  Defect engineering in two-dimensional semiconductors has been exploited to
tune the optoelectronic properties and introduce new quantum states in the band
gap. Chalcogen vacancies in transition metal dichalcogenides in particular have
been found to strongly impact charge carrier concentration and mobility in 2D
transistors as well as feature sub-gap emission and single-photon response. In
this letter, we investigate the layer-dependent charge state lifetime of Se
vacancies in WSe$_2$. In one monolayer WSe$_2$, we observe ultrafast charge
transfer from the lowest unoccupied orbital of the top Se vacancy to the
graphene substrate within (1.0 $\pm$ 0.2) ps measured via the current
saturation in scanning tunneling approach curves. For Se vacancies decoupled by
TMD multilayers, we find a sub-exponential increase of the charge lifetime from
(62 $\pm$ 14) ps in bilayer to few nanoseconds in four-layer WSe$_2$, alongside
a reduction of the defect state binding energy. Additionally, we attribute the
continuous suppression and energy shift of the dI/dV in-gap defect state
resonances at very close tip--sample distances to a current saturation effect.
Our results provide a key measure of the layer-dependent charge transfer rate
of chalcogen vacancies in TMDs.
