Two-Dimensional Transition-Metal Dichalcogenides

Chi Sin Tang, PhD, is currently Research Fellow at the Singapore Synchrotron Light Source, National University of Singapore (NUS) under the NUS Emerging Scientist Fellowship.Xinmao Yin is Professor at the Physics Department of Shanghai University, China.Andrew T. S. Wee is a class of '62 Professor of Physics at the National University of Singapore.

• Preface xi1 Two-dimensional Transition Metal Dichalcogenides: A General Overview 1Chi Sin Tang and Xinmao Yin1.1 Introduction to 2D-TMDs 11.2 Crystal Structures of 2D-TMDs in Different Phases 21.2.1 Other Structural Phases 31.2.2 Phase Stability 41.3 Electronic Band Structures of 2D-TMDs 71.3.1 Electronic Band Structures of the 1H, 1T, and 1T ′ Phase 81.3.2 Indirect-to-Direct Bandgap Transition 111.3.3 Spin-Orbit Coupling and Its Effects and Optical Selection Rules 131.4 Excitons (Coulomb-Bound Electron-Hole Pairs) 151.4.1 Exciton Binding Energy 161.4.2 Excitons and Other Complex Quasiparticles 181.4.3 Resonant Excitons in 2D-TMDs 191.5 Experimental Studies and Characterization of 2D-TMDs 201.5.1 Synthesis of 2D-TMDs 211.5.1.1 Chemical Vapour Deposition 211.5.1.2 Molecular Beam Epitaxy 221.5.2 Optical Characterization 231.5.2.1 Photoluminescence 231.5.2.2 Spectroscopic Ellipsometry 251.5.2.3 Raman Characterization 291.5.3 Electronic Bandgap 351.5.3.1 Angle-Resolved Photoemission Spectroscopy 351.5.3.2 Scanning Tunneling Spectroscopy (STS) 371.5.4 Conclusions 40References 402 Synthesis and Phase Engineering of Low-Dimensional TMDs and Related Material Structures 61Bijun Tang, Jiefu Yang, and Zheng Liu2.1 Introduction 612.2 Structure of 2D TMDs 622.3 Synthesis of 2D TMDs 642.3.1 Top-Down Method 652.3.2 Bottom-Up Method 662.4 Phase Engineering of 2D TMDs 662.4.1 Direct Synthesis of TMDs with Targeted Phases 682.4.1.1 Precursor Selection 682.4.1.2 Catalyst 702.4.1.3 Temperature Control 722.4.1.4 Alloying 742.4.2 External Factor-Induced Phase Transformation 792.4.2.1 Ion Intercalation 792.4.2.2 Thermal Treatment 812.5 Conclusion 82References 833 Thermoelectric Properties of Polymorphic 2D-TMDs 87H. K. Ng, Yunshan Zhao, Dongzhi Chi, and Jing Wu3.1 Introduction to 2D Thermoelectrics 873.1.1 Why 2D over 3D? 883.1.2 Why 2D Semiconductors? 893.2 Thermoelectric Transport 893.2.1 Boltzmann Transport Equation 903.2.2 Scattering Parameter for Different Mechanism 923.2.2.1 Ionized/Charged Impurity Scattering 923.2.2.2 Phonons Scattering 933.2.2.3 Carrier–Carrier Scattering 943.2.2.4 Surface Roughness Scattering 953.3 Experimental Characterization TE in 2D 953.3.1 Electrical Measurements 953.3.1.1 FET Measurements 953.3.1.2 Hall Measurements 963.3.2 Seebeck Measurement 963.3.2.1 ΔT Calibration 973.3.2.2 V Tep Measurement 973.3.3 Thermal Conductivity 983.3.3.1 Raman Spectrometer 993.3.3.2 Tdtr (fdtr) 1013.3.3.3 Thermal Bridge Method (Electron Beam Heating Technique) 1023.3.3.4 Other Thermal Property Measurement Methods 1043.4 Manipulation of TE Properties in 2D 1063.4.1 Tuning of Carrier Concentration 1073.4.2 Strain Engineering 1073.4.3 Band Engineering 1103.4.3.1 Layer Thickness and Band Convergence 1103.4.4 Phase Transition 1123.5 Future Outlook and Perspective 115References 1174 Emerging Electronic Properties of Polymorphic 2D-TMDs 127Tong Yang, Zishen Wang, Jiaren Yuan, Jun Zhou, and Ming Yang4.1 Electronic Structure and Optical Properties of 2D-TMDs 1274.1.1 Electronic and Optical Properties of 1H-Phase 2D-TMDs 1274.1.2 Electronic and Optical Properties of 1T-Phase 2D-TMDs 1314.2 Polaron States of 2D-TMDs 1334.2.1 Holstein Polarons in MoS 2 1334.2.1.1 Experimental Characterizations of Holstein Polarons 1334.2.1.2 Theoretical Simulations of the Spectral Functions 1364.2.2 Asymmetric