Graphene for Transparent Conductors

Qingbin Zheng • Jang-Kyo Kim

Graphene for Transparent Conductors

Synthesis, Properties, and Applications

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ISBN 978-1-4939-2768-5 ISBN 978-1-4939-2769-2 (eBook)DOI 10.1007/978-1-4939-2769-2

Library of Congress Control Number: 2015938323

Springer New York Heidelberg Dordrecht London© Springer Science+Business Media New York 2015This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed.The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use.The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made.

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Qingbin ZhengLeibniz Institute of Polymer Research DresdenDresdenGermany

Jang-Kyo KimDepartment of Mechanical and Aerospace EngineeringThe Hong Kong University of Science and TechnologyKowloonHong Kong SAR

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Preface

Transparent conductors (TCs) have been used in a wide variety of optoelectronic and photovoltaic devices, such as liquid crystal displays (LCDs), solar cells, optical communication devices, and solid-state lighting. Thin films made from indium tin oxide (ITO) have been the dominant source of TCs, and demand of indium from the explosive growth of LCD computer monitors, television sets, and smart phones has risen rapidly in recent years, which now account for 50 % of indium consumption. In 2002, the price was US$ 94 per kg, and it rose to over US$ 1000 per kg recently, a 1000 % increase in 10 years. Graphene, a two-dimensional monolayer of sp2-bonded carbon atoms, has attracted significant interests recently because of the unique transport properties. Due to the high optical transmittance and electrical conductivity, thin film electrodes made from graphene have been considered an ideal candidate to replace the currently used expensive ITO films. Compared with the ITO films, graphene films have high mechanical strength, flexibility, chemical stability, and are much cheaper to produce.

A key to success in such applications is to develop methods to produce large-size graphene sheets with high yields and deposit them onto a substrate layer-by-layer in an orderly manner. The graphene sheets in current study for the fabrication of TCs are very small, mostly with an area of hundreds of square micrometers. The small graphene sheets result in high intersheet contact resistance due to a large amount of intersheet junctions. To reduce the number of intersheet tunneling barriers, production of inherently large-size graphene sheets are highly desirable. Although mechanical cleavage of graphite was shown to prepare high quality graphene with a millimeter size, the yield of this method is extremely low, being unsuitable for mass production. Alternatively, graphitization of Si-terminated SiC (0001) in an argon atmosphere could produce monolayer graphene films with a domain size of several tens of micrometers. However, the graphene obtained thereby was difficult to transfer to other substrates and the yield was very low. The chemical vapor deposition (CVD) technique has been extensively explored to grow extremely large-area graphene on Ni films or Cu foils. This technique usually requires specific substrate materials that have to be removed chemically after the growth of graphene. The high cost of single crystal substrates and the ultrahigh vacuum conditions necessary to maintain for the CVD growth significantly limit the use of the CVD method for large-scale applications. In spite of the significant

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progress for CVD-grown graphene achieved so far, these important challenges must be overcome before the industry applications.

Owing to the scalability of production and the convenience in processing, graphene oxide (GO) has been considered an important precursor for the fabrication of TCs. GO sheets are hydrophilic and can produce stable and homogeneous colloidal suspensions in aqueous and various polar organic solvents due to the electrostatic repulsion between the negatively charged GO sheets. These GO dispersions are easy to be processed to produce TCs on a substrate. Transparent conducting films (TCFs) containing GO or chemically reduced GO sheets have been deposited via several well-established techniques, including spin or spray coating, transfer printing, dip coating, electrophoretic deposition, and the Langmuir–Blodgett (L–B) assembly, followed by chemical reduction and/or thermal annealing.

While there are some books that specialize in fabrication processes and properties of graphene and GO, very few books are available specifically dealing with the following topics for their application in transparent conductors: (i) how to produce TCs by using CVD grown graphene, (ii) how to synthesize GO with different size and control their surface functionalities to enhance the electrical conductivity, (iii) how to incorporate these nanostructured materials into thin films with layered structure, and (iv) how to improve the conductivity and transparency. In light of the authors’ experiences on graphene fabrication and application for TCs in the past few years, this book is aimed to provide a comprehensive overview of traditional and novel techniques in producing and functionalizing graphene for highly conductive transparent thin films. It will offer a systematic presentation of the principles, theories, and technical practices behind the structure–property relationship of the thin films, which we believe to be the key for the development of high-performance TCs.

