  Structural defects in 2D materials offer an effective way to engineer new
material functionalities beyond conventional doping in semiconductors.
Specifically, deep in-gap defect states of chalcogen vacancies have been
associated with intriguing phenomena in monolayer transition metal
dichalcogenides (TMDs). Here, we report the direct experimental correlation of
the atomic and electronic structure of a sulfur vacancy in monolayer WS2 by a
combination of CO-tip noncontact atomic force microscopy (nc-AFM) and scanning
tunneling microscopy (STM). Sulfur vacancies, which are absent in as-grown
samples, were deliberately created by annealing in vacuum. Two energetically
narrow unoccupied defect states of the vacancy provide a unique fingerprint of
this defect. Direct imaging of the defect orbitals by STM and state-of-the-art
GW calculations reveal that the large splitting of 252 meV between these defect
states is induced by spin-orbit coupling. The controllable incorporation and
potential decoration of chalcogen vacancies provide a new route to tailor the
optical, catalytic and magnetic properties of TMDs.
