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 MX 2 (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 WS 2 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 WS 2 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 H 2 SO 4 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 WS 2 and provided insight into the chemical modifications induced by the vapor treatment. Room-temperature PL measurements show that pristine WS 2 exhibits a sharp yet weak emission centered near 626 nm. After H 2 SO 4 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 H 2 SO 4 vapor passivation for WS 2 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 WS 2 as an optically active material for photoelectric conversion, where reduced nonradiative recombination and improved optical quality are critical.