Structure of monolayer tin oxide films on Pt(111) formed using NO₂ as an efficient oxidant

Submonolayer Sn deposits on Pt(111) and Sn incorporated in a (√3 × √3)R30° Sn/Pt(111) surface alloy were oxidized by NO₂ under ultrahigh-vacuum conditions. The oxide films formed were characterized by Auger-electron spectroscopy, low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM). Four different surface morphologies were identified, depending on the preparation conditions, each of them exhibiting a distinct LEED pattern. STM revealed two ordered epitaxial overlayers. One is interpreted as the adsorption of SnO pseudomolecules at preferential sites to form a (4 × 4) coincidence lattice with the substrate. The other structure forms an incommensurate tin oxide overlayer, exhibiting a long-range Moiré pattern. The remaining LEED patterns are associated with the formation of a regular stress-relief pattern that can transform into an ordered array of tin oxide islands upon repeated oxidation. This ordered island array exhibited a (5 × 5) superlattice with respect to the Pt(111) substrate. The variety of SnO_x overlayer morphologies is attributed to subtle differences in the oxide stoichiometry and alterations in the oxide/metal interface, in particular Pt-Sn alloying.