Epitaxial growth of tin oxide on Pt„111…: Structure and properties of wetting layers and SnO 2 crystallites Matthias Batzill, 1 Jooho Kim, 2 David E. Beck, 2 and Bruce E. Koel 2, * 1 Department of Physics, Tulane University, New Orleans, Louisiana 70118, USA 2 Department of Chemistry, University of Southern California, Los Angeles, California 90089-0482, USA Received 20 November 2003; published 2 April 2004 Tin-oxide films were grown on Pt111 substrates by oxidation of Sn/Pt surface alloys using NO 2 exposures or by deposition of Sn in an NO 2 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 mul- tilayers 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 SnO 2 . 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 energeti- cally 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 spectros- copy confirmed a SnO 2 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 SnO 2 crystallites, although only a few monolayers high and tens of nanometers in width, exhibit bulklike vibrational and electronic properties. DOI: 10.1103/PhysRevB.69.165403 PACS numbers: 68.55.Jk, 68.35.-p, 73.22.-f I. INTRODUCTION Interfaces, regions of transition from one material to an- other, often exhibit structures and properties that are different from the bulk planes of materials. This is a result of adjust- ments of the atoms at the interface to minimize new free energy boundary conditions and smoothen alterations in electronic structure, e.g., ionic character, of materials across the interface. This may result in distinct structure and prop- erties of the interface region. Similar to that at surfaces, i.e., the interface between a condensed phase and gas or vacuum, broken bonds may result in reordering at the interface com- pared to bulk planes. In contrast to surfaces, however, inter- faces allow for new bonds to be formed between atoms in the two adjacent materials. This may result for epitaxial systems in interface structures that are different from any bulk planes of either material. Such interface structures may extend over several atomic layers and impose a different structure and/or crystallographic orientation of multilayer films from that of bulk structures or films that are not in contact with a dissimi- lar material. Monolayer films are a special class of interfaces that may differ yet from interfaces between two bulk mate- rials. This is because monolayers have altered bonding envi- ronments on both sides of the layer, i.e., towards the sub- strate and the gas/vacuum for a monolayer surface. Monolayers are an interesting subset of interfaces for sev- eral reasons. Notably, they provide the only interface that is directly accessible by a variety of surface science techniques that can be used to obtain information about these interfaces. Also, monolayers constitute an important class of surfaces that are different from bulk surfaces. Since most interactions of materials with the environment take place at surfaces, these unique surfaces are relevant to important materials problems related to coatings, sensors, and catalysts. Here we report on studies of a metal/metal-oxide inter- face. Such investigations of metal oxide interfaces are com- plicated by the fact that oxides can exist with numerous sto- ichiometries and structures in their bulk form already. At interfaces between a metal-oxide and a dissimilar metal sub- strate, it is impossible to predict what stable interface struc- ture will be formed. Frequently, more than one stable and/or metastable structure is observed, depending only on subtle differences in the preparation of the sample. The variety of these different structures however may not just be a nui- sance, but enriches the number of materials with special properties that can be fabricated and exploited for devices. In fact some of these metal/oxide interfaces may already un- knowingly play an important role in sensors and catalysts, e.g., due to the so-called strong metal support interaction SMSI. This phenomenon has been known for a long time and plays an important role in the chemistry of dispersed metal nanoclusters on certain oxide supports common for commercial catalysts. Due to this interaction these metal clusters may be encapsulated in an oxide layer that pro- foundly changes the catalyst activity and selectivity. Re- cently the TiO x film that formed on top of Pt metal clusters was imaged and characterized in an scanning tuneling mi- croscopy STM study of platinum clusters supported on titania. 1,2 PHYSICAL REVIEW B 69, 165403 2004 0163-1829/2004/6916/16540311/$22.50 ©2004 The American Physical Society 69 165403-1