Paper SS2-ThA9
Surface Structures and Alloy Formation for Ge Deposition on Pt

Thursday, October 23, 2008, 4:40 pm, Room 208

We have investigated surface structures formed by Ge deposition on Pt(111) and (100) single-crystal substrates by using STM, LEED, ALISS, and XPD. This combination of techniques provides an atomic level description of the surface structures necessary to understand the “site-directed” chemistry of these surfaces and can be used to distinguish formation of adlayers and surface alloys. We find large apparent differences between the behavior of Ge on Pt(100) compared to Pt(111) substrates. An ordered p(√19x√19)R23.4°-Ge/Pt(111) surface alloy was formed on Pt(111) by annealing at 900 K. This structure substitutionally incorporates 4%-ML Ge atoms into the Pt surface layer located very close to substitutional Pt atomic positions, without any corrugation or “buckling”. In contrast, and despite extensive 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. We interpret the c(2x2) Ge overlayer that is produced by 0.5 ML Ge deposition and annealing at 600 K as formation of a unique surface alloy or “layer compound”. The Pt(100) substrate thus is covered by a surface alloy film with the structure of a body-centered tetragonal Pt₂Ge layer compound. TPD shows that both CO and NO adsorb more weakly on the Pt-Ge bimetallic surfaces than on the clean Pt surfaces.