  While n-type semiconductor behavior appears to be more common in as-prepared
two-dimensional (2D) transition metal dichalcogenides (TMDCs), substitutional
doping with acceptor atoms is typically required to tune the conductivity to
p-type in order to facilitate their potential application in different devices.
Here, we report a systematic study on the equivalent electrical "doping" effect
of - single sulfur vacancies (V1S) in monolayer WS2 and MoS2 by studying the
interface interaction of WS2-Au and MoS2-Au contacts. Based on our first
principles calculations, we found that the V1S can significantly alter the
semiconductor behavior of both monolayer WS2 and MoS2 so that they can exhibit
the character of electron acceptor (p-type) as well as electron donor (n-type)
when they are contacted with gold. For relatively low V1S densities
(approximately < 7% for MoS2 and < 3% for WS2), the monolayer TMDC serves as
electron acceptor. As the V1S density increases beyond the threshold densities,
the MoS2 and WS2 play the role of electron donor. The significant impact V1S
can have on monolayer WS2 and MoS2 may be useful for engineering its electrical
behavior and offers an alternative way to tune the semiconductor TMDCs to
exhibit either n-type or p-type behavior.
