Many-Body Effects on Two-dimensional Materials: Electronic and Optical Properties of Hexagonal Boron Nitride and Related Systems

Two-dimensional semiconductors are the excellent platforms to develop the new generation ultra-thin and small devices as well as exploring novel physics. Due to the changes in confinement and screening in reduced dimensions, the electronic and optical properties of them are highly different from the bulk materials. In this dissertation, we use ab initio calcualtions based on many-body perturbation theory to explore and predict the quasiparticle and optical properties of two-dimensional hexagonal boron nitride (h-BN) and related systems. Within the GW formalism, we describe the quasiparticle properties, while the optical properties such as correlated electron-hole pairs are taken into consideration by the GW plus Bethe Salpeter equation approach. We find monolayer h-BN is a novel insulator hosting simultaneously bright s-like and p-like excitons, of which have never been found in other materials. Moreover, by utilizing the defects in the twisted interface of two h-BN flakes, we achieve a 100% control of the single-photon emitter. Using h-BN as substrate, we successfully create the anisotropic valley excitons whose helicity parameter could change continuous from zero to one. This dissertation is organized as follows:

In chapter 1, we introduce the methodologies, including the Hartree-Fock equations, density-functional theory, GW formalism and GW plus Bethe Salpeter equation.

In chapter 2, we calculate the electronic and optical properties of monolayer h-BN. We find the perfect circular dichroism that present in monolayer transition-metal dichalcogenides is lost in h-BN. Moreover, we find the simultaneous bright s-like and bright p-like excitons in it, as well as an intervalley 2s-2p excitonic hybridization.

In chapter 3, we present our experimental results that a very bright single photon emitter at 300 nm can be tuned at the twisted interface of two h-BN flakes. We explain this phenomenon by theoretical calcualtions while both trap and defect states are existed in the system.

In chapter 4, we come up with the idea of “anisotropic valley excitons”, which are realized in h-BN/black Arsenic/h-BN heterostructure. We find the anisotropic valley excitons no longer host circular dichroism, which linear dichroism still persist. The idea of elliptical dichroism are then introduced.

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