Structural studies of Sn Pt(100) surfaces: conditions for alloy formation

The deposition of Sn onto Pt(100) and the thermal stability of these surfaces was studied primarily by using low energy alkali ion scattering spectroscopy (ALISS) combined with AES, LEED and CO adsorption in order to elucidate conditions for preparing ordered surface alloys of PtSn. Distinct c(2 × 2) and (3√2 × √2)R45° LEED patterns were observed for specific Sn precoverages and annealing temperatures. Several real space structures that could be responsible for each of the LEED patterns are proposed and investigated. The results of the ALISS studies show that both overlayer Sn and incorporated Sn atoms can produce the c(2 × 2) LEED pattern with the resultant structure determined by the initial coverage and, critically, by the thermal history. Although the overlayer c(2 × 2)-Sn Pt(100) structure is present from 500-700 K, which is over most of the temperature range of the c(2 × 2) LEED pattern, it is not the most stable structure. Annealing to 700-750 K causes Sn adatoms to become incorporated into the first Pt layer and forms the c(2 × 2)-Sn Pt(100) surface alloy. Heating to 750-850 K produces a more stable PtSn alloy structure characterized by a (3√2 × √2)R45° LEED pattern. For Sn incorporated into the first Pt layer to form a surface alloy, a buckling distance of 0.19 ± 0.02 A ̊ is determined. This indicates that an almost planar PtSn alloy surface is achieved, at least locally, in the first layer and that the buckling distance is independent of the substrate orientation for Pt(100) and Pt(111) surfaces.