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(232)-Ge adlayer forms.