Results

Because of their ultrathin nature, the optoelectronic, photonic and quantum properties of layered two-dimensional (2D) semiconductors 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 modifies monolayer MoS₂ carrier density to extents previously only achievable on ultrafast timescales or in electrically connected devices. The enhanced performance is enabled by coupling monolayer MoS₂ with indium tin oxide (ITO) nanocrystals demonstrating the remarkable ability to store multiple electrons per nanocrystal. When this hybrid system is illuminated with UV radiation, MoS₂ electrons are transferred to the photo-generated unoccupied valence band states in the nanocrystals, effectively photo-doping the n-type MoS₂ with holes. MoS₂ photoluminescence reveals that the dominant exciton species evolves from trions to neutral excitons, evidence of electron extraction from the MoS₂. Reductions in carrier density by approximately 6×10¹² cm^-2 are observed, and spatial-dependent measurements reveal long-range changes proliferating up to 40 µm away from a localized point of photodoping. Single ITO nanocrystals in this simple architecture can store upwards of 40 electrons. These studies reveal a new all-optical method to control the carrier density in 2D semiconductors, enabling unprecedented manipulation of their optoelectronic properties and innovative energy storage technologies.