinning.

3.1.1 Natural polymers These polymers are either derived from plant or animal bodies and are considered as excellent renewable resource possessing biodegradability and biocompatibility finding its application toward biomedical and tissue engineering fields. Natural biopolymers including DNA, silk fibroin, chitosan, chitin, hyaluronic acid, collagen, alginate, dextran, etc., have been successfully electrospun into nanofibers. Besides plant and animal based polymers, those obtained from sea weeds were also employed. For instance, alginates with cross links are used as a potential biomaterial in fabricating scaffolds and drug cargoes [71]. Similarly, chitosan, an excellent biomaterial is obtained by the deacetylation of chitin where the extent of deacetylation and pH define its charge density. Notably the positively charged polymer has increased affinity toward negatively charged drugs or proteins that facilitates its wide applications in drug delivery [72]. Hyaluronic acid is also emerging as one of the promising candidates investigated for tissue regeneration application. Hyaluronan is a linear anionic polysaccharide

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Table 2 Polymers and their appropriate solvents used in electrospinning for the generation of nanofibers. Polymers (natural/synthetic)

Solvents used

References

Carboxyethyl chitosan/PVA Alginate/polyvinyl alcohol Silk fibroin/polyethylene glycol Chitosan Collagen/chitosan and collagen/ zein Cellulose/polyethylene vinyl acetate Collagen and chitin PLGA/collagen Laminin 1 Gelatin

H2O H2O H2O H2O/CH3COOH CH3COOH

[56] [57] [58] [59] [60,61]

CH3COOH

[62,63]

Hexafluoro-2-propanol Hexafluoro-2-propanol Hexafluoro-2-propanol Hexafluoro-2-propanol and trifluoroethanol Trifluoroethanol CH3OH/dimethylformamide

[64,65] [66] [67] [68]

Polyurethane/gelatin Hyperbranched polyglycerol

[69] [70]

largely found in human tissues in the form of nonsulfated glycosaminoglycans (GAG), mainly involved in the regulation of cell adhesion, proliferation and differentiation. It is one of the chief components of the extracellular matrix (ECM) having larger interactions with the key proteins present in the ECM. They are widely investigated for the fabrication of scaffolds for tissue engineering applications. Moreover, due to its polyelectrolytic nature the solution turns more viscous making electrospinning very difficult. To achieve critical chain entanglement, HA fibers were introduced into sodium hydroxide-dimethylformamide system to generate fibers of 100 nm [73,74]. Collagen and its derivatives such as gelatin and other polypeptides are considered as natural polymers predominantly found in the connective tissues of humans, spider silk and mori silk. They are investigated for their multifunctional polymeric properties. Converting collagen into fibers offer good mechanical properties, porosity and biocompatibility. They have been largely employed in tissue regeneration applications such as artificial skin graft, vasculature, tissue (cartilage) repair, periodontal restoration, wound dressings, etc. [75].

3.1.2 Synthetic polymers There are over 100 different types of synthetic polymers used for the direct generation of nanofibers adopting electrospinning. Some of them are commercially used, in particular, polystyrene and poly(vinyl chloride) are used for environmental applications. Alnaqbi et al. [76] have generated nanofibrous sorbents adopting polymer blending strategy for the removal of oil spills. Indeed biocompatible, biodegradable, nontoxic and mechanically stable synthetic polymers are largely been employed for the generation of nanofibers using electrospinning and used for biomedical applications. Few of them include poly lactic

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acid (PLA) and PLGA which can be directly spun into fibers and used as scaffolds in tissue engineering [77]. PLA offers the advantage of dissolving easily in various solvents such as acetone, chloroform, dichloromethane, dimethylformamide, and tetrahydrofuran. It is prepared using condensation and ring opening polymerization techniques. As, the former method would produce polymer with low molecular weight and poor mechanical properties, the latter would be the most preferred technique that introduces excellent mechanical stability [78,79]. Alongside the derivatives of PLA, namely poly-L-lactic acid (PLLA) and PLGA occurs via copolymerization with L-lactide and polyglycolic acid, respectively [80,81]. These polymers when converted into nanofibers have profound implications in biosensors, molecular filtrations and used even in preserving biological specimens [82,83].

3.1.3 Conducting polymers Conducting polymers are the recently emerged classes of polymers possessing remarkable properties compared with contemporary polymers. Their characteristic π-conjugated backbone confers enough electrical conductivity upon charge transition by redox reactions. They possess typical characteristics of both the metallic (electrical and optical properties) and the polymeric materials (biocompatibility, good processability, chemical stability) [84]. Polyacetylene was the first recognized conductive polymer in the 1970s and had been widely explored for its biophysical properties [85]. Due to the unstable nature and difficulty in processing of the polyene, alternative forms have been introduced (polyaniline, polypyrrole, polythiophene, and poly (3,4-ethyelenedioxythiophene)) with improved thermal stability and conductivity [86]. Moreover, the biodegradable property has been introduced into conducting polymers by the copolymerization of aniline. Notably, during electrospinning these conducting polymers are mixed with biodegradable polymers and electorspun into nanofibers for scaffold fabrication. Besides innumerous conducting polymers, there are only few polymers that have been successfully developed into nanofibers. For instance, polypyrrole was subjected to preprocessing to attain adequate molecular weight by dissolving it in dimethylformamide and by the addition of di(2ethylhexyl). Upon electrospinning, the polypyrrole was developed into nanofibers endowed with a thinner diameter of 70 nm [87]. Secondly, direct electrospinning of PANI with PLA/gelatin was developed into nanofibers imparting an electrical conductivity of 4.2  103 S cm1 and has been used for cardiac tissue engineering applications [88,89]. Besides, there are different functional polymers such as polyvinylidene fluoride and polyvinlylidene fluoride-co-trifluoroethylene with enhanced piezoelectric and pyroelectric properties that can be directly electrospun for energy harvesting and biosensor applications [90,91].

