  The ultrathin nature of two-dimensional monolayer semiconductors yields
optoelectronic properties which are highly responsive to changes in
free-carrier density, making it imperative to masterfully control their doping
levels. We report a new photo-doping scheme that quasi-permanently dopes the
monolayer MoS2 to extents competing with electrostatic gating. The photo-doping
is achieved by coupling monolayer MoS2 with indium tin oxide nanocrystals that
can store multiple electrons per nanocrystal after UV illumination. In the
hybrid structure, the photo-generated valence band holes in the nanocrystals
are filled by MoS2 electrons, photo-doping the MoS2 with holes. Reductions in
carrier density by ~6x10^12 cm^-2 are observed, equivalent to the storage of
~40 electrons per nanocrystal. Long-range changes proliferating up to 40
micrometers away from the localized photodoping result from local bandstructure
variations in MoS2. These studies reveal novel all-optical carrier density
control in monolayer semiconductors, enabling remote-control of local charge
density and innovative energy storage technologies.
