ABSTRACT
The electronic structure of 2D materials, such as few-layer MoS2 and magic-angle twisted bilayer graphene, is of great importance to their related applications. When the bulk MoS2 is thinned to a monolayer, it has a direct bandgap in its electronic structure, together with unusual luminescence. At the same time, the modification of the electronic structure influeneces its transport properties. Another topic that has attracted global attention is magic-angle twisted bilayer graphene (MATGB), due to its strong correlated states and the unconventional superconductivity contributed by the flat bands near the charge neutrality point. This thesis examines 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. It has has been widely used to protect 2D materials in labs from oxidation and contamination, and is often regarded to have no effect on the electronic properties of encapsulated flakes. 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. Furthermore, I forced the top layer separated away from the other two layers gradually. As the interlayer distance increases, the energy difference between K valley and Q valley increases. Furthermore, when the oxygen plasma is applied, the monolayer MoS2 undergoes a phase transition from 2H to 1T. The former is a semiconductor while the latter is a metallic state.
Finally, we explored the electronic structure of magic-angle twisted bilayer graphene with ripples. By combining a classical force field and a tight-binding model, which is validated by density functional theory, we studied how ripples affect the electronic structure of the twisted bilayer graphene. We generated ripples by applying compressive strain and 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. Additionally, we extended our tight-binding model to accurately calculate the band structure of twisted trilayer graphene. We believe that this model could be extended to other multi-layer graphene systems. Meanwhile, we studied the effect of relaxation on the band structure of twisted bilayer graphene with high-order magic angle.