Flexible and Stretchable Triboelectric Nanogenerator Devices: Toward Self-Powered Systems 9783527345724, 3527345728, 9783527820146, 9783527820160, 9783527820153

Table of contents :
Content: Preface xvPart I Fundamentals of Triboelectric Nanogenerator 11 Overview of Triboelectric Nanogenerators 3Xiaosheng Zhang1.1 Energy Crisis of Microsystems 31.2 Microenergy Technologies 51.2.1 Photovoltaic Effect 71.2.2 Thermoelectric Effect 71.2.3 Electromagnetic Effect 81.2.4 Piezoelectric Effect 81.3 Triboelectric Nanogenerators 91.3.1 Principle of Triboelectric Nanogenerators 91.3.2 Key Factor: Triboelectric Series 111.3.3 Material Progress of Triboelectric Nanogenerators 111.3.4 Challenges of Triboelectric Nanogenerators 141.4 Summary 14Abbreviations 15References 152 Structures of Triboelectric Nanogenerators 19Haixia Zhang2.1 Operation Mechanisms of TENGs 192.1.1 Contact-Separation (CS) Mode 212.1.2 Relative-Sliding (RS) Mode 212.1.3 Single-Electrode (SE) Mode 222.1.4 Freestanding (FS) Mode 222.2 Typical Structures of TENGs 242.2.1 Plane-Shaped TENGs 242.2.2 Arch-Shaped TENGs 262.2.3 Zig-Zag-Shaped TENGs 302.2.4 Wavy-Shaped TENGs 332.2.5 Tank-Shaped TENGs 332.2.6 Rotor-Shaped TENGs 332.3 Summary 37Abbreviations 37References 383 Fabrication of Triboelectric Nanogenerators 41Bo Meng3.1 Mass Fabrication Technologies for Triboelectric Nanogenerators 413.1.1 Soft Lithography 413.1.2 Flexible Printed Circuit Manufacture 443.1.3 Roll-to-Roll Manufacture 453.1.4 3D Printing 463.1.5 Textile Manufacture 493.2 Performance Enhancement for Triboelectric Nanogenerators 503.2.1 Plasma Treatment 513.2.2 Wrinkle-Structured Surface 513.2.3 Chemical Synthesis 533.3 Summary 54Abbreviations 55References 554 Characterization of Triboelectric Nanogenerators 59Yu Song4.1 Electrical Operating Cycles of Triboelectric Nanogenerators 604.1.1 V-Q Plot and Its Characteristics 604.1.2 Operating Cycles of Energy Output 614.1.3 Measurements of Operating Cycles 644.2 Standard and Figure of Merits for Quantifying Triboelectric Nanogenerators 664.2.1 Figure of Merits of Triboelectric Nanogenerators 664.2.2 Structural Figure of Merits of Triboelectric Nanogenerators 674.2.3 Material Figure of Merit for Triboelectric Nanogenerators 704.3 Summary 73Abbreviations 74References 745 Power Management of Triboelectric Nanogenerators 77Xiaoliang Cheng5.1 Theoretical Analysis of Power Transmittance of TENGs 775.1.1 Resistive Load Characteristics of TENGs 785.1.2 Capacitive Load Characteristics of TENGs 785.2 The Progress in TENG Power Management 815.2.1 Using Inductive Transformers 815.2.2 Using Capacitive Transformers 825.2.3 Using LC Oscillation Circuit 835.3 Summary 90Abbreviations 90References 91Part II Approaches to Flexible and Stretchable Device 956 Overview of Flexible and Stretchable Approaches 97Mengdi Han6.1 Intrinsically Flexible or Stretchable Materials 976.1.1 Nanomaterials in Different Dimensions 976.1.2 Organic Materials 1006.1.3 Other Materials 1026.2 Structural Designs for Flexible and Stretchable Electronics 1036.2.1 Structural Design for Flexible Electronics 1036.2.2 2D Structural Design for Stretchable Electronics 1056.2.3 3D Structural Design for Stretchable Electronics 1076.3 Summary 107Abbreviations 107References 1087 Flexible and Stretchable Devices from 0D Nanomaterials 113Zongming Su7.1 0D Nanomaterials 1147.1.1 Quantum Dots 1147.1.2 Carbon Quantum Dots 1157.1.3 Gold Nanoparticles 1167.2 Thin Films Using 0D Nanomaterials 1177.2.1 Casting 1177.2.2 Dip Coating 1187.2.3 Langmuir-Blodgett Deposition 1207.3 Patterning Methods and Applications 1217.3.1 Screen Printing 1217.3.2 Inkjet Printing 1217.3.3 Microcontact Printing 1227.4 Applications of 0D Nanomaterials 1237.4.1 Electrodes 1247.4.2 Light-Emitting Diodes 1257.4.3 Transistors 1257.5 Summary 128Abbreviations 128References 1298 Flexible and Stretchable Devices from 1D Nanomaterials 133Liming Miao8.1 Carbon Nanotubes 1338.1.1 Fabrication Methods for CNTs 1338.1.1.1 CNT-Based Bulk Materials 1348.1.1.2 CNT-Based Surface Materials 1348.1.2 Application of CNTs 1368.2 ZnO Nanowires 1388.2.1 Synthesis of ZnO Nanowires 1398.2.2 Applications of ZnO Nanowires 1418.3 Ag Nanowires 1428.3.1 Fabrication Methods for Ag Nanowires 1428.3.2 Applications of Ag Nanowires 1438.4 Summary 145Abbreviations 145References 1469 Flexible and Stretchable Devices from 2D Nanomaterials 149Jinxin Zhang9.1 2D Nanomaterials 1499.1.1 Graphene 1509.1.2 TMDs 1519.1.3 Boron Nitride 1519.2 Synthesis of Graphene 1529.2.1 Micromechanical Exfoliation 1529.2.2 Epitaxial Growth 1539.2.3 Chemical Exfoliation 1539.3 Graphene Transfer 1549.3.1 Mechanical Exfoliation 1549.3.2 Polymer-Assisted Transfer 1549.3.3 Roll-to-Roll Transfer 1569.3.4 "Transfer-Free" Method 1569.4 Applications of Graphene 1579.4.1 Flexible and Stretchable Transparent Electrodes 1579.4.2 Nanogenerators 1589.5 Summary 160Abbreviations 161References 16110 Flexible and Stretchable Devices from Unconventional 3D Structural Design 165Hangbo Zhao and Mengdi Han10.1 Stretchable 3D Ribbon and Membrane Structures Formed by Basic Buckling 16510.1.1 3D Nanoribbons 16610.1.2 3D Nanomembranes 16710.1.3 3D Bridge-Island Structures 16710.2 Deterministic 3D Assembly 16710.2.1 Basic Approach of Deterministic 3D Assembly 16910.2.2 3D Kirigami Structure in Micro-/Nanomembranes 17210.2.3 Buckling Control Assisted by Stress and Strain Engineering 17210.2.4 Multilayer 3D Structures 17310.2.5 Freestanding 3D Structures 17510.2.6 Morphable 3D Structures by Multistable Buckling Mechanics 17610.3 Flexible and Stretchable Devices from 3D Assembly 17710.3.1 Electronic Devices and Systems 17710.3.2 Optical and Optoelectronic Devices 17710.3.3 Scaffolds as Interfaces with Biological Systems 17810.4 Summary 180Abbreviations 181References 18111 Flexible and Stretchable Devices from Other Materials 183Haotian Chen11.1 Polymer-Based Conductive Materials 18311.1.1 PANI 18411.1.2 PPy 18511.1.3 PEDOT : PSS 18511.1.4 Organic Nanowires 18511.2 Composite-Based Conductive Materials 18911.2.1 Conductive Fillers Blended into Stretchable Elastomers 18911.2.2 Conductive Film Embedded into Stretchable Elastomer 19111.3 Textile-Based Conductive Materials 19511.3.1 Fiber-Based Conductive Materials 19511.3.2 Textile-Based Conductive Materials 19611.4 Summary 199Abbreviations 199References 200Part III Self-Powered Smart System 20312 Active Sensors 205Xuexian Chen12.1 Active Touch Sensors 20512.1.1 Static and Dynamic Pressure Sensor 20612.1.2 Tactile Imaging Sensor 20612.1.3 Single-Electrode Touch Sensor 20712.2 Active Vibration Sensors 21012.2.1 Vibration Sensor for Quantitative Amplitude Measurement 21012.2.2 Vibration Acceleration Sensor 21212.2.3 Vibration Direction Sensor 21312.2.4 Acoustic Sensor 21312.3 Active Motion Sensors 21512.3.1 Linear Displacement Sensor 21512.3.2 Angle Sensor 21712.3.3 Omnidirectional Tilt Sensor 21712.4 Active Chemical/Environmental Sensors 21912.4.1 Chemical Sensor 21912.4.2 UV Sensor 22112.5 Summary 222Abbreviations 222References 22313 Hybrid Sensing Technology 227Xiaosheng Zhang, Yanyuan Ba, and Mengdi Han13.1 Dual Hybrid Power Technology 22713.1.1 Triboelectric-Piezoelectric Nanogenerator 22813.1.2 Triboelectric-Photovoltaic Nanogenerator 23113.1.3 Triboelectric-Electromagnetic Nanogenerator 23313.2 Multiple Hybrid Power Technology 23413.2.1 Triple Hybrid Generators 23413.2.2 Four-Mechanism Hybrid Generators 23513.3 Hybrid Sensors and Applications 23813.3.1 Piezoelectric-Triboelectric Hybrid Sensors 23913.3.2 Electromagnetic-Triboelectric Hybrid Sensors 24213.3.3 Multiple Hybrid Sensors 24713.4 Summary 249Abbreviations 250References 25114 Smart Actuators 253Xiaosheng Zhang and Zhaohui Wu14.1 Actuators in Optics 25414.1.1 Laser Controller 25414.1.2 Tunable Optical Membranes 25814.2 Actuators in Biomedicine 26114.2.1 Bladder Illness Curation 26114.2.2 Drug Delivery 26414.3 Actuators in Industrial Application 26714.3.1 Electrospinning System 26814.3.2 Syringe Printing 27014.4 Actuators in Microfluidic Manipulation 27214.4.1 Droplet Motion Drive 27214.4.2 Microfluidic Transport 27414.5 Summary 276Abbreviations 276References 27715 Flexible and Stretchable Electronic Skin 281Mayue Shi and Hanxiang Wu15.1 Design of Electronic Skin 28115.2 Electronic Skin for Mechanical Sensing 28515.2.1 Pressure Sensing 28515.2.2 Sliding Sensing 28815.2.3 Bending Sensing 28815.2.4 Location Sensing 28915.2.5 Strain Sensing 29015.3 Electronic Skin for Physiological Sensing 29415.3.1 Multimodal Sensing 29415.3.2 Physiological Monitoring 29615.3.3 Signal Transmission 29815.3.4 Reliability 29815.4 Summary 301Abbreviations 301References 302Part IV Applications of Flexible and Stretchable Self-Powered Smart System 30516 All-in-One Self-Powered Microsystems 307Xiaosheng Zhang and Danliang Wen16.1 All-in-One Energy Harvester 30816.1.1 One-Structural Triple-mechanism Energy Harvester 30916.1.2 One-Structural Flexible Energy Harvester 31016.1.3 One-Structural Multi-mechanism Energy Harvester 31216.2 All-in-One Power Unit 31616.2.1 Connection of TENGs and Traditional Circuits 31616.2.2 Integration of TENGs and Flexible Supercapacitors 32016.3 All-in-One Self-Powered Microsystems 32616.3.1 All-Fiber-Based Self-Powered Microsystem 32616.3.2 All-in-One Self-charging Smart Bracelet 32616.3.3 Other Research of All-in-One Self-Powered Microsystems 32716.4 Summary 335Abbreviations 335References 33617 Applications in Biomedical Systems 339Cunman Liang and Mengdi Han17.1 Power Sources of Implantable Medical Devices 34017.1.1 Power Source for Pacemakers 34017.1.2 Power Source for Medical Lasers 34217.1.3 Hybrid Power Source for Medical Applications 34417.2 Active Monitoring 34517.2.1 Nanogenerators for Cardiac Monitoring 34517.2.2 Multifunctional Real-Time Monitoring 34717.2.3 Versatile Energy Conversion and Monitoring 35017.2.4 Self-Powered Wireless Body Sensor Network 35217.3 Self-Powered System for Electric Stimulation in Tissue Engineering 35317.3.1 Self-Powered Electrical-Stimulation-Assisted Neural Differentiation System 35317.3.2 Biodegradable TENG for in Vivo Short-Term Stimulation 35417.3.3 Absorbable Bioresorbable in Vivo Natural-Materials-Based TENGs 35517.4 Summary 356Abbreviations 357References 35718 Applications in Internet of Things and Artificial Intelligence 359Mayue Shi and Hanxiang Wu18.1 Applications in Internet of Things 35918.1.1 Internet of Things 35918.1.2 Self-Powered Sensing Nodes 36018.1.3 Wireless Communication 36318.1.4 Power Management Circuit 36418.2 Applications in Artificial Intelligence 36718.2.1 Artificial Intelligence 36718.2.2 Electronic Skin 36818.2.3 Robotic Prosthetics 37118.2.4 Human-Machine Interfaces 37418.3 Summary 376Abbreviations 376References 37719 Applications in Environmental Monitoring/Protection 379Hang Guo and Wei Tang19.1 Self-powered EnvironmentalMonitoring System 37919.1.1 Phenol Detection 38019.1.2 Dopamine Detection 38219.1.3 Heavy Metal Ion Detection 38319.2 Self-powered Environmental Protection 38419.2.1 Degradation of AAB 38419.2.2 Degradation of Methyl Orange (MO) System 38419.2.3 Removing Fly Ash and SO2 38519.2.4 Seawater Desalination (SD) and Electrolysis (SE) System 38619.3 Self-powered Electrochemistry System 38819.3.1 Water Electrolysis Units 38819.3.2 Electrochemical Polymerization System 38919.3.3 Electrochemical Reduction System 39019.4 Self-powered Anticorrosion 39119.4.1 Driven by Mechanical Energy 39219.4.2 Driven by Wave Energy 39319.5 Summary 394Abbreviations 394References 395Index 399

Citation preview

Flexible and Stretchable Triboelectric Nanogenerator Devices

Flexible and Stretchable Triboelectric Nanogenerator Devices Toward Self-powered Systems

Edited by Mengdi Han Xiaosheng Zhang Haixia Zhang

Editors

Northwestern University Center for Bio-Integrated Electronics B371, Technological Institute 2145 Sheridan Road Evanston, IL United States

All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Prof. Xiaosheng Zhang

Library of Congress Card No.:

University of Electronic Science and Technology of China School of Electronic Science and Engineering B322, Research Building B No. 2006 Xiyuan Ave West Hi-Tech Zone 611731 Chengdu China

applied for

Dr. Mengdi Han

Prof. Haixia Zhang

Peking University Department of Microelectronics No. 5 Yiheyuan Road Haidian District 100871 Beijing China

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library. Bibliographic information published by the Deutsche Nationalbibliothek

The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at . © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany

Cover Images:

© Iaremenko Sergii/Shutterstock, © piick/Shutterstock

All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Print ISBN: 978-3-527-34572-4 ePDF ISBN: 978-3-527-82014-6 ePub ISBN: 978-3-527-82016-0 oBook ISBN: 978-3-527-82015-3 Typesetting SPi Global, Chennai, India Printing and Binding

Printed on acid-free paper

10 9 8 7 6 5 4 3 2 1

v

Contents Preface xv

Part I

Fundamentals of Triboelectric Nanogenerator 1

1

Overview of Triboelectric Nanogenerators 3 Xiaosheng Zhang

1.1 1.2 1.2.1 1.2.2 1.2.3 1.2.4 1.3 1.3.1 1.3.2 1.3.3 1.3.4 1.4

Energy Crisis of Microsystems 3 Microenergy Technologies 5 Photovoltaic Effect 7 Thermoelectric Effect 7 Electromagnetic Effect 8 Piezoelectric Effect 8 Triboelectric Nanogenerators 9 Principle of Triboelectric Nanogenerators 9 Key Factor: Triboelectric Series 11 Material Progress of Triboelectric Nanogenerators 11 Challenges of Triboelectric Nanogenerators 14 Summary 14 Abbreviations 15 References 15

2

Structures of Triboelectric Nanogenerators 19 Haixia Zhang

2.1 2.1.1 2.1.2 2.1.3 2.1.4 2.2 2.2.1 2.2.2 2.2.3

Operation Mechanisms of TENGs 19 Contact-Separation (CS) Mode 21 Relative-Sliding (RS) Mode 21 Single-Electrode (SE) Mode 22 Freestanding (FS) Mode 22 Typical Structures of TENGs 24 Plane-Shaped TENGs 24 Arch-Shaped TENGs 26 Zig-Zag-Shaped TENGs 30

vi

Contents

2.2.4 2.2.5 2.2.6 2.3

Wavy-Shaped TENGs 33 Tank-Shaped TENGs 33 Rotor-Shaped TENGs 33 Summary 37 Abbreviations 37 References 38

3

Fabrication of Triboelectric Nanogenerators 41 Bo Meng

3.1 3.1.1 3.1.2 3.1.3 3.1.4 3.1.5 3.2 3.2.1 3.2.2 3.2.3 3.3

Mass Fabrication Technologies for Triboelectric Nanogenerators 41 Soft Lithography 41 Flexible Printed Circuit Manufacture 44 Roll-to-Roll Manufacture 45 3D Printing 46 Textile Manufacture 49 Performance Enhancement for Triboelectric Nanogenerators 50 Plasma Treatment 51 Wrinkle-Structured Surface 51 Chemical Synthesis 53 Summary 54 Abbreviations 55 References 55

4

Characterization of Triboelectric Nanogenerators 59 Yu Song

4.1 4.1.1 4.1.2 4.1.3 4.2

Electrical Operating Cycles of Triboelectric Nanogenerators 60 V –Q Plot and Its Characteristics 60 Operating Cycles of Energy Output 61 Measurements of Operating Cycles 64 Standard and Figure of Merits for Quantifying Triboelectric Nanogenerators 66 Figure of Merits of Triboelectric Nanogenerators 66 Structural Figure of Merits of Triboelectric Nanogenerators 67 Material Figure of Merit for Triboelectric Nanogenerators 70 Summary 73 Abbreviations 74 References 74

4.2.1 4.2.2 4.2.3 4.3

5

Power Management of Triboelectric Nanogenerators 77 Xiaoliang Cheng

5.1 5.1.1 5.1.2 5.2 5.2.1 5.2.2 5.2.3

Theoretical Analysis of Power Transmittance of TENGs Resistive Load Characteristics of TENGs 78 Capacitive Load Characteristics of TENGs 78 The Progress in TENG Power Management 81 Using Inductive Transformers 81 Using Capacitive Transformers 82 Using LC Oscillation Circuit 83

77

Contents

5.3

Summary 90 Abbreviations 90 References 91

Part II

Approaches to Flexible and Stretchable Device 95

6

Overview of Flexible and Stretchable Approaches 97 Mengdi Han

6.1 6.1.1 6.1.2 6.1.3 6.2 6.2.1 6.2.2 6.2.3 6.3

Intrinsically Flexible or Stretchable Materials 97 Nanomaterials in Different Dimensions 97 Organic Materials 100 Other Materials 102 Structural Designs for Flexible and Stretchable Electronics Structural Design for Flexible Electronics 103 2D Structural Design for Stretchable Electronics 105 3D Structural Design for Stretchable Electronics 107 Summary 107 Abbreviations 107 References 108

7

Flexible and Stretchable Devices from 0D Nanomaterials 113 Zongming Su

7.1 7.1.1 7.1.2 7.1.3 7.2 7.2.1 7.2.2 7.2.3 7.3 7.3.1 7.3.2 7.3.3 7.4 7.4.1 7.4.2 7.4.3 7.5

0D Nanomaterials 114 Quantum Dots 114 Carbon Quantum Dots 115 Gold Nanoparticles 116 Thin Films Using 0D Nanomaterials 117 Casting 117 Dip Coating 118 Langmuir–Blodgett Deposition 120 Patterning Methods and Applications 121 Screen Printing 121 Inkjet Printing 121 Microcontact Printing 122 Applications of 0D Nanomaterials 123 Electrodes 124 Light-Emitting Diodes 125 Transistors 125 Summary 128 Abbreviations 128 References 129

8

Flexible and Stretchable Devices from 1D Nanomaterials 133 Liming Miao

8.1 8.1.1

Carbon Nanotubes 133 Fabrication Methods for CNTs 133

103

vii

viii

Contents

8.1.1.1 8.1.1.2 8.1.2 8.2 8.2.1 8.2.2 8.3 8.3.1 8.3.2 8.4

CNT-Based Bulk Materials 134 CNT-Based Surface Materials 134 Application of CNTs 136 ZnO Nanowires 138 Synthesis of ZnO Nanowires 139 Applications of ZnO Nanowires 141 Ag Nanowires 142 Fabrication Methods for Ag Nanowires 142 Applications of Ag Nanowires 143 Summary 145 Abbreviations 145 References 146

9

Flexible and Stretchable Devices from 2D Nanomaterials 149 Jinxin Zhang

9.1 9.1.1 9.1.2 9.1.3 9.2 9.2.1 9.2.2 9.2.3 9.3 9.3.1 9.3.2 9.3.3 9.3.4 9.4 9.4.1 9.4.2 9.5

2D Nanomaterials 149 Graphene 150 TMDs 151 Boron Nitride 151 Synthesis of Graphene 152 Micromechanical Exfoliation 152 Epitaxial Growth 153 Chemical Exfoliation 153 Graphene Transfer 154 Mechanical Exfoliation 154 Polymer-Assisted Transfer 154 Roll-to-Roll Transfer 156 “Transfer-Free” Method 156 Applications of Graphene 157 Flexible and Stretchable Transparent Electrodes 157 Nanogenerators 158 Summary 160 Abbreviations 161 References 161

10

Flexible and Stretchable Devices from Unconventional 3D Structural Design 165 Hangbo Zhao and Mengdi Han

10.1

Stretchable 3D Ribbon and Membrane Structures Formed by Basic Buckling 165 3D Nanoribbons 166 3D Nanomembranes 167 3D Bridge-Island Structures 167 Deterministic 3D Assembly 167 Basic Approach of Deterministic 3D Assembly 169 3D Kirigami Structure in Micro-/Nanomembranes 172 Buckling Control Assisted by Stress and Strain Engineering 172

10.1.1 10.1.2 10.1.3 10.2 10.2.1 10.2.2 10.2.3

Contents

10.2.4 10.2.5 10.2.6 10.3 10.3.1 10.3.2 10.3.3 10.4

Multilayer 3D Structures 173 Freestanding 3D Structures 175 Morphable 3D Structures by Multistable Buckling Mechanics Flexible and Stretchable Devices from 3D Assembly 177 Electronic Devices and Systems 177 Optical and Optoelectronic Devices 177 Scaffolds as Interfaces with Biological Systems 178 Summary 180 Abbreviations 181 References 181

11

Flexible and Stretchable Devices from Other Materials 183 Haotian Chen

11.1 11.1.1 11.1.2 11.1.3 11.1.4 11.2 11.2.1 11.2.2 11.3 11.3.1 11.3.2 11.4

Polymer-Based Conductive Materials 183 PANI 184 PPy 185 PEDOT:PSS 185 Organic Nanowires 185 Composite-Based Conductive Materials 189 Conductive Fillers Blended into Stretchable Elastomers 189 Conductive Film Embedded into Stretchable Elastomer 191 Textile-Based Conductive Materials 195 Fiber-Based Conductive Materials 195 Textile-Based Conductive Materials 196 Summary 199 Abbreviations 199 References 200

Part III

176

Self-Powered Smart System 203

12

Active Sensors 205 Xuexian Chen

12.1 12.1.1 12.1.2 12.1.3 12.2 12.2.1 12.2.2 12.2.3 12.2.4 12.3 12.3.1 12.3.2 12.3.3 12.4

Active Touch Sensors 205 Static and Dynamic Pressure Sensor 206 Tactile Imaging Sensor 206 Single-Electrode Touch Sensor 207 Active Vibration Sensors 210 Vibration Sensor for Quantitative Amplitude Measurement 210 Vibration Acceleration Sensor 212 Vibration Direction Sensor 213 Acoustic Sensor 213 Active Motion Sensors 215 Linear Displacement Sensor 215 Angle Sensor 217 Omnidirectional Tilt Sensor 217 Active Chemical/Environmental Sensors 219

ix

x

Contents

12.4.1 12.4.2 12.5

Chemical Sensor 219 UV Sensor 221 Summary 222 Abbreviations 222 References 223

13

Hybrid Sensing Technology 227 Xiaosheng Zhang, Yanyuan Ba, and Mengdi Han

13.1 13.1.1 13.1.2 13.1.3 13.2 13.2.1 13.2.2 13.3 13.3.1 13.3.2 13.3.3 13.4

Dual Hybrid Power Technology 227 Triboelectric–Piezoelectric Nanogenerator 228 Triboelectric–Photovoltaic Nanogenerator 231 Triboelectric–Electromagnetic Nanogenerator 233 Multiple Hybrid Power Technology 234 Triple Hybrid Generators 234 Four-Mechanism Hybrid Generators 235 Hybrid Sensors and Applications 238 Piezoelectric–Triboelectric Hybrid Sensors 239 Electromagnetic–Triboelectric Hybrid Sensors 242 Multiple Hybrid Sensors 247 Summary 249 Abbreviations 250 References 251

14

Smart Actuators 253 Xiaosheng Zhang and Zhaohui Wu

14.1 14.1.1 14.1.2 14.2 14.2.1 14.2.2 14.3 14.3.1 14.3.2 14.4 14.4.1 14.4.2 14.5

Actuators in Optics 254 Laser Controller 254 Tunable Optical Membranes 258 Actuators in Biomedicine 261 Bladder Illness Curation 261 Drug Delivery 264 Actuators in Industrial Application 267 Electrospinning System 268 Syringe Printing 270 Actuators in Microfluidic Manipulation 272 Droplet Motion Drive 272 Microfluidic Transport 274 Summary 276 Abbreviations 276 References 277

15

Flexible and Stretchable Electronic Skin 281 Mayue Shi and Hanxiang Wu

15.1 15.2

Design of Electronic Skin 281 Electronic Skin for Mechanical Sensing 285

Contents

15.2.1 15.2.2 15.2.3 15.2.4 15.2.5 15.3 15.3.1 15.3.2 15.3.3 15.3.4 15.4

Pressure Sensing 285 Sliding Sensing 288 Bending Sensing 288 Location Sensing 289 Strain Sensing 290 Electronic Skin for Physiological Sensing Multimodal Sensing 294 Physiological Monitoring 296 Signal Transmission 298 Reliability 298 Summary 301 Abbreviations 301 References 302

294

Part IV Applications of Flexible and Stretchable Self-Powered Smart System 305 16

All-in-One Self-Powered Microsystems 307 Xiaosheng Zhang and Danliang Wen

16.1 16.1.1 16.1.2 16.1.3 16.2 16.2.1 16.2.2 16.3 16.3.1 16.3.2 16.3.3 16.4

All-in-One Energy Harvester 308 One-Structural Triple-mechanism Energy Harvester 309 One-Structural Flexible Energy Harvester 310 One-Structural Multi-mechanism Energy Harvester 312 All-in-One Power Unit 316 Connection of TENGs and Traditional Circuits 316 Integration of TENGs and Flexible Supercapacitors 320 All-in-One Self-Powered Microsystems 326 All-Fiber-Based Self-Powered Microsystem 326 All-in-One Self-charging Smart Bracelet 326 Other Research of All-in-One Self-Powered Microsystems 327 Summary 335 Abbreviations 335 References 336

17

Applications in Biomedical Systems 339 Cunman Liang and Mengdi Han

17.1 17.1.1 17.1.2 17.1.3 17.2 17.2.1 17.2.2 17.2.3

Power Sources of Implantable Medical Devices 340 Power Source for Pacemakers 340 Power Source for Medical Lasers 342 Hybrid Power Source for Medical Applications 344 Active Monitoring 345 Nanogenerators for Cardiac Monitoring 345 Multifunctional Real-Time Monitoring 347 Versatile Energy Conversion and Monitoring 350

xi

xii

Contents

17.2.4 17.3 17.3.1 17.3.2 17.3.3 17.4

Self-Powered Wireless Body Sensor Network 352 Self-Powered System for Electric Stimulation in Tissue Engineering 353 Self-Powered Electrical-Stimulation-Assisted Neural Differentiation System 353 Biodegradable TENG for in Vivo Short-Term Stimulation 354 Absorbable Bioresorbable in Vivo Natural-Materials-Based TENGs 355 Summary 356 Abbreviations 357 References 357

18

Applications in Internet of Things and Artificial Intelligence 359 Mayue Shi and Hanxiang Wu

18.1 18.1.1 18.1.2 18.1.3 18.1.4 18.2 18.2.1 18.2.2 18.2.3 18.2.4 18.3

Applications in Internet of Things 359 Internet of Things 359 Self-Powered Sensing Nodes 360 Wireless Communication 363 Power Management Circuit 364 Applications in Artificial Intelligence 367 Artificial Intelligence 367 Electronic Skin 368 Robotic Prosthetics 371 Human–Machine Interfaces 374 Summary 376 Abbreviations 376 References 377

19

Applications in Environmental Monitoring/Protection 379 Hang Guo and Wei Tang

19.1 19.1.1 19.1.2 19.1.3 19.2 19.2.1 19.2.2 19.2.3 19.2.4 19.3 19.3.1 19.3.2 19.3.3

Self-powered Environmental Monitoring System 379 Phenol Detection 380 Dopamine Detection 382 Heavy Metal Ion Detection 383 Self-powered Environmental Protection 384 Degradation of AAB 384 Degradation of Methyl Orange (MO) System 384 Removing Fly Ash and SO2 385 Seawater Desalination (SD) and Electrolysis (SE) System 386 Self-powered Electrochemistry System 388 Water Electrolysis Units 388 Electrochemical Polymerization System 389 Electrochemical Reduction System 390

Contents

19.4 19.4.1 19.4.2 19.5

Self-powered Anticorrosion 391 Driven by Mechanical Energy 392 Driven by Wave Energy 393 Summary 394 Abbreviations 394 References 395 Index 399

xiii

xv

Preface Since 1960, electronics has played an important role in modern society, either as computing, sensing devices for information collection or as mobile terminates for data exchange. Further wider applications essentially require overcoming the restriction of traditional rigid, unsustainable power sources, thereby promoting the favorable properties of stability, high-output, being maintenance-free, flexibility, or even stretchability for the most sophisticated electronics. Quantitative comparison and qualitative analysis prove that emerging triboelectric nanogenerators (TENGs), invented by Zhonglin Wang of the Georgia Institute of Technology in 2012, can be a powerful and promising approach to address these challenges. Mostly, the TENG is fabricated by polymer-based materials which make it flexible and stretchable naturally. More than a traditional energy harvester, the TENG can not only scavenge the mechanical energy from an ambient environment, based on the combination of contact electrification and electrostatic induction, but also serve as an active sensor from its rich information output with external stimulation. With all these merits, TENGs are developed as a flexible and stretchable self-powered system for many applications. In this book, we introduce in four parts the principle and progress of flexible and stretchable devices based on the triboelectric nanogenerator – the fundamentals of TENGs, approaches to flexible and stretchable structures, self-powered smart systems, and their applications in various areas. The major contents of each part are listed as follows. PART I: Fundamentals of triboelectric nanogenerators, including principle, working modes and structures, fabrication technologies, characterization, and power management. PART II: Approaches to flexible and stretchable, including commonly used materials (0D, 1D, 2D nanomaterials) and unconventional 3D structural design and other recent progresses. PART III: Self-powered smart systems, including active and hybrid sensors, smart actuators, electronic skin, and all-in-one self-powered microsystems. PART IV: Applications of self-powered smart systems, including biomedical systems, Internet of Things and artificial intelligence, environmental monitoring/protection, etc. Therefore, this book provides a coherent viewpoint of this attractive field that is difficult to obtain solely by reading individual journal papers. The book also gives comparisons between methods, designs, and materials that typically do not

xvi

Preface

appear in journal publications. The book will act as a resource to those who aspire to further extend either the science and technologies of flexible and stretchable electronics and self-powered systems or future applications. This book was written based mainly on numerous journal papers we and our collaborators have authored since 2012. I thank Alice Wonderlab’s current and former members and partners who have made countless contributions in relative fields (list not in any particular order): Xiaosheng Zhang, Mengdi Han, Haotian Chen, Zongming Su, Xiaoliang Cheng, Xuexian Chen, Mayue Shi, Hanxiang Wu, Jinxin Zhang, Liming Miao, Yu Song, Bo Meng, Hang Guo, Ji Wan, Chen Xu, Haobin Wang, Wen Liu, Fuyun Zhu, Quan Yuan, Wei Tang, Xuming Sun, Xiuhan Li, Hangbo Zhao, Cunman Liang, Yanyuan Ba, Zhaohui Wu, Danliang Wen, B N Chandrashekar, Smitha A S, Jianbo Fu, and Zijian Song, etc. We thank our collaborators and coworkers worldwide: John A Rogers, Juergen Brugger, Jingfu Bao, Zhongfan Liu, Zhihong Li, Shuxiang Dong, Tianling Ren, and Wei Gao, etc. Particularly, I express my thanks to Prof. Zhonglin Wang for his continuous support and encouragement in the past 10 years, for me Prof. Wang is not just an outstanding scientist in research, a great mentor in career, but also a living model in life. In addition, I thank my iCAN team, Yiqing Wang, Zhonghua Yu, Baoqin Chen, Jianping Xing, Yalin Ren, Xiangjun Xin, Jiaxin Cui, Sulan Li, Esashi Masayoshi, Chihming Ho, and others. iCAN is the original force in my mind, to make something different, to make something great, and to make something meaningful. With your long-term support and co-efforts, we are on the way to make this great dream come true. This book is a demonstration in academy. Last but not least, I thank my harmonious four-generation family, my dearest grandmother, and my father and mother, my husband Herbert and my daughter Lily, without their powerful support and full understanding nothing would have come out in my life. This book is my sincere gift to all of you. Haixia(Alice) Zhang Microelectronics Department Peking University Beijing, China

1

Part I Fundamentals of Triboelectric Nanogenerator

3

1 Overview of Triboelectric Nanogenerators Xiaosheng Zhang University of Electronic Science and Technology of China, School of Electronic Science and Engineering, No. 2006, Xiyuan Ave, West Hi-Tech Zone, Chengdu 611731, P.R. China

Ambient energy is abundant in the environment and takes various forms, such as solar irradiation [1], thermal gradient [2], mechanical deformation [3], and so on, which can be converted into electricity using ambient energy-harvesting techniques [4]. The approach to harvesting energy from the environment is one of the ideal solutions to respond to the energy demands of distributed autonomous microsystems, which should be sustainable, renewable, and of high performance [4]. Ambient energy-harvesting technology provides an attractive future vision to realize fully integrated self-powered microsystems that overcome the drawbacks of batteries, which currently need to be frequently replaced, or of laying out long wires for power supply [5]. In 2012, a new ambient energy-harvesting mechanism named triboelectric nanogenerator (TENG) was developed by combining the triboelectrification effect and electrostatic induction [6, 7]. This novel technology has proved to be a robust power source to directly power commercial electronics and even regular light bulbs [7]. There has been a remarkable growth of TENG research in the past years due to its unique properties, including high-output performance, cleanness, sustainability, etc. [8, 9]. Mechanical energy from sources such as wind, raindrops, and ocean waves, as well as body motions can be efficiently converted to electric power using TENGs. Therefore, this chapter summarizes the current progress of microenergy technologies and then introduces an overview of TENG.

1.1 Energy Crisis of Microsystems In the past half century, as the benefit of the rapid development of electronic science and technology, human society gradually changed to automation, both intelligent and digitization, and various electronic devices have become part of our life and are distributed everywhere. The featured size of modern electronic devices becomes smaller year by year down to the millimeter, and even to micrometer levels, which induces a continuous decrease in power consumption down to milliwatts and even to microwatts. Consequently, the Flexible and Stretchable Triboelectric Nanogenerator Devices: Toward Self-powered Systems, First Edition. Edited by Mengdi Han, Xiaosheng Zhang, and Haixia Zhang. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

4

1 Overview of Triboelectric Nanogenerators

great reforms in small-size and low-power consumption promote the integration of multifunctional electronic devices to realize microsystems. Microsystems experienced a blooming development in the past decades resulting from their unique features, i.e. portable, smart, and miniature. However, the further development of microsystems suffered several critical challenges, especially the exploration of appropriate power sources. At present, batteries are still the first option, especially some of the flexible batteries, but the problems of sustainability and pollution caused by batteries cannot be ignored. In the meantime, distributed autonomous microsystems also have some new energy demands such as sustainability, renewability, high performance, and even flexibility or stretchability. As an essential example of microsystems, Internet of things (IoT) is expected to play an important role in economic and social development of the next generation, as shown in Figure 1.1. Thus, IoT is taken as an example to describe the energy crisis of microsystems [10]. In principle, an IoT system is composed of three main parts: sensing network, interconnection network, and terminal network. The sensing network detects the various changes of environmental factors and transforms them as electronic signals. Subsequently, the electronic signals are transferred to the interconnection network and treated to form control signals. Eventually, these control signals are delivered to the terminal network to drive functional electronic devices to respond to the corresponding changes. Therefore, the sensing network serving as the interface media between environment and client is the essential component of the IoT. The sensing network consists of trillions of sensors, which are widely distributed in the environment, especially in autonomous states. Consequently, exploring an appropriate power approach for the sensing network is an urgent issue for the rapid development of IoT. Micro energy

Internet of things (IoT) Multiple functions Autonomous remote and operation

Sensing network

Terminal network

Harsh environment

High-performance Sustainability Maintains free Self-powered operation Microenergy source

Interconnection network

Figure 1.1 Schematic view of the Internet of things (IoT) and its power supply requirements. It is of great significance for the development of IoT to realize the appropriate microenergy source that fits the unique requirements.

1.2 Microenergy Technologies

sources harvesting energy from the ambient have been proven as one of the attractive methods. By using piezoelectric, thermoelectric, photovoltaic effects, etc., microenergy harvesters can accumulate energy in various forms and convert them to electricity to power miniature devices and systems. There micro energy-harvesting technologies are clean, sustainable, and low-cost. Moreover, it provides the feasibility to integrate functional electronic components with these microenergy sources.

1.2 Microenergy Technologies A promising way to satisfy the energy demands of low-power-consumption microsystems is to collect energy from the living environment. Because the featured size and the electric output are at milliscale or even microscale levels, the ambient energy-harvesting technologies are also named as microenergy technologies. They possess attractive advantages to realize fully integrated, self-powered devices which do not need replacement of batteries or laying out long wires for charging. Previous research work has exploited several techniques using different mechanisms, such as photoelectric conversion, piezoelectric effect, thermoelectric effect, biochemical effect, etc. These techniques can be used to collect various forms of environmental energy such as light, mechanical change, temperature difference, variation of electromagnetic field, etc. Herein, we summarize and compare five essential technologies for ambient energy-harvesting in Figure 1.2 and Table 1.1. Since the specific application WPT

100

90

90

80

80

70

70

60

60

MC

50

50

40

40 1 cm

30

30

20

20

10 0

Energy conversion efficiency* (%)

Power density* (mW/cm2)

100

10 N/A

Photovoltaic Thermoelectric Electromagnetic Piezoelectric

0 Triboelectric

Figure 1.2 Summary of technical progress of five promising methods for harvesting energy from the environment. Source: Reproduced with permission from Zhang et al. [4]. Copyright 2018, Elsevier.

5

Table 1.1 Comparison of five technologies for ambient energy harvesting.

Type

Photovoltaic (PV)

Schematic view

Illumination

Voltage (V)

Current (A)

Power density (mW/cm2 )

0.5–0.9

100–500

5–30

Efficiency (%)

Pros versus cons

0.3–46

High output power, continuous DC output, good basis of industrial fabrication For the flexible organic solar cell, the conversion efficiency is still very low, only works under light

Thermoelectric (ThE)

Heating

0.1–1

5–30

0.01–3

0.1–25

Sustainably working as a DC power source, easy to scale down, no moving component Low conversion efficiency, low output performance, large temperature difference

Electromagnetic (EM)

Electromagnetic

0.1–10

N/A

N/A

5–90

High conversion efficiency, high current with low voltage, resistive impedance For wireless power transmission: short range, working at certain frequency; For magnet moving: heavy weight and big size, complexity

Piezoelectric

Pressure

1–200

0.01–10

0.001–30

0.01–21

Highly sensitive to external excitation, easily integrates and miniaturizes in micro-/nanoscale Low conversion efficiency, low output performance, pulse output, high impedance

Triboelectric (TrE)

3–1500

10–2000

Friction

Source: Reproduced with permission from Zhang et al. [4]. Copyright 2018, Elsevier.

0.1–100

10–85

High output and energy conversion efficiency, no materials limitation, remarkable flexibility Pulse output, high impedance, friction damage

1.2 Microenergy Technologies

area is limited for self-powered flexible microsystems, only flexible or wearable configurations of these five technologies are emphasized. 1.2.1

Photovoltaic Effect

The solar cell is one of most popular power sources based on photovoltaic effect. The basic law of photovoltaic effect is that electrons overcome the potential barrier and are excited to a higher energy state by absorbed lights. The frequency of light must exceed a certain range in order to possess sufficient energy to overcome the potential barrier for excitation, and then the separation of charges leads to the establishment of an electric potential [4, 11]. So far, solar cells can be divided into five main categories, including multijunction cells, single-junction GaAs, crystalline silicon cells, thin-film technologies, and emerging others [12]. The first three types possess better performance, with the energy conversion efficiency (ECE) ranging from 21.2% to 46%, and silicon-based solar cells dominate the commercial market [12]. Nevertheless, because of fragile and nonflexible characterization, it is difficult to apply them in wearable microsystems. By contrast, the latter two classifications show good flexibility by fabricating specific functional materials on polymeric substrates, but their ECEs are still at a relatively low level, with copper indium gallium selenide (CIGS), perovskite, and dye-sensitized solar cells reaching the highest values of 23.3%, 22.1%, and 11.9%, respectively [12]. 1.2.2

Thermoelectric Effect

As is known, the human body itself is a perfect energy source to provide thermal dissipation and physical movement [13, 14], which is regarded as an attractive solution to satisfy the power demand of wearable electronics. The thermoelectric effect is employed to transform the energy generated by the thermal dissipation of the human body to electricity. When a temperature difference is applied to thermoelectric devices, an electrical potential that can drive the flow of electrons in the circuit loop to generate the electricity will be established, which is named the Peltier–Seebeck effect [4, 15]. Thermoelectric generators can be classified into inorganic and organic. Inorganic thermoelectric generators are made from inorganic materials, such as several alloys and intermetallic compounds based on elements like Bi, Te, Sb, Pb, etc., which are, as a matter of fact, toxic [16]. Organic thermoelectric generators are usually manufactured using conductive polymers (i.e. conjugated polymers and certain coordination polymers) and small molecules (i.e. charge-transfer complexes and molecular semiconductors) [15]. They have attracted much attention because of the properties of light weight and outstanding flexibility. However, the ECE needs to be strengthened, which is still at a relatively low level of less than 25%. The ECE of the thermoelectric generator is defined as a function of the figure of merit (ZT), average working temperature, and the temperature difference between the hot and cold ends. Thus, compared with ECE, the figure of merit, i.e. ZT = S2 𝜎T/k, is more important

7

8

1 Overview of Triboelectric Nanogenerators

to characterize the performance of the thermoelectric generator, where S is the Seebeck coefficient or thermopower, 𝜎 is electrical conductivity, 𝜅 is thermal conductivity, and T is the absolute temperature [17]. 1.2.3

Electromagnetic Effect

The electromagnetic effect is a well-known Faraday’s law of electromagnetic induction, which can be traced back to 1831 [4]. It reveals that the voltage induced in a closed loop is proportional to the change rate of the magnetic flux through the annular region. This is the operational principle of a traditional magnetic generator, which is the cornerstone of modern society. The ECE of the magnetic generator can reach 90%, which has a desirable output power. But when we attempt to use it for wearable applications, this traditional magnetic generator is obviously unsuitable because of its heavy weight and large size. Thus, electromagnetic microgenerators were developed by adopting the microfabrication technology, which makes the device miniaturized and partially realizes its flexibility by fabricating flexible coils [18–20]. However, the properties of a hard magnet make it impossible to create fully flexible electromagnetic microgenerators. Another promising alternative is wireless power transfer (WPT) based on electromagnetic induction, which can be used to transfer electrical power among multiple points without requiring a physical connection [21, 22]. The method endows the powered electronics with the maximum freedom, and has been proved as a wireless power source in both laboratory and industry. Although relay coils have been developed to cope with these obstacles, the limitations of power transmission direction and short-range distance still pose challenges for further applications [23]. 1.2.4

Piezoelectric Effect

The piezoelectric generator is an important approach to scavenging biomechanical energy, which has been proved to be a clean energy source [24–26]. The fundamental mechanism is a piezoelectric effect, which, as an electric potential, is established at the end of piezoelectric materials, and under external pressure is a reversible process [24]. Several kinds of materials, including specific crystals, ceramics, polymers, and biological matter, have been discovered to possess piezoelectric property. They can be simply classified into two main categories: inorganic and organic materials. The most well-known inorganic materials are piezoelectric crystals and ceramics, such as PZT (lead zirconate titanate), BaTiO3 (barium titanate), ZnO (zinc oxide), quartz, etc. [4, 27]. The typical organic material is PVDF (polyvinylidene fluoride), which is flexible and suitable for integration with wearable electronics. In order to obtain piezoelectric property, it is necessary to implement a post process of polarization by applying an ultra-strong electric field, which requires specific equipment and manufacturing processes. Piezoelectric coefficient, also named piezoelectric constant, is one of the essential parameters to quantify the piezoelectric property of materials, whose

1.3 Triboelectric Nanogenerators

variation ranges from tens to thousands. Therefore, the performance of a piezoelectric generator has a direct relation with the piezoelectric property of the selected material. The remarkable linear characteristic between input pressure stimulation and output electric signal makes piezoelectric generators suitable to play the part of self-powered transducers [5]. It deserves to be mentioned that piezoelectric nanogenerators (PENGs) based on ZnO nanowires have developed vigorously in the past decades [28].

1.3 Triboelectric Nanogenerators The traditional techniques proposed for microenergy sources are still hindered more or less by the following limitations, such as low output performance, strict environmental requirement, and low conversion efficiency. In 2012, a novel ambient energy-harvesting technology was developed. Named TENG, it combines triboelectrification effect and electrostatic induction [4, 6]. The charge generated at the friction interface of two different materials (i.e. the triboelectric pair) is defined as the triboelectrification effect. Although the observation and description of the electrification effect can be traced back more than 3000 years ago, the question is how to accumulate charges, generate electricity, and minimize the size in an efficient way. By setting two electrodes on the back surface of a triboelectric couple, electrification effect and electrostatic induction are combined for effective power conversion [4, 6]. In the past five years, TENGs have aroused widespread interest due to excellent properties of high-output performance, low cost, being maintenancefree, sustainability, and green power performance. In order to strengthen and extend the capabilities of TENGs, a variety of techniques have been developed, and the power density has reached tens of mW/cm2 level [9]. Furthermore, the maximum power conversion efficiency was achieved at 85% [29]. Since the electrification effect exists between nearly all of the two different materials used, this technique has great tolerance with material selection, and plenty of polymers and organic materials can be selected to achieve flexible and even stretchable devices. Besides, micro-/nanopatterned surfaces are usually adopted to maximize the effective friction area and enhance the output performance, and TENGs serve to emphasize it [30–34]. 1.3.1

Principle of Triboelectric Nanogenerators

The original prototype of TENG had a triboelectric pair made of two different materials and two electrodes placed at the back. The operation principle can be described using contact-separation-mode TENG, as shown in Figure. 1.3. Firstly, the whole TENG is electrically neutral and the triboelectric pair materials are separate. Under an action of external force, the pair makes contact and generates friction, and then surface charges are generated at the friction interface. Because of the difference in capabilities of losing or capturing electrons during electrification, one material of the triboelectric pair loses electrons and shows positive potential, while another captures electrons and shows negative potential.

9

10

1 Overview of Triboelectric Nanogenerators

Electrode 1 Triboelectric material 1 Triboelectric material 2 Electrode 2 (a)

(b)

(c) Approaching

Separating

–I (e)

(d)

Figure 1.3 The working principle of contact-separation-mode triboelectric nanogenerators (TENGs). (a) In the beginning, the triboelectric pair made of triboelectric materials 1 and 2 are separated, and the whole device shows an electrically neutral state. (b) When a compressive force is applied, the top structures will move toward the bottom structures and have a friction with each other. Due to the phenomenon of triboelectrification effect, positive and negative charges of equal amount will be generated on the surfaces of triboelectric pair, respectively. (b–d) When the compressive force is removed, the triboelectric pair will separate from each other as a result of the mechanical recovery force. And then, an internal electric potential is established, which changes accordingly as the distance of the triboelectric pair increases and decreases. Consequently, due to the electrostatic induction, opposite charges are generated on the back electrodes, and a current (I) can be detected in the loop resulting from the charges flowing from one electrode to the other. When the triboelectric pair works cyclically, an electric output power will be generated continuously. Source: Reproduced with permission from Zhang et al. [4]. Copyright 2018, Elsevier.

In principle, the total charge amounts on the surface of the triboelectric pair are equal. Secondly, after removing the external force, the triboelectric pair separates and an internal potential is established owing to the electrostatic induction. During the separation process, this internal potential will drive charges to flow from one electrode to the other through the connection loop to make the change of electrical potential balanced. And then, a positive current is formed, namely, electricity is generated. This is named the process of separating.

1.3 Triboelectric Nanogenerators

Thirdly, when applying an external force to the TENG again, the triboelectric pair moves toward each other, which will cause charges to flow back due to the opposite change of electrical potential. Thus, a negative current is formed, and this process is named approaching. Finally, the triboelectric pair makes contact again and there is friction, and then a new cycle begins. Thus, when TENG repeats this separating–approaching cycle, it produces a periodical electric output with positive and negative parts. The output performances of CS-mode TENGs were not so advantageous and it remained at a low level of several or tens of volts at the beginning. Subsequently, an arch-shaped geometry design was introduced, whereby surface roughness was increased. It significantly expanded the output voltages and power densities of CS-mode TENGs to hundreds of volts and several mW/cm2 [9]. The fundamental mechanism for enhancing output by increasing surface roughness is to maximize the effect friction area, and more surface charges can therefore be generated [30]. As for the arch-shaped optimization, the basic rule of output enhancement can be roughly described by introducing a capacitance equivalent mode [35]. Assuming the surface charges (Q) are constant, the potential difference (U = Q/C) between two electrodes increases sharply when the capacitance (C) decreases sharply due to the stronger mechanical restoring force from the arch-shaped design. The CS-mode TENGs have the advantage of high output voltage, and their simple geometric structure makes for easy utilization, for instance, to convert most mechanical energies, such as pressing, impacting, bending, shaking, vibration, etc. However, using them for harvesting mechanically rotational energy is hard to achieve, and the frequency effect study also figured out that the CS-mode TENGs are not suitable for high-frequency applications [30]. 1.3.2

Key Factor: Triboelectric Series

TENG development is based on the progress of materials. As described in Section 1.3.1, the key of electrification is the ability difference of losing or capturing electrons between the triboelectric pairs. Basically, if the ability relative difference of triboelectric pairs becomes larger, then the output performance of fabricated TENG becomes better as a result of the enhancement of generated surface charges. A table of triboelectric series, which qualitatively points out these ability differences, was established, and is summarized in Table 1.2. The triboelectric series table is essential for constructing high-performance TENGs, since it quantitatively figures out the relative ability difference of losing/capturing electrons during the triboelectrification effect [36]. If the triboelectric pairs have a larger difference in this table, it means that it is easier for electrons to transfer from one to the other during the triboelectrification effect. 1.3.3

Material Progress of Triboelectric Nanogenerators

In the past few years, great progress was made in different triboelectric pairs for TENGs, as listed in Table 1.3. In this table, the materials in the left column, highlighted in yellow, indicate that they are easy to lose electrons and show positive

11

12

1 Overview of Triboelectric Nanogenerators

Table 1.2 Triboelectric series of different materials. Triboelectric series

POSITIVE

1

Materials

No.

