Oxidation of Monolayer WS₂ in Ambient Is a Photoinduced Process

Area of Science:

• Materials Science
• Solid State Physics
• Surface Chemistry

Background:

• Semiconducting transition metal dichalcogenides (S-TMDs), like WS₂, are crucial for next-generation electronics.
• Understanding their stability under ambient conditions is vital for reliable device performance and research.
• Previous studies have not fully elucidated the mechanisms of ambient degradation in WS₂.

Purpose of the Study:

• To investigate the ambient air oxidation of chemical vapor deposition (CVD) grown monolayer WS₂.
• To identify the conditions and mechanisms driving this oxidation process.
• To assess the impact of light exposure on WS₂ stability.

Main Methods:

• Utilized optical microscopy, laser scanning confocal microscopy (LSCM), photoluminescence (PL) spectroscopy, and atomic force microscopy (AFM).
• Exposed monolayer WS₂ samples to ambient conditions with and without light.
• Conducted low-irradiance/fluence experiments using various excitation wavelengths (532 nm, 660 nm, 760 nm).

Main Results:

• Monolayer WS₂ exposed to ambient light shows detectable oxidation damage via LSCM and AFM.
• Oxidation is correlated with high-intensity edges and red-shifted PL areas, suggesting sulfur vacancy involvement.
• Samples stored in darkness showed no oxidation for up to 10 months; light exposure at specific wavelengths (≥ trion energy) induced rapid oxidation, even at low light levels.

Conclusions:

• Ambient oxidation of WS₂ is a photo-oxidation process initiated by photon-mediated electronic band transitions.
• The process is strongly wavelength-dependent and not significantly dependent on irradiance.
• Findings have critical implications for the measurement, storage, and manipulation of S-TMDs in ambient environments.