Graphene for flexible lighting and displays 9780081024836, 0081024835, 9780081024829

Table of contents :
Content: 1. Introduction2. Structure and properties of graphene 3. Preparation of graphene electrod4. Graphene doping for electrode application 5. Technical issues and integration scheme for graphene electrode OLED panels 6. Graphene-based buffer layers for light-emitting diodes 7. Graphene-based Quantum Dot Emitters for Light-Emitting Diodes8. Graphene-Based composite emitter 9. Stretchable graphene electrodes 10. Conclusions and outlook

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Graphene for Flexible Lighting and Displays

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Woodhead Publishing Series in Electronic and Optical Materials

Graphene for Flexible Lighting and Displays Edited by

Tae-Woo Lee Department of Materials Science and Engineering, 1 Gwanak-ro, Gwanak-gu, Seoul, Republic of Korea

Woodhead Publishing is an imprint of Elsevier The Officers’ Mess Business Centre, Royston Road, Duxford, CB22 4QH, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom Copyright © 2020 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-08-102482-9 For information on all Woodhead Publishing publications visit our website at https://www.elsevier.com/books-and-journals Publisher: Matthew Deans Acquisition Editor: Kayla Dos Santos Editorial Project Manager: Peter Adamson Production Project Manager: Joy Christel Neumarin Honest Thangiah Cover Designer: Victoria Pearson

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Contents

Contributors About the editor Preface Acknowledgments

1.

2.

3.

4.

Introduction Tae-Woo Lee and Sung-Joo Kwon References Structure and properties of graphene Yong Seok Choi, Je Min Yoo and Byung Hee Hong 2.1 Structure of graphene 2.2 Synthesis of graphene 2.3 Electronic band structure of graphene 2.4 Optical properties of graphene 2.5 Electrical properties of graphene 2.6 Mechanical properties of graphene References Further reading

vii ix xi xiii

1 2 5 5 8 11 14 16 22 23 26

Preparation of graphene electrode Wencai Ren 3.1 Solution casting of graphene oxide 3.2 Transfer methods of CVD grown graphene References

27

Graphene doping for electrode application Amirhossein Hasani and Soo Young Kim 4.1 Chemical doping of graphene 4.2 Metal oxide doping of graphene 4.3 Stability of the doped graphene electrodes Acknowledgments References

59

27 33 52

59 65 67 70 70

vi

5.

6.

7.

8.

9.

10.

Contents

Technical issues and integration scheme for graphene electrode OLED panels Jaehyun Moon, Jin-Wook Shin, Hyunsu Cho, Jun-Han Han, Byoung-Hwa Kwon, Jeong-Ik Lee and Nam Sung Cho 5.1 Introduction 5.2 Graphene preparation for OLED applications 5.3 Technical issues of OLEDs having graphene film electrodes 5.4 Integration schemes for realizing large area graphene electrode OLED panels 5.5 Summary and future outlook Acknowledgments References Further reading Graphene-based buffer layers for light-emitting diodes Quyet Van Le and Soo Young Kim 6.1 Introduction 6.2 Graphene oxide buffer layer 6.3 Graphene-based composite buffer layer 6.4 Conclusion References

73

73 74 74 87 94 95 95 98 99 99 99 110 113 113

Graphene-based quantum dot emitters for light-emitting diodes Park Minsu, Yoon Hyewon and Jeon Seokwoo 7.1 Introduction to graphene quantum dots 7.2 Synthetic strategies for GQDs 7.3 Toward highly efficient fluorescence from GQDs 7.4 Lighting applications of GQDs 7.5 Summary and outlooks References

117

Graphene-based composite emitter Hong Hee Kim and Won Kook Choi 8.1 Grapheneemetal/metal oxide hybrid composite 8.2 Graphene-based composite emitter References

151

Stretchable graphene electrodes Shuyan Qi and Nan Liu 9.1 Introduction 9.2 Preparation of stretchable graphene electrodes 9.3 Applications of stretchable graphene electrodes 9.4 Summary and outlook References

175

Conclusions and outlook References

205 207

Index

209

117 121 133 138 143 145

151 152 170

175 177 191 199 201

Contributors

Hyunsu Cho Flexible Device Research Group, Electronics and Telecommunications Research Institute (ETRI), Daejeon, Republic of Korea Nam Sung Cho Flexible Device Research Group, Electronics and Telecommunications Research Institute (ETRI), Daejeon, Republic of Korea Yong Seok Choi Department of Chemistry, College of Natural Sciences, Seoul National University, Gwanak-Gu, Seoul, Republic of Korea Won Kook Choi Center for Optoelectronic Materials and Devices, Post-Si Semiconductor Institute, Korea Institute of Science and Technology (KIST), Seoul, Korea Jun-Han Han Flexible Device Research Group, Electronics and Telecommunications Research Institute (ETRI), Daejeon, Republic of Korea Amirhossein Hasani School of Chemical Engineering and Materials Science Chung-Ang University, Seoul, Republic of Korea Byung Hee Hong Department of Chemistry, College of Natural Sciences, Seoul National University, Gwanak-Gu, Seoul, Republic of Korea; Program in Nano Science and Technology, Graduate School of Convergence Science and Technology, Seoul National University, Yeongtong-Gu, Suwon-Si, Republic of Korea Yoon Hyewon Department of Materials Science and Engineering, KAIST Institute for the NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Republic of Korea Hong Hee Kim Center for Optoelectronic Materials and Devices, Post-Si Semiconductor Institute, Korea Institute of Science and Technology (KIST), Seoul, Korea Soo Young Kim Department of Materials Science and Engineering, Korea University, Seongbuk-gu, Seoul, Republic of Korea Sung-Joo Kwon Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyungbuk, Republic of Korea Byoung-Hwa Kwon Flexible Device Research Group, Electronics and Telecommunications Research Institute (ETRI), Daejeon, Republic of Korea

viii

Contributors

Tae-Woo Lee Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro Gwanak-gu, Seoul, Republic of Korea Jeong-Ik Lee Flexible Device Research Group, Electronics and Telecommunications Research Institute (ETRI), Daejeon, Republic of Korea Nan Liu

College of Chemistry, Beijing Normal University, Beijing, P.R., China

Park Minsu Department of Materials Science and Engineering, KAIST Institute for the NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Republic of Korea Jaehyun Moon Flexible Device Research Group, Electronics and Telecommunications Research Institute (ETRI), Daejeon, Republic of Korea Shuyan Qi

College of Chemistry, Beijing Normal University, Beijing, P.R., China

Wencai Ren Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, Liaoning, China Jeon Seokwoo Department of Materials Science and Engineering, KAIST Institute for the NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Republic of Korea Jin-Wook Shin Flexible Device Research Group, Electronics and Telecommunications Research Institute (ETRI), Daejeon, Republic of Korea Quyet Van Le Nang, Vietnam

Institute of Research and Development, Duy Tan University, Da

Je Min Yoo Department of Chemistry, College of Natural Sciences, Seoul National University, Gwanak-Gu, Seoul, Republic of Korea

About the editor

Dr. Tae-Woo Lee is a professor in the Department of Materials Science and Engineering at Seoul National University, Korea. He received his PhD in Chemical Engineering from Korea Advanced Institute of Science and Technology, Korea, in 2002. He joined Bell Laboratories, Lucent Technologies, USA, as a postdoctoral researcher in 2002 and then worked at Samsung Advanced Institute of Technology as a member of research staff (2003e08). He was an assistant and associate professor in the department of materials science and engineering at Pohang University of Science and Technology, Korea until August 2016. He received a prestigious Korea Young Scientist Award from the President of Korea in 2008 and the Scientist of the Month Award from the Ministry of Science, ICT and Future Planning in 2013. He is author and coauthor of 205 papers in prestigious journals including Science, Nature Photonics, Science Advances, Nature Communications, PNAS, Energy and Environmental Science, Angewandte Chemie, Advanced Materials, and ACS Nano. He is also the inventor or coinventor of 375 patented technologies (187 Korean patents and 188 international patents). He currently serves as an editorial board member on the Journals of FlatChem (Elsevier), EcoMat (Wiley), and Semiconductor Science and Technology (IOP). His research focuses on organic, organiceinorganic hybrid perovskite and carbon materials, and their applications to flexible electronics, printed electronics, displays, solid-state lightings, solar energy conversion devices, and bioinspired neuromorphic devices. His work in graphene-related fields includes environmentally benign synthesis of graphene from solid carbon sources such as inexpensive polymers and carbon wastes, chemical doping of graphene for tuning of its work function and conductivity, preparation of graphene quantum dots, and fabrication of efficient organic and halide perovskite light-emitting diodes using graphene electrodes.

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Preface

The demand for lighting and display technologies is driving research to diversify the forms of devices. Future lighting and displays should be bendable, foldable, and stretchable to satisfy consumers’ desire for convenience and efficient use of space. Flexible components of lighting and display devices should be mechanically tolerant of repeated severe flexion. However, conventional light-emitting diodes (LEDs) for lighting and displays are mostly fabricated on a brittle transparent conducting oxide electrode (e.g., indium tin oxide [ITO]), which has poor tolerance to mechanical strain. Therefore, alternative flexible transparent conducting materials have been evaluated to replace ITO. Graphene is a two-dimensional single-atom-thick sheet of carbon atoms in an sp2-bonded hexagonal configuration. Graphene’s unique structure yields excellent electrical and optical properties as well as mechanical robustness, so it is regarded as a strong candidate for use as a flexible electrode in lighting and displays. However, pristine graphene has several characteristics that limit its practical applications in flexible self-emissive LEDs. Its sheet resistance Rs is too high, and its work function (WF) is too low for graphene to be effective as an anode in LEDs. Chemical doping of graphene can control Rs and WF, so the charge injection from graphene electrode to overlying layers in LEDs can be significantly improved. As a result of appropriate chemical doping, especially with organic and polymeric dopants, the luminous properties of organic light-emitting diodes (OLEDs) based on graphene electrodes have been increased to be comparable to those of OLEDs that use ITO electrodes. The doping can also make graphene very stable against moisture, organic solvents, and acids. Graphene-based materials can be used as an interfacial layer to improve the charge injection in LEDs. Pristine graphene has no bandgap, but chemical functionalization (e.g., oxidation, hydrogenation) can induce one and provide an intermediate step for charge injection, so the luminous properties of LEDs can be improved. Graphene quantum dots (QDs) can also themselves be light emitters. The graphene QDs have advantages (e.g., nontoxicity, high chemical stability, high carrier mobility) over inorganic QDs. Graphene can also be stretchable, so it can be used in stretchable electronics and displays. To date, several structural modifications or composite with other stretchable conducting materials have increased the stretchability of graphene and allowed it to be used in stretchable electronics.

xii

Preface

This book will cover the fundamental electrical, optical, and mechanical properties of graphene; the preparation of pristine graphene, doped graphene, graphene-derived interfacial and graphene QD emitting materials and their composites; and treatments to modify its electrical properties by adsorbed molecules and deposited films. Then the book presents the use of flexible or stretchable electrodes with graphene or its composites in various LEDs for lighting and displays (e.g., OLEDs, inorganic LEDs, QD-LEDs, and halide perovskite LEDs). It will also describe the use of graphenederived materials as interfacial buffer layers or as light-emitting layers in LEDs. This book is written by leading experts who are working on graphene-based materials and optoelectronic devices. It provides in-depth information on use of graphene in light-emitting devices. The overall goal of this book is to provide comprehensive information about fundamental properties of graphene; on methods to synthesize graphene; on techniques to prepare graphene electrodes and composite electrodes; on methods to dope graphene electrodes; and on applications of graphene-based flexible electrodes, interfacial buffer layers, nanoscale emitters, and graphene-based stretchable electrodes. The ultimate objective is to inspire further research on practical optoelectronics applications of graphene. This book will be of interest to the large community of researchers who are working on applying graphene in various electronic and optoelectronic devices and may stimulate research to develop practical uses of graphene sheets in next-generation displays and lightings. The book will additionally provide future prospects and suggest further directions for research on graphene-based next-generation displays and lightings. Therefore, this book will be helpful for students, professors, researchers, and engineers who work on graphene or graphene-derived materials or on graphene-based displays and lighting technology.

Acknowledgments

The publication of this book is possible only because of the support of many people. I start by thanking the authors who contributed to this book although they are very busy with their research projects and education. I would like to say a big thanks to all the members of my research group (Printed Nano-Electronics and Energy Laboratories) and especially the students who performed research related to this field: Dr. Tae-Hee Han, Dr. Hong-Kyu Seo, and Dr. Sung-Joo Kwon. I sincerely thank to Kayla Dos Santos, an Acquisitions Editor in charge of the field of Electronic, Magnetic, and Optical Materials in Elsevier’s Science and Technology Books. She encouraged me to write this book and monitored its progress to ensure that it was published on time. I also thank Dr. Peter W. Adamson, Editorial Project Manager in Elsevier, who handled manuscript and cover design and other related tasks of book editing. Finally, I would like to express my sincere thanks and gratitude to my family members including my wife, Mun-Hee, and my daughters, Seohyun and Chaehyun, for their immense understanding, support, and encouragement that were crucial and invaluable for successful achievement of research projects and this book.

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Introduction 1

2

1

Tae-Woo Lee , Sung-Joo Kwon 1 Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro Gwanak-gu, Seoul, Republic of Korea; 2Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyungbuk, Republic of Korea

Humans have always had need for lighting and have recently developed a reliance on information displays (e.g., computer screens) based on lighting. The invention of the incandescent light bulb drove a revolutionary change in human life by freeing us from dependence on fire. However, this technology has poor luminous efficiency and severe heat generation, so it is about to be supplanted by light-emitting diodes (LEDs). Significant advance in LED research has achieved blue LEDs and white LEDs with luminous efficiency >100 lm/W [1]. Therefore, LEDs have diverse applications, including use in displays to visualize information for human interpretation. Advances in information technology have increased humans’ need to share information, so emphasis on display technology has increased. Organic lighteemitting diodes (OLEDs) are also promising next-generation light sources. OLEDs have advantages such as light weight, easy color tunability, designable form, and suitability for large-area fabrication, so they also have applications in displays [2]. Alternative emitters (e.g., quantum dots (QDs), perovskite) are being evaluated for use in LEDs, and their emissive properties have been continuously improved. The luminous properties of display and lighting technology have been advanced remarkably, and the forms of lighting and display devices have been diversified in response to the demands of industry. To satisfy future requirements, lighting and displays should be bendable, foldable, and stretchable. Conventional lighting and displays are mostly fabricated on a transparent electrode that is formed from a conducting oxide (e.g., indium tin oxide (ITO)), which is brittle. To achieve flexible lighting and displays, these brittle elements must be replaced with flexible components. Therefore, many flexible transparent conductors (e.g., graphene, carbon nanotubes, metal nanowires, and conducting polymers) have been evaluated as materials to replace ITO [3e6]. Graphene has remarkable electrical, optical, and mechanical properties and, therefore, has good potential as a flexible electrode to replace ITO in lighting and display devices (Fig. 1.1). However, pristine graphene has high sheet resistance (RS) and low work function (WF), so OLEDs with graphene electrode have poor luminous properties [3]. Chemical doping of graphene can modify Rs and the WF, and thereby substantially improve the charge injection from the graphene electrode to overlying layers. By this approach, the luminous properties of OLEDs based on graphene electrodes have been increased to levels comparable with those of OLEDs that use ITO electrodes [3,7,8].

Graphene for Flexible Lighting and Displays. https://doi.org/10.1016/B978-0-08-102482-9.00001-0 Copyright © 2020 Elsevier Ltd. All rights reserved.

2

Graphene for Flexible Lighting and Displays

Flexible lighting

Flexible display

Figure 1.1 Schematic drawings of future lighting and displays using graphene electrodes.

Further modifications in the properties of graphene are possible. Insertion of a graphene-based interfacial layer can improve the charge injection and increase the luminous properties of LEDs that use graphene. Chemical functionalization (e.g., oxidation, hydrogenation) can induce a bandgap in the electronic structure of graphene [9,10]; the bandgap can provide an intermediate step for charge injection, so the luminous properties of LEDs can be improved. Graphene can also be used as an emitting layer. After chemical functionalization by surface passivation, a graphene QD emitter is less toxic, more chemically stable, and has higher carrier mobility than conventional inorganic QDs [11]. Ideally, flexible electronics should also be stretchable. Graphene has outstanding mechanical flexibility, but strong in-plane stiffness (340 N/m) and Young’s modulus (0.5 TPa), which impede the use of graphene in stretchable electronics that requires stretchability above 10% [12]. Mechanical stress cannot be dissipated in the graphene lattice because of the strong bonding between the carbon atoms. For instance, CVD grown graphene on elastic substrate lose its electrical conductivity under 6% of mechanical strain [13]. Several structural modifications or combinations with other stretchable conducting materials have improved the stretchability of graphene and have been used in stretchable electronics [12,14e16]. In this book, we first introduce the electrical, optical, and mechanical properties of graphene and present the preparation of graphene and treatments to modify its electrical properties. Then we examine the use of flexible or stretchable electrodes with graphene in various lighting and displays (e.g., LEDs, OLEDs, QD-LEDs, perovskite LEDs). We also review the use of graphene in interfacial materials or emitting materials of LEDs.

References [1] Y. Narukawa, J. Narita, T. Sakamoto, K. Deguchi, T. Yamada, T. Mukai, Ultra-high efficiency white light emitting diodes, Jpn. J. Appl. Phys. 45 (41) (2006) L1084eL1086. [2] M.-H. Park, T.-H. Han, Y.-H. Kim, S.-H. Jeong, Y. Lee, H.-K. Seo, H. Cho, T.-W. Lee, Flexible organic light-emitting diodes for solid-state lighting, J. Photonics Energy 5 (1) (2015) 053599.

Introduction

3

[3] T.-H. Han, Y. Lee, M.-R. Choi, S.-H. Woo, S.-H. Bae, B.H. Hong, J.-H. Ahn, T.-W. Lee, Extremely efficient flexible organic light-emitting diodes with modified graphene anode, Nat. Photonics 6 (2012) 105e110. [4] E.C.-W. Ou, L. Hu, G.C.R. Raymond, O.K. Soo, J. Pan, Z. Zheng, Y. Park, D. Hecht, G. Irvin, P. Drzaic, G. Gruner, Surface-modified nanotube anodes for high performance organic light-emitting diode, ACS Nano 3 (8) (2009) 2258e2264. [5] Z. Yu, Q. Zhang, L. Li, Q. Chen, X. Niu, J. Liu, Q. Pei, Highly flexible silver nanowire electrodes for shape-memory polymer light-emitting diodes, Adv. Mater. 23 (5) (2011) 664e668. [6] M. Cai, Z. Ye, T. Xiao, R. Liu, Y. Chen, R.W. Mayer, R. Biswas, K.-M. Ho, R. Shinar, J. Shinar, Extremely efficient indiumetin-oxide-free green phosphorescent organic lightemitting diodes, Adv. Mater. 24 (31) (2012) 4337e4342. [7] T.-H. Han, S.-J. Kwon, N. Li, H.-K. Seo, W. Xu, K.S. Kim, T.-W. Lee, Versatile p-type chemical doping to achieve ideal flexible graphene electrodes, Angew. Chem. Int. Ed. 55 (21) (2016) 6197e6201. [8] S.-J. Kwon, T.-H. Han, T.Y. Ko, N. Li, Y. Kim, D.J. Kim, S.-H. Bae, Y. Yang, B.H. Hong, K.S. Kim, S. Ryu, T.-W. Lee, Extremely stable graphene electrodes doped with macromolecular acid, Nat. Commun. 9 (2018) 2037. [9] T.-H. Han, S.-J. Kwon, H.-K. Seo, T.-W. Lee, Controlled surface oxidation of multilayered graphene anode to increase hole injection efficiency in organic electronic devices, 2D Mater. 3 (2016) 14003. [10] J. Son, S. Lee, S.J. Kim, B.C. Park, H.-K. Lee, S. Kim, J.H. Kim, B.H. Hong, J. Hong, Hydrogenated monolayer graphene with reversible and tunable wide band gap and its fieldeffect transistor, Nat. Commun. 7 (2016) 13261. [11] Z. Luo, G. Qi, K. Chen, M. Zou, L. Yuwen, X. Zhang, W. Huang, L. Wang, Microwaveassisted preparation of white fluorescent graphene quantum dots as a novel phosphor for enhanced white-light-emitting diodes, Adv. Funct. Mater. 26 (16) (2016) 2739e2744. [12] N. Liu, A. Chortos, T. Lei, L. Jin, T.R. Kim, W.-G. Bae, C. Zhu, S. Wang, R. Pfattner, X. Chen, R. Sinclair, Z. Bao, Ultratransparent and stretchable graphene electrodes, Sci. Adv. 3 (9) (2017) e1700159. [13] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, J.-H. Ahn, P. Kim, J.-Y. Choi, B.H. Hong, Large-scale pattern growth of graphene films for stretchable transparent electrodes, Nature 457 (2009) 706e710. [14] M.K. Blees, A.W. Barnard, P.A. Rose, S.P. Roberts, K.L. McGill, P.Y. Huang, A.R. Ruyack, J.W. Kevek, B. Kobrin, D.A. Muller, P.L. McEuen, Graphene kirigami, Nature 524 (2015) 204e207. [15] M. Chen, T. Tao, L. Zhang, W. Gao, C. Li, Highly conductive and stretchable polymer composites based on graphene/MWCNT network, Chem. Commun. 49 (2013) 1612e1614. [16] M.-S. Lee, K. Lee, S.-Y. Kim, H. Lee, J. Park, K.-H. Choi, H.-K. Kim, D.-G. Kim, D.Y. Lee, S. Nam, J.-U. Park, High-performance, transparent, and stretchable electrodes using grapheneemetal nanowire hybrid structures, Nano Lett. 13 (6) (2013) 2814e2821.

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Structure and properties of graphene

2

Yong Seok Choi 1 , Je Min Yoo 1 , Byung Hee Hong 1,2 1 Department of Chemistry, College of Natural Sciences, Seoul National University, Gwanak-Gu, Seoul, Republic of Korea; 2Program in Nano Science and Technology, Graduate School of Convergence Science and Technology, Seoul National University, Yeongtong-Gu, Suwon-Si, Republic of Korea

2.1

Structure of graphene

2.1.1

Atomic structure of graphene

Graphene, the world’s thinnest two-dimensional material with hexagonal carbonbased honeycomb network, is one of the major sp2-hybridized carbon allotropes along with 0D fullerenes (wrapped-up graphene), 1D carbon nanotubes (rolled-up graphene monolayers), and 3D graphite (stacked-up graphene monolayers). Graphene has been an important material in various academic areas due to many of its unprecedented properties including electron mobility, thermal conductivity, atomic thinness, optical transparency, and mechanical strength based on the elongated p conjugation [1e8]. To understand the atomic structure of graphene, it is necessary to comprehend the orbital hybridization feature of carbon atoms. Conventionally, four sp3 hybrid orbitals are formed from one 2s and three 2p orbitals (2px, 2py, and 2pz). The four valence electrons on each carbon atom are thus occupied in a single sp3 orbital. In graphene, on the other hand, one 2s and two 2p orbitals participate in the formation of sp2-hybridized orbitals, leaving 2pz orbital unoccupied. The sp2 orbitals are oriented in xey plane with trigonal planar shape, and the remaining 2pz orbital is perpendicularly positioned to the plane. The sp2 carbon atoms form covalent in-plane s bonds with adjacent carbon atoms, and p bond is formed by overlapping with two unhybridized 2pz orbitals (Fig. 2.1(b)). Fig. 2.1(c) shows the hexagonal lattice of graphene with distinctive armchair and zigzag edges. Graphene consists of two carbon atoms per unit cell, resulting in two nonequivalent carbon atom sublattices (A and B). The real space basis vectors of unit cell are written as follows: a1 ¼

pffiffiffi a
pffiffiffi a
3; 3 and a2 ¼ 3;  3 2 2

Graphene for Flexible Lighting and Displays. https://doi.org/10.1016/B978-0-08-102482-9.00002-2 Copyright © 2020 Elsevier Ltd. All rights reserved.

