Suppressed surface alloying for a bulk miscible system: Ge on Pt(100)

Structural characterization by scanning tunneling microscopy and alkali low-energy ion scattering spectroscopy of Pt(100) surfaces covered by less than 0.5-ML Ge is reported. Despite bulk solubility of Ge in Pt, it was found that Ge remains on top of the Pt(100) surface as adatoms even after annealing to 1200 K. Furthermore, Pt adatoms produced by lifting of the Pt(100)-hex reconstruction in the vicinity of adsorbed Ge do not intermix with the Ge but rather segregate to form pure-Pt islands with Ge adsorbed on top. Ge adatoms disperse across the surface and form a disordered adlayer at low coverages. We propose that this is because of a significant charge transfer from the substrate to the adsorbate which causes a dipole-dipole repulsive interaction between the Ge adatoms. At a coverage of 0.5-ML Ge, an ordered c(2×2)-Ge adlayer forms.