The book is intended primarily for an audience of graduate students, research scientists, and professors in the area of carbon materials, transparent conductors, and related fields, as well as to professionals from the electronic and chemical manufacturing industries. Nanotechnology, as an emerging new subject, has been established as a major in postgraduate level in many universities and research institutes. This book would be well suited as a textbook for an intermediate level class in nanotechnology and/or materials science and engineering as part of such a program or as a stand-alone course. It will be accessible equally to readers with either science or engineering background. At the same time, the unique perspectives provided in the applications of graphene as TCs will serve as a useful guide for design and fabrication of these thin film materials for specific applications.

The authors are grateful for the assistance, discussion, and encouragement offered during the preparation of this book by past and current colleagues and friends, including Dr. B Zhang, Prof. QZ Xue, Prof. ZG Li, Prof. PC Ma, Prof. JH Yang, Dr. J Li, Dr. ZD Huang, Dr. Y Geng, Dr. X Shen, Dr. XY Lin, as well as the research group members who produced the research outputs quoted in this book. Dr. Zheng was partly supported by the Hong Kong University of Science & Technology (HKUST) Postgraduate Studentships (PGS), Finetex-HKUST R & D Center at HKUST, Research Grant Council (RGC) of Hong Kong, Shanghai Pujiang

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Talent Project, and the Alexander von Humboldt Foundation during the course of completing this book. The authors specially thank Prof. E. Mäder, Dr. SL Gao, Dr. HS Qi, Dr. C Scheffler, and Dr. U Gohs at Leibniz Institute of Polymer Research Dresden (IPF) for stimulating discussion on this book. The authors are also grateful to Drs. David Packer, Ho Ying Fan, and Kanchan Kumari at Springer for their kind reviewing and processing of our manuscript.

Dr. Qingbin Zheng, Prof. Jang-Kyo Kim

Clear Water Bay, Hong KongJanuary, 2014

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Contents

1 Introduction to Transparent Conductive Films ........................................ 11.1 Applications of Transparent Conductive Films (TCFs) ........................ 11.2 Transparent conducting oxides (TCOs) ................................................ 5

1.2.1 ITO ............................................................................................. 51.2.2 ITO Substitutes .......................................................................... 7

1.3 Transparent Conducting Polymers ........................................................ 91.3.1 Polythiophene (PT) .................................................................... 91.3.2 Poly(para-phenylene vinylene) (PPV) ....................................... 101.3.3 Polypyrrole (PPy) ...................................................................... 141.3.4 Polyaniline (PANI) .................................................................... 141.3.5 Poly(3,4-ethylenedioxythiophene) (PEDOT) ............................ 15

1.4 Transparent Conducting Metals ............................................................ 171.4.1 Metal Nanogrids ........................................................................ 171.4.2 Metal Nanowires ....................................................................... 17

1.5 Transparent Conducting Carbon ........................................................... 181.5.1 Carbon Nanotubes (CNTs) ........................................................ 181.5.2 Graphene .................................................................................... 19

References ...................................................................................................... 21

2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide .................................................................................... 292.1 Introduction ........................................................................................... 292.2 Synthesis Methods of Graphene and Graphene Oxide ......................... 31

2.2.1 Mechanical Cleavage ................................................................ 312.2.2 Epitaxial Growth ....................................................................... 322.2.3 Chemical Vapor Deposition (CVD) .......................................... 322.2.4 Total Organic Synthesis............................................................. 362.2.5 Chemical Methods .................................................................... 38

2.3 Preparation of Large-Size GO ............................................................... 432.3.1 Mild Oxidation and Sonication ................................................. 432.3.2 Edge-Selective Functionalization of Graphite (EFG) ............... 45

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2.3.3 Elimination of Sonication ....................................................... 452.3.4 Size Separation Methods ......................................................... 47

2.4 Structures of Graphene and GO .......................................................... 512.5 Properties of Graphene and GO .......................................................... 55