3.1.4 Small molecules Small molecules are the most challenging materials to be directly made into nanofibers using electrospinning. Their unique self-assembling characteristics with significant chain entanglement properties were found to be capable of stabilizing the electrified jet and suppression of Rayleigh instability to obtain nanofiber. Notably,

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the small molecules’ structure–concentration interrelationship and the type of solvent decide the success of electrospinning for the generation of fibers. Mostly, the small molecules involved in the development of nanofibers constitute amphiphiles and cyclodextrin derivatives [92]. Most importantly, lecithin, a zwitterionic mixture comprising glycerophospholipids and phosphatidic acid was the first reported small molecule to be developed into nanofibers. Its hydrophobicity and surface tension reduction potential have made them the suitable candidates for regenerative applications with modified structure and surface area of the scaffolds [93]. Secondly, their self-assembling properties tend to form spherical micelles above the critical micelle concentration (CMC). Upon increasing the concentration, the morphology undergoes a transition from spherical to columnar structures which again overlaps to form chains resembling fibers. The criticality relies on the concentration of lecithin to the amount of solvent used in achieving the desirable fiber diameter. For instance, the ratio of lecithin:CHCl3:DMF at 70:30:43 (wt%) yielded continuous fiber with an average microscale diameter of 2.8 μm. Whereas by increasing the concentration of DMF to 50% (>CMC), the fibers generated had a diameter of 5.9 μm [94]. In addition, Gemini ammonium surfactants N,N0 -didodecyl-N,N,N0 ,N0 -tetramethyl-N,N0 ethanediyldiammonium dibromide (12-2-12) in H2O-CH3OH have been successfully electrospun into micellar microstructured hydrophilic nanofibers with diameters in the range of 0.9–7 μm [95,96]. Recently, peptide derivatives of the pyrazole-isothiazole scaffold have been specifically fabricated using electrospinning. Phospholipid amphiphiles, tetraphenylporphyrin compounds, and cyclodextrin small molecular system have been successfully electrospun into nanofibers [97]. As mentioned earlier, the type of the CD used and its concentration decide the morphology and the fiber diameter. The disadvantage of the formation of bead-like nanofibers could be overcome by using CD owing to its tendency to form aggregates via hydrogen bonding and exhibiting high solution viscosity and viscoelastic solid-like characteristics [98]. Moreover, the CD offers truncated cone-shaped and relatively hydrophobic cavity in which the drug of choice can form inclusion complex with hydrophobic drugs with hydrophilic exterior thereby increasing the water-solubility of drugs for prolonged delivery applications [99]. Xiang et al. [100] have developed hydroxypropyl-β-cyclodextrin-polyvinylpyrrolidone-loaded resveratrol nanofibers to enhance the water solubility of resveratrol and to remain stable under UV irradiation. In vitro studies conducted have revealed its slow and sustained release via nanofiber extrusion. The ratio of HP-β-CD to PVP at 1:2 yielded nanofibers with smooth surface and uniform thickness. Further, upon optimizing the concentration of resveratrol to 5%, the solution viscosity corresponded to 4.06 Pa s attributing for good fiber morphology with an average diameter of 500 nm [100].

3.1.5 Colloids Colloidal particles comprises of homogenous noncrystalline substance either found in a dispersed or continuous phase. They may include gels, sols and emulsions where they can be successfully electrospun into nanofibers once they are capable of entanglement among the dispersed particles to form a jet. This colloid-electrospinning has

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been a widely used technique to immobilize the particles in fibers at nanoscale dimension. The essential criteria for electrospinning rely on the size and viscosity facilitated by hydrolysis or condensation process. Most importantly, the viscosity parameter decides the size/thickness of the fibers generated. Conversely, sol-gel method was the alternative and widely used method for the generation of nanofibers. For instance, by using tetraethyl orthosilicate (TEOS), distilled water, ethanol and HCl at the molar ratio of 1:2:2:0.01, silica sol was successfully electrospun into fibers employing acidic catalysis reaction [101]. The thickness of the fibers generated worked out to be in the size range of 200–600 nm with an applied voltage of 12–16 kV. Similarly, lithium-cobalt acetate nanofibers with diameter ranging from 0.5 to 2 μm were generated by calcination method [102]. This method was employed to synthesize metal oxide nanofibers encompassing aluminum, zinc, nickel and cobalt at micrometer scale dimension owing to limited control over the size and uniformity of fibers influenced by the rheological properties of the sol [103]. Further, metal nanoparticle comprising silver known for its antimicrobial properties have been electrospun by integrating with synthetic polymers such as PEO, PVA, PVP and polyacrylonitrile [104,105] or with natural polymers such as chitosan, gelatin and N-carboxyethylchitosan [106]. For instance, in situ reduction of silver nanoparticles using formic acid as solvent was integrated with polyamide 6 and electrospun. This nanoparticle incorporation has attributed for enhanced resonance spectroscopy (SRS) [107]. It was Chen et al. [108] who unraveled the mechanism behind the nanoparticle-polymer interaction where the π–π coordination between the silver moiety and polymer facilitates photoexcitations and charge transfer endorsing optic based application.

3.1.6 Metal oxide nanoparticles (MONPs) Metal oxide nanoparticles are the engineered materials destined for a wide range of industrial applications. These properties have been harnessed efficiently in fabricating gas sensors, solar cells, water purification systems, and waste water treatments applications. There are certain advantages of MONPs over the conventional metal colloids, such as high stability, simple preparation protocols, easy manipulation, no swelling variations, facile functionalization and ease of incorporation into hydrophobic and hydrophilic systems making them a promising tool for biomedical applications [109]. Alongside, the development of MONPs has successfully been employed as biomaterial in tissue therapy, immunotherapy, diagnosis, dentistry, regenerative medicine, wound healing and bio-sensing applications. Such metal oxide nanoparticles are classified into zero-, one-, two- and three-dimensional structures based on their size, shape, purity, stability and surface properties (Fig. 4). These are the important parameters that make the nanophase assemblies indispensable tools in biomedical engineering sector. The prerequisites that these MONPs as potential biomaterial are as follows. As a nanocarrier for efficient drug delivery applications, they must possess kinetics that comply with the treatment of specific infection and biodegradability to get rid of surgical intervention. Some of the broad spectrum MONPs such as TiO2, ZnO, CuO,

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0D (NPs / Nanoclusters / QDs)

1D

3D (Dendrimers / Nano-pillars / -flowers)

MONPs

(Nano-rods / -tubes / wires / fibres)

2D (Nanosheets / coatings / films)

Fig. 4 Metal oxide nanoparticles and their classification into zero-, one-, two-, and threedimensional structures based on size, shape, surface, and functional properties.

