Charging Single Sulfur Vacancies in Monolayer WS₂ on Graphite

Point defects in Van der Waals semiconductors can be engineered in many ways beyond their bulk counterparts since screening is reduced when collapsed to the 2D limit. By using scanning tunneling microscopy and spectroscopy (STM/STS), we study the electronic structures of sulfur vacancies in monolayer WS₂ on graphite as a function of their charging state.  For charge-neutral sulfur vacancies, two in-gap states above the Fermi level are observed, consistent with previously reported results.  Moreover, we found a variety of complex defects, including negatively charged point defects which produce pronounced band bending within a length scale of 1-2 nm. Some negatively charged defects create two in-gap states with one above and one below the Fermi level. Some, however, create far more complicated in-gap states. By introducing known chemical species to deterministically dope the sulfur vacancies, we intend to unveil the evolution of the electronic structures associated with sulfur vacancies as a function of their charging state.