6

(a)

(b) 2px 2py 2pz

sp3 hybrid orbitals

sp2 hybrid orbitals 2pz

Orbital hybridisation

2s 1s

1s

Ground state

1s

Diamond

(c)

Graphene

(d) A δ1

δ3 a1

B

ky

b1 K

Γ

δ2

a2

kx

b2

Figure 2.1 Schematic representation of major carbon allotropes. (a) Graphene can be considered as the basic building block for other carbon allotropes including 0D fullerene (wrapped-up), 1D carbon nanotube (rolled-up) and 3D graphite (stacked). (Reproduced with the permission from Macmillan Publishers Ltd: Nat. Mater., Ref. [1]). (b) Ground state atomic orbital of a carbon atom, sp3 hybridized orbitals in diamond and sp2 hybridized orbitals in graphene. (c) Crystal structure of graphene. 2D hexagonal lattice of graphene in real space with vectors a1 and a2. Two nonequivalent carbon atoms A and B compose the unit cell. (d) The first Brillouin zone with high symmetry points G K, M and the reciprocal lattice with lattice vector b1 and b2 is indicated (Reproduced with the permission from American Physical Society, Ref. [9]).

Graphene for Flexible Lighting and Displays

M K’

Structure and properties of graphene

7

where a ¼ 1.42 Å is the distance from adjacent carbon atoms. The corresponding reciprocal lattice vectors are as follows: b1 ¼

pffiffiffi 2p
pffiffiffi 2p
1; 3 and b2 ¼ 1;  3 3a 3a

The structure of graphene can be expressed in reciprocal lattice geometry with the two high symmetry points K and K0 within the Brillouin zone, which are known as the Dirac points. Their vectors in K-space can be expressed as follows: K¼

pffiffiffi 2p
pffiffiffi 2p
1; 3 and K0 ¼ 1;  3 3a 3a

This atomic structure of graphene results in a zero bandgap, where the conduction band and the valence band meet at the cone-shaped Dirac point. This enables thorough investigations on the properties of single-layer graphene. In addition, graphene’s infinite carbon network structure would ideally give rise to various applications including flexible display, solar cell, and transparent electrode for its perfect crystallinity and high chemical stability and impermeability.

2.1.2

Nanoscale morphology of graphene

Although graphene is only a single atomethick material, it is easily observable with an optical microscope. Fig. 2.2(a) shows chemical vapor deposition (CVD) graphene on SiO2 wafer with multidots, wrinkles, and residues of polymethyl methacrylate (PMMA) supporting layer. Although multidots, wrinkles, and PMMA residues may degrade the electrical properties and induce undesirable doping effects, these attributes are inevitably accompanied during the transfer process. Atomic force microscopy (AFM) analysis provides more detailed surface information including the thickness and morphology. While the thickness of graphene is 0.34 nm in theory, the average thickness of a single-layer graphene ranges from 0.7 to 1.0 nm due to the bonding force between substrate and graphene, as well as the PMMA residues (Fig. 2.2(b)). Transmission electron microscopy (TEM) analysis is employed to explore not only the thickness but also the grain boundaries, diffraction patterns, and carbon atomic lattice networks of graphene. Fig. 2.2(c) shows TEM image with an atom-by-atom analysis of graphene (mid inset). The diffraction pattern analysis indicates that the atomic structures of graphene are highly crystalline (lower inset). Scanning tunneling microscopy (STM) topography also gives evidence of graphene’s high crystallinity and shows the honeycomb structure expected for the full hexagonal symmetry of graphene (Fig. 2.2(d)). The absence of observable defects in the STM image with equivalently positioned carbon atoms with the same intensity indicates the high quality of graphene. Raman spectroscopy is also one of the most widely exploited tools in investigating the crystallinity of graphene, which will be discussed in the later section.

8

Graphene for Flexible Lighting and Displays

(a)

(b)

Residue Wrinkle

(c)

(d)

0.1 nm

Figure 2.2 The atomic structure and morphology of graphene. (a) Representative OM and (b) AFM images of monolayer CVD graphene with multi-dots, wrinkles and PMMA residues transferred on SiO2 substrate with designated arrows. Inset image of (b) represents the height profile of graphene (Reproduced with the permission from American Chemical Society, Ref. [10] and IOP, Ref. [11]). (c) HR-TEM image of CVD graphene showing the carbon atom network. The mid inset shows the hexagonal honeycomb lattice of graphene through atomby-atom analysis. The bottom inset represents the diffraction pattern of graphene indicating the high crystallinity of graphene (Reproduced with the permission from American Chemical Society, Ref. [2]) (d) HR-STEM image of graphene on an insulating surface (Copyright (2007) National Academy of Sciences, USA, Ref. [12]).

2.2

Synthesis of graphene

As graphite is composed of stacked graphene layers through the van der Waals interactions, British scientists were able to obtain graphene flakes by mechanically exfoliating graphite with Scotch tape in 2004 for the first time [9]. Although the highest quality monolayer graphene can be produced by mechanical exfoliation (Fig. 2.3), the production scale is restricted to micrometer range only; further applications for flexible display are limited. Among a number of approaches to produce large-scale graphene film, the CVD method is considered to be optimal to synthesize large-scale monolayer graphene with respectable quality and is thus deemed to be the most

Structure and properties of graphene

9

Single-layer graphene

Figure 2.3 Representative experimental procedures of mechanical exfoliation (Reproduced with the permission from Royal Society of Chemistry, Ref. [14]).

suitable for display applications. The very first attempt to produce CVD graphene was rather a serendipitous discovery while researchers attempted to synthesize CVD diamond by adsorbing carbon gas precursor (i.e., methane) on copper foil, which led to the formation of very thin yet highly crystalline graphite. Along with the previous findings in the 1960s that carbon gas precursors may be adsorbed in a highly crystalline, sp2-hybridized structure onto a hot metal catalyst, this sparked a renewed interest of researchers to synthesize large-scale graphene film through the CVD method. In the early years, some researchers reported the synthesis of micrometer-scale monocrystalline graphene on top of monocrystalline ruthenium catalyst, which still did not overcome the issues with dimension and uniformity [10,11]. Later, other scientists suggested the epitaxial growth of graphene on silicon carbide substrate [12]. However, in addition to the fact that the price of silicon carbide substrates is extremely costly, the processing conditions are difficult to achieve (high vacuum, high temperature (1300 C)), and selective transfer of graphene from the substrate is nearly impossible, epitaxially grown graphene cannot be processed for practical applications. Through further efforts, researchers have demonstrated that a few transition metal catalysts provide the optimal conditions to produce large-scale CVD graphene film and play pivotal role in understanding the growth mechanisms based on the difference in carbon solubility [13e19]. The growth of graphene on transition metal catalysts occur by two major mechanisms: (1) segregation and (2) surfacemediated reaction (Fig.2.4). In case of nickel catalysts, which exhibit relatively high carbon solubility, carbon-based gas precursors are decomposed into individual carbon atoms and are subsequently dissolved and diffused in nickel at high temperature. During cooling, dissolved carbon atoms emerge and segregate on the surface. The thickness of graphene is strongly related to the cooling rate; exceedingly fast

10

(a)

Graphene for Flexible Lighting and Displays

(c)

Hydrocarbon metal gas Carbon dissolving

CH4

H

Surface Body Extremely fast cooling

Fast/medium cooling

Slow cooling

(b)

(d) 1 5 2

1 2

3

4

Segregation, Ni

Surface-mediated reaction, Cu

Figure 2.4 Schematic representation of the growth mechanism of graphene by segregation and surface-mediated reaction on Ni and Cu substrates, respectively. (a, b) Schematic illustration of graphene synthesis mechanism on Ni foil. (Reproduced with the permission from American Institute of Physics, Ref. [15]) (c, d) Schematic illustration of graphene synthesis mechanism on Cu foil (Reproduced with the permission from American Chemical Society, Ref. [16]), (Reproduced with the permission from Elsevier, Ref. [17]).

cooling provokes the loss of carbon mobility from bulk to result in quenching effect, and slow cooling provides sufficient time for the carbon atoms to diffuse into bulk to prevent surface segregation of them. For copper catalysts, which possess relatively low carbon solubility, the formation of graphene undergoes surface-mediated reaction. Copper’s fully filled 3d-shell gives rise to its low carbon affinity and prevents the formation of carbide phase. For such reasons, decomposed carbon atoms form seeds and laterally nucleate on the surface until the entire copper surface is covered with graphene. It can be thus inferred that copper-mediated CVD growth is preferable for the production of large-scale monolayer graphene. It must be noted, however, that as-grown graphene on copper foil is intrinsically polycrystalline; nucleated seeds inevitably form grain boundaries and defects that impede the electrical properties of graphene. Researchers have endeavored to increase the size of individual grains to minimize the number of grain boundaries, which could thus enhance the properties. Based on the fact that the crystallinity of copper foil is one of the decisive factors to determine the grain size of graphene, some researchers physically extended copper foil in the gravitational orientation to increase its crystal grain size. As copper undergoes recrystallization during annealing process, the crystalline domain size of graphene increases accordingly, which eventually contribute to the formation

Structure and properties of graphene

(a)

11

(b) Jig

Cu foil Weight Vertical

Vertical

(c)

Vertical w/ tension Gas in

Horizontal - Vertical Switchable

Gas out

Gas in

Gas out

Figure 2.5 (a,b) Representative images of a vertical CVD system. The gravitactic tension is applied to Cu foils with additional force applied by weight. (c) Schematic representation of a horizontal/vertical convertible CVD system (Reproduced with the permission from IOP, Ref. [25]).

of fewer grain boundaries and thus superior electrical properties (Figs. 2.5 and 2.6). A recent publication from Professor Hong group demonstrated vertical synthesis of graphene with larger grain size by applied tension to copper foil through a vertical CVD system. In addition, the report suggested the continuous mass production of graphene film using a roll-to-roll vertical CVD system, which enables the synthesis of large-scale, high quality graphene with minimized grain boundaries (Fig. 2.7) [20].

2.3

Electronic band structure of graphene

The energy band structure of graphene is closely related to its p electron system. The electrons of graphene behave relativistically from the graphene Dirac equation, and the band structure was first obtained in 1947 by P.R. Wallace [21]. E ðkÞ z  vF jqj

12

Graphene for Flexible Lighting and Displays

(a)

(b)

Horizontal

20

30

3

10 5

20

Counts

Counts

Counts

Vertical w/ tension

25

15

0

(c)

Vertical

15 10

2 1

5 0

5 10 15 20 25 30 35 40 45 2

Grain size (μm )

0

0

5 10 15 20 25 30 35 40 45 2

Grain size (μm )

0

0

5 10 15 20 25 30 35 40 45

Grain size (μm2)

Figure 2.6 Representative analyses of grain boundaries in CVD graphene by scanning diffraction mapping in TEM, indicating that the size of grain boundary can be increased with applied tension to Cu foils (Reproduced with the permission from IOP, Ref. [25]).

(a)

(b)

(c)

Figure 2.7 Representative images of vertical roll-to-roll (R2R) CVD system with a tension control unit. The tension is controlled by the gravitational force and the winding roll. This vertical R2T CVD system enables the synthesis of high quality large-scale graphene at a speed of 300 mm per min (Reproduced with the permission from IOP, Ref. [25]).

Structure and properties of graphene

13

The charge carriers near the Dirac point are massless, and Dirac Fermions act with a Fermi velocity of vF w 1  106 m/s. The pluseminus represents p*(unoccupied or conduction) and p (occupied or valence) bands, respectively. The energy band structure of graphene with the nearest neighbor is shown in Fig. 2.8 and exhibits a linear dispersion at the Fermi energy at the high symmetry K and K0 points of the Brillouin zone [22]. The conduction band and the valence band are in contact with the coneshaped vertices, and each vertex meets at the K-point of the Brillouin zone to form the Fermi level. Unlike ordinary semimetallic materials, graphene has no bandgap. However, it is still deemed semimetallic as it has zero density of state of electrons at the Fermi level. Because of this unique electronic structure, graphene has a bipolar conduction characteristic that can be easily changed by the type of doping. While doping in semiconductors is achieved by implanting impurities, the electrical properties of graphene can be modulated by inducing chemical doping on the surface. When doping with an electron donor, called n-type doping, the electron levels become higher and the conductivity is increased. The upward-shifted Fermi level leads to decreased work function (Fig. 2.9) [23]. Conversely, doping with an electron acceptor, called p-type doping, induces graphene with increased conductivity as well, but the downward-shifted Fermi level results in increased work function. In summary, chemical dopingeinduced changes in the Fermi level of graphene enables the work function control and subsequent electrical properties. Doping can thus be regarded as one of the

4

2

EK 0 4 2

–2 –4

–2

0

kx

0 –2 ky 2

4

–4

Figure 2.8 Representative electronic band structure of graphene. The conduction band and valence band meet at the Dirac point. Circled inset shows a cone-shaped linear dispersion (Reproduced with the permission from MDPI, Ref. [27]).

EF p - doped Intrinsic n - doped

C-PDOS

E(k)

EF n - doped

Intrinsic

p - doped

E

Figure 2.9 Schematic representation of the Fermi level and Dirac point shifts according to the type of doping (Reproduced with the permission from American Chemical Society, Ref. [28]).

14

Graphene for Flexible Lighting and Displays

important techniques for employing graphene in various applications. Studies on graphene doping have been vastly carried out, and the most widely employed chemical doping method is the use of strong acids such as nitric acid, a strong p-dopant. pdoped graphene has a lower sheet resistance than pristine graphene, and the Dirac point shifts from 0 V to (þ) direction in graphene field effect transistor (FET) devices [24].

2.4

Optical properties of graphene

2.4.1

Transparency of graphene

Because of the growing worldwide demand for transparent display electrodes, relevant studies have been vastly carried out. As of today, indium tin oxide (ITO) is the most widely employed transparent electrodes for display, touch panel, and solar cell applications. However, the unit price of ITO has been steeply rising owing to the depletion of indium; developing alternative materials is highly demanded. In addition, ITO’s fragility and inflexibility have complicated its uses for broader applications. Graphene has thus attracted much attention as a promising material for the next-generation transparent electrode because of its flexibility and stretchability, along with its relatively facile synthesis and patterning processes. If mass production of graphene is achievable potentially through roll-to-roll method, it is expected to bring technological breakthroughs on the next-generation flexible electronics industry. Fig. 2.10(a) shows that the UV-vis transmittance spectra on a quartz substrate with increasing number of graphene layers. Theoretically, monolayer graphene reduces the light transmittance by 2.3% in the visible region and the transmittance decreases as the layer number increases. Fig. 2.10(b) shows comparative UV-vis transmittance spectra of graphene

Transmittance (%)

95

(b)

No. of layers (Tr at 550 nm) = 1 (97.4%) 2 (95.1%) 3 (92.9%) 4 (90.1%)

90 85 80 75 70 200

100

95

1 2 3

90

80 60

Graphene only Graphene on PET ITO on PET

40

Graphene/PET ~220Ω/sq.

20

4 λ=550 nm

0

200 400 600 800 Wavelength (nm)

800 400 600 Wavelength (nm)

100

Transmittance (%)

100

Transmittance (%)

(a)

1,000

400

ITO/PET ~180 Ω/sq.

500 600 700 Wavelength (nm)

800

Figure 2.10 Optical characterization of graphene film. (a) UV-vis transmittance spectra of graphene film with an increasing number of layers on quartz substrates. The inset shows the transmittance with and without HNO3 doping (Reproduced with the permission from American Chemical Society, Ref. [2]). (b) UV-vis transmittance spectra of graphene film and ITO on PET substrates. ITO is less transparent in the short visible wavelengths (Reproduced with the permission from Macmillan Publishers Ltd: Nat. Nanotechnol., Ref. [29]).

Structure and properties of graphene

15

and ITO films on polyethylene terephthalate (PET) substrate. While graphene-based film is transparent for all visible ranges, ITO film is relatively opaque in short visible wavelengths, which is observed to be slightly yellowish with naked eyes.

2.4.2

Raman spectroscopy analysis

Raman spectroscopy is a widely used technique for investigations on the molecular structures and bonding effects in various areas including physics, chemistry, and materials science. It is known to be the most effective and facile method to analyze the vibrational structure and electronic properties of different materials without damaging the sample. In particular, due to the unique electron band structure of graphene, the thickness, crystallinity, and doping state can be easily and rapidly analyzed. In addition, the Raman scattering of graphene is amplified by the charge resonance phenomenon, and the surface remains intact despite its exposure to strong laser due to graphene’s high chemical and thermal resistance. Graphene generally exhibits three characteristic bandsdG, D, and 2D bandsdwhich can be exploited to identify a number of different properties (Figs. 2.11 and 2.12) [25]. The G band near 1580 cm1 is commonly detected in graphitic materials because of the phonon vibration mode corresponding to the stretching of the carbonecarbon bond. As a result, the carbon atoms in a hexagonal structure oscillate in opposite directions to the adjacent atoms. Because the energy level of the G band is determined by the density of surplus charge doped in graphene, it is possible to quantify the electrons or holes injected during doping. Furthermore, the change of the Fermi level can be estimated by using the electronic density state of graphene. Namely, the doping information can be obtained by analyzing the position and full width half maximum (FWHM) of the G band. In

(a)

(b) a

Conduction band

non

n

n

Phono

non Pho Ph on o

K

hω 0

Valence band

b

Intervalley D’ band

a c

Pho

Def

h ω vib

K f

i

i

(c)

(d) a

Phonon

b K’

Defect

Intervalley D band

K f

Intervalley 2D band

a

Phonon

b K’

Phonon

c

K f i

Figure 2.11 Resonant scatterings of the valence and conduction bands. (a) Non-resonant phonon scattering. (b) Second-order Raman process for intravalley scattering of the D’ band in graphene (c) Second-order Raman process for the D band. (d) Second-order Raman process for the 2D band. Two phonons have opposite wavevectors to conserve the total momentum in the scattering process (Reprinted with the permission from MDPI, Ref. [32]).

16

Graphene for Flexible Lighting and Displays

(a)

(b) G band

2D band

4

Graphite

Raman intensity

Graphene Graphene +LCCs

532 nm

5 layers

I(2D)/I(G)

3 4 layers 3 layers

2

2 layers 1 layer

1200

1600

2000

2400

2800

3200

–1

Raman shift (cm )

1 1 2

FL Graphene layers

ML

Figure 2.12 (a) Representative Raman spectra of graphene film with an increasing number of layers (Reproduced with the permission from, Springer Science+Business Media, Ref. [33]). (b) The peak intensity ratio of 2D/G as a function of number of ayers (Reproduced with the permission from Elsevier, Ref. [34]).

both p- and n-doping, the G band is blue shifted and exhibits narrower FWHM. In addition, as the FWHM decreases, the intensity is increased with decreased relative 2D/G ratio (Figs. 2.13 and 2.14) [26,27]. The D band near 1380 cm1 appears when there is a bond with sp2 crystal structure. Generally, graphene obtained through mechanical exfoliation has a high crystallinity and hardly presents any D band. However, if graphene is impaired from chemical reaction or physical treatment, the intensity of D band increases; it is thus often used as an indicator of graphene defects. The 2D band is located at w2780 cm1 due to the secondary scattering where two phonons of D band are emitted. This band is determined by the double resonance phenomenon caused by the electronic structure of graphene, so the thickness and number of layers of graphene can be measured through the ratio of the G band and the 2 D band. Fig. 2.12 shows increased G/2D ratio by decreased 2D band intensity with an increasing number of graphene layers [28,29]. Unlike the G band, doping-induced 2D band shifts are determined by the type of doping; the position is either red shifted (n-type) or blue shifted (p-type). Because doping process changes the equilibrium lattice parameters, n-doping expands the lattice that softens the 2D mode, while p-doping induces shrinkage of lattice and phonon stiffening.

2.5 2.5.1

Electrical properties of graphene Graphene field effect transistor

The FET is a device that controls the source and drain currents by adjusting the flow of electrons/holes to the channel by applying a gate voltage to the electrode. In 2004,

Structure and properties of graphene

(a)

(b)

Fermi energy (meV) –703 –574 –406

0

406 574 703 811

Fermi energy (meV) –703 –574 –406

1,610

2,700

1,605

2,690

Pos(G) (cm–1)

Pos(G) (cm–1)

17

1,600 1,595 1,590

p-doping

1,585

n-doping

0

406 574 703 811

2,680 2,670

p-doping

n-doping

2,660

1,580 –3

–2

–1

0

1

2

3

–3

4

Electron concentration (×1013 cm–2)

(c)

(d) p-doping

16 14 12 10 8

Fermi energy (meV) –703 –574 –406 0 406 574 703 811 3.5 3.0

n-doping /(2D)/I(G)

FWHM(G) (cm–1)

18

–2 –1 0 1 2 3 4 13 –2 Electron concentration (×10 cm )

p-doping

n-doping

2.5 2.0 1.5 1.0

6

0.5

4 –3

–2 –1 1 2 3 4 0 Electron concentration (×1013 cm–2)

–3

–2

–1

0

1

2

3

4

Electron concentration (×1013 cm–2)

Figure 2.13 Raman spectral analysis of graphene depending on the doping type. The blue lines indicate the theoretical values and the black lines represent the experimental data. (a) The position shifts of the G band and (b) the 2D band. (c) The FWHM of the G band depending on the doping type. (d) 2D/G Raman spectra intensity ratio depending on the doping type (Reproduced with the permission from Macmillan Publishers Ltd: Nat. Mater., Ref. [35]).

Novoselov and Geim of the University of Manchester showed that graphene-based devices exhibit linearly increasing conductivity by applied gate voltage, confirming that graphene has the properties of FETs [9]. In FETs with graphene channel, the position of the Dirac voltage varies by the doping state of graphene, and the degree of graphene doping level and state can thus be analyzed. Fig. 2.15 shows a typical graphene FET structure and IeV curve (currentevoltage curve) with applied gate voltage [30]. When a gate voltage is applied to a graphene FET fabricated with pristine graphene, the current flow changes according to the applied gate voltage. If a negative gate voltage is applied, positive charge is induced in the graphene channel, and the current flows through the holes as major carriers. When the gate voltage is increased to 0 V, the Fermi level is located in the zero energy state. Ideally, the current does not flow, and this point is the Dirac voltage of the IeV curve. On the contrary, if a positive voltage is applied, a negative charge is induced in graphene and the current flows by electrons as major carriers. On the other hand, the Dirac voltage of doped graphene

18

Graphene for Flexible Lighting and Displays

(b)

240

2700

1595

180

120

60

0 1500

4.5

I(2D)/I(G) ratio

4.0 3.5

2696

1590

2692

1585 2688

1580

2684 1600 2600 2700 –1 Raman shift(cm )

2800

2900

Prisitine

EDA doped

DETA doped

TETA doped

(d) Pristine EDA doped DETA doped TETA doped

3.0 2.5 2.0

Pristine EDA doped DETA doped TETA doped

18

FWHM (cm–1)

(c)

G peak position (cm–1)

Intensity (arb. unit)

–1

2D peak position (cm )

(a)

15 12 9

1.5 1.0 1584 1586 1588 1590 1592 1594 1596 1598 –1 G peak position (cm )

1584

1587 1590 1593 –1 G peak position (cm )

1596

Figure 2.14 Raman spectral analysis of n-doped graphene with different n-dopants having an increasing number of amine groups. (a) Representative Raman spectra of pristine and ndoped graphene with different n-dopants. (b) The G and 2D band position shifts for pristine and n-doped graphene. (c) 2D/G intensity ratio changes as function of the G band position with different n-dopants. (d) The FWHM of the G band with pristine and n-doped graphene with different n-dopants (Reproduced with the permission from American Chemical Society, Ref. [38]).

