Atomically-resolved exciton emission from single defects in MoS$_2$

Understanding how atomic defects shape the nanoscale optical properties of two-dimensional (2D) semiconductors is essential for advancing quantum technologies and optoelectronics. Using scanning tunneling spectroscopy (STS) and luminescence (STML), we correlate the atomic structure and optical fingerprints of individual defects in monolayer MoS$_2$. A bilayer of hexagonal boron nitride (hBN) effectively decouples MoS$_2$ from the graphene substrate, increasing its band gap and extending the defect charge state lifetime. This enables the observation of sharp STML emission lines from MoS$_2$ excitons and trions exhibiting nanoscale sensitivity to local potential fluctuations. We identify the optical signatures of common point defects in MoS$_2$: sulfur vacancies (Vac$_\text{S}^-$), oxygen substitutions (O$_\text{S}$), and negatively charged carbon-hydrogen complexes (CH$_\text{S}^-$). While Vac$_\text{S}^-$ and O$_\text{S}$ only suppress pristine excitonic emission, CH$_\text{S}^-$ generate defect-bound exciton complexes ($A^-X$) about 200\,meV below the MoS$_2$ exciton. Sub-nanometer-resolved STML maps reveal large spectral shifts near charged defects, concurrent with the local band bending expected for band-to-defect optical transitions. These results establish an atomically precise correlation between structure, electronic states, and optical response, enabling deterministic engineering of quantum emitters in 2D materials.