Elucidation of luminescent mechanisms of size-controllable MoSe2 quantum dots

Transition metal dichalcogenides such as MoS2 and WS2 quantum dots (QDs) have been found to show a dramatic enhancement of photoluminescence (PL) quantum efficiency as compared with their planar sheet counterparts. However, the mechanisms of PL enhancement remain not to be very clear. In this work, MoSe2 QDs with the size ranging from about 5.30 nm to 1.55 nm were prepared by a probe-assistant ultrasonication exfoliation approach. The as-prepared MoSe2 QDs are strongly fluorescent, suggesting the existence of quantum confinement effects, and show two distinct PL emissions in the ultraviolet and visible ranges, which are attributed to a band-edge state and a surface related defect state, respectively. We observed blue shifts of the PL peak position and the absorption band edge with the change in the QD size, and the discrepancy of the shifted energies between the PL emission and the estimation based on documented models is briefly addressed.