In situ Raman study on sulfur vacancies in monolayer MoS₂

Monolayer MoS ₂ combines a direct optical bandgap with an atomically thin geometry, making it a promising platform for defect engineering. Raman studies of ion-irradiated MoS ₂ are often complicated by high ion energies, incomplete defect quantification, and uncontrolled adsorbates at defect sites. Here, we irradiate large-area monolayer MoS ₂ with low-energy (600 eV) Ar ⁺ ions in a ultrahigh vacuum chamber and perform in situ Raman spectroscopy over a range of fluences. Atomic-resolution scanning transmission electron microscopy reveals predominantly randomly distributed sulfur vacancies as the dominant defect type. With increasing fluence, Raman spectra show a downshift and broadening of the E (Formula presented) (Formula presented) mode, a slight upshift and broadening of the A (Formula presented) (Formula presented) mode, and the emergence of defect-activated features, including a prominent LA(M) mode. A controlled ambient exposure followed by remeasurement separates intrinsic defect signatures from extrinsic doping: an additional A (Formula presented) (Formula presented) upshift and linewidth narrowing indicate a modest, largely reversible p-doping contribution from weak physisorption at vacancy sites, corresponding to an apparent charge transfer of ∼0.02 e per STEM-counted vacancy. Within the sensitivity of our in situ Raman measurements, oxidation-related signatures remain negligible, and adsorbate effects largely vanish upon returning to vacuum and under laser illumination. These results establish Raman fingerprints of sulfur-vacancy ensembles in monolayer MoS ₂ and provide quantitative guidance for defect engineering and metrology under controlled vacuum conditions.