Fe2O3 (ferric oxide), and Fe3O4 (ferrous oxide) has already staunched their footage in biomedical sector and accomplished their safety when used in vivo [110]. For instance, iron oxide nanoparticles (IONPs) have broad range medical applications that include magnetic resonance imaging, magnetic hyperthermia, drug delivery, cell separation and detection. Their remarkable properties such as superparamagnetism, size and their tendency to accept biocompatible coating make them potential candidate keeping pace with medical and biotechnological advancements [111]. Nanofabrication approaches have been projected in the fabrication of metal-oxide nanofibrous meshes for a wide range of industrial, environmental, tissue engineering and biomedical applications. There are several methods that have been successfully demonstrated viz. vapor-liquid-solid (VLS), nanocarving and electrospinning, among which electrospinning is the most sought after approach for its adaptability. In addition, this approach permits nanofabrication of mesh from organic, inorganic and biological materials. Secondly, precision control over the diameter, surface properties, pore morphology, orientation and composition of the resulting metal oxide nanofiber would be achieved [112]. In electrospinning, mostly coaxial, colloid-, melt- and solution electrospinning are preferred to produce metal-oxide micro/nanofibers. But, the presence of metal-oxide precursor colloids in the feed along with the meshing agent, viz., a polymer would

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complicate the spinning process owing to the physiochemical and process parameters’ incompatibility. There are strategies devised to overcome such pitfalls aimed at fabrication of multicomponent nanofibers. One approach is by adding a metal-oxide precursor along with polymer during electrospinning solution preparation and another approach is by introducing the solution after electrospinning process [113,114]. These approaches would sideline the general calcination process of metal-oxide precursor derived electrospun fibers centered on the oxidative changes of the polymer by sequential heating. In order to overcome polymer degradation, these metal-oxides are introduced into the electrospun fibers by either in situ or ex situ in the form of a surface coating onto a suitable polymeric/carbon core. This could be accomplished by coaxial electrospinning using an appropriate sol-gel or organometallic metal-oxide precursor mixed with suitable polymer solution. Alternatively by dipping the polymeric nanofibers into a solvent bath containing desired metal-oxide precursor would eventually result in surface coating of metal-oxides. For instance, zinc, silica and titania nanoparticles were subjected to electrospinning to produce a polymeric fibers with a blend of polymer and dispersed nanoparticles. To the preformed polymeric fibers, ZnO/SiO2/TiO2-based precursors mixed with compatible carrier polymer were added to produce electrospun metal-oxide (ZnO/SiO2/TiO2) nanofibers [114,115]. In recent times, bi-component metal oxides comprising ZnO and TiO2 nanofibers and metal-metal oxide nanofibers (Ag/ZnO) produced using electrospinning have also been reported. Moreover, colloid electrospinning technique was employed to fabricate metal oxide/silica nanofibers [116]. In this line, Mondal et al. [117] have used the solgel precursor of TiO2, viz., titanium isopropoxide along with a template polymer PVP to demonstrate the fabrication of electrospun carbon doped TiO2 nanofibers.

Metal/carbonaceous nanofibers Metal oxide–carbon nanocomposite fibers constitute another class of nanofibers in which metal oxides such as Fe(acac)3 magnetite dissolved in polyacrylonitrile (PAN/N) and N-dimethylformamide (DMF) were subjected to carbonization for the successful fabrication of MO/carbon nanocomposite fibers. This could be accomplished by electrospinning technique using PAN/DMF as medium for dispersing the already synthesized metal-oxide nanoparticles. The electrospun nanofibers would be functionalized and developed using composite nanostructures for desired applications [118,119]. Carbon nanomaterial-based scaffolds set an example among wide range of biocompatible advanced materials used in bone tissue engineering applications. For instance, their biocompatibility, mechanical stability and commercial availability make them potential candidates in replacing and repairing damaged and defective tissues in human body [120]. There have been studies undertaken to investigate the use of carbon-based nanomaterials for bone tissue engineering in vivo. In this line, Sitharaman et al. [121] have employed CNT-biodegradable polymer nanocomposites for bone regeneration in a rabbit model. More interestingly, single-walled carbon nanotubes (SWCNTs) especially ultra-short SWCNTs were utilized to fabricate polymeric scaffolds. It was inferred that the composition of scaffold had a greater

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influence on the cell behavior and growth highlighting the criticality in bone cell proliferation and growth. Effective functionalization of CNT remains one of the important factors to enhance the cell-scaffold interaction and cell spreading on the scaffold surface microenvironment. To achieve the same, strategies such as covalent and noncovalent functionalization approaches have been adopted. But there are several instances where these strategies could not be applied to all sort of materials, such as polymers and ceramics owing to the poor mechanical strength, brittleness and Young’s modulus. However, Shokri et al. [122] have demonstrated fabrication of a nanocomposite scaffold using a composite of bioactive glass, CNTs and CS in different ratios for bone tissue engineering. Interestingly, it was found that by using this combinatorial approach, there was a significant increase in mechanical, chemical and cellstimulating properties most appropriate for repairing trabecular bone tissue. Furthermore, Hirata et al. [123] have made an extensive investigation on 3D-carbon scaffolds coated with MWCNTs for improved cell adhesion properties. Upon analysis of actin stress fibers after 7 days of culture, stressed Saos2 cells were evidenced. Furthermore, MWCNTs-coating served to be the most suitable 3D scaffold for cell culture on a par with MWCNTs. Following this, Bhattacharya et al. [124] have studied the impacts on layer-by-layer assembled CNT-composite on osteoblasts both in vitro and in vivo using rat bone tissue. It was found that CNT-composite coated materials significantly increased cell differentiation as measured from alkaline phosphatase activity. In continuity, CNTs were also studied for their osteoblast proliferation and bone formation. From the study, it was reported that those CNTs carrying a neutral charge was found capable of increasing the rate of cell growth with plate-like crystals attributing for the attachment of osteoblast cells cultured on MWCNTs [125]. A step forward, Chen et al. [126] have demonstrated synthesis of surface modified PCL-PLA acid scaffolds using self-assembled CNTs and insulin like growth factor-1 (IGF-1) for analyzing the antisenescence functionality. In bone tissue engineering, these heterojunction CNTs could efficiently accelerate the bone healing ability with extremely low toxicity in vivo. Due to the limited in vivo applications of CNTs, 3Dmicroporous structures in CNTs are the most sought after to enhance cell adhesion, migration, growth and tissue formation in scaffolds used for tissue engineering applications. Indeed, metal/metal oxide–nanofiber composites are commonly used in tissue engineering scaffolds to improve their physical, chemical and biological functionalities supporting the regeneration, repair and growth of damaged tissue. Besides their beneficial effects, cytotoxicity and adverse environmental effects need extensive studies. There is no doubt that carbon based metal oxide nanoscaffolds hold great promise in clinical applications. Cytotoxicity issues pertaining to artificial bone tissues shall be acceptable but the risk of normal cells getting affected with disruption of pathways need further investigations. The prime ability of carbon based nanomaterials to undergo surface modification with desirable functional groups or chemical compositions would gain control over cell-scaffold and cell-to-cell interaction. Another advantage is that carbon based nanomaterial shall greatly help to overcome mechanical issues of scaffolds fabricated using labile polymeric materials. Finally, the