Intervalley Polaron Effects on Band Edges of 2D-TMDs 1374.2.3 Polaron Effects on the Band Gap Size of 2D-TMDs 1394.3 Valley Properties of 2D-TMDs 1434.3.1 Circularly Polarized Light 1474.3.2 External Field 1484.3.3 Magnetic Metal Doping 1484.3.4 Magnetic Substrate 1494.4 Charge Density Waves of 2D-TMDs 1514.4.1 Charge Density Waves in TMDs 1514.4.2 Effects of CDW on Electronic Properties 1544.4.3 Mechanisms in CDW Transitions 1554.4.4 Manipulation of CDWs 1584.5 Janus Structures of 2D-TMDs 1594.5.1 Fabrication Approaches for Janus 2D TMDs 1594.5.2 Emerging Properties of Janus 2D TMDs 1604.5.3 Potential Applications of Janus 2D TMDs 1604.6 Moiré Superlattices of 2D-TMDs 161References 1655 Magnetism and Spin Structures of Polymorphic 2D TMDs 181Meizhuang Liu, Zuxin Chen, Jingbo Li, Yuli Huang, Kuan Eng Johnson Goh, and Andrew T. S. Wee5.1 Two-dimensional Ferromagnetism 1825.2 Cr-based Magnetic Materials and Device Applications 1835.3 Polymorphic 2D Cr-based Magnetic TMDs 1915.4 Magnetism in 2D Vanadium, Ion, Manganese Chalcogenides 2005.5 Conclusions and Outlook 204Acknowledgements 204References 2056 Recent Progress of Mechanical Exfoliation and the Application in the Study of 2D Materials 211Yunyun Dai, Xinyu Huang, Xu Han, Jiangang Guo, Xiangfan Xu, Lei Wang, Luqi Liu, Ningning Song, Yeliang Wang, and Yuan Huang6.1 Introduction 2116.2 Different Ways for Preparing 2D Materials 2136.2.1 Chemical Vapor Deposition (CVD) 2136.2.2 Mechanical Exfoliation (ME) 2136.3 New Mechanical Exfoliation Methods 2146.3.1 Oxygen Plasma Enhanced Exfoliation 2146.3.2 Gold Film Enhanced Exfoliation 2186.4 Application of Mechanical Exfoliation Method 2226.4.1 Electrical Properties and Devices 2226.4.1.1 Screening of Disorders 2236.4.1.2 Electrical Contacts of 2D Materials 2256.4.2 Optical Properties and Photonic Devices 2276.4.2.1 Photodetectors 2276.4.2.2 Optical Modulators 2286.4.2.3 Single Photon Emitters 2286.4.3 Moiré Superlattice and Devices 2306.4.3.1 Graphene/h-BN Moiré Superlattice 2306.4.3.2 Twisted Graphene Moiré Superlattice 2316.4.3.3 Twisted TMD Moiré Superlattice 2316.4.4 Magnetic Properties and Memory Devices 2326.4.4.1 Ferromagnetism in 2D Materials 2356.4.4.2 Antiferromagnetism in 2D Materials 2376.4.5 Thermal Conduction 2406.4.6 Superconductors 2446.4.6.1 2D Superconductors and Their Characteristics 2446.4.6.2 Regulation Methods 2476.5 Summary and Outlook 249Acknowledgments 249References 2507 Applications of Polymorphic Two-Dimensional Transition Metal Dichalcogenides in Electronics and Optoelectronics 267Yao Yao, Siyuan Li, Jiajia Zha, Zhuangchai Lai, Qiyuan He, Chaoliang Tan, and Hua Zhang7.1 Field-Effect Transistors (FETs) 2687.1.1 Homojunction-based FETs Formed by Phase Transition 2697.1.2 Homojunction-based FETs Formed by Direct Synthesis 2707.2 Memory and Neuromorphic Computing 2727.3 Energy Harvesting 2757.4 Photodetectors 2777.5 Solar Cells 2827.6 Perspectives 284References 2858 Polymorphic Two-dimensional Transition Metal Dichalcogenides: Modern Challenges and Opportunities 293Chi Sin Tang, Xinmao Yin, and Andrew T. S. Wee8.1 Summing up the Chapters 2938.2 Projecting the Future: Challenges and Opportunities 2958.3 Global Challenges and Threats 2968.3.1 Clean and Renewable Energy Sources 2978.3.2 Water Treatment and Access to Clean Water 2998.3.3 Healthcare and Pandemic Intervention 3028.3.4 Food Safety and Security 3058.3.4.1 Agricultural Production, Sustainability, Productivity, and Protection 3068.3.4.2 Roles of 2D-TMDs in Food Packaging and Preservation 3068.4 Exponential Growth in Demands for Modern Computation 3078.4.1 Deep Learning and Artificial Intelligence 3078.4.2 Internet of Things and Data Overload 3088.5 Conclusion 312References 312Index 325