Materials

No.

Materials

Aniline–formol 17 Styrene-acrylonitrile resin copolymer Polyformaldehyde 18 Styrene-butadiene 1.3–1.4 copolymer

33 Polyacrylonitrile

3

Ethylcellulose

19 Wood

35 Polybisphenol carbonate

4

Polyamide 11

20 Hard rubber

36 Polychloroether

5

Polyamide 6-6

21 Acetate, Rayon

6

Melanimeformol

22 Polymethyl methacrylate (Lucite)

37 Polyvinylidene chloride (Saran) 38 Poly(2,6-dimethyl polyphenyleneoxide)

7

Wool, knitted

23 Polyvinyl alcohol

39 Polystyrene

8

Silk, woven

24 Polyester (Dacron) (PET)

40 Polyethylene

9

Polyethylene glycol succinate

25 Polyisobutylene

41 Polypropylene

10 Cellulose

26 Polyurethane flexible sponge

42 Polydiphenyl propane carbonate

11 Cellulose acetate

27 Polyethylene terephthalate

43 Polyimide (Kapton)

12 Polyethylene glycol adipate

28 Polyvinyl butyral

44 Polyethylene terephthalate

13 Polydiallyl phthalate

29 Formo-phenolique, hardened

45 Polyvinyl chloride (PVC)

14 Cellulose (regenerated) sponge

30 Polychlorobutadience

46 Polytrifluorochloroethylene

15 Cotton, woven

31 Butadiene-acrylonitrile 47 Polytetrafluoroethylene copolymer (Teflon)

16 Polyurethane elastomer

32 Natural rubber

2

34 Acrylonitrile-vinyl chloride

NEGATIVE

No.

Source: Reproduced with permission from Zhang et al. [4]. Copyright 2018, Elsevier.

potential. The materials in the top row, highlighted in blue, indicate that they are relatively easy to capture electrons and show negative potential. Currently, the widely used materials are PTFE (polytetrafluoroethylene), PDMS (polydimethylsiloxane), PI (polyimide), and FEP (fluorinated ethylene propylene), composed of 14, 11, 11, and 8 triboelectric pairs, respectively, as shown in Table 1.3. The main reason is that they occupy the bottom-tier levels in the triboelectric series, which leads to their outstanding abilities of capturing electrons during the electrification effect. The sequence of electron-capturing ability is listed as PTFE > PDMS > PI [7], and FEP is almost like PTFE with little difference in molecular structure. Among known materials, PTFE, also known as Teflon, has the strongest ability to capture electrons, and possesses ultrastable chemical and physical properties.

Table 1.3 The configuration of triboelectric pairs (up to October 2017). Alginate Human

Triboelectric pairs PTFE PDMS Polyimide FEP Parylene PVC Rubber PFA Graphene Epoxy Polyolefin PVDF Polyester Ground Metal

Al Ag Au Cu Ni Steel

2013.04 2012.11 2012.12 2013.08 2013.04 2013.06 2013.08 2016.08 2014.03

ITO TiO2 Al2 O3

2016.05 2013.02 2013.04 2014.01

SiO2 Graphene oxide

2014.05 2013.07 2014.04

Polymer PET PMMA Nylon PU PPy Urethane PVA EVA Biodegradable Polymer

2013.065 2012.05 2012.01 2012.08 2013.04 2015.11 2015.05 2015.12

Oxide

Others

Liquid

2013.09 2016.03 2014.03 2015.05 2013.01

2014.11 2014.03 2016.01

201 7 . 0 4 2015.06

2 0 1 4. 0 2

PU

PET sodium skin Silicone

2015.09

2014.06

PP 2017.03

2013.09 2015.03

2015.03

2015.03 2015.01

2015.01 2013.11 2017.01

2016.06 2014.07

2017.01 2016.11 2014.12

2016.03

Latex Carbon Black CNT 2014.04 Cellulose Silk fibroin Paper 2017.01 Human 2013.08

Total pairs

2013.05

PE Polyamide

2014.05 2013.01 14

2014.07

2013.12 2016.06

2016.09 2 01 6 . 1 1

2015.12 2014.05

2015.11 11

2016.09

2014.04 3

2016.07 3

2015.02 2

1

EVA: ethylene-vinyl acetate, PP: polypropylene, PPy: polypyrrole, PU: polyurethane, FEP: fluorinated ethylenepropylene, PFA: polyfluoroalkoxy, PVC: polyvinyl chloride, ITO: indium tin oxide, CNT: carbon nanotubes, PVDF: polyvinylidene fluoride, PDMS: poly(dimethylsiloxane), PMMA: poly(methyl methacrylate), PE: polyethylene, PET: polyethylene terephthalate, Biodegradable polymer: poly(l-lactide-co-glycolide) (PLGA), poly(3-hydroxybutyricacid-co-3-hydroxyvaleric acid) (PHB/V), poly(caprolactone) (PCL), poly(vinyl alcohol) (PVA). The date in the form represents the first time this triboelectric pair was reported in publications. The materials in the left and the right columns mean the positive and negative parts in each triboelectric parts in each triboelectric pair, respectively. Source: Reproduced with permission from Zhang et al. [4]. Copyright 2018, Elsevier.

14

1 Overview of Triboelectric Nanogenerators

PDMS is a kind of biocompatible material which can be micro-/nanopatterned easily using the molding cast process, and it also maintains a good stretchable capability. PI, also known as Kapton, can be processed into elastic thin films and made to work under high-temperature conditions. Regarding the negative part of triboelectric pairs, materials can be classified into four groups: metals, oxides, polymers, and others. Metals can work as triboelectric pairs and electrodes at the same time [37–39], which simplifies the structure of TENGs. Oxide compounds are widely used, including ITO (indium tin oxide), TiO2 , Al2 O3 , SiO2 , and graphene oxide. Among the rest, ITO exhibits a unique feature of conductivity and an outstanding property of transparency [40]. Polymers are the most important type of triboelectric materials; therefore, TENG is also called organic nanogenerator, which is firstly used to collect mechanical energy as organic materials [7]. One of the most attractive benefits of TENG is its tolerance to the material selection, since almost all of the two different kinds of materials are able to generate the electrification effect. Therefore, as is shown in Table 1.3, there are a great number of triboelectric pairs made up of different materials. However, the electrical performance of fabricated TENG depends on the ability difference of losing or capturing electrons. From this point of view, the materials are expected to possess the excellent electrical property of easily losing or capturing electrons. Exploring novel triboelectric materials still exists as an important research topic. In addition, triboelectric materials are expected to achieve excellent flexibility or even stretchability, environmental friendliness, or even biocompatibility, which fulfills the demand for constructing flexible and wearable self-powered microsystems. 1.3.4

Challenges of Triboelectric Nanogenerators

So far, the rapid development of TENG is still confronted with two challenges: one is the requirement to enhance power density, and the second is to simplify the structure to realize easier integration. Although simply enlarging the size can enhance the output power of TENG, the area power density remains constant, which is closely relevant to the fundamental mechanism of the triboelectrification effect, especially the triboelectric series that qualitatively depends on the ability of the triboelectric pair to lose or capture electrons [4]. Moreover, although four types of TENGs with different working principles have already been developed [4], such as contact-separation mode and relative-sliding mode, integrating TENG with other components to realize fully integrated self-powered microsystems still requires a lot of effort.

1.4 Summary Through the abovementioned analyses and comparisons, we can summarize that TENGs possess much more attractive potentials and are considered as one of the promising ambient energy-harvesting approaches. In the latter part of this book, we summarize the emerging technology of TENG which serves as an important component of self-powered flexible and wearable microsystems, including

References

the innovation of working principle and material selection, the functionalization of sensing and actuating, and the future development direction of the all-in-one concept.

Abbreviations CNT CS ECE EM EVA FEP FS IoT ITO PCL PDMS PE PET PFA PHB/V PLGA PMMA PP PPy PU PV PVA PVC PVDF PZT RS SE TENG ThM WPT

carbon nanotubes contact-separation mode energy conversion efficiency electromagnetic ethylene-vinyl acetate fluorinated ethylenepropylene free-standing mode Internet of things indium tin oxide poly(caprolactone) poly(dimethylsiloxane) polyethylene polyethylene terephthalate polyfluoroalkoxy poly(3-hydroxybutyricacid-co-3-hydroxyvaleric acid) poly(l-lactide-co-glycolide) poly(methyl methacrylate) polypropylene polypyrrole polyurethane photovoltaic poly(vinyl alcohol) polyvinyl chloride polyvinylidene fluoride lead zirconate titanate relative-sliding mode single-electrode mode triboelectric nanogenerator thermoelectric wireless power transfer

References 1 Lewis, N.S. (2007). Toward cost-effective solar energy use. Science 315:

798–801. 2 Snyder, G.J., Lim, J.R., Huang, C.K., and Fleurial, J.P. (2003). Thermo-

electric microdevice fabricated by a MEMS-like electrochemical process. Nature Materials 2: 528–531.

15

16

1 Overview of Triboelectric Nanogenerators

3 Lee, J.H., Lee, K.Y., Gupta, M.K. et al. (2014). Highly stretchable

4

5 6 7

8

9

10 11 12 13

14

15 16

17 18

19

20

piezoelectric-pyroelectric hybrid nanogenerator. Advanced Materials 26: 765–769. Zhang, X.S., Han, M., Kim, B. et al. (2018). All-in-one self-powered flexible microsystems based on triboelectric nanogenerators. Nano Energy 47: 410–426. Wang, Z.L. and Wu, W.Z. (2012). Nanotechnology-enabled energy harvesting for self-powered micro-/nanosystems. Angewandte Chemie 51: 11700–11721. Fan, F.R., Tian, Z.Q., and Wang, Z.L. (2012). Flexible triboelectric generator. Nano Energy 1: 328–334. Wang, Z.L. (2013). Triboelectric nanogenerators as new energy technology for self-powered systems and as active mechanical and chemical sensors. ACS Nano 7: 9533–9557. Zhang, X.S., Han, M.D., Meng, B., and Zhang, H.X. (2015). High performance triboelectric nanogenerators based on large-scale mass-fabrication technologies. Nano Energy 11: 304–322. Wang, Z.L., Chen, J., and Lin, L. (2015). Progress in triboelectric nanogenerators as a new energy technology and self-powered sensors. Energy & Environmental Science 8: 2250–2282. Zhang, X.S. (2016). Micro/nano integrated fabrication technology and its applications in microenergy harvesting. Springer, ISBN:978-3-662-48814-0. Würfel, P. (2009). Physics of Solar Cells: From Basic Principles to New Concepts, 2e. Wiley. ISBN: 978-3-527-40857-3. Best Research – Cell Efficiencies (2018). National Renewable Energy Laboratory, NREL. https://www.nrel.gov/pv/assets/images/efficiency-chart.png. Riemer, R. and Shapiro, A. (2011). Biomechanical energy harvesting from human motion: theory, state of the art, design guidelines, and future directions. Journal of Neuro Engineering and Rehabilitation 8: 22. Park, J., Lee, Y., Ha, M. et al. (2016). Micro/nanostructured surfaces for self-powered and multifunctional electronic skins. Journal of Materials Chemistry 4: 2999. Zhang, X. and Zhao, L.D. (2015). Thermoelectric materials: energy conversion between heat and electricity. Journal of Materiomics 1: 92–105. Zhang, Q., Sun, Y., Xu, W., and Zhu, D. (2014). Organic thermoelectric materials: emerging green energy materials converting heat to electricity directly and efficiently. Advanced Materials 26: 6829–6851. Yang, J., Yip, H.L., and Jen, A.K.Y. (2013). Rational design of advanced thermoelectric materials. Advanced Energy Materials 3: 549–565. Sun, X., Peng, X., Zheng, Y. et al. (2014). A 3-D stacked high-Q PI-based MEMS inductor for wireless power transmission system in bio-implanted applications. Journal of Microelectromechanical Systems 23: 888–898. Han, M., Yuan, Q., Sun, X., and Zhang, H. (2014). Design and fabrication of integrated magnetic MEMS energy harvester for low frequency applications. Journal of Microelectromechanical Systems 23: 204–212. Wang, Y., Zhang, Q., Zhao, L. et al. (2016). Vibration energy harvester with low resonant frequency based on flexible coil and liquid spring. Applied Physics Letters 109: 203901.

References

21 Valenta, C.R. and Durgin, G.D. (2014). Harvesting wireless power. IEEE

Microwave Magazine 15: 108–120. 22 Sun, X., Zheng, Y., Peng, X. et al. (2014). Parylene-based 3D high per-

23

24 25

26 27

28 29

30

31 32 33

34

35

36

37

formance folded multilayer inductors for wireless power transmission in implanted applications. Sensors and Actuators A: Physical 208: 141–151. Yang, C.L., Chang, C.K., Lee, S.Y. et al. (2017). Efficient four-coil wireless power transfer for deep brain stimulation. IEEE Transactions on Microwave Theory and Techniques 65: 2496–2507. Wang, Z.L. (2011). Nanogenerators for Self-powered Devices and Systems. Georgia Institute of Technology. ISBN: 978-1-4507-8016-2. Hu, Y. and Wang, Z.L. (2015). Recent progress in piezoelectric nanogenerators as a sustainable power source in self-powered systems and active sensors. Nano Energy 14: 3–14. Wang, X. (2012). Piezoelectric nanogenerators – harvesting ambient mechanical energy at the nanometer scale. Nano Energy 1: 13–24. Espinosa, H.D., Bernal, R.A., and Minary-Jolandan, M. (2012). A review of mechanical and electromechanical properties of piezoelectric nanowires. Advanced Materials 24: 4656–4675. Wang, Z.L., Zhu, G., Yang, Y. et al. (2012). Progress in nanogenerators for portable electronics. Materials Today 15: 532–543. Xie, Y., Wang, S., Niu, S. et al. (2014). Grating-structured freestanding triboelectric-layer nanogenerator for harvesting mechanical energy at 85% total conversion efficiency. Advanced Materials 26: 6599–6607. Zhang, X.S., Han, M.D., Wang, R.X. et al. (2013). Frequency-multiplication high-output triboelectric nanogenerator for sustainably powering biomedical microsystems. Nano Letters 13: 1168–1172. Han, M., Zhang, X.S., Meng, B. et al. (2013). r-shaped hybrid nanogenerator with enhanced piezoelectricity. ACS Nano 7: 8554–8560. Yang, Y., Lin, L., Zhang, Y. et al. (2012). Self-powered magnetic sensor based on a triboelectric nanogenerator. ACS Nano 6: 10378–10383. Wang, S., Lin, L., and Wang, Z.L. (2012). Nanoscale triboelectric-effect-enabled energy conversion for sustainably powering portable electronics. Nano Letters 12: 6339–6346. Zhong, J., Zhong, Q., Fan, F. et al. (2013). Finger typing driven triboelectric nanogenerator and its use for instantaneously lighting up LEDs. Nano Energy 2: 491–497. Niu, S., Wang, S., Lin, L. et al. (2013). Theoretical study of contact-mode triboelectric nanogenerators as an effective power source. Energy & Environmental Science 6: 3576–3583. Diaz, A.F. and Felix-Navarro, R.M. (2004). A semi-quantitative tribo-electric series for polymeric materials: the influence of chemical structure and properties. Journal of Electrostatics 62: 277–290. Jiang, T., Yao, Y., Xu, L. et al. (2017). Spring-assisted triboelectric nanogenerator for efficiently harvesting water wave energy. Nano Energy 31: 560–567.

17

18

1 Overview of Triboelectric Nanogenerators

38 Zhang, X.S., Han, M.D., Wang, R.X. et al. (2014). High-performance triboelec-

tric nanogenerator with enhanced energy density based on single-step fluorocarbon plasma treatment. Nano Energy 4: 123–131. 39 Jin, Y., Seo, J., Lee, J.S. et al. (2016). Triboelectric nanogenerator accelerates highly efficient nonviral direct conversion and in vivo reprogramming of fibroblasts to functional neuronal cells. Advanced Materials 28: 7365–7374. 40 Meng, B., Tang, W., Too, Z.H. et al. (2013). A transparent single-friction-surface triboelectric generator and self-powered touch sensor. Energy & Environmental Science 6: 3235–3240.

19

2 Structures of Triboelectric Nanogenerators Haixia Zhang Institute of Microelectronics, Peking University, Department of Microelectronics, No. 5 Yiheyuan Road, Haidian District, Beijing 100871, China

As we know, electrification effect is one of the most common effects in this world; therefore, electric charge can be introduced from solid friction, liquid friction, and also from air friction. Triboelectric nanogenerators (TENGs) can be applied in various environments. There are four modes of TENGs to harvest the energy based on operation mechanisms, i.e. contact-separation (CS), relative-sliding (RS), single-electrode (SE), and freestanding (FS). Based on these modes, there are many different structures for various environmental applications, such as plane-shaped, arch-shaped, zig-zag-shaped, wavy-shaped, and rotor-shaped structures. In this chapter, we introduce the four major operation modes and several typical structures of TENGs.

2.1 Operation Mechanisms of TENGs In principle, all types of TENGs harvest environmental mechanical energy based on the combination of triboelectrification effect and electrostatic induction. TENGs can be classified into four main types based on different operating mechanisms, as shown in Figure 2.1. • The CS-mode TENGs can generate a high-voltage pulse output with simple geometries [2]. • The RS-mode TENGs show a fascinating ability to generate continuous electricity at high frequency, although the surface friction damage remains a big challenge [3]. • The SE-mode TENGs are constructed with only one triboelectric surface, while human skin serves as another surface, and this simplified geometry makes it easy to integrate with other electronics [4]. However, the output electricity is relatively low by contrast. • The FS-mode TENGs possess remarkable energy conversion efficiency, but the large size and the structural complex set up obstacles for the integration of self-powered electronics [5]. Flexible and Stretchable Triboelectric Nanogenerator Devices: Toward Self-powered Systems, First Edition. Edited by Mengdi Han, Xiaosheng Zhang, and Haixia Zhang. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

High-frequency, continuous output Surface friction damage

High-voltage simple geometry Pulse output

Contact-separation mode

Relative-sliding mode

Freestanding mode

Single-electrode mode

Easy for integration Relatively low output

Graphite

High conversion efficiency Complex for integration

Figure 2.1 Four operation mechanisms of triboelectric nanogenerators (TENGs). Source: Reproduced with permission from Zhang et al. [1]. Copyright 2018, Elsevier.

2.1 Operation Mechanisms of TENGs

Among these four types, the CS-mode TENG was the first mode proposed in 2012 by F.R. Fan et al. in Georgia Tech [6]. The others were invented later by different research groups worldwide based on different applications. Details of the working principle are introduced in the following sections. 2.1.1

Contact-Separation (CS) Mode

In Section 1.3.1, we introduced the principle of TENGs based on the CS mode; here, a detailed description of this most used TENG mode is shown in Figure 2.2 [7]. In the initial stage, there is no electric charge on the surface of the two friction layers. As the friction layers get in contact with each other, normally in the vertical direction, based on the electrification effect, the negative and positive surfaces will be charged equally and will stay on the surface stably. When the two friction layers start separating, the charge on the surface will be introduced to the electrodes on the top. This charge flow between the two electrodes eventually transfers the mechanical energy into electric energy. 2.1.2

Relative-Sliding (RS) Mode

The RS-mode TENGs were developed in April 2013 by Z.L. Wang’s group to overcome the frequency limitation that exists in the CS-mode TENGs [8, 9], and subsequently several optimized RS-mode TENGs were implemented to obtain the rotational energy as well as air-flow energies [3, 10–15]. The principle of RS-mode TENGs is shown in Figure 2.3 [16], in which its charge-introducing process is similar to that of the CS mode. The difference from the CS mode is that the RS-mode device works in the horizontal direction, in which two friction layers start to stack. Based on the electrification effect, this overlap area will introduce the charge redistribution on the electrodes, which will realize the energy transformation from the mechanical to the electrical domain.

I

I

Figure 2.2 Mechanism of CS-mode triboelectric nanogenerators.

21

22

2 Structures of Triboelectric Nanogenerators

I

I

Figure 2.3 Mechanism of RS-mode triboelectric nanogenerators.

The most attractive advantage of the RS-mode TENG is that it is suitable for high-frequency applications and then generating continuous output electricity. However, this unique operation method of RS also poses a severe challenge to friction damage on the surfaces of the triboelectric pair. Although several studies have reported the considerable reliability of RS-mode TENGs, the long-term stability in practical applications remains a major problem. 2.1.3

Single-Electrode (SE) Mode

Not always can we find enough space for the generator to be operated with CS/RS modes, for example, in cell phones. The cell phone is one of the most popular handsets, but there is no space except on the screen or its back. These demands to further simplify the configuration of TENGs by removing one of the triboelectric pair or the electrodes motivated the invention of new modes. The SE mode was first invented by our group in Peking University for such applications [4] in August 2013; it is also named single-friction (SF) mode in some cases [4, 16–20]. The principle of the SE mode is shown in Figure 2.4. Only one electrode directly interacts with the moving triboelectric layer, while the other is just a reference electrode working as a source for electrons, which can be a large conductor or just the ground. The major advantage of this SE/SF-mode TENG is its simple structure, which suits many applications. 2.1.4

Freestanding (FS) Mode

The FS mode TENG was thus proposed by the Z.L. Wang group in January 2014, whose operating principle is similar to that of RS-mode TENG except

2.1 Operation Mechanisms of TENGs

I

I

Figure 2.4 Mechanism of SE-mode triboelectric nanogenerators.

I

I

for continuous sliding friction [5, 21]. It can harvest energy from a free-moving triboelectrically charged object and generate an alternative output when the outer object moves between the two electrodes. The principle of FS-mode TENG is shown in Figure 2.5. Since electrostatic induction acts more importantly than does the electrification effect, the energy conversion efficiency of FS-mode TENG can achieve up to 100% theoretically. But the FS design of the movable triboelectric layer makes it difficult to integrate with other electronic devices and systems.

Figure 2.5 Mechanism of FS-mode triboelectric nanogenerators.

23

24

2 Structures of Triboelectric Nanogenerators

Compared with other prototypes made of two triboelectric layers (i.e. the triboelectric pairs), both SE-mode and FS-mode TENGs are only made by one triboelectric layer while using human skins or other existing objects as the other triboelectric materials to form a triboelectric pair. The sole electrode is connected to the ground electrode through a load. If the relative motion of the triboelectric pair occurs cyclically, a periodical current will be generated between them accordingly. SE/FS-mode TENGs are outstanding for portable electronics integration, and one of the most promising potentials is to be integrated with electronics screens to collect the biological mechanical energy from sliding, typing, and touch screens which often occur in daily life [4, 18]. The difference between the SE and FS modes is that the FS-mode TENG can not only save space but also have much higher efficiency [22]. What is worth mentioning here is that several other constructions also were existed in last few years, including liquid-metal TENGs [23, 24], textile TENGs [25, 26], and so on, but their working principles still belong to one of the four prototypes mentioned in this session. Liquid metal provides sufficient omnidirectional friction which minimizes the side effect of energy loss and results in an energy conversion efficiency of 70.6% till date [24]. Textile-based TENGs exhibit outstanding features of flexibility and stretchability due to the employment of fibers and fabrics [25–27]. As an attractive vision for the future, clothes fully or partially made of textile-based TENGs will come into reality, thus converting body motion energy to electricity for powering wearable electronics.

2.2 Typical Structures of TENGs After analyzing CS, RS, SE, and FS modes of TENGs, we can find that both vertical and horizontal directional contact can achieve power generation based on suitable operation mechanisms. In this section, we list and analyze the typical TENG structures, such as plane-shaped, arch-shaped, zig-zag-shaped, wavyshaped, rotor-shaped, etc., besides introducing successful applications. 2.2.1

Plane-Shaped TENGs

Plane-shaped TENG is the most common structure. It can be divided into two groups: 2D and 3D. The Z.L. Wang group demonstrated a simple 3D plane-shaped TENG [28], in which two different friction layers are separated by a spacer, as shown in Figure 2.6a. This structure is suitable for vertical operations based on the CS mode and has a higher output with the help of the spacer shown in Figure 2.6b. In order to save space, our group in Peking University proposed a large-area TENG with scatter distributed spacers, as shown in Figure 2.6c [29]. With the roll-to-roll fabrication method, this device has a very low cost of $2/m2 , showing its great commercial potential in the market. This large-area TENG can be used as smart household mat to harvest energy from human walking, and the output of this TENG in normal daily life is shown in Figure 2.6d.

Spacer

PMMA

Kapton

Aluminum

Kapton nanowires

(a)

Short-circuit current (µA)

Glass

Open-circuit voltage (V)

120 80 40 0 –40 –80

6 4 2 0 –2 –4 –6

–120 0

1

2

3

4

5

6

2

0

7

Step on ~0.045 s

80 Upper layer

4

6

8

Time (s)

Time (s)

(b)

Walk away ~0.06 s

Current (µA)

60

EVA spacer Lower layer

40 20 0

PET A1

(c)

–20 3.6

PVC EVA

(d)

3.7

3.8

3.9

4.0

4.1

4.2

Time (s)

Figure 2.6 3D plane-shaped TENG structures. (a) Diagram of a TENG with spacer around and (b) its output voltage and current. Source: Reproduced with permission from Zhu et al. [28]. Copyright 2012, American Chemical Society. (c) Diagram of a large-area TENG with scatter distributed spacers and (d) its function of harvesting human walking energy. Source: Reproduced with permission from Cheng et al. [29]. Copyright 2016, Elsevier.

26

2 Structures of Triboelectric Nanogenerators

The key factor of the 3D plane-shaped design is the spacer, and it must be reliable and stable for multi-cycle operations to separate the friction layers; otherwise, the generator will automatically out of function. The 2D plane-shaped design is another common structure, in which two different friction materials are distributed on the surface and separated by a rationally designed pattern, making it suitable for the RS mode. As shown in Figure 2.7a, we designed a 2D plane-shaped super-flexible and lightweight TENG based on the parylene–metal–parylene–metal structure [30]. Due to this specially designed structure, it can convert multiple types of mechanical motions into electricity, including contacting, sliding, and rotating, Figure 2.7b gives details of their outputs, in which we see that rotating motion has higher efficiency than the other two motions. Another 2D plane-shaped TENG, integrating all the electrodes and friction components in a single film, was fabricated by our group in 2014, as shown in Figure 2.7c [17]. Due to the interdigitated structure of the electrodes, power output becomes possible regardless of the material used to rub the device. The commercial flexible copper-clad laminate fabrication process enables the potential mass production of the TENG. As a demonstration, fixed onto a mouse pad, this device can harvest energy from the mechanical energy of moving the mouse on the pad to light 20 LEDs and an LCD at the same time, as shown in Figure 2.7d,e. The key factor of the 2D floor-shaped design is the pattern of surface which defines its performance. The significance of both 2D and 3D plane-shaped TENGs is that they are flat and simple and easy to fabricate; therefore, they are widely used in TENG design. 2.2.2

Arch-Shaped TENGs

Arch-shaped is one of the most developed structures of TENGs to achieve complete contact and separation upon pressing and releasing and then improve charge transferring efficiency, which is not easy to achieve with plane-shaped structures. The arch-shaped TENG from the Z.L. Wang group enhanced the performance notably, as shown in Figure 2.8 [31]. It has a very high power output of 3.56 mW/cm2 and 128 mW/cm3 with open-circuit voltage (V oc ) of 230V and short-circuit current (I sc ) of 0.13 mA, respectively. Besides the arch shape, we designed a counterarch-shape TENG, which also improves the output efficiency by ensuring complete contact of the triboelectric pair, as shown in Figure 2.9 [32]. The output of this device achieved even higher V oc at 450 V and I sc of 75 μA than the arch-shaped one. Then, we stacked the arch/counterarch-shaped pairs together to improve the performance of TENG. Figure 2.10a shows the schematics of this design [32]. Four different connections of each of the two arch-shaped pairs are investigated in Figure 2.10b. As for the C1 connection, two cells are connected in parallel, which increased the V oc to some degree and doubled the I sc and the transferred charges’ total amount of one single arch-shaped device. On the contrary, the C2 connection is destructive, for both V oc and I sc decreased dramatically, and the charges’ amount was far less than that of the C1 connection. As for the C3 connection, the current was generated and transferred inwards for each cell, and thus

100 Charge (nC)

Z Y Overlapped

X

Top view

Contact

75 50

Slide

25

Bottom view

0 Cross-section view 1 and 2

(a)

Rotate Contact

Slide

Rotate

0s

(i)

(ii)

1.5 s

(iii)

Sliding direction

…

(i)

(ii)

(iii) … …

(v)

Top view

Bottom view

(iv)

…

Pl

Polyimide Copper

Cu

(c)

1s

(b)

(d)

(e)

Figure 2.7 2D plane-shaped TENG structures. (a) Diagram of a super-flexible and lightweight TENG and (b) its output performance with multiple types of mechanical motions. Source: Reproduced with permission from Han et al. [30]. Copyright 2015, IEEE. (c) Diagram of an unmovable single-layer TENG. (d) Working principle of the TENG and (e) its application demonstration. Source: Reproduced from Ref. [17]. Copyright from 2014 Elsevier.

Al PDMS Kapton SiO2 Electrode Resist

100 Short-circuit current (µA)

Open-circuit voltage (V)

200 100 0

–100

50

0

–50

–200 –100 0

2

4

6 Time (s)

8

10

0

2

4

6 Time (s)

8

10

Figure 2.8 Arch-shaped high-performance TENG. Source: Reproduced with permission from Wang et al. [31]. Copyright 2012, American Chemical Society.

2.2 Typical Structures of TENGs

1

400

0.8

300 200 100 0

Voltage (V)

500

Current (100 µA)

(a)

0.6

Double peak

0.4 0.2 0

–100 –0.2

–200 –300 –8

Time (ms) –6

–4

–2

0

2

4

6

8

–0.4 –2

10

Time (ms) –1

0

1

2

3

4

(c)

(b)

Figure 2.9 Counterarch-shaped triboelectric nanogenerators. Source: Reproduced with permission from Tang et al. [32]. Copyright 2013, Elsevier.

C1 (a)

C2

C3

C4

(b) 100

3 Hz

50 0

Voltage (V)

–50 –100 –10.5 –10–9.5 –9 –8.5 –8 –7.5 –7 –6.5 –6 100

Switching-polarity test 3 Hz

50 0 –50

(c)

–100 –10.5 –10–9.5 –9 –8.5 –8 –7.5 –7 –6.5 –6

(d)

Time (s)

Figure 2.10 Stacked arch-shaped TENG. (a) Diagram of the stacked arch-shaped TENG and (b) four connection modes. (c) Diagram of stacking more TENG cells and (d) their output improvement. Source: Reproduced with permission from Tang et al. [32]. Copyright 2013, Elsevier.

29

30

2 Structures of Triboelectric Nanogenerators

no output was observed. In the C4 connection, two cells were connected in series, but no obvious enhancement was found compared to the single cell. By stacking more arch-shaped TENGs together, the output can be improved obviously, as shown in Figure 2.10c,d. In addition, the arch-shaped TENGs can be easy optimized to other designs, such as sandwich-shaped TENG from our group in 2013, as shown in Figure 2.11a [33]. This device can produce frequency-multiplication output signal due to the twice contact electrifications between two polymer membranes and the middle metal film within one cycle of external force. The corresponding working mechanism is demonstrated in Figure 2.11b. In general, arch-shaped or counterarch-shaped pairs or different stacked structures are among the most utilized designs of TENGs, and they can enhance the charge-transferring efficiency and achieve high output. 2.2.3

Zig-Zag-Shaped TENGs

Similar to the arch-shaped TENGs, the zig-zag-shaped ones can be recognized as foldable arch shapes which are easier to fabricate and assemble than the arch-shaped ones. The first zig-zag-shaped TENG was developed by our group in 2013, as shown in Figure 2.12a [34].

(a)

PDMS

A1

PET/ITO

(iii)

(ii) Releasing 1

Equilibrium

(iv)

(vii) Pressing 2

Releasing 2

(v)

(vi) (b)

Pressing 1

(i)

Si

Equilibrium

Figure 2.11 Sandwich arch-shaped TENG. (a) Schematic and (b) its working mechanism. Source: Reproduced with permission from Zhang et al. [33]. Copyright 2013, American Chemical Society.

(i)

(ii)

(iii)

(a) (iii) 600 400

Voltage (V)

300 200 100 0 –100

0.5

1.0

1.5

Time (s)

2.0

2.5

200 0 –200

–200 0.0

(b)

(ii)

Voltage (V)

Voltage (V)

(i) 250 200 150 100 50 0 –50 –100 –150 –200

–400 0.0

0.5

1.0

1.5

Time (s)

2.0

2.5

0.0

0.5

1.0 1.5 Time (s)

2.0

2.5

Figure 2.12 Zig-zag-shaped triboelectric nanogenerators. (a) The diagram of the zig-zag-shaped TENGs with different numbers of cells and (b) their corresponding output voltage. (c) Photograph of the real device and (d) its capability to light up 100 LEDs. Source: Reproduced with permission from Meng et al. [34]. Copyright 2013, Elsevier.

(i)

(ii)

Fingers typing

TENG

100 LEDs

Current (µA)

Voltage (V)

12 160 120 80 40

Figure 2.12 (Continued)

(d)

4 0

0

(c)

8

0

1

2 3 Time (s)

4

5

0

1

2 3 Time (s)

4

5

2.2 Typical Structures of TENGs

Multiple friction surface pairs were integrated on a single TENG. Each of the two kinds of friction surfaces shares a common induction electrode, simplifying the device’s structure. As the amount of friction pairs increased, the instantaneous output power increased significantly and the charge generated in a single CS cycle increased linearly, as shown in Figure 2.12b. This design is suitable for CS operation mode and can be fabricated by industrial manufacturing technology in mass production, such as the flexible printed circuit board (FPCB) process, as shown Figure 2.12c. Owing to the ease of mounting arbitrary electronic components on the FPCB, a load circuit can be easily integrated with the TENG to form a self-powered FPCB board. The ready integration of load circuits with the TENG was demonstrated with a simple test device. Using the TENG as the power source, 100 LEDs on the self-powered FPCB board were illuminated when the TENG was compressed with finger pressure (Figure 2.12d). 2.2.4

Wavy-Shaped TENGs

The wavy-shaped structure, beneficial for its built-in space and self-recovery characteristics to increase integration, is very favorable in assembling TENGs. In Figure 2.13 [35] is shown the wavy-shaped TENG with a poly(vinylidenefluoride-co-trifluoroethylene) P(VDF-TrFE) film invented by our group, which achieved the effect of interaction enhancement. The wavy-shaped structure is especially suitable for harvesting flow-based energy, including air flow and liquid flow, etc. 2.2.5

Tank-Shaped TENGs

To harvest the energy from liquid form, the tank-shaped structure was introduced. For example, Figure 2.14 [36] shows a surface-patterned tank-shaped TENG we designed with encapsulated liquid. It generates electric output when the liquid moves inside, based on water–solid electrification and electrostatic induction. Due to the rationally designed electrodes, mechanical movement of the water from all directions can be effectively utilized to generate electricity. The tank-shaped structure is especially suitable for harvesting flow-based energy in confined spaces. 2.2.6

Rotor-Shaped TENGs

For RS-mode TENGs, the rotor-shaped structure is the most suitable one. It can be designed in many different forms. In 2014, the Z.L. Wang group designed a coaxial cylindrical-structured rotating TENG to harvest mechanical energy from rotation analogous to an electromagnetic-induction-based generator, as shown in Figure 2.15a [10]. This device, with eight strip units on the surface, could achieve a power density

33

Hybrid

Triboelectric

Piezoelectric

Hybrid PET-ITO

(a)

Kapton

Copper P(VDF-TrFE)

PDMS

PET-ITO

PDMS

Kapton

Cu

P(VDF-TrFE)

(b)

Figure 2.13 Wavy-shaped TENG structures. (a) Diagram of the wavy-shaped hybrid nanogenerator and (b) its working mechanism. Source: Reproduced with permission from Chen et al. [35]. Copyright 2017, Royal Society of Chemistry.

2.2 Typical Structures of TENGs

(a)

(b)

Polyethylene Copper Liquid Positive charge Negative charge

300 nm (c)

(d)

(e)

Distance change –1e6

–8e5

–6e5

(f)

Permittivity change

Area change –4e5

–2e5

0

2e5

4e5

6e5

8e5

Electric field, Y component (V/m)

Figure 2.14 Tank-shaped TENG. (a) Diagram of the tank-shaped TENG with encapsulated liquid. (b) SEM image of the surface of the tank. (c) The electrification process of the double layer. (d)–(f ) The working mechanism of harvesting wave energy. Source: Reproduced with permission from Han et al. [36]. Copyright 2015, Royal Society of Chemistry.

of 36.9 W/m2 (I sc of 90 μA and V oc of 410 V) at a linear rotational velocity of 1.33 m/s (rotation rate of 1000 r/min). Higher output can be achieved if more strip units are fabricated and/or a higher linear rotational velocity is applied. Another rotor-shaped TENG from the Z.L. Wang Group is a planar device [3], as shown in Figure 2.15b, which has two radial-arrayed fine electrodes complementary on the same plane. Operating at a rotation rate of 3000 r/min, this rotor-shaped TENG can produce high V oc of 850 V and I sc of 3 mA at a frequency of 3 kHz. Both these cylindrical and planar rotor-shaped structures increase the charge-transferring times of TENGs, thus enhancing the efficiency. Another form of the rotor-shaped structure is disk-like, as shown in Figure 2.16a,b [37]. We developed the disk-shaped TENG to distinguish different polymers according to the different abilities of attracting electrons. By coating different kinds of materials on the surface of the disk, the device incorporates a three-layer structure, friction layer, electrode layer, and substrate, respectively, as shown in Figure 2.16c. Polymers can be distinguished according to the output signals in turn with well-selected friction layers. Figure 2.16d illustrates the relative order of several polymers in the triboelectric series. From negative to positive, the order is polytetrafluoroethylene (PTFE), polydimethylsiloxane (PDMS), polyimide (PI), polyethylene (PE), polystyrene (PS), and polyethylene terephthalate (PET), respectively.

35

2 Structures of Triboelectric Nanogenerators

Acrylic PTFE Copper Foam tape Lead wire

1 µm

LED bulbs ON 400 V

Rotate speed (r/min)

Cu

PTFE Cu

0V –100 V

(a)

Copper

Gold

FEP

Acrylic

(Rotator)

Electrode A Electrode B

ode B

(Stator)

Ele

ctro de A

Electr

36

(b)

Figure 2.15 Rotor-shaped TENG. (a) A coaxial cylindrical-structured rotating TENG. Source: Reproduced with permission from Bai et al. [10]. Copyright 2013, American Chemical Society. (b) A planar rotor-shaped TENG. Source: Reproduced with permission from Zhu et al. [3]. Copyright 2014, Springer Nature.

Abbreviations

(d)

(a)

PI #1 PS #1

PI #2 PS #2

Voltage (V)

0.4

(b)

0.0 –0.4 –0.8

0000

–1.2 1.2

Friction layers of different polymers

Voltage (V)

0.6

(c)

0.0 –0.6

Electrodes

1100

–1.2 1.2

Substrate

Electrodes

0.8 Voltage (V)

Pads

0.4 0.0

1111

–0.4

Figure 2.16 Disk-shaped TENG. (a, b) Photograph of the disk-shaped TENG. (c) The diagram of the TENG with a coating of different materials on the surface, with a rationally designed pattern. (d) Outputs of the TENG when contacting with different materials. Source: Reproduced with permission from Meng et al. [37]. Copyright 2015, IEEE.

2.3 Summary This chapter describes the four major working modes of TENGs, introduces the plane-shaped, arch-shaped, zig-zag-shaped, wavy-shaped, tank-shaped, and rotor-shaped structures which are widely used in TENG design and their applications in different environments.

Abbreviations FPCB LED PDMS PE PET PI PS PTFE P(VDF-TrFE) TENG

flexible printed circuit board light-emitting diode polydimethylsiloxane polyethylene polyethylene terephthalate polyimide polystyrene polytetrafluoroethylene poly(vinylidenefluoride-co-trifluoroethylene) triboelectric nanogenerator

37

38

2 Structures of Triboelectric Nanogenerators

References 1 Zhang, X.S., Han, M., Kim, B. et al. (2018). All-in-one self-powered flexi-

2

3 4

5

6 7 8 9 10 11

12

13

14

15

16

17

ble microsystems based on triboelectric nanogenerators. Nano Energy 47: 410–426. Zhang, X.S., Su, M., Brugger, J., and Kim, B. (2017). Penciling a triboelectric nanogenerator on paper for autonomous power MEMS applications. Nano Energy 33: 393–401. Zhu, G., Chen, J., Zhang, T. et al. (2014). Radial-arrayed rotary electrification for high performance triboelectric generator. Nature Communications 5: 3426. Meng, B., Tang, W., Too, Z.H. et al. (2013). A transparent single-friction-surface triboelectric generator and self-powered touch sensor. Energy & Environmental Science 6: 3235–3240. Wang, S., Xie, Y., Niu, S. et al. (2014). Freestanding triboelectric-layer-based nanogenerators for harvesting energy from a moving object or human motion in contact and non-contact modes. Advanced Materials 26: 2818–2824. Fan, F.-R., Tian, Z.-Q., and Wang, Z.L. (2012). Flexible triboelectric generator. Nano Energy 1: 328–334. Wang, S., Lin, L., and Wang, Z.L. (2015). Triboelectric nanogenerators as self-powered active sensors. Nano Energy 11: 436–462. Zhu, G., Chen, J., Liu, Y. et al. (2013). Linear-grating triboelectric generator based on sliding electrification. Nano Letters 13: 2282–2289. Wang, S., Lin, L., Xie, Y. et al. (2013). Sliding-triboelectric nanogenerators based on in-plane charge-separation mechanism. Nano Letters 13: 2226–2233. Bai, P., Zhu, G., Liu, Y. et al. (2013). Cylindrical rotating triboelectric nanogenerator. ACS Nano 7: 6361–6366. Lin, L., Wang, S., Xie, Y. et al. (2013). Segmentally structured disk triboelectric nanogenerator for harvesting rotational mechanical energy. Nano Letters 13: 2916–2923. Shang, W., Gu, G.Q., Yang, F. et al. (2017). A sliding-mode triboelectric nanogenerator with chemical group grated structure by shadow mask reactive ion etching. ACS Nano 11: 8796–8803. Jing, Q., Zhu, G., Bai, P. et al. (2014). Case-encapsulated triboelectric nanogenerator for harvesting energy from reciprocating sliding motion. ACS Nano 8: 3836–3842. Zhang, C., Tang, W., Pang, Y. et al. (2015). Active micro-actuators for optical modulation based on a planar sliding triboelectric nanogenerator. Advanced Materials 27: 719–726. Kwak, S.S., Lin, S., Lee, J.H. et al. (2016). Triboelectrification-induced large electric power generation from a single moving droplet on grapheme/polytetrafluoroethylene. ACS Nano 10: 7297–7302. Yang, Y., Zhang, H., Chen, J. et al. (2013). Single-electrode-based sliding triboelectric nanogenerator for self-powered displacement vector sensor system. ACS Nano 7: 7342–7351. Liu, W., Han, M., Sun, X. et al. (2014). An unmovable single-layer triboelectric generator driven by sliding friction. Nano Energy 9: 401–407.

References

18 Yang, Y., Zhang, H., Lin, Z.H. et al. (2013). Human skin based triboelectric

19

20 21

22

23

24

25

26

27 28 29

30

31

32 33

34

nanogenerators for harvesting biomechanical energy and as self-powered active tactile sensor system. ACS Nano 7: 9213–9222. Su, Y., Yang, Y., Zhong, X. et al. (2014). Fully enclosed cylindrical single-electrode-based triboelectric nanogenerator. ACS Applied Materials & Interfaces 6: 553–559. Meng, B., Cheng, X., Zhang, X. et al. (2014). Single-friction-surface triboelectric generator with human body conduit. Applied Physics Letters 104: 103904. Lin, L., Wang, S., Niu, S. et al. (2014). Noncontact free-rotating disk triboelectric nanogenerator as a sustainable energy harvester and self-powered mechanical sensor. ACS Applied Materials & Interfaces 6: 3031–3038. Niu, S.M., Wang, S.H., Liu, Y. et al. (2014). A theoretical study of grating structured triboelectric nanogenerators. Energy & Environmental Science 7: 2339–2349. Tang, W., Jiang, T., Fan, F.R. et al. (2015). Liquid-metal electrode for high-performance triboelectric nanogenerator at an instantaneous energy conversion efficiency of 70.6%. Advanced Functional Materials 25: 3718–3725. Zhang, B., Zhang, L., Deng, W. et al. (2017). Self-powered acceleration sensor based liquid metal triboelectric nanogenerator for vibration monitoring. ACS Nano 11: 7440–7446. Pu, X., Song, W.X., Liu, M.M. et al. (2016). Wearable power-textiles by integrating fabric triboelectric nanogenerators and fiber-shaped dye-sensitized solar cells. Advanced Energy Materials 6: 1601048. Chen, J., Huang, Y., Zhang, N.N. et al. (2016). Micro-cable structured textile for simultaneously harvesting solar and mechanical energy. Nature Energy 1: 16138. Shi, M., Wu, H., Zhang, J.Z. et al. (2017). Self-powered wireless smart patch for healthcare monitoring. Nano Energy 32: 479–487. Zhu, G., Pan, C., Guo, W. et al. (2012). Triboelectric-generator-driven pulse electrodeposition for micropatterning. Nano Letters 12: 4960–4965. Cheng, X., Song, Y., Han, M. et al. (2016). A flexible large-area triboelectric generator by low-cost roll-to-roll process for location-based monitoring. Sensors and Actuators A: Physical 247: 206–214. Han, M., Yu, B., Cheng, X. et al. (2015). A super-flexible and lightweight membrane for energy harvesting. 2015 18th International Conference on Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS) (21–25 June 2015). Wang, S., Lin, L., and Wang, Z.L. (2012). Nanoscale triboelectric-effect-enabled energy conversion for sustainably powering portable electronics. Nano Letters 12: 6339–6346. Tang, W., Meng, B., and Zhang, H.X. (2013). Investigation of power generation based on stacked triboelectric nanogenerator. Nano Energy 2: 1164–1171. Zhang, X.-S., Han, M.-D., Wang, R.-X. et al. (2013). Frequency-multiplication high-output triboelectric nanogenerator for sustainably powering biomedical microsystems. Nano Letters 13: 1168–1172. Meng, B., Tang, W., Zhang, X. et al. (2013). Self-powered flexible printed circuit board with integrated triboelectric generator. Nano Energy 2: 1101–1106.

39

40

2 Structures of Triboelectric Nanogenerators

35 Chen, X., Han, M., Chen, H. et al. (2017). A wave-shaped hybrid piezoelectric

and triboelectric nanogenerator based on P(VDF-TrFE) nanofibers. Nanoscale 9: 1263–1270. 36 Han, M., Yu, B., Qiu, G. et al. (2015). Electrification based devices with encapsulated liquid for energy harvesting, multifunctional sensing, and self-powered visualized detection. Journal of Materials Chemistry A 3: 7382–7388. 37 Meng, B., Cheng, X.L., Han, M.D. et al. (2015). Triboelectrification based active sensor for polymer distinguishing. 28th IEEE International Conference on Micro Electro Mechanical Systems (MEMS2015) (18–22 January 2015).

41

3 Fabrication of Triboelectric Nanogenerators Bo Meng Shenzhen University, College of Physics and Optoelectronic Engineering, 3688 Nanhai Ave., Shenzhen, Guangdong 518061, China

The rapid expansion of triboelectric nanogenerators (TENGs) has benefited from the extensive available manufacturing procedures for micro-/nanofabrication and flexible electronics. This chapter introduces the developed fabrication technologies for TENGs, especially focused on those that may lead to an industrial mass production and performance enhancement of TENGs.

3.1 Mass Fabrication Technologies for Triboelectric Nanogenerators Different mass fabrication technologies have been employed for the highthroughput manufacture of TENGs. In this section, several typical large-scale fabrication technologies are illustrated, including soft lithography, 3D printing procedure, flexible printed circuit (FPC) board technique, roll-to-roll manufacture, and textile manufacture, etc. 3.1.1

Soft Lithography

Soft lithography is one of the most commonly used processes to fabricate micro-/nanopatterned structures on a friction surface. The essential benefit of soft lithography is mass production with low cost due to the reusability of the mold and the simple process [1–3]. In Ref. [4], we reported the development of a flexible TENG using the soft lithography process in 2013 at Peking University (Ref. [4]), as is shown in Figure 3.1. First, a silicon mold with micro-/nanohierarchical structures was prepared by combining the conventional microfabrication process and the improved deep reactive ion etching (DRIE) process. The conventional microfabrication processes, including low-pressure chemical vapor deposition (LPCVD), reactive ion etching (RIE), photolithography, and KOH wet etching, were employed to fabricate microstructure arrays on the silicon substrate. And the improved DRIE Flexible and Stretchable Triboelectric Nanogenerator Devices: Toward Self-powered Systems, First Edition. Edited by Mengdi Han, Xiaosheng Zhang, and Haixia Zhang. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

42

3 Fabrication of Triboelectric Nanogenerators

Al

PDMS (a)

(i)

(iv) (b)

(v)

PET/ITO (ii)

Si (iii)

(vi) (c)

Figure 3.1 Structure and fabrication process of a TENG fabricated using soft lithography. (a) Schematic view of the 3D structure of the sandwich-shaped TENG, (b) the process flow of soft lithography, and (c) the high-magnification view of the soft lithography. Source: Reproduced with permission from Zhang et al. [4]. Copyright 2013, American Chemical Society.

process was used to fabricate a nanopillar forest atop microstructure arrays to form micro-/nanohierarchical structures. Then, this silicon mold was dipped in the liquid PDMS mixture, prepared by the base solution and the curing agent with a quantity ratio of 10 : 1. After being heated at a high temperature, this liquid mixture changed to solid PDMS film, and then the PDMS film with micro-/nanohierarchical structures was peeled off from the silicon mold. These surface-textured PDMS films are widely used to build high-performance TENGs, which serve as the triboelectrification surface [2–5]. The key point of the abovementioned soft-lithography process is employing the improved DRIE process, which can significantly reduce the surface energy of the silicon mold due to both minimizing the solid–liquid contact area and depositing a thin fluorocarbon polymer. Therefore, an ultralow-surface-energy silicon mold can be realized, and the replication process is single-step and surfactant-free, which actually reduces the cost, enhances the pattern transfer precision, and avoids surface pollution. Thus, wafer-level surface-textured PDMS film was easily fabricated and the micro-/nanostructures were accurately replicated from the silicon mold to the PDMS film, as is shown in Figure 3.2. Furthermore, this soft-lithography process based on the improved DRIE process is also suitable to fabricate other soft materials with a micro-/nanotextured surface [6]. As a commercialized soft-lithography method, nanoimprinting has been widely used for the manufacture of nanostructures, and it has been employed for the fabrication of TENG as well [7, 8]. In Ref. [7], D.S. Kim’s group from Pohang University of Science and Technology reported a transparent and flexible TENG which was fabricated using the thermal nanoimprinting process. Thermal nanoimprinting is a low-cost and mass-producible nanofabrication method for replicating nanocavities on a mold to a thermoplastic polymer

3.1 Mass Fabrication Technologies for Triboelectric Nanogenerators

1 μm

1 μm

10 μm

10 μm

5 μm

5 μm

10 μm

10 μm

Figure 3.2 Photographs and SEM images of the films fabricated using the soft-lithography process. Source: Reproduced with permission from Zhang et al. [6]. Copyright 2013, American Chemical Society.

substrate by applying heat and pressure. An innovative aspect of using the thermal nanoimprint process is a one-step fabrication, with decreased manufacturing costs and no requirement for any postprocessing, such as transfer or integration, as shown in Figure 3.3. The principle of this process is the same as that of conventional thermal nanoimprinting, except that three films (fluorinated ethylene propylene [FEP], indium tin oxide [ITO], and FEP, in sequence) are sandwiched between the mold and a platen for simultaneous integration. A conventionally available FEP film is used as a contact layer in the TENG. Among the various fluoropolymers, FEP is easy to process thermally because of its good moldability and thermoplastic properties. As an electrode layer, a conventionally available ITO-deposited flexible polyethylene terephthalate (PET) film is used to add highly flexible and transparent characteristics to the TENG. The bottom FEP film is used for achieving stable adhesion and robustness of the resulting TENG. Applying heat and pressure, the nanopore arrays on the mold are replicated to the contact layer of the TENG and the three films are naturally thermal bonded at the same time.

