Experiments and Consideration about Surface Nonstoichiometry of Few-Layer MoS2 Prepared by Chemical Vapor Deposition

A detailed chemical and structural characterization of the 2H-MoS2 prepared by chemical vapor-phase method is presented. The nanosized MoS2 (five to eight layers and the lateral planes area from 17 to 55 nm) was confirmed by atomic force microscopy, X-ray powder diffraction, and Raman spectroscopy. Here the main attention was given to active edge sites and vacancy defects in the nanosized MoS2. To settle the problem, we pioneered the application of an effective combination of adequate techniques such as high-resolution differential dissolution, X-ray photoelectron spectroscopy with different excitation energies, and IR spectroscopy coupled to thermal gravimetric analysis. The results indicate that the sulfur vacancies at the topmost surface layers are created by the incongruent MoS2 evaporation in vacuum at a temperature above 1000 K and the active chemically undercoordinated Mo atoms on the (110) plane react with residual oxygen to produce the surface-oxidized layers reconstructed relative to the perfect MoS2 lattice. The structure of the surface-oxidized layers on the MoS2 particles is presented by Mo with a varying number of nearest neighboring oxygen atoms. The sulfate species are anchored to the active sites of the (013) planes. The thickness of the surface coating was found to vary as a function of the sintering temperature, and the conditions to control composition and thickness of the altered top layers are given.