Fabrication and High-Resolution Structural Characterisation of 2D Material Moiré Structures

Sammanfattning

Atomically thin two dimensional (2D) materials have become a major research topicsince the discovery of graphene. In particular, 2D transition metal dichalcogenides(TMDs), such as MoS2, have attracted extensive research interest due to their uniqueelectrical and optical properties suited for a future-generation of electronic and opticaldevices. One unique property of 2D materials is that atomically thin layerscan be artificially stacked together building tunable structures, for example, Moiréstructures. Moiré structures constructed by atomically thin TMDs can be designedto generate and control new material properties, including superconductivity, ferroelectricity,and quantum hall effect. Understanding the fabrication of 2D TMDMoiré structures by stacking atomically thin layers, and determining the structureof 2D TMD Moiré structures with high spatial resolution, are of critical importancefor developing and using the novel structures of 2D materials.In this thesis work, an experimental setup and protocol for fabricating twisted TMDMoiré structures were successfully developed and completed. The fabrication procedureinvolves the preparation of atomically thin TMD layers from bulk crystals,accurate determination of thicknesses, and mechanical stacking of individual 2DTMD layers in a controllable manner. Then, the structure of twisted TMD Moiréstructures was characterized at the nanometre and atomic scales using transmissionelectron microscopy (TEM) and scanning TEM (STEM). The focus was on stackedbilayer-bilayer and monolayer-monolayer MoS2 Moiré structures. Electron diffractionand TEM dark field (DF) imaging were used to reveal nanoscale domain structuresresulting from atomic reconstruction in the Moiré structures. Moiré structureswith parallel and anti-parallel stacking showed distinctly different domain structures.Furthermore, STEM imaging provided a direct visualization of domain and domainboundary structures at the atomic scale. DFT calculations were used to proposean hypothesis for the possible atomic arrangement at the Moiré domain boundaries.Possible electrical polarization resulting from changes in the atomic stacking of thestructures was also studied using TEM selected area electron diffraction patterns.Evidence of in-plane polarization in MoS2 Moiré structures was observed in electrondiffraction, while out-of-plane polarization in MoS2 with rhombohedral stacking wasinvestigated using differential phase Contrast (DPC) STEM imaging.