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biodegradable property of carbon based material though appreciable, the cellular behavior, nutrient exchange efficiency and the cellular microenvironment on the surface of scaffold are greatly influenced by the biodegradability.

4

Applications of electrospun nanofibers in biomedical & environmental sector

Prior to thorough understanding on the basics related to physical and functional properties of electrospinning materials, this section attempts to bring out the applications of nanofiber technology toward biomedical and environmental sectors. Nanofibers are mostly applied in fabricating scaffolds for either tissue regeneration or effective drug delivery. Conventionally, tissue regeneration was made possible by involving autoand allografts. Autograft, although genetically feasible, challenge toward the availability of donor sites in case of larger affected area could not solve the purpose and in turn the donor may suffer heavy damage due to the removal of tissues. Besides, the possibility of rejection would be greater when a genetically different material is introduced in case of an allograft due to immunological response. To counterbalance the regeneration and compatibility, this novel tissue regeneration approach has established a biocompatible arena for the host tissues to adhere, proliferate and differentiate into specific tissues that need to be repaired [127]. This could be achieved by the nanofibers for its enhanced surface area and porosity that drives the regeneration efficacy of the scaffolds [128].

4.1 Tissue engineering applications The requirements for successful growth of tissues via nanofiber scaffolds produced using electrospinning have made this technology the most preferred one. Properties such as biocompatibility, biodegradability, large surface area, maintaining the structural integrity, high porosity and high mechanical stability have improved the growth and differentiation of cells [129]. Mostly nanocomposite materials mimicking extracellular matrix (ECM) proteins (collagen and glycosaminoglycans) are preferred for improved cell function and cell–cell/cell–ECM affinity. Notably, thinner fibers with diameter in the range of 50 to 200 nm have been reported to enhance cell adhesion, proliferation and alkaline phosphatase activity [130]. Furthermore, the unique properties conferred by these core-shell nanofibers and their ease of manipulation pertaining to mechanical and electrical properties have augmented the scope for tissue engineering (Table 3). The incorporation of antimicrobial nanoparticles, viz., Ag, Au, Zn, Ti, Mg, Cu, SWCNT, graphene, etc. [139–141] and polymers (chitosan) [142] into the nanofibers via electrospinning has profound implications in the fabrication of biomedical devices, textiles, and tissue engineering (Table 3). In tissue engineering applications, there is a novel in situ approach that holds promise in the regeneration of functional blood vessels following the guidance of vascular scaffolds. The tubular design with multilayered wall mimicking the native blood

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Table 3 Nanofiber scaffolds and its application in tissue engineering. Nanofiber scaffolds

Tissues

Functional properties

References

PLGA

Cartilage (mouse fibroblast cells)

[131]

PCL

Bone (neonatal rat bone marrow mesenchymal stem cells) Cartilage (human articular chondrocytic cell line)

Confers mechanical properties for the desired tissue Increase in production of ECM

Improves adhesion, proliferation, infiltration of chondrocytes. Establishment of pseudopodia Increases the production of ECM

[133]

Improvement in the physical properties

[135]

High porosity conferred increased adhesion property and mechanical strength Induced proangiogenic characteristics. Promotes functional and morphological recovery in peripheral nerve generation Improved internal flexibility, adhesion, proliferation and differentiation. Good candidate for soft tissue replacement

[136]

Collagen type II

Polyurethane (PU)

Collagen type I, elastin and poly(D,Llactide-coglycolide) Poly(εcaprolactone) and polyethylenimine

Ligament/tendon (human ligament fibroblast) Blood vessels (endothelial cells and smooth muscle cells) Fibroblast cells

Graphene oxide/ polycaprolactone

Neurite (sciatic nerve defect cells)

Bi-layered PCL scaffolds coated with collagen

Bone (human bone mesenchymal stem cells)

[132]

[134]

[137]

[138]

vessel architecture has been the most sought after model [143]. It is in this model, the tunica intima takes care of the functional properties by accelerating endothelialization and preventing thrombus. Secondly, the media confers mechanical stability to the vascular scaffold at the anastomotic sites and avoids architecture remodeling. In order to achieve better endothelialization, critical evaluation on the inner and middle layers of the tubular shape needs much consideration. This greatly helps in supporting increased blood flow without leakage in the lumen and checks for anticoagulation

