High-temperature strain-mediated oxidation of 2D MoS2

MoS2 is 2D material applicable for electronics, photo-detectors, light-emitting diodes, and solar cells. Since these applications operate at elevated temperatures, determining and controlling the environmental stability of MoS2 is essential. This study uses Raman spectroscopic experiments at elevated temperatures to observe reversible and irreversible changes in 2D MoS2 films with varying thickness, morphology, and substrate to explore the limits of its thermal stability. Molecular dynamics simulations and in-situ X-ray diffraction confirm that thermal expansion causes the Raman reversible spectral shifts. Further analysis reveals irreversible spectral shifts associated with oxidation and strain effects caused by oxidation with two distinct modes. The former is due to the general homogenous thinning of layers from the top layer down through randomly spaced sulfur vacancies. The latter is due to inhomogeneous oxidation, which known to occur as localized pitting at grain boundaries, void formation from nucleation centers, or stress corrosion cracking starting from edges under strain. Segregation of doping and strain contributions confirms higher strain in the edges. Annealing increases the spread of the strain distribution. These results highlight the critical role of strain, resulting from thermal expansion coefficient mismatch between substrates and 2D MoS2 films, on their environmental stability.