XPS investigation of preferential sputtering of S from MoS2 and determination of MoSx stoichiometry from Mo and S peak positions

The preferential sputtering of S from bulk MoS2 standard samples exposed to 3 keV Ar+ ion bombardment has been studied by XPS. The MoSx stoichiometry decreases from MoS2 to MoS1.12 with a concomitant reduction in the Mo 3d5/2 binding energy from 229.25 to 228.35 eV. The altered layer extends to a depth of 3.8 nm and is proposed to consist of a single amorphous MoSx phase in which Mo has a varying number of nearest neighbour S atoms. Using peak positions alone it is possible to determine the MoSx stoichiometry to an accuracy of x±0.1 from a plot of MoSx stoichiometry against (Mo 3d5/2–S 2p3/2) binding energy. The results are of strong current interest for coating analysis applications as MoS2 is a compound capable of providing low friction properties when incorporated into hard coatings.