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properties in preventing stenosis and occlusion that determines the lumen patency [144]. For instance, nanofibers with introduction/loading of heparin or arginine-glycineaspartic acid have been used to fabricate lumen to evade early thrombosis [145]. Recently, Eilenberg et al. [146] have designed a novel degradable thermoplastic polycarbonate urethane (dPCU) grafts for small vessel replacement in rodents. They have reported an upregulated antiinflammatory signaling in dPCU conduits offering excellent patency rates of 92.9% without causing any adverse effects in rat model. When compared to expanded polytetrafluoroethylene (ePTFE), dPCU grafts accelerated transmural ingrowth of vascular cells into a structured neovessel around the graft with gradual reduction of graft material. In addition, natural biopolymer silk has shown promise in the fabrication of vascular scaffolds using a bilayered Anthereae assama (AA) and Bombyx mori (BM) silk. This hybrid model with its inner layer measuring 40 μm and interconnected pores has accelerated cellular infiltration whereas the outer dense layer offers mechanical stability. Human adipose tissue-derived stromal vascular fraction seeded silk vascular graft was implanted surgically in Lewis rats as an abdominal aortic interposition graft. It was inferred that AA silk laden vascular graft showed superior animal survival and graft patency after 8 weeks. Further, these silk laden vascular grafts degrade into amino acids and their resorbable byproducts well elucidates its remodeling ability [147]. This approach remains to be the futuristic vascular alternative for bypass and reconstructive surgeries. The fabrication of nanoparticle-nanofiber composites in recent years is drawing much attention for their ease of use, simple methodology and scalability. For instance, fabrication of novel nanofibrous scaffolds made of PCL loaded with magnesium phosphate (Mg3(PO4)2) and magnesium oxide (MgO) nanoparticles showed significant biocompatibility, cell viability with augmented cell spreading ability demonstrated using mesenchymal stem cells. Such potential scaffolds find their application in bone tissue engineering [148,149]. In this line, PLA nanofibers fabricated along with chitosan were loaded with different types of metal (Au, Pt) and metal-oxide (TiO2) nanoparticles to confer chemical structure, swelling and crystallinity. The incorporation of metal and metal-oxides showed significant bioactivity with promising features for application in bone tissue engineering [150]. Among the metal oxides, ZnO remains one of the widely investigated materials in tissue engineering applications owing to its antibacterial properties, cell growth promoting activity, proliferation and differentiation. The properties have been well documented using pure ZnO nanostructures and their integration with composite materials such as polymers and ceramics [151,152]. This has been accomplished by electrospinning technique for the production of hybrid polymeric nanostructures [153]. There are innumerous biopolymers that have been used in combination with the ZnO among which poly(ε-caprolactone) (PCL) (approved by the Food and Drug Administration—FDA) has been the most widely used owing to its biodegradability and biocompatibility for tissue engineering applications. Their antibacterial and tissue regeneration efficacy of PCL-ZnO NPs and a PCL gel-ZnO NPs electrospun scaffold have been studied using periodontal pathogens, viz., Porphyromonas, gingivalis, and Fusobacterium nucleatum. It was inferred that

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upon increasing the concentration of ZnO to 30% of its weight, there was a significant antibacterial effect but with little cytotoxicity. This effect was further compromised when the concentration of ZnO was reduced to 15% [154]. Furthermore, electrospun PCL-hydroxyapatite-ZnO nanofibers have been used as potential biomaterials in bone tissue engineering. The developed nanofibrous scaffolds with ZnO NPs at a concentration range of 1–30 wt% balanced both biocompatibility and hastened osteoregulation. The increased concentration of ZnO NPs in the PCL-hydroxyapatite scaffold imparted potent antimicrobial activity in a dose dependent fashion but ultimately resulted in reduced cell viability. In order to balance cell viability and mechanical properties of the scaffold, ZnO concentrations were brought down to 10 wt% so that optimal biocompatibility and bio-mineralization could be achieved [155]. Recently, electrospun PCL fibers were loaded with coumarin and ZnO NPs to coat the commercially available AZ31 Mg alloy to study the in vitro effects on the macrophage inflammatory response and osteoclastogenic process. From the X-ray photoelectron spectroscopy (XPS) analysis, it was inferred that AZ31 was perfectly coated with PCL fibers loaded with coumarin and ZnO NPs exhibiting sustained release of the active ingredient. The degradative properties of PCL-ZnO and PCL-coumarinZnO coated samples were found to exhibit best corrosion behavior. This study showed that PCL-coumarin-ZnO coated sample exhibited best behavior in terms of inducing inflammatory response and activating the receptor for nuclear factor kappa-B ligandmediated differentiation of RAW 264.7 macrophages into osteoclasts [156]. Facile synthesis and characterization of calcium oxide (CaO) NPs loaded electrospun matrices for osteomyelitis treatment and bone tissue engineering applications have been reported [157]. Polymeric solutions containing PCL and PCL/gelatin (1:1, w/w) were blended with CaO NPs and electrospun into fibrous matrices and their osteoprecursor cell response, alkaline phosphatase (ALP) expression and their infiltration efficiencies were evaluated. The results revealed successful incorporation of CaO NPs into the fibers at a reduced concentration exhibiting low antibacterial activity thereby enhancing the cell viability and differentiation capacity. CaO NPs-loaded matrices containing PCLX/gelatin showed an increased ALP expression on a par with individual PCL and gelatin counterparts.

4.2 Wound healing/dressing Skin, front-line defense of human and the largest organ is the most susceptible part responding toward physical means of injury. The severity of the wound determines the type of approach adopted to hasten healing process. For instance, minor wounds would heal faster through intrinsic repair mechanism whereas large-scale or fullthickness wounds (burns, diabetes related wounds) require scaffolds to aid migration, proliferation and maturation of repairable cells [153,158]. Besides wound healing, antiinflammation, antiinfection, scar formation, and conditions leading to skin cancer need timely resolution. This has been enticed by the nanofibers developed using electrospinning to enhance healing of wounds. These nanofibers with its typical topography mimicking basketweave-like pattern of collagen have been fabricated to accelerate migration and infiltration of repairable cells.