43

44

3 Fabrication of Triboelectric Nanogenerators

P

-m AAO nano

old

Heating and imprinting

FEP film ITO film film P E F P

Figure 3.3 Schematics of the fabrication process of a TENG through thermal nanoimprinting with the AAO nano-mold. Source: Reproduced with permission from Choi et al. [7]. Copyright 2015, John Wiley & Sons.

3.1.2

Flexible Printed Circuit Manufacture

Well-developed technology usually used to produce flexible circuits and electronics, such as FPC manufacture [9–11], inkjet printing [12], screen printing [13] and roll-to-roll patterning [14, 15], provide promising approaches for the mass production of TENGs. Based on the FPC technology, we report in Ref. [9] a self-powered FPC board with an integrated TENG. The schematic and fabrication process of the FPC-based TENG is outlined in Figure 3.4a. The thin layers of epoxy adhesive on the polyimide substrate serve as one kind of friction surface. Gold-coated microcubic arrays are patterned as the other kind of friction surface to enhance contact electrification. The two surfaces of each tooth of the zigzag structure form one friction pair, and multiple friction pairs are integrated in a single TENG cell. The induction electrodes have been designed as two counter combs and are placed between two layers of polyimide. Owing to the excellent and adjustable elasticity, the zigzag structure can serve as a natural spring. This simplifies the excitation of the TENG. The fabrication procedure starts from two sheets of commercial flexible copper-clad laminate (FCCL), one double-sided and the other single-sided. The FCCL consists of a flexible polyimide substrate and one or two copper films with thin layers of epoxy adhesive in between. The copper films were patterned by photolithography and etched by FeCl3 solution to fabricate the counter comb electrodes and cubic copper arrays. Using an industrial ENIG surface plating process, a thin layer of electroless nickel was plated on copper, and then covered with a thin layer of immersion gold. The two patterned and coated FCCLs were then aligned and permanently bonded by an epoxy adhesive layer with the electrodes amid two layers of polyimide. The bonded multilayer laminate was then folded into a zigzag shape (Figure 3.4b). The TENG with 10 friction pairs forms as a spring in the natural state, and can be pressed into a thickness of 1.64 mm in the compressed state (Figure 3.4c). The top-view scanning electron microscopic (SEM) image of the microcubic arrays is shown in Figure 3.4d.

3.1 Mass Fabrication Technologies for Triboelectric Nanogenerators

(a)

(i)

(ii)

A friction pair

Corresponding to polyimide

Corresponding to (b)

(i)

(ii) gold-coated patterns

(iii)

(iv)

Gold Polyimide Copper

(v)

(c) (i)

(ii)

(d)

500 μm

Figure 3.4 Schematic and fabrication process of an FPC-based TENG. (a) Schematic of an FPC-based TENG and the two counter comb electrodes. (b) The FPC-based mass fabrication process of the TENG. (c) Photograph of a fabricated FPC-based TENG. (d) Top view SEM image of the gold-coated micro-cubic patterns. Source: Reproduced from Ref. [9]. Copyright from 2013 Elsevier.

The industrial manufacturing process for FPC has been adopted to realize the mass production of TENGs. This FPC manufacture procedure is efficient, low-cost, and has high yield and high reliability. The use of FPC technology makes it simple to integrate electronic components with the TENG. 3.1.3

Roll-to-Roll Manufacture

The roll-to-roll process has played an important role in industrial flexible electronics manufacturing, taking advantage of the efficient fabrication of large-scale patterning on thin films. It has been adopted in the fabrication of TENGs as well. In Ref. [14], C.K. Lee’s group from the National University of Singapore developed a large-scale TENG using roll-to-roll ultraviolet embossing method to

45

46

3 Fabrication of Triboelectric Nanogenerators

pattern PET sheets. Patterned PET and copper films are used as two triboelectric layers; for the fabrication of patterned PET triboelectric layers, roll-to-roll UV embossing is used, as shown in Figure 3.5. The setup for the roll-to-roll fabrication process is shown in Figure 3.5a. The UV embossing system consists of four modules: unwinding module for supplying the PET substrate film, coating module for depositing UV curable resin on the PET substrate, UV embossing module for patterning microstructures on the PET substrate, and the rewinding module for peeling off and collecting the embossed PET film. UV curable resin is coated on PET films using a slot die in the coating module. The coating thickness of 20–60 μm with thickness uniformity better than ±10% can be obtained for the layer. Large-area patterned PET film is fabricated via roll-to-roll UV embossing inside the UV embossing module with UV exposure. Figure 3.5b,c shows images of the fabrication setup and embossing roller, respectively. Optical and SEM images of a patterned PET film with line patterns is shown in Figure 3.5d,e respectively. Large patterned films were cut into sizes of 40 cm × 40 cm sheets. These patterned PET sheets are used to enhance the performance of triboelectric-effect-based contact electrification. The patterned PET film acts as one of the triboelectric layers in the large-scale TENG. In Ref. [15], our group in Peking University reported a flexible large-area TENG using the roll-to-roll process with commercially available and low-cost materials. In this work, a prepatterned PVC thin film with microstructure on top was laminated on a PET substrate under a roll-to-roll process, serving as a friction surface. TENGs with a size up to 30 cm × 30 cm were obtained and showed potential applications in energy harvesting in daily life and location-based monitoring.

3.1.4

3D Printing

Besides the different kinds of planar printing methods, 3D printing technology has been adapted to fabricate TENGs [12, 16, 17]. In Ref. [16], Z.L. Wang’s group reported to have developed a three-dimensional ultraflexible TENG based on the 3D printing method. Printing was performed using a UV curing 3D printer, comprised of an automatic pressure injection device, an ink extrusion and UV curing system, a precision positioning platform, and a hardware/software control system. As is schematically illustrated in Figure 3.6a, this is a new method of hybrid 3D printing for producing ultraflexible parts, in which the liquid photopolymer resins and support materials are directly printed using a single manufacturing process within an integrated platform. Printed parts with additive internal supporting structure are intended to keep the high accuracy of the ultraflexible printed parts and shape unchanged, and then the internal supporting structure is removed by corrosive reagents without affecting the other parts. Finally, the ultraflexible parts are obtained to act as the electrification materials of TENGs, as shown in Figure 3.6b.

Unwinding module

Coating module

UV embossing module

Rewinding module Embossing roller

Exhaust

Slot deposition process

PET film Guide rollers

Backup roller

Backup roller UV lamp

PET film UV lamp

(a)

(b)

500 μm (c)

(d)

200 μm

50 μm

(e)

Figure 3.5 Fabrication process of a roll-to-roll UV embossing manufactured TENG. (a) Schematic of roll-to-roll UV embossing fabrication setup. (b) Photograph of the roll-to-roll fabrication setup. (c) A mold made of polyurethane-based photopolymer attached to an embossing roller. (d) Optical image of a line-patterned film. (e) SEM image of the line-patterned film. Inset shows the SEM image of the cross section. Source: Reproduced with permission from Dhakar et al. [14]. Copyright 2016, Springer Nature.

48

3 Fabrication of Triboelectric Nanogenerators

Support material ink Automatic pressure injection system and UV curable inks Controlling electronics hardware

Buld

UV ray

Ink extrusion and UV curing system

Moves in X and Y

platfo

PC and controlling software Moves in Z

(a)

Buld material ink

rm

Part Part supports

Precision positioning system

100 μm

500 μm

(b)

1 cm

(c)

Ultraflexible 3DTENG

(i)

(ii)

(iii)

(iv)

PAAm-LiCl hydrogel 3D printed composite resin I

(d)

3D printed composite resin II

(e)

Figure 3.6 Structure of the ultraflexible 3D-TENG and its fabrication process. (a) Overall schematic of the hybrid 3D printing system used in this study. (b) Digital image of the final 3D printed ultraflexible parts. (c) SEM images of the ultraflexible part. (d) The design of ultraflexible 3D-TENG. (e) Ultraflexible test of the ultraflexible 3D printed parts. Source: Reproduced with permission from Chen et al. [16]. Copyright 2018, Elsevier.

The SEM analysis about the micromorphology of the ultraflexible printed parts shows that the UV curing forms a layer-upon-layer structure along the printing direction as the object is fully realized (Figure 3.6c). In this approach, a digital light processing UV curing system was used to partially set the extruded resins and provide high accuracy, allowing a designed performance of the ultraflexible part relevant to application requirements. The ultraflexible 3D-TENG uses printed resin parts and ionic hydrogel as the electrification material and electrode, respectively, which together make up one

3.1 Mass Fabrication Technologies for Triboelectric Nanogenerators

columnar array of triboelectric interfaces. Its structure and assembly process are schematically illustrated in Figure 3.6d, and the flexibility tests are presented in Figure 3.6e. The fabrication process of this ultraflexible 3D-TENG includes developing an ultraflexible 3D printer, material preparation (composite resins), 3D modeling, printing setup, and 3D printing and integration assembly. Finally, the copper wire is attached to the hydrogel for electrical connection. In Ref. [12], J.W. Han’s group in NASA’s Ames Research Center reported the development of an all-printed TENG fabricated by combining 3D printing and inkjet printing. 3D printing was used to form the structural frame, which serves as a core–shell structure and effectively converts external vibrations into a continuous sliding motion. Meanwhile, inkjet printing was employed to fabricate the contact layers with a precisely patterned grating structure.

3.1.5

Textile Manufacture

Wearable electronics have attracted significant attention and developed rapidly over the past years. Meanwhile, TENGs have been applied in multiple wearable electronic systems, and the fabricating of TENGs has been carried out based on textile manufacture as well [13, 18–21]. In Ref. [18], Z.L. Wang’s group reported the development of a machinewashable textile TENG through loom weaving of metallic yarns. This textile-based TENG is fabricated by direct weaving of Cu-coated polyethylene terephthalate (Cu-PET) warp yarns and PI-coated Cu-PET (PI-Cu-PET) weft yarns on an industrial sample weaving loom. The fabrication of this textile TENG is fully compatible with high-throughput textile processing. As illustrated in Figure 3.7a, the textile TENG consists of 2-ply Cu-PET yarns and PI-Cu-PET yarns woven as warp and weft, respectively. To prepare the metallic yarns, commercial PET yarn (298 dtex) was chosen as the substrate. The 1-ply Cu-PET was first fabricated using a polymer-assisted metal deposition method previously developed. Briefly, the PET yarns were at first modified with a thin layer of poly[(2-(methacryloyloxy)ethyl) trimethylammonium chloride (PMETAC). After loading catalytic palladium salt on the quaternary ammonium groups of PMETAC, about 1-μm-thick Cu was then deposited by electroless deposition to obtain 1-ply Cu-PET yarn (Figure 3.7b). This 1-ply Cu-PET yarn exhibited a very low linear resistance and a high breaking strength. Subsequently, two pieces of 1-ply Cu-PET yarns were Z-twisted into a single 2-ply Cu-PET yarn (Figure 3.7c) to further lower the resistance and increase the breaking strength. As-made 2-ply Cu-PET yarns were used as warp. To fabricate the weft yarns, 2-ply Cu-PET yarns were dipped into a PI precursor and subsequently cured to obtain PI-Cu-PET yarns (Figure 3.7d). The PI film with a thickness of about 20–50 μm formed a continuous and smooth coating, wrapping around the 2-ply Cu-PET yarns (Figure 3.7e). The textile TENG was fabricated by plain weaving 2-ply Cu-PET yarns as warp and PI-Cu-PET yarns as weft.

49

50

3 Fabrication of Triboelectric Nanogenerators

3

(a)

2

Pl-Cu-PET yarn

2

22

22 2

1 Pristine PET yarn Weaving 2 as warp yarn and 3 as weft yarn

2 2-ply Cu-PET yarn

Cu (b)

(e)

Pl

3 3 3

3 3 3

PET Pl

(f)

(g)

2-ply Cu-PET

(c) (h) (d)

(i) Weft Warp

Figure 3.7 Structure and fabrication process of a textile TENG. (a) Schematic illustration of the preparation of yarns and structure of the woven textile TENG. (b, c, d) SEM images of 1-ply Cu-PET yarn, 2-ply Cu-PET yarn, and PI-Cu-PET yarn (scale bar: 1 mm). (e) Cross section of PI-Cu-PET yarn (scale bar: 250 μm). (f ) Photographs of a weaving loom. (g) Metallic yarns wound on a cone. (h) As-woven textile TENG on the loom. (i) SEM image of the cross section of the woven structure (scale bar: 1 mm). Source: Reproduced from Ref. [18]. Copyright from 2016 Wiley.

The high tensile strength of the as-made metallic yarns ensures their weaving ability on an industrial sample weaving loom (SL8900S, CCI) (Figure 3.7f ). As proof of concept, the metallic yarns were wound onto a cone (Figure 3.7g), which was then loaded onto the weaving loom for automatic weaving. The resulting textile TENG (Figure 3.7h) comprised 9 pieces of 2-ply Cu-PET warp yarns and 60 pieces of PI-Cu-PET weft yarns. The SEM image in Figure 3.7i presents a typical cross section of the single-layer textile TENG, where the weft and warp yarns interlace up and down with each other.

3.2 Performance Enhancement for Triboelectric Nanogenerators Employing physical or chemical surface treatment processes to enhance the performance of TENGs have been widely investigated, and played an important role in developing universal techniques for the mass production of high-performance TENGs [1, 22–31].

3.2 Performance Enhancement for Triboelectric Nanogenerators

3.2.1

Plasma Treatment

The process of plasma treatment has been widely used in surface cleaning, surface activation, surface modification, etc. Currently, plasma-treatment-induced fluorinating was developed as a universal approach to improve the performance of TENGs [23–25]. In Ref. [23], our group in Peking University reported the development of a high-performance TENG with enhanced energy density based on a single-step fluorocarbon plasma treatment process. Figure 3.8a illustrates the schematic view of the fluorocarbon plasma treatment process. After the optimization of plasma treatment cycles, the maximum instantaneous energy area density of the TENG with micro/nano-hierarchical structures is enhanced by 278–4.85 mW/cm2 , with a peak output voltage of 265 V and current density of 18.3 μA/cm2 , as is shown in Figure 3.8b. As in the principle analysis shown in Figure 3.9a,b, first-principle calculations were employed to calculate the vertical ionization energy of PDMS and the fluorocarbon layer. Obviously, the model of complex C4 F8 shows larger ionization energy than the model of complex PDMS, which are 12.31 eV versus 8.98 eV, respectively. This clearly reveals that it is easier for the former to obtain one electron than the latter, consistent with the observed experimental results of enhancing the TENG output performance by C4 F8 plasma treatment. The reliability of this calculation model of the fluorocarbon polymeric layer was also demonstrated by the Fourier transform infrared (FTIR) spectra, and the calculated spectrum is highly consistent with the measured one, as is shown in Figure 3.9c. 3.2.2

Wrinkle-Structured Surface

Meanwhile, unique wrinkle patterns on the polymer surface are available using surface treatment [26–28]. In Ref. [26], we reported a high-performance TENG e

260 d

Voltage (V)

240 220 b

180 140 a

0 (b)

g

c

200

160

(a)

f a–0 b–1 c–2 c–4 c–8 c – 10 c – 20

20 5 10 15 Cycle of plasma treatment

Figure 3.8 Analysis and measurement of TENG enhanced by fluorocarbon plasma treatment. (a) Schematic view of the fluorocarbon plasma treatment process; (b) characterization of the output performance of the TENG treated with different plasma treatment cycles under external force, with a frequency of 5 Hz. Source: Reproduced from Ref. [23]. Copyright from 2014 Elsevier.

51

3 Fabrication of Triboelectric Nanogenerators

Cation –e

Cation 8.98 eV

–e

Neutral

683

Calculation

12.31 eV IR Intensity

Neutral

1195 732

Measurement

(a)

F (b)

O

H (c)

1800

C

1200 1400 1600

Si

600 800 1000

1240 400

52

Wavenumber (cm–1)

Figure 3.9 Theoretical calculation and FTIR spectra analysis of the plasma treatment. Theoretical calculation results of vertical ionization energy of model complexes of (a) PDMS and (b) fluorocarbon layer deposited by the C4 F8 plasma treatment; (c) FTIR spectra of the fluorocarbon layer. Source: Reproduced from Ref. [23]. Copyright from 2014 Elsevier.

with large-scale wrinkle nanostructure which was fabricated using single-step fluorocarbon plasma treatment. Figure 3.10 illustrates the fabrication process of this device. It starts with an ITO-coated PET film. The vacuum degassed PDMS mixture is then spin coated on it. A C4 F8 plasma treatment process is carried out in an inductively coupled plasma etching machine before PDMS is cured. The wrinkle structure would be formed in this process. After the PDMS is cured, the processed PDMS film is assembled with another PET/ITO film to form an arch-shaped TENG, which could make the two layers of this device periodically separate and contact under the cycle force. The detailed fabrication diagram of the wrinkle structure is shown in Figure 3.10b. In the C4 F8 plasma treatment process, the fluorocarbon polymer would deposit on the uncured PDMS surface with a velocity. The deposited fluorocarbon polymer is a stiff layer on the PDMS, and its velocity deforms the PDMS. Then the PDMS recovers to its undeformed state, resulting in the occurrence of the wrinkle topologies on this multilayered film. The photograph in Figure 3.10c shows the fabricated TENG with a coin, which has a size of 40 mm × 50 mm. The gap size was fixed at about 3 mm for all TENGs. The processed wrinkle structure was characterized by SEM and its 3D view was attained by laser scanning confocal microscope (LSCM), which is shown in Figure 3.10d,e. Disordered wrinkle topologies are shown with an average width of about 15 μm and height of about 12 μm. After the optimization of plasma treatment, the maximum surface charge density is about 165 μC/m2 . Compared with untreated TENG, the wrinkle structure makes the current and surface charge density increase by 810% and 528%, respectively. Further, Ref. [27] reports the controlled fabrication of the nanoscale wrinkle structure developed by our group using fluorocarbon plasma treatment on prestretched PDMS/Solaris thin films. Highly transparent films with wrinkle

3.2 Performance Enhancement for Triboelectric Nanogenerators

(i)

PET/ITO film (iii)

(ii)

(i)

Spin-coating PDMS (iv)

(ii)

C4F8 plasma

C4F8 plasma (iii)

TENG

ICP treatment (a)

PET

ITO

(b) PDMS

Fluorocarbon polymer

100 μm (c)

(d)

0 (e)

7 14 Height (μm)

30 μm Length

Figure 3.10 Fabrication process of a wrinkle-structure-patterned TENG. (a) Schematic diagram of the fabrication process of this TENG, including the spin-coating PDMS on PET/ITO film, C4 F8 plasma treatment to form the wrinkle structure, and its assembly process. (b) Deposition of fluorocarbon polymer and the formation process of the wrinkle structure. (c) Photograph of a fabricated TENG with a coin, which has a size of 40 mm × 50 mm × 2 mm. (d) SEM photo of the wrinkle structure and (e) its 3D view by LSCM. Source: Reproduced from Ref. [26]. Copyright from 2016 Wiley.

structures of different scales and topologies are available by adjusting the duration time of plasma treatment. Due to the ultralow surface energy of the fluorocarbon polymer on the surface, this fabricated wrinkle pattern can be used as a transfer mold as well. 3.2.3

Chemical Synthesis

Chemical synthesis is another common method for surface modification of the polymer layers, which is helpful to enhance the surface charge density. In Ref. [29], Y. Feng et al. from the Chinese Academy of Sciences reported a high-output polypropylene (PP) nanowire array TENG using surface structural control and chemical modification. In this work, friction layers with PP nanowire arrays were prepared by a simplified hot processing technique using porous anodic aluminum oxide (AAO) as template, as shown in Figure 3.11. Typically, a flat PP film with the thickness of 50 μm and an AAO template material with different thicknesses were placed in

53

54

3 Fabrication of Triboelectric Nanogenerators

Anodized

Place in turn

Heat and press Modified by fluoropolymer

Remove AAO template

Figure 3.11 Schematic depiction of the fabrication process of PP nanowires prepared by hot processing technique and post surface modification with fluorinated compounds. Source: Reproduced from Ref. [29]. Copyright from 2016 Elsevier.

the middle of two glass plates, and then moved to an oven at 200 ∘ C by applying a load of 2 N/cm2 onto the top glass plate. After maintaining the temperature at 200 ∘ C for one hour and cooling down without releasing the load, the prepared PP nanowire–AAO composite was put into a hot NaOH solution (1.0 M) at 50 ∘ C for three hours to remove the AAO template. Lastly, after dissolving the AAO template, PP nanowire arrays with different lengths were obtained. In Ref. [30], X.D. Wang’s group from the University of Wisconsin-Madison reported developing an efficient TENG based on sequential infiltration synthesis (SIS) of doped polymer films. An internal AlOx doping of several polymers was proceed via SIS, including PDMS, polyimide, and PMMA. It has been showed that SIS can introduce AlOx molecules ∼3 μm deep into these polymers, which effectively tuned the bulk electrical property of the film. TENG devices using the modified polymer films exhibited enhanced power output; and this enhancement remained effective after the surface of the polymer film was polished off for more than 2 μm.

3.3 Summary In this chapter, the developed mass fabrication technologies and surface treatment approaches for TENGs are summarized. The advantages of mass production technologies, such as soft lithography, 3D printing procedure, FPC board technique, and roll-to-roll and textile manufacturing, were taken up to realize the efficient ways of manufacturing TENGs. Surface treatment processes, including different kinds of plasma treatment processes and chemical synthesis methods, were employed as a universal technique to enhance the performance of TENGs on a large scale. The fabrication of TENGs has shown great promise in achieving cost-effective industrial manufacturing.

References

Abbreviations AAO DRIE ENIG FCCL FEP FPC FTIR ICP ITO LPCVD LSCM PDMS PET PMETAC PMMA PP RIE SEM SIS TENG

anodic aluminum oxide deep reactive ion etching electroless nickel immersion gold flexible copper-clad laminate fluorinated ethylene propylene flexible printed circuits Fourier transform infrared inductively coupled plasma indium tin oxide low-pressure chemical vapor deposition laser scanning confocal microscope polydimethylsiloxane polyethylene terephthalate poly[(2-(methacryloyloxy)ethyl) trimethylammonium chloride poly(methylmethacrylate) polypropylene reactive ion etching scanning electron microscopic sequential infiltration synthesis triboelectric nanogenerator

References 1 Zhang, X., Han, M., Meng, B., and Zhang, H. (2015). High performance tri-

2

3

4

5

6

boelectric nanogenerators based on large-scale mass-fabrication technologies. Nano Energy 11: 304–322. Fan, F.R., Lin, L., Zhu, G. et al. (2012). Transparent triboelectric nanogenerators and self-powered pressure sensors based on micropatterned plastic films. Nano Letters 12: 3109–3114. Wang, S., Lin, L., and Wang, Z.L. (2012). Nanoscale triboelectric-effect-enabled energy conversion for sustainably powering portable electronics. Nano Letters 12: 6339–6346. Zhang, X.S., Han, M.D., Wang, R.X. et al. (2013). Frequency-multiplication high-output triboelectric nanogenerator for sustainably powering biomedical microsystems. Nano Letters 13: 1168–1172. Meng, B., Tang, W., Too, Z.H. et al. (2013). A transparent single-friction-surface triboelectric generator and self-powered touch sensor. Energy & Environmental Science 6: 3235–3240. Zhang, X.S., Zhu, F.Y., Han, M.D. et al. (2013). Self-cleaning poly(dimethylsiloxane) film with functional micro/nano hierarchical structures. Langmuir 29: 10769–10775.

55

56

3 Fabrication of Triboelectric Nanogenerators

7 Choi, D., Yoo, D., and Kim, D.S. (2015). One-step fabrication of transpar-

8

9 10 11

12 13

14

15

16 17 18

19 20

21

22

23

ent and flexible nanotopographical-triboelectric nanogenerators via thermal nanoimprinting of thermoplastic fluoropolymers. Advanced Materials 27: 7386–7394. Kwon, Y.H., Shin, S.H., Jung, J.Y., and Nah, J. (2016). Scalable and enhanced triboelectric output power generation by surface functionalized nanoimprint patterns. Nanotechnology 27: 205401. Meng, B., Tang, W., Zhang, X. et al. (2013). Self-powered flexible printed circuit board with integrated triboelectric generator. Nano Energy 2: 1101–1106. Liu, W., Han, M.D., Sun, X.M. et al. (2014). An unmovable single-layer triboelectric generator driven by sliding friction. Nano Energy 9: 401–407. Han, C., Zhang, C., Tang, W. et al. (2015). High power triboelectric nanogenerator based on printed circuit board (PCB) technology. Nano Research 8: 722–730. Seol, M., Hang, J.W., Moon, D. et al. (2018). All-printed triboelectric nanogenerator. Nano Energy 44: 82–88. Cao, R., Pu, X., Du, X. et al. (2018). Screen-printed washable electronic textiles as self-powered touch/gesture tribo-sensors for intelligent human–machine interaction. ACS Nano 12: 5190–5196. Dhakar, L., Gudla, S., Shan, X. et al. (2016). Large scale triboelectric nanogenerator and self-powered pressure sensor array using low cost roll-to-roll UV embossing. Scientific Reports 6: 22253. Cheng, X., Song, Y., Han, M. et al. (2016). A flexible large-area triboelectric generator by low-cost roll-to-roll process for location-based monitoring. Sensors and Actuators A: Physical 247: 206–214. Chen, B., Tang, W., Jiang, T. et al. (2018). Three-dimensional ultraflexible triboelectric nanogenerator made by 3D printing. Nano Energy 45: 380–389. Qiao, H., Zhang, Y., Huang, Z. et al. (2018). 3D printing individualized triboelectric nanogenerator with macro-pattern. Nano Energy 50: 126–132. Zhao, Z., Yan, C., Liu, Z. et al. (2016). Machine-washable textile triboelectric nanogenerators for effective human respiratory monitoring through loom weaving of metallic yarns. Advanced Materials 28: 10267–10274. Hu, A., Pu, X., Wen, R. et al. (2017). Core–shell-yarn-based triboelectric nanogenerator textiles as power cloths. ACS Nano 11: 12764–12771. Lai, Y., Deng, J., Zhang, S.L. et al. (2017). Single-thread-based wearable and highly stretchable triboelectric nanogenerators and their applications in cloth-based self-powered human-interactive and biomedical sensing. Advanced Functional Materials 27: 1604462. Lin, Z., Yang, J., Li, X. et al. (2018). Large-scale and washable smart textiles based on triboelectric nanogenerator arrays for self-powered sleeping monitoring. Advanced Functional Materials 28: 1704112. Yu, Y. and Wang, X. (2016). Chemical modification of polymer surfaces for advanced triboelectric nanogenerator development. Extreme Mechanics Letters 9: 514–530. Zhang, X.S., Han, M.D., Wang, R.X. et al. (2014). High-performance triboelectric nanogenerator with enhanced energy density based on single-step fluorocarbon plasma treatment. Nano Energy 4: 123–131.

References

24 Shin, S.H., Kwon, Y.H., Kim, Y.H. et al. (2015). Triboelectric charging

25

26

27

28

29

30

31

sequence induced by surface functionalization as a method to fabricate high performance triboelectric generators. ACS Nano 9: 4621–4627. Li, H.Y., Su, L., Kuang, S.Y. et al. (2015). Significant enhancement of triboelectric charge density by fluorinated surface modification in nanoscale for converting mechanical energy. Advanced Functional Materials 25: 5691–5697. Cheng, X., Meng, B., Chen, X. et al. (2016). Single-step fluorocarbon plasma treatment-induced wrinkle structure for high-performance triboelectric nanogenerator. Small 12: 229–236. Cheng, X., Miao, L., Su, Z. et al. (2017). Controlled fabrication of nanoscale wrinkle structure by fluorocarbon plasma for highly transparent triboelectric nanogenerator. Microsystems & Nanoengineering 3: 16074. Cheng, X., Song, Z., Miao, L. et al. (2018). Wide range fabrication of wrinkle patterns for maximizing surface charge density of a triboelectric nanogenerator. Journal of Microelectromechanical Systems 27: 106–112. Feng, Y., Zheng, Y., Ma, S. et al. (2016). High output polypropylene nanowire array triboelectric nanogenerator through surface structural control and chemical modification. Nano Energy 19: 48–57. Yu, Y., Li, Z., Wang, Y. et al. (2015). Sequential infiltration synthesis of doped polymer films with tunable electrical properties for efficient triboelectric nanogenerator development. Advanced Materials 27: 4938–4944. Zhao, L., Zheng, Q., Ouyang, H. et al. (2016). A size-unlimited surface microstructure modification method for achieving high performance triboelectric nanogenerator. Nano Energy 28: 172–178.

57

59

4 Characterization of Triboelectric Nanogenerators Yu Song Peking University, Institute of Microelectronics, National Key Lab of Nano/Micro Fabrication Technology, Building of Micro/Nanoelectronics, No. 5 Yiheyuan Road, Beijing 100871, China

As a mechanical–electrical transducer, the performance of a triboelectric nanogenerator (TENG) is normally characterized by electrical and mechanical properties. Therefore, improving the performance of a TENG needs efforts from both electrical and mechanical fields, such as enhancement of the surface charge density 𝜎 [1, 2] and development of new structures/modes [3–7]. However, it is difficult to evaluate the performance of a TENG through a general standard and optimal characterization either in mechanical or electric areas. As introduced in previous chapters, four basic modes of TENGs have been developed, including the vertical contact-separation (CS) mode, relative-sliding (RS) mode, single-electrode (SE) mode, and freestanding (FS) mode [4, 8–10]. Each mode has its own structure and materials selection as well as specific mechanical triggering configurations. For other power generators, standards have been widely discussed and established, such as ZT factor for thermoelectric materials [11], Carnot efficiency for pyroelectric nanogenerators [12], and energy conversion efficiency for solar cells [13]. Therefore, to evaluate and compare the performance of TENGs in different structures/modes, a universal standard is needed to quantify the performance of the TENG regardless of its operation mode. In this chapter, starting from the plot of built-up voltage–total transferred charges (V –Q), the TENG operation cycle with maximized energy output is firstly proposed. Then, the performance figure of merit (FOM) is proposed to evaluate TENG design considering both the maximized energy conversion efficiency and average output power. Meanwhile, both structural FOM and material FOM were demonstrated for different structures of TENGs and various materials, respectively.

Flexible and Stretchable Triboelectric Nanogenerator Devices: Toward Self-powered Systems, First Edition. Edited by Mengdi Han, Xiaosheng Zhang, and Haixia Zhang. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

4 Characterization of Triboelectric Nanogenerators

4.1 Electrical Operating Cycles of Triboelectric Nanogenerators 4.1.1

V–Q Plot and Its Characteristics

The fundamental mechanism of the TENG is the coupling effect of triboelectrification and electrostatic induction [14–16]. For a basic TENG, there is at least one pair of triboelectric layers for creating opposite triboelectric charges via physical contacts. Meanwhile, these two electrodes (for SE mode, the ground is considered as the second/reference electrode [17]) are carefully insulated from each other. Through the external load, the free electrons could transfer easily between two electrodes. For the electrical operating cycle, as shown in Figure 4.1, triggered by the external mechanical force, there is a periodical relative motion between the triboelectric layers that breaks the balanced distribution of electrostatic charges. As a result, free electrons are driven to flow between the electrodes in order to build a new equilibrium. Therefore, the governing equations of TENGs can be developed on the basis of the relationship among the transferred charges between the electrodes Q, the built-up voltage V , and the relative displacement x between the triboelectric layers. Usually, we define the status while one pair of the triboelectric layers is fully contacting with each other as x = 0, and x = xmax is defined at the largest achievable displacement, which is fixed inside a range dependent on the structure design. We use the most commonly utilized minimum achievable charge reference state [19], so both the absolute short-circuit transferred charges QSC (x) and the absolute open-circuit voltage V OC (x) at x = 0 position are set to be 0. For the RS-mode TENG, the definitions of the displacement x and the two electrodes are illustrated in Figure 4.1a. The maximum of QSC,max and V OC,max are expected to be reached at x = xmax for these basic modes of TENGs. For a continuous periodic mechanical motion, the electrical output signal from the TENG is also periodically time-dependent. The average output power

x x = xmax + + – – – – + + (a)

80 TENG First period Second period R 60 Following periods 40 20 0 –20 –40 Qc QSC,max –60 0 5 10 15 20 25 30 35 (b) Charge (nC)

Steady state of CEO

4.4 GΩ 80 with various R 250 MΩ 60 100 MΩ 44 MΩ 40 4.4 MΩ 20 0 –20 –40 QSC,max –60 0 5 10 15 20 25 30 35 (c) Charge (nC)

Voltage (V)

CEO with R = 100 mΩ

Electrodes x=0 + + + + – – – – Dielectric

Voltage (V)

60

Figure 4.1 Electrical operation cycles of TENG. (a) Schematic diagram of the RS-mode TENG with displacement x = 0 and x = x max . (b) The cycle for energy output (CEO) with load resistance R = 100 MΩ. The total cycling charge QC is marked and the inset shows the operation circuit. (c) The steady state of CEO with various load resistances. Source: Reproduced with permission from Zi et al. [18]. Copyright 2015, Springer Nature.

4.1 Electrical Operating Cycles of Triboelectric Nanogenerators

P, related to the load resistance, is used to determine the merits of the TENG. Given a certain period of time T, the output energy per cycle E can be derived as T

E = PT =

∫0

t=T

VI dt =

∫0

V dQ =

∫

V dQ

(4.1)

Therefore, the electrostatic states and the energy output of TENGs can be represented by the plot of built-up voltage V against the transferred charges Q. Based on the V –Q plots, different electrical operation cycles were developed to quantitatively describe the working status of TENGs. Currently reported cycles include that of energy output and maximized energy output for TENGs connected with a load resistance [18], and the direct charging cycle and the designed charging cycle for TENGs to charge a battery or a capacitor [20–22]. In the following sections, we mainly focus on the former two which are closely related to the FOMs as proposed [20]. The V–Q plot for an RS-mode TENG was firstly simulated by the finite element method (FEM) under external load resistance of 100 MΩ, starting from (Q, V ) = (0, 0). From the V –Q plot, we notice that the operation of the TENG will reach its stable state after only a few periods (Figure 4.1b), and thus we can directly focus on the output of the steady-state operation. Since the steady-state output signal of the TENG is periodic in responding to the mechanical triggering, the V –Q plot should be a closed loop. As indicated by Eq. (4.1), the output energy per cycle E can be calculated as the encircled area of the closed loop in the V –Q plot. The steady-state V –Q plots for this RS-mode TENG were also simulated by FEM under various external loads, as shown in Figure 4.1c. From the encircled areas of these V –Q plots, we found out that the output energy per cycle E could be optimized by applying a matched load resistance [23]. The cycles given here could be called as “cycles for energy output” (CEOs). For each CEO, the difference between the maximum and the minimum transferred charges in its steady state is defined as the total cycling charge QC , as marked in Figure 4.1b. 4.1.2

Operating Cycles of Energy Output

From Figure 4.1b,c, it has been noticed that for each CEO, the total cycling charge QC was always less than the maximum transferred charges QSC,max , especially for cycles under large external load resistances. If we could maximize the QC to be QSC,max for these cycles, the output energy per cycle E would be further enhanced. Considering that QC = QSC,max occurs in short-circuit condition, the following repeated steps were designed to achieve instantaneous short-circuit conditions during operations, assisted by a switch in parallel with the external load (as shown in Figure 4.2a) [18]. In step 1, the triboelectric layers move relatively from x = 0 to x = xmax at switch off. In step 2, turn the switch on to enable Q = QSC,max , and then turn the switch off. In step 3, the triboelectric layers move relatively from x = xmax to x = 0 at switch off. In step 4, turn the switch on to enable Q = 0, and then turn the switch off.

61

4 Characterization of Triboelectric Nanogenerators CMEO with R = 100 MΩ

80

60

TENG

R

ep St

20

1

x = xmax

Ste

p2

QC = QSC,max

0

x=0

–20

Step

TENG

–40

4

p Ste

3

R

TENG R

40

0

5

0

(QSC,max, 0)

–20 (0, 0) –40

10 15 20 25 30 35 Charge (nC)

Infinite 250 MΩ 100 MΩ 44 MΩ 4.4 MΩ

20

CMEO with various R

–60

–60

(a)

Voltage (V)

R

40

(0, VOC,max)

80

TENG

60 Voltage (V)

62

0

(b)

5

(QSC,max–V′max)

10 15 20 25 30 35 Charge (nC)

Figure 4.2 CMEO of TENG. (a) The CMEO with load resistance R = 100 MΩ, where the insets show the corresponding status of the switch in circuits during different steps. (b) The CMEO with various load resistances. Source: Reproduced with permission from Zi et al. [18]. Copyright 2015, Springer Nature.

Therefore, the maximized total cycling charge QC = QSC,max was enabled by the instantaneous short-circuit conditions in steps 2 and 4, controlled by the switch. Under different load resistances, the simulation results of the RS-mode TENG were plotted as shown in Figure 4.2b. These cycles can be named as “cycles for maximized energy output” (CMEOs). Clearly, having benefited from the maximized total cycling charges, the output energy per cycle of the CMEO was always higher than that of the CEO with the same load resistance R, observed in the encircled areas in Figures 4.1c and 4.2b. It is obvious that for the CMEO, the larger the external load resistance, the higher the output energy per cycle. Therefore, the maximized output energy per cycle can be obtained at R = +∞, which is equivalent to the product of the maximum open-circuit voltage (V OC ) and short-circuit transferred charges (QSC ). We simulated this maximized output energy by simply removing the external load and operating the remaining part of the circuit. The corresponding V –Q curve was plotted as the CMEO with infinite load resistance, as shown in Figure 4.2b. The plot of CMEO changes from an oval to a trapezoid shape, the vertices of which mainly depend on the maximum short-circuit transferred charge QSC,max , the maximum open-circuit voltage V OC,max , and the maximum achievable absolute voltage V ′ max at Q = QSC,max . It can be easily proved that the V –Q plots for all kinds of TENG operations are limited inside the four edges of this trapezoid, as follows. Here, we only consider the charge Q with 0 ≤ Q ≤ QSC,max since the motion part only operates between x = 0 and x = xmax , which represents two of the four boundary lines. Then, at arbitrary displacement x, the capacitance was fixed at C(x). Therefore, as the electrical potential superposition of the open-circuit voltage and the voltage drop due to the charge transfer, the total voltage was derived as V =−

Q + VOC (x) C(x)

(4.2)

4.1 Electrical Operating Cycles of Triboelectric Nanogenerators

Here, we define a variable V ′ (x), which is the absolute voltage value when Q = QSC,max at displacement x. So from the definitions, at fixed x, (Q, V ) plots (Eq. (4.2)) should include points of (QSC (x), 0), (0, V OC (x)), and (QSC,max , −V ′ (x)). By placing these points into Eq. (4.2) we can get | QSC,max | QSC,max | | V ′ (x) = |− + VOC (x) |= − VOC (x) | C(x) | C(x) | | QSC,max 0=− + VOC (x) C(x)

(4.3) (4.4)

Then, the relationship among QSC (x), VOC (x), C(x), and V ′ (x) can be shown as C(x) =

QSC,max − QSC (x) QSC (x) = VOC (x) V ′ (x)

(4.5)

If we replace C(x) in Eq. (4.2) by Eq. (4.5), we get Q V + =1 VOC (x) QSC (x) QSC,max − Q V − ′ + =1 V (x) QSC,max − QSC (x)

(4.6)

Obviously, V OC (x) and QSC (x) always increase with the increase in x. Therefore, at x = 0, V OC (x) and QSC (x) should achieve their minimum value V OC (0) = 0 and QSC (0) = 0; and at x = xmax , V OC (x) and QSC (x) should achieve their maximum value V OC (xmax ) = V OC,max and QSC (xmax ) = QSC,max , respectively. Therefore, when 0 ≤ QSC (x) ≤ QSC,max , 0 ≤ V OC (x) ≤ V OC,max . We noticed that if we define a new displacement x′ = xmax − x, and redefine this new coordinate system as V OC ′ (x′ ) = 0 and QSC ′ (x′ ) = 0 at x′ = 0, there would be QSC ′ (x′ ) = QSC,max − QSC (x) and C ′ (x′ ) = C(x) from the definitions. Therefore, ′ (x′ ) VOC

=

Q′SC (x′ ) C ′ (x′ )

=

QSC,max − QSC (x) C(x)

= V ′ (x)

(4.7)

Therefore, as the open-circuit voltage is at this new charge reference state, V′ OC (x′ ) = V ′ (x′ ) should achieve the minimum value 0 at x′ = 0 and the maximum value V ′ max at x′ = xmax . Then, there would be 0 ≤ V ′ (x) ≤ V ′ max . For arbitrary (Q, V ) with displacement x, it satisfies Eq. (4.6), When V ≥ 0, VV′ + Q Q ≥ 0 is always valid, and, besides, SC,max

max

Q Q V V + ≤ + =1 VOC,max QSC,max VOC (x) QSC (x) When V < 0,

V VOC,max

+

Q QOC,max


1, then this material is more negative than FEP.

4.3 Summary In this chapter, we have discussed methods for standardized evaluations of the performance of TENGs. Based on the maximum output energy per cycle, and considering both the maximized energy conversion efficiency and the maximized average output power, the FOMP was derived to evaluate each TENG design. For different structures of TENGs, the FOMS was simulated by analytical formulas and FEM, showing the maximum value of FOMS for each TENG structure. The standard evaluation of the material FOM was also demonstrated by measuring triboelectric surface charge density via contacting the materials with liquid metals, where the normalized triboelectric charge density 𝜎 N and FOMDM were defined and derived for various materials. Therefore, both the standards and evaluation methods lay the solid foundation on the development of TENG in practical applications and in industrialization.

73

74

4 Characterization of Triboelectric Nanogenerators

Abbreviations Al CEO c-FS CMEO CS Cu FEM FEP FOM FS PE PVDF RS SE s-FS TENG

aluminum cycle for energy output contact freestanding cycles for maximized energy output contact-separation copper finite element method fluorinated ethylene propylene figure-of-merit freestanding polyethylene polyvinylidene fluoride relative-sliding single-electrode sliding freestanding triboelectric nanogenerator

References 1 Zhu, G., Pan, C., Guo, W. et al. (2012). Triboelectric-generator-driven pulse

electrodeposition for micropatterning. Nano Letters 12: 4960–4965. 2 Meng, X.S., Zhu, G., and Wang, Z.L. (2014). Robust thin-film generator based

3 4 5 6

7

8

9

on segmented contact-electrification for harvesting wind energy. ACS Applied Materials & Interfaces 6: 8011–8016. Zhu, G., Chen, J., Liu, Y. et al. (2013). Linear-grating triboelectric generator based on sliding electrification. Nano Letters 13: 2282–2289. Wang, S., Lin, L., Xie, Y. et al. (2013). Sliding-triboelectric nanogenerators based on in-plane charge-separation mechanism. Nano Letters 13: 2226–2233. Zhu, G., Chen, J., Zhang, T. et al. (2014). Radial-arrayed rotary electrification for high performance triboelectric generator. Nature Communications 5: 3426. Yang, Y., Zhang, H., Chen, J. et al. (2013). Single-electrode-based sliding triboelectric nanogenerator for self-powered displacement vector sensor system. ACS Nano 7: 7342–7351. Wang, S., Niu, S., Yang, J. et al. (2014). Quantitative measurements of vibration amplitude using a contact-mode freestanding triboelectric nanogenerator. ACS Nano 8: 12004–12013. Zhang, X., Han, M., Wang, R. et al. (2013). Frequency-multiplication high-output triboelectric nanogenerator for sustainably powering biomedical microsystems. Nano Letters 13: 1168–1172. Meng, B., Tang, W., Too, Z. et al. (2013). A transparent single-friction-surface triboelectric generator and self-powered touch sensor. Energy & Environmental Science 6: 3235–3240.

References

10 Wang, S., Xie, Y., Niu, S. et al. (2014). Freestanding triboelectric-layer-based

11 12

13

14 15

16 17

18

19 20 21

22 23 24 25

26

27

nanogenerators for harvesting energy from a moving object or human motion in contact and non-contact modes. Advanced Materials 26: 2818–2824. Tritt, T. and Subramanian, M. (2006). Thermoelectric materials, phenomena, and applications: a bird’s eye view. MRS Bulletin 31: 188–198. Alpay, S., Mantese, J., Trolier-McKinstry, S. et al. (2014). Next-generation electrocaloric and pyroelectric materials for solid-state electrothermal energy interconversion. MRS Bulletin 39: 1099–1111. Scharber, M., Mühlbacher, D., Koppe, M. et al. (2006). Design rules for donors in bulk-heterojunction solar cells-towards 10% energy-conversion efficiency. Advanced Materials 18: 789–794. Wang, Z.L. (2017). On Maxwell’s displacement current for energy and sensors: the origin of nanogenerators. Materials Today 20: 74–82. Zhang, X., Han, M., Meng, B., and Zhang, H. (2015). High performance triboelectric nanogenerators based on large-scale mass-fabrication technologies. Nano Energy 11: 304–322. Fan, F., Tian, Z., and Wang, Z.L. (2012). Flexible triboelectric generator. Nano Energy 1: 328–334. Niu, S., Liu, Y., Wang, S. et al. (2014). Theoretical investigation and structural optimization of single-electrode triboelectric nanogenerators. Advanced Functional Materials 24: 3332–3340. Zi, Y., Niu, S., Wang, J. et al. (2015). Standards and figure-of-merits for quantifying the performance of triboelectric nanogenerators. Nature Communications 6: 8376. Niu, S., Liu, Y., Chen, X. et al. (2015). Theory of freestanding triboelectric-layer-based nanogenerators. Nano Energy 12: 760–774. Zi, Y., Wang, J., Wang, S. et al. (2016). Effective energy storage from a triboelectric nanogenerator. Nature Communications 7: 10987. Cheng, X., Miao, L., Song, Y. et al. (2017). High efficiency power management and charge boosting strategy for a triboelectric nanogenerator. Nano Energy 38: 438–446. Song, Y., Wang, H., Cheng, X. et al. (2018). High-efficiency self-charging smart bracelet for portable electronics. Nano Energy 55: 29–36. Niu, S., Liu, Y., Wang, S. et al. (2013). Theory of sliding-mode triboelectric nanogenerators. Advanced Materials 25: 6184–6193. Cheng, G., Lin, Z.H., Lin, L. et al. (2013). Pulsed nanogenerator with huge instantaneous output power density. ACS Nano 7: 7383–7391. Qin, H., Cheng, G., Zi, Y. et al. (2018). High energy storage efficiency triboelectric nanogenerators with unidirectional switches and passive power management circuits. Advanced Functional Materials 28: 1805216. Wang, Z.L. (2013). Triboelectric nanogenerators as new energy technology for self-powered systems and as active mechanical and chemical sensors. ACS Nano 7: 9533–9557. Wang, Z.L. (2015). Triboelectric nanogenerators as new energy technology and self-powered sensors–principles, problems and perspectives. Faraday Discussions 176: 447–458.

75

76

4 Characterization of Triboelectric Nanogenerators

28 Shao, J., Jiang, T., Tang, W. et al. (2018). Structural figure-of-merits of tribo-

electric nanogenerators at powering loads. Nano Energy 51: 688–697. 29 Baytekin, H.T., Patashinski, A.Z., Branicki, M. et al. (2011). The mosaic of sur-

face charge in contact electrification. Science 333: 1201512. 30 Burgo, T., Ducati, T., Francisco, K. et al. (2012). Triboelectricity: macroscopic

charge patterns formed by self-arraying ions on polymer surfaces. Langmuir 28: 7407–7416. 31 Tang, W., Jiang, T., Fan, F.R. et al. (2015). Liquid-metal electrode for high-performance triboelectric nanogenerator at an instantaneous energy conversion efficiency of 70.6%. Advanced Functional Materials 25: 3718–3725. 32 Chiechi, R., Weiss, E., Dickey, M., and Whitesides, G. (2008). Eutectic gallium-indium (EGaIn): a moldable liquid metal for electrical characterization of self-assembled monolayers. Angewandte Chemie International Edition 47: 142–144. 33 Dickey, M., Chiechi, R., Larsen, R. et al. (2008). Eutectic gallium-indium (EGaIn): a liquid metal alloy for the formation of stable structures in microchannels at room temperature. Advanced Functional Materials 18: 1097–1104.

77

5 Power Management of Triboelectric Nanogenerators Xiaoliang Cheng Peking University, Institute of Microelectronics, National Key Lab of Nano/Micro Fabrication Technology, No. 5 Yiheyuan Road, 100871 Beijing, China

Due to the unique features of electrical output signals of triboelectric nanogenerators (TENGs), such as large impedance, ultrahigh voltage, and low current, the electrical power generated by TENGs is hard to be delivered to the load efficiently or stored directly by high-capacity energy storage devices. Thus, this chapter focuses on the power management of TENGs. Firstly, the theoretical analysis of power transmittance of TENGs is discussed to help readers understand the underlying mechanism of TENG power management. Subsequently, the current progress in power management for TENGs is summarized and classified into several categories, including inductive transformers, capacitive transformers, and inductor–capacitor (LC) oscillation circuits.

