Ultrahigh vacuum high resolution transmission electron microscopy of sputter-deposited MoS₂ thin films

High resolution electron microscopy has been used to characterize the structure of sputter-deposited MoS₂ coatings under both conventional and ultrahigh vacuum (UHV) conditions. As deposited, the films have a mixture of short-range ordered basal-plane and edge-plane oriented grains near the film substrate interface; structural changes were characterized in a UHV transmission electron microscope as a function of two processing variables: temperature and Au deposition. Annealing in an oxygen environment was also carried out to assess chemical stability. During thermal annealing in UHV and in oxygen, substantial long-range ordering of the basal islands followed by grain growth was observed. Inhomogeneous oxidation resulting in the formation of MoO₃ in the initial stages followed by grain growth, yielding the final morphology of a mixture of MoO₃ crystallites of 5-50 nm size was seen on annealing in an oxidizing atmosphere. Au nucleation and growth on both thermally annealed and as-deposited films were seen to follow the Volmer-Weber mode, i.e. three-dimensional islands; these islands were also seen to be highly textured. Also, in comparison with carbon and SiO substrates, Au demonstrated higher stability on MoS₂ to electron beam fluxes, suggesting higher bonding strengths to the substrate. These experiments demonstrated the paramount need for UHV conditions during both deposition and characterization to avoid uncertain contamination artifacts.