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Most importantly, the orientation of the fibers play a key role in expediting the process of wound healing. Intercross nanofibers on a par with random or uniaxially aligned fibers exhibit best healing performance by accelerating the infiltration of fibroblasts and keratinocytes [159]. Pal et al. [160] have demonstrated the wound healing efficiency (3 weeks) in rat model by fabricating chitosan-PCL core-sheath fibers using emulsion electrospinning. Further, the use of 3D scaffold in the regeneration of dermal ECM was found critical. To overcome infiltration intricacies, sandwich-type scaffold was preferred over 3D in skin regeneration application. Here, the radially aligned nanofibers are placed at the bottom; a mat comprising an array of square shaped microwells at the top and the microlevel skin tissues placed in between the two layers. Their applicability as a promising wound dressing facilitates enhanced cell infiltration thereby preventing drainage at wound site [161]. Besides 2D nanofibers, fabrication of 3D nanofiber scaffold to promote cellular infiltration with controlled thickness and porosity has been demonstrated by Jiang et al. [162] using depressurized subcritical CO2 fluid. Such modified 3D scaffold not only formed layered structures but also retained the fluorescent intensity and antibacterial efficacy of coumarin 6 and LL-37 peptide. This helped to surpass nanotopographical cues and significantly accelerated cell infiltration and neotissue formation aided by subcutaneous implantation. Promising results in the formation of blood vessel within 2–4 weeks with significant antimicrobial effect and tissue regeneration have been reported using extended 3D scaffolds on a par with the traditional 2D scaffold. For diabetic related wound healing, Chen et al. [163] have developed 3D vertical/radially aligned nanofiber scaffolds in bone marrow mesenchymal stem cells (BMSC) transplant. It offers the advantage of shape-recovery upon compression in atmospheric and aquatic conditions fitting in for a variety of type 2 diabetic wounds. These BMSC embedded scaffold has potentially enhanced granulation, angiogenesis and collagen deposition. In addition, they were also found to inhibit the formation of M1-type macrophage and pro-inflammatory cytokines IL-6 and TNF-α and thereby promoting M2-type macrophage and expression of IL-4 and IL-10. In another study, Lv et al. [164] have reported on the wound healing property of PCL/gelatin nanofibrous scaffold containing silicate based bioceramic particles (NAGEL) fabricated using coelectrospinning process. The uniform distribution of bioceramic particles in the PCL-gelatin nanofibers aided Si ions in sustained release during their degradation. Significantly, they promoted cell adhesion, proliferation and migration by activating epithelial/endothelial to mesenchymal transition pathway both in vitro and in vivo. Such synergistic effect of the functional biomaterials involving conductive nanocomposite scaffold have opened new vistas in wound healing. Similarly, Ren et al. [165] have reported PILA electrospun nanofiber impregnated with dimethyloxalylglycine-decorated mesoporous silica NPs for wound healing by accelerating the expression of human umbilical vein endothelial cells (HUVECs). It is noteworthy to mention that several metal nanoparticles with antimicrobial potential are electrospun into nanofiber for counteracting multidrug-resistant bacteria and for wound-healing application. Yang et al. [166] have developed wound dressings using 6-aminopenicillanic acid decorated-gold nanoparticles to inhibit the growth of

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MDR bacteria. These materials are then electrospun into PCL-gelatin fibers to challenge MDR bacteria and promoting faster wound-healing capability. In this line, Xi et al. [167] have developed an elastomeric, photoluminescent, biocompatible and antimicrobial polypeptide based PCE-PCL nanofiber to inhibit MDR bacteria. There was also significant enhancement in skin-thickness wound healing and tissue regeneration in mouse model attributing for a competitive multifunctional wound dressing (Table 4).

Table 4 Patent products of Nanofiber enabled technology for biomedical and environmental applications. Product

Specifications

Country

Patent ID/year

A nanofiber product for wound dressing

A mesh comprising oxidized polysaccharide (polyanhydroglucuronic acid) and a fiber-forming polymer (PVA) Three nanofibers were electrospun with three different functionalities. The middle layer comprises herbal extract of Melilotus officinalis. Various additives are added to control the release profile of the extract One or more nanofibers electrospunned by the integration of active agents comprising plant extracts for the prevention of skin diseases Fabrication of nanofibrous highly porous scaffolds (100– 1000 nm) including chitosan and honey for drug delivery, bacteriophage, wound dressing, cancer treatment and antibacterials Fibers prepared by a masterbatch of polymer pellets (PET), silver, and copper salts. Potent inhibitors of athlete’s foot, antibacterial effective against drug resistant strains

Dublin

WO2008010199A2/ 2007

United States

US9101508B2/2011

Australia

AU2012305986A1/ 2012

United States

WO2015003155A1/ 2014

United States

US9908987B2/2014

Electrospun nanofibrous wound dressing

Bioactive nanofibers

Bio-compatible apitherapeutic nanofibers

Antimicrobial and antifungal polymer fibers and fabrics

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Table 4 Continued Product

Specifications

Country

Patent ID/year

Nanofiber cover for wounds

A hybrid mixture of polyvinyl alcohol-pure water-natural honey electrospun into fibers. The nanofibers comprised only honey molecules and are applied for covering wound and ambustion Absorbable polymers such as PLA, PGA and PCL were used for their biocompatibility and biodegradability along with oil based polyesteramide integrated with a pharmaceutical drug for faster wound healing Electrospun nanofiber ultrafiltration membrane for application in tangential filtration mode. Purification of biological materials Hybrid nanofibers comprising A. vera and synthetic polymer, viz., poly3-hydroxybutyrate-co-3hydoxyvalerate (PHBV), PLLA and polydioxanone (PDS). The mean diameter ranges from 0.3 to 1.5 μm. Applied for tubular prostheses prior to peripheral nerve axotomy, dressings, and sutures. They are also applied for enhanced recovery from lesions

Konya, Turkey

WO2015183228A1/ 2015

India

WO2015159305A1/ 2015

United States

US10675588B2/ 2016

Spain

ES2579161B2/2016

Bioactive oil based polyesteramide nanofibers

Nanofiber ultrafiltration unit

Aloe vera hybrid nanofibers

There has been intense research in the development of new and effective wound dressing materials in the area of wound care management. Chhabra et al. [110] have developed ZnO NPs electrospun into scaffolds of gelatin and polymethyl vinyl etheralt-maleic anhydride (PMVE/MA) for skin tissue engineering applications. The scaffold stability enhanced by using glutaraldehyde vapor cross-linking was found to exhibit enhanced antibacterial activity with optimum biocompatibility when tested