(either n- or p-type) is shifted to the negative or positive directions depending on the doping state. In case of p-doped (hole-induced) graphene FETs, the current flows with more holes induced by a negative gate voltage. Even when the voltage becomes 0 V, the current continues to flow due to the presence of holes. At a positive voltage, electrons begin to appear and combine with the holes and thus provide the lowest current value; this point becomes the Dirac voltage of a p-doped graphene. When a negative voltage is applied to n-doped graphene, the current flows by holes as the major carriers up to some point, depending on the degree of doping. However, if the gate voltage exceeds a certain value, the holes are equaled out with electrons in n-doped graphene. Likewise, the point with the lowest current value becomes the Dirac voltage of an n-doped graphene FET. Fig. 2.16 shows the Dirac voltages of pristine graphene and n- and p-doped graphene. In brief, the changes in the Dirac voltage position of doped graphene can be analyzed by the current flow at different gate voltages. The Dirac voltages of n- and p-doped graphene are located in the negative and positive voltage regions, respectively. The graphene FETs are thus one of the most important

Structure and properties of graphene

19

(a)

(b)

100

Sourc

80

e

ID(μA)

Drain Gate

Diele

ctric p++ Si

60 40 20 0

SiO2 Right after fabrication (atmosphere) After 10 h vacuum (in vacuum) After annealing (in vacuum)

–20

0

20

VG(V)

Figure 2.15 (a) Schematic image of a graphene FET device. (b) Representative I-V curve from a graphene FET (Reproduced with the permission from Elsevier, Ref. [37]).

(a)

(b)

20

60

Pristine FET 3 min 15 min 8 h (overnight)

2 –1 –1 μ = 550 cm V s

50 40

σ (e2/h)

Isd(μA)

15 10 5 0

n-doping dirac point shift –180 –150 –120 –90 –60 –30

Vg(V)

3100 cm2V–1s–1

30 20

p-doping dirac point shift

10 0

30

0

0

10

20

30

40

50

60

Vg(V)

Figure 2.16 The shifts in the Dirac voltage in (a) n-doped and (b) p-doped graphene FET devices (Reproduced with the permission from American Chemical Society, Ref. [38, 39]).

techniques for analyzing the electrical properties of graphene because the doping levels can be validated from the shifts in Dirac position (Fig. 2.16) [31,32]. The graphene FETs can also be used to evaluate the electrical conductivity and resistivity. The electrical conductivity and resistivity of graphene are linearly related to the gate voltage near the Dirac point. Therefore, the slope of the IeV curve near the Dirac voltage indicates the carrier mobility, which can be calculated by the following equation: m¼

1 ds Ci dVG

where Ci is capacitance of SiO2, s is conductivity of graphene, and VG is applied gate voltage. Although CVD graphene has relatively high mobility, it cannot reach the

20

Graphene for Flexible Lighting and Displays

theoretical level owing to the presence of defects, grain boundaries, ripples, and cracks. Graphene FET devices may also be fabricated on flexible substrates such as polydimethylsiloxane (PDMS) and PET films and are more advantageous in terms of transparency and flexibility. Like the ones on SiO2 substrate, graphene FETs on PET have a source, drain, and gate electrodes system. While SiO2-based FETs are dielectric, PET-based FETs utilize ion gelebased top gate system. Fig. 2.17 shows a representative graphene FET on PET film. Although the device is transparent and flexible, the mobility is much lower than that of SiO2-based FETs because the surface morphology of PET is much rougher [33,34].

2.5.2

Sheet resistance

Because graphene is a two-dimensional film, the electrical properties can be evaluated by measuring the sheet resistance. It may also provide the information on doping as doping generally induces decreased sheet resistance from increased amount of electrons/holes (Fig. 2.18(a),(b)) [35,36]. The linear resistance is measured with two probes at a certain distance, but in the case of sheet resistance, a four-point probe is employed. The four-point probe technique is the most widely used method for measuring the sheet resistance of semiconductors, especially the resistivity of a metal film formed on an insulator, as it is a simple and accurate method that does not require complicated calibration procedures. The most significant feature of a four-point probe is that the probes are linearly aligned with consistent intervals (Fig. 2.18(c)). A constant current is applied between probes 1 and 4, and the voltage between probes 2 and 3 is measured to calculate the sheet resistance using the ratio between voltage and current through the equation: r ¼ 2psðV = IÞ

(a)

Graphene transfer

Ion gel prepolymer drop-casting

(b)

UV exposure Mask

PET

Graphene patterning

PEDOT:PSS transfer

Figure 2.17 Representative fabrication processes and characterization of ion gel-gated flexible, transparent graphene FETs (Reproduced with the permission from IOP, Ref. [40] and American Chemical Society, Ref. [41]).

Structure and properties of graphene

(b)

Reduction of Rs after doping

100

10

60

65

70

75

80

85

350

Pristine graphene

300 250 200 150 50 50

90

1

2

(c)

A V

4

3

Number of layers

Transmittance at 550 nm (%)

(d)

100

1600

90

1400

80 Sheetresistance (Ω/sq)

1000

Sheet resistance (ohm/sqr)

Sheet resistance (Ω/sq)

(a)

21

1200

70

1000

60 50

800

40 600

30

400

20 10

200 10

20

30

40

50

60

70

80

90 100

0

Figure 2.18 (a, b) Representative sheet resistance data with and without doping. (Reproduced with the permission from IOP, Ref. [42] and Hindawi, Ref. [43]). (c) Schematic representation of a linear 4-point probe. (d) Representative sheet resistance mapping image of a large-scale graphene film using the eddy currents.

where V is the voltage, I is the current, r is the resistance, and s is the distance between the probes. The correction factor (CF) can be applied to calculate the sheet resistance ðRs based on the resistance measured using a four-point probe. The CF is the value of which the size, the thickness, and temperature of the sample are reflected. Therefore, the sheet resistance of graphene can be expressed as follows: Rs ¼ 4:532  rðohm = sq.Þ where 4.532 is the CF used for graphene-based films. In like manner, the van der Pauw method provides an average resistance using a four-point probe placed as a square form. The sheet resistance can be calculated by the following formula [37]: Rs ¼

pR ln2

where R is measured resistance. Probe-based sheet resistance measurement can provoke damage to sample owing to the direct contact between probe and sample surface and is unsuitable for large-scale films as well. Therefore, noncontact sheet resistance measurements using eddy currents may be alternatively used for transparent electrode

22

Graphene for Flexible Lighting and Displays

and display applications. This is additionally advantageous for large-scale analysis without significant damages. Fig. 2.18(d) shows sheet resistance of large-scale CVD graphene film (10  10 cm) on PET measured using eddy currents.

2.6

Mechanical properties of graphene

As mentioned above, although ITO is widely employed in touch screen, transparent electrode, and display, the applications in metal-based electrodes in foldable, stretchable electronic devices are intrinsically unachievable as they break even with a strain of 0.1%. On the other hand, although graphene consists of only one layer of atoms, the sp2 carbon atoms are connected through covalent s-bonds, which are known as the strongest bond. Therefore, graphene exhibits excellent stretchability and flexibility. The intrinsic strength of graphene was first revealed by Hone Group. They punched holes in the SiO2 wafer to fabricate suspended graphene structure and analyzed with AFM. Its theoretical breaking strength is w40 N/m, and the elastic stiffness is w1.0 Tpa [38]. Based on these properties, researchers have demonstrated a number of studies which employed large-scale graphene film as a flexible, bendable, and stretchable transparent electrode. The mechanical flexibility was investigated by measuring the electrical resistance change against mechanical deformation. To evaluate the flexibility of graphene, Kim et al. transferred graphene on PET and measured the resistance change with the bending radius (Fig. 2.19(a),(b)) [40]. As shown in Fig. 2.19(a), the resistance of graphene changed only slightly for the bending radius up to 2.3 mm (tensile strain of 6.5). When the film was bent 0.8 mm (tensile strain of 18.7%), the resistance sharply increased initially but restored the original value as it returned to the original state. In addition, graphene was transferred to longitudinally prestrained PDMS substrate, and the tensile strain was measured (Fig. 2.19(b)). The resistance of graphene film of both horizontal and vertical directions remained stable with 11% strain, and 25% stretching induced only a single-order change. Notably, the results were consistent through repeated bending cycles. Fig. 2.19(c) indicates the mechanical properties of graphene-based touchscreen devices compared with ITO/PET electrodes, showing the resistance change when tensile strain is applied [24]. In case of ITO electrodes, the resistance change increases sharply to 2%e3% strain value because ITO cannot endure the strain. On the other hand, graphene-based panel is intact up to 6% strain. It can thus be concluded that the elasticity and flexibility of graphene is far superior to that of ITO or other metal candidates. Bunch et al. fabricates a graphene balloon by expanding suspended graphene using the pressure difference between inside and outside of graphene membrane (Fig. 2.19(d)) [41]. This demonstrates the strong mechanical strength and impermeability of graphene [3,5,41,42]. Based on these properties, researchers have endeavored to exploit graphene as a selective membrane and stretchable encapsulation barrier for organic lighteemitting diode applications.

Structure and properties of graphene 108

(b)

9 8

107 106

6

Resistance (Ω)

Resistance (kΩ)

7 5 0.0

4

0.4

0.8

1.2

Curvature, k (mm–1)

3 2

Bending

1

Recovery

2nd

3rd

0 3 6 0 3 6 0 3 6 Stretching (%)

104

Ry

103

10

Flat 3.5 2.7 2.3 1.0 0.8 Flat Bending radius (mm)

1st

Stretching cycles

102

0

(c)

105

Resistance (Ω)

(a)

23

Rx

Stable

1

0

5

10 15 20 Stretching (%)

25

30

(d)

200 3 2

150

1

ΔR/R0

0

100

2 4 Strain (%)

600 Deflection (nm)

0

50 0 1

2

3 4 Strain (%)

5

6

500 400 300 200 100 0

–3 –2 –1 0 1 y (μm)

2

3

Figure 2.19 (a) Representative resistance changes with respect to the bending radius and (b) strain. Left inset of (b) shows the stretching cycling test results. (Reproduced with the permission from Macmillan Publishers Ltd: Nature, Ref. [46]). (c) Representative resistance changes of graphene-based touch-panel compared with ITO/PET films in bent and flat states (Reproduced with the permission from Macmillan Publishers Ltd: Nat. Nanotechnol., Ref. [29]). (d) Representative images of graphene balloon fabricated by the pressure difference of the suspended graphene membrane (Reproduced with the permission from Macmillan Publishers Ltd: Nat. Nanotechnol., Ref. [47]).

References [1] A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6 (3) (2007) 183. [2] J. Ryu, Y. Kim, D. Won, N. Kim, J.S. Park, E.-K. Lee, D. Cho, S.-P. Cho, S.J. Kim, G.H. Ryu, Fast synthesis of high-performance graphene films by hydrogen-free rapid thermal chemical vapor deposition, ACS Nano 8 (1) (2014) 950e956. [3] K. Choi, S. Nam, Y. Lee, M. Lee, J. Jang, S.J. Kim, Y.J. Jeong, H. Kim, S. Bae, J.-B. Yoo, Reduced water vapor transmission rate of graphene gas barrier films for flexible organic field-effect transistors, ACS Nano 9 (6) (2015) 5818e5824. [4] J.-H. Ahn, B.H. Hong, Graphene for displays that bend, Nat. Nanotechnol. 9 (10) (2014) 737. [5] D. Shin, J.B. Park, Y.-J. Kim, S.J. Kim, J.H. Kang, B. Lee, S.-P. Cho, B.H. Hong, K.S. Novoselov, Growth dynamics and gas transport mechanism of nanobubbles in graphene liquid cells, Nat. Commun. 6 (2015) 6068.

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Graphene for Flexible Lighting and Displays

[6] U. Sim, J. Moon, J. An, J.H. Kang, S.E. Jerng, J. Moon, S.-P. Cho, B.H. Hong, K.T. Nam, N-doped graphene quantum sheets on silicon nanowire photocathodes for hydrogen production, Energy Environ. Sci. 8 (4) (2015) 1329e1338. [7] S. Shin, H.-H. Choi, Y.B. Kim, B.-H. Hong, J.-Y. Lee, Evaluation of body heating protocols with graphene heated clothing in a cold environment, Int. J. Cloth. Sci. Technol. 29 (6) (2017) 830e844. [8] S. Lee, I. Jo, S. Kang, B. Jang, J. Moon, J.B. Park, S. Lee, S. Rho, Y. Kim, B.H. Hong, Smart contact lenses with graphene coating for electromagnetic interference shielding and dehydration protection, ACS Nano 11 (6) (2017) 5318e5324. [9] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (5696) (2004) 666e669. [10] E. Sutter, P. Albrecht, P. Sutter, Graphene growth on polycrystalline Ru thin films, Appl. Phys. Lett. 95 (13) (2009) 133109. [11] S. Marchini, S. G€unther, J. Wintterlin, Scanning tunneling microscopy of graphene on Ru (0001), Phys. Rev. B 76 (7) (2007) 075429. [12] C. Riedl, U. Starke, J. Bernhardt, M. Franke, K. Heinz, Structural properties of the graphene-SiC (0001) interface as a key for the preparation of homogeneous large-terrace graphene surfaces, Phys. Rev. B 76 (24) (2007) 245406. [13] Q. Yu, J. Lian, S. Siriponglert, H. Li, Y.P. Chen, S.-S. Pei, Graphene segregated on Ni surfaces and transferred to insulators, Appl. Phys. Lett. 93 (11) (2008) 113103. [14] H. Mehdipour, K. Ostrikov, Kinetics of low-pressure, low-temperature graphene growth: toward single-layer, single-crystalline structure, ACS Nano 6 (11) (2012) 10276e10286. [15] C.-M. Seah, S.-P. Chai, A.R. Mohamed, Mechanisms of graphene growth by chemical vapour deposition on transition metals, Carbon 70 (2014) 1e21. [16] X. Li, W. Cai, L. Colombo, R.S. Ruoff, Evolution of graphene growth on Ni and Cu by carbon isotope labeling, Nano Lett. 9 (12) (2009) 4268e4272. [17] W. Cai, Y. Zhu, X. Li, R.D. Piner, R.S. Ruoff, Large area few-layer graphene/graphite films as transparent thin conducting electrodes, Appl. Phys. Lett. 95 (12) (2009) 123115. [18] A. Reina, X. Jia, J. Ho, D. Nezich, H. Son, V. Bulovic, M.S. Dresselhaus, J. Kong, Large area, few-layer graphene films on arbitrary substrates by chemical vapor deposition, Nano Lett. 9 (1) (2008) 30e35. [19] X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, Large-area synthesis of high-quality and uniform graphene films on copper foils, Science 324 (5932) (2009) 1312e1314. [20] I. Jo, S. Park, D.J. Kim, J. San Moon, W.B. Park, T.H. Kim, J.H. Kang, W. Lee, Y. Kim, D.N. Lee, Tension-controlled single-crystallization of copper foils for roll-to-roll synthesis of high-quality graphene films, 2D Mater. 5 (2) (2018). [21] P.R. Wallace, The band theory of graphite, Phys. Rev. 71 (9) (1947) 622. [22] A. Maffucci, G. Miano, Electrical properties of graphene for interconnect applications, Appl. Sci. 4 (2) (2014) 305e317. [23] G. Jo, M. Choe, S. Lee, W. Park, Y.H. Kahng, T. Lee, The application of graphene as electrodes in electrical and optical devices, Nanotechnology 23 (11) (2012) 112001. [24] S. Bae, H. Kim, Y. Lee, X. Xu, J.-S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H.R. Kim, Y.I. Song, Roll-to-roll production of 30-inch graphene films for transparent electrodes, Nat. Nanotechnol. 5 (8) (2010) 574. [25] A. Merlen, J.G. Buijnsters, C. Pardanaud, A guide to and review of the use of multiwavelength Raman spectroscopy for characterizing defective aromatic carbon solids: from graphene to amorphous carbons, Coatings 7 (10) (2017) 153.

Structure and properties of graphene

25

[26] A. Das, S. Pisana, B. Chakraborty, S. Piscanec, S. Saha, U. Waghmare, K. Novoselov, H. Krishnamurthy, A. Geim, A. Ferrari, Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor, Nat. Nanotechnol. 3 (4) (2008) nnano. 2008.67. [27] I. Jo, Y. Kim, J. Moon, S. Park, J. San Moon, W.B. Park, J.S. Lee, B.H. Hong, Stable ntype doping of graphene via high-molecular-weight ethylene amines, Phys. Chem. Chem. Phys. 17 (44) (2015) 29492e29495. [28] Y. Liu, Z. Liu, W.S. Lew, Q.J. Wang, Temperature dependence of the electrical transport properties in few-layer graphene interconnects, Nanoscale Res. Lett. 8 (1) (2013) 335. [29] L. D’Urso, G. Forte, P. Russo, C. Caccamo, G. Compagnini, O. Puglisi, Surface-enhanced Raman scattering study on 1D-2D graphene-based structures, Carbon 49 (10) (2011) 3149e3157. [30] S.K. Jang, J. Jeon, S.M. Jeon, Y.J. Song, S. Lee, Effects of dielectric material properties on graphene transistor performance, Solid State Electron. 109 (2015) 8e11. [31] Y. Kim, J. Ryu, M. Park, E.S. Kim, J.M. Yoo, J. Park, J.H. Kang, B.H. Hong, Vapor-phase molecular doping of graphene for high-performance transparent electrodes, ACS Nano 8 (1) (2013) 868e874. [32] B. Lee, Y. Chen, F. Duerr, D. Mastrogiovanni, E. Garfunkel, E. Andrei, V. Podzorov, Modification of electronic properties of graphene with self-assembled monolayers, Nano Lett. 10 (7) (2010) 2427e2432. [33] S.-K. Lee, B.J. Kim, H. Jang, S.C. Yoon, C. Lee, B.H. Hong, J.A. Rogers, J.H. Cho, J.H. Ahn, Stretchable graphene transistors with printed dielectrics and gate electrodes, Nano Lett. 11 (11) (2011) 4642e4646. [34] S.-K. Lee, S.H. Kabir, B.K. Sharma, B.J. Kim, J.H. Cho, J.-H. Ahn, Photo-patternable ion gel-gated graphene transistors and inverters on plastic, Nanotechnology 25 (1) (2013) 014002. [35] K.K. Kim, A. Reina, Y. Shi, H. Park, L.-J. Li, Y.H. Lee, J. Kong, Enhancing the conductivity of transparent graphene films via doping, Nanotechnology 21 (28) (2010) 285205. [36] S.-H. Chan, S.-H. Chen, W.-T. Lin, C.-C. Kuo, Uniformly distributed graphene domain grows on standing copper via low-pressure chemical vapor deposition, Adv. Mater. Sci. Eng. 2013 (2013). [37] L. Van der Pauw, A method of measuring specific resistivity and Hall effect of discs of arbitrary shape, Philips Res. Rep. 13 (1958) 1e9. [38] C. Lee, X. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science 321 (5887) (2008) 385e388. [39] X. Wang, X. Li, L. Zhang, Y. Yoon, P.K. Weber, H. Wang, J. Guo, H. Dai, N-doping of graphene through electrothermal reactions with ammonia, Science 324 (5928) (2009) 768e771. [40] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, J.-H. Ahn, P. Kim, J.-Y. Choi, B.H. Hong, Large-scale pattern growth of graphene films for stretchable transparent electrodes, Nature 457 (7230) (2009) 706. [41] S.P. Koenig, N.G. Boddeti, M.L. Dunn, J.S. Bunch, Ultrastrong adhesion of graphene membranes, Nat. Nanotechnol. 6 (9) (2011) 543. [42] L. Wang, L.W. Drahushuk, L. Cantley, S.P. Koenig, X. Liu, J. Pellegrino, M.S. Strano, J.S. Bunch, Molecular valves for controlling gas phase transport made from discrete ångstr€om-sized pores in graphene, Nat. Nanotechnol. 10 (9) (2015) 785.

26

Graphene for Flexible Lighting and Displays

Further reading [1] A.C. Neto, F. Guinea, N.M. Peres, K.S. Novoselov, A.K. Geim, The electronic properties of graphene, Rev. Mod. Phys. 81 (1) (2009) 109. [2] S.J. Kim, T. Choi, B. Lee, S. Lee, K. Choi, J.B. Park, J.M. Yoo, Y.S. Choi, J. Ryu, P. Kim, Ultraclean patterned transfer of single-layer graphene by recyclable pressure sensitive adhesive films, Nano Lett. 15 (5) (2015) 3236e3240. [3] C.J. Shearer, A.D. Slattery, A.J. Stapleton, J.G. Shapter, C.T. Gibson, Accurate thickness measurement of graphene, Nanotechnology 27 (12) (2016) 125704. [4] E. Stolyarova, K.T. Rim, S. Ryu, J. Maultzsch, P. Kim, L.E. Brus, T.F. Heinz, M.S. Hybertsen, G.W. Flynn, High-resolution scanning tunneling microscopy imaging of mesoscopic graphene sheets on an insulating surface, Proc. Natl. Acad. Sci.U.S.A. 104 (22) (2007) 9209e9212. [5] M. Yi, Z. Shen, A review on mechanical exfoliation for the scalable production of graphene, J. Mater. Chem. 3 (22) (2015) 11700e11715. [6] W.H. Lee, J.W. Suk, J. Lee, Y. Hao, J. Park, J.W. Yang, H.-W. Ha, S. Murali, H. Chou, D. Akinwande, Simultaneous transfer and doping of CVD-grown graphene by fluoropolymer for transparent conductive films on plastic, ACS Nano 6 (2) (2012) 1284e1290.

Preparation of graphene electrode

3

Wencai Ren Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, Liaoning, China

3.1

Solution casting of graphene oxide

Among the various synthetic ways, graphene-based transparent conductive films (G-TCFs) can be fabricated by solution casting of graphene oxide (GO) from solution and subsequent reduction through thermal of chemical ways. The properties of final G-TCFs are closely related with the properties of the raw GO, the methods for film formation and reduction.

3.1.1

Properties of GO and GO solution

Because of high aspect ratio, large specific surface area, and strong van der Waals attraction, graphene sheets tend to stick together. Well-graphitized graphene have a surface that is inert to most commonly used solvents, so the interaction between graphene sheets and solvents is hard to balance against the attraction between graphene sheets, which results in poor dispersion or even reaggregation [1e4]. But the properties of GO is much different from that of pristine graphene. GO is usually produced by exfoliation of graphite oxide, which is a highly oxidized graphite produced by intercalating and oxidizing with a strong acidic oxidant. During oxidation, the graphene layers of the graphite are decorated with a large amount of oxygen containing groups, e.g., epoxy, hydroxyl and carboxyl, etc. (Fig. 3.1(a)) with a typical carbon/oxygen atomic ratio lower than two. This structure changes the surface of the graphene sheets from hydrophobic to hydrophilic and enables them to be well dispersed in polar solvents such as water (Fig. 3.1(b)), N, N-dimethylformamide, and 1-methyl-2pyrrolidinone. As a result, graphite oxide is much easier to exfoliate into mono- or few-layer sheets in a solvent (mostly water), and these sheets are named as GO. GO sheets are intrinsically insulating due to the disturbed long rangeeconjugated structure during oxidation, and reduction treatment is necessary before or after film formation to restore the conductivity of the films. Figs. 3.1(c,d) show the typical optical microscope images of GO sheets before and after reduction. After reduction, accompanying with the elimination of oxygencontaining groups, part of the insulated sp3 structures can be restored into conductive sp2 structures, which restores the long-range conductivity of reduced GO (rGO).

Graphene for Flexible Lighting and Displays. https://doi.org/10.1016/B978-0-08-102482-9.00003-4 Copyright © 2020 Elsevier Ltd. All rights reserved.

28

Graphene for Flexible Lighting and Displays

(a)

(b)

OH OH OH O

OH O

HO

O O

O

OH

OH

O OH HO

O

OH

O OH

OH O

HO OH O

O OH

OH

O

HO

OH HO

HO

O

(c)

HO

O OH

HO O

O

(d)

Figure 3.1 (a) LerfeKlinowski model of graphene oxide (GO) with the omission of minor groups (carboxyl, carbonyl, ester, etc.) on the periphery of the carbon plane of the GO, (b) GO solution, and (c,d) GO and reduced GO (rGO) sheets on a 300 nm SiO2/Si substrate [5].

