Bound excitons at the edges in monolayer tungsten disulfide

Defects play a significant role in tailoring the optical properties of two-dimensional materials. Optical signatures of defect-bound excitons are important tools to probe defective regions and thus interrogate the optical quality of as-grown semiconducting monolayer materials. We have performed a systematic study of defect-bound excitons using photoluminescence spectroscopy combined with atomically resolved scanning electron microscopy and first-principles calculations. Spatially resolved photoluminescence spectroscopy at low temperatures revealed bound excitons that were present only on the edges of the triangular islands and not in the interior. Atomic-resolution images reveal that the areal density of mono-sulfur vacancies is much larger near the edges ($0.92\pm0.45$ nm$^{-2}$) than in the interior ($0.33\pm0.11$ nm$^{-2}$). First-principles calculations confirm that sulfur mono-vacancies introduce mid-gap states that host optical transitions with finite matrix elements. These results demonstrate that bound exciton emission induced by mono-sulfur vacancies is concentrated near the edges in as-grown monolayer tungsten disulfide.