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using NIH3T3 cells. Further, the wound healing efficacy of nanofibrous scaffolds enriched with endothelial progenitor cells (EPCs) was assessed in vivo by topical application on wounds induced in mice. The scaffolds showed significant and sustained release of the active ingredient that resulted in faster wound healing finding a prominent application in skin tissue engineering. Zinc remains one of the metals having an important role in influencing the cellular activity specifically synthesis of DNA and RNA; regulating cell replication, differentiation and transcription. It also has a crucial role in regulating enzyme systems determining the fate of cell division and proliferation. They are well studied for their antimicrobial effect, ability to induce proliferation of fibroblasts and angiogenesis. Metal oxides of zinc and its nanoform have been widely studied for biomedical implications as wound dressings. In this prospect, ZnO NPs have been coupled with chitinchitosan/PVA nanofibers to develop a wound dressing material for diabetic ulcers. The promising antibacterial activity, faster healing rate and higher ROS generation boost cell migration and proliferation thereby hastening the wound healing process [168,169]. Similarly, Chen et al. [170] have fabricated a nanofibrous antibacterial wound dressing by incorporating ZnO NPs into gelatin nanofibers. The generation of Superoxide radicals by these nanocomposite nanofibers contributed at large for the antibacterial effect. By drastically deteriorating the cell wall of bacteria followed by the leakage of cell content, characteristic bactericidal effect was induced. In addition, the ROS generation was maximized by UV-irradiation accounting for higher level of bacterial cell death. Among many other metal oxide nanoparticles studied for their antimicrobial properties in enhancing the wound healing process, zirconium dioxide (ZrO2) has found clinical relevance owing to its potent antibacterial and antifungal properties. Using electrospinning technique, polyurethane (PU)-ZrO2 and PU-zeolite nanofiber mats were developed to study the antimicrobial property, cytotoxicity and biocompatibility. There was a significant antimicrobial action toward Candida albicans and S. aureus but demonstrated very low inhibitory effect toward Pseudomonas aeruginosa. By increasing the concentration of PU, ZrO2 and zeolite in the nanofibers, there was an increase in the antimicrobial activity, cell proliferation and viability making them the suitable wound dressing material with potent antimicrobial action [171]. Silver has been known as a potent antimicrobial agent whose contribution toward developing antibacterial nanofibrous blend has been used to enhance healing of diabetic wounds. For instance, Dong et al. [172] demonstrated the wound healing efficacy of electrospun nanofiber membrane loaded with AgNPs using Wistar rats. The study showed reduced inflammation accelerating faster wound healing with low cytotoxicity and long-term antibacterial action. Furthermore, there are studies on development of AgNPs-chitosan-oligosaccharide-poly(vinyl alcohol) nanofibers as wound dressings and their preclinical data reported. This nanofabricated assembly showed significant antibacterial activity in vitro and enhanced wound healing in vivo finding a potential place for clinical use as a bioactive wound dressing [173]. Nanofibers constituting chitosan/gelatin hybrid was reinforced by the integration of Fe3O4 nanoparticles for the development of nanofibrous dressings with high robustness and antibacterial function. Such deposition of nanoparticles on the nanofiber

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surface could lead to their release into the human system either bringing in beneficial effect or undesired consequence [174]. Hitherto, there are several challenges pertaining to the use of nanocomposite systems which shows the possibility of aggregation/deposition inside the human system could affect the sensitive organ and/or alter the biochemical pathways. Metal oxides like TiO2 exhibits antibacterial activity when subjected to UV irradiation. This approach help tailor the functional modality by altering the duration of exposure, intensity, wavelength, environmental factors, morphology of the nanoparticle, oxygenation and hydroxylation degree and ROS retention time. Furthermore, the photocatalytic antibacterial efficacy of TiO2 could be enhanced by doping with metals/metal oxides or nonmetals blended with biocompatible polymer-based nanofibers. In this line, a bilayer wound dressing was developed using TiO2-chitinchitosan nanofibrous layer superimposed onto a human adipose-derived ECM sheet. The rich hydroxyl and glucosidic groups of chitin-chitosan has the tendency to chelate titanium ions thereby forming a robust bond between the nanoparticles and nanofibers. Wound dressings of such type would not only exhibit potent antibacterial action toward E. coli and S. aureus but also help reduce the wound size up to 17% compared to 23% seen in normal wound dressing [175,176].

4.3 Drug delivery Electrospinning has enabled easy encapsulation of bioactive molecules/drugs and has prevented its loss facilitating sustained release to exhibit its maximum activity. The objective in delivering a predetermined dose of drug efficiently, specific to tissue/cell for a defined period of time has been achieved using electrospinning. They have also been applied to treat various diseases via oral and topical routes of administration of poorly soluble or insoluble drugs. Typically, drugs that undergo rapid metabolism, extensive degradation and with low solubility and instability (mostly antiinflammatory and antioxidant drugs) have been electrospun into fibers for sustained release [177]. The electrospun nanofibers mediated scaffolds are delivered either by viral or nonviral nucleic acids. Subsequently, immobilizing the drugs onto the nanofibers pose a great challenge and which is overcome by the most commonly adopted Entrapment method. In case of nanofibers, the drugs are entrapped through the crosslinking of the polymeric fibers or by an intermediate carriers attributing for core-sheath encapsulation. For instance, alginate when cross linked with calcium acts as common polymer for entrapment of drugs in bulk [178]. In core-sheath approach, polymers (PCL/PLGA) would encapsulate the drugs with BSA-dextran/chitosan core conferring stability [179]. Multidrug delivery, a novel approach comprising multiple drugs with or without similar remedial properties have been electrospun into desirable polymers. Wang et al. [180] have developed a novel controlled drug release system using Chitosan NPs-PCL polymer electrospun fibers. Further, small molecule rhodamine B and naproxen have been successfully loaded in the core-sheath region for sustained release. Similarly, MPEG-b-PLA micelles-chitosan-PEO has been electrospun with both hydrophobic and hydrophilic drugs viz. 5-FU and Ceftradine. This model

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exhibited the final release proportion of about 91.4% prior to continuous exposure for 109 h. HepG-2 cells treated with the micellar-loaded nanofibers showed 45.9% viability prior to 3 days of exposure with 21.6 μg 5-FU [181]. With the core-shell nanofibers composed of PVA crosslinked PAN, water- and organic solvent-soluble drugs namely diclofenac sodium and gentamicin sulfate have been loaded successfully at concentrations 1%–2% (w/w) PAN/GEN that enabled deep penetration of PAV/DS into the nanofibers [182]. Recently, Nagiah et al. [183] have developed high tensile tripolymeric triaxial electrospun fibrous matrix for delivering multiple drugs. The polymers used were PCL as core and PLGA as sheath with an intermediate gelatin layer. They have demonstrated the dual release of small molecule Rhodamine B and a model protein FITC-BSA incorporated in the sheath and the intermediate gelatin layers. They were able to support adhesion, migration and proliferation of mesenchymal stem cells. By this approach, the shrinkage encountered in conventional electrospinning technique was reduced with additive biomechanical stability [183]. Nanofibers with excellent physicobiochemical properties have been developed using electrospinning technique. They have been considered as an effective multipurpose drug delivery approach serving as a carrier of various therapeutic agents ranging from nanomedicines to macromolecules such as proteins and DNA. This has been achieved by using biocompatible and biodegradable polymers (as single component or blended ones) to fabricate nanofibers for desired applications and to overcome several pitfalls faced by conventional approaches [184]. Their application in anticancer therapeutics has helped overcome several challenges such as imperfect solubility, impermanence in the circulatory bloodstream, poor outreach in cancer cells, toxicity to normal cells, low working efficacy and excess elimination profile. These electrospun nanofibers ensure superior drug loading and delivery capability which in turn amplify the therapeutic efficacy and potency of loaded drugs resulting in target-specificity [185,186]. Recently, electrospun nanofibers have been widely used for effective delivery of poorly soluble anticancer drugs. It is achieved by crystallization of anticancer drugs within the fiber thereby reducing the dissociation rate of the drug in the biological environment. For instance, Paclitaxel, one of the water insoluble anticancer drugs was loaded with mesoporous hollow stannic oxide nanofiber and the rate of change of dissociation was evaluated in vitro. The dissolution study on drug integrated with stannic oxide nanofiber showed 8.34 times higher dissolution of Paclitaxel on a par with naked ones over a period of 5 min. Moreover, the cumulative release of pure Paclitaxel was found to be 16.77  2% whereas the one integrated with the nanofiber showed a high dissolution rate of 80  2.64% in 1 h [187]. In order to reduce the undesirable side effects and toxicity to the adjacent normal cells, nanofibers were fabricated to achieve target-specific and pH-mediated drug delivery. One such strategy is the surface modification of electrospun nanofibers using ligands capable of targeting specific receptors overexpressed on tumor cells driven by pH-mediated drug release. For instance, a chitosan-PLA solution was used to ensheathe graphene oxide-TiO2-Doxorubicin into nanocomplexes through electrospinning. Once inside the tumor environment prior to the encountering of acidic pH, there was a typical release of Doxorubicin. When the drug release study

