Electron spectroscopic studies of vapor-deposited Co layers on MoO3 surfaces

The modification of polycrystalline MoO3 surfaces by vapor-deposited Co ato ms was studied by means of X-ray photoelectron spectroscopy (XPS). Thin MoO 3 layers react with evaporated Co atoms already at 90 K. At 300 K this proc ess proceeds with a significantly higher reaction rate. Besides the Mo6+ 3d signal, two new signals are observed in the Mo 3d XP spectra, which are as signed to Mo4+ and Mo5+. Cobalt is initially oxidised to Co2+ in the surfac e layer, but with increasing evaporation time the Co-0 intensity steadily i ncreases. During evaporation at a substrate temperature of 800 K, after an initial increase, a steady state of the Mo5+ and Mo6+ intensities develops. Thereafter at a constant evaporation rate no significant variations of the concentrations are detected within the escape depth of the photoelectrons. This behavior is attributed to interdiffusion processes from the surface a nd interface layers into the bulk. A satisfying fit of the Mo 3d spectra ca n only be obtained, when an additional Mo6+ species is assumed, which is as signed to a cobalt molybdate. (C) 2001 Elsevier Science B.V. All rights res erved.