Photoluminescence of CVD Grown WS ₂ Monolayers Treated with H ₂ SO ₄ Vapor: Implications for Photoelectric Conversion Materials

Two-dimensional transition metal dichalcogenides (TMDCs), such as MX2 (M = Mo, W; X = S, Se), have emerged as promising materials for optoelectronic applications owing to their high carrier mobility and the transition from an indirect to a direct band gap at the monolayer limit. Among them, monolayer WS2 is particularly attractive; however, films grown by chemical vapor deposition (CVD) commonly exhibit weak and spatially nonuniform photoluminescence (PL) because of intrinsic surface defects. In this work, monolayer WS2 was synthesized via a bottom-up CVD process using a three-zone furnace, producing well-defined triangular crystals with average lateral dimensions of ∼20–25 μm and a maximum size of ∼75 μm. To improve its light-emission performance, surface defect passivation using H2SO4 vapor was systematically explored. Comprehensive structural, morphological, and chemical characterization using AFM, SEM, EDX, Raman spectroscopy, and XPS confirmed the formation of high-quality monolayer WS2 and provided insight into the chemical modifications induced by the vapor treatment. Room-temperature PL measurements show that pristine WS2 exhibits a sharp yet weak emission centered near 626 nm. After H2SO4 vapor exposure, the PL intensity increases by up to 20-fold, accompanied by a red shift of the emission peak to ∼634 nm. Deconvolution of the PL spectra reveals a pronounced suppression of trion emission and a corresponding enhancement of neutral exciton recombination. Consistent with these observations, first-principles calculations indicate that oxidation-driven passivation of sulfur vacancies eliminates mid-gap defect states and induces band gap renormalization. Overall, this study demonstrates, for the first time, the effectiveness of H2SO4 vapor passivation for WS2 monolayers and introduces a simple, scalable, and relatively mild strategy for enhancing excitonic purity and radiative efficiency under ambient conditions. These findings strengthen the potential of monolayer WS2 as an optically active material for photoelectric conversion, where reduced nonradiative recombination and improved optical quality are critical.