Dilute Impurities and Bound Excitons in 2D Semiconductors

Owing to reduced screening, point defects in 2D semiconductors such as monolayer transition metal dichalcogenides (TMDs) can serve as optically addressable quantum dots. However, the density of common defects in TMDs are excessively high, making it challenging to address individual defect states for quantum operations. Accordingly, the physical origin of the localized states also remains elusive, preventing strategic quantum defect engineering. We introduce and optically probe a variety of atomic defects in monolayer TMDs in the dilute limit with the aim to accessing their quantum nature. One example is Nb-doped monolayer WS₂, which is found to exhibit bright sub-gap emission even at ppm concentrations. We show that such emission arises from ionized-acceptor-bound excitons, a three-body charge complex analogous to a negative trion. These bound exciton complexes exhibit sizeable valley selectivity reflecting their partial free exciton character. We further discuss a scanning probe approach to rapidly quantifying impurities in the ultra-dilute limit (<10¹⁰cm^-2) in ambient conditions.