Because of the increase of free electrons for conductivity, the transparency of rGO sheets decreases, which increases the contrast as compared in Fig. 3.1(c,d). Generally, the stable and homogeneous GO suspensions allow the easy assembly of ultrathin film with thickness as low as several nanometers, and the reducibility of GO enables the fabrication of GO-based TCF and subsequent applications.

3.1.2

Reduction of GO film

As-fabricated GO films are insulating. As a result, the reduction of the GO after film assembly is unavoidable for the fabrication of GO-based TCFs. Generally, three types of reduction methods have been used, chemical reduction (CR), high temperature annealing (HTA), and a combination of both CR and HTA, to realize the reduction of GO, and some typical results are listed in Table 3.1. For details on the methods and mechanism of GO reduction, the reader can refer to our review on the reduction of GO [16]. Specific to the production of TCFs, hydrazine was the most frequently used reduction reagent. The reduction treatment can remove most of the oxygen-containing groups attached to the carbon plane and the conjugated structure of graphene can be partly restored to make rGO electrically conductive [17]. However, reduction by hydrazine alone is not sufficient to achieve maximum reduction; a subsequent annealing can well improve the conductivity of rGO-TCFs

sDC/sOP

References

85

2.2

[5]

4.4

81

0.38

[6]

e

30

80

0.05

[7]

SnCl2/EtOH

e

0.82

83

1.55

[8]

Hydrazine

e

1.4
10

92

80

Preparation of graphene electrode

Table 3.1 Optoelectrical property comparison of rGO-TCFs produced by different reduction methods.

29

30

Graphene for Flexible Lighting and Displays

[14,18]. An rGO-TCF with Rs of 102e103 U/sq at an 80% transparency was obtained by a combination of hydrazine reduction and annealing at 1100 C [18]. HTA also is found to be efficient to reduce the electrical conductivity of rGO films, and the reduction effect is significantly affected by the heating temperature [12,18]. For GO films with the same thickness, the conductivity is 50, 100, and 550 S/cm when the annealing temperature is 500, 700, and 1100 C, respectively. The rGO-TCF reduced at 1100 C has an Rs of 1.8 kU/sq at 70.7% transparency. Although it is highly effective, HTA is not suitable for the production of TCFs. One important obstacle is that the most commercial transparent substrate materials, e.g., glass and polymers, cannot stand a temperature higher than 500  C. As a result, CR at low temperature is important for rGO-TCFs. Sodium borohydride (NaBH4) [6,19] and hydroiodic acid (HI) [20,21] were reported to be more effective than hydrazine to reduce GO, especially GO films. An rGO-TCF reduced by NaBH4 has an Rs of 4.4 kU/sq at 81% transparency [6], while an rGO-TCF reduced by HI has a lower Rs of 1 kU/sq at 85% transparency [5]. Recently, Ning et al. reported another effective CR method using SnCl2 as reductant, which results in an rGO film with 820 U/sq sheet resistance and 83% transmittance [8].

3.1.3 3.1.3.1

Fabrication of GO-based TCF Fabrication methods of solution casting

Research on nanocarbon-based TCFs started in 2004 when Wu et al. [22] and Saran et al. [23] reported using single-walled carbon nanotube films (SWCNTs) to make TCFs by filtration transfer and dip coating, respectively. After that, hundreds of papers have published on this topic. Till now, more than 10 methods for the TCF fabrication have proposed, and the basic processes are shown in Fig. 3.2. They demonstrate the innovation of researchers as well as the inspiration from a highly developed printing industry on film formation using traditional materials. Hu et al. have done a detailed review of the methods for the fabrication of CNT-TCFs [24]. These methods are also suitable for the fabrication of GO-based TCFs because the dispersion of graphene and GO has very similar characteristics to those of CNTs. After 10 years of research and development, some methods are still widely considered to be lab-scale only, such as filtration transferring [22,25e29], spin coating [30e32], and LangmuireBlodgett (LB) coating [33,34], because these methods are mainly limited by a low-coating efficiency, and the nature of the equipment is intrinsically hard to scale up. Some methods have potential to be scaled up on industrial scale, like dip coating [23,35], spray coating [22,36e40], blade coating, and rod coating [41,42]. Although lab-scale fabrication methods are less likely to be used to realize the industrial production of G-TCFs, most fundamental research on TCFs such as percolation behavior, temperature- and frequency-dependent transport, and factors that affect their properties are usually based on films fabricated using these methods. This is because the filtration transferring method leads to uniform and reproducible films, and the network density of G-TCFs can be precisely controlled by varying the dispersion concentration. The LB-coating method can be used to preciously

Preparation of graphene electrode

31

GO solution Filtration membrane Vacuum

GO film Substrate

Filtration transfering

Spin coating

LB/Dip coating

Gas Ink

Spray coating

Rod coating

Impression cylinder

Blade coating

Curtain die Ink

Blade

Gravure cylinder

Substrate Ink

Gravure printing

Curtain printing

Figure 3.2 Depiction of various solution casting methods for graphene oxideebased TCFs.

control the film thickness down to a monolayer of graphene sheets. These features facilitate the fabrication of TCFs with precisely controlled microstructures that may have desired properties.

3.1.3.2

Fabrication of GO-based hybrid TCFs

To summarize the properties of graphene-based TCFs fabricated by various solution casting methods, G-TCFs, no matter whether using directly exfoliated graphene or rGO, mostly have a sDC/sOP value lower than 1, which is far from what is required for applications. As a result, in recent years, the fabrication of GO-based hybrid TCFs has been the main trend for this topic. Silver nanowires (AgNWs) have been utilized as a transparent conductive electrode in organic photovoltaic devices because of their excellent conductivity and transparency [43]. However, the AgNWs deposited on the substrate had a relatively low adhesion force and a low resistance to corrosion because they are composed of a metal and are quite fragile because their dimensions, which are several tens of nanometers in diameter and micrometers in length. To overcome this problem, Ahn et al. [44,45] improved electrodes based on AgNWs by combining them with graphene.

32

Graphene for Flexible Lighting and Displays

In particular, rGO could prevent chemical reactions and thermal oxidation of AgNWs at high temperature and humidity. Zhang et al. reported a kind of solution-processible TCF consisting of rGO sheets and AgNWs. The sandwiched structure of rGO/AgNWs/rGO is readily deposited via layer-by-layer blade coating on the rigid glass or flexible PET substrate at mild annealing temperature. The rGO nanosheets sandwiching the AgNW networks not only induce close contact of the AgNWs networks as a clipper to improve the conductivity but also link the discrete AgNWs as a connector to improve the uniformity of the film conductivity (Fig. 3.3). The transparent rGO/AgNWs/rGO film exhibits sheet resistance as low as 14.29 U/,, with transmittance over 90% at 550 nm. Moreover, the rGO/AgNWs/rGO composite film shows good ambient stability due to the rGO coverage and the existing charge interaction between AgNWs and rGO [46,47]. Kim et al. reported the hybrid coating based on the rGO, CNTs, and AgNWs using a spraying method. The overall characteristics of multilayers based on rGO, CNTs, and AgNWs were found to be much better than those of the single-layer AgNW coating. The rGO and CNT layers served to protect the AgNW layer from damage due to bending, contact sliding motions, corrosion, and oxidation due to the variation of temperature and humidity. Furthermore, the rGO and CNT layers showed a reduction in the haze of 20%e55% compared with the single layer of AgNWs [48].

(a)

(b)

(c)

(d)

Figure 3.3 SEM images of (a) reduced graphene oxide (rGO) nanosheets, (b) pristine Silver nanowires (AgNWs), (c) rGO/AgNWs, and (d) rGO/AgNW/rGO structures on glass substrates [46].

Preparation of graphene electrode

33

Hu et al. reported a fabrication of poly(3, 4-ethylenedioxythiophene):poly(styrene sulfonate):graphene:ethyl cellulose (PEDOT:PSS:G:EC) hybrid electrodes by roll-toroll (R2R) process, which allows for the elimination of strong acid treatment. The high-performance flexible printable electrode includes a transmittance of 78% at 550 nm and a sheet resistance of 13 U/sq with excellent mechanical stability. These features arise from the PSS interacting strongly with the ethoxyl groups from EC promoting a favorable phase separation between PEDOT and PSS chains, and the highly uniform and conductive G:EC enable rearrangement of the PEDOT chains with more expanded conformation surrounded by G:EC via the pep interaction between G:EC and PEDOT (Fig. 3.4). The hybrid electrodes are fully functional as universal electrodes for outstanding flexible electronic applications [49].

3.1.3.3

Continuous fabrication of GO-based TCFs

The scaling-up of TCF production relies on the design of highly automatic equipment with intelligent programming based on some lab-scale fabrication methods. The continuous fabrication of SWCNT-TCFs now has been realized through an R2R printing process by slot casting and printing can be performed at a high speed up to 100 m/ min on flexible or rigid substrates with several meters wide [50]. Some other methods that have been widely used in the printing industry, such as a gravure press [8], reverse roll painting, and fountain/curtain coating, are also proposed to realize the industrial production of CNT- and G-TCFs [51]. Recently, fabrication of PEDOT:PSS:G:EC hybrid electrodes by R2R allows for the elimination of strong acid treatment. The hybrid electrodes are fully functional as universal electrodes for outstanding flexible electronic applications (Fig. 3.5) [49]. Hu et al. reported the continuous fabrication of hybrid rGO-TCF by the shear force of slot-die printing during R2R process for fabricating oriented PEDOT:PSS:rGO composite. The subsequent HI acid posttreatment can remove insulating content of conductive polymer as well as reduce the GO to enhance the conductivity of TCFs to 1949 S cm⁻1 with a sheet resistance of 51 U/sq and 82% transmittance. In addition, these FTEs demonstrate remarkable flexural endurance even under extreme bending situation. The TCFs possess a low cost of only 2.8 $ for per square meter due to the carbon materials and R2R technologies [52].

3.2

Transfer methods of CVD grown graphene

Chemical vapor deposition (CVD) on metals has been extensively investigated to prepare high-performance flexible transparent electrode because highly conductive and transparent graphene film can be synthesized in a scalable way [53]. CVD graphene electrode has demonstrated its great potential in a wide range of flexible lighting and display devices [54]. For all these applications, CVD-grown graphene must be transferred from metals to flexible substrates. Although high-quality graphene can be readily grown on metals, the transferred graphene is prone to structural damage and contamination. As a result, the device performances fall far behind the

34

O

O

(01

O O

O

O S

0)

OH O O S

200

H-bond OH

O O S

O

PSS n

Ionic bond

180 160

180

PEDOT:PSS PEDOT:PSS:G:EC Thickness

150

140

120

120

90

100 80

60

60

30

40 20 0 0.2

δ

0 0.4 0.6 0.8 1.0 Coating radio (web speed/roll speed)

1.2

Quinoid

Figure 3.4 Diagram of the interaction between PEDOT chains and G:EC, and the electrical properties and thickness of PEDOT:PSS composite films via roll-to-roll process at different coating ratio [49].

Graphene for Flexible Lighting and Displays

PEDOT

Benzene

210

220

O O

OH

EC

(100)

Sheet resistance (Ω sq–1)

O

O

Thickness (nm)

O

Preparation of graphene electrode

35

(II)Gravure printing: PEDOT:PSS:G:EC (I) Cleaning and pretreatment for PET substrates (IV) Annealing treatment and encapsulation (III) Slot die: ethylene glycol

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

Figure 3.5 Process and devices of the four steps in the roll-to-roll process for a PEDOT:PSS:G: EC film. The system mainly comprised of unwinding, cleaning, rectification, corona, gravure printing, slot die coating, thermal annealing, and rolling units [49].

expectations as compared with those fabricated by using the exfoliated graphene flakes [55,56]. Therefore, improving the transfer process has currently become the bottleneck to realizing the application of CVD graphene as high-performance flexible transparent electrode. The application in flexible lighting and display devices poses several challenges to the transfer of CVD graphene. It is a fundamental requirement to retain the structural

36

Graphene for Flexible Lighting and Displays

integrity and uniformity of graphene by using a controllable transfer, which is especially difficult for large-area film. For some applications such as organic lighte emission diode (OLED), clean surfaces are needed to avoid current leakage and even electrical short between electrodes without the residue contamination such as polymer or metal particles. A continuous and low-cost method is equally important in terms of scalable production of large-area graphene electrode. Furthermore, some stringent requirements must be satisfied in specific applications. For example, intact graphene needs to be transferred onto a rough thin-film transistor array substrate if it is integrated with typical liquid crystal display panel. The above challenges stimulate extensive research efforts in the development of more effective and efficient transfer methods, and significant advances have been made in recent years. In this section, we introduce the representative progresses in the transfer methods of CVD graphene in terms of the different interaction between graphene, supporting layer, and target substrate. Supporting layers are widely used to improve both the structural integrity of graphene and the consistence of transfer, which has a significant impact on the performances of transferred graphene. For simplicity, typical transfer methods are classified into support-dissolving transfer, adhesion-mediated transfer, and target-supported transfer according to different separation/attachment mechanisms of graphene.

3.2.1

Support-dissolving transfer

Support-dissolving transfer is the most representative method for CVD-grown graphene, which allows the transfer of graphene onto either rigid or flexible substrate in a versatile and reproducible manner [57]. A typical transfer process involves the deposition of supporting layer on graphene, separation of graphene from metal, attachment of supported graphene onto the target substrate, and removal of supporting layer (Fig. 3.6(a)). Chemical etching is generally used as a relatively mild method to separate graphene by completely dissolving metal substrates with etchant solution. Thin polymer films such as poly(methyl-methacrylate) (PMMA) are preferred as the supporting layer because they are not only readily removed by organic solvents but also flexible with sufficient mechanical strength and stable to the etchant solution [58]. The typical PMMA-supported transfer involves spin coating and curing PMMA film, etching metal substrate, water rinsing graphene/PMMA film, attaching graphene/PMMA onto target substrate, baking, dissolving PMMA, and drying. The quality of transferred graphene is determined by several factors such as the property of PMMA film, interfacial contact between graphene and target substrate, as well as the posttreatment process. An adequate thickness is crucial for simultaneously realizing a flexible and strong PMMA film. However, structural damages such as cracks and tears are frequently observed in the transferred graphene even with the use of PMMA layer. This issue was attributed to the formation of gaps (e.g., under wrinkles or folds) between graphene and target substrate, which will lead to the damage of graphene on removing the PMMA layer [57]. Redissolution of PMMA coating was reported to be effective in reducing cracks arising from the unmatched topography of PMMA by mechanically releasing PMMA/graphene layer to improve its contact

(b)

As-grown Gr on Cu

Low surface tension liquid

Bad transfer

High surface tension liquid PMMA/graphene

PMMA/graphene

Deposit PMMA and cure

Transfer

Organic liquid substrate

Water substrate

Etch away Cu

Cracks Wash PMMA/Gr in DI water

Direction of liquid flow Direction of film deformation

Contact Old

Place PMMA/Gr on substrate

Pinned TCL

Remove PMMA with acetone

New

Preparation of graphene electrode

(a)

Pinned TCL

Lamination

Redeposit PMMA and cure

Remove PMMA and acetone

Lamination wrinkles

Graphene

Flat

Dry

Good transfer

Quartz PMMA removal

(c)

N2 blowing EVA /graphene

(d)

Graphene folds and cracks

Defect-free graphene

N2 plasma

PMMA

Capillary bridges PMMA

Si

Si

Cu etching

Cu sputter and CVD growth

Cu

Rough substrate Tight attachment without void

Si

Si Cu etching

Without pretreatment

Delamination PMMA

PMMA

Si

Si

Baking

Rough substrate Removal of EVA

Si Si

Rough substrate

37

Figure 3.6 (a) Processes for transfer of graphene films with and without the redissolution of poly(methyl-methacrylate) (PMMA) layer [57]. (b) Comparison of the PMMA/graphene film transfer process onto low surface energy substrates using liquids of low (heptane) or high (water) surface tension [60]. (c) Transfer of graphene onto a rough surface with the ethyleneevinyl acetate supporting layer [63]. (d) Illustration of the faceto-face method for transferring graphene mediated by capillary bridges [64].

38

Graphene for Flexible Lighting and Displays

with target substrate. However, the presence of interfacial water is responsible for the gaps in most cases, which originates from the remaining water used for rinsing etchants [59]. To solve this problem, several methods have been developed. The use of hydrophilic target substrate together with enhanced baking at 150 C facilitates the spreading and then the evaporation of trapped water, thus yielding nearly crack-free graphene on silicon wafer [59]. Alternatively, water can be replaced by volatile liquids with low surface tensions such as heptane, which spread readily over different solid surfaces (Fig. 3.6(b)) [60]. As a result, high-quality and uniform graphene was transferred onto a variety of target substrates, which showed significantly improved carrier mobility. Support-dissolving transfer is preferred in transferring graphene onto holey [61,62] or rough substrates [63] because the mild support-dissolving process can minimize the damage of suspended graphene caused by removing the support and the use of thin-supporting layer can improve the contact between graphene and rough surface. Particularly, ethyleneevinyl acetate (EVA) was found to be superior to PMMA in transferring graphene onto rough surface in terms of enabling conformal contact between graphene and large surface steps, which was attributed to its lower contact stiffness and elastic modulus (Fig. 3.6(c)) [63]. In general, the typical support-dissolving transfer is complicated and timeconsuming from the perspective of transfer efficiency. The quality of graphene attachment is also susceptible to the variation of operation skill. A face-to-face technique was developed to realize the spontaneous transfer of wafer-scale graphene onto target substrate, which uses graphene grown on metal film deposited onto plasma-treated target substrate (Fig. 3.6(d)) [64]. In contrast to the independent separating and attaching processes, PMMA-supported graphene was spontaneously attached to the underlying silicon substrate by the capillary bridges of nascent gas bubbles during etching the metal film. This strategy is promising for batch production of wafer-scale graphene with improved structural integrity and consistence of large-area graphene. As the target substrate is involved in the CVD growth, however, it is difficult to transfer graphene onto flexible plastic substrate using this method. The major problem in the support-dissolving transfer is the formation of contamination including the residue of support layer, particles of etching product, and interfacial water. In particular, residues of polymer support such as PMMA not only degrade the electrical performances of electronic devices (e.g., FET) by increasing the scattering of carriers and the contact resistance to metal electrode but also lower the yield and stability of lighting and display devices (e.g., OLED) by causing large current leakage and even electrical short of graphene electrode. This issue becomes even worse when multilayer graphene was prepared by using the layer-by-layer transfer, in which the surface roughness is multiplied due to the accumulation of residues. Although this issue can be fundamentally solved by using the support-free transfer, it is difficult to scale-up with a typical transferred film of centimeter size [65,66]. Therefore, most of research efforts focus on the techniques to reduce residue or alternative supporting material for clean transfer. The common methods to reduce the PMMA residue include the composition modification and posttreatments. It is easier to dissolve the cured PMMA layer by decreasing the concentration of precursor solution or using UV irradiation [67,68].

Preparation of graphene electrode

39

In the latter case, the intermolecular interaction between PMMA and graphene can be weakened by cleaving the side chain of ester groups in PMMA. Posttreatments typically involve the enhanced solvent rinsing, vacuum annealing, thermal oxidation, or radiolized water etching [55,68e74]. PMMA residues were reduced by using prolonged immersion in acetone, but they cannot be completely dissolved [71]. The dissolution can be significantly enhanced by applying both high pressure (w1 MPa) and heating (e.g., 140 C) [70]. Annealing was generally used after solvent rinsing to further reduce the polymer residue, which was reported to enable a twofold increase in carrier mobility [55]. However, such thermal treatment limits the available substrate to materials with high melting points such as silicon wafer or TEM grid. More importantly, even such harsh treatment cannot completely remove polymer residue and a thin layer of PMMA was found to remain on the surface of graphene [69]. Therefore, it is desirable to identify new support materials that are not only robust but also easy to be removed. Although alternative polymers such as poly(bisphenol A carbonate) and polystyrene were reported to form less residue than PMMA [75,76], small organic molecules seems to allow cleaner transfer due to their significantly lower sublimation temperatures and higher solubility in solvents. For instance, cyclododecane was found to be a clean support material for graphene because its low sublimating point allowed it to be removed by simple air exposure after transfer [77]. Yet, the transferred graphene suffered from structural damage with the presence of D peaks in its Raman spectra, which might be related to its relatively low mechanical strength. Similarly, polycyclic aromatic hydrocarbons such as pentacene also allow clean transfer as sublimable supporting layers [78,79]. A novel solvent intercalation strategy was used to exfoliate pentacene layer from graphene rather than the sublimation treatment that was less effective in removing the pentacene (Fig. 3.7(a)) [79]. Wafer-scale transfer was demonstrated with this intercalation strategy. The absence of D peaks in Raman spectra and high carrier mobility together with a nearly zero Dirac point voltage indicated a clean and intact transfer. The added advantage of aromatic hydrocarbon supports is that the residue would not change the band structure and Fermi level of graphene because their weak interaction with graphene causes negligible charge transfer [79]. Recently, ultraclean and damage-free transfer of graphene was accomplished by using rosin as a novel support layer (Fig. 3.7(b)) [80]. This method enabled large-area intact graphene film with a low surface roughness of 0.66 nm, thus yielding a uniform sheet resistance with significantly reduced deviation of 1% over an area of 10
10 cm2. The superior effect of rosin-supported transfer was attributed to its good solubility, weak interaction with graphene, and adequate support strength. The use of rosin-transferred graphene electrode allowed the fabrication of four-inch flexible graphene-based OLED, which further demonstrated the clean and intact transfer. Interestingly, even liquid organics such as hexane can be used as a clean support in the form of an organic/aqueous biphasic configuration [81]. Obviously, this transfer method is more suitable for small size applications such as conductive coating of AFM tip or support film of TEM grid because it is difficult to provide strong and uniform support for large area film with liquid layer.

40

(a)

Pentacene on graphene

Pentacene on graphene Cu etching transfer

Cu foil

Pentacene/ graphene

Substrate

Pentacene removal (thermal or chemical)

Graphene

Substrate

Graphene Graphene for Flexible Lighting and Displays

(c)

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and transfer

20 nm 0 nm 5

Gold/Gr/Cu Gr/Cu

Dis

3

ce

tan

PMMA/Gr/substrate

1

)

(μm

0

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and removal of gold

n

Dista

10 8

(cm

)

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0

th

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ng

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Le

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1 8 0 ) (cm dth

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uare)

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Patterned Gr/substrate

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20 um

20 um

20 um

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Sheet resista

555

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PMMA spin coating

4

565

Etching gold layer

Copper etching

Preparation of graphene electrode

(b)

0 um

10 um

20 um

0 um

10 um

20 um

Figure 3.7 (a) Transfer procedure of graphene from Cu foil to an arbitrary substrate using the pentacene-supporting layer [79]. Photographs of pentacene/graphene and graphene before and after pentacene removal (left) and the transferred graphene on 6-inch SiO2/Si wafer with corresponding optical microscopy images after chemical removal of pentacene layer (right). (b) Photograph of a 10
10 cm2 monolayer graphene film transferred onto PET using rosin with its surface roughness characterized by AFM and sheet resistance map; a four-inch flexible green organic lighteemitting diode with the rosin-transferred graphene anode [80]. (c) Schematic of the transfer methods and surface topography of Gr layers by PMMA (left) and gold (right)-supporting layers [84].

41

42

Graphene for Flexible Lighting and Displays

Noble metal film such as gold can also be used as alternative to PMMA layer because it is not only readily dissolved by a special solution but also highly stable to water and transfer etchants [82e84]. Typically, thin film of gold ( 140°C

T > 300°C

O2 trapping/stability Configuration coordinate

Figure 4.10 (a) Schematic distribution of stable doping state populations as a function of energy. The temperatures reported in the scheme represent the onset and completion process temperature determined experimentally. (b) Schematic representation of the doping state energy as a function of a generalized configuration coordinate where the barrier of activation and trap were determined theoretically. Reproduced with permission A. Piazza, F. Giannazzo, G. Buscarino, G. Fisichella, A.L. Magna, F. Roccaforte, M. Cannas, F. Gelardi, S. Agnello, Graphene p-type doping and stability by thermal treatments in molecular oxygen controlled atmosphere, J. Phys. Chem. C 119(2015) 22718e23., Copyright 2015 American Chemical Society.

