Tin-oxide overlayer formation by oxidation of Pt-Sn(111) surface alloys

Ordered (2X2) and (root3 X root3)R30 degrees Pt-Sn(1 1 1) surface alloys we re oxidized by NO2 exposure at 400 K under ultrahigh vacuum conditions. The evolution of the surface morphology with annealing temperature was charact erized by using low energy electron diffraction (LEED), scanning tunneling microscopy, Auger electron spectroscopy, and x-ray photoelectron spectrosco py. Both oxidized surface alloys form a SnOx overlayer that wets the substr ate. However, the SnOx film does not completely cover the surface for the o xidized (2X2) surface alloy. For the oxidized ( 3 X root3)R30 degrees surfa ce alloy, an ordered (4X4) LEED pattern is formed upon flash annealing abov e 900 K. The formation of this ordered SnOx adlayer coincides with Sri segr egation from the bulk to the interface region. A model for the (4X4) struct ure is discussed. The SnOx overlayer formed by oxidation of the (2X2) surfa ce alloy is significantly less thermally stable than the oxidized (,3 X roo t3)R30 degrees surface alloy. Exothermic alloying of Sn with Pt may facilit ate the decomposition of the oxide overlayers. Differences in the amount of subsurface tin and its segregation to the surface is proposed to explain t he thermal stabilities of the oxide layers. The incompleteness of the SnOx overlayer and less subsurface tin for the oxidized (2X2) surface alloy is p roposed to explain its significant lower thermal stability. (C) 2001 Americ an Vacuum Society.