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Monolayer WS₂ exhibits exceptional optical and electronic properties, however, it also oxidizes in ambient conditions leading to the degradation of its optical and electrical performance over time.

In this work, we reveal that the oxidation of WS₂ in ambient conditions is driven by bandgap photo-excitation (i.e. a photo-oxidation effect) and we describe a possible oxidation chemical reaction pathway. Through a series of controlled experiments, WS₂ monolayers grown via chemical vapor deposition were exposed to low power light (10³ to 10⁴ mW/m²) with wavelengths of 532nm, 650nm and 760nm for 7 days. Our findings suggest that WS₂ is only oxidised in ambient conditions when exposed to light with enough energy to excite an optical bandgap transition.

Furthermore, we find that even limited exposure to above-bandgap illumination in ambient – at levels routine for photoluminescence or Raman spectroscopy characterization – causes significant oxidation. We predict that this photo-oxidation effect may be universal across all monolayer semiconducting transition metal dichalcogenides, and thus, these results could have far reaching consequences to past, present and future studies.