Abstract
Submonolayer Sn deposits on Pt(111) and Sn incorporated in a $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ Sn/Pt(111) surface alloy were oxidized by ${\mathrm{NO}}_{2}$ 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 $(4\ifmmode\times\else\texttimes\fi{}4)$ coincidence lattice with the substrate. The other structure forms an incommensurate tin oxide overlayer, exhibiting a long-range Moir\'e 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 $(5\ifmmode\times\else\texttimes\fi{}5)$ superlattice with respect to the Pt(111) substrate. The variety of ${\mathrm{SnO}}_{x}$ overlayer morphologies is attributed to subtle differences in the oxide stoichiometry and alterations in the oxide/metal interface, in particular Pt-Sn alloying.
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