Impurity Luminescence from a 2D Semiconductor in the Ultradilute Limit

For optically active impurities in 2D semiconductors to exhibit quantum nature, their concentration must be sufficiently low such that individual defects can be selectively addressed. While this requires impurity concentration to be lower than ~0.1 ppm, the lowest concentration reported to date for intentionally introduced impurities in a 2D semiconductor is ~1000 ppm, far greater than the desired limit. In this work, we demonstrate ultralow Nb doping of monolayer WS₂ by liquid-phase precursor dilution and systematically study the impact of doping concentration on impurity emission characteristics over three orders of magnitude of dilution down to 0.5 ppm. The Nb-induced emission, which is observed ~130 meV below the A exciton emission, exhibits reduction of inhomogeneous broadening with decreasing Nb concentration. Scanning transmission electron microscopy (STEM) reveals that majority of Nb impurities are isolated, and decoupled to other defects, suggesting that negatively charged Nb atoms are responsible for the observed emission. We further conduct density functional theory (DFT) and GW-Bethe−Salpeter equation (GW-BSE) calculations to gain insight into the physical nature of the localized states.