Novel transport behavior in two-dimensional semiconducting and superconducting transitional metal dichalcogenides

Abstract

Atomically layered transitional metal dichalcogenides (TMDs) manifest many fascinating properties such as atomic-scale thickness, favorable mechanical, electronic and optoelectronic properties and strong spin-orbit coupling. In terms of electronic properties, the TMDs range from insulating or semiconducting to metallic or semi-metallic. Some of them also exhibit exotic electronic phases such as charge density waves and superconductivity. Recent advances in nano-materials characterization and device fabrication, in particular, fabrication of high quality van der Waals heterostructures, have boosted studies on two-dimensional layers of thin TMDs for purpose of both fundamental research and industrial applications. In this thesis, I present a series of experiments investigating electronic and optoelectronic properties of semiconducting TMDs MoS2 and WSe2. I also demonstrate technical advances in fabrication of van der Waals heterostructures, which enables high-quality encapsulated thin TMDs devices with ionic liquid introduced as electrolyte. I further show that phase transitions in superconducting TMDs such as 2H-TaS2 can be greatly impacted by dimensionality reduction. A substantial enhancement of superconducting T, and a suppression of the CDW transition are observed in 2H-TaS2 in the 2D limit. At last, I present a machine learning algorithm to realize pixel-wise classification on laboratory acquired images of various 2D materials, which might open up new opportunities for full automation of nano-material search and device fabrication.