Extended Data Fig. 4: XPS analysis of MoS₂ samples with varying crystallinity.

a. XPS spectra of three MoS₂ samples with different crystallinity. The top spectrum represents MoS₂ single crystal, the middle one corresponds to polycrystalline MoS₂ grown with a grain size of 500 nm, and the bottom spectrum is for polycrystalline MoS₂ used in this study with a grain size of less than 100 nm. While the MoS₂ single crystal exhibits minimal presence of Mo 6+ or 3+ peaks, the other two MoS₂ samples show observable peaks for both Mo 6+ and 3+ states. Here, the intensity of the 6+ peak indicates the extent of oxidation to MoO₃, while the size of the 3+ peak is directly related to the sulfur vacancy concentration. b. The Mo:S ratio and sulfur vacancy concentration extracted from the XPS spectra for each sample. Here, the Mo:S ratio has been normalized to the crystal sample. It clearly indicates that the polycrystalline sample indeed has a sulfur-deficient environment. Furthermore, the increase in sulfur vacancy concentration as the grain size decreases, coupled with a decrease in the Mo:S ratio, indirectly suggests that sulfur vacancies are generated at grain boundaries.