MoS 2 oxidative etching caught in the act: formation of single (MoO 3 ) n molecules.

We report the presence of sub-nm MoO x clusters formed on basal planes of the 2H MoS 2 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 x clusters are predominantly single MoO 3 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 MoO 3 /MoO x species covering the MoS 2 surface only sparsely. The X-ray absorption near edge spectroscopy data confirm the MoO 3 stoichiometry. Taken together, our results show that oxidative etching and removal of Mo atoms at the atomic level follow predominantly via formation of single MoO 3 molecules. Such findings confirm the previously only proposed oxidative etching stoichiometry.