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 a nd ultrahigh vacuum (UHV) conditions. As deposited, the films have a m ixture of short-range ordered basal-plane and edge-plane oriented grai ns near the film substrate interface; structural changes were characte rized in a UHV transmission electron microscope as a function of two p rocessing variables: temperature and Au deposition. Annealing in an ox ygen environment was also carried out to assess chemical stability. Du ring thermal annealing in UHV and in oxygen, substantial long-range or dering of the basal islands followed by grain growth was observed. Inh omogeneous oxidation resulting in the formation of MoO3 in the initial stages followed by grain growth, yielding the final morphology of a m ixture of MoO3 crystallites of 5-50 nm size was seen on annealing in a n oxidizing atmosphere. Au nucleation and growth on both thermally ann ealed and as-deposited films were seen to follow the Volmer-Weber mode , i.e. three-dimensional islands; these islands were also seen to be h ighly textured. Also, in comparison with carbon and SiO substrates, Au demonstrated higher stability on MoS, to electron beam fluxes, sugges ting higher bonding strengths to the substrate. These experiments demo nstrated the paramount need for UHV conditions during both deposition and characterization to avoid uncertain contamination artifacts.