Oxidation of Monolayer WS2 in Ambient Is a Photoinduced Process

We have studied the ambient air oxidation of chemical vapor deposition (CVD) grown monolayers of the semiconducting transition metal dichalcogenide (S-TMD) WS2 using optical microscopy, laser scanning confocal microscopy (LSCM), photoluminescence (PL) spectroscopy, and atomic force microscopy (AFM). Monolayer WS2 exposed to ambient conditions in the presence of light (typical laboratory ambient light for weeks or typical PL spectroscopy map) exhibits damage due to oxidation which can be detected with the LSCM and AFM, though may not be evident in conventional optical microscopy due to poorer contrast and resolution. Additionally, this oxidation was not random and was correlated with "high-symmetry" high intensity edges and red-shifted areas in the PL spectroscopy map, areas thought to contain a higher concentration of sulfur vacancies. In contrast, samples kept in ambient and darkness showed no signs of oxidation for up to 10 months. Low-irradiance/fluence experiments showed that samples subjected to excitation energies at or above the trion excitation energy (532 nm/2.33 eV and 660 nm/1.88 eV) oxidized in as little as 7 days, even for irradiances and fluences 8 and 4 orders of magnitude lower (respectively) than previously reported. No significant oxidation was observed for 760 nm/1.63 eV light exposure, which lies below the trion excitation energy in WS2. The strong wavelength dependence and apparent lack of irradiance dependence suggests that ambient oxidation of WS2 is initiated by photon-mediated electronic band transitions, that is, photo-oxidation. These findings have important implications for prior, present, and future studies concerning S-TMDs measured, stored, or manipulated in ambient conditions.