Electronic structure studies of 2D materials : MoS2 and twisted bilayer graphene

Abstract

The electronic structure of 2D materials, such as few-layer MoS₂ and magic-angle twisted bilayer graphene (MATBG), is of great importance to the application of electronic devices. This thesis studies the effects of several factors on the electronic structures of these two typical 2D materials. First, we studied the effects of encapsulated hexagonal boron nitride on the electronic structure of few-layer MoS₂. By using both density functional theory calculations and experimental Raman spectroscopy, we found that hBN encapsulation can induce tensile strain. This induced strain can then affect the electronic structure of few-layer MoS₂. In detail, it may cause the K-Q crossover in the conduction bands of few-layer MoS₂. We then found that an external electric field also plays a significant role in modifying the electronic structure of few-layer MoS₂. For trilayer MoS₂, an electric field of 162 meV/nm is large enough to force the conduction band minimum to transit from Q valley to K valley. Additionally, when the oxygen plasma is applied, the monolayer MoS₂ undergoes a phase transition from 2H (semiconductor) to 1T (metal). Finally, we found that the ripples may induce electric dipole moments that are perpendicular to the 2D plane, which may significantly affect the band structure of the twisted bilayer graphene close to the first magic angle. More importantly, the tight-binding model developed by us in calculating the electronic structure is believed to be extended to other moiré systems.

Date of Award	2022
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Ding PAN (Supervisor)