Graphene doping for electrode application

Pristine graphene

69

Doped state

Annealed state

Pristine graphene

Au complex doping Au3+, AuX, Au0

Annealing AuX, Au0 aggregation X2 sublimation or evaporation

Figure 4.11 The proposed mechanism of annealing-induced degradation of an Au complexe doped graphene. Yellow spheres, orange spheres, and green spheres represent Au0, Au3þ, and X anions, respectively. Ref. Effect of anions in Au complexes on doping and degradation of graphene. Reproduced with permission (Kwon et al. 2011), Copyright 2013 The Royal Society of Chemistry.

long-term stability of graphene-doped materials [31]. In another work, Kwon et al. increased the thermal stability of doped graphene electrode through the formation of graphene overlayers. The Rs of the metal chlorideedoped graphene electrode without overlayers increased from 460 to 24,473 U/sq. after thermal annealing at 400 C. The origin of this enhanced stability can be described by the following equations [32]: 2graphene þ 2MeCl3 /2grapheneþ þ MeCl2  þ MeCl4  ðdominantÞ

(4.13)

3MeCl2  / 2Me0 Y þ MeCl4  þ 2Cl

(4.14)

MeCl4  þ 3graphene/3grapheneþ þ Me0 þ 4Cl

(4.15)

Grapheneþ þ Cl /Graphene  Cl

(4.16)

Grapheneþ þ MeCl4  /Graphene  MeCl4

(4.17)

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Graphene for Flexible Lighting and Displays

The composition of graphene chlorine and graphene metal chlorides can be adjusted by annealing process. The chlorine vapor was generated from the unstable part of MeCl 4 . The amount of electronegativity in the combined chlorineecarbon atoms is higher than carbon atoms in the graphene nanosheet. As a result, temporary dipole electrons are formed by the difference of the electronegativity which transformed the graphene to p-type (Eqs. (4.16) and (4.17)). Chlorine gas is also produced by MeCl 2 during the annealing process. Therefore, the improved stability originated from the annealing process and application of an increased number of overlayers in graphene.

Acknowledgments This research was supported in part by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT(NRF2017M3D1A1039379) and in part by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. 2017K1A3A1A67014432).

References [1] T. Wehling, K. Novoselov, S. Morozov, E. Vdovin, M. Katsnelson, A. Geim, A. Lichtenstein, Molecular doping of graphene, Nano Lett. 8 (2008) 173e177. [2] X. Wang, X. Li, L. Zhang, Y. Yoon, P.K. Weber, H. Wang, J. Guo, H. Dai, N-doping of graphene through electrothermal reactions with ammonia, Science 324 (2009) 768e771. [3] H. Liu, Y. Liu, D. Zhu, Chemical doping of graphene, J. Mater. Chem. 21 (2011) 3335e3345. [4] K.C. Kwon, K.S. Choi, C. Kim, S.Y. Kim, Effect of transition-metal chlorides on graphene properties, Phys. Status Solidi (A) 211 (2014) 1794e1800. [5] Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier, H. Dai, Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction, Nat. Mater. 10 (2011) 780. [6] S. Agnoli, M. Favaro, Doping graphene with boron: a review of synthesis methods, physicochemical characterization, and emerging applications, J. Mater. Chem. 4 (2016) 5002e5025. [7] H. Wang, Y. Zhou, D. Wu, L. Liao, S. Zhao, H. Peng, Z. Liu, Synthesis of boron-doped graphene monolayers using the sole solid feedstock by chemical vapor deposition, Small 9 (2013) 1316e1320. [8] X. Miao, S. Tongay, M.K. Petterson, K. Berke, A.G. Rinzler, B.R. Appleton, A.F. Hebard, High efficiency graphene solar cells by chemical doping, Nano Lett. 12 (2012) 2745e2750. [9] K.C. Kwon, S. Kim, C. Kim, J.-L. Lee, S.Y. Kim, Fluoropolymer-assisted graphene electrode for organic light-emitting diodes, Org. Electron. 15 (2014) 3154e3161. [10] K.C. Kwon, K.S. Choi, B.J. Kim, J.-L. Lee, S.Y. Kim, Work-function decrease of graphene sheet using alkali metal carbonates, J. Phys. Chem. C 116 (2012) 26586e26591. [11] Y. Shi, K.K. Kim, A. Reina, M. Hofmann, L.-J. Li, J. Kong, Work function engineering of graphene electrode via chemical doping, ACS Nano 4 (2010) 2689e2694.

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[12] H.-J. Shin, W.M. Choi, D. Choi, G.H. Han, S.-M. Yoon, H.-K. Park, S.-W. Kim, Y.W. Jin, S.Y. Lee, J.M. Kim, Control of electronic structure of graphene by various dopants and their effects on a nanogenerator, J. Am. Chem. Soc. 132 (2010) 15603e15609. [13] D.-C. Choi, M. Kim, Y.J. Song, S. Hussain, W.-S. Song, K.-S. An, J. Jung, Selective AuCl3 doping of graphene for reducing contact resistance of graphene devices, Appl. Surf. Sci. 427 (2018) 48e54. [14] K.C. Kwon, K.S. Choi, S.Y. Kim, Increased work function in few-layer graphene sheets via metal chloride doping, Adv. Funct. Mater. 22 (2012) 4724e4731. [15] C.W. Jang, J.H. Kim, J.M. Kim, D.H. Shin, S. Kim, S.-H. Choi, Rapid-thermal-annealing surface treatment for restoring the intrinsic properties of graphene field-effect transistors, Nanotechnology 24 (2013) 405301. [16] A. Kasry, M.A. Kuroda, G.J. Martyna, G.S. Tulevski, A.A. Bol, Chemical doping of largearea stacked graphene films for use as transparent, conducting electrodes, ACS Nano 4 (2010) 3839e3844. [17] Q.B. Zheng, M.M. Gudarzi, S.J. Wang, Y. Geng, Z. Li, J.-K. Kim, Improved electrical and optical characteristics of transparent graphene thin films produced by acid and doping treatments, Carbon 49 (2011) 2905e2916. [18] L. D’Arsié, S. Esconjauregui, R.S. Weatherup, X. Wu, W.E. Arter, H. Sugime, C. Cepek, J. Robertson, Stable, efficient p-type doping of graphene by nitric acid, RSC Adv. 6 (2016) 113185e113192. [19] J.K. Wassei, K.C. Cha, V.C. Tung, Y. Yang, R.B. Kaner, The effects of thionyl chloride on the properties of graphene and grapheneecarbon nanotube composites, J. Mater. Chem. 21 (2011) 3391e3396. [20] Y. Lu, W. Chen, Y. Feng, P. He, Tuning the electronic structure of graphene by an organic molecule, J. Phys. Chem. B 113 (2008) 2e5. [21] K.C. Kwon, B.J. Kim, C. Kim, J.-L. Lee, S.Y. Kim, Comparison of metal chloride-doped graphene electrode fabrication processes for GaN-based light emitting diodes, RSC Adv. 4 (2014) 51215e51219. [22] D.B. Farmer, R. Golizadeh-Mojarad, V. Perebeinos, Y.-M. Lin, G.S. Tulevski, J.C. Tsang, P. Avouris, Chemical doping and electron hole conduction asymmetry in graphene devices, Nano Lett. 9 (2008) 388e392. [23] A. Gupta, G. Chen, P. Joshi, S. Tadigadapa, P. Eklund, Raman scattering from highfrequency phonons in supported n-graphene layer films, Nano Lett. 6 (2006) 2667e2673. [24] A. Das, S. Pisana, B. Chakraborty, S. Piscanec, S. Saha, U. Waghmare, K. Novoselov, H. Krishnamurthy, A. Geim, A. Ferrari, Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor, Nat. Nanotechnol. 3 (2008) nnano. 2008.2067. [25] A. Kuruvila, P.R. Kidambi, J. Kling, J.B. Wagner, J. Robertson, S. Hofmann, J. Meyer, Organic light emitting diodes with environmentally and thermally stable doped graphene electrodes, J. Mater. Chem. C 2 (2014) 6940e6945. [26] J. Meyer, P.R. Kidambi, B.C. Bayer, C. Weijtens, A. Kuhn, A. Centeno, A. Pesquera, A. Zurutuza, J. Robertson, S. Hofmann, Metal oxide induced charge transfer doping and band alignment of graphene electrodes for efficient organic light emitting diodes, Sci. Rep. 4 (2014) 5380. [27] Y. Fan, L. Kang, W. Zhou, W. Jiang, L. Wang, A. Kawasaki, Control of doping by matrix in few-layer graphene/metal oxide composites with highly enhanced electrical conductivity, Carbon 81 (2015) 83e90. [28] C.A. Klein, STB model and transport properties of pyrolytic graphites, J. Appl. Phys. 35 (1964) 2947e2957.

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[29] Y. Hong, M. Wu, G. Chen, Z. Dai, Y. Zhang, G. Chen, X. Dong, 3D printed microfluidic device with microporous Mn2O3-modified screen printed electrode for real-time determination of heavy metal ions, ACS Appl. Mater. Interfaces 8 (2016) 32940e32947. [30] A. Piazza, F. Giannazzo, G. Buscarino, G. Fisichella, A.L. Magna, F. Roccaforte, M. Cannas, F. Gelardi, S. Agnello, Graphene p-type doping and stability by thermal treatments in molecular oxygen controlled atmosphere, J. Phys. Chem. C 119 (2015) 22718e22723. [31] K.C. Kwon, B.J. Kim, J.-L. Lee, S.Y. Kim, Effect of anions in Au complexes on doping and degradation of graphene, J. Mater. Chem. C 1 (2011) 2463e2469. [32] K.C. Kwon, S.Y. Kim, Extended thermal stability in metal-chloride doped graphene using graphene overlayers, Chem. Eng. J. 244 (2014) 355e363.

Technical issues and integration scheme for graphene electrode OLED panels

5

Jaehyun Moon, Jin-Wook Shin, Hyunsu Cho, Jun-Han Han, Byoung-Hwa Kwon, Jeong-Ik Lee, Nam Sung Cho Flexible Device Research Group, Electronics and Telecommunications Research Institute (ETRI), Daejeon, Republic of Korea

5.1

Introduction

Thanks to its versatile chemical bonding capacity, carbon can form many allotropes. Among known eight allotropes, graphene is the only two-dimensional sheet and may be considered as a large assembly of aromatic molecule. Graphene is a two-dimensional film, in which carbon atoms are arranged in a hexagonal array [1]. Since its isolation in 2004, graphene research has experienced the hype stage [2e4]. Graphene has been extensively investigated from atomistic level to device level. As an active device component, its applications have been probed virtually in all device area. Graphene was proposed as a novel material that will push the boundaries of existing technologies to the extreme. Various platforms for transistors, sensors, energy components, shielding layer, structural components, and biomedical applications have been suggested and reported [5]. In this chapter, we aim to explore the technical issues relevant to organic lighteemitting diodes (OLEDs) [6]. To be specific, we address processing issues relevant to the pattering of graphene films and their integration into large area OLED panels as transparent electrodes [7]. Graphene can offer optically transparency, electrically conductivity, chemical stability, and mechanically flexibility. Thus, from the perspective of optoelectronic applications, of which OLED belongs to, graphene emerges as a fitting choice for replacing the dominantly used transparent electrode material indium tin oxide (ITO) [8]. However, the reported majority of outstanding graphene properties are based on single-domain graphene which has not undergone device-level processing. In real applications, singledomain graphene is rarely used, and the characteristics of a graphene containing unit device cannot fully represent the overall performances of a panel-level device array. In fact, not the graphene properties but the processing and integration strongly matter to bring graphene electrode OLED panel into reality. In this chapter, we first explore important technical features of graphene anode OLEDs. Here, we contrast the difference between ITO anode OLEDs. Next, we examine the technical hurdles which impede the realizations of pixelated graphene anode OLED on large area. We suggest and demonstrate a systematic procedure which

Graphene for Flexible Lighting and Displays. https://doi.org/10.1016/B978-0-08-102482-9.00005-8 Copyright © 2020 Elsevier Ltd. All rights reserved.

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Graphene for Flexible Lighting and Displays

allows the implementation of display compatible processes [7]. Finally, we discuss the possibility of applying graphene for flexible OLED panels. For a longtime, graphene has been contemplated as an alternative electrode material to its brittle ITO counterpart to realize flexible OLEDs.

5.2

Graphene preparation for OLED applications

Because we aim to use graphene in OLEDs as a transparent electrode, the choice of graphene preparation is important. Graphene can be prepared by numerous methods [9]. Technologies related to graphene growth and transfer are quite extensive and can be easily amounted to an independent chapter. Reader interested in this field may consult a published article [10]. To use graphene or multilayer graphene in OLEDs, several requirements must be met. First, it must be possible to have graphene over a large area. Second, as a transparent electrode, graphene must bear acceptable levels of electrical and optical properties. Third, graphene must be patternable into geometrically accurate shapes and size. So far, the patterning issue of graphene has not been a major scientific interest. However, establishing a reliable patterning process is of very high importance in realizing commercial-level products. We will return to this topic in the forthcoming sections. For OLED application, chemical vapor deposition (CVD) is the preferred graphene growth method [11,12]. In the CVD method, large area graphene films are obtained mainly by thermally decomposing carbon-containing gas species (e.g., CH4). Carbon atoms absorb on a catalytic metallic surface on which graphene nucleate and grow isothermally in a polydomain fashion. The choice of catalytic metal is Cu. Compared with Ni, Cu gives monolayeric graphene with excellent surface flatness and uniformity. Moreover, layer-by-layer doping technique can be applied to lower the sheet resistance of the graphene film. Thanks to the advancement in CVD growth and isolation/transfer process, graphene and multilayer graphene film can be formed on the substrate of choice on large area. Graphene film grown by CVD method offers acceptable sheet resistance (w50 U/Sq.), mechanical compliance, and transmittance (w80%) [13]. CVD-grown graphene can be transferred on plastic substrates. This makes CVD graphene as a unique choice for the use in flexible graphene OLEDs [14].

5.3 5.3.1

Technical issues of OLEDs having graphene film electrodes Actual examples. Graphene versus ITO OLEDs

In this section, we investigate the characteristics of graphene anode OLEDs. At first glance, replacing ITO anode to graphene might look trivial. However, because graphene and ITO are different in many aspects, one needs to pay close attention to the differences to make most of the graphene anode. In particular, optical and interfacial considerations are of high importance [15e18].

Technical issues and integration scheme for graphene electrode OLED panels

75

To grasp the difference, we present actual OLED characteristics measured from real devices [6]. Fig. 5.1 summarizes the device performance of bottom emissione type phosphorescent OLEDs. As the emissive layer, we used a green dopant of tris(2-phenylpyridinato-C2,N) iridium(III) (Ir(ppy)3) and a host of 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (DCzPPy). The transparent electrodes were 70 nm ITO and four-layered graphene film. Our graphene film is grown by a CVD method on a Cu foil. The isolated monolayer graphene was p-doped using benzimidazole (C7H6N2)containing solution [19]. Four-layered graphene film was obtained by layer-by-layer transfer. The direct transmittance (DT) and sheet resistance of our graphene film were 83% (at 550 nm) and 65 U/Sq., respectively. For convenience, we refer an OLED with ITO anode as ITO OLED and an OLED with graphene film anode as graphene OLED. Both the current density (J) and luminance (L) levels of ITO OLED were observed to be higher than those of the graphene OLED (Fig.5.1(a)). Accordingly, the ITO OLED showed higher external quantum efficiency (EQE, %) level than

(a) ITO OLED Graphene OLED

16

10

10 10

10

10

10

14

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EQE (%)

2

L (cd/m )

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(b)

10

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ITO OLED Graphene OLED

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10 –1

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5 4 3 2 Applied voltage (V)

(c)

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10 10 2 L (cd/m )

(d )

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Graphene OLED 60° 40° 20° 0° Norm. Int.

Norm. Int.

ITO anode OLED 60° 40° 20° 0°

10

400

500

600

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700

400

500

600

700

Wavelength (nm)

Figure 5.1 (a) The JVL characteristics of ITO anode and graphene film anode OLEDs. (b) The EQEs of ITO anode and graphene film anode OLEDs. (c) The EL spectra of ITO anode OLED as a function of viewing angle. (d) The EL spectra of graphene film anode OLED as a function of viewing angle. EL, electroluminescence; EQE, external quantum efficiency; ITO, indium tin oxide; OLED, organic lighteemitting diode. Reproduced by permission from Elsevier, J. Moon, J.W. Shin, H. Cho, J.H. Han, N.S. Cho, T.J. Lim, S.K. Park, H.K. Choi, S.Y. Choi, J.H. Kim, M.J. Maeng, J. Seo, Y. Park, J.I. Lee, Technical issues in graphene anode organic light emitting diodes, Diam. Relat. Mater. 57 (2015) 68e73.

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Graphene for Flexible Lighting and Displays

that of graphene OLED (Fig.5.1(b)). The EQE of graphene OLED is approximately 90% of ITO OLED at luminance (L) level of 1000 cd/m2. It is important to notice that for a given organic stack structure, the EQE of graphene OLED is always lower than ITO OLED. The results of Fig.5.1(a),(b) can be attributed to various factors. The hole injection from graphene surface into the adjacent hole transport layer (HTL) is not necessarily effective as the case of ITO. The hole injection is closely related to the energy alignment of the work function of the anode and the highest occupied molecular orbital (HOMO) level of the adjacent organics. This part calls for interfacial modification to enhance hole injection and charge transport characteristics. The optical properties of graphene film can be a factor. Generated light passes through the transparent anode. Thus, the extinction coefficient (k), which is related to absorption, of the anode is expected to have effect on the OLED efficiency. We will return to the absorption issue later. Fig. 5.1 (c),(d) shows the angular electroluminescence (EL) spectra of ITO and graphene OLEDs. In the case of ITO OLED, shoulder development in the EL spectrum is apparent as the viewing angle changes. In addition, the main peak shifts slightly toward lower wavelength as the viewing angle increases. However, in the case of graphene OLED, all EL spectral lines almost superimpose, indicating negligible angular dependency. The presence of angular EL spectra dependency is a demerit of light source quality [20,21]. From practical viewpoint, the absence of angular dependency is highly preferred because the perceived light or color will be uniform. The results strongly indicate that ITO and graphene OLEDs are different in electrical performance and EL spectral characteristics, which have to be considered in designing graphene OLEDs.

5.3.2

Optical issues

In this part, we explore the aspect of internal optics relevant to graphene OLED [22,23]. In a simplified picture, conventional bottom emissiveetype OLEDs can be structurally described as a vertical stack of a metallic cathode, organic layers, and a transparent anode. Typical organic thickness does not exceed 1 mm. At such thickness, internal interference or microcavity strongly matters. This situation is schematically described in Fig. 5.2(a). Also an actual scanning electron microscope (SEM) image of a graphene OLED is shown (Fig. 5.2(b)). The light generated in the emissive organic layer travels downward (DEO) and upward (UEO). The optical component U EO reflects at the highly reflective metallic cathode, resulting in an optical component U ER. Because of the difference in the refractive indices of organics and transparent anode, there exists a weak mirror surface at the HTL/anode interface. The DEO contributes to the appearance of DER. As one may easily imagine, all optical components interfere, and the internal optics is fairly complicated to describe. Reflection takes places at the surface where the optical contrast is large. Thus, among many factors governing the internal optics, the reflectance of the electrodes is of special interest. The reflectance of Al, which is typical cathode material, exceeds 93% in the visible range. Thus, the reflectance of interest is that of anode/HTL interface. If the reflectance at the interface is low, it is not easy to form a cavity structure.

Technical issues and integration scheme for graphene electrode OLED panels

(a)

77

(b) Cathode ETL

UE o

UE R

Aluminum

EL* DE o

HTL

Organic layers DE R

Anode Glass

Graphene 500 nm

*:Emissive layer

Figure 5.2 (a) Schematics of simulation cell and optical components. (b) An SEM image of a graphene organic lighteemitting diode. (a) Reprinted by permission from Elsevier, J. Moon, J.W. Shin, H. Cho, J.H. Han, N.S. Cho, T.J. Lim, S.K. Park, H.K. Choi, S.Y. Choi, J.H. Kim, M.J. Maeng, J. Seo, Y. Park, J.I. Lee, Technical issues in graphene anode organic light emitting diodes, Diam. Relat. Mater. 57 (2015) 68e73. (b) Reproduced by permission from American Institute of Physics, J. Hwang, H.K. Choi, J. Moon, T. Kim, J.W. Shin, C.W. J. Joo, J.H. Han, D.H. Cho, J.W. Huh, J.I. Lee, H.Y. Chu, Multilayered graphene anode for blue phosphorescent organic light emitting diodes, Appl. Phys. Lett. 100 (2012) 133304e133307.

The reflectance of anode/HTL can be calculated using a transfer matrix method. In turn, the reflectance is related to the cavity enhancement factor (Gcav(l)) as the following [24]: pffiffiffiffiffi 2 Tt ð1 þ Rr Þ   Gcav ðlÞf pffiffiffiffiffiffiffiffiffiffi 2 pffiffiffiffiffiffiffiffiffiffi D4 ð1  Rt Rr Þ þ 4 Rt Rr sin2 2 where Rt and Rr are the reflectance of the transparent electrode and the reflective electrode, respectively, and Tt is the transmittance of the transparent electrode. Df is phase change taking place on reflection. Constructive interference takes place when the Df equals 2mp, where m is an integer. The calculated profile of Gcav(l) can be applied to deduce the emission augmentation or reduction at a specific wavelength. In addition, for given wavelength, the influence of reflectance can be estimated. Fig. 5.3 shows the calculated reflectance of ITO/organics and graphene/organics interfaces. Also the calculated Gcav(l)s are also shown. In the calculation course, a constructive interference condition at a wavelength of 520 nm is assumed. While the reflectance of ITO/organics is showing variation, the reflectance of graphene OLED is hardly varying. Also the reflectance value is fairly low. Accordingly, the Gcav(l) at the wavelength of 520 nm, there exists noticeable difference between two devices. In the ITO case, the Gcav(l) is approximately 1.3, while the corresponding value of graphene case does not exceed one.

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ITO Graphene

8

1.4 1.2 1.0 0.8

4

0.6 2

0.4 0.2

0 400

Gcav (a.u.)

Reflectance (%)

6

500

600

0.0 700

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Figure 5.3 The simulated reflectance from organic layer and cavity enhancement factor of indium tin oxide (ITO) and graphene electrode. Reproduced by permission from IEEE, H. Cho, J.W. Shin, N.S. Cho, J. Moon, J.H. Han, Y.D. Kwon, S. Cho and J.I. Lee, Optical effects of graphene electrodes on organic light-emitting diodes, IEEE J. Sel. Top. Quantum Electron., 22 (2016) 7230237.

To verify the results of Fig. 5.3, we performed optical simulations on devices, which are based on multiple interference theory and dipole oscillation theory (Fig. 5.4). The light source or the dipole was positioned at the emissive layer (EML) side of the electron transport layer (ETL)/EML interface. The theoretical background of the simulations can be found elsewhere [24]. The optical constants of graphene were obtained from published literature. In the simulation courses, we have varied the thickness of the HTL, while fixed the ETL thickness as 60 nm. The thicknesses of metallic Al cathode and EML were 100 and 20 nm, respectively. In the ITO case, the efficiency (EQE, %) shows evident sinusoidal oscillatory behavior as the HTL thickness changes. In contrast, in the graphene case, the oscillatory behavior is low and the efficiency changes slightly in narrow widow. The results of Fig. 5.4 strongly indicate that the graphene/organics interface cannot form a strong mirror, which is indispensable to induce microcavity effect in OLEDs. In other words, the possibility of enhancing the emission or efficiency is limited when graphene is used as a transparent electrode. Fig. 5.5(a) shows the normalized angular emission profile of ITO and graphene OLEDs. Experimental angular emission profiles obtained under different organic thicknesses are useful to gain insights on the microcavity and anode/HTL reflectance on device level. The OLED structure relevant to Fig. 5.5 can be found elsewhere [18]. The HTL thickness was varied to alter the microcavity condition. The internal interference or microcavity is a function of the length, which corresponds to the total organic thickness in a given angle. If the length in a specific angle matches the constructive condition of a wavelength, the emission will be enhanced at the corresponding angle. First, we explore the ITO OLEDs. The OLEDs with HTL thicknesses of 70 and

Technical issues and integration scheme for graphene electrode OLED panels

ITO anode (70 nm) Graphene anode (1 nm) Graphene anode (2 nm)

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15

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0

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Figure 5.4 Simulated EQEs of ITO anode and graphene film anode OLEDs as a function of HTL thickness. Inset is the actual OLED structure used in simulations. EQE, external quantum efficiency; HTL, hole transport layer; ITO, indium tin oxide; OLED, organic lighteemitting diode. Reproduced by permission from Elsevier, J. Moon, J.W. Shin, H. Cho, J.H. Han, N.S. Cho, T.J. Lim, S.K. Park, H.K. Choi, S.Y. Choi, J.H. Kim, M.J. Maeng, J. Seo, Y. Park, J.I. Lee, Technical issues in graphene anode organic light emitting diodes, Diam. Relat. Mater. 57 (2015) 68e73.

