XPS investigation of ion beam treated MoS2–Ti composite coatings

Abstract

MoS2 coatings incorporating Ti were deposited on Si substrates primarily for compositional characterisation and chemical structure studies. For comparison, MoS2 layers and natural molibdenite crystals were also involved in this work. The MoS2 and the MoS2–Ti composite layers were co-deposited by unbalanced close field DC magnetron sputtering using simultaneously operating MoS2 and Ti targets in Ar plasma. The coatings are almost amorphous by XRD. The overall composition and the in-depth homogeneity of the coatings were determined by NBS. XPS studies show that the surface of the coatings stored in ambient air was partially oxidised and carbon contaminated. The majority of Mo and S were found to be in sulphide environment and part of the Ti in oxide (TiO2) and part in sulphide states. A small part of the S proved to be in a newly discovered sulphidic environment, different from that found in MoS2. No evidence of such state could be detected for the molibdenite crystal but developed in it at Ar+ treatment. The major effect of the Ar+ ion bombardment was the severe preferential sputtering of the sulphide type S lowering the ratio of sulphide S to Mo-S states from the initial value of 2 down to approx. 1.5 for molibdenite. This effect was more pronounced for both coatings lowering the S/Mo-S ratio to about 1. The experimental results demonstrate that the atomic scale chemical structure of the MoS2 coatings was also affected by energetic particles in agreement of their amorphous microstructure.