Metastable surface structures of the bimetallic Sn/Pt(100) system

Abstract

An ordered Sn overlayer on Pt(1 0 0) and several metastable Sn/Pt(1 0 0) surface alloys were studied using scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). These surfaces were studied previously using alkali-ion scattering spectroscopy (ALISS), LEED, and CO chemisorption, in order to determine conditions for alloy formation [Surf. Sci. 330 (1995) 193]. In those earlier studies, structural transitions with increasing annealing temperatures from a c(2 × 2)-Sn overlayer to a c(2 × 2)-Sn/Pt(1 0 0) surface alloy and finally to a (3√2×√2)R45° alloy structure were observed. Here STM is used to determine surface morphologies and atomic scale structures of these alloys. Sn (0.67 ML) deposited on the reconstructed Pt(1 0 0)-hex surface and annealed to 550 K formed a c(2 × 2)-Sn overlayer with compact, monolayer-high islands with step edges oriented along the [1 0 0] and [0 1 0] azimuths. Annealing the sample to 750 K initiated alloying and a c(2 × 2) surface alloy formed locally. However, the c(2 × 2) alloy structure does not appear to be stable even at this early stage of alloying and narrow channels along the 〈1 0 0〉 crystallographic directions form. For higher annealing temperatures, these channels evolve to form a two-domain structure with almost equidistant channels along the [1 0 0] and [0 1 0] directions, respectively. A dominant channel separation of 3√2 times the surface-lattice constant of Pt(1 0 0) is observed. This channel separation gives rise to a streaky (3√2×√2)R45° LEED pattern. The channels are characterized by three missing atomic rows. Between these channels are narrow terraces that are predominantly three-atomic-rows wide. The step edges of these terraces are Pt-terminated, while Sn is alloyed in the center of these terraces. A local p(2 × 2) ordering of alloyed Sn is often observed for wider terraces. Annealing to 1000 K results in “fragmentation” of the narrow terraces to form small, often square, islands with side-lengths as small as √2 times the surface-lattice constant of Pt(1 0 0). The structural characterization of these Sn/Pt(1 0 0) surfaces is used to interpret previous chemical adsorption studies.