MoS₂ oxidative etching caught in the act: formation of single (MoO₃) molecules

We report the presence of sub-nm MoO clusters formed on basal planes of the 2H MoS₂ crystals during thermal oxidative etching in air at a temperature of 370 °C. Using high resolution non-contact atomic force microscopy (AFM) we provide a histogram of their preferred heights. The AFM results combined with density functional theory (DFT) simulations show remarkably well that the MoO clusters are predominantly single MoO₃ molecules and their dimers at the sulfur vacancies. Additional Raman spectroscopy, and energy and wavelength dispersive X-ray spectroscopies as well as Kelvin probe AFM investigations confirmed the presence of the MoO₃/MoO species covering the MoS₂ surface only sparsely. The X-ray absorption near edge spectroscopy data confirm the MoO₃ stoichiometry. Taken together, our results show that oxidative etching and removal of Mo atoms at the atomic level follow predominantly via formation of single MoO₃ molecules. Such findings confirm the previously only proposed oxidative etching stoichiometry.