Point defects in semiconductors are of great technological significance in semiconductor industry and have been proposed more recently as a room temperature qubit platform. In two-dimensional semiconductors such as transition metal dichalcogenide (TMD) monolayers, defects have an even larger impact on material properties, but offer exciting possibilities as atomic quantum systems. In this talk, I will present a comprehensive study of point defects in monolayer WS2, and other TMDs. The atomic, electronic, vibronic and optical properties of single defects were probed using a combination of high-resolution scanning probe microscopy techniques. We identified isoelectronic chalcogen and transition metal substitutions as the dominant defects based on their unique electronic fingerprint [1,2]. Most importantly, we found four defining effects that govern the in-gap defect states in TMDs: Crystal-field splitting, spin-orbit coupling, electron-phonon interaction and charge localization. Specifically, we observed large spin-orbit splitting at sulfur vacancies  and chromium substituting tungsten , local strain at substitutional defects , charge localization and associated hydrogenic bound states  and vibronic excitation of quasi-local modes . Moreover, we demonstrate electron-induced luminescence at individual point defects. Luminescence maps and bias-dependent optical spectra suggest that the in-gap defect states are final states in an inelastic electron tunneling process, which is mediated by the tip plasmon . The atomic-scale characterization of atomic, electronic and optical defect properties will guide future efforts of targeted defect engineering and doping of TMDs.
Speaker: Bruno Schuler, Lawrence Berkeley National Lab
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