Dose-Resolved Oxygen Passivation and Vacancy Formation in WS₂ and WSe₂ under Ambient X-ray Irradiation

Defect chemistry critically shapes the optical response of atomically thin WQ₂ (Q = S, Se), yet the interplay between ambient oxygen and ionizing radiation remains poorly quantified. Here, we map dose-resolved variations in monolayer and bilayer WQ₂ exposed to in-air X-rays while tracking Raman and photoluminescence evolution. All four systems (monolayer/bilayer WQ₂) show a nonmonotonic evolution: vibrational peaks blue-shift and intensities increase at low cumulative doses, then red-shift and weaken at higher doses; exciton and trion emissions follow the same rise-and-fall. The transition occurs at an X-ray dose of ∼120-150 Gy for WS₂ and ∼60 Gy for WSe₂, with bilayers requiring higher doses than monolayers, indicating chemistry-dependent and monolayer-bilayer thickness-dependent tolerance. Atomic force microscopy of these flakes reveals no intrinsic topographic change in this window. First-principles calculations rationalize these trends: chalcogen vacancies introduce midgap states that quench emission, whereas oxygen substitution removes subgap states and slightly narrows the band gap; binding-energy analysis identifies selenium in monolayers as the easiest to remove, while bilayers and oxygen-substituted sites are more robust. The combined experiment-theory framework establishes a practical strategy for enhancing optical emission through oxygen-assisted passivation without incurring irreversible radiation damage.