Optical properties of Transition Metal Dichalcogenides in the Defect-Free Limit

Transition metal dichalcogenides (TMDs) of the form MX₂ exhibit a direct optical gap in single layer with promising applications. Monolayer optical experiments are highly sensitive to defects, both those intrinsic to bulk, and those occurring in the substrate. While substrate defects can be passivated, intrinsic defects limit TMD performance. In this work, we quantify defect densities of TMD materials using scanning tunneling microscopy (STM). We show that the best crystals have a defect density of 0.01%, while typical crystals have a density of 0.1% or higher. Tunneling spectroscopy was then used to map the local bandgap on scales relevant for optical measurements. Based on STM results, the optical properties of exfoliated monolayers of known defect density were compared using photoluminescence (PL) spectroscopy. Our primary finding is that defects cause non-radiative decay of excitons, reducing the intensity of the observed PL by up to an order of magnitude at 0.1% defect concentrations. Spin splitting in the TMD conduction band additionally breaks the exciton into an optically bright and dark state. Since the PL is proportional to the population of excitons in each state, signatures of intervalley scattering to the dark exciton are observed in temperature-dependent PL spectra.