The Electron Hole System in Two-Dimensional Semiconductors and Their Heterostructures

The discovery of two-dimensional (2D) transition metal dichalcogenides (TMDCs), a new group of direct band gap semiconductors, and their heterostructures, provides unprecedented opportunities to the research and application of exciton and related species. The strong Coulomb interaction in those materials correlated the photo-excited electron hole system and generates series of exotic electronic phases. In this dissertation, I will focus on two of such systems: the interlayer exciton in TMDC heterobilayers and the trion in TMDC monolayers. With the first generation TMDC samples, the carrier dynamics in MoS2/WSe2 heterobilayers was studied as a function of twist angle. The twist angle independence of the ultrafast charge transfer indicates a hot carrier mediated charge transfer mechanism, while that of charge recombination was attributed to defect-mediated non-radiative charge recombination. The development of second generation TMDC samples, characterized by BN encapsulation and flux growth of bulk crystals, facilitates the revelation of intrinsic properties of those materials. In MoSe2/WSe2 heterobilayers, the Mott transition from insulating interlayer exciton to conducting charge s