Substitutional and Adsorptive Defects Induce Local Bandgap Modulation and Defect Emission in Monolayer MoS₂

Defect-related photoluminescence in monolayer MoS₂ has long been attributed to sulfur vacancies, yet the microscopic origins of its multiple emission components remain controversial. Low-temperature PL of monolayer MoS₂ reveals three defect-related emissions (D₁-D₃) beyond the neutral exciton peak. Laser exposure experiments show a pronounced quenching of D₂ and D₃, indicating their origin from weakly adsorbed gas molecules bound to sulfur vacancies, whereas the stable D₁ emission arises from substitutional defects. To further clarify their microscopic nature, MoS₂ samples were irradiated with O, Si, and Cl ions. Only O ion irradiation reproduces all three emissions observed in as-grown samples, confirming the oxygen-related nature of these defects. The results demonstrate that oxygen substitution locally narrows the bandgap and facilitates exciton funneling, giving rise to bound exciton emission. This study provides a unified understanding of substitutional and adsorptive defect luminescence and highlights the critical role of oxygen chemistry in tuning excitonic behavior in two-dimensional semiconductors.