5.1 Theoretical Analysis of Power Transmittance of TENGs Understanding the theoretical model is the first step to design a proper power management system for TENG. We primarily discuss the fundamental model of TENG in this section. The fundamental working mechanism of TENGs is a conjugation of contact electrification and electrostatic induction [1–7]. Since the most basic device based on electrostatic is a capacitor, principally TENG will have an inherent capacitive behavior. For any mode of TENG, the governing equation can be given by [8] V =−

1 Q + VOC (x) C(x)

(5.1)

where V is the total voltage difference between the two electrodes, C and Q represent the capacitance between the two electrodes and the already transferred charges, and V OC is the open-circuit voltage between the two electrodes. From this governing equation, the lumped equivalent circuit model can be derived and represented by a serial connection of an ideal voltage source and Flexible and Stretchable Triboelectric Nanogenerator Devices: Toward Self-powered Systems, First Edition. Edited by Mengdi Han, Xiaosheng Zhang, and Haixia Zhang. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

78

5 Power Management of Triboelectric Nanogenerators

Figure 5.1 Theoretical model of any mode of TENGs. Source: Reproduced with permission from Niu and Wang [1]. Copyright 2014, Elsevier. C

VOC

Arbitrary load circuits

TENG

a capacitor, as shown in Figure 5.1. Using this lumped equivalent circuit model, the load characteristics of the TENG for any type of devices could be obtained by simply replacing the diagram of arbitrary load circuits with the load connected with the TENG. Resistive load and capacitive load are two common circuits when using a TENG as a power source. Therefore, the power transmittance of TENGs using resistive and capacitive loads is introduced subsequently. 5.1.1

Resistive Load Characteristics of TENGs

When TENGs are connected with a resistive load R, the equivalent circuit of the whole system is as shown in Figure 5.2a [1]. Employing Kirchhoff ’s law, the governing equation of the whole system can be easily given by dQ 1 (5.2) = − Q + VOC dt C The values of the voltage, current, and power on different loads of typical TENGs are shown in Figure 5.2b,c. According to works by Niu and Wang from the Georgia Institute of Technology, the operation of the TENG can be divided into three working regions [1], as plotted in Figure 5.2b. Region I, in which the resistance is low (0.1–1000 Ω), the peak value of current has dropped a little from the short-circuit state, while the maximum voltage is nearly proportional to the outer resistor. In Region II, the maximum current decreases with the resistance, while the voltage behaves in an opposite trend, and the TENG obtains its maximum output power as shown in Figure 5.2c. In Region III, when the resistor is larger than 1 GΩ, the maximum voltage saturates at V OC . It can be noted that the inherent impedance of the TENG mainly comes from its inherent capacitance, which is in the level of nanofarads. This led to an internal high impedance from the small inherent capacitance. R

5.1.2

Capacitive Load Characteristics of TENGs

Capacitive load is another common load circuit for TENG, which is employed when storing the electric energy of the TENG to a capacitor or battery [9]. When the TENG is connected with a capacitive load C, the equivalent circuit model of

5.1 Theoretical Analysis of Power Transmittance of TENGs

+

V

R

+

Region I 100

Region II

1000 Region III

800 600 400

50

200

VOC –

TENG

Current (μA)

+Q

Voltage (V)

C

1200

Max I Max V

150

–Q

0

0

–

(a)

(b)

101 103 105 107 109 1011 1013 Resistor (Ω)

Power Power (mW)

2

1

0 (c)

101 103 105 107 109 1011 1013 Resistor (Ω)

Figure 5.2 Resistive load characteristics of a TENG. (a) The equivalent circuit of the whole system when a TENG is connected with a resistance load. (b) The values of the voltage and current on different loads of typical TENGs. (c) The power-on of typical TENGs on varying resistance. Reproduced with permission from Niu and Wang [1]. Copyright 2014, Elsevier.

the system is as shown in Figure 5.3a [1, 3]. Here, Q is defined as the transferred charges from the primary electrode to the reference electrode and QC is defined as the charges on the top plate of the load capacitor. When the motion TENG reaches its maximum separation distance (i.e. xmax ), the final voltage and charge on the capacitor can be given by V (x = xmax ) =

QSC,max

CL + CT CL QSC,max QC (x = xmax ) = CL + CT

(5.3) (5.4)

While the total stored energy in the capacitor (EC ) can be determined by C

E =

CL Q2SC,max 2(CL + CT )2

(5.5)

where V OC,max and QSC,max are the open-circuit voltage and short-circuit transferred charge of the TENG when x = xmax , and C T is the intrinsic capacitive impedance of the TENG. The voltages, currents, and powers on different capacitive loads are shown in Figure 5.3b,c. When C L is much smaller than the impedance of C T , the applied

79

5 Power Management of Triboelectric Nanogenerators

300 –Q

V –QC

VOC

CL

Voltage (V)

+Q

200

60

150

40

100

Max V Max Q

50

–

0

0

(a)

(b)

20

Charge (pC)

CT

+QC

80

250

+

10–3 10–2 10–1 100 101 CL/CTENG

102

103

3

Energy (nJ)

80

2

1

0

(c)

Stored energy 10–3 10–2 10–1 100 101 CL/CTENG

102

103

Figure 5.3 Capacitive load characteristics of a TENG. (a) The equivalent circuit of the whole system when the TENG is connected with a capacitive load. (b) The values of the voltage and current on different capacitive loads. (c) The power-on of typical TENGs on varying capacitance. Source: Reproduced with permission from Niu and Wang [1]. Copyright 2014, Elsevier.

voltage on C L is approximately equal to V OC , while the stored charge is still very small (Figure 5.3b). Instead, when C L is much larger than the impedance of C T , the voltage applied on C L is almost equal to 0. However, the total stored energy is still very small because of the low voltage on C L . When C L = C T , the impedance match state is reached and the maximum value of total stored energy on C L is obtained (Figure 5.3c). In summary, working as an electrostatic device, TENG has an inherent capacitive behavior. The intrinsic capacitance of the TENG is usually very small (i.e. in nanofarad levels) [9]. Therefore, TENG has a relatively high equivalent resistance in the order of mega ohm, leading to low-power transmittance efficiency when supplying power for a low-impedance circuit. As with the resistive load characteristics, the maximum power when charging a capacitor using a TENG is reached only when the load capacitance is equal to the intrinsic capacitance of TENG. Considering the capacitance of the energy storage unit (i.e. much larger than the microfarad level) is much larger than the intrinsic capacitance of TENG, the charging efficiency for the energy storage unit would be very low.

5.2 The Progress in TENG Power Management

5.2 The Progress in TENG Power Management As mentioned earlier, the ultrahigh voltages and impedance are the main obstacles for using TENGs as power sources for traditional electronics. In this section, we focus on the current progress and works on reducing the voltage as well as the impedance of TENGs, which primarily include inductive transformers, capacitive transformers, and LC oscillation circuit. 5.2.1

Using Inductive Transformers

The inductive transformer is the common method for dropping voltage, which is also employed by previous works to reduce the high voltage from TENGs [10–21]. Zhu et al. from the Georgia Institute of Technology presented a power management circuit consisting of a transformer, a rectifier, a voltage regulator, and capacitors, which is shown in Figure 5.4a [10]. Using this circuit, the high voltage from the TENG is reduced from over 400 to 5 V, which can deliver a direct current (DC) output at a constant voltage of 5 V in less than 0.5 s after the TENG starts to operate, as shown in Figure 5.4b. Since 5 V is the standard charging voltage for most of the commercial portable electronics, for example, a cellphone 6 Output voltage (V)

0.01 μF

1000 μF

0.01 μF

40 : 1

1000 μF

0.001 μF

TENG

5V

5 4 3 2

TENG starts

1 0 0.0

(a)

(b)

0.5

1.0 1.5 Time (s)

2.0

(c)

Figure 5.4 Using a TENG to charge a cellphone via a transformer. (a) Circuit diagram of the complete power management circuit using a transformer. (b) Output voltage of the system reaches a constant value of 5 V in less than 0.5 s. (c) Photograph of a cellphone that is being charged by the power supplying system. Source: Reproduced with permission from Zhu et al. [10]. Copyright 2014, Springer Nature.

81

106 Z1 = n–2 ZO CO

ZB n: 1

VOC

Ideal battery

Rectifier

Battery

TENG

100 600 rpm 250 rpm

105

Efficiency at 250 rpm

104 10

6.1

102

1

1

24.4

10

ZLFP-LTO = 1.7 Ω @ 1500 Hz

0

10

1

(a)

10 Transformer coil ratio

(c)

Transformer coil ratio n

10

3

Discharging capacity (mAh)

Charging power (mW)

36.7

0.1 100

Current amplitude (mA)

Voltage amplitude (V)

Charging time (min)

1.0

2.0 ± 0.07

391.4 ± 1.8

125

1.1

2.4 ± 0.08

6.1

11.6 ± 0.4

61.9 ± 0.4

125

12.9

16.1 ± 0.6

8.4 ± 0.3

24.4

48.4 ± 1.2

15.4 ± 0.1

23

10.7

76.9 ± 1.9

40.1 ± 1.0

36.7

73.5 ± 0.9

9.2 ± 0.4

10

9.3

139.0 ± 1.7

72.4 ± 0.9

Power utilization efficiency (%)

5 Power Management of Triboelectric Nanogenerators

Matched impedance (Ω)

82

Power utilization efficiency (%) 1.2 ± 0.04

(b)

Figure 5.5 The factor of coil turn ratio of a transformer. (a) Circuit diagram. (b) The effect of the transformer coil ratio on matched impedances of the TENG. (c) Charging a lithium battery by the TENG with different transformers. Source: Reproduced with permission from Pu et al. [11]. Copyright 2016, John Wiley & Sons.

automatically turned on once the voltage output shot to 5 V due to the operation of the TENG, as visualized in Figure 5.4c. The coil turn ratio of the transformer is a key factor deciding its performance [12]. Previous work by Xiong Pu et al. from the Chinese Academy of Sciences investigated the influence of coil ratio, as shown in Figure 5.5a [11]. When the coil ratio was increased from 1 to 36.7, the current increased dramatically from 2.0 to 73.5 mA, and voltage decreased from 391.4 to 9 V, as listed in Figure 5.5b. Meanwhile, the charging efficiency by a transformer with a coil ratio of 36.7 can be obviously improved from 1.2% to about 72.4%, the details of which are shown in Figure 5.5c. The transformer was also found to be an effective method in decreasing the large impedance of TENGs. Using a transformer with a coil ratio of n, the impedance will be decreased to Z1 = n−2 Z0 . In their work, with a transformer (n = 36.7), the matched impedance decreased to about 110 Ω, as plotted in Figure 5.5c [11]. When employed as the power management unit for a TENG, the inductive transformer shows the advantage of reducing the voltage as well as the matched impedance, resulting in higher current and power utilization efficiency. However, as the transformer has a central frequency, the efficiency of the transformer would dramatically decrease when off the center frequency. Here, it must be mentioned that this kind of transformer design only suits the rotary-shape TENG with a relatively high and stable working frequency. 5.2.2

Using Capacitive Transformers

Different from the inductive transformer working at a typical frequency, Wei Tang et al. from the Chinese Academy of Sciences proposed a capacitive

5.2 The Progress in TENG Power Management

transformer with an array of self-connection switches, which can manage the short pulse at a variable frequency output of a TENG [22, 23]. By integrating a contact-separation-mode TENG with an array of selfconnection-switching capacitors that are connected in serial when being charged and then in parallel during discharging, they developed a power-transformedand-managed triboelectric nanogenerator (PTM-TENG), as shown in Figure 5.6a–c. It was found that the PTM-TENG’s output voltage can be tunably decreased and its output current and charges increased per applied load (impact). When the number of capacitors in the circuit is N, the output charges and voltage are described as follows, Qout = NQ0 V Vout = 0 N

(5.6) (5.7)

where V 0 and Q0 represent the total generated charge voltage and charges in the serial capacitors. Their experimental results validated the given equations, as shown in Figure 5.6d,e, which realize the purpose of reducing the voltage. However, because of the complex mechanical layout, this design has limited the number of switch and transformer ratio. Y.L. Zi et al. from the Georgia Institute of Technology designed similar solutions for sliding-mode TENG, as shown in Figure 5.7a,b, based on switches between serial-connected and parallel-connected capacitors during the operation of the TENG, so that the output voltage is lowered but the output charge is raised in proportion [23]. The working of the TENG is along with mechanical motions, which is able to automatically trigger switches for proper operations of the capacitors. The 3D diagram of a sliding TENG and a power management unit with N = 2 (N is the capacitor number of the circuits) is shown in Figure 5.7a. Using an acrylic board with metal areas and electric brushes built on the moving part of the TENG, the power management unit can realize the switch between the serial-connected mode and the parallel-connected mode. The top view design of the metal areas and electric brushes is shown in Figure 5.7b. By designing a specific contact pad at the two ends of the metal areas, they realized the switch between these two conditions. Utilizing a design in the power management unit with N = 5, they demonstrated the charging of a 5 mF supercapacitor with a charging rate of about five times as that without the power management unit (through a traditional full-wave bridge rectifier), as plotted in Figure 5.7c. The experimental output energies per cycle (Eout ) are plotted in Figure 5.7d. The highest Eout during their experiments was 1.74 μJ, and the corresponding experimental maximum energy transfer efficiency was calculated to be 23.0%. 5.2.3

Using LC Oscillation Circuit

For improving the universality of the power management circuit for TENG, S.M. Niu et al. from the Georgia Institute of Technology designed the following charging strategy for maximized energy storage efficiency [24].

83

TENG

(a)

OUT

Voltage (V)

100

0

(b) (ii)

104

++ ––

Released

++ ––

e–

Pressed

++ ++ –– ––

2e–

(iv)

e–

(iii) Pressing

+ ++ – –– OUT

OUT + ++ – –– 2e–

++ ++ –– ––

(c)

106

107

108

Resistor (Ω) 101

OUT

OUT + –

105

(d)

Energy on load (μJ)

+ –

Releasing

T8

50

e–

(i)

T0 T2 T4

100 10–1 T0 T2

10–2

T4

10–3

T8

10–4

e–

10

(e)

4

10

5

10

6

10

7

10

8

109

1010

Resistor (Ω)

Figure 5.6 Capacitive transformer for a TENG. (a) A 3D structure of the PTM-TENG. (b) The pressed PTM-TENG’s cross-sectional view and the equivalent circuit diagram. (c) A full working cycle of the PTM-TENG. (d) Output voltages of the T0, T2, T4, and T8 under various load resistances. (e) Output energy (energy supplied to the load) of the T0, T2, T4, and T8 under various load resistances. Source: Reproduced with permission from Tang et al. [22]. Copyright 2016, IOP.

5.2 The Progress in TENG Power Management Metal

Dielectric

Metal areas

Acrylic

NG TE

Electric brushes

1+ 2+ A1 1– 2– A2

0+

2+

1+ 2+ B1 1– 2– B2

0+

2–

0+

1+ 1–

0–

X=0

2

1

Power manag ement unit

(a)

X =Xmax 1+

TENG

(b) 100

0–

1–

2+ A1, B1 A2, B2 A1, B2 2– A2, B1

20 Through our device: ~0.4 mV/s 804 nC per half cycle

80

Energy per cycle (μJ)

Voltage (mV)

0+

60 40 Through a rectifier: ~0.08 mV/s 164 nC per half cycle

20

Load

AC output

or Load

DC output

Calculated Experimental

15 10 5 0

0 0

(c)

50

100

150

Time (s)

200

250

1

(d)

10

100

External total capacitance (nF)

Figure 5.7 Capacitive transformer for a lateral sliding TENG. (a) The 3D illustration of the power management unit. (b) The design of metal areas in the board and the electric brushes of the motion-triggered power management unit. (c) The charging plot for a 5 mF supercapacitor. (d) The experimental and calculated total output energy per cycle. Source: Reproduced with permission from Zi et al. [23]. Copyright 2017, Elsevier.

This theoretical charging cycle can be realized by a two-stage power management circuit, as shown in Figure 5.8a. In the first stage, a temporary capacitor C temp is charged by a TENG through a bridge rectifier. In this single step, the charging efficiency for this capacitor can theoretically reach 75%. The second stage is for energy transfer from C temp to the final energy storage unit. Since transferring electrostatic energy directly from a small capacitor to a large capacitor (or a battery) results in huge energy loss, two automatic electronic switches (controlled by a logic control unit, and the power of both switches and their logic control unit is supplied from the final energy storage unit) and a coupled inductor are utilized in the second stage. The detailed operation process is shown here to achieve efficient energy transfer in this stage. First, both the switches J 1 and J 2 are open to avoid interference of the charging process of C temp . When the voltage of C temp reaches its maximum value, electronic switch J 1 closes. As a result, the energy starts to transfer from C temp to inductor L1 and V temp starts to drop. When the energy is fully transferred to L1 , switch J 1 opens and J 2 closes. As a consequence, the current of L1 falls to 0 instantaneously. The current of L2 will suddenly rise, corresponding to the energy transfer from L1 to L2 . Finally, the energy stored in L2 will automatically transfer to the final energy storage unit because of the closure of J 2 . When the energy stored in L2 is fully sent out, J 2 is open again and another charging cycle begins.

85

TENG D1

J1

D3

J2 +

+ D4

L1

Vtemp

D3

–

L2

Energy storage

Vstore

Ctemp

Functional circuits

–

Vstore (V)

6.4 6.0

20 kΩ 30 kΩ 39 kΩ 60 kΩ 70 kΩ

5.6 5.2 0

(b)

2

6

8 (c)

0.202 mW DC power at 180 kΩ load

0.30 0.25 0.20 0.15 Measurement Interpolation

0.10 0.05

4 Time (s)

6.05

0.35 Vstore (V)

1.044 mW DC power at 39 kΩ load

6.8

Average power (mW)

(a)

1

10 Resistive load (MΩ)

6.04 6.03 RL = 180 kΩ

6.02 0 (d)

5 10 Time (s)

15

Figure 5.8 A temporary capacitor and LC oscillation circuit. (a) Circuit diagram. (b) The maximum DC power of this system driven by human biomechanical energy. (c) Measurement of the AC-harvested power from a resistor. (d) Measurement of the DC-harvested power from the power management circuit. Source: Reproduced with permission from [24]. Copyright 2015, Springer Nature.

5.2 The Progress in TENG Power Management

The most important parameter of the power management circuit is the total efficiency Ztotal , which is defined as the ratio of the maximum DC power stored in the storage unit to the maximum alternating current (AC) power delivered to a resistive load. To measure Ztotal , first, the maximum AC power delivered to a resistive load can be extracted by the TENG resistance-matching measurement. As shown in Figure 5.8b–d, the maximum AC energy generated by TENG is 0.3384 mW at an optimum load resistance of 4.26 MΩ. Second, the maximum DC power delivered through the power management board can be measured using the method shown earlier, which is 0.2023 mW. Therefore, Ztotal is calculated as 59.8%. This work provides a universal method for TENG’s power management via a temporary capacitor, a logical controlled switch, and a coupled inductor. However, at least 25% energy loss would occur between energy transfer from the TENG to the temporary capacitor, and this capacitor has to be optimized for TENGs with different parameters, making it inefficient and unpractical. Our group from the Peking University proposed in their work a simpler method without using the temporary capacitor [25, 26]. A two-step strategy was adopted, (1) Maximizing the output energy of a TENG using the built-up voltage V total transferred charges Q plot applicable to both modes of TENGs; (2) Maximizing the transferred energy from the TENG to the energy storage unit by employing the LC oscillating model. The operating cycles for scavenging maximum energy from a TENG and transferring it to the capacitor via LC oscillation is plotted in Figure 5.9a. A power management module (PMM) was designed and assembled under the guidance of this strategy to manage and regulate the electric outputs from CS-mode TENGand RS-mode TENGs and demonstrated the universality of their method. Using this module, huge improvements were achieved for both the stored charges and the energy in the energy storage unit. As shown in Figure 5.9b, the maximum AC power generated by LS-mode TENG was 0.265 mW at an optimum load resistance of 5 MΩ. The DC power stored in the storage unit was obtained by measuring the DC voltage of a varying parallel resistor, as shown in Figure 5.9c. A maximum DC power of 0.2 mW was reached at an equivalent resistance of 100 KΩ, as shown in Figure 5.9d. Therefore, the total efficiency of PMM for this RS-mode TENG was calculated to be 75.5%. Similar to this circuit diagram, researchers have tried to fabricate a PMM on a flexible substrate using the flexible printed circuit board (FPCB) process as well as a silicon wafer using the complementary metal-oxide-semiconductor (CMOS) transistor process for better integration with TENG, as shown in Figure 5.10 [26, 27]. Yu Song et al. from the Peking University demonstrated a high-efficiency self-charging smart bracelet integrated the power management module as well as a TENG and energy storing component, as seen in Figure 5.10a [26]. Utilizing the FPCB technique, the freestanding TENG and corresponding PMM were fabricated together, which could serve as a bracelet for harvesting human motion

87

5 Power Management of Triboelectric Nanogenerators

(ii) (Q,V) = (QSC,max,0)

(i) (Q,V) = (0,VOC,max)

(iii) (Q,V) = (QSC,max, –V′)

I

I

x = xmax

x = xmax

(vi) (Q,V) = (0,VOC)

(v) (Q,V) = (0,0)

(iv) (Q,V) = (QSC,max, –V′max)

I

I

x=0

x=0

(a) 0.3

AC power curve 4

0 1E-7

(b)

0.1

12

0.3 0.2

8

0.1

4

0.0

0

0.0 1E-3 10 Resistor (MΩ)

DC voltage and current

(c)

1E-3 0.01 0.1 1 10 100 Resistor (MΩ)

Power (mW)

0.2

Power (mW) Voltage (V)

8

1

0.4 16

Current (mA)

12

Current (μA)

88

0.01

Standard PMM

1E-4

1E-6

(d)

1E-5 1E-3 0.1 10 Resistor (MΩ)

Figure 5.9 Power management of a TENG using an LC oscillation circuit. (a) Operating cycles. (b) The AC power curve of an LS-mode TENG. (c) The DC voltage and current of an RS-mode TENG via PMM. (d) The experimental DC power curve via PMM and AC power curve of an RS-mode TENG. Source: Reproduced with permission from Cheng et al. [25]. Copyright 2017, Elsevier. (a)

Smart bracelet

Double-sided MSCs

FPCB (F-TENG + PMM) Ecoflex

Human motion

Mechanical energy

Flexible energy Harvesting device

Cu

Power management

PI

CNT-PDMS

Stretchable energy Storage device

LCD display Electrolyte

Power supply

Electronic component

Wearable/portable electronics

(b)

Freestanding triboelectric nanogenerator

Power management module

Double-sided microsupercapacitors

Figure 5.10 Power management module on flexible and silicon wafer substrates. (a) Schematic diagram of a power management module on a flexible substrate. (b) Photographs of the power management module on the flexible substrate. Source: Reproduced with permission from Song et al. [26]. Copyright 2019, Elsevier.

(a)

(b) I

R

R

Capacitor or QLED I

R

R

(c) Uth = 5 V

UGS (V)

UT Comparator

100

t1 = 38 μs

4 3 2

Tribotronic Energy Extractor

0

–100

1 B

Theoretical Measured

200

–200

0 0

1

2 Time (ms)

3

12 Average power (μW)

6 5

UGS TENG

300

A

U(V)

MOSFET

Managed Direct

10 8 6 4 2

RM = 1 MΩ RD = 35 MΩ

0

–300 0

20

40

60 80 Q (nC)

100 120

0.01

0.1

1 10 100 1000 Resisitor (MΩ)

Figure 5.11 LC oscillation with a passive switch for a TENG. (a) Electrostatic vibrator switch. Source: Reproduced with permission from Yang et al. [29]. Copyright 2018, Elsevier. (b) Unidirectional switch. Source: Reproduced with permission from Qin et al. [30]. Copyright 2018, John Wiley & Sons. (c) Tribotronic energy extractor using a metal-oxide-semiconductor field-effect transistor (MOSFET) switch. Source: Reproduced with permission Xi et al. [31]. Copyright 2018, Elsevier.

90

5 Power Management of Triboelectric Nanogenerators

energy. By seamlessly assembling a flexible supercapacitor with a TENG and PMM, they realized a totally flexible and self-charging bracelet, as shown in the photograph in Figure 5.10b. The TENG could deliver a maximum power of 300.4 μW, and the PMM has an energy transfer efficiency of 69.3%. I. Park et al. from Korea University presented a PMM fabricated using the CMOS process on silicon wafer with a total size of just about 2.2 mm2 [27]. Meanwhile, their design has added the module of dual-input buck converter and root mean square (rms) maximum power point tracking, showing the potential for managing dual inputs from multiple TENGs at the same time. The maximum power conversion efficiency is measured as 51.1% on considering the power consumption of the controlling circuit. Typically, the logic circuit is powered by outer power [25, 28], which limited its application when starting the energy harvesting system without a battery or any initial energy. In this regard, the mechanical switch would be a good candidate. As an example, an electrostatic vibrator switch was introduced as a self-powered switch of a TENG [29], where its vibration is driven by the potential difference of the TENG itself, as shown in Figure 5.11a. A mechanical switch triggered by TENG’s motion was also developed using a unidirectional switch and an inductor [30], which reached an energy storage efficiency of 48.0% in the actual experiments, as seen in Figure 5.11b. Similar to this work, F.B. Xi et al. from the Chinese Academy of Sciences reported a work using a passive tribotronic energy extractor and obtained an efficiency of 85% [31], as shown in Figure 5.11c, which is the highest record, the best till date.

5.3 Summary This chapter discussed the theoretical analysis of power transmittance of TENGs and the recent progress in power management for TENGs. Fundamentally, there is an intrinsic capacitor inside the TENG that makes it high impedance, high voltage, and low current. Since this intrinsic capacitor is usually in the level of a nanofarad, the charging efficiency for an energy storage unit, such as a capacitor or a battery, would be very low. The inductive transformer is a simple and highly efficient solution for rotary TENGs with fixed central frequency, the efficiency of which, however, would be very low when off the central frequency. A capacitive transformer suits the pulsed output of a TENG, but its complexity would exponentially grow with the transformer ratio. The LC oscillation method is a universal method that applies to the uniform output as well as to the pulsed output from TENGs.

Abbreviations TENG LC

triboelectric nanogenerator inductor–capacitor

References

PTM-TENG DC AC PMM CS-TENG RS-TENG FPCB CMOS

power-transformed-and-managed triboelectric nanogenerator direct current alternating current power management module contact-separation-mode triboelectric nanogenerator relative-sliding-mode triboelectric nanogenerator flexible printed circuit board complementary metal-oxide-semiconductor transistor

References 1 Niu, S.M. and Wang, Z.L. (2015). Theoretical systems of triboelectric nano-

generators. Nano Energy 14: 161–192. 2 Niu, S., Liu, Y., Wang, S. et al. (2013). Theory of sliding-mode triboelectric

nanogenerators. Advanced Materials 25: 6184–6193. 3 Niu, S., Liu, Y., Wang, S. et al. (2014). Theoretical investigation and structural

4 5

6 7

8

9

10

11

12

13

optimization of single-electrode triboelectric nanogenerators. Advanced Functional Materials 24: 3332–3340. Niu, S., Liu, Y., Chen, X. et al. (2015). Theory of freestanding triboelectric-layer-based nanogenerators. Nano Energy 12: 760–774. Niu, S., Wang, S., Liu, Y. et al. (2014). A theoretical study of grating structured triboelectric nanogenerators. Energy & Environmental Science 7: 2339–2349. Fan, F.R., Tian, Z.Q., and Wang, Z.L. (2012). Flexible triboelectric generator! Nano Energy 1: 328–334. Fan, F.R., Lin, L., Zhu, G. et al. (2012). Transparent triboelectric nanogenerators and self-powered pressure sensors based on micropatterned plastic films. Nano Letters 12: 3109–3114. Niu, S., Wang, S., Lin, L. et al. (2013). Theoretical study of contact-mode triboelectric nanogenerators as an effective power source. Energy & Environmental Science 6: 3576–3583. Niu, S.M., Liu, Y., Zhou, Y.S. et al. (2015). Optimization of triboelectric nanogenerator charging systems for efficient energy harvesting and storage. IEEE Transactions on Electron Devices 62: 641–647. Zhu, G., Chen, J., Zhang, T.J. et al. (2014). Radial-arrayed rotary electrification for high performance triboelectric generator. Nature Communications 5, https://doi.org/10.1038/ncomms4426. Pu, X., Liu, M., Li, L. et al. (2016). Efficient charging of Li-Ion batteries with pulsed output current of triboelectric nanogenerators. Advanced Science 3: 1500255. Han, C.B., Zhang, C., Tang, W. et al. (2015). High power triboelectric nanogenerator based on printed circuit board (PCB) technology. Nano Research 8: 722–730. Zhong, X.D., Yang, Y., Wang, X., and Wang, Z.L. (2015). Rotating-disk-based hybridized electromagnetic-triboelectric nanogenerator for scavenging biomechanical energy as a mobile power source. Nano Energy 13: 771–780.

91

92

5 Power Management of Triboelectric Nanogenerators

14 Bhatia, D., Lee, J., Hwang, H.J. et al. (2018). Design of mechanical frequency

15

16

17

18

19

20

21

22

23

24

25

26 27

regulator for predictable uniform power from triboelectric nanogenerators. Advanced Energy Materials 8: https://doi.org/10.1002/aenm.201702667. Wang, S., Mu, X., Yang, Y. et al. (2015). Flow-driven triboelectric generator for directly powering a wireless sensor node. Advanced Materials 27: 240–248. Guo, H., Wen, Z., Zi, Y. et al. (2016). A water-proof triboelectric–electromagnetic hybrid generator for energy harvesting in harsh environments. Advanced Energy Materials 6: 1501593. Zhang, K., Wang, X., Yang, Y., and Wang, Z.L. (2015). Hybridized electromagnetic–triboelectric nanogenerator for scavenging biomechanical energy for sustainably powering wearable electronics. ACS Nano 9: 3521–3529. Wang, X., Wang, S., Yang, Y., and Wang, Z.L. (2015). Hybridized electromagnetic–triboelectric nanogenerator for scavenging air-flow energy to sustainably power temperature sensors. ACS Nano 9: 4553–4562. Cao, R., Zhou, T., Wang, B. et al. (2017). Rotating-sleeve triboelectric–electromagnetic hybrid nanogenerator for high efficiency of harvesting mechanical energy. ACS Nano 11: 8370–8378. Yeh, M.H., Guo, H., Lin, L. et al. (2016). Rolling friction enhanced free-standing triboelectric nanogenerators and their applications in self-powered electrochemical recovery systems. Advanced Functional Materials 26: 1054–1062. Zhao, K., Qin, Q., Wang, H. et al. (2017). Antibacterial triboelectric membrane-based highly-efficient self-charging supercapacitors. Nano Energy 36: 30–37. Tang, W., Zhou, T., Zhang, C. et al. (2014). A power-transformed-andmanaged triboelectric nanogenerator and its applications in a self-powered wireless sensing node. Nanotechnology 25, https://doi.org/10.1088/0957-4484/ 25/22/225402. Zi, Y.L., Guo, H.Y., Wang, J. et al. (2017). An inductor-free auto-power-management design built-in triboelectric nanogenerators. Nano Energy 31: 302–310. Niu, S.M., Wang, X.F., Yi, F. et al. (2015). A universal self-charging system driven by random biomechanical energy for sustainable operation of mobile electronics. Nature Communications 6, https://doi.org/10.1038/ncomms9975. Cheng, X.L., Miao, L.M., Song, Y. et al. (2017). High efficiency power management and charge boosting strategy for a triboelectric nanogenerator. Nano Energy 38: 448–456. Song, Y., Wang, H., Cheng, X. et al. (2018). High-efficiency self-charging smart bracelet for portable electronics. Nano Energy 55: 29–36. Park, I., Maeng, J., Lim, D. et al. (2018). A 4.5-to-16μW integrated triboelectric energy-harvesting system based on high-voltage dual-input buck converter with MPPT and 70V maximum input voltage. presented at 2018 IEEE International Solid - State Circuits Conference - (ISSCC) (11–15 February 2018). San Francisco, CA, USA: IEEE.

References

28 Boisseau, S., Despesse, G., Monfray, S. et al. (2013). Semi-flexible

bimetal-based thermal energy harvesters. Smart Materials and Structures 22, https://doi.org/10.1088/0964-1726/22/2/025021. 29 Yang, J.J., Yang, F., Zhao, L. et al. (2018). Managing and optimizing the output performances of a triboelectric nanogenerator by a self-powered electrostatic vibrator switch. Nano Energy 46: 220–228. 30 Qin, H., Cheng, G., Zi, Y. et al. (2018). High energy storage efficiency triboelectric nanogenerators with unidirectional switches and passive power management circuits. Advanced Functional Materials 28: 1805216. 31 Xi, F.B., Pang, Y.K., Li, W. et al. (2017). Universal power management strategy for triboelectric nanogenerator. Nano Energy 37: 168–176.

93

95

Part II Approaches to Flexible and Stretchable Device

97

6 Overview of Flexible and Stretchable Approaches Mengdi Han Northwestern University, Center for Bio-Integrated Electronics, Evanston, IL, USA

The real world is curvilinear, continuously evolving, and may contain soft components. This requires that triboelectric nanogenerators (TENGs) be flexible and stretchable. This chapter focuses on the introduction of flexible and stretchable devices and their approaches, including intrinsically flexible or stretchable materials for electronics including nanomaterials, organic materials, and other materials, as well as structural designs for flexible and stretchable electronics such as nanomembranes (NMs), two-dimensional (2D) serpentine layouts, three-dimensional (3D) buckled mesostructures, and others.

6.1 Intrinsically Flexible or Stretchable Materials This section covers the materials used in the state-of-the-art flexible and stretchable electronics, including nanomaterials in different dimensions, organic materials, fiber-based materials, composite materials, and others. 6.1.1

Nanomaterials in Different Dimensions

Growth and assembly of nanomaterials are promising approaches to build micro-/nanoscale flexible and stretchable electronics. The unique electrical, mechanical, and optical properties of nanomaterials have advanced the development of semiconductor devices [1, 2], microelectromechanical systems (MEMSs) [3, 4], biomedical devices [5, 6], energy harvesters [7, 8], etc. Considering the dimensions of materials in nanoscale, nanomaterials can be classified in three categories. Specifically, zero-dimensional (0D) nanomaterials have all the dimensions within the nanoscale (i.e. 1010 ), photoswitching ability, and low power consumption due to its lower mobility compared with silicon transistors [6, 17]. 9.1.3

Boron Nitride

Composed of equal numbers of boron and nitrogen atoms, boron nitride (BN) shows a lattice structure similar to that of graphene, but totally insulated. The number of boron and nitrogen atoms can be tuned in the h-BN layers, but still in a honeycomb structure, as shown in Figure 9.1. The 2D h-BN layer is extremely chemically stable and insulated, which has been used as the dielectric layer of graphene transistors to enhance the device performance [6, 18]. Due to the improvement in device properties and device mechanics from the combination of plastic and soft substrates and 2D nanomaterials, flexible and stretchable nanoelectronics greatly benefit from the rapid development of 2D nanomaterials. Therefore, the flexible and stretchable nanotechnologies are expected to replace the traditional silicon-based low-cost commodity applications through an integrating nanosystem with not only comparable electronic performance but also mechanical flexibility and stretchability.

151

152

9 Flexible and Stretchable Devices from 2D Nanomaterials

9.2 Synthesis of Graphene In flexible and stretchable electronics, particularly, graphene attracts much more attention than do other 2D materials; therefore, we focus on the topic of graphene in this section. 9.2.1

Micromechanical Exfoliation

As a well-known scientific story, Geim and Novoselov used the Scotch tape to successfully get the superthin platelets of graphite in 2004 [1]. Because 3D graphite is composed of layers of graphene held together by a weak van der Waals force, monolayer graphene can be isolated using adhesive tape [2]. Figure 9.2a displays the graphene generated by this micromechanical exfoliation method [2, 21]. The critical reason for success was that monolayer graphene 0 9 Å 13 Å

20 nm

1 μm (a)

(b)

10 μm (c)

Figure 9.2 Graphene films grown by different methods. (a) AFM image of the graphene generated by the micromechanical exfoliation method. The height of the folded area (around 4 Å) indicates this is a single-layer graphite. Source: Reproduced with permission from Geim and Novoselov [2]. Copyright 2007, Springer Nature. (b) Scanning tunneling microscope (STM) topographs (0.8 V sample bias, 100 pA) of nominally 1 ML epitaxial graphene on SiC(0001) showing large flat regions. Source: Reproduced with permission from de Heer et al. [19]. Copyright 2007, Elsevier. (c) Scanning electron microscope (SEM) image of graphene on silicon/SiO2 substrates after spin coating from hydrazine dispersions. Source: Reproduced with permission from Tung et al. [20]. Copyright 2009, Springer Nature.

9.2 Synthesis of Graphene

became visible when placed on the silicon wafer with a special thickness of SiO2 , which helped researchers find out the single-layer graphene [2]. After that, graphene was found to have a clear signature in Raman microscopy, which increased the speed of thickness inspection. Micromechanical exfoliation is an effective method to generate graphene with excellent electrical properties used for fundamental studies. In addition, many other single-layer 2D nanomaterial films can be fabricated using this method, such as TMDs and black phosphorus (BP). However, this method has not yet shown the ability to generate graphene with a large area. And, the thickness of the films is difficult to control precisely. Therefore, it is of low yield and not suitable for mass production [11]. 9.2.2

Epitaxial Growth

Epitaxial growth provides an approach for producing graphene on a large scale with high quality. Heer et al. from the Georgia Institute of Technology used SiC wafer as the base material, because Si can be sublimated at high temperature (∼1300–1600 ∘ C) in ultrahigh vacuum or atmospheric environment, and still leave carbon on the surface of the SiC [19], as shown in Figure 9.2b. This method can generate wafer-size graphene with a mobility of ∼2000 cm2 /V/s [22]. Although SiC can work as the substrate of the graphene electronic devices, it is rigid, expensive, limited in size, and difficult to transfer graphene to other substrates. Therefore, this SiC method is not suitable for flexible and stretchable applications. In 2009, Ruoff’s group at the University of Texas at Austin successfully grew graphene and few-layer graphene on metal surfaces such as Ni and Cu using chemical vapor deposition (CVD) of hydrocarbons [23]. This method can produce graphene in large scale and at low cost. Recently, different carbon sources (such as CH4 and C2 H2 ) and different solid substrates, including polymer (polystyrene, polyacrylonitrile, polymethyl methacrylate [PMMA]), and metal catalysts (Ni, Cu, Pd, Ru, Ir, and alloys) have been used to grow graphene using the CVD method [23]. Graphene grown using this method also exhibits high quality; the mobility typically ranges from ∼2000 to ∼4000 cm2 /V/s [11], which is lower than that of some graphene flakes carefully made and chosen from micromechanical exfoliation, but still meaningful in practical applications when successfully transferred to desirable substrates. 9.2.3

Chemical Exfoliation

Another method is chemical exfoliation. Graphene oxide (GO) can be obtained by dispersing graphite oxide into several polar solvents, such as dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), and tetrahydrofuran (THF) and then treated using sonication [24]. Because the oxide process disturbs the network structure of the graphene, the GO is electrically insulating [25]. Therefore, one more step is need to “remove the oxygen” and enhance the conductive property. In 2008, Tung et al. from University of California, Los Angeles (UCLA) fabricated reduced graphene oxides (rGOs) by treating GO films with reductants

153

154

9 Flexible and Stretchable Devices from 2D Nanomaterials

(such as hydrazine, dimethylhydrazine, hydroquinone), which showed better electrical property [20], as displayed in Figure 9.2c. Although chemical exfoliation is a solution-based method, of low cost, and easy to manipulate, the graphene derived from reduction of GO can contained some oxygen. Therefore, its electrical properties and transparency are lower than the graphene grown by CVD. The same method can be used to exfoliate other 2D nanomaterials, such as h-BN and TMDs. In the solution mentioned, sonication can only break the weak van der Waals force between layers and then isolate nanosheets.

9.3 Graphene Transfer Graphene can be grown on various metal surfaces on a large scale by epitaxial growth methods, but this method can realize practical significance only when as-grown graphene can be successfully transferred to other substrates without damage [11]. Successful transfer of graphene to desired substrates is an indispensable step for the industrial applications. Researchers developed a lot of feasible solutions in the past 15 years. 9.3.1

Mechanical Exfoliation

John A. Rogers’s group at the University of Illinois at Urbana–Champaign (UIUC) used a bilayer film of gold and polyimide (PI) to successfully transfer graphene grown on the SiC surface to target substrates [26]. The 100-nm gold was first deposited on graphene with electrode beam evaporation, followed by spin coating an amic acid solution and baking at 110 ∘ C to generate a PI/Au/graphene layer-by-layer structure. This structure can be lifted and transferred to target substrates. The whole process is displayed in Figure 9.3a. The transferred graphene was still good enough to fabricate a back-gated FET with hole mobilities of ∼100 cm2 /V/s. This group then replaced Au with Pd and employed the repeated peeling strategy to improve the quality of the transferred graphene [30]. 9.3.2

Polymer-Assisted Transfer

The metal substrates used to grow graphene are typically unwanted after the CVD process, so the transfer step needs to remove the metal layers. Cu and Ni are the most popular substrates and can be etched away in solutions of Fe(NO3 )3 , FeCl3 , and (NH4 )2 S2 O8 [31]. However, after removal of metal substrate in solutions, the ultrathin graphene is easy to tear, wrinkle, and break. Therefore, some researchers tried to use polymers as support to ensure the successful transfer. In 2009, Kim et al. from Sungkyunkwan University used polydimethylsiloxane (PDMS) as the supporting material to assist the transfer process [27]. Because the surface free energy of PDMS is low, when a substance maintained by the PDMS surface contacts with the target substrate, the substance prefers to adhere to the

9.3 Graphene Transfer

(a) Deposit Au/Pl transfer layer

Graphene on SiC

(b)

Peel graphene/Au/ Transfer graphene/Au/Pl Pl film to SiO2/Si

Pattetned Ni layer (300 nm)

Ni/C layer

CH4/H2/Ar

Ar

~1000 °C

Cooling ~RT

Ni Si

Etch Pl and Au layers

SiO2 (300 nm)

PDMS/graphene/Ni /SiO2/Si

PDMS/graphene

Graphene on a substrate

FeCl3(aq) or acids Ni-layer etching

Stamping

(c)

Graphene on polymer support

Polymer support

Graphene on Cu foil

Cu etchant

Released polymer support

Target substrate Graphene on target

(d) Cu sputter and CVD growth

Cu etching

Capillary bridges

Graphene

PMMA Graphene

PMMA Graphene

Cu

Bubbles

Water

SiO2

SiO2

SiO2

SiO2

Graphene SiO2

Si

Si

Si

Si

Si

N2 plasma

Baking

Figure 9.3 Schematics of different graphene transfer methods. (a) Mechanical exfoliation method for transferring graphene grown on the SiC wafer to target substrates. Source: Reproduced with permission from Unarunotai et al. [26]. Copyright 2009, American Chemical Society. (b) Synthesis and transfer of graphene grown on Ni film with the assistance of PDMS. Source: Reproduced with permission from Kim et al. [27]. Copyright 2009, Springer Nature. (c) Roll-to-roll production and transfer of graphene. This process includes adhesion of polymer supports, Cu etching, and dry transfer to other substrates. Source: Reproduced with permission from [28]. Copyright 2010, Nature Publishing Group. (d) Face-to-face graphene transfer process, including N2 plasma treating, CVD growth of graphene, Cu etching, capillary bridge formation, and removal of water and PMMA. Source: Reproduced with permission from [29]. Copyright 2014, Nature Publishing Group.

target surface and release from the PDMS surface; thus, PDMS can work as a soft stamp [32]. As demonstrated in Figure 9.3b, after etching the Ni substrate, the freed graphene on the PDMS surface can be transferred to other substrates, such as SiO2 /Si. Except for patterning graphene on the growing substrate as mentioned, Nuckolls’s group at Columbia University used the patterned PDMS stamp to achieve the desired graphene pattern on the transferred substrates [33]. The single-layer graphene on the Cu surface was first laminated onto a micropatterned PDMS stamp. After etching the copper, the patterned graphene film was transferred to the target substrates through pressing and the PDMS mold was gently removed. PMMA also can be used as the support for graphene transfer [34]. PMMA is first spin coated on the surface of the graphene grown on the substrate, and then the substrate is etched to the target substrate. The PMMA layer is finally removed using acetone. Different from PDMS maintaining graphene by weak van

155

156

9 Flexible and Stretchable Devices from 2D Nanomaterials

der Waals forces, the PMMA coating can generate covalent bonds with graphene. Because PMMA holds the graphene rigidly, it may cause cracks and damage the graphene film. Some researchers applied several methods to modify the process, such as double coating [34] and thermal annealing [35]. 9.3.3

Roll-to-Roll Transfer

Roll-to-roll transfer is a general method used in the paper and metal rolling industry, which is easy to produce in large scale and at low cost. In 2010, Bae et al. from Sungkyunkwan University reported the roll-to-roll production and transfer of monolayer graphene with a 30-inch diagonal dimension [28]. In this process, graphene grown on copper foil is first attached with thermal release tapes by passing through two rollers. After removal of Cu by (NH4 )2 S2 O8 solution, the graphene is transferred on to the polymer film. To transfer graphene to the desired substrate, the polymer with graphene was heated to remove the adhesiveness of the thermal release tapes when it went through the two rollers. The whole process can be seen in Figure 9.3c. A doped four-layer graphene film fabricated by this method and layer-by-layer stacking exhibited conductivity of ∼30 Ω/sq and ∼90% optical transmittance. However, when the transfer substrate is too rigid, the shear stress may cause damage to the graphene, and the thermal release tape may leave a residue. The hot pressing method is an alternative method reported by Kang et al. from Sungkyunkwan University [36], which used a large hot plate to replace the rollers to avoid the shear stress. To realize the industrial application of graphene, roll-to-roll and hot pressing are two promising approaches. 9.3.4

“Transfer-Free” Method

The transfer process may cause damage to the graphene film as mentioned earlier, such as cracks, holes, and wrinkles, which drove researchers to develop the transfer-free method [11, 29]. Directly growing graphene on insulating substrates, like SiO2 /Si wafer, is useful, but it is still in the preliminary stage (only can fabricate an island graphene particle on a small scale) [37]. Growing graphene on a Cu-deposited insulating surface and then evaporating the Cu was a promising approach. However, the high temperature for evaporating Cu greatly hurt the quality of the graphene [38]. Loh’s team from the National University of Singapore developed a face-to-face transfer method which can complete the growth and transfer of graphene on the same wafer-in-wafer scale [29] (Figure 9.3d). The critical step of this method was to first use nitrogen plasma to treat the SiO2 /Si surface and then sputtered Cu for graphene growth. During the etching of Cu, the voids and channels according to the dissolution of Cu can generate a capillary force, which works as capillary bridges between the graphene film and the substrate. After etching, the PMMA/graphene film firmly attached to the underlying wafer. After the transfer, the mobilities of electrons and holes were both ∼3800 cm2 /V/s, which was comparable to the thermal CVD graphene [28]. This method can reduce the defect density to a lower level compared with other transfer methods.

9.4 Applications of Graphene

9.4 Applications of Graphene According to its exceptional properties, including unusual electronic properties, high mobility, and excellent optical transparency and maximum mechanical strength, graphene has been widely used in various fields. Here, we choose two major applications of graphene as examples.

9.4.1

Flexible and Stretchable Transparent Electrodes

With its high conductivity, high mobility, and high optical transmittance, graphene is a superstrong candidate for the new-generation transparent conductive electrodes. Single-layer graphene grown using the CVD method has great conductivity (125 Ω/sq) and transparency (97.4%), which is similar to the most commercially used transparent electrode, indium tin oxide (ITO) with a conductivity of ∼100 Ω/sq and transparency of ∼90%. Although graphene has played an important role in flexible electronics, its stretchable application was limited by its mechanical properties [28]. The carbon network will crack under ∼5% strain because of the high Young’s modulus and in-plane stiffness of graphene [39]. The transferred graphene grown using CVD on the PDMS surface reported by Kim et al. from Sungkyunkwan University loses its conductivity when the strain increases to 6% [27]. According to theoretical analysis, the out-of-plane crumping can soften the graphene, reduce its in-plane stiffness from 340 to 20–100 N/m at room temperature [40]. Multilayer graphene has been reported to exhibit better stretchability due to the strain relaxation effect achieved by sliding among the graphene layers when strain applied [39, 41]. According to Won et al. from the Korea Advanced Institute of Science and Technology (KAIST), the double-layer graphene exhibited a high stretchability of 36.2%, while the resistance change was 13 times smaller than that of single-layer graphene [41]. However, for wearable devices, this stretchability level is still significantly lower than the minimum required level (>50%) [42]. In 2013, Park’s group at the Ulsan National Institute of Science and Technology (UNIST) introduced a hybrid structure to enhance the conductivity and stretchability by spin coating AgNW network on the graphene film [43]. The electrode exhibited a low resistance of 33 Ω/sq at the transmittance of 94%, as shown in Figure 9.4a. The AgNW network can help maintain the conductivity of the electrode, which shows negligible resistance change even when stretched to 100% (Figure 9.4b). Furthermore, a special stretchable structure is another method. McEuen’s group at Cornell University developed a kirigami approach to pattern graphene to achieve highly stretchable graphene-based devices in 2015 [44]. The transistor gated by liquid electrolyte can be stretched to 240%, while the conductivity of graphene did not show notable change, as shown in Figure 9.4c,d. Although the performance of this method is excellent, the manipulation process is much more complicated. Recently, Zhenan Bao’s group at Stanford University used the multilayer graphene and intercalated graphene scrolls formed during the transfer process

157

9 Flexible and Stretchable Devices from 2D Nanomaterials

100

(b) ΔR/R0 (%)

(a)

50 0 –50

–100

0

20

40 60 Strain (%)

80

100

(d) 7

(c)

6 5 G (μS)

158

240% strain

4 3 2 1

VSD = 100 mV

0 0.0

0.1

0.2

0% strain

0.3 0.4 VLG (V)

0.5

0.6 0.7

Figure 9.4 Stretchable and transparent graphene-based electrodes. (a) Photograph of the hybrid film of graphene and AgNWs on PDMS substrate. Scale bar: 5 mm. Source: Reproduced with permission from Lee et al. [43]. Copyright 2013, American Chemical Society. (b) Relationship between relative resistance change of the film and strain. Source: Reproduced with permission from Lee et al. [43]. Copyright 2013, American Chemical Society. (c) Graphene in-plane kirigami spring without strain (upside) and with 70% strain (down). Scale bar: 10 μm. Source: Reproduced with permission from Blees et al. [44]. Copyright 2015, Springer Nature. (d) Comparison of the electrical properties of the stretchable transistor before stretching (blue) and when stretched to 240% (orange). Scale bar: 10 μm. Source: Reproduced with permission from Blees et al. [44]. Copyright 2015, Springer Nature.

to bridge the cracks on the graphene film, which could retain 65% original conductivity when stretched to 100% [39]. The all-carbon transistor using this graphene as electrodes exhibited a transmittance of >90% and retained 60% output when it stretched to 120%.

9.4.2

Nanogenerators

Graphene is suitable for integration with piezoelectric materials, including 1D piezoelectric nanowires (ZnO, GaN, and CdS) and 2D piezoelectric film (PbZr 0.52 Ti0.48 O3 (PZT), and poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE))), to get flexible and transparent nanogenerators, going by its excellent optical and mechanical properties [11]. In 2010, Kim’s group at Sungkyunkwan University successfully grew ZnO nanorods on the graphene surface (grown using CVD) through a low-temperature solution process with no damage [45], as shown in Figure 9.5a. Due to the high carrier mobility

(a) Si

Graphene Ni

(b) Transfer

Graphene grown on Cu

Polymer ZnO nanorod growth

Graphene transfer

Cu etching

Transmittance (%)

100

ZnO nanorods

Integration

80 60 40 20 0 400

(c)

PVDF(TrFE) coating

Releasing

Pressed

R e–

R

500 600 Wavelength (cm–1)

PDMS

Conformal TENGs

Graphene PET

Released R e–

5 μm

PDMS

(d)

Graphene Pressing

700

Skin Skin replica

R

PET thickness (μm) Graphene

EVA/PET

Graphene

PDMS

3

0

4

2 PDMS (1.5 μm)

2

0 –5

–10

0.4 0.2 0

tPDMS tsubstrate

PET

1 Conformal

0

Non-conformal

–0.2 –0.4

–15 0

Uinterface

0.6

5

Current (μA)

Voltage (V)

10

0.5 1.0 1.5 2.0 2.5 3.0

Time (s)

–1 0

0.5

1.0

1.5

2.0

Time (s)

2.5 3.0

2

4

Total thickness (μm)

rmal Confo s TENG

6

Figure 9.5 Graphene-based nanogenerators. (a) Piezoelectric nanogenerator with the structure of ZnO nanorods on the graphene surface. Source: Reproduced with permission from Choi et al. [45]. Copyright 2010, John Wiley & Sons. (b) Fabrication process of a graphene/P(VDF-TrFE)/graphene multilayer film. The transmission spectra of monolayer graphene (red line, 97%) and graphene/P(VDF-TrFE) (black line, 88%). The SEM image of the P(VDF-TrFE) surface. Source: Reproduced with permission from Bae et al. [46]. Copyright 2013, American Chemical Society. (c) Schematic and the output performance of the graphene/EVA/PET-based TENG. Source: Reproduced with permission from Chandrashekar et al. [47]. Copyright 2015, John Wiley & Sons. (d) Schematic, performance, and photographs of the conformal TENG consisting of PDMS, graphene, and PET. Source: Reproduced with permission from Chu et al. [48]. Copyright 2016, Elsevier.

