Optical detection of a highly localized impurity state in monolayer tungsten disulfide

A photoluminescence (PL) peak has been observed from a monolayer of transition metal dichalcogenide (TMD), which is known to be an ideal 2D semiconductor. The PL peak appears near the low-energy side of neutral free excitons with very sharp peak width (∼10 meV) at low temperature (83 K). Systematic temperature-dependent PL measurements reveal that the peak can be explained by bound excitons being trapped by the surface impurities, which results in a highly localized state for the excitons. Since the optically detectable, highly localized impurity state promises to have extensive practical applications for quantum optics, our finding represents an important step in the study of 2D materials for use in quantum computation and information.