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

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