Because of profound applications of MoS2 crystals in electronics, their oxidation is the subject of substantial interest. Particularly so, because pristine 2H-MoS2 crystals are n-type semiconductors, while Mo oxides might display p-type semiconducting behavior. Here, we report on oxidation of the surface of single MoS2 crystals, which were heated at a series of steps either by increasing heating time or by increasing temperature up to 500 °C. In the case of increasing heating time we observed triangular etch pits similarly to earlier MoS2 heating studies. However, while keeping the time constant and by increasing heating temperature a cross-over from a triangular etch pits to a completely different behavior has been observed. Using electron dispersion X-ray spectroscopy (EDS) at ambient conditions, we observed an increase of oxide content with increasing heating temperature and obtained an apparent activation energy for the oxidation process of the order of 1 kcal/mol. This value is at least 8 times smaller than an activation energy for surface formation of MoO3 and according to the literature points rather to physisorbed oxygen species. Our Auger electron spectroscopy (AES) results also pointed out toward the physisorbed oxygen, similarly as our further heating studies within elevated relative humidity conditions. The Mo oxide leftovers after heating in the vicinity of 500 °C on the sample were investigated using atomic force microscopy (AFM) and showed dendritic structures. Surface appearance of those dendrites, their fractal dimension between 1.61 and 1.66, and their surface distribution were reminiscent of the diffusion-limited aggregation (DLA) growth. On the basis of analysis of AFM topographs, we hypothesized that the DLA process was controlled by a surface diffusion of the initially physisorbed oxygen, which had to diffuse to reaction centers in order to facilitate the subsequent chemical conversion of MoS2 layers to volatile Mo oxides.

Local changes to the structure and chemistry of thick MoS2 flakes due to heating

2019 Spring National Meeting

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