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 were oxidized by NO2 exposure at 400 K under ultrahigh vacuum conditions. The evolution of the surface morphology with annealing temperature was characterized by using low energy electron diffraction (LEED), scanning tunneling microscopy, Auger electron spectroscopy, and x-ray photoelectron spectroscopy. Both oxidized surface alloys form a SnOx overlayer that wets the substrate. However, the SnOx film does not completely cover the surface for the oxidized (2X2) surface alloy. For the oxidized ( 3 X root3)R30 degrees surface alloy, an ordered (4X4) LEED pattern is formed upon flash annealing above 900 K. The formation of this ordered SnOx adlayer coincides with Sri segregation from the bulk to the interface region. A model for the (4X4) structure is discussed. The SnOx overlayer formed by oxidation of the (2X2) surface alloy is significantly less thermally stable than the oxidized (,3 X root3)R30 degrees surface alloy. Exothermic alloying of Sn with Pt may facilitate the decomposition of the oxide overlayers. Differences in the amount of subsurface tin and its segregation to the surface is proposed to explain the thermal stabilities of the oxide layers. The incompleteness of the SnOx overlayer and less subsurface tin for the oxidized (2X2) surface alloy is proposed to explain its significant lower thermal stability. (C) 2001 American Vacuum Society.