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was carried out in physiological pH (7.4), the release pattern was found insignificant during a 200 h experiment. It was revealed that the protonation of –NH2 in Doxorubicin might have led to the disintegration of –H bond between the drug molecule and nanofibrous scaffold thereby resulting in higher drug release in the tumor environment [188]. Doxorubicin hydrochloride-loaded electrospun chitosan-cobalt ferriteTiO2 nanofibers were developed and the combinatorial effect of hyperthermia and chemotherapy against melanoma cancer (B16F10) cell lines was investigated [189]. The drug loading efficiency and in vitro drug release of the nanofibers were investigated at both physiological and acidic environments (7.4 and 5.3) by alternating the magnetic field. By applying three cycles of hyperthermia (15 min/24 h) to the drug loaded nanofibers, there was a slower release of the drug from the magnetic nanofibers at physiological pH than its release at acidic pH. Alongside, the release rate of the drug from the nanofibers under the influence of magnetic field was faster than that released from the nanofiber without the magnetic field. It was inferred that strong ionic interaction between chitosan and the drug resulted in slower release at physiological pH. On the other hand, the protonation of the amine group present in chitosan accelerated the deformation of chitosan-cobalt ferrite-TiO2 nanofibers thereby resulting in faster release of doxorubicin at tumor (acidic) environment. Moreover, a novel approach for breast cancer treatment was proposed by Vimala et al. [190] emphasizing the synergistic effect of chemo-photothermal targeted therapy and multifunctional drug delivery system. In continuity, folic acid-functionalized polyethylene glycol encapsulated with ZnO nanosheet was successfully fabricated, characterized and introduced as an effective drug delivery system. Prior to aminic functionalization, the drug Doxorubicin was loaded onto the sheets. From the study, it was revealed that the drug loaded delivery system exhibited sustained release of the drug triggered by heat and pH. Furthermore, the combined therapeutic approach toward breast cancer cells with maximum efficacy on a par with either chemotherapy or photothermal therapy was also reported. Moreover, in vivo studies pertaining to the toxicity induced in mice was found negligible confirming the biocompatibility. Although slight toxicity was observed; the histopathological and morphological analyses on kidney, lungs, brain, heart and testes showed no abnormalities. Electrospun nanofibers have also been used to effectively deliver antibiotics owing to their large surface area, tunable pore size, maximum antibiotic loading capacity and encapsulation efficiency. As these electrospun nanofibers were found capable of releasing the encapsulated drug at a higher rate, Khorshidi et al. [191] have developed an electrospun scaffold for loading and effectively delivering a second generation fluoroquinolone antibiotic, ciprofloxacin by ultrasound-mediated drug release.

5

Conclusion and future outlook

This chapter had thrown some light on several techniques, forms, and materials used in electrospinning for the generation of nanofiber materials and their potential biomedical applications. A paradigm shift toward fabrication of 3D architecture constitutes

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one of the key elements for clinical applications. This is quite different from the conventional electrospun fiber mats produced by direct deposition of fibers onto the substrate. With 3D intervention, the solubility, low porosity, swelling and collapsing of the 2D fibers hindering cell infiltration shall be overcome. As 3D nanofiber mimics typical in vivo settings for cells to adhere, proliferate and differentiate within the matrix, their applications in tissue regeneration are the most promising. These advancements have been achieved by thorough understanding of the spinneret, its control and mechanism involved in the electrospinning for a better control conforming various applications. It is this understanding and the need of the hour to involve diverse materials including polymers, small molecules, nanoparticles, colloids to be electrospun in to fibers. Apart from innovation, technology transfer, expansion and commercialization of postelectrospinning product determines the success of any technology (Table 5). Table 5 Commercial products developed using nanofiber technology. Product

Specification

Manufacturer

Country

Respiratory mask

Highly cellulose nanofiber-based material with disposable filter cartridge capable of removing virus-size nanoparticles and high breathability Excellent filtering efficacy (94%). Reusable. Nanofibers with diameter of 100– 500 nm arranged orthogonally. Water resistant

Queensland University of Technology

Australia

Kim Il-Doo Research Institute (A start-up company) https:// statnano.com/news/ 67527/Nanofiberbased-Face-MaskPreserves-Its-Filtering%E2%80%8EFunctionand-Sturdiness-After20-Washes Yamashin-filter corp. http://www.yamashinfilter.co.jp/eng/

Korea

Metamasks https:// www.metamasks.com/

New Zealand

Nano-filter

YAMASHIN Nano Filter

Nanococo-carbon filter

Fibers developed from synthetic polymers. The 3D structure of the extremely thin nanofibers attributes for super-high trapping properties They are made of natural, organic and sustainable nanofibers. It comprises coconut shell carbon and nanofiber matrix producing

Japan

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Table 5 Continued Product

Nanopoli Nanofiber mask

Nanohack (face mask)

Antiradiation fabric for pregnant women

Specification