  We describe a photogating effect in mono- and few-layer MoS2, which allows
the control of the charge carrier density by almost two orders of magnitude
without electrical contacts. Our Raman studies are consistent with physisorbed
environmental molecules, that effectively deplete the intrinsically n-doped
charge carrier system via charge transfer, and which can be gradually removed
by the exposure to light. This photogating process is reversible and precisely
tunable by the light intensity. The photogating efficiency is quantified by
comparison with measurements on electrostatically gated MoS2.
