Theory of the CH${}_{\text{S}}^{-}$ Defect in MoS${}_2$ Confirming the Origin of the Quantum Emission

Establishing the microscopic origin of localized emission in two-dimensional materials through consistent agreement between experiment and theory remains challenging. Recent STM/STS-STML measurements assigned an emission near 1.8 eV in monolayer MoS${}_2$/hBN(2L)/graphene to the negatively charged CH${}_{\mathrm{S}}^{-}$ defect. However, a comprehensive theoretical description of this emitter is still lacking. Here, we develop such a description and systematically investigate the photophysical properties of CH${}_{\mathrm{S}}^{-}$ in monolayer MoS${}_2$, two bilayer configurations, and a MoS${}_2$/hBN heterostructure. The defect retains a similar local geometry and favors a singlet ground state in all four environments, whereas its electronic transition, zero-phonon line, electron-phonon coupling, radiative lifetime, and transition-dipole orientation depend strongly on the surrounding layers. The isolated-monolayer model exhibits substantial structural reorganization and produces a broad, dominant phonon sideband that is inconsistent with experiment. Adding an adjacent layer, either MoS${}_2$ or hBN, substantially suppresses the phonon sideband, with only a minor dependence on the defect position in bilayer MoS${}_2$. In particular, the MoS${}_2$/hBN model yields a weak phonon sideband and closely reproduces the experimental photoluminescence lineshape. Our results thus provide the first theoretical agreement of CH${}_{\mathrm{S}}^{-}$ emission and further reveal the role of adjacent layers in determining the electronic, vibronic, and optical properties of the quantum emitters in two-dimensional materials.