175 nm exhibited higher luminance in the normal incidence direction (q ¼ 0 ) distributions. In contrast, OLEDs with HTL thicknesses of 105 and 140 nm exhibited higher luminance in the high angle direction (q > 40o). The variations in the luminance distribution as a function of HTL thickness clearly show the presence of a microcavity effect in the ITO OLEDs. The result also strongly indicates the intrinsic presence of mircocavity in ITO OLEDs. In contrast, the angular luminance distributions of graphene OLEDs turned out to be not a strong function of the HTL thickness. The luminance level varies within 10% in the angular range of 0 >1%) without obvious changes in their electrical performance [2]. Conventional materials for electrodes or interconnects in microchips, comprising single-crystal inorganic metals (heavily doped silicon), polycrystalline films of evaporated metals (e.g., copper, nickel, cobalt, and so on), and metal oxides (e.g., indium tin oxide, ITO) [3,4], are not applicable in stretchable electronic devices due to their brittle and rigid nature. To solve this problem, tremendous research efforts have been carried out in developing stretchable conductors. Basically, there are two main strategies, one is to apply new structural layouts on conventional electrodes and the other one is to design and synthesize new materials and composites, which are intrinsically stretchable and can also use special structural layouts to enhance the stretchability. Common structural layouts for stretchable electrodes include “wavy” geometries, percolating networks, serpentine interconnects, coiled structures, and etc. While making conventional electrodes into such configurations, they are able to sustain enough conductivity at high strains. However, because the fabrication processes of conventional conductors involve extreme conditions, such as high temperature or vacuum evaporation, they are highly dependent on the very expensive tools. Moreover, the adhesion of the conductive layer to elastomer substrate is weak, leading to poor

Graphene for Flexible Lighting and Displays. https://doi.org/10.1016/B978-0-08-102482-9.00009-5 Copyright © 2020 Elsevier Ltd. All rights reserved.

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durability. To fabricate large-area stretchable electronic devices, a reliable method with fairly simple and low cost techniques is very attractive. Novel nanomaterial conductors made from carbon nanotubes, graphene, and metal nanowires (mNWs)/nanoparticles are excellent flexible electrodes. However, due to the in-plane stiffness and large Young’s modulus, it is essential to form networks and composites to enable them as stretchable electrodes. For example, randomly assembled networks of one-dimensional mNWs or carbon nanotubes are very robust conductors, which are able to maintain conductivity up to 100% strain. Compared with metal electrodes, carbon-based electrodes have the advantages of being light weight, low cost, highly transparent, and work function tunable although the conductivity is slightly lower. Utilizing randomly formed network of carbon nanotubes, stretchable electronic devices such as transistors, sensors, displays, etc., were successfully fabricated, showing comparable electrical performances as traditional Si-based devices. Graphene is one of the strongest materials in existence. Its theoretical strength is defined as the maximum stress to sustain in the absence of any defects. It has an in-plane stiffness of 340 Nm
1 and a Young’s modulus of 0.5 TPa, which is intrinsically not an ideal material for stretchable electrodes. The strong carbonecarbon network does not provide any energy dissipation mechanisms for applied strain and therefore readily cracks at less than 5% strain. To apply the atomically thick, highly transparent, and highly conductive graphene in stretchable transparent devices, overcoming the mechanical limitations and sustaining its extraordinary properties under strain are desired. Theoretical calculations show that crumpling and interplay between different layers should strongly decrease the stiffness. When bi- or trilayer graphene are stretched at 30% strain, they exhibit 13 times smaller resistance change than that of monolayer graphene [5]. In particular, when graphene is adhered onto certain substrates and applied shear forces, both adhesion and delamination will occur and their intensities vary depending on the surface conditions, such as roughness, moisture, chemical reactivity, etc. The interaction between graphene and target substrates is believed to be van der Waals interactions but will also be complicated by capillary and contamination effects [6]. This is because surface roughness plays a vital role in the interfacial toughness with fracture mix-mode of tractioneseparation relations. A variety of adhesion/ separation energies of graphene to different substrates have been summarized in Table 9.1. When graphene is adhered onto silicon oxide (SiO2), the separation energy is in the range of 0.151e0.45 J/m2. While graphene is transferred onto polyethylene terephthalate (PET), the separation energy dropped to be just 0.54 mJ/m2, suggesting that the friction of graphene/soft polymer is lower than that of graphene/SiO2. Therefore, the maximum strain that can be transferred to graphene by stretching the substrate is dependent on the interfacial shear strength between graphene and the substrate, leading to a higher strain tolerance of graphene on elastomer substrates. In addition, to weaken the strain by using low modulus elastomer as substrate, similar to conventional materials, constructing structural layouts on graphene and utilizing graphene kirigami and origami to enable stretchable graphene electrodes are also very effective. On the other hand, increasing the amount of conductive paths and

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Table 9.1 Measured adhesion/separation energies of exfoliated graphene onto different substrates. Substrate materials

G (J/m2)

No. of layers

References

SiOx

0.45

1

Ref. [7] (2011)

0.31

2e5

Ref. [7] (2011)

0.24

1

Ref. [8] (2016)

0.14

1

Ref. [9] (2013)

w5

Ref. [10] (2010)

1

Ref. [11] (2014)

0.15 PET

0.54  10


3

Reproduced with permission from D. Akinwande, C.J. Brennan, J.S. Bunch, P. Egberts, J.R. Felts, H. Gao, R. Huang, J.-S. Kim, T. Li, Y. Li, K.M. Liechti, N. Lu, H.S. Park, E.J. Reed, P. Wang, B.I. Yakobson, T. Zhang, Y.W. Zhang, Y. Zhou, Y. Zhu, A review on mechanics and mechanical properties of 2D materialsdgraphene and beyond, Extreme Mech. Lett. 13 (2017) 42e77. Copyright 2017 Elsevier Ltd.

forming percolation pathway under strain will also improve the conductivity strain tolerance. Below we mainly introduce the recent progress on stretchable graphene electrodes from the aspects of preparation and application. The section of preparation is further divided into three parts based on the methodology, including “kirigami,” “origami” of graphene, and “formation of percolation networks.” Kirigami and origami are two Japanese terms, originated from Asia paper art of “paper cutting” and “paper folding.” For the atomically thin graphene film, describing the operation of graphene by “Kirigami” and “Origami” are quite appropriate. “Kirigami” focuses on cutting suspended graphene into certain structural layouts and then enable its stretchability, while “origami” can be applied at three stages of graphene during its fabrication process, which are CVD growth, transfer procedure, and posttransfer. Corresponding “origami” methods to enable graphene stretchable electrodes are prepatterning the growth substrate, prestraining the transferred target substrate, and direct engineering the graphene film. In the last part of this section, forming graphene-based composites by combining with other conductors and/or elastomers is also reviewed. The section of application includes the recent progress of stretchable graphene electrodes in optoelectronics, energy-related area and sensors, which is a fast-growing area. In the final section of this chapter, we also provide a comment of current research status and an outlook for further studies related to the future electronic technology.

9.2

Preparation of stretchable graphene electrodes

The unique properties of graphene make it a strong candidate for the next generation of transparent conductive electrodes. While graphene has shown promising results for flexible electronics, its application in stretchable electronics has been limited by its

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mechanical properties. As early as 2009, when Kim et al. [12] for the first time developed chemical vapor deposition technique to grow large-area and high-quality graphene film, they already successfully demonstrated stretchable graphene electrodes. They adopted dry-transfer process to transfer graphene on the unstrained/prestrained elastomer substrate. Surprisingly, the conductivity of transferred graphene on the prestrained substrate keeps stable until w11% stretching (which is w6% stretching for transferred graphene on the unstrained substrate), and only one order of magnitude changes at w25% stretching. It shows that graphene is a promising candidate for stretchable electronics. Then, lots of research efforts have been carried out on how to engineer the graphene structure to improve its stretchability and maintain its remarkable electrical properties. Generally, to make the conductivity of graphene strain tolerance, there are mainly three types of methods, called graphene kirigami, graphene origami, and forming graphene-based composites. Below we will introduce them and discuss their advantages and disadvantages.

9.2.1

Graphene “Kirigami”

Blees et al. [13] first proposed graphene kirigami to achieve the stretchability of graphene (Fig. 9.1(aee)). They released graphene from the surface and found it could be treated like a sheet of atom-thick paper. Inspired by the old Chinese and Japanese art, kirigami, they created simple cuts in graphene with lithography techniques. Experiments show that graphene kirigami behaves like a spring when stretched, the same as paper kirigami. It is also worth noting that the graphene kirigamiebased transistors can be stretched by 240% without the reduction of the electrical performance, which demonstrates that graphene kirigami is suitable for flexible and stretchable electrodes. Theoretical investigations also verify the enhancement in the stretchability of graphene through applying the kirigami approach. Qi et al. [14] used classical molecular dynamics simulations (MD) to study the deformation response of graphene kirigami. Different from pristine graphene, there are four distinct stages preceding fracture, including (1) the interior cuts elongating, flipping, and rotating; (2) the carbon bonds being stretched; (3) yielding beginning; and (4) fracture occurring. The first step only occurs during graphene kirigami stretching, not during graphene stretching. Therefore, the yield and fracture strains of graphene can be enhanced by about a factor of three using kirigami as compared with standard monolayer graphene. The mechanical properties of graphene kirigami can be tuned through tailoring the kirigami geometry. Wei et al. [16] studied the stress distribution on graphene kirigami under different tensile strain and the strain effect on its thermal conductivity. Mortazavi et al. [15] employed classical MD simulations to evaluate the mechanical of graphene kirigami with periodic and curved cuts (Fig. 9.1(f),(g)). The result showed that linear cuts were more favorable for stretchable electronics because they could deflect more than curved cuts in the first stretching stage. These simulation results have been confirmed in an experimental work that kirigami approach can engineer elasticity in nanocomposites through patterned defects [17]. Although the kirigami structure could enhance the stretchability of graphene dramatically, high cost and long time-consuming lithographic fabrication process

(a)

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240% strain

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(g) lc ls

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Figure 9.1 (a,b) Paper and graphene in-plane kirigami springs, respectively. (c) Graphene spring stretched by about 70%. (d) Electrical properties in approximately 10 mM KCl. Conductance G is plotted against liquid-gate voltage VLG at sourceedrain bias VSD ¼ 100 mV before stretching (blue) and when stretched by 240% (orange). The top (orange-boxed) inset is split because the stretched device was larger than the visible area. (e) Threedimensional reconstruction from a z-scan focal series of a graphene spring. The right side remains stuck to the surface and the left side is lifted. Insets show views of sections of the graphene (right) and paper models (left). Top images show side views; bottom images show top views. The thin gray lines are the bounding box from the three-dimensional reconstruction. The aspect ratio of the side-view paper model was compressed 1.8. Scale bars are 10 mm. (f) Schematic of the graphene kirigami, with key geometric parameters labeled. The kirigami is deformed via tensile displacement loading that is applied at the two ends in the direction indicated by the arrows. (g) Atomistic and periodic structure of graphene kirigami, to illustrate the key geometric parameters: the curvature angle (q) and the longitudinal and transverse spacing distances (ls and ts). (e) Reproduced with permission from M.K. Blees, A.W. Barnard, P.A. Rose, S. P. Roberts, K.L. McGill, P.Y. Huang, A.R. Ruyack, J.W. Kevek, B. Kobrin, D.A Muller, P.L. McEuen, Graphene kirigami, Nature 524 (7564) (2015) 204e207. Copyright 2015 Macmillan Publishers Limited. (f) Reproduced with permission from ref Z. Qi, D.K. Campbell, H.S. Park, Atomistic simulations of tension-induced large deformation and stretchability in graphene kirigami, Phys. Rev. B 90 (24) (2014) Copyright 2014 American Physical Society. (g) Reproduced with permission from B. Mortazavi, A. Lherbier, Z. Fan, A. Harju, T. Rabczuk, J.C. Charlier, Thermal and electronic transport characteristics of highly stretchable graphene kirigami, Nanoscale 9 (42) (2017) 16329e16341. Copyright The Royal Society of Chemistry 2017.

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makes it unsuitable for industrial manufacturing. Thus, there is still a big need to achieve graphene kirigami by low cost and more feasible techniques, such as the very controllable and high-throughput chemical etching/cutting.

9.2.2

Graphene “Origami”

Origami is another important method to achieve stretchable graphene. The most popular way of graphene origami is to form wrinkled or crumpled graphene, which is an effective way to make graphene better in terms of stretchability, electrochemical behavior, and hydrophobicity [18]. According to the fabrication process, there are three methods to form wrinkled graphene, namely prestraining the transferred target substrate, prepatterning the growth substrate, and direct engineering the graphene film. Prestraining the transferred target substrate: Prestraining substrate refers to prestrain elastomeric target substrates in the posttransfer step. When the prestrain applied on the target substrate is released, wrinkles form spontaneously on the resulting graphene film. Kim et al., as mentioned above, were the first one to successfully prepare wrinkled graphene using the prestrained substrate method. Zang et al. [19] demonstrated via experiments and theoretical calculations that wrinkles and delaminated buckles were obtained when the substrate was uniaxially released, but crumpled graphene was obtained when biaxially released (Fig. 9.2(aef)). Besides, flat graphene could be unfolded if the relaxed substrate was biaxially stretched back. The crumplingunfolding process was reversible, which made it possible to achieve a set of unprecedented morphologies of graphene. Graphene, with different morphologies, exhibited different mechanical and electrical properties. As an example, its wettability and transparency can be tuned by biaxially prestretching substrates with different levels. Utilizing a balloon-blowing method, Mu et al. [20] demonstrated another example of prestraining the target substrate to make wrinkled graphene (Fig. 9.2(gei)). Firstly, they prestrained a polyacrylic ester (PEA) substrate with a large strain of z300% and tightly attached it on a circular glass dish. Then they heated the glass dish using a hot stage. The air pressure inside the dish would increase and the PEA film would further expand via the expansion force of the hot air inside. When the air pressure is stable, reduced graphene oxide (rGO) was next spray-coated on the surface of the expanded PEA substrate. After cooling down the glass dish and removing the PEA substrate, two types of wrinkles are generated: (1) short-period wrinkles appeared when the glass dish cooled to room temperature; (2) long-period wrinkles formed when the PEA substrate was released from the glass dish. Apart from more wrinkles, there are two great advantages of this balloon-blowing method: (1) the rGO film conformed with the substrate very well and (2) its conductivity is isotropically stable as the relaxation process was isotropic in the horizontal direction. Prepatterning the growth substrate: Using the above “prestraining the transferred target substrate” method, more cracks may produce during the transfer process, which probably results in low conductivity. As-obtained Cu foil naturally contains a series of parallel lines after mechanical processing and metal polishing. And it is surprisingly noticed that the transferred graphene film to some extent maintains the morphology of CVD grown substrates, meaning that the parallel processing lines

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(h) e

tag

ts

Ho

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Glass dish ge

ta

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Ho

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e

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Figure 9.2 (a) Schematic illustration of macroscopic deformation of a graphene sheet on a biaxially prestretched substrate. (bee) SEM images of patterns developed on the graphene sheet: first wrinkles form (b) then delaminated buckles as the substrate is uniaxially relaxed (c) followed by crumples as the substrate is biaxially relaxed (d) which unfold as the substrate is biaxially stretched back (e). (f) Atomistic modeling results of the crumpling of a single-layer graphene under uniaxial compression, and biaxial compression, followed by a visualization of the Mises stress distribution (from left to right). Stress concentrations (visualized in red) are observed near highly deformed regions. (g,h) Schematic representation of hierarchically wrinkled elastic transparent conductor (HWETC) preparation. (i) SEM images and illustrations of typical hierarchical wrinkles (short- and long-period wrinkles) in the N-rGO layer deposited on the released PEA substrate. The fabrication strain was 580%. (f) Reproduced with permission from J. Zang, S. Ryu, N. Pugno, Q. Wang, Q. Tu, M.J. Buehler, X. Zhao, Multifunctionality and control of the crumpling and unfolding of large-area graphene, Nat. Mater. 12 (4) (2013) 321e325. Copyright 2013 Macmillan Publishers Limited. (i) Reproduced with permission from J. Mu, C. Hou, G. Wang, X. Wang, Q. Zhang, Y. Li, H. Wang, M. Zhu, An elastic transparent conductor based on hierarchically wrinkled reduced graphene oxide for artificial muscles and sensors, Adv. Mater. 28 (43) (2016) 9491e9497. Copyright 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

can be reproduced from grown substrates onto the surface of resultant graphene film [21]. By carefully modifying the transfer procedure, Liu et al. for the first time achieved parallel wrinkles on transferred graphene film by copying the morphology of CVD-grown Cu foil onto the graphene film [22]. Similarly, Chen et al. [23] prepatterned substrates to prepare wrinkled graphene (Fig. 9.3). They slid a tweezer, which had a special structure, over a copper foil. The structure of the copper foil surface was consequently the same as that of the tweezer. Then, they used this waved copper as the

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3. Drop coating of a layer PDMS on the as-grown graphene

(c)

4. Remove Cu substrate in FeCl3 aqueous solution

Electrolyte

PDMS substrate

Wrinkled graphene

Figure 9.3 (a,b) Digital photographic images of the copper foil before (a) and after (b) the wrinkle formation. (c) Typical SEM image of the wrinkled graphene sheet on the PDMS substrate from the top view. (d) Schematic representation of the procedures for producing wrinkled graphene sheets for the fabrication of transparent and stretchable supercapacitors. Reproduced with permission from T. Chen, X. Yuhua, A. K. Roy, L. Dai, Transparent and stretchable high-performance supercapacitors based on wrinkled graphene electrodes, ACS Nano 8 (1) (2013) 1039e1046.Copyright 2013 American Chemical Society.

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5. Assmble device by pressing two wrinkled graphene coated with polymer electrolyte together

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183

growth substrate to prepare graphene by CVD method. SEM images showed that the as-prepared graphene was wrinkled, with an interwrinkle distance of about 400 mm. And the transmittance of the as-prepared graphene was 50%e60%, which was comparable to that of planar graphene. But, its electrical resistance increased highly when it was stretched up. To solve this critical problem, the authors coated a layer of polyvinyl alcohol (PVA) onto the surface of wrinkled graphene. It is shown that the resistance of the PVA-coated wrinkled graphene only increased by less than twice when it was stretched up to 40% strain. Xie et al. [24] used a steel rod to prepattern the nickel foam, a catalyst for growing porous graphene. They also deposited a polyaniline (PANI) thin film on the surface of the wrinkled graphene to improve its stretchability and electrochemical properties. In addition to enable uniaxially stretchable graphene, Hong et al. [25] pressed a copper foil with a Fresnel lens to fabricate omnidirectionally stretchable graphene. This indicates that using various patterned grown substrates in principle can allow the fabrication of stretchable and transparent graphene electrodes with mechanical durability and performance reliability. Prepatterning substrates to generate wrinkled graphene is not only suitable for CVD process but also for solution process. Starting from microstructured PDMS, Zhu et al. [26] employed layer-by-layer assembly (LBL) technology to prepare wrinkled graphene on PDMS. First, they prepared a silicon master with recessed pyramid structures by photolithography and silicon etching technologies. Then, a mixture of PDMS elastomers and cross-linker was cast on the surface of the silicon master to form the microstructured PDMS film. Subsequently, the PDMS film was peeled off from the master, and graphene oxide (GO) solution was adsorbed on the film by an LBL assembly method. After being reduced under hydrazine vapor, a reduced GO (rGO) sheet with convex pyramid patterns was finally formed. Together with an ITO/PET film, wrinkled rGO/PDMS could serve as a sensor unit on the artificial hand with high sensitivity and stability. Overall, controlling the morphology of both as-grown substrates and transferred target substrates can tune the morphology of resultant graphene film, achieving wrinkled or crumpled graphene. Although this is an indirect tuning method relying on a second media, it is very effective, cost-efficient, and potentially applied to be massive production. Direct engineering the graphene film: Transfer process of CVD graphene includes etching the copper substrate in the etchant solution, cleaning the graphene in the deionized water, and transferring onto the target substrate. Chen et al. [27] reported a liquid-phase shrink method to prepare wrinkled graphene, which was simple and could efficiently control the density of wrinkles (Fig. 9.4). They soaked the cleaned graphene in the organic solution before it was transferred on the target substrate, such as ethanol, acetone, and so on. The cleaned graphene shrank fast owing to the reduction of the surface and interface energy of the graphene/solution system. Moreover, changing the concentration of the organic solution could control the density of wrinkles. The wrinkled graphene prepared by this method was much more stretchable than the pristine graphene. The sheet resistance only changed a little under 40% stretching, while the transparency of this wrinkled graphene was similar to pristine graphene. It was even conductive under more than 100% strain, which largely

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Figure 9.4 (a) Schematic illustration of the preparation of wrinkled graphene in ethanol solution. (b) 3D schematic of as-prepared wrinkled graphene. (c,d) SEM images of clean graphene and wrinkled graphene (prepared in 1:1 vol % ethanol solution) on a silicon substrate, respectively. Inset shows high-magnification SEM image of the wrinkled graphene. (e,f) AFM images and surface analysis of the clean graphene and wrinkled graphene, respectively. (g) 3D views of wrinkled graphene. Reproduced with permission from W. Chen, X. Gui, B. Liang, M. Liu, Z. Lin, Y. Zhu, Z. Tang, Controllable fabrication of large-area wrinkled graphene on a solution surface, ACS Appl. Mater. Interfaces 8 (17) (2016) 10977e10984. Copyright 2016 American Chemical Society.

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expanded its working range. Thus-obtained wrinkled graphene was an ideal material for sensitive electrochemical sensors. Mechanical deformation of GO into 3D hierarchical structures as electrochemical electrodes shows high hydrophobicity and can improve the electrochemical reactivity and current density. Chen et al. [18] successfully produced wrinkled graphene utilizing the special property of polystyrene (Fig. 9.5). Polystyrene, thermoplastic “shrink film,” tends to uniaxially or biaxially shrink at an elevated temperature. GO was first coated on the pretreated polystyrene and then air-dried and baked at a temperature, which is above the Tg of polystyrene. GO would shrink with polystyrene due to strong intermolecular attraction between the GO and polystyrene. If two sides of the sample were constrained, GO would undergo one-dimensional uniaxial deformation. Next, they transferred wrinkled GO (G1) on another polystyrene and heated them again. G1 became much smaller, and the G2 hierarchy was obtained. After multiple repeated processes and dissolving the substrates in dichloromethane, different hierarchical and free-standing wrinkle/crumple GO could be obtained. The hydrophobicity and electrochemical current density of thus-obtained graphene were systematically improved after GO was reduced. Combined with other literature, they believed that wrinkled graphene processed by sequential mechanical deformation were highly possible in stretchable electronics. Compared with kirigami graphene, the fabrication procedures for origami graphene are relatively simple and low cost, and they are much more suitable for the large-scale production of stretchable graphene. Nevertheless, disadvantages of this method still cannot be ignored. The stretchability of wrinkled graphene prepared by such method is either scalable or reproducible enough for wearable electronic devices. Lots of uncontrollable wrinkles appeared on the surface of graphene, and the repeated processes make it too complex to apply in the actual production.

