Direct characterization of intrinsic defects in monolayer ReSe₂ on graphene

Understanding the characteristics of intrinsic defects in crystals is of great interest in many fields, from fundamental physics to applied materials science. Combined investigations of scanning tunneling microscopy/spectroscopy (STM/S) and density functional theory (DFT) are conducted to understand the nature of Se vacancy defects in monolayer (ML) ReSe₂ grown on a graphene substrate. Among four possible Se vacancy sites, we identify the Se4 vacancy close to the Re layer by registry between STM topography and DFT simulated images. The Se4 vacancy is also thermodynamically favored in formation energy calculations, supporting its common observation via STM. dI/dV spectroscopy shows that the Se4 vacancy has a defect state at around -1.0 V, near the valence band maximum (E_VBM). DOS calculations done for all four Se vacancies indicate that only the Se4 vacancy presents such a defect state near E_VBM, confirming experimental observations. Our work provides valuable insights into the behavior of ML ReSe₂/graphene heterojunctions containing naturally occurring Se vacancies, which may have strong implications in electronic device applications.