Moire superlattice and excitons in two-dimensional transition metal dichalcogenides

Abstract

Semiconducting transition metal dichalcogenides (TMDs) monolayers feature direct bandgap in the visible frequency range, exotic valley physics, strong spin-orbit coupling, and exceptionally strong Coulomb interaction, which has attracted great interest for exploring semiconductor optics and device applications in the atomically thin limit. The monolayer TMDs have degenerate band edges located at the Brillouin zone corner ±K, labeled by the valley pseudospin, an effective quantum degree of freedom of carriers that can be addressed by electrical and optical controls. An exciton formed by Coulomb binding of an electron-hole pair inherits the valley optical selection rules of the band edges. However, the short radiative lifetime and fast valley depolarization of monolayer excitons limit the uses for practical valley-functionaldevices. Heterobilayers of TMDs provide a way to overcome these limitations. Their type-II band alignments lead to the layer separation of the electron and hole components in the exciton. Thus, electron-hole wavefunction overlap is quenched in the interlayer configuration. Consequently, the recombination lifetime and the valley depolarization time are substantially increased. Interlayer excitons have attracted remarkable interest in the exploration of valleytronic and optoelectronic applications based on TMDs heterostructures. In this thesis, I mainly focus on the electronic and optical properties of excitons in TMDs bilayer. After introducing intralayer and interlayer excitons in moire, two ideal models with mass modulations in moire superlattice are built for graphene based bilayers, which are used to investigate the topological edge states in the zero mass domain walls. Minibands reconstruction and Fermi velocity renormalization of electrons in the moire superlattice are also studied. In bilayer TMDs, the constituent monolayer’s twin boundary would be magnified into the moire pattern, forming the moire line defect. The configurations of the moire line defect sensitively depend on the layer relative twist angle, lattice mismatch, and the twin boundary. Those configurations can be exploited to engineer various interface moire exciton modes between the R-stacking and the H-stacking regions. Some quantitative examples of the interface mode dispersions are discussed. The topological interface moire exciton states can manifest the phase shift obtained after crossing the twin boundary. The interlayer coupling would cause hybridization between the interlayer exciton and the intralayer exciton, bringing a new kind of excitons in moire pattern, called the hybridized exciton. Such exciton incorporates the large optical dipole of the intralayer exciton and large electric dipole of the interlayer exciton, providing a way to the quantum control of moire excitons. The hybridized exciton has fine structures in the spectrum, momentum dependent optical properties, and momentum dependent layer polarization. The intervalley coupling is an effective spin-orbit coupling in the intralayer exciton component of the hybridized exciton. When the intervalley coupling is large, there would be a large valley pseudospin Hall effect of hybridized excitons.