160

9 Flexible and Stretchable Devices from 2D Nanomaterials

and the Schottky contact with ZnO nanorods of the graphene electrode, this nanogenerator realized superior performance [49]. Using polyethylene naphthalate (PEN) as the substrate, this device could be fully rollable and transparent (75% transmittance). However, the mechanical durability and flexibility of this device were still limited by the nature of the inorganic piezoelectric materials [46]. Therefore, a polymer piezoelectric material, P(VDF-TrFE), was chosen as an alternative material due to its high transparency and flexibility. Bae et al. from Sungkyunkwan University spin coated P(VDF-TrFE) solution on a graphene film grown using CVD and then transferred another graphene layer on top of the P(VDF-TrFE) film to fabricate the flexible and transparent nanogenerator in large scale (Figure 9.5b), which also could work as an acoustic actuator [46]. The output of this nanogenerator (∼3 V and ∼0.37 μA/cm2 ) was comparable to that of the ZnO- and PZT-based nanogenerators. Recently, electrospinning technology was introduced to fabricate functional nanofibers for wearable electronics at low cost and in high yield. Mandal’s group at Jadavpur University constructed a wearable nanogenerator by electrospun Ce3+ -doped PVDF/graphene composite nanofibers, which also could work as a highly sensitive pressure sensor [50]. Because of the ability of storing the electric charges form electrostatic induction, graphene is suitable to work as the electrode and triboelectric materials of triboelectric nanogenerators (TENGs) [47–49]. Our group at Peking University demonstrated a transparent and flexible TENG fabricated using roll-to-roll graphene growth and transfer process [47], as displayed in Figure 9.5c. The upper layer of the TENG was a graphene/ethylene-vinylacetate (EVA)/polyethylene terephthalate (PET) structure, while the lower layer was PDMS with microstructure. Graphene worked as the friction material of the upper layer and the electrode of the lower layer. The maximum output of this TENG was 22 V and 0.075 μA/cm2 . Toward supplying power for wearable systems in the long term, Ahn’s group at Yonsei University developed a conformal graphene-based TENG, as demonstrated in Figure 9.5d, which could directly form on human skin because its thickness was only 2.4 μm [48]. After plasma and fluorinate treatment, the output of this TENG increased from 3.4 to 130 μW, which could assist to transmit Morse code to a smartphone.

9.5 Summary This chapter introduced the representative members of 2D nanomaterials and their synthesis and transfer processes. The discussion was mainly focused on graphene due to its outstanding properties and huge application potential in the fields of flexible and stretchable electronics. Various applications have been developed to exhibit the enhancement from the incorporation of 2D nanomaterials, such as transparent electrodes and nanogenerators. However, there are still many critical obstructs that need to be addressed to realize the particular application of 2D nanomaterial-based devices.

References

The first is to fabricate 2D nanomaterials with high performance and uniformity in large scale and at low cost. And then transferring the materials to the desired surface with as less defect as possible. Compared with 1D nanomaterials, the structure of 2D nanomaterials is much easier to break when strain increases, which generates an additional requirement for material modification and dexterous pattern design for stretchable electronics. Furthermore, the extremely mechanical and chemical stability and strong adhesion with the target surface are also indispensable. Although many problems exist, graphene and other 2D nanomaterials are still one of the most promising materials for the next-generation stretchable and wearable electronics with higher performance. 2D nanomaterial-based electronics have suffered explosive growth in the past 15 years since the first graphene was isolated in 2004, and will be greatly pushed forward by resolving these problems.

Abbreviations AFM BN BP CVD DMF FET GO ITO LDHs MOFs NMP PEN PET PI PMMA P(VDF-TrFE) QHE rGO SEM STM THF TMDs TMOs

atomic force microscopy boron nitride black phosphorus chemical vapor deposition dimethylformamide field-effect transistor graphene oxide indium tin oxide layered double hydroxides metal-organic frameworks N-methyl-2-pyrrolidone polyethylene naphthalate polyethylene terephthalate polyimide polymethyl methacrylate poly(vinylidene fluoride-co-trifluoroethylene) quantum Hall effect reduced graphene oxide scanning electron microscopy scanning tunneling microscopy tetrahydrofuran transition-metal dichalcogenides transition-metal oxides

References 1 Novoselov, K.S., Geim, A.K., Morozov, S.V. et al. (2004). Electric field effect in

atomically thin carbon films. Science 306 (5696): 666–669.

161

162

9 Flexible and Stretchable Devices from 2D Nanomaterials

2 Geim, A.K. and Novoselov, K.S. (2007). The rise of graphene. Nature Materi-

als 6 (3): 183–191. 3 Zhang, H. (2015). Ultrathin two-dimensional nanomaterials. ACS Nano 9 (10):

9451–9469. 4 Zhuang, X., Mai, Y., Wu, D. et al. (2015). Two-dimensional soft nanomateri-

als: a fascinating world of materials. Advanced Materials 27 (3): 403–427. 5 Yu, X., Cheng, H., Zhang, M. et al. (2017). Graphene-based smart materials.

Nature Reviews Materials 2: 17046. 6 Xu, M., Liang, T., Shi, M., and Chen, H. (2013). Graphene-like

two-dimensional materials. Chemical Reviews 113 (5): 3766–3798. 7 Sun, Z., Liao, T., Dou, Y. et al. (2014). Generalized self-assembly of scalable

8 9

10

11 12

13 14

15 16

17

18 19 20

two-dimensional transition metal oxide nanosheets. Nature Communications 5: 3813. https://doi.org/10.1038/ncomms4813. Osada, M. and Sasaki, T. (2009). Exfoliated oxide nanosheets: new solution to nanoelectronics. Journal of Materials Chemistry 19 (17): 2503–2511. Wang, Q. and Ohare, D. (2012). Recent advances in the synthesis and application of layered double hydroxide (LDH) nanosheets. Chemical Reviews 112 (7): 4124–4155. Rodenas, T., Luz, I., Prieto, G. et al. (2015). Metal-organic framework nanosheets in polymer composite materials for gas separation. Nature Materials 14 (1): 48–55. Zhu, Y., Murali, S., Cai, W. et al. (2010). Graphene and graphene oxide: synthesis, properties, and applications. Advanced Materials 22 (35): 3906–3924. Zhang, Y., Tan, Y.W., Stormer, H.L., and Kim, P. (2005). Experimental observation of the quantum Hall effect and Berry’s phase in graphene. Nature 438 (7065): 201–204. Nair, R.R., Blake, P., Grigorenko, A.N. et al. (2008). Fine structure constant defines visual transparency of graphene. Science 320 (5881): 1308. Lee, C., Wei, X., Kysar, J.W., and Hone, J. (2008). Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 321 (5887): 385–388. Stoller, M.D., Park, S., Yanwu, Z. et al. (2008). Graphene-based ultracapacitors. Nano Letters 8 (10): 3498–3502. Chimene, D., Alge, D.L., and Gaharwar, A.K. (2015). Two-dimensional nanomaterials for biomedical applications: emerging trends and future prospects. Advanced Materials 27 (45): 7261–7284. Mak, K.F., Lee, C., Hone, J. et al. (2010). Atomically thin MoS2 : a new direct-gap semiconductor. Physical Review Letters 105 (13): 136805. https:// doi.org/10.1103/PhysRevLett.105.136805. Dean, C.R., Young, A.F., Meric, I. et al. (2010). Boron nitride substrates for high-quality graphene electronics. Nature Nanotechnology 5 (10): 722–726. de Heer, W.A., Berger, C., Wu, X. et al. (2007). Epitaxial graphene. Solid State Communications 143 (1–2): 92–100. Tung, V.C., Allen, M.J., Yang, Y., and Kaner, R.B. (2009). High-throughput solution processing of large-scale graphene. Nature Nanotechnology 4 (1): 25–29.

References

21 Chung, C., Kim, Y.K., Shin, D. et al. (2013). Biomedical applications of

22

23

24

25 26

27

28

29 30

31 32

33

34

35

36

graphene and graphene oxide. Accounts of Chemical Research 46 (10): 2211–2224. Emtsev, K.V., Bostwick, A., Horn, K. et al. (2009). Towards wafer-size graphene layers by atmospheric pressure graphitization of silicon carbide. Nature Materials 8 (3): 203–207. Li, X., Cai, W., Colombo, L., and Ruoff, R. (2009). Evolution of graphene growth on Ni and Cu by carbon isotope labeling. Nano Letters 12 (12): 4268–4272. Yang, D., Velamakanni, A., Bozoklu, G. et al. (2009). Chemical analysis of graphene oxide films after heat and chemical treatments by X-ray photoelectron and Micro-Raman spectroscopy. Carbon 47 (1): 145–152. Park, S. and Ruoff, R.S. (2009). Chemical methods for the production of graphenes. Nature Nanotechnology 4 (4): 217–224. Unarunotai, S., Murata, Y., Chialvo, C.E. et al. (2009). Transfer of graphene layers grown on SiC wafers to other substrates and their integration into field effect transistors. Applied Physics Letters 95: 202101. https://doi.org/10.1063/1 .3263942. Kim, K.S., Zhao, Y., Jang, H. et al. (2009). Large-scale pattern growth of graphene films for stretchable transparent electrodes. Nature 457 (7230): 706–710. Bae, S., Kim, H., Lee, Y. et al. (2010). Roll-to-roll production of 30-inch graphene films for transparent electrodes. Nature Nanotechnology 5 (8): 574–578. Gao, L., Ni, G.X., Liu, Y. et al. (2014). Face-to-face transfer of wafer-scale graphene films. Nature 505 (7482): 190–194. Unarunotai, S., Koepke, J.C., Tsai, C.L. et al. (2010). Layer-by-layer transfer of multiple, large area sheets of graphene grown in multilayer stacks on a single SiC wafer. ACS Nano 4 (10): 5591–5598. Kang, J., Shin, D., Bae, S., and Hong, B.H. (2012). Graphene transfer: key for applications. Nanoscale 4 (18): 5527–5537. Gates, B.D., Xu, Q., Stewart, M. et al. (2005). New approaches to nanofabrication: molding, printing, and other techniques. Chemical Reviews 105 (4): 1171–1196. Kang, S.J., Kim, B., Kim, K.S. et al. (2011). Inking elastomeric stamps with micro-patterned, single layer graphene to create high-performance OFETs. Advanced Materials 23 (31): 3531–3535. Li, X., Zhu, Y., Cai, W. et al. (2009). Transfer of large-area graphene films for high-performance transparent conductive electrodes. Nano Letters 9 (12): 4359–4363. Cheng, Z., Zhou, Q., Wang, C. et al. (2011). Toward intrinsic graphene surfaces: a systematic study on thermal annealing and wet-chemical treatment of SiO2 -supported graphene devices. Nano Letters 11 (2): 767–771. Kang, J., Hwang, S., Kim, J.H. et al. (2012). Efficient transfer of large-area graphene films onto rigid substrates by hot pressing. ACS Nano 6 (6): 5360–5365.

163

164

9 Flexible and Stretchable Devices from 2D Nanomaterials

37 Chen, J., Wen, Y., Guo, Y. et al. (2011). Oxygen-aided synthesis of poly-

38 39 40

41 42

43

44 45

46 47

48

49 50

crystalline graphene on silicon dioxide substrates. Journal of the American Chemical Society 133 (44): 17548–17551. Ismach, A., Druzgalski, C., Penwell, S. et al. (2010). Direct chemical vapor deposition of graphene on dielectric surfaces. Nano Letters 10 (5): 1542–1548. Liu, N., Chortos, A., Lei, T. et al. (2017). Ultratransparent and stretchable graphene electrodes. Science Advances 3 (9): e1700159. Nicholl, R.J.T., Conley, H.J., Lavrik, N.V. et al. (2015). The effect of intrinsic crumpling on the mechanics of free-standing graphene. Nature Communications 6: 8789. https://doi.org/10.1038/ncomms9789. Won, S., Kim, J.H., Hwangbo, Y. et al. (2014). Double-layer CVD graphene as stretchable transparent electrodes. Nanoscale 6 (11): 6057–6064. Zhao, S., Li, J., Cao, D. et al. (2017). Recent advancements in flexible and stretchable electrodes for electromechanical sensors: strategies, materials, and features. ACS Applied Materials & Interfaces 9 (14): 12147–12164. Lee, M.S., Lee, K., Kim, S.Y. et al. (2013). High-performance, transparent, and stretchable electrodes using graphene-metal nanowire hybrid structures. Nano Letters 13 (6): 2814–2821. Blees, M.K., Barnard, A.W., Rose, P.A. et al. (2015). Graphene kirigami. Nature 524 (7564): 204–207. Choi, D., Choi, M.Y., Choi, W.M. et al. (2010). Fully rollable transparent nanogenerators based on graphene electrodes. Advanced Materials 22 (19): 2187–2192. Bae, S.H., Kahya, O., Sharma, B.K. et al. (2013). Graphene-P(VDF-TrFE) multilayer film for flexible applications. ACS Nano 7 (4): 3130–3138. Chandrashekar, B.N., Deng, B., Smitha, A.S. et al. (2015). Roll-to-roll green transfer of CVD graphene onto plastic for a transparent and flexible triboelectric nanogenerator. Advanced Materials 27 (35): 5210–5216. Chu, H., Jang, H., Lee, Y. et al. (2016). Conformal, graphene-based triboelectric nanogenerator for self-powered wearable electronics. Nano Energy 27: 298–305. Jang, H., Park, Y.J., Chen, X. et al. (2016). Graphene-based flexible and stretchable electronics. Advanced Materials 28 (22): 4184–4202. Garain, S., Jana, S., Sinha, T.K., and Mandal, D. (2016). Design of in situ poled Ce3+ -doped electrospun PVDF/graphene composite nanofibers for fabrication of nanopressure sensor and ultrasensitive acoustic nanogenerator. ACS Applied Materials & Interfaces 8 (7): 4532–4540.

165

10 Flexible and Stretchable Devices from Unconventional 3D Structural Design Hangbo Zhao and Mengdi Han Northwestern University, Center for Bio-Integrated Electronics, Evanston, IL, USA

Many biological systems in nature are three-dimensional (3D), such as plant roots, animal cells, human vascular networks, etc. The 3D feature provides essential functions in various activities including physicochemical reactions and interactions with the environment. Biomimetic recreation and engineering of 3D architectures inspired by nature has been a topic of increasing interest in recent decades, especially in the area of flexible and stretchable devices. 3D structures on such devices enable new and more complex functionalities as compared to 1D or 2D counterparts. Moreover, the flexibility and stretchability of the substrate provide a unique means of forming, controlling, and tuning 3D structures on it. The interplay between mechanics and structures provides plenty of space and fascinating opportunities for the design and fabrication of unconventional 3D structures with novel functionalities. In this chapter, various approaches to forming unconventional 3D structures are first introduced, from basic buckling of ribbons, membranes, and non-coplanar bridge-island structures, to more complex deterministic assembly. A series of strategies for extending the level of control in deterministic assembly is discussed, followed by a collection of exemplary flexible and stretchable electronic and optical devices fabricated using 3D deterministic assembly methods.

10.1 Stretchable 3D Ribbon and Membrane Structures Formed by Basic Buckling Mechanical buckling of thin films under compressive stress provides a powerful route to forming 3D structures. The stretchability of elastomeric substrates is utilized as a platform for buckling micro-/nanostructures. In this section, the fabrication of 3D ribbon and membrane structures based on simple substrate buckling is introduced.

Flexible and Stretchable Triboelectric Nanogenerator Devices: Toward Self-powered Systems, First Edition. Edited by Mengdi Han, Xiaosheng Zhang, and Haixia Zhang. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

10 Flexible and Stretchable Devices from Unconventional 3D Structural Design

10.1.1

3D Nanoribbons

The John A. Rogers group from the University of Illinois Urbana-Champaign exploited compressive buckling to convert nanoribbons into 3D, wavy structures. The process starts with fabricating patterned thin, flat, single-crystal silicon ribbons on a mother wafer [1]. Then, the Si elements are bound to a pre-strained elastomeric substrate. After peeling off the elastomer and releasing the pre-strain, the silicon is transformed into well-controlled, periodic wavy ribbons that are stretchable. Large-area, uniform-array, wavy nanoribbons can be fabricated using this method. The nanoribbons are highly stretchable, with a maximum strain over 10% for both compression and stretching. A single-crystal silicon p–n diode is showcased as an example of applications of such wavy nanoribbons. The same principle of buckling nanoribbons could be applied with spatial control over adhesion sites added to create well-controlled local displacement of the ribbons [2]. The fabrication process is schematically illustrated in Figure 10.1a. (a)

(b) SU8 5 Gold Quartz (i)

Stretch

Photolithography and gold etching (vi)

Compress

(c)

Cast PDMS

UVO mask 10 μm

Wact

Win (ii)

Laminate UVO mask against stretched PDMS UV light (v)

Release PDMS

Air panel

GaAs

L + ΔL

(iii)

PDMS Expose to UV light and removal of mask Activated surface

100 μm

L + ΔL (iv)

Bond GsAs ribbons to pre-strained PDMS

(d)

(e) 6.0

GaAs

Au

4.0

PDMS

V

Current (μA)

166

A

GaAs Infrared irradiation

2.0 0.0

Stretching degree 0% 14.4% 34.3% 44.4% 51.4%

–2.0 –4.0 With light illumination –6.0 –0.4

Au/GaAs

GaAs

Au/GaAs

–0.2

0.0

0.2

0.4

Bias voltage (V)

Figure 10.1 Buckling of semiconductor nanoribbons for stretchable electronics. (a) Schematic illustration of the fabrication process of 3D buckled nanoribbons. (b) Schematic of bucked nanoribbons in response to stretching and compression. (c) SEM image of GaAs nanoribbons formed. (d) Schematic and optical images of a stretchable photodetector (PD) from nanoribbons. (e) I–V characteristics of fabricated PDs under different stretching degrees. Source: Reproduced with permission from Sun et al. [2]. Copyright 2006, Springer Nature.

10.2 Deterministic 3D Assembly

An ultraviolet ozone (UVO) mask is used to pattern the surface chemistry on a poly(dimethylsiloxane) (PDMS) substrate, where bonding between the Si or GaAs nanoribbons and PDMS substrate only occurs in lithographically defined regions. The buckled nanoribbons could be embedded in PDMS and undergo extremely high levels of stretching (up to ∼100%), compression (up to ∼25%), and bending (with curvature radius down to ∼5 mm), as shown in Figure 10.1b. Such stretchable ribbons could be used in the construction of metal–semiconductor–metal photodetectors, as shown in Figure 10.1c–e. The I–V characteristics of the photodetectors are mechanically tunable due to the change of the projected area of the buckled GaAs ribbons. 10.1.2

3D Nanomembranes

In addition to nanoribbons, nanomembranes could also be engineered into 3D configurations using buckling. As shown in Figure 10.2, a silicon thin film, initially on a mother wafer, is transferred to a biaxially pre-strained PDMS slab. After the release of the strain of PDMS, wavy Si nanomembranes are formed with controllable surface topologies. The formed wavy Si nanomembranes are stretchable, and yield various morphologies in response to different strains applied at different orientations, as shown in Figure 10.2b. These wavy membranes provide a useful path to biaxially stretchable devices. 10.1.3

3D Bridge-Island Structures

More complex structural configurations are achievable by combining ribbons with island-like 2D structures. Figure 10.3 shows an example of non-coplanar, bridge-island structures [4]. The ultrathin circuit mesh fabricated using standard semiconductor fabrication is bound to pre-stretched PDMS at the positions of the flat islands, via ozone surface treatment. Using this strategy, an array of complementary metal–oxide–semiconductor (CMOS) inverters is fabricated and encapsulated with PDMS (Figure 10.3b). The fabricated device offers extremely high stretchability, with stretching strain up to ∼140% and twisting pitch as tight as 90∘ in ∼1 cm (Figure 10.3c). The feature size of the stretchable circuit is comparable to conventional 2D devices, while no significant changes in electrical properties are observed owing to the high stretchability of the PDMS substrate and mechanics-guided structural design.

10.2 Deterministic 3D Assembly While the basic buckling approaches described in Section 10.1 are effective in transforming 2D patterns into 3D configurations, the accessible structures are limited to wavy ribbons, membranes, and combinations of them. In this section, a more complex 3D assembly approach is introduced, namely, deterministic 3D assembly. Deterministic 3D assembly combines lithographically

167

Fabricate thin Si membrane film

(ii)

(i)

(iii) Stretching strain

Mother wafer

εst = 0% Contact and transfer to pre-strained PDMS slab

L

L +Δ

L+Δ

100 μm

100 μm

100 μm

L

εst = +1.8%

Peel off the PDMS slab; flip over and release the strain of PDMS

L

L

εst = +3.8%

εst = 0%

(a)

(b)

Figure 10.2 Biaxially stretchable “wavy” nanomembranes. (a) Schematic illustration of the fabrication process. (b) Optical micrographs of wavy Si nanomembranes under different uniaxial strains at three different orientations. Source: Reproduced with permission from Choi et al. [3]. Copyright 2007, American Chemical Society.

10.2 Deterministic 3D Assembly

Fabricate ultrathin circuit mesh

300 μm

Wafer Liftoff; deposit Cr/SiO2

Deform/ release

200 μm

Transfer fo stretched PDMS

PDMS Encapsulate with PDMS (a)

(b)

1 mm

100 μm

Twist

Diag. stretch

Bend/stretch

(c)

Figure 10.3 Stretchable, noncoplanar electronics. (a) Schematic illustration of the fabrication process for circuits with bridge-island structures. (b) SEM images of an array of CMOS inverters fabricated using the method in (a). (c) Optical image of CMOS inverter arrays under large deformation including stretching, twisting, and bending. Source: Reproduced with permission from Kim et al. [4]. Copyright 2008, PNAS.

controlled layouts of 2D structures with patterned adhesion sites to the surface of pre-strained elastomer substrates to enable fast and scalable assembly of a broad range of 3D structures. 10.2.1

Basic Approach of Deterministic 3D Assembly

The basic approach of deterministic 3D assembly is described in detail in Refs. [5–7]. Briefly, the process starts with patterning 2D patterns (precursors) with lithographically defined bonding sites. Then, the 2D precursors are bound to a pre-stretched elastomer slab. Release of the pre-strain induces spatially dependent twisting and bending deformations, transforming the 2D patterns into 3D structures.

169

170

10 Flexible and Stretchable Devices from Unconventional 3D Structural Design

Form 2D precursor structures; stretch the assembly platform

2D

m

icr

o-

/n

an

os

tru

Fr

ct

ee

-s

ta

nd

in

g

Figure 10.4 Schematic illustration of the assembly approach for forming 3D conical coils of single-crystalline Si. Source: Reproduced with permission from Yan et al. [6]. Copyright 2017, Elsevier.

ur

es

sil

ico

ne

St

Transfer with selective bonding Bo nd in g sit es

re

tc

he

d

sil

ico

ne

Re

Release the platform

le

as

ed

sil

ico

ne

As shown in Figure 10.4, 3D conical helices made from single-crystal silicon are fabricated, which were inaccessible 3D architectures previously. The mechanics and shape transformation process can be analyzed and precisely predicted by a finite element analysis (FEA) scheme, and excellent agreement is found between FEA predictions and experiments. A broad class of geometries are realized, including single and multiple helices, conical spirals, flowers, frameworks, and multilevel configurations. Moreover, this 3D assembly approach applies to various material compositions (inorganic semiconductors, polymers, metals, and heterogeneous combinations) and feature sizes (submicrometer to centimeter), as shown in Figure 10.5. This deterministic 3D assembly approach provides a powerful route to sophisticated classes of 3D structures and materials. The compatibility with the CMOS fabrication methods, and device-grade materials, creates many opportunities for achieving new 3D-structure-based electronic and other devices. Depending on the basic 3D assembly method described in the previous sections, a series of strategies and processing techniques have been developed to

5 μm

200 μm

20 cm

2 cm

5 mm

Sub-microscale

Microscale

5 μm

Millimeter scale

200 μm

Centimeter scale

Meter scale

2 cm

20 cm

5 mm

Blood cells (a) SU8

Pl

Ni

Au

SU8–Au

SU8–Si

SU8–Si nanodiscs

1 μm

(b)

Figure 10.5 3D structures fabricated using the deterministic 3D assembly approach with various characteristic dimensions and materials. (a) Starfishjellyfish-like structures. (b) 3D mesostructures of different materials. Source: Reproduced with permission from Zhang et al. [8]. Copyright 2017, Springer Nature.

172

10 Flexible and Stretchable Devices from Unconventional 3D Structural Design

extend the level of control in the deterministic 3D assembly process, which has greatly increased the space of attainable 3D structures. 10.2.2

3D Kirigami Structure in Micro-/Nanomembranes

The John A. Rogers group from the University of Illinois Urbana-Champaign introduced the concept of kirigami to the 3D assembly method, where 2D micro-/nanomembranes have strategically designed geometries and patterns of cuts [9]. Figure 10.6 lists several examples showing how the 2D silicon membranes with engineered kirigami cuts at precisely defined locations are transformed into 3D structures with strategically determined panel deformations. 10.2.3

Buckling Control Assisted by Stress and Strain Engineering

Engineering of stress and strain in the 2D precursors could also influence the 3D buckling process. Figure 10.7 shows two such engineering strategies. In Figure 10.7a, residual stress is introduced to the 2D precursor by patterning a 2D silicon membrane

z

y

Partially assembled 3D structure

3D structure

Experiment

εmax (%)

x

0.0

0.8

(a) y

z

x

(b) z x

y

εmax (%) 0.0

1.4

εmax (%) 0.0

1.4

(c) z y

εmax (%)

x 0.0

0.9

(d) y

z

x

εmax (%) 0.0

1.2

(e)

Figure 10.6 Examples of mechanically driven kirigami for deterministic 3D assembly from 2D micro/nanomembranes. (a) A square cuboid. (b,c) Membranes with first- and second-order cross-cuts. (d,e) Membranes with symmetric and antisymmetric cuts. Scale bars, 200 μm. Source: Reproduced with permission from Zhang et al. [9]. Copyright 2015, PNAS.

10.2 Deterministic 3D Assembly

Tensile FEM residual stress σresidual = +480 MPa

εpre = 80%

Analytical 2.16 mm

εrelease = 30%

60%

100%

Compressive residual stress FEM σresidual = –580 MPa

1.8 mm

Analytical

(a) z x

Transfer print sample onto PVA tape

y

old

te m

stra

Sub

Form engineered substrate on mold

εmax-ribbon (%)

trate

ubs

ds tche

Stre 0.00

s

rsor

2D

εmax-substrate

0.04

0.0

u prec

Form 2D Si/SU8 patterns by photolithography

0.69

Pre-stretch engineered substrate (60% strain); align sample onto substrate Laminate sample with selective bonding

s

εmax-ribbon (%)

ture

truc

3D s

0.0

2.8

s

trate 3D st

ubs

ed s

as Rele

ure ruct

εmax-substrate Release the substrate 0.62 to form 3D structures 0.00

(b)

Figure 10.7 Buckling control assisted by stress and strain engineering. (a) Stress engineering. (b) Strain engineering. SOI, silicon on insulator; PVA, poly(vinyl alcohol). Source: Reproduced with permission from Fu et al. [10] and Nan et al. [11].

2D SiNx film onto a Si substrate prior to the 2D precursors being buckled [10]. This residual stress, either tensile or compressive, induces different buckling mechanics and results in distinct 3D structures. Another strategy involves the engineering of elastomer substrates for spatially nonuniform strain in the substrates for buckling. As Figure 10.7b illustrates, the elastomer substrate, which is nonuniform in thickness via molding, induces spatially varying strain upon release of the pre-strain, thereby creating nonuniform 3D structures [11]. A similar strain-engineered method is also used to fabricate 3D microarchitectures based on growth [12]. 10.2.4

Multilayer 3D Structures

Instead of single-layered structures, multilayers of advanced materials could also be produced using releasable, multilayer 2D precursors. The schematic illustration of the process is shown in Figure 10.8a [13]. Multilayer 2D precursors are transferred to the stretched elastomer using a layer-by-layer transfer printing

173

SiO2

AZ 5214

p Stam

Layer-by-layer transfer

Fabricate 2D presursors SU8

Si SOI wafer

AZ 5214

PVA tape

Laminate samples on pre-stretched elastomer

Gap er

Dissolve PVA tape Stre

tch

ed

e

om last

Remove AZ 5214

(a)

εmax (%)

0.0

(b)

0.27

εmax (%)

0.0

4.33

(c)

Figure 10.8 Deterministic assembly of 3D mesostructures from multilayer 2D precursors. (a) Schematic illustration of the fabrication process. (b and c) Examples of resulting 3D multilayer structures. Scale bars, 400 μm. Source: Reproduced with permission from Yan et al. [13]. Copyright 2016, AAAS.

10.2 Deterministic 3D Assembly

technique. Complex 3D topologies such as dense architectures with nested layouts are constructed (Figure 10.8b,c). The multilayer feature also enhances the structural stability and drives the motion of extended features in those structures. 10.2.5

Freestanding 3D Structures

A potential disadvantage of the 3D assembly method discussed so far is that the elastomeric substrates, necessary for the assembly process, could impose engineering constraints to the system as the 3D structures are mechanically tethered to the substrates. Several techniques have been developed to bypass this limitation in order to fabricate freestanding 3D structures, including interfacial photopolymerization and wax-assisted transfer [14]. The process of interfacial photopolymerization is schematically shown in Figure 10.9a. A thin, sacrificial Al2 O3 layer is deposited between the bonding sites Assembly substrate

Al2O3

SU8 droplet

SU8 base

UV light Assemble 3D structures Drop cast SU8

Release

Backside exposure

(a) Assembled 3D structures

Wax Al2O3

Drop wax and release

Transfer print

Assembly substrate Dissolve wax

Target substrate Adhesive layer

Quartz substrate

Copper substrate

Silicon substrate

(b)

Figure 10.9 Freestanding 3D structures. (a) Interfacial photopolymerization and (b) wax-assisted transfer printing methods for fabricating freestanding 3D structures assembled by compressive buckling. Scale bars, 500 μm. Source: Reproduced with permission from Yan et al. [14]. Copyright 2017, PNAS.

175

176

10 Flexible and Stretchable Devices from Unconventional 3D Structural Design

and the elastomer for subsequent release from the substrate. To mechanically hold the 3D deformed shape, a photodefinable epoxy SU8 is cast and cured onto the 3D structure, followed by patterned backside exposure. Developing of the epoxy and removal of the Al2 O3 layer releases the 3D structures into freestanding objects, with a thin remaining base layer. For the second method shown in Figure 10.9b, transfer printing is employed, assisted by wax encapsulation to transfer the assembled 3D structures onto the target substrate. The wax holds the shape of the deformed structures and is dissolved upon transfer of the structures onto the adhesive layer on the target substrate. 10.2.6

Morphable 3D Structures by Multistable Buckling Mechanics

3D structures that can undergo reversible transformations are important in a broad range of applications such as microelectromechanical systems (MEMSs), biomedical devices, and microrobotics. A scheme in 3D assembly was developed recently to fabricate morphable 3D structures relying on sequential release of the pre-strain in elastomeric substrates [15]. The concept of the scheme and representative SEM images are illustrated in Figure 10.10a,b, respectively. Starting from a biaxially stretched flat state of Shape I

Shape II Experiment

εx = 0% εy = 0%

εx = 0% εy = 0%

εx = 100% εy = 100%

Flat state

εx = 50% εy = 50%

εx = 0% εy = 100% Crease

Intermediate state I

(a)

z x

Intermediate state II

z

z y

Micrometers (SU8 + silicon)

x

y

Tens of micrometers (silicon)

x

y

Millimeter (PET + Cu)

(b)

Figure 10.10 Morphable 3D structures by loading-path-controlled mechanical assembly. (a) Schematic illustration of the strategy with FEA predictions and experimental SEM images of formed 3D structures. (b) 2D shapes, FEA predictions, and corresponding experimental images of morphable 3D structures. Scale bars, 1 mm. Source: Reproduced with permission from Fu et al. [15]. Copyright 2018, Springer Nature.

10.3 Flexible and Stretchable Devices from 3D Assembly

the substrate, the pre-strain could be released in two different loading paths: simultaneous release in both x and y directions or sequential release (y direction first, then x direction). These two loading paths yield two distinct buckling modes and corresponding 3D structures due to the mechanics involved in the buckling process. The entire process is reversible, and thereby the two shape configurations can be switched between each other continuously and repetitively. Again, such reconfigurable 3D structures are achievable in a broad set of materials, multilayer stacks, and over various length scales.

10.3 Flexible and Stretchable Devices from 3D Assembly The deterministic 3D assembly method discussed in Section 10.2 provides a versatile route to the fabrication of 3D structures made of a broad range of materials, complex geometric configurations, and over various length scales. Such 3D structures are integrated on elastomeric substrates upon assembly, making them capable of dynamically and reversibly changing their shapes. This feature makes them promising building blocks for flexible and stretchable devices for applications spanning from microelectronic, to optical, optoelectronic, and biomedical applications. In this section, some exemplary flexible and stretchable devices fabricated using the deterministic 3D assembly are introduced. 10.3.1

Electronic Devices and Systems

As the fabrication process of the 2D precursors is compatible with the most sophisticated CMOS process, a variety of classes of electronic components, devices, and systems could be made on the basis of the 3D assembly method. Furthermore, the mechanical tunability of the 3D structures gives rise to tunable electronic properties of such flexible devices [8, 13–19]. Several exemplary electronics devices and systems are shown in Figure 10.11. A 3D toroidal inductor is fabricated and can be mechanically configured into two distinct shapes by partial and then complete release of the pre-strain (Figure 10.11a) [8]. Good agreement between the FEA-predicted and experimentally measured inductance as a function of frequency is observed. In Figure 10.11b, a pyramidal coil is fabricated as part of a 3D NFC device taking advantage of the multilayer stacking strategy [13]. The 3D NFC device exhibits significantly enhanced Q factor and improved working angle over conventional 2D counterparts. Figure 10.11c shows a network of Si-nanomembrane-based nMOS transistors interconnected by bridge structures [16]. Helical coils fabricated using compressive buckling are used as effective electrical interconnects for soft electronics, as shown in Figure 10.11d [17]. 10.3.2

Optical and Optoelectronic Devices

Complex 3D structures inspired by origami and kirigami are promising platforms for optical and optoelectronic applications. For example, a mechanically

177

10 Flexible and Stretchable Devices from Unconventional 3D Structural Design

1

H (A/m)

300

Measurement Modeling

4

6

3

4

2

2

1

2

4 6 Frequency (GHz)

NFC chip

8

Capacitor 3D coil

20

(a)

15

2D coil

10 Modeling

5

Experiment

0 0.0 0.4 0.8 1.2 1.6 2.0 Width of supporting ribbon wribbon (mm)

nMOS

Bonding site

3D coil

25

0 10

0 0

LED

Q factor

8

Q factor

Inductance (nH)

(b)

500 μm

nMOS

Bonding site

EOG Frequency (Hz)

nMOS

Downside 0 Upside

(c)

0

500 μm

Eyes closed

Eyes opened

12

10

–1

Bonding site

EEG

14

1 1 mm

Amplitude (mV)

178

1

2

3 4 Time

5

6

7

0

1

2 Time

3

4

(d)

Figure 10.11 Exemplary flexible electronic devices fabricated using deterministic 3D assembly. (a) Measured and modeled frequency dependence of the inductance and Q factor of a 3D torsional inductor. Source: Reproduced with permission from Zhang et al. [8]. Copyright 2017, Springer Nature. (b) A 3D NFC device with enhanced Q factor over conventional 2D counterparts. Source: Reproduced with permission from Yan et al. [13]. Copyright 2016, AAAS. (c) A 3D-interconnected bridge structure with an array of Si nanomembrane nMOS transistors. Source: Reproduced with permission from Kim et al. [16]. Copyright 2018, American Chemical Society. (d) A flexible device composed of a network of helical coils as electrical interconnects. Source: Reproduced with permission from Jang et al. [17]. Copyright 2018, Springer Nature.

tunable optical transmission window is demonstrated in Figure 10.12a [9]. As the membrane rotates in response to the strain applied to the elastomer substrate, the amount of normally incident light they block also changes continuously, making the device an optical shutter with well-controlled mechanical tunability. In Figure 10.12b, a 3D hemispherical structure is made by integrating 2D materials (graphene, MoS2 ) with a buckled hemispherical 3D structure [20]. The 2D semiconductor/semimetal materials provide photodetection and light-imaging capabilities on the 3D structural platform. 10.3.3

Scaffolds as Interfaces with Biological Systems

3D structures formed by compressive 3D assembly could also serve as electronic cellular scaffolds for the growth, recording the stimulation of neural networks.

2D precursor (design I, 90% pre-strain) Bonding region

1 mm

100 μm

SU-8 Partially assembled 3D structure Experiment FEA

z

3D optical transmission window

uz (μm) 0

SU-8

855

y

θ = –1.1°, ϕ = 91.1°

(i)

θ = 45.0°, ϕ = 90.0°

Using protractor Experimental

(ii)

θ = 44.3°, ϕ = 91.3° (iii)

P1

X (ii)

43% 66%

P2

P1

θ = –45°→0°→45° ϕ = 0°

Receiver

P1

lph 1 μA

0

y (i)

(a)

θ = 0.0°, ϕ = 90.0°

θ = –45.0°, ϕ = 90.3°

P2

Light spot Optical shutter

θ = –45.0°, ϕ = 90.0° (iii)

Z

x εappl = 0%

MoS2

y x

z

Graphene

P2

(b)

Figure 10.12 An exemplary flexible 3D-structure-based device for optical and optoelectronic applications. (a) A mechanically tunable optical shutter. Scale bars, 500 μm. Source: Reproduced with permission from Zhang et al. [9]. Copyright 2015, PNAS. (b) A 3D photodetection and imaging system. Source: Reproduced with permission from Lee et al. [20]. Copyright 2018, Springer Nature.

180

10 Flexible and Stretchable Devices from Unconventional 3D Structural Design

Schwann cell Axon Satellite cell Pseudounipolar neuron Culture on 3D scaffolds

(a)

(b)

Figure 10.13 3D electronic scaffolds for engineering neural network. (a) Schematic illustration of rat dorsal root ganglion (DRG) and cell populations on 3D structures. (b) Confocal fluorescence micrographs of DRG cells cultured on a 3D cage structure. Scale bars, 100 μm. Source: Reproduced with permission from Yan et al. [14]. Copyright 2017, PNAS.

Figure 10.13 shows an example of a freestanding 3D open-cage structure used as scaffold for engineered dorsal root ganglion (DRG) neural networks [14]. The 3D configuration allows interaction and communication with live cells and tissues in 3D. Experiments demonstrate that such scaffolds facilitate the reorganization of cells into hierarchical cellular constructs. This example shows promising opportunities of 3D structures as cellular and tissue scaffolds in fundamental and applied biological studies.

10.4 Summary This chapter introduces different approaches for forming 3D structures. Bonding nanomembranes in Si or other inorganic materials to a pre-strained elastomer can yield 3D wavy structures with controllable surface topologies upon release of the pre-strain. On this basis, deterministic 3D assembly introduces spatial control of the adhesion sites to further expand the accessible 3D geometries. The final 3D geometries can be tailored through layouts of the 2D patterns, position of the bonding sites, pre-strains, thickness profiles of 2D patterns, loading path, nonuniform elastomeric substrates, and many others. Applications of such 3D mesostructures cover the field of electronics, optoelectronics, optical devices, biointerfaces, energy harvesters, microrobots, and others.

References

Abbreviations CMOS FEA MEMS NFC PDMS SEM

complementary metal–oxide–semiconductor finite element analysis microelectromechanical system near-field communication poly(dimethylsiloxane) scanning electron microscopy

References 1 Khang, D.-Y., Jiang, H., Huang, Y., and Rogers, J.A. (2006). A stretchable form

2

3 4

5

6

7

8

9

10

of single-crystal silicon for high-performance electronics on rubber substrates. Science 311: 208–212. Sun, Y., Choi, W.M., Jiang, H. et al. (2006). Controlled buckling of semiconductor nanoribbons for stretchable electronics. Nature Nanotechnology 1: 201–207. Choi, W.M., Song, J., Khang, D.-Y. et al. (2007). Biaxially stretchable “wavy” silicon nanomembranes. Nano Letters 7: 1655–1663. Kim, D.-H., Song, J., Choi, W.M. et al. (2008). Materials and noncoplanar mesh designs for integrated circuits with linear elastic responses to extreme mechanical deformations. Proceedings of the National Academy of Sciences of the United States of America 105: 18675–18680. Xu, S., Yan, Z., Jang, K.-I. et al. (2015). Assembly of micro/nanomaterials into complex, three-dimensional architectures by compressive buckling. Science 347: 154–159. Yan, Z., Han, M., Yang, Y. et al. (2017). Deterministic assembly of 3D mesostructures in advanced materials via compressive buckling: a short review of recent progress. Extreme Mechanics Letters 11: 96–101. Liu, W., Zou, Q., Zheng, C., and Jin, C. (2019). Metal-assisted transfer strategy for construction of 2D and 3D nanostructures on an elastic substrate. ACS Nano 13: 440–448. Zhang, Y., Zhang, F., Yan, Z. et al. (2017). Printing, folding and assembly methods for forming 3D mesostructures in advanced materials. Nature Reviews Materials 2: 17019. Zhang, Y., Yan, Z., Nan, K. et al. (2015). A mechanically driven form of Kirigami as a route to 3D mesostructures in micro/nanomembranes. Proceedings of the National Academy of Sciences of the United States of America 112: 11757–11764. Fu, H., Nan, K., Froeter, P. et al. (2017). Mechanically-guided deterministic assembly of 3D mesostructures assisted by residual stresses. Small 13: 1700151.

181

182

10 Flexible and Stretchable Devices from Unconventional 3D Structural Design

11 Nan, K., Luan, H., Yan, Z. et al. (2017). Engineered elastomer substrates for

12

13

14

15

16

17 18

19

20

guided assembly of complex 3D mesostructures by spatially nonuniform compressive buckling. Advanced Functional Materials 27: 1604281. Park, S.J., Zhao, H., Kim, S. et al. (2016). Predictive synthesis of freeform carbon nanotube microarchitectures by strain-engineered chemical vapor deposition. Small 12: 4393–4403. Yan, Z., Zhang, F., Liu, F. et al. (2016). Mechanical assembly of complex, 3D mesostructures from releasable multilayers of advanced materials. Science Advances 2: e1601014. Yan, Z., Han, M., Shi, Y. et al. (2017). Three-dimensional mesostructures as high-temperature growth templates, electronic cellular scaffolds, and self-propelled microrobots. Proceedings of the National Academy of Sciences of the United States of America 114: e9455–e9464. Fu, H., Nan, K., Bai, W. et al. (2018). Morphable 3D mesostructures and microelectronic devices by multistable buckling mechanics. Nature Materials 17: 268–276. Kim, B.H., Lee, J., Won, S.M. et al. (2018). Three-dimensional silicon electronic systems fabricated by compressive buckling process. ACS Nano 12: 4164–4171. Jang, K.-I., Li, K., Chung, H.U. et al. (2018). Self-assembled three dimensional network designs for soft electronics. Nature Communications 8: 15894. Han, M., Wang, H., Yang, Y. et al. (2019). Three-dimensional piezoelectric polymer microsystems for vibrational energy harvesting, robotic interfaces and biomedical implants. Nature Electronics 2: 26–35. Nan, K., Wang, H., Ning, X. et al. (2019). Soft three-dimensional microscale vibratory platforms for characterization of nano-thin polymer films. ACS Nano 13: 449–457. Lee, W., Liu, Y., Lee, Y. et al. (2018). Two-dimensional materials in functional three-dimensional architectures with applications in photodetection and imaging. Nature Communications 9: 1417.

183

11 Flexible and Stretchable Devices from Other Materials Haotian Chen Peking University, Institute of Microelectronics, National Key Laboratory of Nano/Micro Fabrication Technology, Building of Micro/Nanoelectronics, No. 5 Yiheyuan Road, Beijing 100871, China

Besides the 0D, 1D, and 2D materials mentioned in previous chapters, there are some other kinds of materials, such as polymer materials, composite materials, and textile materials, which have also been broadly explored to construct flexible and stretchable devices. Their unique and outstanding characteristics, including mechanical compliance, electrical properties, mass production, and low cost, demonstrate great potential in many fields like portable and wearable electronics, smart skin, etc. In this chapter, we introduce these materials and their applications in the flexible and stretchable devices.

11.1 Polymer-Based Conductive Materials Compared to traditional conductive materials like carbon-based materials and metals, polymers have a much better intrinsic flexibility. However, traditional polymers usually are insulators, which hinder their application areas. In 1977, three scientists, Heeger, MacDiarmid, and Shirakawa reported, similar high conductivity in oxidized iodine-doped polyacetylene [1]. Because of this breakthrough research, they were awarded the 2000 Nobel Prize in Chemistry “for the discovery and development of conductive polymers”, which opens a new window and drives the rapid developments in this field [2]. There are two main conditions to get conductive polymers [3]. The first one is that the polymer must consist of alternating single and double bonds, the so-called conjugated double bonds. The second is that the polymer has to be disturbed by removing electrons from the material or by inserting electrons into the material, which is known as the doping process. Three kinds of conductive polymers, polyaniline (PANI) [4], polypyrrole (PPy) [5], and poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) [6], have been commonly used as stretchable conductors.

Flexible and Stretchable Triboelectric Nanogenerator Devices: Toward Self-powered Systems, First Edition. Edited by Mengdi Han, Xiaosheng Zhang, and Haixia Zhang. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

184

11 Flexible and Stretchable Devices from Other Materials

11.1.1

PANI

PANI was reported with high conductivity by De Surville et al. [7]. It not only has very good electrical conductivity but also is a common pseudocapacitive material which can increase the electrochemistry performance of a supercapacitor. In Ref. [4], the Yang Chai group from the Hong Kong Polytechnic University reported a stretchable all-solid-state supercapacitor utilizing PANI and graphene as electrodes, as shown in Figure 11.1. By introducing the wavy structure and using H3 PO4 -polyvinylalcohol (H3 PO4 -PVA) as electrolyte, the whole supercapacitor exhibits high flexibility and stretchability. What is more, the electrochemical performance of the device can remain almost unchanged even under high bending (180∘ ) and 30% tensile strain. Besides the supercapacitor, PANI has been applied in various areas including gas vapor sensors [8], biological sensors [9], and electrochromic devices [10] because of its high surface area and short diffusion length inherent in this morphology. (a)

(b)

Electrode Electrolyte Electrode

Ecoflex

200 μm

Ecoflex Fabric

(c)

Bending –180°

(d)

Bending 180°

(e)

Stretching 0%

(f)

Stretching 30%

Figure 11.1 All-solid-state stretchable supercapacitor using PANI as electrode. (a) Cross-sectional SEM image of the device. (b) Stretchable supercapacitors encapsulated in Ecoflex and Ecoflex/fabric. Insets are cross-sectional schematics of the devices. Optical images of the bent all-solid-state supercapacitor at (c) −180∘ and (d) 180∘ . Optical images of the stretched all-solid-state supercapacitor under strains of (e) 0% and (f ) 30%. Source: Reproduced with permission from Xie et al. [4]. Copyright 2014, Royal Society of Chemistry.

11.1 Polymer-Based Conductive Materials

11.1.2

PPy

Like PANI, PPy is also a kind of common conductive polymer due to its easy preparation, high conductivity, nontoxicity, and good adhesion with diverse substrates. In 1963, B.A. Bolto et al. reported derivatives of PPy [11] with resistivity as low as 1 Ω cm, which has attracted a lot of researchers since that time. The Dong Wang group from the Wuhan Textile University fabricated a strain sensor for human breath detection by coating PPy onto a polyurethane (PU) elastomer to get a stretchable conductor with very good sensitivity and repeatability [5] (Figure 11.2). The electrical resistivity of this stretchable conductor could reach 8.364 Ω cm2 with the maximum elongation of 420%. Similar to PANI, PPy is not only a conductive material but also a good candidate material for gas sensors (CO2 , N2 , CH4 , H2 S, NH3 , etc.) [12, 13], which is beneficial for the stretchable gas sensor or the electronic nose. 11.1.3

PEDOT:PSS

Apart from PANI and PPy, PEDOT:PSS is another promising electrode material, which was first developed at the Bayer AG Research Laboratories in Germany during the second half of the 1980s [14]. PEDOT:PSS has a lot of attractive properties including its excellent air and thermal stability, high transparency in the visible spectral region, and tunable conductivity (10−4 –10−3 S/cm). The Zhenan Bao group from Stanford University has done a lot of inspiring work with PEDOT:PSS [12, 15]. They carefully investigated the behavior of a thin film of PEDOT:PSS, mixed with Zonyl fluorosurfactant, on PDMS substrates as stretchable transparent electrodes, as shown in Figure 11.3. The PEDOT:PSS film can retain significant conductivity up to 188% strain and are reversibly stretchable up to 30% strain. What is more, samples treated with different methods such as oxygen plasma and ultraviolet/ozone (UV/O3 ) are compared to achieve optimal treatment. PEDOT:PSS has also been applied in various applications such as stretchable sensors [16], organic light-emitting diodes (OLEDs) [17], and stretchable transistors [18], etc. 11.1.4

Organic Nanowires

Organic semiconducting nanowire (NW) is also one of the promising approaches to overcome the problem of compatibility of stretchability and conductivity. The mechanical properties including elastic modulus, ductility, and toughness can be easily modified by the polymer formulation. What is more, applying strainengineered structures like serpentine geometry to the nanowire can further enhance mechanical reliability of the organic semiconductors. The Zhenan Bao group presented a deformable organic semiconductor nanowire field-effect transistor (FET) [19], as shown in Figure 11.4. The organic NWs comprise a homogeneous mixture of fused thiophene diketopyrrolopyrrole

185

(d)

(b)

(a)

Original length 10% elongation 50% elongation 100% elongation Microcracks

(c) Stretching 300%

(e)

(f) Waistband

0.5

0.5 Elongation

belt

Rigid

belt

Breath in Maginification Releasing

Releasing

Stretching

Stretching

0.4

0.4

0.3

0.3

0.2

0.2

0.1

0.1

Sensitivity (ΔR/R0)

Rigid

Sensitivity

PPy/PU elastomer

Elongation (ΔL/L0)

Fasten

Breath out PPy/PU elastomer

Microcracks

PPy/PU elastomer

0.0

0.0 0

1

2

3

4

5

6

7

8

9

10

11

Cycles

Figure 11.2 Stretchable strain sensor using PPy/PU as stretchable conductor with very high sensitivity and repeatability. (a) Photographs of porous PU substrate before polymerization, (b) PPy/PU elastomer after surface in situ polymerization, and (c) surface and cross section of PPy/PU elastomer. (d) Stretchability of PPy/PU elastomer with various elongations. (e) Schematic diagram of the waistband-like human breath detection sensor built from the PPy/PU elastomer. (f ) Elongation and sensitivity of the waistband-like human breath detection sensor. Source: Reproduced with permission from Li et al. [5]. Copyright 2014, American Chemical Society.