2.5.1 Electrical/Electronic Properties ............................................... 552.5.2 Thermal Properties .................................................................. 632.5.3 Optical Properties .................................................................... 672.5.4 Mechanical Properties ............................................................. 68

2.6 Common Tools for Characterization of Graphene and Its Derivatives ............................................................................... 732.6.1 Atomic Force Microscopy ....................................................... 732.6.2 Scanning Electron Microscopy (SEM) ................................... 742.6.3 Transmission Electron Microscopy (TEM) ............................. 742.6.4 Scanning Tunneling Microscope (STM) ................................. 762.6.5 Raman Spectroscopy ............................................................... 79

References .................................................................................................... 81

3 Fabrication of Graphene-Based Transparent Conducting Thin Films ................................................................................................... 953.1 Introduction ......................................................................................... 953.2 CVD-Grown Graphene-Based TCs ..................................................... 95

3.2.1 Etching Method ....................................................................... 953.2.2 Stamping Method .................................................................... 973.2.3 Thermal Release Method ........................................................ 1013.2.4 Photoresist Method.................................................................. 1013.2.5 Roll-to-Roll Transfer Method ................................................. 1033.2.6 Challenges of Transferred CVD-Grown Graphene for TCs ... 105

3.3 Fabrication of GO-based TCs ............................................................. 1053.3.1 Electrophoretic Deposition...................................................... 1093.3.2 Spin Coating ............................................................................ 1103.3.3 Spray Coating .......................................................................... 1113.3.4 Dip Coating ............................................................................. 1123.3.5 Transfer Printing of GO Films ................................................ 1143.3.6 Langmuir–Blodgett Method .................................................... 1143.3.7 Rod Coating ............................................................................ 1173.3.8 Inkjet Printing.......................................................................... 117

References .................................................................................................... 119

4 Improvement of Electrical Conductivity and Transparency ................. 1234.1 Introduction ......................................................................................... 1234.2 Chemical Doping ................................................................................ 124

4.2.1 Chemical Doping of Carbon Materials ................................... 124 4.2.2 Chemical Doping of Graphene................................................ 1274.2.3 Stability of Doped Graphene Films......................................... 136

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4.3 Hybridization ....................................................................................... 1384.3.1 Hybridization with CNTs ........................................................ 1384.3.2 Hybridization with Metal Wires ............................................. 1514.3.3 Hybridization with Metal Grids .............................................. 157

4.4 Using UL-GO ...................................................................................... 1584.4.1 Solution Casting of UL-GO .................................................... 1584.4.2 Dip Coating of UL-GO ........................................................... 1594.4.3 L–B Assembly of UL-GO ....................................................... 161

References .................................................................................................... 173

5 Application of Graphene-Based Transparent Conductors (TCs) .......... 1795.1 Introduction ......................................................................................... 1795.2 Touch Screen ....................................................................................... 1795.3 Displays ............................................................................................... 181

5.3.1 Liquid Crystal Displays .......................................................... 1815.3.2 Light-Emitting Diodes ............................................................ 181

5.4 Solar Cells ........................................................................................... 1845.5 Transistors ........................................................................................... 1845.6 Other Applications .............................................................................. 187

5.6.1 Electromagnetic Interference (EMI) Shielding ...................... 1875.6.2 Functional Glasses .................................................................. 1905.6.3 Transparent Loudspeakers ...................................................... 1925.6.4 Transparent Heaters ................................................................ 1925.6.5 Transparent Actuators ............................................................. 1945.6.6 Transparent Sensors ................................................................ 1965.6.7 Transparent Supercapacitors ................................................... 198

References .................................................................................................... 200

6 Conclusions and Perspectives ................................................................... 205References .................................................................................................... 210

Index .................................................................................................................. 215

