Epitaxial growth of tin oxide on Pt(111):: Structure and properties of wetting layers and SnO2 crystallites -: art. no. 165403

Tin-oxide films were grown on Pt(111) substrates by oxidation of Sn/Pt surface alloys using NO2 exposures or by deposition of Sn in an NO2 ambient gas. Structural aspects of monolayer tin-oxide films were reported previously [Phys. Rev. B 64, 245402/1 (2001)]. At elevated substrate temperatures, growth of tin-oxide multilayers proceeds in a Stranski-Krastanov mode, i.e., the Pt substrate is covered with a monolayer thick tin-oxide wetting-layer before Sn-oxide crystallites form. The crystallites were tens to hundreds of nanometers in lateral size and were identified by scanning tunneling microscopy to be rutile SnO2. These had a height of a few monolayers exposing the (011) crystal plane parallel to the Pt substrate. The low misfit of this crystal face with respect to the Pt(111) lattice apparently stabilizes this plane which is otherwise relatively energetically unfavorable. These studies demonstrate the importance of metal substrates in imposing structure and crystallographic orientation on oxide films. X-ray photoelectron spectroscopy studies of the tin-oxide films confirmed the existence of three Sn states that have been labeled previously as metallic, "quasimetallic," and oxidic Sn. We conclude that the "quasimetallic" state results from oxidized Sn that is still alloyed within the Pt surface layer. Ultraviolet photoelectron spectroscopy of the valance band and electron energy loss spectroscopy confirmed a SnO2 stoichiometry for multilayer tin-oxide films. High-resolution electron energy loss spectroscopy was used to identify characteristic vibrational modes for the different monolayer films. The SnO2 crystallites, although only a few monolayers high and tens of nanometers in width, exhibit bulklike vibrational and electronic properties.