9.2.3

Graphene-based composites

Composites are very popular among materials scientists, because they always exhibit fascinating characters without changing excellent properties of the original materials. Thanks to its extraordinary electrical and mechanical properties, graphene has been extensively designed to combine with other materials to achieve new functions. The strainetolerance conductivity of some graphene-based composites is enhanced greatly, which can meet the requirements of soft electronics working for robots, electronic skin, and other flexible/stretchable devices. Many polymers are excellent stretchable substrates, and graphene in particular wrinkles, ripples, and crumples has strong mechanical interlocking with polymer chains. To enable highly conductive and stretchable graphene-based conductors, Chen et al. [29] initiated from high-quality 3D CVDegraphene grown on Ni foam and combined it with polymers (Fig. 9.6(aej)). They coated PMMA thin film on the as-prepared 3D graphene, which acted as a supporting layer to keep graphene foam from collapsing when etching the nickel skeleton. Then, they dissolved PMMA with acetone and immersed the free-standing graphene foam into PDMS to obtain composites. The graphene sheets are seamlessly interconnected into 3D flexible

GO dispersion

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Figure 9.5 (a) Schematic illustration of the fabrication process to generate multiscale GO structures. (b) Controlling surface morphology of the G2 hierarchy through different combinations of shrinking orientation. The G2 structures in this figure are shrunk from 20-nm G0 coatings. SEM images of hierarchical G2 structures formed with (b) 2D-2D, (c) 2D-1D, (e) 1D-2D, (f) 1Dt1D, and (g) 1Djj1D shrinking orientations are shown. Yellow arrows indicate the wrinkled feature from the G1 uniaxial shrinkage. Scale bars in the first row are 20 mm; 2 mm in the second row; and 1 mm in the third row. Reproduced with permission from P.Y. Chen, J. Sodhi, Y. Qiu, T.M. Valentin, R.S. Steinberg, Z. Wang, R.H. Hurt, I.Y. Wong, Multiscale graphene topographies programmed by sequential mechanical deformation, Adv. Mater. 28 (18) (2016) 3564e3571. Copyright 2016 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim.

(b) CH4/H2/Ar

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Figure 9.6 (a,b) CVD growth of graphene films (NieG, (b)) using a nickel foam (Ni foam, (a)) as a 3D scaffold template. (c) An as-grown graphene film after coating a thin PMMA supporting layer (NieG-PMMA). (d) A graphene foam (GF) coated with PMMA (GF-PMMA) after etching the nickel foam with hot HCl (or FeCl3/HCl) solution. (e) A free-standing GF after dissolving the PMMA layer with acetone. (f) A GF/PDMS composite after infiltration of PDMS into a GF. All the scale bars are 500 mm. (g) Photograph of a 170  220 mm2 free-standing GF. (h) SEM image of a GF. (i) Low-magnification TEM image of a GF. (j) High-resolution TEM images of graphene sheets with different numbers of layers in a GF; the interlayer spacing of bilayer (2L) and trilayer (3L) graphene is w0:34 nm. Reproduced with permission from Z. Chen, W. Ren, L. Gao, B. Liu, S. Pei, H.M. Cheng, Three-dimensional flexible and conductive interconnected graphene networks grown by chemical vapour deposition, Nat. Mater. 10 (6) (2011) 424e428. Copyright 2011 Macmillan Publishers Limited.

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networks, showing 30% increase of resistance at 50% uniaxial tensile strain. The unique network structure, high specific surface area, and outstanding electrical and mechanical properties of grapheneefoamePDMS composites should enable many applications including high-performance electrically conductive polymer composites, elastic and flexible conductors. In addition to 3D graphene network grown by Ni foam, the highly conductive graphene to enable stretchable devices can also be achieved by intercalating graphene scrolls (w1e20 mm long, w 0.1e1 mm wide, and w 10e100 nm high) in between graphene layers (Fig. 9.7), referred to as multilayer G/G scrolls (MGG). The scrolls were naturally formed during the transfer process, which is from unprotected backside graphene grown on Cu foil and rolled up under the effect of surface tension in etchant solution. They do not require additional synthesis or process. It is hypothesized that these graphene scrolls could provide conductive paths to bridge cracks in the graphene sheets, thus maintaining high conductivity under strain. By using MGG graphene stretchable electrodes (source/drain and gate) and semiconducting CNTs, they were able to demonstrate highly transparent and highly stretchable all-carbon transistors, which can be stretched to 120% strain (parallel to the direction of charge transport) and retain 60% of its original current output. This is the most stretchable transparent carbon-based transistor so far, and it provides sufficient current to drive an inorganic LED. mNWs are another ideal materials for stretchable electrodes by virtue of its low sheet resistance, high transparency, and outstanding mechanical robustness. However, disadvantages of mNWs, such as typically high NWeNW junction resistance, instability in harsh environment, and so on, have limited its further development. Thus, Lee et al. [31] hybridized Ag nanowires with graphene to prepare novel composites to lower the junction resistance (Fig. 9.8(a,b)). They spun a suspension of AgNWs onto a CVDegraphene layer, whose interaction in between is van der Waals forces. The integration of two-dimensional graphene and one-dimensional NWs in the hybrid film can significantly enhance electrical properties, and the formation of percolation pathways allow simultaneous charge transport even at strain up to 100%, presenting negligible resistance change and great optical transparency of >94% transmittance in the visible range. In addition, due to the thermal oxidation stability of graphene, such-made composites are much more chemically inactive than AgNWs only. Direct engineering of graphene for stretchable conductors not only applies for CVD graphene but also works for liquid phase GO/rGO. Yan et al. [32] mixed the crumpled graphene and nanocellulose to get nanopapers via vacuum filtration (Fig. 9.8(eej)). This nanopaper had certain mechanical strength so that they would not crack and delaminate from the filter membrane and allowed for further processing into stretchable form. Embedded it into elastomeric matrix, such as PDMS, these nanopapers became more stretchable, which could be stretched up to 100% without mechanical failure. The stretchability is determined by the mechanical fracture limit of the elastomer used. The relative resistance change of 710% was observed at 100% strain, suggesting an excellent piezoresistive effect and an ideal material for strain sensors. In summary, graphene as a novel electrode candidate has drawn extensive attention for the application in stretchable electronics. It is highly conductive, transparent, and

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Figure 9.7 (a) Schematic illustration of the fabrication procedure for MGGs as a stretchable electrode. During the graphene transfer, backside graphene on Cu foil was broken at boundaries and defects, rolled up into arbitrary shapes, and tightly attached onto the upper films, forming nanoscrolls. The fourth cartoon depicts the stacked MGG structure. (b,c) High-resolution TEM characterizations of a monolayer MGG, focusing on the monolayer graphene (b) and the scroll (c) region, respectively. The inset of (b) is a low-magnification image showing the overall morphology of monolayer MGGs on the TEM grid. Insets of (c) are the intensity profiles taken along the rectangular boxes indicated in the image, where the distances between the atomic planes are 0.34 and 0.41 nm. (d) Carbon K-edge EEL spectrum with the characteristic graphitic p* and s* peaks labeled. (e) Sectional AFM image of monolayer G/G scrolls with a height profile along the yellow dotted line. (fei) Optical microscopy and AFM images of trilayer G without (f,h) and with scrolls (g,i) on 300-nm-thick SiO2/Si substrates, respectively. Representative scrolls and wrinkles were labeled to highlight their differences. Reproduced with permission from N. Liu, A. Chortos, T. Lei, L. Jin, T.R. Kim, W.G. Bae, C. Zhu, S. Wang, R. Pfattner, X. Chen, R. Sinclair, Z. Bao, Ultratransparent and stretchable graphene electrodes, Sci. Adv. 3 (9) (2017) e1700159. Copyright 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science.

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Figure 9.8 (a) Photograph of graphene
AgNW hybrid film on a PET substrate. The scale bar indicates 2 cm. The inset shows a SEM image of this hybrid (scale bar, 5 mm). (b) Dependence of AgNW density (in the area of 100  100 mm2) on spin rate. (c) Optical transmittance spectra of the hybrid films where AgNWs are coated with different spin rates. (d) Log-scale plots of the sheet resistances as a function of NW density. (e) Schematic illustrations of the fabrication processes for stretchable graphene nanopapers. (fei) Example images of the free-standing flexible nanopaper (f,g) and stretchable nanopaper (h,i). (j) Water adsorption comparison of crumpled graphene paper, planar graphene paper, and commercial graphite paper. The scale bars in (fej) are 10 mm. (d) Reproduced with permission from M.S. Lee, K. Lee, S.Y. Kim, H. Lee, J. Park, K.H. Choi, H.K. Kim, D.G. Kim, D.Y. Lee, S. Nam, J.U. Park, High-performance, transparent, and stretchable electrodes using graphene-metal nanowire hybrid structures, Nano Lett. 13 (6) (2013) 2814e2821. Copyright 2013 American Chemical Society. (fej) Reproduced with permission from C. Yan, J. Wang, W. Kang, M. Cui, X. Wang, C.Y. Foo, K.J. Chee, P.S. Lee, Highly stretchable piezoresistive graphene-nanocellulose nanopaper for strain sensors, Adv. Mater. 26 (13) (2014) 2022e2027. Copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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light weight and work function tunable. Graphene kirigami, origami, and combining with other conductors or elastomers to form composites are three typical methods for fabricating stretchable electrodes. Mostly, graphene obtained by these three methods can achieve high conductivity at certain strain, but in terms of easy preparation and fracture-strain limit, there is still a big room to improve.

9.3

Applications of stretchable graphene electrodes

From “siri” to self-driving car, AI is developing at a rate that is faster than many experts imagine. Intelligent machines that are demanded to work and react like humans make flexible and wearable materials increasingly important. Stretching is one of the manifestations of flexibility and is the basic mode of muscle activity. Tremendous efforts have been done to make stretchable electronic devices during several decades, which is a core of intelligent machines. Stretchable graphene electrodes are a star material for flexible, wearable smart devices due to their extraordinary mechanical properties, electrical properties, transparency, and so on. Nowadays, they have already been applied in sensors, digital electronics, and energy storage devices. Sensors: There are many types of sensors, such as tactile sensors, temperature sensors, chemical sensors, photodetectors, and so on. Tactile sensors, which translate a mechanical deformation into an electrical signal to monitor human body motion, are the most difficult one to mimic skin functions among various sensors. Here we chose strain sensors as an example of tactile sensors to discuss the application of stretchable graphene. According to different electrical signals, strain sensors usually are divided into three types: piezo-voltage, piezo-capacitive, and piezo-resistive. Piezo-voltage strain sensors are much more accurate, which can detect small strains with high strain gauge. Nevertheless, it is not suitable for monitoring human body motions due to its low stretchability. Piezo-capacitive strain sensors possess a more linear response and a lower hysteresis, but a smaller strain gauge. Relatively, piezo-resistive strain sensors are much more popular because they are simple to produce and test. Larimi et al. [33] developed a simple and low-cost method to fabricate an ultrastretchable, sensible strain sensor based on stretchable graphene (Fig. 9.9). They infused a solution of graphene nanoflakes (GNFs), methanol, and water into a holerich, rubber-like adhesive pad, which was pretreated in acetone before. After being dried overnight at room temperature, the GNF-pad was washed to remove residuals. Surprisingly, the GNF-pad exhibited outstanding mechanical properties. It could be stretched up to 350%, twisted from 0 to 180 degrees, bent (0e90 degrees), and pressed, which was due to the flexibility of the adhesive pad and relatively slide between GNF. On the other hand, its electrical properties and sensitivity were before other strain sensors. Its initial resistance was as low as 8 kU, and gauge factor was as high as 161. In addition, its stretchability and sensitivity were robust. After repeated tests, they did not show obvious change. For practical applications, they changed to another skin-friendly ultrastretchable substrate (Ecoflex 00e50). As expected, the results showed that the GNF-pad could not only sense small strains, like the pulse

192

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Figure 9.9 (a) Heartbeat monitoring using GNF-Pad implanted in Ecoflex wristband. (b) Finger pose reading. Three GNF-Pad were assembled on a finger pad made out Ecoflex and used to measure the strain caused by change in the angle of each finger joints during bending the finger. (c) Wireless knee band to track the knee angle changing during different activities, including walking, running, and sitting down and standing up. Reproduced with permission from S.R. Larimi,H. Rezaei Nejad, M. Oyatsi, A. O’Brien, M. Hoorfar, H. Najjaran, Low-cost ultra-stretchable strain sensors for monitoring human motion and bio-signals, Sens. Actuators A Phys. 271 (2018) 182e191. Copyright 2018 Elsevier B.V.

of the artery, but also large strains, for example, muscle movements during running and walking. Monitoring human body motions is just the first step. Combined strain sensors with data processors, it will have even greater practice value. Larimi et al. found the GNFpad could control the robot’s movements and gestures, which means this fabricated GNF-pad was an ideal material for humanemachine interfacing. In another work done by Wang et al. [34], they expected that after acquiring and recognizing sound signal with graphene-based strain sensors, utilizing big data analysis, intelligent machine could help people who had trouble in speaking express their feelings by just moving relative muscles.

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Photodetectors are sensors of light, which can translate light signals into electrical signals. Flexible photodetectors have been actively applied in many areas, including sensing biological systems and wearable integrated optoelectronics. “Electrical eyes” have recently been put forward, which are used on robots to simulate human eyes to sense light or on people who have trouble in watching to help them sense obstacles. Apparently, conventional materials for photodetectors are too rigid to stretch and hardly suitable for “electrical eyes.” Because of the broadband absorption from ultraviolet and terahertz frequency, flexible graphene has been considered as an attractive material for optoelectronics. Kang et al. [35] successfully fabricated stretchable, high photoresponsivity photodetectors with crumpled graphene (Fig. 9.10). The crumpled graphene was prepared by prestraining the transferred target substrate as we have mentioned before and used as photocurrent generation. The resultant extinction spectrum of the crumpled graphene at varying uniaxial tensile strains (ε tensile, x) ranging from 0% to 200% and flat graphene showed that the crumple could not only enhance graphene stretchability but also its photoabsorption. Photocurrent measurements showed that the photoresponsivity (Rph) at ε tensile ¼ 0% was estimated to be w0.11 mA W
1, which was 370% larger than that of a flat graphene photodetector and two times larger than that at ε tensile, x ¼ 200%. Only small variance has been found in measured photocurrents and currentevoltage curves at varying stretchinge releasing cycles. Besides, the rise time (son) of this stretchable photodetectors were not more than 300 ms. The capacity of strain-tunable photoadsorption enhancement can be modulated by changing the crumple density, height, and pitch of the 3D crumpled graphene. This graphene photodetector could be attached on arbitrary substrates, such as the surface of human face and heart models. And the stretchability can be extended to a very large strain in that it is determined by the predesigned strains of the elastomeric substrate on which the device is fabricated. To further improve the responsivity of stretchable graphene photodetectors, hybrid systems based on photonic or plasmonic nanostructures have been introduced [36,37]. They demonstrated that stretchable graphene photodetectors may find broad applications as conformable and flexible optical sensors. Digital electronics: Transistor is the fundamental building component in digital electronics, which is important in stretchable applications because they enable sophisticated sensor readout and signal analysis. Because of the atomic thickness, high transparency, and extraordinary electrical performance, graphene has been considered as a strong candidate for the next-generation flexible transistors. A variety of stretchable graphene prepared by the aforementioned three methods, including graphene kirigami, origami, and composites, have been demonstrated in stretchable transistors [38e42]. The most stretchable and strain-tolerance transistor was achieved by kirigami graphene, which is gated by a liquid electrolyte and can be stretched to as much as 240%. However, kirigami method requires suspended graphene, which complicates the fabrication process. Transistors on PDMS and other stretchable elastomers are relatively easier applied for wearable health monitoring sensors and electronic skin. Lee et al. for the first time fabricated stretchable and transparent transistors using multilayer graphene as source/ drain electrodes and channel material, which can only maintain electrical function up

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Figure 9.10 (a) Schematic illustration of a stretchable photodetector and the experiment setup. (b) Fabrication procedures of a stretchable textured graphene photodetector. (c) Dynamic photoresponse of the device at varying uniaxial tensile strains (εtensile, x) from 0% to 200%. Measured photocurrent is normalized with the photocurrent at ε tensile, x ¼ 0%. (d) Comparison of measured photocurrent of the textured graphene photodetector at the varying strains over two cycles and measured photocurrent of a flat graphene photodetector. The inset shows an optical microscope image of the textured graphene photodetector. The value of 0.27 indicates the measured photocurrent of the flat graphene photodetector normalized with that of the textured graphene photodetector at εtensile, x ¼ 0%. (e) Highly stretchable and conformal photodetector on the surface of a human brain model. (f) Conformal photodetector on a curved surface. The inset shows a photograph of the fabricated device on an Ecoflex substrate. (g) Dynamic photoresponse of the device at different bending strains to an incident 405 nm illumination. Photocurrent was measured in three oneoff cycles. Reproduced with permission from P. Kang, M.C. Wang, P.M. Knapp, S. Nam, Crumpled graphene photodetector with enhanced, strain-tunable, and wavelength-selective photoresponsivity, Adv. Mater. 28 (23) (2016) 4639e4645. Copyright 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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to 5% strain [41]. By combining graphene with single-walled carbon nanotube (SWNT) as electrodes, Chae et al. fabricated stretchable and transparent field-effect transistors with SWNT-network channel and a geometrically wrinkled inorganic dielectric layer [43]. The devices exhibited an excellent on/off ratio of w105, a high mobility of w40 cm2 V
1 s
1, and a low operating voltage of less than 1 V. The transistors can be functioned at 20% strain without appreciable leakage current increases or physical degradation. No significant performance loss was observed after stretching and releasing the devices for over 1000 times. However, this stretchability is still significantly below the minimum required value for electronic skin (w50%) [44,45]. Liu et al. fabricated all-carbon stretchable and transparent transistors using MGG as bottom gate and source/drain contacts, polymer-sorted semiconductive CNT as semiconductor, and SEBS as dielectric layer (Fig. 9.11) [30]. The preparation of MGG has been introduced in the section of “preparation methods of stretchable graphene.” The measured on/off ratio is greater than 103 and the mobility of the stretchable transistor is about 5.6 cm2/Vs, similar to the same polymer-sorted CNT transistors on rigid Si substrates with 300 nm SiO2 as a dielectric layer. When the transparent, all-carbon device was stretched in the direction parallel to the charge transport direction, minimal degradation was observed up to 120% strain. During stretching, the mobility continuously decreased from 5.6 cm2/Vs at 0% strain to 2.5 cm2/Vs at 120% strain. Notably, at strain as large as 105%, all these transistors still exhibited high on/off ratio (>103) and mobility (>3 cm2/Vs). As an application of the fully transparent and stretchable transistor, they used it to control a LED’s switching. While stretching the transistor up to w100%, the LED light intensity does not change. This demonstrates that these highly stretchable and transparent graphene transistors could enable sophisticated stretchable optoelectronics. Energy-storage devices: With the emergence of intelligent machines, stretchable energy storage devices are key components for the fabrication of complete and independent stretchable systems. It is very challenging to fabricate stretchable energystorage devices, and few work has been reported. Conductive polymers [46], carbon nanotubes [47], and graphene [48] were used in fabricating stretchable supercapacitor electrodes, and stretchable graphene are particularly prominent. Zang et al. [49] developed stretchable all-solid-state supercapacitors based on crumpled graphene (Fig. 9.12). They prepared the crumpled-graphene-papers (CG-papers) by biaxially/uniaxially prestretching the target substrate. The cyclic voltammetry (CV) curves of CG-papers electrodes showed that there was a typical rectangular shape at a certain scan rate, which indicated that it is an ideal double-layer electrochemical capacitor. The CV curves only changed slightly when the CG-papers electrodes are under large deformation. Calculated from the discharge slops at different charge/discharge current densities, CG-paper electrodes present remarkable gravimetric capacitance in the range of 166e196 F g
1 at the operation rate of 1 A g
1, which are comparable to other high-performance but unstretchable supercapacitor electrodes. Furthermore, cycling stressestrain and galvanostatic charge/discharge tests under large deformation verified the stability of CG-paper electrodes. Integrated the two stretchable CG-paper electrodes with a stretchable polymer gel, all-solid-state supercapacitors present extraordinary stretchability and outstanding electrochemical properties.

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Figure 9.11 (a) Scheme of graphene-based stretchable transistor. SWNTs, single-walled carbon nanotubes. (b) Photo of the stretchable transistors made of graphene electrodes (top) and CNT electrodes (bottom). The difference in transparency is clearly noticeable. (c, d) Transfer and output curves of the graphene-based transistor on SEBS before strain. (e, f) Transfer curves, on and off current, on/off ratio, and mobility of the graphene-based transistor at different strains. Reproduced with permission from N. Liu, A. Chortos, T. Lei, L. Jin, T.R. Kim, W.G. Bae, C. Zhu, S. Wang, R. Pfattner, X. Chen, R. Sinclair, Z. Bao, Ultratransparent and stretchable graphene electrodes, Sci. Adv. 3 (9) (2017) e1700159. Copyright 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science.

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Figure 9.12 (aec) Subjected to uniaxial strains of 0%, 100%, 200%, and 300% and (def) biaxial strains of 0%  0% and 200%  200%. (a,d) Cyclic voltammetry curves at 50 mV s
1, (b,e) galvanostatic charge/discharge curves at 5 A g
1, and (c,f) gravimetric capacitance measured at different charge/discharge current densities (Is ¼ 0.5, 1.0, 2.0, 5.0, 10, 20, 50, and 80 Ag
1). The tests were carried out in 1.0 M H2SO4. The thickness of the graphene paper is w2 mm measured at dehydrated state. g, A schematic diagram of the supercapacitor using crumpled-graphene-paper electrodes with a polymer electrolyte gel as the electrolyte and separator. (h) The CV curves of the supercapacitor collected at a scan rate of 10 mV s
1. (i) Galvanostatic charge/discharge curves at a current density of 1 A g
1, under uniaxial strains of 0%, 50%, 100%, and 150%. The thickness of the graphene paper is w0.8 mm measured at dehydrated state. ( j) Reproduced with permission from J. Zang, C. Cao, Y. Feng, J. Liu, X. Zhao, Stretchable and high-performance supercapacitors with crumpled graphene papers, Sci. Rep. 4 (2014) 6492. (m) Reproduced with permission from X. Zang, M. Zhu, X. Li, X. Li, Z. Zhen, J. Lao, K. Wang, F. Kang, B. Wei, H. Zhu, Dynamically stretchable supercapacitors based on graphene woven fabric electrodes, Nano Energy 15 (2015), 83e91. Copyright 2015 ElsevierLtd.

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Another example of stretchable supercapacitor is fabricated with graphene-woven fabric electrodes (GWFs) done by Zang et al. [50]. GWF was prepared on the copper mesh through chemical vapor deposition method. The CV curves of the supercapacitor assembled with two identical GWF electrodes exhibited nearly rectangular at varying scan rates increased from 0.06 to 1 V/s and the capacitance was up to 17 mF/cm2 at the scan rate of 0.06 V/s. The galvanostatic charge/discharge curve was nearly linear and symmetrical. They also transferred GWF on prestretched substrates to make electrodes more stretchable and added a conducting polymer (PANI) into GWF to enhance the electrochemical properties of supercapacitors. These stretchable supercapacitors with GWF-PANI electrodes present good galvanic ability and excellent electrochemical properties not only under the static mode but also the dynamic mode.

