Microscale Insight into Oxidation of Single MoS2 Crystals in Air

Because of profound applications of MoS2 crystals in electronics, their microscale oxidation is the subject of substantial interest. We report on oxidation of single MoS2 crystals, which were oxidized within a precision muffle furnace at a series of increasing temperatures up to 500 degrees C. 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 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 topography, 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.