Title: The Electron Hole System in Two-Dimensional Semiconductors and Their Heterostructures
Description: 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 separated electron/hole plasmas was investigated by photoluminescence, transient reflectance, photoconductivity and diffusion measurements and directly observed in time and space. The high carrier density of more than 1014 cm-2 can be optically generated under both continuous wave and pulsed excitation conditions. This work paves the way towards predicted high-temperature exciton condensate in TMDC heterostructures. In MoSe2 monolayers, the nature of trion was revealed by time and energy resolved photoluminescence imaging. The trion binding energy is exceptionally tolerant to dielectric disorder, the temperature dependence of which disfavors the virtual trion theory. The higher diffusion constant of trion than exciton supports that it is a mobile charged species in contradiction to the exciton polaron theory. The trionic resonance is robust against Mott transition leading to the trionic emission and the ring diffusion pattern at high excitation densities.

Our observations demonstrate that the trion in monolayer MoSe2 is a robust and mobile carrier of charge and energy.