In the past decade, ultra-thin films of transition metals deposited on single crystal metal substrates have attracted considerable interest due to their catalytic, electronic, electrochemical, and magnetic properties. The interfacial interactions between the two metals can lead to preferential surface orientations, surface relaxation, surface reconstruction, order/disordered effects, and surface alloying. Ni-Pd presents a complete solid solution in the bulk, but significant Pd surface enrichment was found in bulk alloys and alloy films. In this work, ultra-thin films of Pd and Ni were deposited on the Ni(111) and Pd(111) surfaces, respectively, and then were characterized by X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and X-ray photoelectron diffraction (XPD).
For a 1.5 ML Pd film deposited on Ni(111) at room temperature, XPS measurements indicated that Pd does not diffuse on the Ni substrate, growing in a layer-by-layer fashion. LEED exhibits a reconstructed pattern, which can be attributed to a distribution of bi-dimensional islands on the surface with a lateral lattice parameter different from that of Ni(111). The same film annealed at 650 °C presented a (1x1) LEED pattern suggesting Pd diffusion and alloy formation. By using a systematic XPD analysis, we were able to determine that Pd diffused at least to the fourth layer into the Ni(111) substrate in low concentrations (10-20%), and that 75% of the surface remained covered by Pd bi-dimensional islands [1].
The complementary system, Ni on Pd(111), presented similar LEED and XPS results. The comparison between experimental and theoretical XPD results indicated that the surface was partially covered by Ni islands (60-70%), and the other part was formed by random NixPd1-x surface alloy [2].
[1] M.F. Carazzolle, S.S. Maluf, A. de Siervo, P.A.P. Nascente, R. Landers, G.G. Kleiman, Sur. Sci. 600 (2006) 2268-2274.
[2] M.F. Carazzolle, S.S. Maluf, A. de Siervo, P.A.P. Nascente, R. Landers, G.G. Kleiman, J. Electron Spectrosc. Relat. Phenom. (in press, 2006), doi: 10.1016/j.elspec.2006.11.052.
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