Surface alloy formation and the structure of c(2 x 2)-Sn/Ni(100) determined by low-energy alkali-ion scattering

Ge overlayer and surface alloy structures on Pt(100) studied using alkali ion scattering spectroscopy, x-ray photoelectron spectroscopy and x-ray photoelectron diffraction

We have investigated surface structures formed by deposition of Ge on a Pt(100) substrate by using a multi-technique approach utilizing alkali ion scattering spectroscopy (ALISS), x-ray photoelectron spectroscopy (XPS), and x-ray photoelectron diffraction (XPD). ALISS was used to distinguish Ge overlayers from incorporated alloy layers for the surface structures reported, and to supply structural information about the surface alloy or 'layer compound' formed by the deposition of 1.5-ML Ge. A Ge adlayer forms following the deposition of 0.2-ML Ge on Pt(100) and annealing at 600 K. ALISS revealed that Ge adatoms in these overlayers had 1D (incomplete c(2 × 2)) Ge-Ge ordering along [010] and equivalent directions, even though this was not directly apparent in observations using LEED and STM. A c(2 × 2)-Ge overlayer was produced after 0.5 ML-Ge deposition on Pt(100) and annealing at 600 K. Deposition of 1.5-ML Ge on Pt(100) and annealing at 600 K caused extensive Ge interdiffusion into the third (subsurface) layer, while the first and second layers remained as a c(2 × 2) Ge overlayer and (1 × 1) Pt layer, respectively. We propose that the Pt(100) substrate thus is 'capped' by an alloy film with the structure of a body-centered tetragonal Pt2Ge layer compound, which is terminated by a pure-Ge layer that is indistinguishable from a c(2 × 2)-Ge adlayer. This explains the apparently 'strange' result that even though extensive Ge interdiffusion was occurring deeply into the Pt bulk during annealing at 900 and 1200 K, a Ge overlayer remained on the surface. XPS spectra showed a +0.5 eV binding energy shift of the Ge 3d core level and a small (0-0.1 eV) positive shift of the Pt 5d core level compared to Ge(100) and Pt(100) surfaces for the c(2 × 2)-Ge overlayer. There was no effect on these binding energies upon formation of the Pt2Ge layer compound at the surface, and this indicates similar Ge-Pt interactions in the two cases. Compared to other overlayers of Group-IV atoms on metal surfaces, the Ge overlayer on Pt(100) was extraordinarily stable.