Crystallographic structure of ultra-thin films of Pd on Ni(1 1 1) and Ni on Pd(1 1 1) studied by photoelectron diffraction

Abstract

We report studies of ultra-thin films of Pd and Ni deposited on Ni(1 1 1) and Pd(1 1 1) surfaces, respectively, using 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(1 1 1) at room temperature, XPS indicates that Pd grows in a layer-by-layer fashion and does not diffuse on the Ni substrate, whereas 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(1 1 1). Annealing the film at 650 °C produces a (1 × 1) LEED pattern, which suggests 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(1 1 1) substrate in low concentrations (10–20%), and that 75% of the surface remained covered by Pd bi-dimensional islands. The complementary system, Ni on Pd(1 1 1), presented similar LEED and XPS results. The comparison between experimental and theoretical XPD results indicated that the surface was partially covered by Ni islands (50–60%), and the other part was formed by random Ni x Pd 100− x surface alloy.