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Acronyms and Symbols

Acronyms

AFM Atomic force microscopyAR-XPS Angle-resolved X-ray photoelectron spectroscopyAZO Aluminum-doped zinc oxideBA BenzylamineBDM Bubble deposition methodBHJ Bulk-heterojunctionBSE Backscattered electronsCB Carbonaceous byproductsCCFT Cold cathode fluorescent lampCGOWs Concentrated graphene oxide wrinklesCMOS Complementary metal–oxide–semiconductorCNS Carbon nanoscrollCNT Carbon nanotubeCRT Cathode ray tubesC-rUL-GO Chemically doped, reduced ultra-large graphene oxideCSA Chlorosulfonic acidCVD Chemical vapor depositionDC Direct currentDCB o-dichlorobenzeneDCE DichloroethaneDEG Diethylene glycolDGU Density gradient ultracentrifugationDI DeionizedDMA DimethylacetamideDMF N, N-DimethylformDMSO Dimethyl sufoxideDSLR Digital single lens reflexEBA Ethylbenzoic acidED Electron diffractionEMI Electromagnetic interference

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EPD Electrophoretic depositionEFG Edge selective functionalization of graphiteFDP Flat panel displayFLG Few-layer-grapheneFTIR Fourier transform infraredFTO Fluorine tin oxideFTS Fluoroalkyl trichlorosilaneFWHM Full width at half maximumGIC Graphite intercalation compoundsGNP Graphite nanoplateletGNR Graphene nano ribbonsGO Graphene oxideGOP Graphene oxide paperGOW Graphene oxide wrinklesGP Graphene paperHF Hydrogen florideHI Hydrogen iodineHNO3 Nitric acidHOPG Highly ordered pyrolytic graphiteHRTEM High-resolution transmission electron microscopyICO Indium-doped cadmium-oxideIPA IsopropanolITO Indium tin oxideL–B Langmuir–BlodgettLbL Layer-by-layerLCDs Liquid crystal displaysLED Light-emitting diodeL-GO Large graphene oxideMS Magnetron sputteringMSA Methanesulfonic acidMSD Magnetron sputtering depositionMDs Molecular dynamicsMLG Multilayer grapheneMMs Molecular mechanicsMWCNT Multi-walled carbon nanotubeMQW Multiple quantum wellsNGO Nanosized graphene oxide(NH4)2S2O8 Ammonium persulfateNMP N-methyl-2-pyrrolidoneNPs NanoparticlesNWs NanowiresOLEDs Organic light emitting diodesOPVs Organic photovoltaicsPAHs Polyacrylic hydrocarbons

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PANI PolyanilinePDMS PolydimethylsiloxanePDP Plasma display panelPEDOT/PSS Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)PEI PolyethyleneiminePEG Polyethylene glycolPET Polyethylene terephthalatePLD Pulsed laser depositionPMMA Poly(methyl methacrylate)PmPV Poly(m-phenylene vinylene-co-2,5-dioctyloxy-pphenylene vinylene)P2O5 Phosphorus pentoxidePOPT Poly(2,5-dioctyloxy-1,4-phenylene-alt-2,5-thienylene)PPA Polyphosphoric acidPPV Poly(para-phenylene vinylene)PPy PolypyrrolePTs PolythiophenesPVDF Polyvinylidene fluoridePVP Poly(4-vinylphenol)RAM Radar absorbing materialsrGO Reduced graphene oxideRF Radio frequencyRP Rear-projectionSAED Selected area electron diffractionSE Secondary electronsSEM Scanning electron microscopeSDS Sodium dodecyl sulfateS-GO Small graphene oxideSRL Self-release layerSPL Sound pressure levelSTM Scanning tunneling microscopeSWCNT Single-walled carbon nanotubeTCs Transparent conductorsTCFs Transparent conductive filmsTCOs Transparent conducting oxidesTHD Harmonic distortionToF-SIMS Time-of-flight secondary ion mass spectrometryUL-GO Ultralarge graphene oxideUHV Ultra-high vacuumUV-Vis Ultraviolet–visible spectroscopyVAPE Vacuum arc plasma depositionVL-GO Very large graphene oxideWHM Width-at-half-maximumXPS X-ray photoelectron spectroscopyXRD X-ray diffraction

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Symbols

Rs Sheet resistance (Ω/sq)t Thickness (nm)T Transparency (%)U Strain energy (kcal/mol)Z Impedance of free space (= 377 Ω)ε Strain (%)

/DC Opσ σ DC to optical conductivity ratio