Interacting Excitons and Electrons in van der Waals Heterostructures Open Access
Li, Weijie (Summer 2023)
Published
Abstract
Layered materials, such as 2D semiconducting transition metal dichalcogenide (TMD), have garnered tremendous excitement for investigating physics in two dimensions. Photoexcitations in TMDs--excitons, can provide deep insights into the rich electronic properties of the host crystals. In this dissertation, we explore the interplay among excitons and electrons in TMD based van der Waals heterostructures. These low-dimensional systems have been shown to enhance few-body and many-body interactions among excitons, electrons, and exciton-electron mixture. To study these interactions, we focus on optical excitations in TMD heterostructures, viz. interlayer excitons (IXs), where electrons and holes are spatially separated into different layers due to type-II band alignment, giving rise to a permanent out-of-plane dipole moment. We observed dipole-dipole repulsive interaction-induced single photon nonlinearity for localized IXs in WSe2/MoSe2. Furthermore, at large exciton densities, we observed many-exciton exchange interaction-induced out-of-equilibrium magnetic field, which enables optical control of valley Zeeman splitting.
In addition, in an electron-doped TMD heterostructures, the interactions between electrons and localized IXs allow for local sensing of electron distribution. We used localized IXs to optically probe the charge orders of correlated electronic states in TMD heterostructures WSe2/MoSe2/WSe2 with a high spatial resolution (~10 nm). More recently, we observed a new quasiparticle in a heterotrilayer (WS2/WSe2/WS2)--quadrupolar exciton, a superposition of oppositely orientated dipolar IXs. We further observed quadrupolar to dipolar IX transition involving the modification of its internal structure, through electric field and many-body interactions. This series of works related to dipolar IXs reveal the rich physics of interactions in TMD van der Waals heterostructures.
Table of Contents
1 Introduction 1
1.1 Scope of the Dissertation 3
2 VdW Semiconductors: Background 7
2.1 Monolayer Semiconductors–Transition Metal Dichalcogenides 7
2.1.1 Lattice structure of ML TMDs 8
2.1.2 Electronic structure of ML TMDs 9
2.1.3 Valley physics in ML TMDs 11
2.1.4 Optical excitations in ML TMDs—exciton complexes 14
2.2 VdW Heterostructures 19
2.2.1 Artificial supercell–moiré superlattice 19
2.2.2 Moiré bands and strongly correlated electrons 21
2.2.3 Interlayer excitons and moiré exciton complexes 27
2.2.4 Excitonic insulator 31
3 Experimental Methods 34
3.1 PDMS-based all dry transfer 34
3.2 PC-based pick-up technique 36
3.3 Cryogenic optical measurement setup 40
4 Few-body Interactions in 2D Materials 44
4.1 Previous measurements on localized interlayer excitons 44
4.2 Experimental details 46
4.3 Observation of repulsive dipole-dipole interactions between localized IXs 48
4.4 Electric field tunability and valley dynamics 55
4.5 Multi-exciton complexes and configurations 58
4.6 Conclusion and Outlook 60
5 Many-body Interactions in 2D Materials 61
5.1 Introduction 61
5.2 Experimental details 67
5.3 Observation of many-exciton exchange interactions induced splitting 68
5.4 Interplay between exchange field and external magnetic field 75
5.5 Conclusion and Outlook 78
6 Quantum Sensing of Correlated Electrons in vdW Heterostructures 79
6.1 Introduction 79
6.1.1 Previous detection tools for correlated electronic states 79
6.1.2 Previous study on strongly correlated interlayer excitons and electrons 81
6.1.3 Previous study on doping dependence of localized interlayer excitons 82
6.2 Experimental details 83
6.3 Sharp and non-jittering localized dipolar excitons as electric and magnetic field sensors 84
6.4 Probing charge order reconfiguration with tunable carrier density 86
6.5 Monte-Carlo simulation of charge orders in multi-orbital lattices 92
6.6 Interplay between charge order state and LIX valley polarization 93
6.7 Probing charge order reconfiguration with external electric field 98
6.8 Reproducibility of the localized IXs-based quantum sensing technique 98
6.9 Conclusion and Outlook 102
7 Hybridized Quadrupolar Excitons with Tunable Oscillation Strength 103
7.1 Previous work on exciton hybridization 103
7.2 Experimental details 104
7.3 Observation of quadrupolar excitons in heterotrilayers 108
7.4 Electric field tunable oscillation strength of quadrupolar excitons 114
7.5 Many-body interaction driven quadrupolar to dipolar transitions 115
7.6 Conclusion and Outlook 120
8 Summary and Outlook 121
Appendix A Supplementary Information for moiré band calculation 124
Appendix B Extended data for few-body interaction studies 126
Appendix C Extended discussions for many-body interaction studies 129
Appendix D Extended data for quantum sensing 136
Appendix E Extended data for quadrupolar excitons 139
Bibliography 150
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