Evolution of spectroscopy features in layered MoS_xSe_(2-x) solid solutions

Abstract

In this work we report the structural and spectroscopic characterization of the bulk MoSxSe(2-x) solid solution synthesized by chemical vapor transport. The bulk crystals were analyzed by scanning electron microscopy (SEM), x-ray diffraction (XRD), energy dispersive spectroscopy (EDS), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Electron microscopy evaluation of the layered materials shows two distinct types of crystals: flat and easily exfoliable hexagonal microcrystals up to 50 µm in size, and agglomerated irregular crystals of 5-10 µm in size. XRD shows a consistent increase in interplanar spacing as the Se content is increased in the sample series. Raman spectra of bulk MoSxSe(2-x) obtained with three different excitation energies revealed first order phonon modes associated with pure MoS2 (x=2) and MoSe2 (x=0) along with a complex behavior of vibrational modes when x had intermediate values. XPS Mo3d line scans indicate a slight shift towards lower binding energies as the Se/S ratio increases, consistent with the expected energies of MoSe2. A simple and direct relationship can be stablished between the characteristic Raman peaks and the value of x, which can be useful for identifying the compositions of TMD crystals.