Growth and morphology of Rh deposits on an alumina film under UHV conditions and under the influence of CO

Adsorption of certain gases may alter the structure of small metal particles deposited on a surface. If the particles are prepared by vapour deposition, the morphology may also be changed by the presence of these gases during growth. We have investigated the growth and morphology of Rh under ultrahigh vacuum (UHV) conditions and under the influence of CO using a thin, well-ordered Al2O3 film as substrate. First, the growth modes at UHV pressures were characterized with the help of spot profile analysis low energy electron diffraction (SPA-LEED) and scanning tunnelling microscopy at two substrate temperatures: 90 and 300 K. At 300 K, we observe the decoration of domain boundaries between different antiphase domains of the film. The particles are mainly disordered, only a small fraction is crystalline. Keeping the substrate temperature at 90 K, smaller (disordered) aggregates form, which are mostly located inside the domains. If the deposits are exposed to CO, in both cases structural changes are detectable: SPA-LEED results point to a spreading of the particles, resulting in an expansion of their surface area. Exposing the sample to CO during deposition, on the other hand, leads to the formation of a carbonyl-like species in the case of large amounts of Rh. Its decomposition gives rise to a characteristic peak in the thermal desorption spectrum. (C) 1997 Elsevier Science B.V.