→10%→0%

0%→25%→0%

Pressure

PDMS

PDMS substrate

Ecoflex d PDMS

PEDOT: PSS

1. Plasma oxidize 20 μm →50%→0%

0%→100%→0%

Crack→

300

0.05

250

0.04

200

P (kPa)

2. Spin PEDOT:PSS

20 μm

0.03

150

0.02

100

0.01

50 0 20 μm

(b)

20 μm

0 100

0.04

0% Z, plasma (180 s)

81

1% Z, plasma (180 s)

61

ΔC/C0

R/R0

50 t (s)

0.05

101

4. Relax

0

(e)

3. Stretch

ΔC/C0

(d)

Buckle→

SiOx layer

d′

0% Z, plasma (10 s)

41

1% Z, plasma (10 s) 1% Z, UV/O3 (20 min)

21

0.03 0.02 0.01 0

1 (a)

(c)

0

10

20

ε (%)

30

40

0

50 (f)

100 200 P (kPa)

300

Figure 11.3 Electronic properties of transparent conductive films of PEDOT:PSS on stretchable substrates. (a) Proposed mechanism of reversible stretchability of films of PEDOT:PSS on plasma-treated PDMS. (b) Optical microscopic images of PEDOT:PSS films on PDMS after stretching and releasing the films from strains of 10%, 25%, 50%, and 100% along the horizontal axis. (c) Plots of normalized resistance versus strain for five combinations of methods of surface activation (oxygen plasma for 180 or 10 s or UV/O3 for 20 min) and addition of Zonyl fluorosurfactant. (d) Cross section of the region of overlap between the two strips. (e and f ) Overlaid plots of pressure P and normalized change in capacitance ΔC/C 0 versus time t over two cycles of applied pressure. Source: Reproduced with permission from Lipomi et al. [6]. Copyright 2012, American Chemical Society.

11 Flexible and Stretchable Devices from Other Materials C10H21

(a)

C8H17 N

N

S

OSC NW

O n

S

S

n

S

O

PEO (binder)

H

O C H 17 25

S

Semiconducting polymer

S C8H17

50 μm

C17H35

OH

Drain Gate

Dielectric

(c) 10–8

0.2 10–9 0.1

10–10

Avg. mobility 2 0.0 = 7.46 cm /V s –30

(e)

Pristine

Max. drain current (–nA)

Drain voltage = –30 V 10–7 Drain current (–A)

(Drain current)1/2 (mA)1/2

(b) 0.3

50 μm

C10H21

Source

0 –20 –10 Gate voltage (V)

400

C

300

500 nm

250 nm

200 100 0

10–11

10

(d)

500

1

3

O

S

250 nm

250 nm

7

5

Number of wires

Length-directional Stretching

Release

Pristine (0% strain)

(f) Pristine and Release

Width-directional Stretching

Pristine (0% strain)

Stretching (100% strain)

Release (0% strain)

10 μm

10 μm

Stretching (100% strain) 10 μm

Width-directional stretching and release

(g)

50 μm

50 μm

Length-directional stretching and release

Pristine (0% strain) EGain gate SEBS dielectric CNT S/D

Pristine NW

SEBS substrate

–11

10

–13

10

Length direction 0.01 0 20 40 60 80 100 Strain (%)

10

–7

10

–9

25% strain 50% strain 100% strain

100

1 –11

10

–13

10

Length direction 0

10 20 30 40 50 Number of stretches

0.01

(j)

–7

100

10

–9

10

1

10

–11

10

–13

Width direction 0

Mobility (cm2/V s)

1

(i)

Mobility (cm2/V s)

–9

Mobility (cm2/V s)

10

100

Max. drain current (–A)

(h)10–7

Wrinkled NW

Stretched NW

Max. drain current (–A)

Folded NW

Max. drain current (–A)

188

0.01 25 50 75 100 Release Strain (%)

11.2 Composite-Based Conductive Materials

Figure 11.4 (a) Schematic illustration of organic semiconducting NW FET. Chemical structures (upper left) and optical microscope and polarized optical microscope images (upper right) of organic semiconducting NW. (b) Transfer curves of NW FET with a PVDF-TrFE dielectric. (c) Maximum drain current as a function of the number of NWs. (d) Transmission electron microscope image of NW and its elemental mapping using energy dispersive X-ray spectroscopy. (e) Length-directional stretching demonstration. (f ) Width-directional stretching demonstration. (g) Schematic illustration of deformable FET with straight NW and SEBS dielectric with length and width-directional stretching. (h) Mobility and maximum drain current of deformable FET with straight NW under various strains and (i) after repeated stretching cycles at 25%, 50%, and 100% strains in the channel length direction. (j) Mobility and maximum drain current of deformable FET with straight NW under width-directional strain and after releasing to its original state. Source: Reproduced with permission from Lee et al. [19]. Copyright 2018, John Wiley & Sons.

(FT4-DPP)-based semiconducting polymer and polyethylene oxide (PEO) (7 : 3 w:w). Single FT4-DPP:PEO NWs were observed to be highly ductile and flexible, without breaking on the elastic substrate during stretching up to 100% strain along channel length and width directions and even after release back to their original state. The NWs were further strain engineered by depositing on a prestretched elastic substrate and released to give a serpentine-like geometry. This structure further improved the device’s mechanical durability during multiple deformations. Furthermore, the Tae-Woo Lee group from the Pohang University of Science and Technology reported a high-speed electrohydrodynamic organic nanowire printer to print large-area organic semiconducting nanowire arrays directly on device substrates in a precisely, individually controlled manner, which enables large-area nanowire lithography for electronics [20].

11.2 Composite-Based Conductive Materials A composite-based conductive material, also called a conductive composite, is another widely used method for fabricating stretchable electronic devices. They are usually developed by introducing conductive fillers into elastomers, which combine the electrical performance of the conductive fillers and the mechanical stretchability of the elastomers [21]. The biggest problem is that the traditional conductive fillers and the elastomers have huge differences in elastic properties [22]. Plenty of efforts have been made to harmonize the mismatches and integrate materials with widely different properties as a whole system. 11.2.1

Conductive Fillers Blended into Stretchable Elastomers

Blending conductive fillers into an insulating elastomeric matrix to form a conductive composite is the common approach toward stretchable conductors. Several materials like carbon nanotubes (CNTs) [23], metal particles [24], and conductive polymers [25] were used to blend into different elastomers. The electrical conductivity can be analyzed by the percolation theory [26]: 𝜎 = 𝜎0 (Vf − Vc )s

(11.1)

189

190

11 Flexible and Stretchable Devices from Other Materials

where 𝜎 is the electrical conductivity, 𝜎 0 is the conductivity of the conductive filler, V f is the volumetric fraction of the filler, V c is the volumetric fraction at the percolation threshold, and s is the fitting exponent. The percolation threshold is the key parameter to control the electrical conductivity of composites. The size and shape can strongly influence the percolation threshold. Compared to the particle fillers, 1D nanomaterials like CNT have a lower percolation threshold due to their high aspect ratio. CNT is the most common conductive filler material for fabricating conductive composites because of its comparatively high conductivity and low cost. Our group developed a method by blending CNT into PDMS to obtain the stretchable conductive composite (CNT–PDMS) [23], as shown in Figure 11.5. With the help of toluene, the CNTs can be dispersed into PDMS uniformly. What is more, the shape of the CNT–PDMS can be designed using the molding method, the structure of which can be obtained by 3D printing technology. In this way, a triboelectric generator (TENG)-based self-powered sliding sensor with four spiral-shaped CNT–PDMS electrodes is fabricated, which can detect the sliding direction and speed at the same time. Metal particle is another kind of conductive filler due to its high conductivity. The group led by Alexandre Larmagnac from ETH Zurich presented a simple, low-cost and large-scale process to produce soft printed circuit boards (PCBs) by mixing Ag into PDMS (Ag-PDMS) [24] (Figure 11.6). This composite can be stencil printed or screen printed with a resolution of 150 mm for large-scale fabrication. Double-sided soft PCBs had, in average, a low electrical resistance of 2 Ω/cm. What is more, Ag-PDMS can be directly connected to commercial electronic components, which build a bridge between the novel materials science and the traditional electronic products. Besides CNT and metal particles, conductive polymers can also work as conductive fillers to mix with elastomers to achieve stretchable conductors. For example, the Niels B. Larsen group from the Technical University of Denmark (a)

(c)

PDMS

CNT-PDMS

Toluene + CNT

(b) CNT-PDMS

PDMS

Figure 11.5 (a) The fabrication process of CNT–PDMS. (b) The pattern method of CNT–PDMS. (c) Photograph of the fingerprint shaped CNT–PDMS. Source: Reproduced with permission from Chen et al. [23]. Copyright 2018, Elsevier.

11.2 Composite-Based Conductive Materials

(a)

(c)

(e)

(g)

(b)

(d)

(f)

(h)

Figure 11.6 (a) Picture showing the stencil printing of Ag-PDMS composites. (b) SEM picture of the stencil printed lines with highest achieved resolution. (c) Picture showing the setup for screen printing on 8 in. wafers. (d) Microscopic image of finest screen printed lines (black) and spacing (white). (e) Picture of a ribbon cable with eight conductors clamped on both ends by a commercial connector. (f ) Picture of a custom-made miniature connector with 12 contacts. (g) Pictures of chip resistors bonded on conductive tracks. (h) Picture of a large array of SMD LEDs bonded on a soft PCB. Source: Reproduced with permission from Larmagnac et al. [24]. Copyright 2014, Springer Nature.

reported a conductive composite by blending PEDOT with PU (PEDOT-PU) [25], as shown in Figure 11.7. This material exhibits a high conductivity of 100 S/cm even if stretched over 100%. Furthermore, it shows good adhesion to many types of materials such as metal, glass, polymer, and fabric, under both dry and wet conditions. 11.2.2

Conductive Film Embedded into Stretchable Elastomer

As discussed earlier, mechanical blending could not effectively achieve highly conductive and transparent composites. To solve this problem, an embedding approach was developed, in which the conductive film either directly deposited on the elastomer or transferred from a traditional substrate (such as glass or silicon) to the target elastomer. The Inkyu Park group from the Korea Advanced Institute of Science and Technology proposed a sandwich-structure composite (i.e. silver nanowire (AgNW) thin film embedded between two layers of PDMS) to fabricate a strain sensor [27], as shown in Figure 11.8. This device shows an excellent performance to both static and dynamic loads with a high linearity and negligible hysteresis even for a large strain level and possesses a strong piezoresistivity with tunable gauge factors (GFs) in the ranges of 2–14 depending on the density of AgNWs with a maximum stretchability up to 70%. The Yong Zhu group from North Carolina State University reported a stretchable and reversibly deformable antenna by embedding the AgNWs in the surface layer of an elastomeric substrate [28], as shown in Figure 11.9. The pattern of the conductive film can be obtained by stencil mask. The device can maintain the same spectral properties under severe bending, twisting, and rolling due to the stretchable conductor. The material and fabrication technique reported here could be extended to other types of stretchable devices with more complex patterns and multilayer structures.

191

Current electrodes

PEDOT /PUR

PUR Potential electrodes Elongation

Elongation

10 mm

10 mm

(a)

(b) 50 wt% PEDOT stored at 21° C

1.5

1

1

Conductivity

50% PEDOT 40% PEDOT 33% PEDOT

1

2

3

4

5

6

7

8

Number of elongations

9

0.5

10

Relative conductivity

1.5

0

(c)

Resistance

0.5

40 wt% PEDOT stored at 21° C

2 Relative conductivity σ/σ0

Relative resistance

2

0

(d)

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 101

33 wt% PEDOT stored at 21° C 50 wt% PEDOT stored at 60° C 40 wt% PEDOT stored at 60° C 33 wt% PEDOT stored at 60° C

102

103

104

Time (h)

Figure 11.7 (a) Schematic of the PEDOT/PU/PEDOT film sandwich with crossing copper wires for establishing a four-point resistance measurement. (b) Photograph of the actual stretching setup. (c) The relative resistance and conductivity of the 50% PEDOT, 40% PEDOT, and 33% PEDOT samples, respectively, cyclically elongated by 50% and relaxed to their unstretched state 10 times. (d) The temporal development of the conductivity for the PEDOT/PU blends stored at 21 ∘ C and 60 ∘ C. Source: Reproduced with permission from Hansen et al. [25]. Copyright 2007, John Wiley & Sons.

(a)

Patterned AgNW network

AgNW solution

Partial curing of PDMS @ 70 °C

(e) 50

Middle finger Index finger

40

Patterned glass slide Polyimide tape

ΔR/R0 (%)

30

Thermal annealing @ 200 °C

10 0

–10

Peeling off

Silver paste

Curing of PDMS @ 70 °C

20

–20 –30

0

4

8

12

16

20

Time (s)

Embedded AgNW network

(b)

(c)

(f) (d)

Top-view

Cross-view

PDMS PDMS

~ 5 μm

~ 3 mm

Patterned Ag-PDMS nanocomposite thin film

PDMS PDMS

100 μm

50 μm

Figure 11.8 (a) Fabrication process of the strain sensors. (b) Photographs of the strain sensor before and after stretching of 𝜀 = 100%. (c) Photographs of the strain sensor under bending and twisting. (d) Optical microscope images on top and cross-section of the sandwich-structured strain sensor. (e) Motion detection of index and middle fingers. (f ) Control of avatar fingers in the virtual environment using wireless smart glove system. Source: Reproduced with permission from Amjadi et al. [27]. Copyright 2014, American Chemical Society.

Cast NW solution and dry it

Wsub

0

W

–5

Lsub L

AgNWs

Reflection coefficient (dB)

Si substrate

Si substrate 50 Ω microstrip feed line

AgNW/PDMS Liquid PDMS

W

AgNW/PDMS

Impedance transformer

Cured PDMS (a)

(d)

Lsub

Cure the PDMS and peel off the substrate

3% 6% 9%

–25

12%

Frequency (GHz) 3.10 3.08

50 Ω feeding network

(b)

0%

–20

–30 2.75 2.80 2.85 2.90 2.95 3.00 3.05 3.10 3.15 3.20 3.25

y

L

Si substrate

Wsub

–15

15%

z

x

Resonant frequency (GHz)

Coat liquid PDMS

–10

3.06 3.04 3.02 3.00 2.98 2.96 2.94

Simulation

2.92

1st strech 1st release

2.90 2.88

(c)

(e)

1.0

2.5

4.0 5.5

7.0

8.5 10.0 11.5 13.0 14.5 16.0

Strain (%)

Figure 11.9 (a) Fabrication procedure for the AgNW/PDMS flexible patch antenna and the schematics for (b) microstrip patch antenna and two-element patch array. (c) Photographs of a stretchable microstrip patch antenna composed of AgNW/PDMS flexible conductor. (d) Measured frequency response of reflection coefficient for the AgNW/PDMS microstrip patch antenna under tensile strains from 0% to 15%. (e) Comparison of simulated (red dotted) and measured resonant frequency during stretch (blue solid) and release (green solid) for the AgNW/PDMS microstrip patch antenna under tensile strain from 0% to 15%. Source: Reproduced with permission from Song et al. [28]. Copyright 2014, American Chemical Society.

11.3 Textile-Based Conductive Materials

11.3 Textile-Based Conductive Materials The development of textiles reflects the civilization of humans. From natural materials such as silk and cotton to manmade fiber materials like nylon, the appearance of textiles has benefited our lives over history. Of late, textiles are expected to exhibit additional functionalities with the advancement of electronics and the Internet [29, 30]. For example, the self-power technology is hoped to be realized if clothes can convert energy sources such as mechanical energy [31, 32] and solar energy [33] into electric energy. What is more, sensing technology is also desired to be integrated with the clothes [34]. To achieve all these goals, textile-based conductive materials like single fibers, yarns, and fabrics have been developed and investigated. Some of them are made from CNTs [35] and metals [36]; others are made from dielectric textiles using different coating methods [37]. 11.3.1

Fiber-Based Conductive Materials

Fiber is the fundamental component for constructing textiles. Many groups concentrate on the fabrication of stretchable conductive fibers and hope to obtain a highly stretchable textile by weaving these fibers together. The Taeyoon Lee group from Yonsei University fabricated a conductive fiber by coating poly(styrene-butadiene-styrene) (SBS) polymer on the surface of poly(p-phenylene terephthalamide) (Kevlar) fiber, followed by converting a huge amount of silver ions into Ag nanoparticles directly in the SBS polymer [38], as shown in Figure 11.10. The obtained conductive fiber has an excellent electrical property of 0.15 Ω/cm owing to the dense electrical connection of the Ag nanoparticles. The conductive fiber also exhibited outstanding stability against repeated external deformations (3000 bending tests) with the help of the elastic rubber materials, which act as elastic scaffolds. SBS Ag precursor absorption

SBS coating

Kevlar fiber

SBS-coated Kevlar fiber

SBS/precursor composite Precursor reduction

SBS/precursor-compositecoated Kevlar fiber

1.2 Resistance (Ω/cm)

Conductive fiber

Kevlar fiber

SBS/AgNP composite

SBS/AgNP/ composite-coated Kevlar fiber Conductive fibers

0.8 0.4 0.0 –0.4 –0.8 0

500 1000 1500 2000 2500 3000 Number of foldings

Figure 11.10 Ag-based conductive fiber. Source: Reproduced with permission from Lee et al. [38]. Copyright 2015, John Wiley & Sons.

195

196

11 Flexible and Stretchable Devices from Other Materials

(a)

Top view

Side view

Contacting intersection (b)

m l Cut in half

Cu

PI

PET

l = km (k > 1) Original d

(c)

x

d

(d)

––––– Pressure applied

X

–––––

Pressure released d+x

––––– –––––

Figure 11.11 (a) Schematic diagram of the side view and top view of each yarn crisscross intersection in the TENG. (b) The morphology of the contacting intersection and the conceptual flat ellipse with two radii along the two axes. (c) The cross-sectional view of the yarn crisscross intersection. The right parts illustrate the two conditions when pressure is applied onto or released from the yarn crisscross intersection, with m = d at the original state. (d) Illustration showing the charge distribution at yarn crisscross intersection when contacting area changes. Source: Reproduced with permission from Zhao et al. [34]. Copyright 2016, John Wiley & Sons.

The Youfan Hu group from Peking University reported a textile TENG fabricated using direct weaving of copper (Cu)-coated polyethylene terephthalate (Cu-PET) warp yarns and polyimide (PI)-coated Cu-PET (PI-Cu-PET) weft yarns on an industrial sample weaving loom [34] (Figure 11.11). More importantly, this TENG shows a remarkable washing durability, which can withstand standard machine washing tests. 11.3.2

Textile-Based Conductive Materials

Conductive textiles, on the one hand, can be fabricated by weaving plenty of fibers together; on the other hand, they can also be obtained by coating conductive materials on the traditional textile, which provides a low-cost fabrication method. The Xiao-Ming Tao group from the Hong Kong Polytechnic University reported an elastic conducting fabric electrode made from segmented polyurethane multifilament yarns and silver-coated polyamide multifilament yarn [39], as shown in Figure 11.12. With this fabric-based electrode, an all-fiber piezoelectric nanogenerator is fabricated, which can convert low-frequency (5 Hz)

(a)

(c)

(d)

PDMS packaging 5.00 mm

Silver-coated polyamide yarn

Fabric electrode Piezoelectric nonwoven fabric 20 kV

× 30

500 μm

Segmented polyurethane yarn

11 50 SEI

(e) 1Hz

Voltage (V)

(b)

3Hz

2 0 –2

1.0 cm

6

5Hz

Current (μA)

4

1Hz

3Hz

5Hz

4 2 0

–2 0

2

4

6

8 10 12 14 16 18

Time (s)

0

3

6

9

12

15

18

Time (s)

Figure 11.12 (a) Schematic structure of the all-fiber electric power nanogenerator. (b) Photo of a fully packaged generator device. (c) SEM of the cross-section of the free-standing knitted fabric electrode. (d) Schematic structure of the composite conducting yarn, the blue part corresponds to the highly stretchable segmented polyurethane yarn which is wrapped by a silver coated polyamide filament yarn. (e) Open circuit voltage and current output of the all-fiber generator. Source: Reproduced with permission from Zeng et al. [39]. Copyright 2013, Royal Society of Chemistry.

11 Flexible and Stretchable Devices from Other Materials

(a)

e–

e–

e–

Energy-harvesting STEG

e–

e–

e– Energy storage Supercapacitor (iii)

(b) (i)

STEG

(ii) (iv) Pristine cotton

Drop-drying CNT Supercapacitor

Cotton

Separator

Dielectric

CNT/cotton

PVA/H3PO4 electrolyte

(c)

20 μm

(d)

STEG

20 μm

(e) 100 Voltage (mV)

198

50

Supercapacitor 0 0

3 Time (min)

6

Figure 11.13 (a) Schematic of the all-fabric-based self-charging power cloth, which is made of a wearable STEG and a flexible supercapacitor. (b) Fabrication process with a general CNT/cotton fabric electrode. (c) SEM images of pristine and CNT coated cotton fabric. (d) Wearable display of device attached among the cloth surface. (e) Initial charging curves during the running motions. Source: Reproduced with permission from Song et al. [40]. Copyright 2017, American Institute of Physics.

Abbreviations

compression energy into electricity. Under a compressive pressure of 0.2 MPa, the device can generate a stable and high open-circuit peak voltage of 3.4 V, and a peak current of 4.4 mA. What is more, as the whole nanogenerator is fabricated using fabric, it can retain excellent performance even after 1 000 000 compression-recovery cycles, demonstrating its great promise in supplying power to portable or wearable electronic devices by harvesting a portion of the mechanical energy from the walking motion of humans. Furthermore, conductive textiles can also be used for fabricating energy storage devices such as supercapacitors. Our group presented a prototype of an all-fabric-based self-charging power cloth by integrating a wearable singleelectrode triboelectric nanogenerator (STEG) and a flexible supercapacitor with a general CNT/cotton fabric electrode [40], as shown in Figure 11.13. The CNT/cotton fabric electrodes are simply fabricated using the drop-drying method. Woven into daily clothes, STEG could scavenge the human mechanical energy effectively and convert the mechanical energy into electrochemical energy, which is stored in the supercapacitor.

11.4 Summary In this chapter, we mainly discussed about flexible and stretchable devices made from polymers, composite materials, and textiles. Their properties such as intrinsic mechanical compliance, mass production, and low cost provide more choices for fabricating different functional devices including sensors, supercapacitors, antennae, nanogenerators, and transistors, which are expected to work as a whole system.

Abbreviations AgNW CNT Cu FET FT4-DPP GF LDW OLED PANI PCB PDMS PEDOT:PSS PEO PET PI PPy

silver nanowire carbon nanotube copper field-effect transistor fused thiophene diketopyrrolopyrrole gauge factor laser direct write organic light-emitting diode polyaniline printed circuit board poly(dimethylsiloxane) poly(3,4-ethylenediox-ythiophene):poly(styrene sulfonate) polyethylene oxide polyethylene terephthalate polyimide polypyrrole

199

200

11 Flexible and Stretchable Devices from Other Materials

PU PVA SBS STENG TENG

polyurethane polyvinylalcohol poly(styrene-butadiene-styrene) single-electrode triboelectric nanogenerator triboelectric nanogenerator

References 1 Shirakawa, H., Louis, E.J., MacDiarmid, A.G. et al. (1977). Synthesis of electri-

2

3

4

5

6

7 8 9

10 11

12

13

cally conducting organic polymers - halogen derivatives of polyacetylene, (Ch)X. Journal of the Chemical Society, Chemical Communications (16) 578–580. Heeger, A.J., MacDiarmid, A.G., and Shirakawa, H. (2000). The Nobel Prize in Chemistry, 2000: conductive polymers. Stockholm, Sweden: Royal Swedish Academy of Sciences 1–16. Gerhard, W. (1981). Polymers with metal-like conductivity—a review of their synthesis, structure and properties. Angewandte Chemie International Edition in English 20: 361–381. Xie, Y., Liu, Y., Zhao, Y. et al. (2014). Stretchable all-solid-state supercapacitor with wavy shaped polyaniline/graphene electrode. Journal of Materials Chemistry A 2: 9142–9149. Li, M.F.F., Li, H.Y., Zhong, W.B. et al. (2014). Stretchable conductive polypyrrole/polyurethane (PPy/PU) strain sensor with netlike microcracks for human breath detection. ACS Applied Materials & Interfaces 6: 1313–1319. Lipomi, D.J., Lee, J.A., Vosgueritchian, M. et al. (2012). Electronic properties of transparent conductive films of PEDOT:PSS on stretchable substrates. Chemistry of Materials 24: 373–382. De Surville, R., Jozefowicz, M., Yu, L.T. et al. (1968). Electrochemical chains using protolytic organic semiconductors. Electrochimica Acta 13: 1451–1458. Virji, S., Huang, J., Kaner, R.B., and Weiller, B.H. (2004). Polyaniline nanofiber gas sensors: examination of response mechanisms. Nano Letters 4: 491–496. Ndangili, P.M., Waryo, T.T., Muchindu, M. et al. (2010). Ferrocenium hexafluorophosphate-induced nanofibrillarity of polyaniline–polyvinyl sulfonate electropolymer and application in an amperometric enzyme biosensor. Electrochimica Acta 55: 4267–4273. Lu, W., Fadeev, A.G., Qi, B. et al. (2002). Use of ionic liquids for π-conjugated polymer electrochemical devices. Science 297: 983–987. Bolto, B., McNeill, R., and Weiss, D. (1963). Electronic conduction in polymers. III. Electronic properties of polypyrrole. Australian Journal of Chemistry 16: 1090–1103. Suri, K., Annapoorni, S., Sarkar, A.K., and Tandon, R.P. (2002). Gas and humidity sensors based on iron oxide–polypyrrole nanocomposites. Sensors and Actuators B: Chemical 81: 277–282. Geng, L., Wang, S., Zhao, Y. et al. (2006). Study of the primary sensitivity of polypyrrole/r-Fe2 O3 to toxic gases. Materials Chemistry and Physics 99: 15–19.

References

14 Groenendaal, L., Jonas, F., Freitag, D. et al. (2000).

15 16

17

18 19 20 21

22

23

24 25

26

27

28

29 30

Poly(3,4-ethylenedioxythiophene) and its derivatives: past, present, and future. Advanced Materials 12: 481–494. Wang, S., Xu, J., Wang, W. et al. (2018). Skin electronics from scalable fabrication of an intrinsically stretchable transistor array. Nature 555: 83. Borghetti, M., Serpelloni, M., Sardini, E., and Pandini, S. (2016). Mechanical behavior of strain sensors based on PEDOT:PSS and silver nanoparticles inks deposited on polymer substrate by inkjet printing. Sensors and Actuators A: Physical 243: 71–80. Kim, J., Kanwat, A., Kim, H.-M., and Jang, J. (2015). Solution processed polymer light emitting diode with vanadium-oxide doped PEDOT:PSS. Physica Status Solidi A 212: 640–645. Xu, J., Wang, S., Wang, G.-J.N. et al. (2017). Highly stretchable polymer semiconductor films through the nanoconfinement effect. Science 355: 59–64. Lee, Y., Oh, J.Y., Kim, T.R. et al. (2018). Deformable organic nanowire field-effect transistors. Advanced Materials 30: 1704401. Min, S.-Y., Kim, T.-S., Kim, B.J. et al. (2013). Large-scale organic nanowire lithography and electronics. Nature Communications 4: 1773. Yu, X.W., Mahajan, B.K., Shou, W., and Pan, H. (2017). Materials, mechanics, and patterning techniques for elastomer-based stretchable conductors. Micromachines 8: 7–35. Zhao, S.F., Li, J.H., Cao, D.X. et al. (2017). Recent advancements in flexible and stretchable electrodes for electromechanical sensors: strategies, materials, and features. ACS Applied Materials & Interfaces 9: 12147–12164. Chen, H., Song, Y., Guo, H. et al. (2018). Hybrid porous micro structured finger skin inspired self-powered electronic skin system for pressure sensing and sliding detection. Nano Energy 51: 496–503. Larmagnac, A., Eggenberger, S., Janossy, H., and Vörös, J. (2014). Stretchable electronics based on Ag-PDMS composites. Scientific Reports 4: 7254. Hansen, T.S., West, K., Hassager, O., and Larsen, N.B. (2007). Highly stretchable and conductive polymer material made from poly(3, 4-ethylenedioxythiophene) and polyurethane elastomers. Advanced Functional Materials 17: 3069–3073. Chun, K.Y., Oh, Y., Rho, J. et al. (2010). Highly conductive, printable and stretchable composite films of carbon nanotubes and silver. Nature Nanotechnology 5: 853–857. Amjadi, M., Pichitpajongkit, A., Lee, S. et al. (2014). Highly stretchable and sensitive strain sensor based on silver nanowire-elastomer nanocomposite. ACS Nano 8: 5154–5163. Song, L.N., Myers, A.C., Adams, J.J., and Zhu, Y. (2014). Stretchable and reversibly deformable radio frequency antennas based on silver nanowires. ACS Applied Materials & Interfaces 6: 4248–4253. Weng, W., Chen, P., He, S. et al. (2016). Smart electronic textiles. Angewandte Chemie International Edition 55: 6140–6169. Abouraddy, A., Bayindir, M., Benoit, G. et al. (2007). Towards multimaterial multifunctional fibres that see, hear, sense and communicate. Nature Materials 6: 336.

201

202

11 Flexible and Stretchable Devices from Other Materials

31 Wang, Z.L. (2013). Triboelectric nanogenerators as new energy technology

32 33

34

35 36

37 38

39

40

for self-powered systems and as active mechanical and chemical sensors. ACS Nano 7: 9533–9557. Wang, Z.L. and Song, J. (2006). Piezoelectric nanogenerators based on zinc oxide nanowire arrays. Science 312: 242–246. Green, M.A., Ho-Baillie, A., and Snaith, H.J. (2014). The emergence of perovskite solar cells. Nature Photonics 8: https://doi.org/10.1038/nphoton.2014 .134. Zhao, Z., Yan, C., Liu, Z. et al. (2016). Machine-washable textile triboelectric nanogenerators for effective human respiratory monitoring through loom weaving of metallic yarns. Advanced Materials 28: 10267–10274. Zhang, X., Li, Q., Holesinger, T.G. et al. (2007). Ultrastrong, stiff, and lightweight carbon-nanotube fibers. Advanced Materials 19: 4198–4201. Lee, S., Shin, S., Lee, S. et al. (2015). Ag nanowire reinforced highly stretchable conductive fibers for wearable electronics. Advanced Functional Materials 25: 3114–3121. Hamedi, M., Forchheimer, R., and Inganäs, O. (2007). Towards woven logic from organic electronic fibres. Nature Materials 6: 357. Lee, J., Kwon, H., Seo, J. et al. (2015). Conductive fiber-based ultrasensitive textile pressure sensor for wearable electronics. Advanced Materials 27: 2433–2439. Zeng, W., Tao, X.-M., Chen, S. et al. (2013). Highly durable all-fiber nanogenerator for mechanical energy harvesting. Energy & Environmental Science 6: 2631–2638. Song, Y., Zhang, J., Guo, H. et al. (2017). All-fabric-based wearable self-charging power cloth. Applied Physics Letters 111: 073901.

203

Part III Self-Powered Smart System

205

12 Active Sensors Xuexian Chen National Key Lab of Nano/Micro Fabrication Technology, Institute of Microelectronics, Peking University, No. 5 Yiheyuan Road, Beijing 100871, China

As a new power generation technology, triboelectric nanogenerators (TENGs) can convert mechanical energy into electricity; on the other hand, by analyzing the electrical output signals (including V oc , J sc , frequency, etc.) of TENGs, the information regarding the mechanical input (magnitude, frequency, etc.) can be successfully retrieved. Since this sensing technology originates from the output signal of the TENG itself, no external power source is required to apply on the device, which is a unique advantage over conventional sensor technologies. By correlating the mechanical input with different parameters, a lot of prototypes have been realized for various applications, including pressure detection, vibration sensing, chemical sensing, environmental monitoring, etc. In this chapter, we introduce several kinds of active sensors based on the TENG and their practical applications.

12.1 Active Touch Sensors As mentioned in Chapter 2, TENGs have four operation modes, i.e. contactseparation (CS), relative-sliding (RS), single-electrode (SE), and freestanding (FS). Each of them can not only harvest energy from an ambient environment but also its output has plenty of information relative to the external supply. Based on the working principles of TENGs, their output performance (open-circuit voltage, short-circuit current, etc.) is largely affected by the external mechanical stimuli (magnitude/frequency), among which normal pressing is one of the most common types. In this regard, the most straightforward applications for TENG-based active sensors would be the monitoring of external touch motion applied to the TENG. Different modes are suitable for different touch motions, for example, SE-mode TENG has outstanding advantages in the detection of touch from a foreign object due to its unique structure that the moving apart does not need the attachment of an electrode [1]. However, the SE-mode TENG is only suitable for detecting dynamic pressure change, while the CS-mode TENG can differentiate both Flexible and Stretchable Triboelectric Nanogenerator Devices: Toward Self-powered Systems, First Edition. Edited by Mengdi Han, Xiaosheng Zhang, and Haixia Zhang. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

206

12 Active Sensors

static and dynamic pressure [2]. Furthermore, through integrating multiple active pressure/touch sensor units into a sensor array, the as-fabricated sensor matrix can identify various touching modes (i.e. single-point touch, sliding, and multipoint touch) and mapping the local pressure distribution on the device with distinguishable spatial profiles [2, 3]. 12.1.1

Static and Dynamic Pressure Sensor

For a CS-mode TENG, it has been proved that the relative variation of the opencircuit voltage (V oc ) has a directly linear relationship with the applied pressure, while the short-circuit current (J sc ) of TENG is determined by the speed at which the gap of friction layers was squeezed. Therefore, the V oc can be used for static measurement of magnitude of the applied pressure and the J sc is suitable for dynamic measurement on the loading rate of the pressure/force being applied. Based on this basic principle, several prototypes that display high sensitivity, fast response, and low power consumption have been developed [2, 4, 5]. To gain a quantitative understanding and characterization about the contactseparation-mode TENG-based active pressure sensor, Lin et al. at the Georgia Institute of Technology developed a triboelectric active sensor (TEAS) to reveal its basic sensing capabilities [2]. As shown in Figure 12.1a, the device has a double-electrode structure which contains a micropatterned polydimethylsiloxane (PDMS) membrane (Figure 12.1b) and a piece of Al with the composite of Ag nanowires and nanoparticles (Figure 12.1c) as friction layers. Figure 12.1d is the real-time measurement of V oc under a series of different pressures. It could be observed that the V oc stays at the maximum level (V 0 = 60 V) when there is no pressure, and decreases to a lower level once the external pressure is applied. The relationship between relative voltage variation (V 0 − V )/V 0 and magnitude pressure is plotted in Figure 12.1e. In the low-pressure region (Region I), the sensitivity of the TEAS is 0.31 kPa−1 , which is much higher than that of the high-pressure region (0.01 kPa−1 ). On the other hand, the J sc was not only dependent on the applied pressure but also related to the loading rate of the external force, which enables its application for dynamic pressure detection, as shown in Figure 12.1f,g. 12.1.2

Tactile Imaging Sensor

By further assembling many of the triboelectric pressure sensors into a matrix, the sensing array can map the pressure distribution on a 2D plane. Figure 12.2a shows the schematic diagram of a pressure mapping array based on the TEAS mentioned earlier [2]. When no pressure is loaded on the matrix, uniform output voltages distribute through the area, as is shown in Figure 12.2b. Once some of the pixels in the array are pressed simultaneously, the voltage peaks would be detected only from those pixels, while the output profile of the other channels remains almost unchanged in the same period. As a demonstration, a predesigned plastic architecture with the calligraphy of the letters “TENG” was loaded on the sensing matrix. Figure 12.2c–f shows the two-dimensional contour plotting of the peak value of the voltages measured through the matrix.

12.1 Active Touch Sensors

(b)

(a)

50 μm (c) Au electrode PDMS Ag nanowires Al film (e)

Pressure off

70

I

II

0.85 kPa 0.92 kPa 1.00 kPa 1.10 kPa 1.18 kPa 1.38 kPa 1.50 kPa

50 40

(V0–V)/V0

0.8

60

0

4

12 Time (s)

8

(f)

3.50

9.23 kPa 8.10

4.55

2.00

3 1.55

2 0.80 0.40

1

0

20

(g) 7.00

4

Experimental data Fitting of Region I Fitting of Region II

0.4

0.0 16

6 5

0.6

0.2

Pressure on

30

Rectified Jsc (mA/m2)

1.0

1.28

Rectified Jsc (mA/m2)

Voc (V)

(d)

1 μm

2

4 6 8 10 Pressure (kPa)

6

12

14

5 4 3 2 Increasing pressure Decreasing pressure

1 0

0 0

2

4

6 Time (s)

8

10

0

2

4 6 Pressure (kPa)

8

10

Figure 12.1 Self-powered pressure sensor based on CS-mode TENG. (a) Schematic structure of the triboelectric active sensor. (b) SEM image of the PDMS pyramid microstructure. (c) SEM image of the silver nanowire/nanoparticles. (d) The output open-circuit voltage of the triboelectric active sensor upon different pressures. (e) The relative change of the open-circuit voltage with different pressures. (f ) The rectified output short-circuit current with variable pressures. (g) The reversible behavior of the rectified short-circuit current with variable pressures. Source: Reproduced with permission from Lin et al. [2]. Copyright 2013, American Chemical Society.

12.1.3

Single-Electrode Touch Sensor

With the advantage of a vast choice of materials, TENGs can be fabricated to an ultrasimple structure and convert gentle touch to electric signal. Based on this property, a single-electrode TENG (SE-mode TENG) was first developed to work as a self-powered touch sensor in 2013 by our group at Peking University [6]. Figure 12.3a shows the schematic diagram. The device is fabricated on

207

12 Active Sensors

6 TEAS unit

Pixel number

5 4 Voltage (V)

3

0 2.1

2

4.2 6.3

1 0

1

2 3 4 Pixel number

5

6

0

1

2 3 4 Pixel number

5

6

0

1

2 3 4 Pixel number

5

6

(b) 6

6

5

5 Pixel number

Pixel number

(a)

4 3 2

4 3 2 1

1

0

0 0

1

(c)

3 2 4 Pixel number

5

6 (d) 6

5

5 Pixel number

6

4 3 2 1

4 3 2 1

0 0 (e)

8.4

0

Common electrode

Pixel number

208

1

3 2 4 Pixel number

5

0

6 (f)

Figure 12.2 Self-powered pressure mapping based on the TEAS array. (a) A schematic illustration of the TEAS array device. (b) The background signal with no pressure applied on the matrix. (c–f ) The 2D voltage contour plot from the multichannel measurement of the TEAS matrix with an external pressure uniformly applied onto the device through architecture with calligraphy of “T”, “E”, “N”, and “G”. Source: Reproduced with permission from Lin et al. [2]. Copyright 2013, American Chemical Society.

12.1 Active Touch Sensors

(i)

(ii)

Ground Ground (a)

PDMS

PET

ITO

Copper

(i)

(ii) 20 μm (b)

(c)

LCD

STEG1 STEG2 STEG3 STEG4

1

2

3

4

Diodes STEG1 STEG2 STEG3 STEG4

(d)

(e)

Figure 12.3 Self-powered touch sensor. (a) Schematic of the device using a microstructured PDMS friction surface with the grounded reference electrode placed (i) beside or (ii) beneath the induction electrode. (b) SEM image of the micropatterned PDMS film. (c) Photographs of a PDMS surface and PET surface showing their high transparency and flexibility. (d) The logic circuit diagram and (e) photograph of the self-powered touch sensor. Source: Reproduced with permission from Meng et al. [6]. Copyright 2013, Royal Society of Chemistry.

a 125-μm-thick polyethylene terephthalate (PET) substrate. A PDMS film patterned with an array of micropyramids serves as the friction surface. A transparent indium tin oxide (ITO) induction electrode is coated on the back of the PET substrate to work as the electrode. As required for different applications, the reference electrode can be placed beside or beneath the induction electrode, as shown in Figure 12.3a(i), (ii).

209

210

12 Active Sensors

Figure 12.3b shows the SEM image of the micropatterned PDMS film. The photographs of the fabricated device in Figure 12.3c illustrate the flexibility and transparency of the device. By employing four SE-mode TENGs as touch pads and integrating with a rational logic circuit, a visualized touch sensor with self-powered display is demonstrated. Figure 12.3d,e shows the logical diagram and photograph of the self-powered touch sensor. When the SE-mode TENG touch pad is tapped with a finger, the LCD displays which touch pad was touched. SE-mode TENGs can be produced using a very simple fabrication process and can be easily integrated with other objects, which makes possible the use of the TENG in an extended range of applications.

12.2 Active Vibration Sensors Vibration is one of the most common mechanical motion forms in an ambient environment, such as the swaying of trees, vibrations of buildings or bridges, and ocean wave oscillations, etc. Detecting vibrations is of critical importance in equipment maintenance and environment monitoring. As an effective approach to convert vibration motions into electricity, TENGs can also serve as selfpowered active sensors for vibrations [7, 8]. Among the four basic modes of TENGs, the CS-mode and FS-mode are particularly suitable for this purpose. 12.2.1

Vibration Sensor for Quantitative Amplitude Measurement

Generally, vibrations are characterized by two main parameters: frequency and amplitude. Since each cycle of vibration corresponds to one cycle of triboelectric output, the measurement of vibration frequency can be easily realized through CS-mode by counting the frequency of triboelectric waveforms. However, a quantitative measurement of the amplitude is relatively more difficult because the electric generation behavior of the CS-mode is not linear to the separation distance, due to the nature of the changing capacitance between the two electrodes during its operation [9]. Fortunately, freestanding-mode TENG offers a valuable opportunity for this purpose owing to the characteristic that the capacitance between the electrodes remains constant during the operation [10]. It has been proved that the opencircuit voltage change (V oc ) of the contact-mode freestanding TENG has a linear relationship with the distance change Δx, and the equation can be expressed as ΔVoc =

2𝜎0 Δx 𝜀0

(12.1)

where 𝜎 0 is the charge density on the electrode and 𝜀0 is the vacuum permittivity. In 2014, Z.L. Wang and coworkers at the Georgia Institute of Technology developed a new design of TENG for quantitatively measuring the amplitude of vibration. Figure 12.4a demonstrates the structure design of a vibration-enabled contact-mode freestanding TENG (c-FS-TENG) [11]. It has two Al-deposited acrylic plates serving as the two stationary electrodes. In between, another

12.2 Active Vibration Sensors

Al

Spring Acrylic

FEP

1 μm

(a) 30

80

1.0

60

0.5

40

Region II: saturation

20

0.0

0.4

20 0.3

10

0.2 0.1

0

0.0

0

(b)

Isc amplitude (μA)

1.5

0.5 15 Hz

2.0

Isc amplitude (μA)

15 Hz 120 Region I linear 100

Voc peak-to-peak (V)

Voc peak-to-peak (V)

140

–0.5 0

1

3 4 5 6 2 Vibration amplitude (mm)

7

0.00

(c)

0.05

0.10

0.15

0.20

0.25

0.30

Vibration amplitude (mm)

Figure 12.4 Self-powered vibration sensor. (a) Schematic diagram showing the typical device structure of an FS-TENG. Insets: schematic diagram and SEM image of the nanowire structure on the surface of the FEP films. (b) V oc and Isc from the CF-TENG triggered by vibrations with different amplitudes, when the vibration source is at the resonating frequency (15 Hz) of the FS-TENG. (c) Enlarged plot of Figure 12.3b with vibration amplitude in the range below 0.3 mm. Source: Reproduced with permission from Wang et al. [11]. Copyright 2014, American Chemical Society.

acrylic sheet is employed as the vibration resonator, with its four corners connected by eight springs to the two ends of the acrylic skeleton. As shown in Figure 12.4b, when the frequency is at the resonant frequency of the device and the vibration amplitude of the shaker is within a small range (below 1.5 mm, i.e. Region I shown in Figure 12.4b, both the peak-to-peak value of the V oc and the amplitude of the I sc increase almost linearly with the increase in the vibration amplitude. Figure 12.4c is the enlarged plot of the curves in Figure 12.4b below the vibration amplitude of 0.3 mm. It can be found that the both the V oc and the I sc have very good linear responses, proving the capability of c-FS-mode TENG as a quantitative measurement vibration sensor. Because of the unique characteristic of the c-FS-TENG, the amplitudes of the generated electric signals are proportional to the amplitude of the vibration under the same frequency. This device is also the first triboelectrification-based self-powered vibration sensor that can quantify the amplitude, as well as the frequency.

211

12 Active Sensors

12.2.2

Vibration Acceleration Sensor

Acceleration is one of the most important parameters in the characterization of an object’s movement. Based on the noncontact-freestanding TENG, a novel self-powered accelerometer sensor with the sleeve-tube structure for vibration detection was proposed recently by Z.L. Wang’s group [12]. As shown in Figure 12.5a, this structure consists of an outer transparent sleeve-tube and an inner cylindrical inertial mass suspended by a highly stretchable silicone fiber. It has been proved that the quantitative relationship of output voltage, vibration frequency, and acceleration can be expressed by the following equation: Voc = k

1 a f2

(12.2)

where the parameters are defined as follows: the output voltage of triboelectric accelerometer V oc , vibration frequency f , constant coefficient k, and acceleration a (m/s2 ). Theoretically, the output voltage of the device has a linear relationship with the acceleration. In order to evaluate the performance of the proposed triboelectric accelerometer, the output voltage of the device at different vibration accelerations are measured at a fixed frequency. As shown in the curve of Figure 12.5b, the amplitude of output voltage is proportional to the vibration acceleration, which demonstrates an obvious linear relationship by fitting the data with a correlation coefficient of 0.975 and a slop of 3.903. So this sensor has a high sensitivity of 0.391 (V S2 )/m. This triboelectric-based accelerometer has a more superior performance in the low vibration frequency range (below 5 Hz) compared with the traditional piezoelectric accelerometer, which can be used to monitor the infrastructure health in a real-time bridge displacement monitoring system. 25 Experimental data Fitting

Silicon rubber Cu Acrylic FEP

20 Voltage (V)

212

15

10

1 (a)

2

3 4 Acceleration (g)

5

(b)

Figure 12.5 Self-powered triboelectric accelerometer. (a) Schematic diagram showing the typical device structure of the proposed triboelectric accelerometer. (b) The relationship between the measured positive peak value of output voltage and the corresponding acceleration. Source: Reproduced with permission from Yu et al. [12]. Copyright 2017, John Wiley & Sons.

12.2 Active Vibration Sensors

12.2.3

Vibration Direction Sensor

Except for the vibration frequency and amplitude, direction is also a vital parameter for characterizing the vibration motion. In 2018, Haixia Zhang’s group at Peking University proposed a newly designed TENG with eight petal electrodes to actively measure the vibration direction [13]. As shown in Figure 12.6a, the device employs eight petal electrodes to constitute four freestanding TENGs, which can measure the vibration at eight directions by analyzing the output signals from the four TENGs. At the center of the discoid shell, a magnet mass with a diameter of 3 cm is mobile and suspended by four identical springs. When the device vibrates along an arbitrary in-plane direction, corresponding electrodes along the motion direction will generate the highest output (Figure 12.6b). Figure 12.6c–f shows the comparisons of output voltage and current among the four pair of electrodes when the device is excited along the direction of 11′ , 22′ , 33′ , and 44′ , respectively. Obviously, both the output voltage and current are the largest for the pair of electrodes along the motion direction. To prove the capability of the device as a self-powered vibration direction sensor, a “Hit Hamster” game was designed and implemented with the device as the human–machine interface. The superior vibration-sensing capacity enables the device to detect many features of vibration such as direction and frequency, showing its great potential in the field of self-powered sensing systems including alarms, environmental/ infrastructure monitoring, and motion recognition. 12.2.4

Acoustic Sensor

Acoustic vibration is one of the most common vibration forms, with a wide range of frequencies and strengths. With the advantages of a vast choice of materials and flexible structure designs, TENGs provide an effective means for converting acoustic vibration into electric output. The first organic thin-film-based TENG was developed by Z.L. Wang’s group to not only scavenge ambient acoustic energy as a sustainable power source but also measure the acoustic waves as a selfpowered active acoustic sensor in the low-frequency range (Figure 12.7a) [14]. Utilizing a Helmholtz cavity, the sensitivity of the as-fabricated device was as high as 9.54 V/Pa in the acoustic pressure range from 70 dBSPL (0.063 Pa) to 110 dBSPL (6.32 Pa). Besides, since the acoustic waves decay in the course of propagation, the distance of the measured device to the acoustic source shows a tremendous impact on the electric output. Therefore, the TENG sensor can be used to measure the location of acoustic source. Figure 12.7b shows the relationship of the opencircuit voltage and the distance. It is worth noting that the electric output attenuates at a rate of 6 dB each time the distance from the acoustic source doubles. Furthermore, by combining several devices with different natural frequencies, the TENGs can be used as a self-powered microphone by analyzing the record signal with short-time Fourier transform. Figure 12.7c shows an as-fabricated nanogenerator working as a self-powered microphone for sound recording.

213

3 2

4

E3 E4

E2

E 1′

Voltage (V)

Voltage (V)

40

0.2

E22′

E33′

Electrode

(d)

0.8

40

0.4

0.0

0

E44′

E11′

E22′

E33′

Electrode

E44′

(b)

80

0.8

40

0.4

0.0

0

(e)

80

E11′

E22′

E33′

Electrode

E44′

0.0

0

(f)

Current (μA)

0.4

0.0 E11′

NdFeB magnet

80

Current (μA)

0.4

Stainless steel

4′ 3′

Current (μA)

40

Current (μA)

0.8

0

(c)

Al 1.2

2′

Voltage (V)

Kapton

E4′

E 3′

Voltage (V)

Cu

PMMA 80

1

E1

E 2′

(a)

1′

E11′

E22′

E33′

E44′

Electrode

Figure 12.6 Self-powered vibration direction sensor. (a) Schematic diagram of the device. (b) Schematic illustration showing the motion of mass. (c–f ) The comparison of maximum output voltage and current of the four TENGs under the motion direction of 11′ , 22′ , 33′ , and 44′ , respectively. Source: Reproduced with permission from Chen et al. [13]. Copyright 2018, Elsevier.

(b)

Short-circuit current (μA)

Open-circuit voltage (V)

12.3 Active Motion Sensors

16 14 12 10 8 6 4 2 0 –2

60 50 40 30 20

1

10 Distance (cm)

100

10 0 1

10 Distance (cm)

100

Self-powered microphone Acrylic

PTFE

PET

Copper

Aluminum

(a)

(c)

Figure 12.7 Self-powered acoustic sensor. (a) Schematic diagram of the active acoustic sensor. (b) The self-powered acoustic sensor for distance measurement as well as ambient acoustic source detection. (c) Demonstration of the nanogenerator as a self-powered microphone. Source: Reproduced with permission from Yang et al. [14]. Copyright 2014, American Chemical Society.

This design presents an adaptable, mobile, and cost-effective technology for harvesting acoustic energy from the ambient environment, with huge potential in the applications of infrastructure monitoring, sensor networks, military surveillance, and environmental noise reduction.

