Unraveling Sulfur Vacancy Evolution Mechanism during Low-Temperature H2 and S Vapor Annealing in Monolayer Mo(1-x)WxS2 Alloys

This study systematically modulated the healing and creation of S vacancies in monolayer Mo(1-x)WxS2 alloys via low-temperature H2 and S vapor annealing, observing three distinct types of photoluminescence (PL) evolutions. Type I featured intensified PL, a decreased X-/X0 integrated-area ratio, and a blue-shift, indicating S vacancy healing. Type II showed intensified PL along with an increased X-/X0 ratio and a red-shift, suggesting the repair of S vacancy types distinct from those in type I. Type III exhibited PL quenching alongside a reduced X-/X0 ratio and blue-shift, corresponding to the formation of additional S vacancies. First-principles calculations reveal that selective healing or creation of specific vacancy types modulates the concentration of isolated S monovacancies, which in turn governs the X-/X0 ratio and drives the distinct PL intensity-energy trajectories. Experimentally observed Raman shifts corroborate this mechanism: PL enhancement coincides with S vacancy healing and red-shift of the Raman modes, whereas PL quenching correlates with increased S vacancy density and blue-shifts. This work clarifies the microstructural origins of annealing-driven PL and Raman responses, including single-to-double vacancy transitions, providing key insights for tailoring the optoelectronic properties of two-dimensional transition-metal dichalcogenides.