9.4

Summary and outlook

In this chapter, we mainly introduced the ways to prepare stretchable graphene and its applications in sensors, digital electronics, and energy storage devices. Graphene exhibits highly desirable properties of atomic thickness, high transparency, and high conductivity, but its implementation in stretchable applications has been inhibited by its tendency to crack at small strains. Overcoming the mechanical limitations of graphene could enable new functionality in stretchable transparent devices. Basically, there are three methods to obtain stretchable graphene, which are graphene kirigami, origami, and combining with 1D conductive species and/or polymers to form composites. Among them, kirigami on suspended graphene can maintain most superior properties of graphene, such as atomic thickness and high transparency, but the fabrication process is very complicated and impossible to be massive production. Graphene origami has been broadly used, in particular folding it into the very simple “wavy” structure. According to the fabrication process, graphene origami has been divided into three categories, including prestraining the transferred target substrate, patterning the growth substrate, and direct engineering the graphene film. Another rapidly developing method to enable stretchable graphene is to form composites by combining graphene with conductive species on polymers. The composites take advantages of high conductivity and transparency as well as inert chemical reactivity of graphene and avoid its small fracture strain limit by combining with conductive networks or polymers. Utilizing methods of “graphene origami and forming composites,” large-scale and high-performance stretchable sensors and supercapacitors have been demonstrated. As an outlook for future studies, a few topics of interest are summarized as follows. Theoretical understanding of the mechanisms of stretchable graphene: Graphene intrinsically is in-plane stiff and can only be bent due to its atomic thickness. Stretchable graphene has been experimentally achieved through origami, kirigami, and forming composites. Fundamental research on the mechanical properties of graphene has made significant progress over the last decade, but there is limited understanding on the mechanisms of stretchable graphene, such as the lateral forces beyond

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Figure 9.13 (a) Strainetime curve at strain rate of 6%/s (left) and corresponding CV curve (right). (b) Strainetime curve at strain rate of 12%/s (left) and corresponding CV curve (right). (c) Schematic illustration of GWF-based electrode in dynamic stretching process.

van der Waal interactions, the effects of surface roughness, and capillary bridging of graphene under strain. Developing more reliable fabrication methods to achieve stretchable graphene: Ideally, stretchable graphene electrode should be a superthin, transparent, and conductive film. So far, most fabricated stretchable graphene film cannot satisfy all those merits at the same time, in particular being superthin which is the main characteristic of graphene. Thus, there is still a lot of room to develop fabrication methods to achieve stretchable graphene as well as being large-scale, robust, and highly reproduced. After deeper understanding of the mechanisms of current preparation methods of stretchable graphene, a more reliable method may be put forward.

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Extending to other 2D materials: The family of 2D materials has grown beyond graphene, and together they hold great promise for a wide range of applications. Intrinsically, the atomic composition, mechanical structure, and electrical properties of other 2D materials are more complicated than graphene. Roads toward stretchable graphene beyond 2D materials must be very challenging but of more interest.

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This book has reviewed application of graphene in various display and lighting devices (e.g., organic lighteemitting diodes (OLEDs), inorganic LEDs, quantum dot LEDs (QD-LEDs), perovskite LEDs (PeLEDs)). Graphene has superior electrical, optical, and mechanical properties, and there have been significant advances in synthesis and transfer of high-quality graphene with large area, so it has been widely used as the electrode in various LEDs. Especially, the wide-angle spectral stability in OLED using graphene electrodes is a strong merit for its use in transparent electrode for display applications [1]. However, pristine graphene has high RS and too low (high) work function (WF) to be used as an anode (cathode); these traits limit the hole (electron) injection from graphene electrode to emitting layers, and thereby increase operating voltage and reduce the luminous efficiency of LEDs. Thus, various researchers have attempted to improve the electrical properties of graphene, so that graphene electrodes can be used practically in electrodes of LEDs. Early studies of chemical graphene doping used small-molecule inorganic acid (e.g., HNO3) or metal chloride (e.g., AuCl3) [2]; the dopants substantially improved the electrical properties of graphene. However, these dopants are severely unstable in ambient conditions and, therefore, are not practically applicable in graphene electrodes in LEDs. Therefore, fluorinated organic acids [3,4] and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzoimidazole derivatives [5,6] were used as stable p-type and n-type chemical dopants; they effectively modify the WF of graphene with substantially improved doping stability. Graphene has been successfully used as electrodes in various LEDs. The technical standard of OLEDs with graphene electrode exceeds demonstration level and achieves high luminous properties that are comparable to or higher than those of OLEDs that use an ITO electrode. As the technology to fabricate graphene-based OLEDs has been advanced, research on graphene-based OLEDs has expanded from developing high-efficiency OLEDs to demonstration of OLEDs arrays and to panel integration. To proceed from proof-of-concept to the commercialization level, graphene-based OLEDs should be used in active matrix display. In this aspect, it is essential to achieve patterning technology of graphene so that graphene is elaborately pixelized without defect generation. Patterning using O2 plasma etching with shadow mask usually leaves partial damage at the edge of the unexposed region. Patterning using laser ablation still shows poor yields, and the laser-exposed region protrudes severely. Photolithography is a promising patterning process due to its predominance in the display industry, but graphene adheres weakly to its substrate, so photolithographic patterning of graphene film leaves a nonuniform surface. Graphene’s weak adhesion has been increased

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using a liquid bridging method; it enables photolithographic patterning and pixelated arrays of OLEDs with stable operation [7]. Graphene-based materials have also been used as interfacial layer and emitting layers for OLEDs. For those purpose, solution-processable graphene derivatives were mostly used. These approaches have been actively studied due to the low cost, simple synthetic routes of solution process, and they have achieved significant advances in employment of graphene-based derivatives for interfacial layers and emitting layers in LEDs. A graphene-based interfacial layer acts as an intermediate step between electrode and emitting layer and improves charge injection to the emitting layer and increases the luminous properties of OLEDs. In addition, graphenebased emitters including graphene quantum dots and its composite form were developed, and they exhibited continuous progress in luminous properties and synthetic ways ranging from top-down to bottom-up approaches. As a result, LEDs were developed using the graphene-based emitters. Moreover, graphene can be used in encapsulating layers in OLED devices [8]. Because graphene is composed of atomically fine carbon lattice, graphene can effectively block the penetration of oxygen and moisture that causes degradation of OLEDs. Graphene has also been evaluated as an electrode in other LEDs (e.g., inorganic LEDs, QD-LEDs, PeLEDs). In these LEDs also, energy-level engineering and chemical doping of the graphene electrode substantially increase the luminous properties. To achieve inorganic LEDs with graphene that exhibit highly luminous properties, the crystal quality of epitaxially grown inorganic semiconductor layer must be maintained. QDs have a deeper VBM than other emitters, and the emitting layer must be thin. Thus, to achieve efficient hole injection from anode to QD layers, considerable design of materials and devices is necessary, and the surface must be smooth before deposition of QD layers. Halide perovskites have low exciton-binding energies, so to achieve highly luminous PeLEDs, luminescence quenching caused by metallic species must be prevented. Graphene is also promising as an electrode material because of its chemical stability. Graphene does not generate chemical and metallic species that cause luminescence quenching when it was exposed to acidic environments, whereas ITO generates metallic species causing quenching in those conditions. The research direction in graphene-based LEDs has diversified from improvement of luminous properties of LEDs to various other subjects including stretchable LEDs, transparent LEDs, and transferable LEDs. Considering the flourishing progress in graphene-based LEDs, graphene electrodes will become practically useable in LEDs in near future if (i) a method can be developed to synthesize and transfer highquality graphene and (ii) safe and inexpensive mass production of graphene can be achieved. High-quality graphene that has large graphene grains with few defects has superior electrical conductivity and would yield further improvement of LEDs that use graphene electrodes. Productivity of graphene must be increased before commercialization of graphene-based LEDs is feasible. To synthesize high-quality graphene, chemical vapor deposition (CVD) method is generally used. During the CVD process, high-purity hydrocarbon gases (e.g., CH4, C2H2) are mostly used for graphene synthesis, but they are expensive and explosive; this is a great impediment to inexpensive

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mass production of graphene. Graphene has been synthesized using inexpensive carbon precursors (e.g., coal tar pitch, waste plastics, leaves) that are not explosive [9]. Introducing these carbon sources in a synthesis process will be safe and substantially reduce the production cost of graphene synthesis; this approach would substantially improve the productivity of graphene. After the quality and productivity of graphene are improved, it can be practically used in transparent electrodes in LEDs of nextgeneration displays and lightings.

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Index Note: ‘Page numbers followed by “f ” indicate figures, “t” indicates tables’. A Adhesion-mediated transfer method biological graphene coatings, 45e46 poly(vinyl alcohol) (PVA) carrier layer, 47 polydimethylsiloxane (PDMS), 45, 47 prepatterned silicone/PET film, 45e46, 46f silicone-based pressure-sensitive adhesive film (PSAF), 46e47 stamping transfer, 45e46 thermal release tape (TRT), 45, 47 Amino-functionalized GQDs (af-GQDs), 123e125 Annealing process, 9e11, 11fe12f Atomic force microscopy (AFM) analysis, 7 Atomic structure, graphene, 5e7, 6f B Balloon-blowing method, 180 BeereLambert law, 83 Benzene, atomic structure, 119e120, 121f Block copolymers (BCPs), 130 Bubble-free electrochemical delamination method, 44 C Carbon dots (CDs), 121e123 Carbon nanotubes (CNTs), 59e61 Chemical etching, 36e38, 42 Chemical vapor deposition (CVD), 7e9, 8fe9f, 12f, 59, 60f, 74, 128e130, 177e178, 206e207 adhesion-mediated transfer. See Adhesionmediated transfer method applications, 33e35 liquid crystal display panel, 35e36

structural integrity and uniformity, 35e36 support-dissolving transfer. See Supportdissolving transfer method target-supported transfer, 48e51, 49f Continuous fabrication method, 33 Conventional electrodes, 175 Crumpled-graphene-papers (CG-papers), 196 D Density functional theory (DFT), 165 Digital electronics, 193 Doping chemical doping carbon nanotubes (CNTs), 59e61 chemical reactions, 59e61 conductivity and carrier concentration, 62e63, 63f electronic structure, 59e61, 61f by metal chloride, 61, 62f negative Gibbs free energy, 59e61 photovoltaic devices, 59e61 poly-(methyl methacrylate) (PMMA), 62 single-wall carbon nanotubes (SWCNTs), 62e63 tetracyanoethylene (TCNE), 63, 64f thionyl chloride, reaction mechanism, 62e63, 64f transfer and device fabrication process, 59e61, 60f work function (WF), 59e61 chemical vapor deposition (CVD), 59, 60f metal oxide application, 67 Fermi-level energies, 66 Seebeck coefficient, 65e66

210

Doping (Continued) thermally evaporated molybdenum trioxide (MoO3) layers, 65, 65f zero-gap two band model, 65e66 substitution method, 59 E Electrical eyes, 193 Electrical resistance, 22, 23f Electrochemical cutting method, 128e130, 129f Electroluminescence (EL) spectra, 75e76 Electroluminescent devices, 140e143, 141fe142f, 144f Electron paramagnetic resonance (EPR), 158 Energy band structure, graphene, 11 Energy-storage devices, 196 Etching-free bubbling transfer process, 42e44, 43f External quantum efficiency (EQE), 140 F Fermi energy, 13e14, 13f Few layered graphene (FLG), 153 Few layered graphene (FLG)/ ZnO nanorod (NR) composite, 154 Field effect transistor (FET) devices, 13e14, 16e20, 19fe20f Filtration transferring method, 30e31 Flexible photodetectors, 193 Fluorine-doped tin oxide (FTO), 152 Free exciton emission (FEE), 157 G Graphene adhesion/separation energies, 176, 177t in-plane stiffness, 176 low modulus elastomer, 176e177 surface roughness, 176 theoretical calculations, 176 theoretical strength, 176 Graphene-based composite emitter graphene oxide (GO)-zinc oxide (ZnO) composite, 154e157 band alignment, 161e162 charge transfer mechanism, 161f core-shell model, 161f

Index

electron paramagnetic resonance (EPR), 158, 159f photoluminescence spectra, 158, 161f grapheneeSnO2 hybrid composite, 162e163 semiconducting oxide NPs hybridization, 152 thiol-functionalized reduced graphene oxide (TrGOeZnO) hybrid materials band alignment, 161e162 charge transfer mechanism, 161f electron paramagnetic resonance (EPR), 158, 160f oxygen vacancy surface defects, 161e162 photoluminescence spectra, 161f tungsten oxideereduced graphene oxide (WO3erGO) nanocomposite, 163 zinc oxide (ZnO)egraphene quantum dot LED density functional theory (DFT), 165 energy level, 170f graphite power, 163e165 green light emission diode, 168e169 passive matrix, 168, 168fe169f photoluminescence spectra, 165, 166f schematic energy band diagram, 165e168, 167f synthetic process, 164f Zn acetate dehydrate, 163e165 zinc oxide (ZnO) nanoneedles, 154f zinc oxide (ZnO) nanowires (NWs), 156e157 Graphene-based composites CVD growth, 187f 3D graphene, 185e188 fabrication process, 186f graphene sheets, 185e188 metal nanowires (mNWs), 188 multilayer G/G scrolls (MGG), 188, 189f Ni foam, 188 silver nanowires (AgNWs), 188, 190f Graphene-based interfacial layer, 206 Graphene-based organic lighteemitting diodes (OLEDs) chemical vapor deposition (CVD), 206e207 energy-level engineering and chemical doping, 206

Index

graphene-based interfacial layer, 206 luminescence quenching, 206 photolithographic patterning, 205e206 solution-processable graphene derivatives, 206 Graphene-based transparent conductive films (G-TCFs) continuous fabrication, 33 hybrid TCFs, 31 lab-scale fabrication methods, 33 poly(3, 4ethylenedioxythiophene):poly(styrene sulfonate):graphene:ethyl cellulose (PEDOT:PSS:G:EC) hybrid electrodes, 33, 34fe35f reduced graphene oxide (rGO) nanosheets, 32, 32f roll-to-roll (R2R) process, 33 silver nanowires (AgNWs), 31e32, 32f solution casting, 30e31, 31f GrapheneeCdSe composite, 153 GrapheneeCdSe (GeCdSe) quantum dot (QDs), 153, 153f Graphene electrodes, 1, 2f chemical vapor deposition (CVD). See Chemical vapor deposition (CVD) doping, stability annealing-induced degradation, 68e69, 69f electronegativity, 70 energy configuration, 68e69 environmental conditions, 67 p-type doping, 68e69 sheet resistance, 67, 67f graphene-based transparent conductive films (G-TCFs), 27. See also Graphene-based transparent conductive films (G-TCFs) graphene oxide (GO) carbon/oxygen atomic ratio, 27 chemical reduction (CR), 28 graphene sheets, 27 high temperature annealing (HTA), 28, 30 LerfeKlinowski model, 27, 28f optoelectrical property, 28, 29t oxygen-containing groups, 28e30

211

reduced GO (rGO), 27e30 van der Waals attraction, 27 organic lighteemitting diodes (OLEDs). See Organic lighteemitting diodes (OLEDs) Grapheneemetal composites, 151e152 Grapheneemetal/metal oxide hybrid composite, 152 Graphene nanoflakes (GNFs), 191e192 Graphene origami balloon-blowing method, 180 graphene film engineering, 183e185 growth substrate prepatterning, 180e183 polyacrylic ester (PEA) substrate, 180 prestraining substrate, 180, 181f Graphene oxide quantum dots (GOQDs), 121e123 Graphene quantum dots (GQDs), 151 amorphous carbon materials, 119 atomic structure, benzene, 119e120, 121f block copolymers (BCPs), 130 bottom-up synthesis, 130e131, 132f carbon dots (CDs), 121e123 C60 molecules, 131e132, 133f DFT calculation, 119e120, 122f Dirac cone, 117e118 electrical and optical properties, 117 electrochemical cutting, 128e130, 129f p-electrons, 117e118 emission wavelength, 118e119, 118f energy levels, 117 fluorescence chemical exfoliation, photoluminescence spectra, 134, 135f controlled oxidation, 136e138, 137f defects/functional groups, 133 electroluminescence (EL) devices, 133 optical applications, 133 structural models, 134e136, 136f graphene oxide quantum dots (GOQDs), 121e123 graphite intercalation compounds, 127, 128f Hummers’ method, 123 hydrothermal/solvothermal cutting amino-functionalized GQDs (af-GQDs), 123e125 chemical reduction, 125e127

212

Graphene quantum dots (GQDs) (Continued) mechanism, 123e125, 124f preparation of, 125, 126f thermal deoxidization, 123e125 lighting applications down-converting white light-emtting diodes (WLEDs), 138e140, 139f electroluminescent devices, 140e143, 141fe142f, 144f properties, 138 microwave-assisted cutting, 127e128, 129f photo-Fenton reaction, 130, 131f photoluminescence (PL), 119, 120f quantum confinement effect, 118e119 semiconductor quantum dot, 117 top-down and bottom-up approach, 123 UV irradiation, 130 Graphene-TiO2 (G-TiO2), 152 Graphene-woven fabric electrodes (GWFs), 199 Grapheneezinc oxide (ZnO) hybrid composite band-edge emission, 154e156 band-to-band transition, 154e156 few layered graphene (FLG)/ZnO nanorod (NR) composite, 154, 155f free exciton emission (FEE), 157 localized surface plasmon (LSP), 154e156 metalorganic vaporephase epitaxy (MOVPE), 154, 155f p-Si/ZnO/graphene hybrid structure, 154e156 rGO/ZnO nanorods, 156, 157f schematically depicted photoluminescence enhancement path, 156f Graphene-ZnO (G-ZnO), 152 G-TCFs. See Graphene-based transparent conductive films (G-TCFs) H High temperature annealing (HTA), 28, 30 Hummers method, 103, 104f, 123 Hydrothermal/solvothermal cutting method amino-functionalized GQDs (af-GQDs), 123e125 chemical reduction, 125e127 mechanism, 123e125, 124f preparation of, 125, 126f thermal deoxidization, 123e125

Index

I Indium tin oxide (ITO), 1, 14e15 L Lab-scale fabrication methods, 30e31, 33 LangmuireBlodgett film, 108 Layer-by-layer doping technique, 74 LerfeKlinowski model, 27, 28f Light-emitting diodes (LEDs), 1 graphene-based buffer layers atomic-scale features, 99e101 composite buffer layer, 110e113, 111fe112f, 112t electrical properties, 101 graphite powder, 103 hole injection layer (HIL), 99 Hummers’s method, 103, 104f indium tin oxide (ITO), 99 ionization energy, 99 optical properties, 101 optoelectronics, 103e110, 105fe107f, 107te108t, 109fe110f oxidation and exfoliation process, 99e101 structure models, 99e101, 100f thermal reduction, 99e101, 102fe103f work function (WF), 99 graphene-based quantum dot emitters. See Graphene quantum dots (GQDs) Liquid bridging concept, 89e91, 90f M Metal nanowires (mNWs)/nanoparticles, 176 Microwave-assisted cutting method, 127e128, 129f Monocrystalline ruthenium catalyst, 9e11 N Nanocarbon-based transparent conductive films, 30 Nitrogen-doped GQDs (N-GQDs), 128e130 O Optoelectronics LangmuireBlodgett film, 108 organic light-emitting diodes (OLED), 103e108, 105f performance of, 108, 108t

Index

polymer light-emitting diodes (PLEDs), 103e108, 106fe107f device performance, 103e108, 107t quantum dot light-emitting diodes (QLEDs), 108e110, 109fe110f Organic lighteemitting diodes (OLEDs), 1, 103e108, 105f, 140, 141f BeereLambert law, 83 chemical bonding, 73 chemical vapor deposition (CVD), 74 direct transmittances, 83e84, 84f electrical issues, 81e83, 82f external quantum efficiencies, 83e84, 84f graphene-pixel electrode fabricated integration panel, 92e94, 93f flexible substrate, 94, 94f layout of, 92, 93f photolithography process, 91, 92f process, 91, 91f indium tin oxide (ITO), 74e76, 75f layer-by-layer doping technique, 74 liquid bridging concept, 89e91, 90f luminance distribution, 83e84, 84f optical issues angular emission, 78e81, 80f cavity enhancement factor, 76e77 dipole oscillation theory, 78 hole transport layer (HTL), 78, 79f microcavity approach, 78e81 multiple interference theory, 78 optical contrast, 76e77 reflectance of, 76e77, 78f scanning electron microscope (SEM), 76, 77f optoelectronic applications, 73 panel-level device array, 73 patterning hurdles, 87e88, 88fe89f patterning process, 74 performance of, 108, 108t proof-of-concept level, 87 random scattering layer (RSL) effect of, 85, 86f light extraction structure, 84e85, 85f surface planarization, 84e85 P Perovskite lighteemitting diodes (PeLEDs), 206

213

Photodetectors, 193, 195f Photo-Fenton reaction, 130, 131f Photolithography, 91, 92f, 205e206 Photoluminescence (PL), 119, 120f, 151 Piezo-capacitive strain sensors, 191 Piezo-resistive strain sensors, 191 Piezo-voltage strain sensors, 191 Poly(methyl-methacrylate) (PMMA), 36e38, 37f, 42 Polyacrylic ester (PEA) substrate, 180 Polyaniline (PANI), 180e183 Poly(vinyl alcohol) (PVA) carrier layer, 47 Polydimethylsiloxane (PDMS), 45, 47 Polymer light-emitting diodes (PLEDs), 103e108, 106fe107f, 107t, 140e143, 142f device performance, 103e108, 107t properties, 110e113, 112t Poly(methyl methacrylate) (PMMA), 7, 62 Poly(3, 4-ethylenedioxythiophene): poly(styrene sulfonate):graphene: ethyl cellulose (PEDOT:PSS:G:EC) hybrid electrodes, 33, 34fe35f Polystyrene, 185 Polyvinyl alcohol (PVA), 180e183 Q Quantum dot light-emitting diodes (QLEDs), 108e110, 109fe110f Quench factors (QFs), 153 R Raman spectroscopy analysis, 15e16, 15fe18f Random scattering layer (RSL) effect of, 85, 86f light extraction structure, 84e85, 85f S Scanning tunneling microscopy (STM) topography, 7 Seebeck coefficient, 65e66 Semiconductor quantum dot, 117 Sheet resistance, 20e22, 21f Silicone-based pressure-sensitive adhesive film (PSAF), 46e47 Silver nanowires (AgNWs), 31e32, 32f

214

Single-wall carbon nanotubes (SWCNTs), 30, 62e63, 193e196, 197f Solution casting methods, 30e31, 31f Solution-processable graphene derivatives, 206 Stretchable graphene electrodes carbon nanotubes, 176 conventional materials, 175 digital electronics, 193 2D materials, 201 energy-storage devices, 196 fabrication process, 175e176 mechanisms, 199e200 metal nanowires (mNWs)/nanoparticles, 176 “origami” methods, 177 preparation chemical vapor deposition technique, 177e178 dry-transfer process, 177e178 graphene-based composites, 185e191 graphene kirigami, 178e180, 179f graphene origami, 180e185 reliable fabrication methods, 200 sensors, 191 structural layouts, 175e176 Support-dissolving transfer method annealing, 38e39 bubble-free electrochemical delamination method, 44 chemical etching, 36e38, 42 Cu foil, 42 cyclododecane, 39 etching-free bubbling transfer process, 42e44, 43f face-to-face technique, 38 hydrogen bubbles, 44

Index

layer-by-layer transfer, 38 pentacene-supporting layer, 39, 41f poly(methyl-methacrylate) (PMMA), 36e38, 37f, 42 silicon substrate, 38 structural continuity and efficiency, 44e45 ultraclean and damage-free transfer, 39 UV irradiation, 38e39 vacuum evaporation technique, 42 van der Waals interaction, 45 Surface-mediated reaction, 9e11, 10f Surface segregation, 9e11, 10f T Tactile sensors, 191 Target-supported transfer method, 48e51, 49f Tetracyanoethylene (TCNE), 63, 64f Thermal release tape (TRT), 45, 47 Transmission electron microscopy (TEM) analysis, 7 Transparent display electrodes, 14e15 Tungsten oxideereduced graphene oxide (WO3erGO) nanocomposite, 163 U UV-vis transmittance spectra, 14e15, 14f V Vacuum evaporation technique, 42 van der Waals interactions, 8e9 W White light-emitting diodes (WLEDs), 138e140, 139f, 143, 144f Wrinkled graphene, 183e185, 184f