12.3 Active Motion Sensors Mechanical motions are generally characterized by a series of parameters, such as movement distance, velocity, accelerations, and angles, etc. Monitoring and detecting these parameters of moving objects is very important for building up smart mechanical systems. Since the frequency, amplitude, and total period of the generated electric signals are all related to the characteristics of the input mechanical motion, TENGs with various structure designs have been developed to monitor these parameters and serve as self-powered motion sensors [15, 16]. In this section, the RS-mode TENG has played an important role because its output is related to the relative sliding area. 12.3.1

Linear Displacement Sensor

Measurement of displacement and speed has ubiquitous applications in the scientific and industrial fields such as manufacturing, automation, robotics, and nano-manipulation. TENGs with a segmentally structured design show potential

215

12 Active Sensors

to serve as self-powered linear motion sensors since the periods of the electrode are fixed. By counting the cycles of output waveform or measuring the time interval of the adjacent waveform, the displacement or speed of objects during this period can be calculated. In 2014, Z.L. Wang’s group proposed a nanometer-resolution self-powered motion sensor based on micro-grated triboelectrification (Figure 12.8a) [17]. During the movement, the open-circuit voltage measured between the two electrodes (Figure 12.8b) alternates periodically with a frequency of about 5 Hz. Since each cycle corresponds to one period of the gratings, which is 200 μm in this device, the real-time displacement can be calculated by accumulating an extra 200 μm once one more peak is detected. As shown in the blue curve in Figure 12.8c, the detected displacement increases linearly with time. The real-time motion speed can also be derived by dividing the width of one pitch by the time interval between two adjacent voltage peaks. This displacement sensor opens a new paradigm for displacement/speed sensing through a combination of self-powered, nano-resolution, high-robustness, nonoptical simple structures as well as low cost, which has wide potential applications in automation, manufacturing, process control, etc. Aluminum

SiO2 Silicon

ion Mot

n ctio dire

SU-8 Parylene Glass

ITO

(a) 200

10

OC voltage Position (mm)

50

1.0

4

0.8

2

0.6

0

0 0

(b)

1.2

6

2

4

8 6 Time (s)

10

12

0

(c)

Speed (mm/s)

100

1.4

Position Velocity

8

150 Voltage (mV)

216

2

4

6 8 Time (s)

10

12

Figure 12.8 Self-powered linear displacement sensor. (a) Schematic structure of the TENG-based self-powered displacement sensor. (b) The open-circuit voltage signals acquired from a displacement of 9.2 mm at a speed of 1 mm/s. (c) The real-time displacement and speed derived from the measured voltage signal. Source: Reproduced with permission from Zhou et al. [17]. Copyright 2014, John Wiley & Sons.

12.3 Active Motion Sensors

12.3.2

Angle Sensor

Besides linear motion sensing, TENGs, with their rational structure design, can also detect the absolute angular position, angular velocity, and acceleration. In 2015, Z.L. Wang’s group presented the first self-powered angle measurement sensor based on SE-mode TENG, as shown in Figure 12.9a [18]. The device consists of two parts, a rotator and a stator. The rotator employs a collection of radially arrayed sectors separated by equal-degree intervals in between as the electrification layer (Figure 12.9b). The stator consists of a layer of Cu as electrode and a layer of fluorinated ethylene propylene (FEP) spread out on the Cu substrate, working as another electrification layer (Figure 12.9c). The operation of the TENG sensor relies on the relative rotation between the rotator and the stator, in which the Cu sectors on different rings of the rotator pass the Cu electrodes on the stator in different sequences, generating different waveforms on the four electrodes. For demonstration, the sensor is mounted with a steering wheel for monitoring and mapping the angle of the steering wheel in real time. Figure 12.9d presents the measured output voltage and real-time location mapping when the steering wheel rotates counter clockwise to 0∘ , 90∘ , 180∘ , 270∘ , and 360∘ , respectively. The resolution of the sensor is 22.5∘ and can be further improved by increasing the number of channels. This design demonstrated a new principle in the field of angular measurement, which can be used for monitoring and mapping the angle of the steering wheel in real time. 12.3.3

Omnidirectional Tilt Sensor

Except for the regular motion, a TENG that can measure omnidirectional tilt angle was also demonstrated by the group of Haixia Zhang at Peking University [19]. In the sensing system, two magnetic-assisted TENGs are employed. The structure design of the TENG is shown in Figure 12.10a, which includes the top steel mass, top NdFeB permanent magnet, silica layer, spiral-shaped electrode wrapped by polyimide, and the bottom NdFeB permanent magnet. The top magnet also serves as the top electrode of the TENG and has a polarity opposite to the bottom magnet, thus providing a magnetic repulsive force. The photograph of the whole TENG is shown in Figure 12.10b. For omnidirectional sensing, two TENGs perpendicular to each other were mounted to a cube. As illustrated in Figure 12.10c, in the original state, the tilt angles of the two TENGs are 0∘ and 90∘ , respectively. With an arbitrary incline (i.e. tilted state in Figure 12.10c), the tilt angles of the two TENGs will be changed, thus affecting their output voltages. Experimental measurement was conducted under 16 tilt angles in different directions, as shown in Figure 12.10d. By integrating the tilt sensor with LCD screen, a visualized self-powered tilt sensing system can be achieved. This self-powered visualized system eliminates the complicated measure process, and brings great opportunity for the further development of self-powered systems.

217

12 Active Sensors

(a)

(b)

(c) L1 L2 L3 L4

Foam tape

FEP

Copper

5

15

(d) –5 0 L4 0°

Open-circuit voltage (V)

218

10

Acrylic

Time (s) 20 25 30

35

40

270°

45

50

90°

180°

0

0

0

0

0

0

1

0

1

0

0

1

1

0

0

0

0

1

1

0

55

360°

L3

L2

L1

I

II

III

IV

V

Figure 12.9 Self-powered angle sensor. (a) Schematic illustration of the self-powered angle sensor. (b) Photograph of the rotator. (c) Photograph of the stator. (d) Measured output voltage and real-time location mapping when the steering wheel rotates counter clockwise to (I) 0∘ , (II) 90∘ , (III) 180∘ , (IV) 270∘ , and (V) 360∘ . Source: Reproduced with permission from Wu et al. [18]. Copyright 2015, John Wiley & Sons.

12.4 Active Chemical/Environmental Sensors

Mass NdFeB magnet Silica PTFE Polyimide Copper electrode (b)

y′

v x Original state

Tilted state

(c)

35 90 10 90

60 60 φ (°) 30

30

)

x′

60

(°

Z′

θ

z

Peak voltage (V)

(a)

0 0

(d)

Figure 12.10 Self-powered tilt sensor. (a) Schematic diagram and (b) photograph of the magnetic-assisted TENG. (c) Schematic diagram of the omnidirectional tilt sensor. (d) Output peak voltage of the two TENGs at 16 different tilt angles. Source: Reproduced with permission from Han et al. [19]. Copyright 2014, Springer Nature.

12.4 Active Chemical/Environmental Sensors As discussed in previous chapters, a key factor that determines the output performance of a TENG is the triboelectric charge density generated on the material surface. As revealed in Section 3.2, surface modification of certain chemical species and alteration of environment factors will greatly affect the triboelectric charge density and the output power. Therefore, by monitoring the changes of output performance, properly designed TENGs can serve as self-powered chemical and environmental sensors for detection of ion concentration, UV illumination, humidity, etc. [20–22] 12.4.1

Chemical Sensor

The first triboelectric chemical sensor was developed by Z.H. Lin et al. from the Georgia Institute of Technology for the detection of Hg2+ ions [20]. Based on a typical contact-mode TENG, the sensor has a layered structure based on two plates, as shown in Figure 12.11a. Glass was selected as the substrate, On the lower side, Au nanoparticles were assembled onto the metal plate to improve the contact area, and the NPs were further modified with 3-mercaptopropionic acid (3-MPA) molecules with strong Au–S interactions. Figure 12.11b–d shows the SEM images of Au NPs with three different sizes. The Hg2+ ions in the buffer solution would be bound on 3-MPA molecules and the surface polarity of the metal plate would be subject to change, which

219

12 Active Sensors

(a)

(b)

(i)

(ii) 1 μm

(c)

Glass substrate Au film Au nanoparticle (13, 32, and 56 nm) PDMS film

1 μm

(d) (iii)

1 μm

(f)

1.0

0.8

0.8

0.6

0.6

0.4 0.2 0

M nM M M M M M 10 n 100 500 n 1 μ 5 μ 10 μ 100 μ Mercury ion concentration

0.4 0.2 0

+

1.0

(I0–I)/I0

(e)

Na + K 2+ Mg 2+ Ca 2+ Sr 2+ Ba Mn2+ 2+ Fe 3+ Fe Co2+ 2+ Ni 2+ Cu 2+ Zn Cd2+ Hg2+ 2+ Pb 3+ Ai

(iv)

(I0–I)/I0

220

Metal ions

Figure 12.11 Self-powered mercury ion sensor. (a) Fabrication process of the TENG. (b–d) SEM images of the Au film modified with (b) 13 nm, (c) 32 nm, and (d) 56 nm Au NPs. (e) Sensitivity and (f ) selectivity of the as-fabricated TENG for the detection of Hg2+ ions. Source: Reproduced with permission from Lin et al. [20]. Copyright 2013, John Wiley & Sons.

then tuned the surface charge density accordingly. Hence, the detection of the Hg2+ concentration was enabled by real-time monitoring of the TENG’s output performance. A linear relationship between the relative variation of J sc and the concentration of Hg2+ ions indicated an effective detection range from 100 nM to 5 mM, and the selectivity of the developed system toward the Hg2+ ion detection as compared to other metal ions was confirmed with a control experiment, as shown in Figure 12.11e,f. As different ions, molecules, and materials have unique triboelectric polarities, the TENG can become either an electrical turn-on or turn-off sensor when the analytes selectively bind to the modified electrode surface. This work is a stepping stone for related TENG studies and inspires the development of TENGs toward other metal ions and biomolecules.

12.4 Active Chemical/Environmental Sensors

12.4.2

UV Sensor

With a similar structure design, Z.H. Lin et al. from the Georgia Institute of Technology proposed an active UV photodetector [21]. In their work, 3D dendritic TiO2 nanostructures were synthesized as the built-in UV photodetector as well as the contact material of the TENG. The fabrication process of the device is shown in Figure 12.12a. The resistance of the built-in photodetector varies upon UV light irradiation with different intensities, which will consequently influence the measured output of the TENG from the two leads. In the dark, the resistance of the built-in photodetector is larger, and hence the output current will be lower. Under UV light irradiation, the resistance of the built-in photodetector decreases. The output current of the self-powered photodetector when sensing UV light at different intensity is displayed in Figure 12.12b. The dependence of the output current on the incident light power density can be seen more clearly in Figure 12.12c. It is found that the output current has a linear relationship with the incident light power density from 20 μW/cm2 to 7 mW/cm2 .

ic drit den res D 3 u of uct wth nostr Gro na TiO 2

ITO thin-film deposition

PDMS layer coating Glass ITO thin-film PDMS layer 3D dendritic TiO2 nanostructures Ni thin-film

(a)

Photodetector electrode (Ni) deposition

Cu wire

0.8

Current ( A)

Current ( A)

00

0 –0.4

Built-in photodetector

TENG

0.4

A

10–1

10–2 –0.8 0

(b)

10

20

30 Time (s)

40

50

60

10–2

(c)

10–1 100 Intensity (mW/cm2)

101

Figure 12.12 Self-powered UV sensor. (a) Fabrication process of the self-powered UV photodetector. (b) Output current of the self-powered sensor under UV light illumination with various power intensities. (c) The dependence of the output current of the self-powered UV sensor on the different light intensities. Source: Reproduced with permission from Lin et al. [21]. Copyright 2014, John Wiley & Sons.

221

222

12 Active Sensors

The UV sensor skillfully integrates UV sensing material and TENG together, achieving self-powered UV detection in an easily fabricated, cost-effective, and robust way. This work demonstrates the great potential of TENG-based self-powered active sensors.

12.5 Summary In this chapter, we have introduced several kinds of TENG-based active sensors. By monitoring the relationship between output signals and mechanical motion (i.e. touching, linear sliding, vibration, rotation, and so on), TENGs can serve as self-powered active sensors for all these different types of mechanical motions. Besides, since the amplitude of the electrical signals generated by the TENGs are proportional to the surface triboelectric charge density that is determined by the chemical state of the surface, the TENGs can also be utilized as active chemical sensors. TENGs also have been demonstrated in many practical applications, such as self-powered smart microphones, infrastructure health monitors, and for mapping the angle of the steering wheel, etc. However, the detection ranges of TENG-based active sensors are not limited to these introductions; there are also many other applications in the monitoring of impact [23], magnetic [24], angular motion [25], body motion [26–28], alcohol/ethanol [29, 30], biomedical signals [31–33], etc. With the rapid development of electronic systems and Internet of Things, we believe that the TENG-based active sensors will be more and more favorable due to their easily fabricated, low-cost, and sustainable characteristics.

Abbreviations 3-MPA Au–S CF-TENG CS-TENG Cu FEP FS-TENG Hg ITO LCD NdFeB NPs PDMS PET RS-TENG SE-TENG SEM

3-mercaptopropionic acid gold-sulfur contact-mode freestanding triboelectric nanogenerator contact-separation-mode triboelectric nanogenerator copper fluorinated ethylene propylene freestanding-mode triboelectric nanogenerator mercury indium tin oxide liquid-crystal display neodymium iron boron nanoparticles polydimethylsiloxane polyethylene terephthalate relative-sliding-mode triboelectric nanogenerator single-electrode-based triboelectric nanogenerator scanning electron microscopy

References

TENG TEAS TiO2 UV

triboelectric nanogenerator triboelectric active sensor titanium dioxide ultraviolet

References 1 Yang, Y., Zhang, H., Lin, Z. et al. (2013). Human skin based triboelectric

2

3

4

5

6

7

8

9

10 11

12

13

nanogenerators for harvesting biomechanical energy and as self-powered active tactile sensor system. ACS Nano 7: 9213–9222. Lin, L., Xie, Y., Wang, S. et al. (2013). Triboelectric active sensor array for self-powered static and dynamic pressure detection and tactile imaging. ACS Nano 7: 8266–8274. Wang, X., Zhang, H., Dong, L. et al. (2016). Self-powered high-resolution and pressure-sensitive triboelectric sensor matrix for real-time tactile mapping. Advanced Materials 28: 2896–2903. Fan, F., Lin, L., Zhu, G. et al. (2012). Transparent triboelectric nanogenerators and self-powered pressure sensors based on micropatterned plastic films. ACS Nano 12: 3109–3114. Lee, K., Yoon, H., Jiang, T. et al. (2016). Fully packaged self-powered triboelectric pressure sensor using hemispheres-array. Advanced Energy Materials 6: 1502566. Meng, B., Tang, W., Too, Z. et al. (2013). A transparent single-friction-surface triboelectric generator and self-powered touch sensor. Energy & Environmental Science 6: 3235–3240. Hu, Y., Yang, J., Jing, Q. et al. (2013). Triboelectric nanogenerator built on suspended 3D spiral structure as vibration and positioning sensor and wave energy harvester. ACS Nano 11: 10424–10432. Chen, J., Zhu, G., Yang, W. et al. (2013). Harmonic-resonator-based triboelectric nanogenerator as a sustainable power source and a self-powered active vibration sensor. Advanced Materials 25: 6094–6099. Niu, S., Wang, S., Lin, L. et al. (2013). Theoretical study of contact-mode triboelectric nanogenerators as an effective power source. Energy & Environmental Science 6: 3576–3583. Niu, S., Liu, Y., Chen, X. et al. (2015). Theory of freestanding triboelectric-layer-based nanogenerators. Nano Energy 12: 760–774. Wang, S., Niu, S., Yang, J. et al. (2014). Quantitative measurements of vibration amplitude using a contact-mode freestanding triboelectric nanogenerator. ACS Nano 8: 12004–12013. Yu, H., He, X., Ding, W. et al. (2017). A self-powered dynamic displacement monitoring system based on triboelectric accelerometer. Advanced Energy Materials 7: 1700565. Chen, X., Guo, H., Wu, H. et al. (2018). Hybrid generator based on freestanding magnet as all-direction in-plane energy harvester and vibration sensor. Nano Energy 49: 51–58.

223

224

12 Active Sensors

14 Yang, J., Chen, J., Liu, Y. et al. (2014). Triboelectrification-based organic film

15 16

17

18

19

20 21 22

23

24 25 26

27

28

29

30

nanogenerator for acoustic energy harvesting and self-powered active acoustic sensing. ACS Nano 8: 2649–2657. Su, Y., Zhu, G., Yang, W. et al. (2014). Triboelectric sensor for self-powered tracking of object motion inside tubing. ACS Nano 4: 3843–3850. Yi, F., Lin, L., Niu, S. et al. (2014). Self-powered trajectory, velocity, and acceleration tracking of a moving object/body using a triboelectric sensor. Advanced Functional Materials 24: 7488–7494. Zhou, Y., Zhu, G., Niu, S. et al. (2014). Nanometer resolution self-powered static and dynamic motion sensor based on micro-grated triboelectrification. Advanced Materials 26: 1719–1724. Wu, Y., Jing, Q., Chen, J. et al. (2015). A self-powered angle measurement sensor based on triboelectric nanogenerator. Advanced Functional Materials 25: 2166–2174. Han, M., Zhang, X., Sun, X. et al. (2014). Magnetic-assisted treiboelectric nanogenerator as self-powered visualized omnidirectional tilt sensing system. Scientific Reports 4: 4811. Lin, Z.H., Zhu, G., Zhou, Y. et al. (2013). A self-powered triboelectric nanosensor for mercury ion detection. Angewandte Chemie 125: 5169–5173. Lin, Z.H., Cheng, G., Yang, Y. et al. (2014). Triboelectric nanogenerator as an active UV photodetector. Advanced Functional Materials 24: 2810–2816. Guo, H., Chen, J., Tian, L. et al. (2014). Airflow-induced triboelectric nanogenerator as a self-powered sensor for detecting humidity and airflow rate. ACS Applied Materials & Interfaces 6: 17184–17189. Zhu, G., Lin, Z., Jing, Q. et al. (2013). Toward large-scale energy harvesting by a nanoparticle-enhanced triboelectric nanogenerator. Nano Letters 13: 847–853. Yang, Y., Lin, L., Zhang, Y. et al. (2012). Self-powered magnetic sensor based on a triboelectric nanogenerator. ACS Nano 6: 10378–10383. Shi, Q., Wu, H., Wang, H. et al. (2017). Self-powered gyroscope ball using a triboelectric mechanism. Advanced Energy Materials 7: 1701300. Yi, F., Lin, L., Niu, S. et al. (2015). Stretchable-rubber-based triboelectric nanogenerator and its application as self-powered body motion sensors. Advanced Functional Materials 25: 3688–3696. Chen, X., Song, Y., Chen, H. et al. (2017). An ultrathin stretchable triboelectric nanogenerator with coplanar electrode for energy harvesting and gesture sensing. Journal of Materials Chemistry A 5: 12361–12368. Chen, X., Miao, L., Guo, H. et al. (2018). Waterproof and stretchable triboelectric nanogenerator for biomechanical energy harvesting and self-powered sensing. Applied Physics Letters 20: 203902. Zhang, H., Yang, Y., Su, Y. et al. (2013). Triboelectric nanogenerator as self-powered active sensors for detecting liquid/gaseous water/ethanol. Nano Energy 2: 693–701. Wen, Z., Chen, J., Yeh, M. et al. (2015). Blow-driven triboelectric nanogenerator as an active alcohol breath analyzer. Nano Energy 16: 38–46.

References

31 Ma, Y., Zheng, Q., Liu, Y. et al. (2016). Self-powered, one-stop, and

multifunctional implantable triboelectric active sensor for real-time biomedical monitoring. Nano Letters 16: 6042–6051. 32 Lai, Y., Deng, J., Zhang, S. et al. (2017). Single-thread-based wearable and highly stretchable triboelectric nanogenerators and their applications in cloth-based self-powered human-interactive and biomedical sensing. Advanced Functional Materials 27: 1604462. 33 Chen, X., Song, Y., Su, Z. et al. (2017). Flexible fiber-based hybrid nanogenerator for biomechanical energy harvesting and physiological monitoring. Nano Energy 38: 43–50.

225

227

13 Hybrid Sensing Technology Xiaosheng Zhang 1 , Yanyuan Ba 1 , and Mengdi Han 2 1 University of Electronic Science and Technology of China, School of Electronic Science and Engineering, No. 2006, Xiyuan Ave, West Hi-Tech Zone, 611731 Chengdu, China 2 Technological Institute, Northwestern University, Center for Bio-Integrated Electronics, 2145 Sheridan Road, Evanston, IL 60208, USA

In earlier chapters, triboelectric nanogenerators (TENGs) have been proved as an effective approach to sense the changes of environmental factors. Similarly, according to the quantitative or qualitative relations between electric outputs and external inputs, nanogenerators (NGs) based on other working principles can be utilized as sensors too. Actually, hybrid nanogenerators based on the combination of different energy harvesting mechanisms have developed rapidly in the past decade. Briefly, the energy-harvesting mechanisms can be classified into five categories [1–5], as discussed in Chapter 1: triboelectric nanogenerators (TENGs), piezoelectric nanogenerators (PNGs), electromagnetic generators (EMGs), thermoelectric nanogenerators (ThNGs), and photovoltaic cells (PVCs). In order to enhance the performance of single-principle devices, hybrid TENGs were developed using the combination of TENGs and other energy-harvesting approaches. Later, researchers proposed many hybrid multifunctional sensors based on these hybrid TENGs to detect the variation of environmental factors in the wider range. This chapter focuses on hybrid sensors, such as piezoelectric–triboelectric hybrid sensors, thermoelectric–triboelectric hybrid sensors, and even triple-mechanism or quadruple-mechanism hybrid sensors, etc (Figure 13.1).

13.1 Dual Hybrid Power Technology Generally, four approaches, including the introduction of new materials, the introduction of new structures, surface patterning, and the hybrid with multiple harvesting mechanisms, were adopted to optimize TENGs. In this section, some commonly used dual-mechanism hybridization methods are introduced, such as triboelectric–piezoelectric effect [6–9], triboelectric–photovoltaic effect [10–12], triboelectric–thermoelectric effect [13], triboelectric–electromagnetic effect [13–15], etc. Flexible and Stretchable Triboelectric Nanogenerator Devices: Toward Self-powered Systems, First Edition. Edited by Mengdi Han, Xiaosheng Zhang, and Haixia Zhang. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

13 Hybrid Sensing Technology

r ete ram pa ng ori nit Mo

Multiple hybrid sensor

Ac cu rac y

228

Dual hybrid sensor

Individual sensor

Figure 13.1 The illustration of hybrid sensors.

13.1.1

Triboelectric–Piezoelectric Nanogenerator

Hybrid triboelectric–piezoelectric nanogenerators (TPNGs) can generate both triboelectricity and piezoelectricity at the same time to enhance the energy conversion efficiency, resulting in the enhancement of output performance. The commonly used piezoelectric materials include ZnO, polyvinylidene fluoride (PVDF), piezoelectric ceramic transducer (PZT), BaTiO3 , ZnSnO3 , etc. At present, several TPNG prototypes have been reported and demonstrate some specific properties. The device structure of different piezoelectric materials may be different. Figure 13.2 shows the schematic diagrams of several TPNG prototypes. M.D. Han et al. from Peking University reported an r-shaped triboelectric–piezoelectric nanogenerator [6], as shown in Figure 13.2a. Zhonglin Wang’s group from the Georgia Institute of Technology demonstrated a hybrid triboelectric–piezoelectric nanogenerator for harvesting water wave energy [7], as shown in Figure 13.2b. Dong Sun and coworkers from North University of China reported an arch-shaped triboelectric–piezoelectric nanogenerator based on BaTiO3 [8]. Y. Guo et al. from École Polytechnique Fédérale de Lausanne fabricated an all-fiber hybrid piezoelectric-enhanced TENG for wearable gesture monitoring [9], as shown in Figure 13.2c. Although their structures have some differences, their working principles are similar. Here, we take the all-fiber hybrid triboelectric–piezoelectric nanogenerator as an example to introduce their operating principle [9]. The operating principle of TPNGs is similar to that of TENGs; however, the difference is that when the top electrode and the bottom electrode contact under external force,

(m)

Aluminum

PVDF

PDMS 20 μm (f)

(b)

FEP

PET

Cu

PTFE

Kapton

500 nm

0

–60

0.2 s

–30 –60 0

1

2

100

3

4

5

6

100 0

–100

0

–200

0.2 s

–100 –200 0

1

2

(g)

3 Time (s)

4

5

6

IE-TENG

60

Voltage (V)

(i)

30

–30

0

200

(n)

(h)

30

40

20

Voltage (V)

PET/ITO

Output voltage (V)

500 nm (e)

60

Voltage (V)

(l)

(d)

Voltage (V)

Output voltage (V)

(a)

60 40 20

0 0

5

10 Time (s)

0 10

11

12

Time (s)

15

13

20

Figure 13.2 The schematic diagrams and outputs of TPNGs. (a–c) The schematic diagrams and (d–k) the surface microstructures of several different TPNGs and their corresponding (l–o) electric output performance. Source: Reproduced with permission from Refs. [6, 7, 9]. Copyright 2013, American Chemical Society, 2014 Elsevier and 2018 Elsevier, respectively.

(j) (c) (o)

600

Silk (k)

Voltage (V)

400 200 0 –200 –400 0 PVDF

Figure 13.2 (Continued)

1

2

3 4 Time (s)

5

6

13.1 Dual Hybrid Power Technology

an additional piezoelectric output is generated. Under a deformation caused by pressure, the PVDF generates piezoelectric bound charges, which cause a built-in potential between the top and bottom surfaces. To neutralize this potential, the electron flow is generated from the top electrode to the bottom electrode. When the external force is removed, the built-in piezoelectric potential gradually decreases and the electrons will flow back to keep the electrostatic neutral state until the PVDF fully recovers. Therefore, the addition of the piezoelectric effect leads to the enhancement of the output performance of the designed devices. Polarization should be applied to piezoelectric materials to make them possess piezoelectric property. The direction of polarization determines the output performance of the TPNG. If the current direction of piezoelectricity after polarizing is accordant with the current direction of triboelectricity, an enhancement in output performance of the designed hybrid nanogenerator can be obtained. On the contrary, when the directions of triboelectricity and piezoelectricity are contrary, the piezoelectric effect will weaken the output performance of the designed hybrid nanogenerator. In this case, when the directions of triboelectricity and piezoelectricity are coincident, the output voltage can reach up to ∼72 V driven by only pressing without separation. Conversely, when the directions are contrary, the output voltage is weakened to ∼39 V. In Ref. [6], the r-shaped TPNG also adopted PVDF as the piezoelectric material, while the arch-shaped TPNG adopted BaTiO3 as the piezoelectric material [8]. Normally, BaTiO3 is mixed in flexible materials. In order to make the device inherit the flexibility of the materials, the concentration of BaTiO3 is limited, resulting in a normal lower output performance of the TPNG based on BaTiO3 . 13.1.2

Triboelectric–Photovoltaic Nanogenerator

As described in Section 1.2.1, under the irradiation of electromagnetic waves up to a certain frequency, electrons in substances will be excited by photons to form electric current, namely, the photovoltaic effect. Hybrid triboelectric–photovoltaic nanogenerators can be used to harvest both light and mechanical energy from the environment. Multi-junction cells, single-junction GaAs, crystalline silicon cells, thin-film technologies, and emerging others are the five main categories of solar cells. Although the first three types show higher energy conversion efficiency (ECE) (21.2–46%) [12], their material properties are frangibility and rigidity, which restrict the development of integrating with the TENGs. The two types behind possess flexibility, but their ECEs are relatively low. Based on these, researchers made a great deal of effort to investigate hybrid triboelectric–photovoltaic nanogenerators. The working principle of the hybrid triboelectric–photovoltaic nanogenerators includes two aspects: the photovoltaic components to generate power from solar energy and the TENGs to convert mechanical movement into electricity. And the two aspects can work independently.

231

232

13 Hybrid Sensing Technology

(a)

(c)

(e) 5 μm

(b)

20 μm

(d) (f)

PTFE

Cu

Mn

PBT

Cul

ZnO\dye

(g)

1 cm

(i) DSSCs LEDs

SLBs

(h)

PLA

ITO

FEP

DSSC

(j)

SLB

LED

Bearing

Figure 13.3 Schematic illustration of triboelectric–photovoltaic nanogenerator. (a,b) Schematic illustration, (c,d) enlarged view, (e) SEM image of the photoanode, and (f ) an optical photograph of the textile-based triboelectric–photovoltaic nanogenerator. Source: (a–f ) Reproduced with permission from Chen et al. [11]. Copyright 2016, Springer Nature. (g–j) Schematic illustration, SEM image of the FEP film, optical photograph of the inside and lateral view of a self-powered lantern, respectively. Source: (g–j) Reproduced with permission from Cao et al. [10]. Copyright 2018, John Wiley & Sons.

Zhonglin Wang’s group fabricated a textile-based microcable-structured triboelectric–photovoltaic nanogenerator [11], as shown in Figure 13.3a–f. Congju Li and coworkers from the Beijing Institute of Nanoenergy and Nanosystems reported a self-powered lantern based on a triboelectric–photovoltaic nanogenerator [10] shown in Figure 13.3g–j. Both of them adopted dye-sensitized solar cells. Figure 13.4a,b exhibits electrical outputs of the textile-based triboelectric–photovoltaic nanogenerator and the lantern-shaped triboelectric–photovoltaic

13.1 Dual Hybrid Power Technology

Voc (V)

lsc (mA)

Light

Light and vibration

Dark and vibration

0.2 0.1 0.0 80 60 40 20 0 0

20

40

(a)

60

80 100 Time (s)

120

140

160

180 Current (μA)

Solar cell

TENG

150

TENG + Solar cell

120 90 60 30 0 0

(b)

10

30 20 Time (s)

40

50

Figure 13.4 Electrical outputs of (a) the textile-based triboelectric–photovoltaic nanogenerator and (b) the lantern-shaped triboelectric–photovoltaic nanogenerator. Source: (a) Reproduced with permission from Chen et al. [11]. Copyright 2016, Springer Nature. (b) Reproduced with permission from Cao et al. [10]. Copyright 2018, John Wiley & Sons.

nanogenerator, which were up to about 250 μA and about 150 μA, respectively. Apparently, the electrical output of the hybrid triboelectric–photovoltaic nanogenerators was higher than that of a single TENG or a single photovoltaic nanogenerator, indicating the feasibility of and necessity for hybrid energy harvesting.

13.1.3

Triboelectric–Electromagnetic Nanogenerator

The principle of electromagnetic effect is that the relationship between the voltage induced in a closed loop and the change rate of the magnetic flux through the loop area is proportional. In flexible electronics, electromagnetic is integrated with triboelectric [13–15]. Zhonglin Wang and coworkers made great efforts to realize the integration of electromagnetic effect and triboelectric effect, as shown in Figure 13.5 [14]. The maximum output powers of a TENG and an EMG were 4.9 and 3.5 mW, respectively. The corresponding power densities were 5.1 and 3.6 W/m2 with a dimension of 5 cm × 5 cm × 2.5 cm.

233

13 Hybrid Sensing Technology

(b) 60 40

Current (μA)

(a)

20 0 –20 –40

Acrylic Al

Coil Buffer

Magnet

5 μm

10 μm

5 cm

(d)

4

(e)

0

2

4 Time (s)

6

8

2

4 Time (s)

6

8

(c) 0

PDMS

Open voltage (V)

Spring

200 100 0 0

6 4

Open voltage (V)

2 Current (mA)

234

0

–2

2 0 –2 –4

–4 0

1

2

3

4

–6

0

1

Time (s)

2

3

4

Time (s)

Figure 13.5 Output performance of the hybrid triboelectric–electromagnetic nanogenerator. (a) Schematic illustrations of three reported triboelectric–electromagnetic hybrid nanogenerators. (b, c) Open-circuit voltage and short-circuit current of the TENG. (d, e) Open-circuit voltage and short-circuit current of the EMG. Source: Reproduced with permission from Zhang et al. [14]. Copyright 2015, American Chemical Society.

13.2 Multiple Hybrid Power Technology In order to improve the overall power output and maximize the use of multitype sources, individual energy harvesters have recently been integrated into hybrid nanogenerators. The combination of the two energy collection methods was introduced in the previous section, such as PNG-TENG [16], PVC-TENG, ThNG-PNG [17], and EMG-TENG [18], etc. This section introduces the multiple hybrid methods. 13.2.1

Triple Hybrid Generators

As one of the most frequently utilized energies, mechanical energy can be successfully converted into electrical energy using TENGs. However, there is a large amount of energy loss in TENG due to heat dissipation owing to friction. As

13.2 Multiple Hybrid Power Technology

the working time of the device increases, the waste heat energy caused by the triboelectric effect increases significantly in the devices, especially for the high temperature generated by the high-speed rotation. The use of ThNGs to collect waste heat energy is an ideal solution to minimize energy loss in TENGs. Meanwhile, combining PNG and EMG can enhance the harvest effectiveness of mechanical energy. In Ref. [19], the group led by Ya Yang from the Chinese Academy of Sciences developed a triple-hybrid nanogenerator that combines electromagnetic, triboelectric, and thermoelectric effects to collect both mechanical and thermal energies, as shown in Figure 13.6. Using the management circuits, the hybridized nanogenerator can deliver a constant output voltage of 5 V and a pulsed output current peak of about 160 mA. In Ref. [20], Ya Yang’s group reported the fabrication of a triple-hybrid nanogenerator that was highly transparent and flexible based on a PVDF nanowire-PDMS composite film as a friction layer, a polarized PVDF film as a piezoelectric and thermoelectric hybrid layer, and indium tin oxide (ITO) electrodes, as shown in Figure 13.7. In this work, the PVDF layer has three properties: piezoelectric, triboelectric, and thermoelectric. With these materials, multiple energies can be collected simultaneously. This makes the structure of the device simple and the size of the device small.

13.2.2

Four-Mechanism Hybrid Generators

Furthermore, Ya Yang’s group proposed a hybrid nanogenerator composed of four energy-harvesting methods: pyroelectric nanogenerator, photovoltaic cell, triboelectric nanogenerator and piezoelectric nanogenerator to simultaneously scavenge thermal, solar, and mechanical energies. This work is reported in Ref. [21], and the schematic diagram and the photograph are shown in Figure 13.8. To evaluate the performance of this multiple hybrid nanogenerator as a power source, a 0.33 μF capacitor was used for charge storage. Figure 13.9 shows the rectified output currents of three generators and the charging performance of a capacitor charged by four nanogenerators. It can be clearly observed that the coupled output can charge the capacitor faster than just a single output. Due to the combination of multiple energy-harvesting mechanisms, the multiple hybrid nanogenerator has high conversion efficiency and can convert many types of external energy, such as mechanical energy, thermal energy, light energy, etc. We believe that the multiple hybrid nanogenerator can be used to maximize energy collection from the surrounding environment. At present, due to air pollution and petroleum depletion, the combination of various energy harvesters for collecting multiple energy sources is considered one of the most important energy-related technologies. Based on multi-energy acquisition composite technology, the enhancement can occur in simple device stacking and multifunction energy-harvesting materials too. This section describes the hybrid nanogenerators based on multiple energy-harvesting mechanisms. Compared to conventional generators, these hybrid generators are smaller, simpler, more efficient, and less costly.

235

236

13 Hybrid Sensing Technology

Layer 2

Layer 1

(a)

Layer 3

30 mm 30 mm

30 mm Layer 4

30 mm

Layer 5

30 mm

Layer 6

30 mm

(b)

Figure 13.6 The photograph of the hybrid nanogenerator. (a) Photograph of each layer of the device and (b) the photograph of the device. Source: Reproduced with permission from Wang et al. [19]. Copyright 2016, Elsevier.

We believe that the future development trend of composite generators is to introduce multifunctional composite materials into the device to make the device structure simple. With the development of technology, hybrid nanogenerators will be more and more widely used in biological energy harvesting, encompassing fire detection, individual weaponry, environmental monitoring, human health monitoring, etc.

13.2 Multiple Hybrid Power Technology

ΔF

(a)

(b)

ITO

PVDF

PDMS PVDF

ΔT

(c)

2 cm

Figure 13.7 The structure and photograph of the device. (a, b) Schematic diagram and photograph of the hybridized nanogenerator; (c) photograph of PVDF film covering both sides of the ITO electrode. Source: Reproduced with permission from Wang et al. [20]. Copyright 2016, John Wiley & Sons.

(a)

Multi-effect coupled nanogenerator (PENG + PVC TPiENG)

(c)

PENG

PENG + PVC (pyroelectric effect) PENG + TPiENG

PVC (photovoltaic effect)

TPiENG (triboelectricpiezoelectric effect)

5 mm

PVC + TPiENG (b) Nylon PDMS

ITO BTO

FEP

Ag

TE module Acrylic

Figure 13.8 Schematic diagram and photograph of the multiple hybridized nanogenerator. (a) The principle composition of a composite nanogenerator; (b) illustration of the hybridized nanogenerator; (c) photograph of the hybridized nanogenerator. Source: Reproduced with permission from Ji et al. [21]. Copyright 2018, John Wiley & Sons.

237

13 Hybrid Sensing Technology

2.0

18 PVC

1.5

12 Current (nA)

Current (μA)

TPiENG

1.0 0.5

6 0

0.0 0

14

(a)

28 42 Time (s)

56

–6

70

0

600 Time (s)

900

1200

1.5 PENG

45 Voltage (V)

1.0

30 15

PENG + TPiENG + PVC TPiENG PENG PVC

0.5 0.0

0 0 (c)

300

(b)

60

Current (nA)

238

300

600 Time (s)

900

1200

–0.5 0.0 (d)

2.5

5.0 Time (s)

7.5

10.0

Figure 13.9 The output performance of four nanogenerators. (a) Rectified output current of TPiENG at airflow speed of 15 m/s; (b) rectified output current of PVC illuminated by 405 nm LED; (c) rectified output current of PENG while heating rate is 0.98 K\s; (d) voltage curves of a 0.33 μF capacitor charged with the coupled nanogenerator, TPiENG, PENG, and PVC. Source: Reproduced with permission from Ji et al. [21]. Copyright 2018, John Wiley & Sons.

13.3 Hybrid Sensors and Applications Due to the combined effects of various power generation principles, the hybrid generator not only has a great improvement in energy collection but also has excellent performance in other aspects, such as sensing technology. This section details the hybrid sensors that using the electrical signals generated by the energy harvesters to reflect changes in external inputs. Traditional sensors convert external information into electrical quantities such as resistance and capacitance to realize the digitization of natural information. However, the acquisition of such electrical information depends on the external power supply and is a passive measurement. With the rapid development of traditional sensors, long-term reliable power supply was a urgent problem, restricting the widespread use of sensors in daily life. Self-driven active sensors can be realized using the electrical signals generated by the energy harvester to reflect changes in external physical quantities. In these types of devices, such as TENG, PNG, EMG, and ThNG, the energy harvesters themselves act as sensors that generate electrical signals, simplifying the system’s composition. Compared

13.3 Hybrid Sensors and Applications

with a single type of active sensor, the composite sensor has a wider working range and higher measurement accuracy. TENG, PNG, EMG, and ThNG can all serve as active sensors based on their energy-harvesting mechanism. Combining two energy-harvesting mechanisms to form a dual hybrid nanogenerator as a hybrid sensor will inherit the sensing characteristics of the individual nanogenerator and increase the output performance of the device to improve the sensor’s performance. In the following section, dual hybrid sensors and other applications are introduced. 13.3.1

Piezoelectric–Triboelectric Hybrid Sensors

As mentioned earlier, energy harvesters based on PNGs and TENGs are the two kinds of sensors which can convert mechanical energy into electrical energy. At the same time, hybrid sensors were also invented. In Ref. [22], to achieve a reliable real-time self-powered sensor, the group led by Wendong Zhang from North University of China reported a double-arch sensor consisting of a piezoelectric–triboelectric hybrid nanogenerator that can be used as a vibration sensor while converting mechanical energy into electrical energy. The photograph and schematic illustration of the hybrid sensor are shown in Figure 13.10a. Figure 13.10b shows the electronic transfer of the hybrid nanogenerator in each state. Figure 13.11a–d corresponds with the vibration amplitude of 3 and 6 mm at different accelerations, respectively. In the case of the 3-mm vibration amplitude, there was no contact between the PNG and TENG. However, in the case of the 6-mm vibration amplitude, the PNG contacted with TENG, so that the output performances improved. Figure 13.11e–g shows that the output performance had a corresponding relationship with acceleration under the vibration amplitude of 3 or 6 mm. Therefore, this piezoelectric–triboelectric hybrid nanogenerator can be used as a vibration sensor. If the hybrid nanogenerator is based on a flexible material, it can be used as a pressure sensor in the field of flexible wearables. As reported in Ref. [23], the PET (V) Al PVDF Silicone rubber

(a)

(IV)

e–– e

e–

Pressing

(b)

(I)

Al1 PVDF Al2 5 mm Silicone Al3

Original view

Released

(II)

(III) e– Pressured

e– e– Releasing

Figure 13.10 Hybrid sensors based on piezoelectric–triboelectric hybrid nanogenerator. (a) The photograph of the device and materials of each film; (b) working principle of the piezoelectric–triboelectric hybrid NG. Source: Reproduced with permission from Zhu et al. [22]. Copyright 2017, Elsevier.

239

13 Hybrid Sensing Technology 15

2

2 m/s

8 m/s

2

15 m/s

2

30 m/s

2

2

8 m/s

2

15 m/s

2

30 m/s

2

2 Current (μA)

5

Voltage (V)

2 m/s

3

10

0 –5

1 0 –1 –2

–10

–3 –15

(a)

0

20

1

2

2 m/s

2

3

4 5 Time (s) 2 2 8 m/s 15 m/s

6

7 30 m/s

8

(b)

2

Current (μA)

2 m/s

2

3

2

4 5 Time (s)

8 m/s

2

15 m/s

6 2

7 30 m/s

8 2

0 –5

–10

1 0 –1 –2 –3

–15

–4

–20 0

35

1

2

3

4 Time (s)

5

6

3 mm amplitude 6 mm amplitude Current (μA)

30 25 20 15 10

7 6

0

15 20 g (m/s2)

25

30

(f)

0

1

2

3

4 5 Time (s)

250

3 mm amplitude 6 mm amplitude

6

7

8

3 mm amplitude 6 mm amplitude

200 150

2 1

10

(d)

3

0

5

8

5 4

5 0

7

Output power (μW)

Voltage (V)

5

(e)

1

2

10

(c)

0 4 3

15

Voltage (V)

240

0

5

10

15 20 g (m/s2)

25

100 50 0

30

(g)

0

5

10 15 20 25 g (m/s2)

30

Figure 13.11 The output performances of the hybrid sensor. (a, b) The output performances of the device with the vibration amplitude of 3 mm at different accelerations; (c, d) the output performances of the device with a vibration amplitude of 6 mm at different accelerations; (e–g) comparison of output performances at different amplitudes. Source: Reproduced with permission from Chen et al. [22]. Copyright 2017, Elsevier.

group led by Zhonglin Wang from the Chinese Academy of Sciences confirmed that the piezoelectric–triboelectric hybrid nanogenerator can be woven into smart fabrics to capture mechanical energy from human motion and served as a self-powered strain sensor. This fiber-based hybrid nanogenerator (FBHNG) composed of TENG and PNG is shown in Figure 13.12a. Basically, FBHNG is a 3D coaxial and fully integrated device that includes a PNG unit (output 2) in the core and a TENG unit (output 1) in the shell. The microarray of ZnO is shown in Figure 13.12b–d. The FBHNG can also be used as a self-powered strain sensor for monitor health. As shown in Figure 13.13a, the woven FBHNGs were tightly fixed to the elbow. When the elbows were bent at different angles (90∘ , 60∘ , and 30∘ ), the output of FBHNG was different, as shown in Figure 13.13b,c. This is because the smaller the bending angle, the greater the applied bending strain is on ZnO NR, thus resulting in a higher output. It was clearly shown that the piezoelectric potential along the ZnO NRs varies linearly with the strain ratio, which confirmed that the piezoelectric–triboelectric hybrid nanogenerator can function as a pressure sensor.

13.3 Hybrid Sensors and Applications

Output 1: TENG (i)

(ii) (iv) (v)

(a) Output 2: PENG (iv)

(iii)

Carbon fiber

Cu electrode

ZnO seed layer ZnO NRs

PDMS Tube mold

Nylon film

(d)

(c)

(b)

10 μm

1 μm

3 μm

Figure 13.12 The fabrication process step for FBHNG and SEM images for ZnO NRs. (a) Fabrication process flow-chart of FBHNG. (b–d) The SEM images of microarray of ZnO. Source: Reproduced with permission from Li et al. [23]. Copyright 2014, American Chemical Society.

90°

(i)

(ii)

(iii)

(a) Piezo voltage (mV)

Piezo current (nA) (b)

(iv)

25

25 30°

20 60°

15 90°

10 5 0

30°

60°

30° 20

60°

15 10 90°

5 0

0

5

10 Time (s)

15

0 (c)

4

8 12 Time (s)

16

Figure 13.13 The output performances corresponding to different bending angles. (a) The FBHNGs were tightly fixed to the elbow to act as a self-powered strain sensor. (b,c) The piezo current and piezo voltage of FBHNG while the arm was bent at different angles. Source: Reproduced with permission from Li et al. [23]. Copyright 2014, American Chemical Society.

241

242

13 Hybrid Sensing Technology

In general, the piezoelectric–triboelectric hybrid nanogenerator composed of a PNG and a TENG can integrate the energy generated by the two, so that the external mechanical energy can be perceived in a wider range and more accurately. It can be used in many different applications. 13.3.2

Electromagnetic–Triboelectric Hybrid Sensors

The output characteristic of EMG is low voltage and high current, which is opposite to the output characteristic of TENG. When combined, the output characteristics of high voltage and high current can be obtained. This output allows the hybrid nanogenerator to greatly increase sensitivity when used as an active sensor. In Ref. [24], our group at Peking University developed a magnetic-assisted TENG which replaced the traditional mechanical restoring force with magnetic force to prevent mechanical fatigue of TENG. The device consisted of a top steel mass, a top NdFeB permanent magnet, a silicone layer coated under the top NdFeB, a bottom spiral electrode (the top and bottom surfaces are encapsulated in polyimide), and a bottom NdFeB permanent magnet. The upper permanent magnet also served as the upper electrode of the TENG while providing the magnetic field. At the same time, the bottom permanent magnet had polarity opposite to that of the upper permanent magnet, and an electromagnetic repulsion can be generated between the two to balance the gravity generated by the steel mass. The bottom spiral electrode was used for the lower electrode of the TENG, and also acted as a metal coil to generate induced voltage and current by electromagnetic induction. The polyimide package on the surface of the spiral electrode and the silica gel layer applied under the top NdFeB were friction materials to generate a net surface charge. The polyimide surface had a nanostructure for increasing the surface area and enhancing the surface charge density after rubbing. When the device was tilted, as shown in Figure 13.14a, the influence of gravity will be weakened, thereby increasing the speed and distance of separation. The electromagnetic repulsion F M (z) and gravity G(𝜃) can be expressed as [ ] π𝜇 1 1 1 (13.1) FM (z) = 0 M2 R4 2 4 z (z + 2H)2 (z + H)2 G(𝜃) = mg cos 𝜃 + 𝜇mg sin 𝜃

(13.2)

where 𝜇0 is the vacuum permeability, M is the magnetization of the magnet, R and H are the radius and height of the cylindrical permanent magnet, z is the spacing between the two permanent magnets, m is the value of the top mass, and g is gravity acceleration, 𝜃 is the tilt angle of the device, and 𝜇0 is the coefficient of friction between the mass and the polytetrafluoroethylene (PTFE) material. Based on the given formula, the displacement and speed of the moving mass versus time can be calculated through a Simulink model, which consisted of two parts: a mechanics module and an electrical module. The mechanical module obtained the time-domain motion of the top mass of the device according to an external force such as electromagnetic repulsion. In electrical terms, different tilt angles changed the displacement function, which affected the electrical output of the device, as shown in Figure 13.14b,c.

13.3 Hybrid Sensors and Applications

(a)

θ

TENG

e

Diode

Slop

Resistor

Height adjuster (b) Self-powered sensor (c)

Off state

Tilf angle = 60°

Tilf angle = 70°

Tilf angle = 80°

Figure 13.14 Demonstration of the visualized tilt sensing system. (a) Measurement system of the self-powered sensor. (b) Electric circuit of the self-powered visible system. (c) Photographs of the self-powered visible sensor at different operation states. Source: Reproduced with permission from Han et al. [24]. Copyright 2014, Springer Nature.

As the dip increased, the component of gravity decreased, causing an increase in the maximum displacement, as shown in Figure 13.15a. In addition, the time required to reach the maximum displacement also increased as the angle of inclination increases. Since the electromagnetic repulsion was related to the distance, the speed reached a maximum at a certain position during the movement of the mass to the maximum displacement. The velocity versus time curve at different dip angles is shown in Figure 13.15b. In the experimental test, the output of a single device at different tilt angles was first characterized. As shown in Figure 13.15c, the experimental test results of the relationship between the peak value of the output voltage and the tilt angle were consistent with the theoretical analysis. When the inclination was small, the peak value of the output voltage does not change significantly with the inclination angle. However, when the inclination was greater than 30∘ , the peak value of the output voltage increased linearly with the increase in the inclination angle. The time-domain output voltage curve at different dip angles is shown in Figure 13.15d. To achieve omnidirectional tilt detection, the two devices can be placed perpendicular to each other, and the output voltages of the two devices respectively reflected the tilt angle along two different axes, thereby further deriving a specific tilt angle. A schematic diagram of placing the two devices vertically was shown in Figure 13.15e. The left and right sides indicated the device in the original state and the device in the tilted state. When an arbitrary angle of tilt occurred, both devices were tilted, causing both output voltages to change. Here, the inclination at any angle and in any direction was decomposed into two parts at a fixed angle: (i) rotating the 𝜃 angle along the X axis, and (ii) rotating the 𝜑 angle along the

243

13 Hybrid Sensing Technology

12 θ

10 8

3.5

6 4

(a)

10 8 6 2.5

4

0.03

0.06 0.09 Time (s)

0.03

0° 15° 30° 45° 60° 75° 90°

12

2

3.3 0.02

0.00

14 Velocity (cm/s)

0° 15° 30° 45° 60° 75° 90°

0.12

2.0 0.01

0 0.00

0.15

0.03

(b)

Experimental Theoretical Voltage (V)

50

0.02

0.06 0.09 Time (s)

0.12

0.15

0° 15° 30° 45° 60° 75° 90°

0 –50

–100 30 45 60 Angle (°)

z

75

90

z′ x′

y′

y x Original state

0 (d)

Tilted state

(e)

(f)

2

4

6 8 10 Time (s)

12

60 35 90 10 90

60 60 φ (°) 30

30

(° )

15

θ

0 (c)

Peak voltage (V)

Displacement (mm)

14

Normalized voltage

244

0 0

Figure 13.15 Demonstration of the magnetic-assisted TENG as a self-powered tilt sensor. Theoretical analysis of (a) time-domain displacement and (b) velocity in the separation process under different tilt angles. (c) Comparison of the theoretical and experimental output peak voltage. (d) Measured time-domain output voltage at different tilt angles. (e) Schematic diagram of the omnidirectional tilt sensor. (f ) Output peak voltage of the two TENGs at 16 different tilt angles. Source: Reproduced with permission from Han et al. [24]. Copyright 2014, Springer Nature.

Y axis. Thus, the tilt angles TA1 , TA2 of the two devices placed vertically can be expressed as TA1 = 𝛼𝛾 cos[cos 𝜃 cos 𝜑] TA = 90∘ − 𝜑 2

(13.3) (13.4)

By measuring the output voltage of the two devices, the values of TA1 and TA2 can be obtained, and the values of 𝜃 and 𝜑 are obtained according to these two formulas to determine the magnitude and direction of the tilt angle. In the

13.3 Hybrid Sensors and Applications

experimental test, the control 𝜃 and 𝜑 changed from 0 to 90∘ , respectively, and the output voltage peaks of the two devices in different angle combinations are shown in Figure 13.15f. The EMG can be used as an acceleration sensor, which has high precision when monitoring small accelerations; but when the acceleration is large, the accuracy is lowered due to the limitation of its structure. By combining the EMG and the TENG, the EMG works independently at a small acceleration. When the acceleration is large, the TENG can collect part of the mechanical energy lost by the EMG to improve the accuracy. In Ref. [25], the group led by Chengkuo Lee from the National University of Singapore developed a broadband and hybrid energy harvester (B-HEH) based on a combination of nonlinear enhancement effect and multimode energy harvesting to obtain high bandwidth over a wide range of accelerations. Figure 13.16 shows the structure and photographs of the device. The electromagnetic–triboelectric hybrid NG can also be used as a selfpowered hybrid sensor in the field of transportation.

