Imaging photo-carrier dynamics and correlated states in two-dimensional semiconductors

Two-dimensional (2D) semiconducting transition-metal dichalcogenides (TMDs) exhibit remarkable electrical and optical properties, such as strong light-matter interaction and high photoresponsivity. Moreover, moiré superlattices composed of TMD offer a fantastic platform to manipulate the correlated quantum states of matter. A comprehensive understanding of photo-excited carrier dynamics and electronic correlation in TMDs and their heterostructures at nanoscale is critical for fundamental science and novel nano-electronic applications. This thesis presents several scientific advances that lead to the successful visualization of nanoscale photoconductivity distribution, defect-mediated photo-carrier dynamics, and correlated insulator states in 2D lattices. First, using light-illuminated microwave impedance microscopy (iMIM), I performed the intrinsic photoconductivity imaging in TMD materials and revealed many interesting results. For instance, we demonstrated that, due to the photo-excited carrier diffusion and confinement into the alloy region, the local photoconductivity in the alloy region could be tailored and enhanced by two orders of magnitude over pure WS₂ in monolayer WS₂/WS₂ [subscript (1-x)] Se₂ [subscript x] /WS₂ lateral heterostructures. Secondly, to reveal the intrinsic time and length scales of photocarriers, I built iMIM and time-resolved iMIM in a helium-flow cryostat. The diffusion length and lifetime of photo-carrier were directly measured by performing spatiotemporal photoconductivity imaging. Time-resolved experiments indicate that the critical process for photo-carriers in TMD materials is the escape of holes from defect-induced trap states, which prolong the apparent lifetime of mobile electrons in the conduction band. Finally, I performed the nanoscale conductivity imaging of correlated electronic states in TMD moiré superlattices. The noncontact microwave probe allows us to observe the Mott insulating state at both hole-doped and electron-doped sides that persists for temperatures up to 150 K. Appreciable inhomogeneity of the correlated states is directly visualized in the hetero-bilayer region, indicative of local disorders in the moiré superlattice potential or electrostatic doping. These works provide essential insights into TMD-based optoelectronics and 2D correlated physics down to the microscopic level.Physic