Upper PTFE layer Triboelectric electrodes

Spacers EMG coil

PDMS spring

Magnets

(a)

(b)

Acrylic

Acrylic

PDMS

PCB

Cu coil

Magnet

PTFE

ITO (c)

Figure 13.16 The photograph and schematic illustration of the electromagnetic–triboelectric hybrid sensor. (a) An overall schematic of the hybrid sensor. (b) A cross-sectional view of the hybrid sensor. (c) A physical photo of the device. Source: Reproduced with permission from Gupta et al. [25]. Copyright 2017, Springer Nature.

245

13 Hybrid Sensing Technology

In Ref. [26], the group led by Jean Zu from the University of Toronto reported a new self-powered device for traffic monitoring purposes. This research specializes in a new hybrid electromagnetic–triboelectric motor configuration for self-powered sensing that has the potential to be utilized in speed bumpers. The novel design of this system overcomes the shortcomings of low displacement and low frequency energy conversion and sensing devices with a combination of both triboelectricity and electromagnetism. The device is shown in Figure 13.17.

Load

PM Steel Coil Rubber Teflon Subtrate PTFE Aluminum

(I)

(II)

(IV)

(III)

Sliding forward

1 2 3 4 5 6 7 8

EMG

TENG

246

6 Sliding backward

7

8

Figure 13.17 The schematic of hybridized nanogenerator and TENG unit working mechanism. Source: Reproduced with permission from Askari et al. [26]. Copyright 2017, Elsevier.

13.3 Hybrid Sensors and Applications

Electromagnetic–triboelectric hybrid NGs include the advantages of electromagnetic and triboelectric mechanisms. Typically, TENG and EMG complement each other in output performance. Even in very small mechanical relative motions, TENG is able to harvest energy. In addition, it can be implemented when there is high impedance in the system. It must be noted that the EMG can harvest enough energy when there is sufficiently relative displacement between the permanent magnet and the coil. In addition, they are commonly used in lowimpedance circuitry. Based on these characteristics of the two generators, the hybrid sensor can be a sensor with a wide range of applications and high sensitivity. In the foreseeable future, it will have a good application in traffic volume sensors, harsh environment monitoring, biomechanical motion, and other aspects. 13.3.3

Multiple Hybrid Sensors

As described in Section 13.1.3, multiple hybrid nanogenerators can collect different types of energy based on various energy-harvesting mechanisms. It also can integrate these different sensor types into a multifunction sensor. In Ref. [27], Zhonglin Wang’s group demonstrated a triboelectric–pyroelectric– piezoelectric hybrid nanogenerator which composes a sliding-mode TENG and a pyroelectric–piezoelectric nanogenerator (PPENG) with an outstanding performance. The multiple hybrid NG was also proved to be a self-powered temperature and pressure sensor to measure the temperature while the device is operating and the external force is applied. Figure 13.18 shows the (a)

(b) Sliding

Triboelectric output (1 sliding cycle) I

II I

1~

III

IV 2~

I Kapton Al Cu

PVDF PTFE

I

Pyroelectric output II

(c)

Heat transport

Compressive force

(d) I

Piezoelectric output (1 sliding cycle) II

I

IV I

I III

IV

III

I

Figure 13.18 The structure and working procession of the multiple hybrid nanogenerator for sensing. (a) The structure of the hybrid nanogenerator; (b–d) triboelectric, pyroelectric, and piezoelectric output working mechanism of the hybrid nanogenerator, respectively. Source: Reproduced with permission from Zi et al. [27]. Copyright 2015, John Wiley & Sons.

247

13 Hybrid Sensing Technology

composition of the generator and also describes the mechanism by which three energy-harvesting methods generate electricity in this device. The thermoelectric generators converted heat into electrical energy, and this feature made it a good temperature sensor. Piezoelectric nanogenerators converted mechanical energy into electrical energy, and this property made it an excellent pressure sensor. In order to better illustrate the performance of the temperature sensor, the structure of the multiple hybrid NG was modified into a structure based on rotation, as shown in Figure 13.19a. Zhonglin Wang and his team placed the PPENG position on the side (#1), the radius (#2), and the center of the disk (#3), and experienced different temperature rises during the same rotational friction. As shown in Figure 13.19c, it can be seen that the output voltage obtained at #1 is the highest. As can be seen from Figure 13.19b, the change in temperature is proportional to the change in voltage and has a linear relationship. In the Figure 13.19d–i, we can also find that the magnitude of the externally applied pressure is linear with the magnitude of the voltage and the charge generated. 500 rpm

27.5

2

26.5

1

Disc 2 Kapton

PPENG

Al

PTFE

27.0

IVoltage| (V)

3

T (°C)

Disc 1 (rotated)

15 10

PPENG #1

5 0

(a)

0

40

(b)

80

120

Time (s)

15 IVoltage| (V)

248

PPENG #1

Force

10

5

PPENG #2 0

PPENG #3 0

(c)

Force meter + motor

40

80 Time (s)

120

Kapton

PPENG

Disc 2

PTFE

(d)

Figure 13.19 The output of the hybrid nanogenerator worked as temperature and normal force sensor. (a) The schematic diagram displays device application as temperature sensor; (b) the voltage and the temperature corresponding variations of the PPENG #1; (c) the voltage variations of the PPENG #1–3. (d) Test schematic of the device; (e–h) the electrical output characteristics in PPENG under the force of about 0.5 N; (i) the relationship between electrical output characteristics and force. Source: Reproduced with permission from Zi et al. [27]. Copyright 2015, John Wiley & Sons.

13.4 Summary 1.2 1.0

3

Voltage (V)

Force (N)

4

2 1

0.8 0.6 0.4 0.2

0

0.0 0

2

(e)

6

4 Time (s)

8

0

2

4 Time (s)

6

8

2

4 Time (s)

6

8

(f) 0.8

20

Charge (nC)

Current (nA)

40

0 –20

0.6 0.4 0.2

–40 0.0 0

4 Time (s)

Charge (nC) voltage (V)

(g)

2

6

(h) Voltage Linear fit of voltage

2.0 1.6 1.2 0.8 1.4 1.2 1.0 0.8 0.6

Charge Linear fit of charge

0 (i)

0

8

1

2 3 Force (N)

4

Figure 13.19 (Continued)

13.4 Summary This chapter discussed hybrid sensing technologies from dual, triple, and quadruple mechanisms. Some examples and their applications have been investigated, including piezoelectric–triboelectric hybrid sensors, thermoelectric– triboelectric hybrid sensors, magnetic–triboelectric hybrid sensors, etc. Since multiple hybrid NGs combine multiple energy-harvesting mechanisms to collect different types of energy, when a multiple hybrid NG is used as a sensor to sense the surrounding environment, the range of targets it perceives will be expanded,

249

250

13 Hybrid Sensing Technology

and the accuracy of sensing will be improved. The ability of simultaneously sense multiple external physical quantities makes multiple hybrid NGs an excellent application prospect in areas such as wearable devices, individual smart equipment, automotive manufacturing, and aerospace, which require a large number of sensors. Conventional sensors used to detect environmental parameters require additional power, which is a huge limitation for sensor miniaturization, sensor life, and reduction of environmentally unfriendly waste from batteries. The invention of the hybrid nanogenerator can solve these problems in a targeted manner, which can be used not only as an efficient multiclass energy harvester but also as a self-power active sensor. However, as a new type of sensor, hybrid sensors have great potential in improving the single-signal accuracy, and there has been no deep exploration in the analysis of multichannel coupled signals. In the author’s opinion, the focus of future research should be on these two aspects of hybrid sensors, and the multifunctional composite material should be also paid attention to reduce the size of device.

Abbreviations AC B-HEH DC ECE EMG FBHNG ITO MNDS NG NR PCM PDMS PET PNG PPENG PTFE PTM PVC PVDF TEG TENG ThNG TPiENG TPNG XRD

alternating current broadband and hybrid energy harvester direct current energy conversion efficiency electromagnetic generator fiber-based hybrid nanogenerator indium tin oxide micro/nano double-scale nanogenerator nanorod phase change material polydimethylsiloxane polyethylene terephthalate piezoelectric nanogenerator pyroelectric–piezoelectric nanogenerator polytetrafluoroethylene power transformed and managed photovoltaic cell poly(vinylidene fluoride) triboelectric generator triboelectric nanogenerator thermoelectric nanogenerator triboelectric–piezoelectric nanogenerator hybrid triboelectric-piezoelectric nanogenerator X-ray diffraction

References

References 1 Bai, P., Zhu, G., Jing, Q. et al. (2014). Membrane-based self-powered tri-

2

3

4

5

6 7

8

9

10

11

12

13

14

boelectric sensors for pressure change detection and its uses in security surveillance and healthcare monitoring. Advanced Functional Materials 24: 5807–5813. Zhao, Z., Pu, X., Du, C. et al. (2016). Freestanding flag-type triboelectric nanogenerator for harvesting high-altitude wind energy from arbitrary directions. ACS Nano 10: 1780–1787. Xing, F., Jie, Y., Cao, X. et al. (2017). Natural triboelectric nanogenerator based on soles for harvesting low-frequency walking energy. Nano Energy 42: 138–142. Ma, Y., Zheng, Q., Liu, Y. et al. (2016). Self-powered, one-stop, and multifunctional implantable triboelectric active sensor for real-time biomedical monitoring. Nano Letters 16: 6042–6051. Sun, C., Shi, J., Bayerl, D.J., and Wang, X. (2011). PVDF microbelts for harvesting energy from respiration. Energy & Environmental Science 4: 4508–4512. Han, M., Zhang, X., Meng, B. et al. (2013). R-shaped hybrid nanogenerator with enhanced piezoelectricity. ACS Nano 7: 8554–8560. Su, Y., Wen, X., Zhu, G. et al. (2014). Hybrid triboelectric nanogenerator for harvesting water wave energy and as a self-powered distress signal emitter. Nano Energy 9: 186–195. Xue, C., Li, J., Zhang, Q. et al. (2015). A novel arch-shape nanogenerator based on piezoelectric and triboelectric mechanism for mechanical energy harvesting. Nanomaterials 5: 36–46. Guo, Y., Zhang, X.S., Wang, Y. et al. (2018). All-fiber hybrid piezoelectric-enhanced triboelectric nanogenerator for wearable gesture monitoring. Nano Energy 48: 152–160. Cao, R., Wang, J., Xing, Y. et al. (2018). A self-powered lantern based on a triboelectric–photovoltaic hybrid nanogenerator. Advanced Materials Technologies 3: 1700371. Chen, J., Huang, Y., Zhang, N. et al. (2016). Micro-cable structured textile for simultaneously harvesting solar and mechanical energy. Nature Energy 1: 16138. Zhang, X.S., Han, M., Kim, B. et al. (2018). All-in-one self-powered flexible microsystems based on triboelectric nanogenerators. Nano Energy 47: 410–426. Quan, T., Wang, X., Wang, Z.L., and Yang, Y. (2015). Hybridized electromagnetic–triboelectric nanogenerator for a self-powered electronic watch. ACS Nano 9: 12301–12310. Zhang, K., Wang, X., Yang, Y., and Wang, Z.L. (2015). Hybridized electromagnetic–triboelectric nanogenerator for scavenging biomechanical energy for sustainably powering wearable electronics. ACS Nano 9: 3521–3529.

251

252

13 Hybrid Sensing Technology

15 Quan, T., Wang, Z.L., and Yang, Y. (2016). A shared-electrodes-based

16

17 18

19

20

21

22

23

24

25

26

27

hybridized electromagnetic–triboelectric nanogenerator. ACS Applied Materials & Interfaces 8: 19573–19578. Han, M., Zhang, X., Liu, W. et al. (2013). Low-frequency wide-band hybrid energy harvester based on piezoelectric and triboelectric mechanism. Science China Technological Sciences 56: 1835–1841. Yang, Y., Guo, W., Pradel, K.C. et al. (2012). Pyroelectric nanogenerators for harvesting thermoelectric energy. Nano Letters 12: 2833–2841. Hu, Y., Yang, J., Niu, S. et al. (2014). Hybridizing triboelectrification and electromagnetic induction effects for high-efficient mechanical energy harvesting. ACS Nano 8: 7442–7450. Wang, X., Wang, Z., and Yang, Y. (2016). Hybridized nanogenerator for simultaneously scavenging mechanical and thermal energies by electromagnetic– triboelectric–thermoelectric effects. Nano Energy 26: 164–171. Wang, S., Wang, Z., and Yang, Y. (2016). A one-structure-based hybridized nanogenerator for scavenging mechanical and thermal energies by triboelectric–piezoelectric–pyroelectric effects. Advanced Materials 28: 2881–2888. Ji, Y., Zhang, K., and Yang, Y. (2018). A one-structure-based multieffects coupled nanogenerator for simultaneously scavenging thermal, solar, and mechanical energies. Advanced Science 5: 1700622–1700630. Zhu, J., Hou, X., Niu, X. et al. (2017). The d-arched piezoelectric–triboelectric hybrid nanogenerator as a self-powered vibration sensor. Sensors and Actuators A: Physical 263: 317–325. Li, X., Lin, Z., Cheng, G. et al. (2014). 3D fiber-based hybrid nanogenerator for energy harvesting and as a self-powered pressure sensor. ACS Nano 10: 10674–10681. Han, M., Zhang, X., Sun, X. et al. (2014). Magnetic-assisted triboelectric nanogenerators as self-powered visualized omnidirectional tilt sensing system. Scientific Reports 4: 4811–4818. Gupta, R., Shi, Q., Dhakar, L. et al. (2017). Broadband energy harvester using non-linear polymer spring and electromagnetic/triboelectric hybrid mechanism. Scientific Reports 7: 41396–41409. Askari, H., Asadi, E., Saadatnia, Z. et al. (2017). A hybridized electromagnetic–triboelectric self-powered sensor for traffic monitoring: concept, modelling, and optimization. Nano Energy 32: 105–116. Zi, Y., Lin, L., Wang, J. et al. (2015). Triboelectric–pyroelectric–piezoelectric hybrid cell for high-efficiency energy-harvesting and self-powered sensing. Advanced Materials 27: 2340–2347.

253

14 Smart Actuators Xiaosheng Zhang and Zhaohui Wu University of Electronic Science and Technology of China, School of Electronic Science and Engineering, No. 2006, Xiyuan Ave, West Hi-Tech Zone, 611731 Chengdu, China

To date, triboelectric nanogenerators (TENGs) have been widely applied in driving microelectronics to achieve self-powered running by harvesting mechanical energy [1–17], such as photoelectric devices [18], transistors [19, 20], radio-frequency transmitters [21], but not yet in microelectromechanical system (MEMS) devices. MEMS devices are usually activated by voltage rather than current. Therefore, high inner resistance and loss are the fundamental features of these devices. However, TENGs work on the basis of capacitive effect, matching the requirements from the MEMS devices. Thus, the application of TENGs in MEMSs is an important introduction in this chapter. Profiting from the improvement in performance and the intrinsic merit of high-output voltage, TENGs can now be used not only as a power source but also as a stimulation signal to drive micro/nano-actuators. Several micro-actuators have been successfully driven using the instantaneous electrical signals generated by TENGs, resulting from its unique feature of ultrahigh voltage at the levels of hundreds or even over 1000 V. Actuators are devices which can realize specific functions with external stimulations in terms of definition, designed in a diversity of appearances, materials and functions, and applied in abundant fields, especially in optical systems and MEMSs. In principle, an MEMS actuator is defined as the component which performs specific functions by movable parts. A piezoelectric actuator produces deformation under the applied voltage, which is widely applied in optical systems to adjust a laser source. Electrostatic mirror arrays, the key functional part of projectors, rotate frequently under the control of an electrostatic field. Besides, this section also discusses nozzle actuators and motion manipulation actuators that are widely used in biomedicine. It is of significance that the TENG serves as a novel technique to support these actuators, which is a promising application. Based on TENGs, these actuators are smart, resulting from the self-powered feature as well as controllability. TENGs play a dual role in these applications. Firstly, the TENG is the power source of actuators and helps them realize their self-power property. Secondly, it provides a control signal to lead the actuators to work in different states. Flexible and Stretchable Triboelectric Nanogenerator Devices: Toward Self-powered Systems, First Edition. Edited by Mengdi Han, Xiaosheng Zhang, and Haixia Zhang. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

254

14 Smart Actuators

Table 14.1 Performance of TENGs when applied in different actuators. Application

V oc (V)

Isc (𝛍A)

Pmax (mW)

Bimorph [22]

1700

—

—

Elastomer [23]

3600

6

—

Shape memory alloy [24]

0.35

—

—

Drug delivery [25]

150

70

—

Electrospinning [26]

1400

5

1.5

Printer [27]

130

15

2

Droplet motion [28]

1800

3.9

—

Microfluidic transport [29]

5000

5

—

In general, we can conclude that there are fields where the TENG can be suitably utilized according to its characterization of high voltage and low current, as shown in Table 14.1. It is an emerging tendency to develop portable and selfpowered MEMS actuators using TENGs. In this chapter, we introduce several typical smart actuators driven by TENGs.

14.1 Actuators in Optics The optical system is a vital part in photo imaging, such as in the optical microscope, optical fiber, laser source, cameras, etc. Following the development of MEMS, optical MEMS has been investigated as a technology that combines MEMS and optics, as well as a promising research in smart communication, in MEMS laser device, MEMS optical display, and MEMS camera. Micro-optics, micromechanics, and microelectronics together form optical MEMS, a novel subject in which the microreflection mirror, microlens, microprism, and micro-optical grating are the fundamental actuators, driven by electrostatic, electromagnetic, or piezoelectric effects. Herein, we concentrate on a piezoelectric bimorph beam to form a laser system driven by the TENG. 14.1.1

Laser Controller

A planar TENG applied in optical modulation was proposed by Chi Zhang et al. from the Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, in 2014 [22]. As shown in Figure 14.1, the TENG was employed as an energy source as well as a signal to excite an optical direction modulator by receiving two-dimensional planar information. The optical direction modulator was constructed by two bimorphs, which were piezoelectric transducers, as shown in Figure 14.1a. The mechanism of the optical direction modulator is simple. A laser beam is illuminated on the bimorphs and the light path changed by reflection. One of the bimorphs controls the X direction and the other the Y direction. Based on the joint action of the bimorphs, a

14.1 Actuators in Optics

Attenuator II

O Piezoelectric bimorph

A

D

Infrared Laser II

Laser B

Wattmeter II

Top +

Screen

47 nF –

TENG Bottom +

Piezoelectric bimorph

Infrared Laser I

Bottom + TENG – –

Top + (a)

Fiber

– 47 nF

Fiber Attenuator I

Wattmeter I

(b)

Figure 14.1 TENGs applied in optical modulation. Source: Reproduced with permission from Chi et al. [22]. Copyright 2015, John Wiley & Sons.

light spot is focused on the screen. Any slight mechanical deformation would be reflected and a deviation would occur as the light spot is displaced. The fabricated planar TENG was assembled into this optical system, and it was found that the system ran well. Firstly, by the function of the planar TENG, the signal of the X- and Y -direction motion could be transferred into electric signals, which were outputs from the top and bottom channels, respectively. Then the output voltages from the two channels drove the two bimorphs; one served as the X-direction optical controller, while the other served as the Y -direction optical controller. Under the stimulation of different triboelectric voltages, the piezoelectric bimorphs produced responses in the form of different displacements. Lasers illuminated the bimorphs and the light spot deflected its original position finally through a functional curve. In general, the motion signals were changed into displacement signals. The TENG and the piezoelectric bimorphs could be regarded as a motion signal amplifier. The optical power modulator was constructed using an infrared laser source, optical fibers, an MEMS attenuator, a TENG, and a parallel capacitance of 47 nF, as shown in Figure 14.1b. Capacitances were used as capacitive voltage dividers to make voltage from TENG variates in appropriate range. It was of most significance that attenuators were controlled by the TENG and different triboelectric voltages induced different attenuation. The attenuators acted as voltagecontrolled optical resistive devices. Herein, researchers invented a planar TENG to change the position signal into a control signal and the bimorph beam’s response. From the introduction, it is known that energy from sliding was successfully demonstrated to drive the optical micro-actuators. As seen in the results, both triboelectric power and voltage were suitable for displacement actuators and power actuators. Toward the optical application, the planar TENG was designed as shown in Figure 14.2. Successively, it was of utmost importance that the relationship between the TENG’s voltage and the displacement of the light spot on the screen was investigated. First of all, the performance of the TENG was characterized. After that, the TENG’s effect on optical direction modulation was revealed quantitatively.

255

14 Smart Actuators

(a)

PTFE Aluminum (top electrodes) Aluminum (bottom electrodes) Acrylic

(b)

Z Y

X

1 μm

Figure 14.2 For the realization of two-dimensional modulation of laser controller, a relative TENG was fabricated with planar sensing function. Separated electrodes are assembled on the top and bottom, active in X and Y directions, respectively. (a) Structure of designed planar triboelectric nanogenerator. (b) SEM image of PTFE nanoparticles embedded into PTFE film to enhance output level. Source: Reproduced with permission from Chi et al. [22]. Copyright 2015, John Wiley & Sons.

2.0

2.0 Top electrodes Bottom electrodes

1.0

1.0

0.5

0.5

0.0

0.0 10

(a)

20

30

40 50 X (mm)

Top electrodes Bottom electrodes

1.5 VOC (kV)

1.5 VOC (kV)

256

60

70

10

(b)

20

30

40 50 Y (mm)

60

70

Figure 14.3 Relationship between V oc and displacement. Source: Reproduced with permission from Chi et al. [22]. Copyright 2015, John Wiley & Sons.

Using a series of tests, the linear relationship between open-circuit voltage and displacement was invented, as shown in Figure 14.3. Despite the high voltage up to kilovolts, the capacitive load obtained hundreds of volts due to the smaller impedance, as shown in Figure 14.4. It is worth noting that piezoelectric bimorphs were applied with a capacitance equivalent to 392 pF. Then, the interests were concentrated on the quantificational relationship between deflection angle and spot displacement. Deflection angle, the most important device parameter, and displacement of the light spot on screen are two main variables to describe system performance; and displacement represents the relationship between system input and output. From Figure 14.5b,d, the linear functions were shown both in bimorph characteristic and system output performance. The relationship of the output is exported, as illustrated in Figure 14.5a,c. Consequently, by replacing 𝜃 with other parameters, calculation formulas of SOA and SOB were listed as follows. As a smart actuator, it is necessary

14.1 Actuators in Optics 400

200 pF 500 pF

1 nF

400 300

5 nF

Vc (V)

300

Vc (V)

10 nF

Top electrodes Bottom electrodes

2 nF

200

Top electrodes Bottom electrodes

200 100

100

0

0 0

10

20

(a)

30 40 Time (S)

50

60

0

2

(b)

4 6 8 Capacitance (nF)

10

l

Uload

h h

θ

Piezoelectric bimorph

(a)

S

Δd

Displacement in screen (mm)

Figure 14.4 Characterization of load capacitance. Source: Reproduced with permission from Chi et al. [22]. Copyright 2015, John Wiley & Sons. 20 OA axis OB axis

15 10 5 0

Screen

0

10

20

D 2θ Laser

(c)

Piezoelectric bimorphs

θ

Displacement in screen (mm)

(b)

50

60

70

30 40 Y (mm)

50

60

70

20 OA axis OB axis

15 10 5 0 0

(d)

30 40 X (mm)

10

20

Figure 14.5 Toward an accurate control, a relationship between input displacement and the output performance of the laser system is required for a mathematical characterization. (a) Piezoelectric bimorph deformation schematic and (b) laser root model are the basics of calculation. (c, d) Output quantitative analysis of the system for a proper fit. Source: Reproduced with permission from Chi et al. [22]. Copyright 2015, John Wiley & Sons.

to give out the mechanism of triboelectric voltage control and describe it quantitatively. SOA = tg2𝜃 ⋅ D ≈ 2𝜃 ⋅ D 3d31 ⋅ Q ⋅ l ⋅ d ⋅L (C0 + Cbimorph ) ⋅ h2 ⋅ L x 3d31 ⋅ Q ⋅ l ⋅ d = ⋅L (C0 + Cbimorph ) ⋅ h2 ⋅ L y

(14.1)

SOA =

(14.2)

SOB

(14.3)

257

258

14 Smart Actuators

14.1.2

Tunable Optical Membranes

Another research is about the application of the TENG in optical modulators to construct a smart optical system with tunable function, which was done by Xiangyu Chen et al., from the Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences [23]. It is a triboelectric tunable smart optical modulator (SOM) based on a conjunction system of TENG and dielectric elastomer actuator (DEA). The transmittance of SOM was regulated triboelectrically to realize a haze effect and applied in privacy protection. As shown in Figure 14.6, silver nanowire (Ag NW) electrodes were deposited on the top and bottom surfaces of elastomers. While high voltage was applied to the electrodes, Coulombic attraction was induced between opposing charges on both sides of the elastomer. Hence, electrostatic force was exerted and the elastomer was squeezed and compressed. Successively, localized deformation, namely, wrinkles in nanoscale, were rooted in electrostatic force and resulted in a series of refraction processes, and the haze effect was attributed to the disordered light paths. Furthermore, to optimize the effect of SOM, the unbalanced density of Ag NWs was investigated. As a result, larger density combined with middle density would output more evident transmittance change, which can refer to relative supporting information. The TENG was assembled using aluminum electrodes and Kapton after inductively coupled plasma (ICP) reactive-ion etching to promote output performance. Dielectric optical electrode (DOE) was developed using acrylic elastomer mixed with Ag NW network, as electrodes located on the top and bottom surfaces. Meanwhile, an optimized method for combining the external power source and TENG was demonstrated. As Figure 14.7 shows, external DC voltage (V ex ) was set up to 1400 V to overcome the threshold, the effective Almost transparent

Refraction Activated

Relaxed

Silver nanowire

Nanowrinkles was generated

Light

Light

Activated

Relaxed

V

Transparent Light

Light

No electric field

Refraction

Electric-field-induced Maxwell force

Figure 14.6 Working mechanism of a dielectric elastomer. With an electric field, silver nanowires are active, which brings about the shrinkage of the elastomer. By this effect, light refraction is in disorder, resulting in a reduction of transmittance. Source: Reproduced with permission from Chen et al. [23]. Copyright 2016, John Wiley & Sons.

14.1 Actuators in Optics

9

Current (μA)

6

Triboelectrification Separation

External circuit

V

External circuit

3 0 –3 –6 –9

(a)

–12

Short circuit

0

10

20

600 Transferred charges (nC)

Voltage (kV)

0 –1 –2 –3 –4 –5 (c)

VOC

0

4

With elastomer

8

12

Time (s)

16

(d)

40

50

With elastomer

QSC

500 400 300 200 100 0

20

30

Time (s)

(b) 1

With elastomer

0

5

10

15

20

Time (s)

Figure 14.7 Output performance of the TENG with or without an elastomer. Actually, the load of the elastomer mainly influences the output voltage. By the application of the TENG, power consumption decreases obviously. (a) Electrons transfer in a friction cycle. (b–d) The electrical features of the nanogenerator. Source: Reproduced with permission from Chen et al. [23]. Copyright 2016, John Wiley & Sons.

voltage to generate minimum transmittance change, by applying an accessory serial DEA covered with silicone grease. When the top electrode of the TENG was in separation, light refraction was generated in the expanding DEA, while the accessory DEA was in shrinking state. In this way, the voltage of the TENG favorably regulated the transmittance of the DEA and avoided the drawbacks of a serial or parallel structure with a power source. For the measurement of output voltage, an Al electrode was attached to a plate guided by a linear motor. Accordingly, the maximum output voltage of the TENG with 100 cm2 of friction surface area can reach up to 3600 V, while there is an approximately 1000 V voltage drop in SOM. The transmittance was first tested when Ag NWs were dispersed on the elastomer and ranged from 65% to 75% due to scattering of the nanostructure. Applied with TENG, the transmittance of SOM could be tuned from 70% to 40%. In practice, 40% is enough for privacy protection to block observation and image information (Figure 14.8). In this SOM, a dielectric elastomer with nanowires is a promising method to realizing transmittance control by refraction of nanostructures. Moreover, the high output voltage of the TENG is adapted to the requirement of the capacitive load of the elastomer. The proposed TENG–DEA structure is suitable not only for SOM but also for MEMSs, human–robots, and Internet of things.

259

(a) Silicon grease

Decrease V2

Increase V1

Transparent

Refraction Shrink

Expansion Vex

80 70 60 50 40 30 20 10 0

Transmittance Strain

20 16 12 8 4

(c)

Optical modulator

(d)

60 50

Transmittance (%)

Transmittance (%)

(b)

Silver nanowire

Transmittance (%)

Optical modulator

V1

V2

40 30 20 10

V2

V1

24

80

20 60

Transmi ttance

16

40

12 Strain

8

Strain (%)

Vex

20 4

0 0.1 0.5 1 2 5 8 Separation height (cm)

0 0.1

0.5 1 5 Motion speed (cm/s)

10

0

0

10

20

30

0 40

Time (s)

Figure 14.8 TENG is combined in the optical modulator system. (a) Structure and actuation principle of a smart optical modulator. (b–d) Measurements of transmittance, regulated by TENG. (b) Transmittance is lower as the separation height of the TENG increases. (c) Transmittance reduces a little with the increasing motion speed. (d) Variation of transmittance is shown in time domain. Source: Reproduced with permission from Chen et al. [23]. Copyright 2016, John Wiley & Sons.

14.2 Actuators in Biomedicine

There are several preconditions when applying TENG technology in the MEMS field. Firstly, it is better that the targeted system is a voltage-control system or a tunable system with voltage because the load system must match the features of TENG, such as high output voltage and low current. Secondly, a rather high inner resistance is required for the terminal to maintain a high-output voltage from the TENG. The dielectric elastic actuator introduced here is based on a stretchable isolative elastomer, which is usually applied in artificial muscles and wearable electronics with a high driven voltage and significant capacitive effect. The best choice is to use a TENG to activate this elastomer because of its high inner resistance.

14.2 Actuators in Biomedicine With the help of flexible materials, wearable medical electronics makes a great advancement in continuous detection and physiological signal processing. For example, flexible electric skin attracts great attention worldwide with its multifunctional sensing, like pressure, temperature, hydration, blood pressure, and the biomarkers in sweat. Many implantable devices are designed for diagnoses and therapy by the application of biocompatible, flexible, and stretchable materials. In the field of biomedicine, the TENG was employed as a sensor and combined with an actuator. Firstly, using biocompatible materials, the TENG can be embedded in the human body as a micro–nano device. Secondly, the TENG, as a new method to harvest energy, is promising in realizing a “self-powered device” in the human body. Thirdly, the pulses produced from the TENG are interesting in realizing drug injection, too. In this section, we introduce the biomedicine actuators based on TENGs. 14.2.1

Bladder Illness Curation

Under active bladder (UAB) was researched by F.A. Hassani et al., from the National University of Singapore [24], and it is remarkable that the TENG served as a smart sensor integrated with an actuator and was tested successfully in in vivo experiments. UAB, or detrusor muscle underactivity, is caused by factors such as aging, neurologic diseases, diabetes, and infection. Traditional treatment methods include double voiding or straining the bladder to void, indwelling or intermittent catheterization, drugs, stem cell or gene therapy, and sacral nerve stimulation. Among these, intermittent catheterization is the common choice for patients with UAB. However, multiple daily insertions may lead to serious infection in the urinary tract or asymptomatic bacteriuria. Using the TENG, a novel method to cure UAB was proposed. In this research work, the TENG played a role in sensing, and was assembled with a shape memory alloy (SMA) activated by an external power source. The integrated TENG detected the fullness of the bladder via the nerves, and it displayed merits such as biological compatibility, low cost, and easy fabrication; the schematic view is shown in Figure 14.9a. Figure 14.9b–d shows that SMA actuators, the TENG, and the bladder are grouped compactly by two pieces of polyvinylchloride (PVC) sheets. SMA actuators were on the top of the bladder to provide compressive force or

261

262

14 Smart Actuators

(a)

(b)

Top flexible PVC sheet

Bladder

PET

Tape PET Y

PVC Copper TENG sensor Y

X

Z

(c)

PDMS Sponge

Tape Copper

Bottom flexible PVC sheet Tape

(d)

Filling of bladder

Sponge

5 mm

Activation of actuator

Bladder

Bladder

X

TENG sensor output wires

Bladder V

V

V

Voiding of bladder

Figure 14.9 Structure and mechanism of the proposed TENG–SMA actuator system. (a) Structure of the SMA-based actuator and side view of the layered triboelectric nanogenerator. (b) Side view of the assembled system for the voiding of the bladder. (c) Image of the test using a rubber balloon. (d) Mechanism of compression and restoration. Source: Reproduced with permission from Arab et al. [24]. Copyright 2018, American Chemical Society.

restoring force, while the TENG was assembled under the bladder for sensing of urine and indicated the bladder fullness. The substrates were 400-μm-thick PVC sheets with a dimension of 4 cm × 2.7 cm for eight 3D-printed cubic anchor points. Thin SMA wires and SMA springs were employed, anchored on two of the anchor points and functioned in compression phase. Thick SMA wires working in restoration phase were anchored on residual anchor points. The TENG under the bladder consisted of copper electrodes, PDMS as dielectric material, and sponge adhered on the PVC sheet. The whole system is shown in Figure 14.10. NiTi was the SMA and was used to fabricate SMA wires and SMA springs. While 4 V DC voltage was applied on the thin SMA wire and SMA spring, they were heated to 80.5 and 67.3 ∘ C, respectively, which resulted in the contraction of the SMA. Consequently, the bladder was voided under the force exerted from the SMA spring. Contrary to the former, thick SMA wires would be contracted working in the restoration phase for the balloon experiment. Filling of the bladder would work out to be worse without the function of the thick SMA wire in the in vivo experiment, referred to Figure 14.10c.

14.2 Actuators in Biomedicine

Stopcock

Syringe pump Oscilloscope

Wires from TENG sensor

Urethra

Balance

Power source

Urine (b)

(a)

ΔVb/Vbm (%)

100 80 60 40 Compression only – actuation 1 Compression only – actuation 2 Restoration-compression – actuation 1 Restoration-compression – actuation 2

20 0

(c)

0

50

100 150 Time (s)

200

Figure 14.10 The whole system of bladder illness curation. (a) In vivo experiment to test the voiding of urine. (b) Experiment on balloon for simulation. (c) Effect comparisons of different actuation methods. Source: Reproduced with permission from Arab et al. [24]. Copyright 2018, American Chemical Society.

TENG was used to indicate the fullness of the bladder by reaching the maximum voltage. The sponge sandwiched in the copper layer and polydimethylsiloxane (PDMS) absorbed liquid to affect the electrostatic charge on copper; meanwhile, different peaks of output voltage were induced in different phases, as illustrated in Figure 14.11. During the compression phase, the TENG was under the force from the actuator and liquid was squeezed out, resulting in reduction of output voltage. During the restoration phase, filling of the bladder, the TENG was released and liquid was absorbed again. As a result, the output voltage was up gradually to the maximum. It is notable that the TENG served as a sensor, and the indicator of fullness state and actuators were activated by a power source manually. Figures 14.10 and 14.11 show the outstanding performances of the TENG and SMA-based actuators, respectively. Compared to traditional therapies, NiTi springs and NiTi wires realized a large voiding ratio up to 80% in the relative test. With respect to TENG, its output voltage varied from 50 to 350 mV and the DC signal was an effective indicator. During the filling of the bladder, the output voltage increased to about 80 mV. During the activation, the output voltage continued to increase, and this was attributed to the pressure from the actuator. Once the activation power was removed, the voltage decreased rapidly in seconds and was down to 25 mV.

263

14 Smart Actuators

200 mV

200 mV

Output voltage (mV)

100 50 0 –50 –100

0.2

0

0.4

0.6

0.8

1

Vb (ml)

(a)

300 250 4

200 150 100 50 0

0

5

(b)

10

15 Time (s)

20

25

Compression voltage (V)

350

Output voltage (mV)

264

30

Figure 14.11 Output performance of the TENG. (a) Output voltage during the filling of the bladder. (b) Output voltage during activation. Source: Reproduced with permission from Arab et al. [24]. Copyright 2018, American Chemical Society.

This research work of combining the triboelectric nanosensor and the biological actuator to cure diseases is of significance for the promising future of TENGs. By realizing portable and low-cost therapy in UAB, using TENGs in the cure of other diseases is practicable. However, herein the TENGs were just a sensing part – and not an energy collector – that needed improvement for excellence. 14.2.2

Drug Delivery

Due to its great merits, the TENG was firstly applied in biomedicine to realize drug delivery [25, 30]. Moonjeong Bok et al. from Dankook University developed an in vitro transdermal drug release method incorporating dissolved microneedles to realize the delivery of salmon deoxyribonucleic acid (SDNA)

14.2 Actuators in Biomedicine

based on the coupling of triboelectric electrification and electrostatic induction toward portable therapy, replacing traditional electrophoresis (EP) with TENG technology. Traditional ways to drug delivery, such as hypodermic needles and oral drug delivery, easily cause pain or drug degradation in the digestive system. Hence, the TENG was applied, as seen in Figure 14.12. Herein, SDNA was used to fabricate TENG and microneedles, which can dissolve inside the skin without any

V/A

Polymer film SDNA film

(a)

Counter electrode

Triboelectric nanogenerator

Vibration

(i)

Distance

Statum corneum Viable epidermis

(ii)

Dermis

Polymer film SDNA microneedle patch containing drug molecules (b)

Figure 14.12 Transdermal drug delivery in porcine cadaver skin actuated by the TENG. (a) Prototype of an SDNA-based TENG for test and (b) a practical device to deliver the drug embedded in SDNA by direct friction between the SDNA and polymer. Source: Reproduced with permission from Bok et al. [25]. Copyright 2018, Royal Society of Chemistry.

265

14 Smart Actuators

biohazardous residuals. It is of great significance that electrostatic force induced by TENG functioned in two aspects: creating pores on the skin and compelling the macro-molecules into the skin. In this work, the TENG was composed of a negatively charged polymer (polyimide or Teflon), a positively charged SDNA film, and electrodes with inductive charges, working in vertical contact-separation (CS) mode. Under the actuation of TENG, the skin was penetrated and SDNA-made microneedles and the embedded drug were dissolved. Logically, by electrical experiment, the designed TENG outputted an exponential decay, voltage at approximately 100 V, duration in few μs to ms, and controllable interval between pluses. All these features showed that the TENG was potential to instead of the EP method as its low cost, portability, easy fabrication as well as performance in drug delivery. For a demonstration of the effects of the concentration of drugs, tests were done and the relative results are shown in Figure 14.13 and Table 14.2. In this approach, hydrolyzed sodium hyaluronate (HA) was used to identify the effectiveness of microneedles. As can be seen, output voltage and current decreased with increasing concentration of HA. Therefore, an appropriate content of HA should be chosen in drug delivery. Another essential step was the fabrication of microneedles. As illustrated in Figure 14.14, the mold of a microneedle stamp was prepared. PDMS was poured

200

200 SDNA/drug molecule composite (HA) 150

150

100 50 0

50 0

Releasing

–50 –100

100

Voltage (V)

Voltage (V)

Pressing

–50 1 wt%

0

2

(a)

6 4 Time (s)

8

–100

10

(b)

200 Maximum voltage (V)

100 50 0 –50

–100

(c)

0

2

4 6 Time (s)

5 wt% 8 10

160

150 Voltage (V)

266

2

4 6 Time (s)

8

120 100 80 60 40 20

10 wt% 0

140

0

10

(d)

1 5 10 Drug molecule concentration (wt%)

Figure 14.13 Effects of concentration of drug molecules in the output of the TENG. Source: Reproduced with permission from Bok et al. [25]. Copyright 2018, Royal Society of Chemistry.

14.3 Actuators in Industrial Application

Table 14.2 Output function of concentration of HA.

TENG samples

Maximum voltage (V)

Maximum current (𝛍A)

SDNA/HA (1 wt%)

150

70

SDNA/HA (5 wt%)

125

30

SDNA/HA (10 wt%)

80

20

Source: Reproduced with permission from Bok et al. [25]. Copyright 2018, Royal Society of Chemistry.

Microneedle stamp

Array

(a)

PDMS

(b)

SDNA solution

(c)

(d)

Figure 14.14 Processes of fabricating SDNA microneedles. Source: Reproduced with permission from Bok et al. [25]. Copyright 2018, Royal Society of Chemistry.

into the stamp and a PDMS mold was formed on solidifying. SDNA solution was poured into the PDMS mold and SDNA-based microneedles were finished, as shown in Figure 14.14d.

14.3 Actuators in Industrial Application Of late, industry has been making great progress, which is supported by a great volume of new precision components. Mechanical actuators, in which the TENG can serve as a portable power source to increase their convenience, are applied in many instruments, in micro/nanoscale. Listed here are examples including spray and printing systems. The common point is that the TENG is used to drive

267

14 Smart Actuators

micro sprinkles. Looking forward to the future, more and more research will concentrate on industry for an reliable power supply. 14.3.1

Electrospinning System

In the research work done by Congju Li et al., from the Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, the TENG was used to drive an electrospinning system for nanofibers [26]. Electrospinning is a technology where polymer solution is ejected from a jet due to the applied voltage, from several to tens of kilovolts. To date, nanofibers fabricated by electrospinning are feasible in many fields, such as drug delivery, tissue filtration, and sensors. A rotating-disk TENG working in rotary freestanding-mode (FS mode) was employed to power the electrospinning with high voltage. In view of the intrinsic properties, the TENG outputs high voltage with low current, which is consistent for the electrospinning system with voltage up to kilovolts as well as current of microamps. Combined with TENG, this electrospinning system is suitable for working in remote places or in the wilderness without a power source. To apply well in the electrospinning system, a voltage-doubling rectifying circuit (VDRC) was inserted between the TENG and the jet. The output voltage from the TENG through the VDRC was transformed to direct voltage up to 8 kV, driving the jet to spin out nanofibers. As seen in Figure 14.15, the anode Vout

–

+ C – 4+

C – 2+

Electrospinning TENG Voltage-doubling rectifying circuit

+

D1

TENG –

(c)

(a)

(i)

Cu (rotator)

Voltage (V)

7000

D2 D3

– +

– +

C1

C3

D4

470 pF 1000 pF 100 000 pF

6000 5000 4000 3000 2000 1000 0

FEP film

(d)

Cu

FEP

Kapton

5

10

15

20

25

30

Amplifying multiple 8000

(ii)

Cu (stator)

Voltage (V)

268

FR4

(b)

6000 4000 2000 0

(e)

470 1000 4700 1E4 1E5 1E6

Capacitance (pF)

Figure 14.15 Electrospinning system employed rotating-disk TENG. (a) System diagram in which a rectifying circuit is exploited. (b) Inner layers of the rotating-disk TENG. (c, d) Topology structure of the voltage-doubling rectifying circuit (VDRC) and its electrical performance with different capacitances. Source: Reproduced with permission from Li et al. [26]. Copyright 2017, American Chemical Society.

14.3 Actuators in Industrial Application

was connected to a spinneret, while the cathode was connected to a metal plate. The high voltage not only charged the polymer solution but also provided enough electrostatic force for spinning. Rotary-disk TENG was assembled using copper strip electrodes and fluorinated ethylene propylene (FEP) film (50 μm in thickness) as negative charged frictional material. The top electrode was a rotator with radial-arrayed Cu strips. Meanwhile, the bottom electrode was two sets of complementary radial-arrayed Cu strips as stator. VDRC, as shown in Figure 14.15, is composed of a capacitor and diodes arrayed in multiple stages of architecture. By a series of experiments, the optimal parameters, magnification, and capacitor value were set at 22 and 100 nF, respectively, to gain kilovolts. It was obvious that the quality of polymer nanofibers was affected by voltage. To avoid intermittent, intertwined, or sparse nanofibers, controlling of voltage by VDRC, the most critical step, was done and contrast identifications are displayed in Figure 14.16.

(a)

(b) Electrospinning

(c)

TENG Voltage-doubling rectifying circuit

(d)

(e) 5 μm

(h) 5 μm

20 μm

5 μm

20 μm

20 μm (g)

(f) 5 μm

20 μm (i)

5 μm

5 μm

20 μm

20 μm

Figure 14.16 Performance of a self-powered electrospinning system. (a) Photograph of a practical system with a hand-cranked TENG. (b, c) High-speed photograph of the needle before and after actuation. (d–i) SEM images of nanofibers with different amplifying multiple, 6, 8, 10, 16, 22, and 28, respectively. The capacitors’ values in VDRC were chosen at 100 nF. Source: Reproduced with permission from Li et al. [26]. Copyright 2017, American Chemical Society.

269

270

14 Smart Actuators

Compared with a commercial system, this self-powered electrospinning technique is useful to produce nanofibers for biomedical applications without power supply, especially in wound therapy. Furthermore, TENGs couple well with electrospinning and realize the function in urgent medical treatment, which is an innovative application. 14.3.2

Syringe Printing

In the field of printers, Bo Chen et al. from the Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, recommended a self-powered printer, which was similar to the abovementioned work on electrospinning [27]. As is known, traditionally, a printer is applied with many drawbacks, such as high energy consumption, immobility, environmental hazards, large size, and high cost. Toward a portable and cost-effective printer, the TENG was employed as the power unit for a homemade printer system to realize that print onto a different substrate with color ink for pattern or Ag NWs for a conductive device. Firstly, the proposed TENG was divided into three layers. Cellulose nanocrystal (CNC)-coated indium tin oxide (ITO) was employed as the top electrode and bottom electrode layers, respectively, while the CNC acted as the positive charged material. In the middle layer, FEP served as the negative charged material and vibrated in the flow of wind or mouth-blown air. As seen in Figure 14.17, when the FEP film moved up or down, induced electrons would flow from the top ITO to the bottom ITO or inversely. The predominant step in the fabrication of the TENG was the preparation of CNC. Firstly, the original material, Kraft pulp, was milled into powder at high speed. After that, the powder was hydrolyzed by 64 wt% sulfuric acid in 45 ∘ C, stirring for 45 minutes. Successively, the cellulose suspension was diluted by distilled water to stop hydrolysis. After removal of the supernatant, the residual was dialyzed in a dialysis membrane with distilled water for four to seven days until a stable PH value was reached. At last, the cellulose suspension was diluted into 0.2 wt% and treated by ultrasonication in 1000 W for five minutes. Under wind speed of 20 m/s, the TENG outputted a performance with maximum power of 2 mW in the load of 13 MΩ. By the function of the rectifier circuit, output voltage could reach at 130 V, 15 μA with a frequency of 556 Hz. When the syringe was actuated by TENG, the droplet was positively charged and the Cu sheet linked to cathode was negatively charged. Because of the electrostatic force and coupling of gravity, the van der Waal’s force between nozzle and droplet was overcome, which led to the droplet falling, as shown in Figure 14.17. Besides, as the achievements, the printed pattern with color ink and LED illumination tested by printed path with Ag nanowires are shown. Overall, the fabricated self-powered printer system displays practicability and feasibility in portable printers with diverse substrates. Not only the conductive material, Ag NWs, but common ink can also be used in this printer. However, it is worth noting that the printer could not move under the control of the TENG, which can be improved as per the relative technique in Section 14.1 by the application of a two-dimensional planar TENG. There is no doubt that it would be excellent. In addition, this kind of printer system can be used as a large-scale printer array actuated by tactile array, which may be an interesting application.

(a)

(b) ITO

e– Moving direction

PET Water CNCs

Deposition

Evaporation

(c) Acrylic PET

ITO

CNC

FEP

(d) e

(1)

–

e–

(2)

(3)

(7)

(6)

(4)

Current (μA)

20 10 0

–10

E

–20

(8)

e– ITO Color ink

CNC

(5)

e–

FEP Cu

ITO

CNC

FEP

Moving direction

Figure 14.17 Schematic view of a syringe printer. (a) The TENG is used to drive the injection of color ink. Furthermore, (b) CNC/ITO electrodes are adopted to enhance the performance of the TENG, which shown in (c). Electrical energy comes from the vibration of the middle electrode, as illustrated in (d), working principle of wind-driven FEP/CNC/ITO-stacked TENG. Source: Reproduced with permission from Chen et al. [27]. Copyright 2017, Elsevier.

272

14 Smart Actuators

14.4 Actuators in Microfluidic Manipulation As is known, electrostatic actuation, based on Coulomb force, has advantages in the driving or manipulating micro/nano objects. Usually it is considered as a low-cost, fast-response, low-consumption, and small charge leakage method compared to electromagnetic and piezoelectric approaches. The effects are better in the situations of small electrode gap, with higher efficiency because of the basic rule of Coulomb force. The TENG is an intrinsic choice to induce strong electric field, moreover, with a smaller volume of device. This section discusses a TENG assembled with electrodes to generate a high Coulomb force and drive the microfluidic motion. 14.4.1

Droplet Motion Drive

In comparison with electromagnetic or piezoelectric methods, actuators based on electrostatic field and Coulomb force are considered the ideal way to manipulate the motion of micro/nano objects. In the research work by Li Zheng et al., a method for motion controlling of microfluid and solid objects was demonstrated [28]. As Figure 14.18 depicts, the TENG consists of a Kapton and an aluminum foil, connected to one of the ITO electrodes beneath the FEP film while another electrode is grounded. It is worth noting that the FEP and Kapton films were both under the treatment of ICP. Kapton, after ICP treatment, became easier to electrify and owned a stronger affinity to electrons, which promoted the output performance of the TENG, while the FEP layer, after ICP treatment, became superhydrophobic. The mechanism of electron flow triggered by CS mode is illustrated in Figure 14.19. It should be known that the droplet was charged due to the friction with air and substrate. Firstly, when the Kapton was contacted to the Al foil, there was no charge induction in the left electrode. Then, separating between Kapton and Al foil, the electrodes were charge attributed to the electrostatic effect and the droplet underwent a Coulomb force to move from left to right. During the contacting period again, the positive charges in the Al foil were neutralized and the droplet was pulled backward under the Coulomb force induced between the FEP and the droplet. As a result, the output open-circuit voltage (V oc ) of TENG was up to 1800 V and the short-circuit transferred charge (Qsc ) was 250 nC. In this work, many parameters such as the initial position, gap between electrodes, volume of droplet, and driven height were investigated. Accordingly, 60 μl and a gap of 20 mm were the optimal values to realize better motion control. In addition, based on the manipulation of one droplet in Figure 14.18a , the controlled confluence of two droplets were designed as shown in Figure 14.18c. In this system, the phenolphthalein droplet and the potassium hydroxide droplet dropped on two electrodes, respectively. Under the actuation by TENG, the two droplets converged on the middle electrode and changed into red from transparent. Besides, as a driven object, a solid pellet was tested by a newly designed electrostatic actuated system (EAS; Figure 14.18b,d). Especially, the second system was assembled by four electrodes correspondingly to control the pellet moving along a circle.

(a)

(b) Objects driven by electrostatic force

Sliding motion Kapton Kapton

Al foil

Electrode

FEP film

Water drop (positively charged)

Target objects (grounded)

(d)

(c)

Kapton

Al foil

Electrode

FEP film

Water drop (positively charged)

Al foil Electrode

Objects driven by electrostatic force

Kapton

Al foil

Target objects (grounded)

Electrode

Figure 14.18 Four structures for the manipulation of different drop motion. (a) Moving, (b) confluence of two drops, (c) combination of two drops, and (d) rotation of pellet, driven by the TENG. Source: Reproduced with permission from Zheng et al. [28]. Copyright 2017, John Wiley & Sons.

274

14 Smart Actuators

Contact electrification

Coulomb force

Electrified by air

Move forward

Separation

E Charge leakage

Charge leakage results Coulomb force in an opposite E′ Move back Return

Contact electrification Coulomb force

Charge saturated

Separation E

E′ (E′