AVS2010 Session SS1-WeM: Oxide Surface Structure

The (1 × 1) hydrogen reconstruction observed on Zn-terminated polar ZnO(0001) surfaces [1] is a prominent surface structure, which has so far defied any explanation attempts: none of the surface reconstructions observed in ab initio based equilibrium surface phase diagrams is consistent with the experimental observations [2]. Extending these equilibrium phase diagrams into the region of non-equilibrium conditions for the hydrogen chemical potential we show the occurrence of a new and so far not reported surface structure. The new structure consists of a (1 × 1) H covered triangular shaped reconstructions. It is stabilised and obeys electron counting, due to a simultaneous protonation of step-edge oxygen and surface terminating Zn atoms. It will be shown that the experimental conditions justify the consideration of hydrogen chemical potential outside the stability range of the H₂ molecule [3].

[1] T. Becker et al., Surf. Sci. 486, L502 (2001).

[2] M. Valtiner et al., Phys. Rev. Lett. 103, 065502 (2009).

[3] M. Valtiner et al., submitted (2010).

Epitaxial heterostructures constitute a large fraction of the materials systems used in current optoelectronics technology. As device dimensions continue to shrink to the nanoscale, atomic interfaces play an increasingly dominant role in their characteristics and performance. Moreover, new classes of devices are envisioned based on novel phenomena emerging from the complex ionic and electronic rearrangements occurring at interfaces. Energy harvesting, catalysis, quantum information processing and smart sensors are but a few of the possible applications. An essential requirement for harnessing these transformative developments is to provide accurate and detailed maps of the structure, chemical composition and strain at epitaxial interfaces prepared by various deposition methods, including molecular beam epitaxy, metallorganic chemical vapor deposition, focused ion beam and pulsed laser deposition. This presentation will describe some of the exciting science that can be done with current and envisioned capabilities in x-ray surface scattering. Examples include the use of direct methods for achieving sub-Ångstrom resolution maps of complex oxide interfaces, quantum-dot tailoring and focused ion beam directed assembly. Such experiments provide a basis for understanding how local structure can give rise to novel properties.

*Supported by DOE Basic Energy Sciences Contract DE-FG02-06ER46273 and the University of Michigan Energy Frontiers Research Center.</P>

Oxide clusters with well-defined size and stoichiometry supported on metal surfaces are interesting nanoscale objects with attractive features for both fundamental research and technological applications. It has been shown recently that (WO₃)₃ cluster molecules, formed by vacuum sublimation of WO₃ powder, can be deposited as monodispersive clusters on TiO₂ (110) surfaces [1]. In the present work, we investigate the interaction and electronic properties of (WO₃)₃ species at the single molecule level with Cu(110) and reconstructed Cu(110)-O surfaces using low-temperature (5K) scanning tunneling microscopy (STM) and spectroscopy (STS). In order to decouple the (WO₃)₃ cluster states from the metal substrate states, the clusters have been deposited also on a NaCl buffer layer (grown on Cu(110)). The comparison of the STM (STS) fingerprints of the (WO₃)₃ clusters on the NaCl and the Cu surfaces, together with respective DFT calculations, allows us to gauge the cluster-Cu interaction and to determine the hybridisation of the cluster orbitals with metal states. At low temperature (<20K) the (WO₃)₃ clusters are stable as individual units on the Cu(-O) surfaces, but at elevated temperature (room temperature and above) the clusters react with Cu substrate atoms. They selforganise via condensation and, depending on the coverage, form 1-D W-oxide line structures (nanowires) or arrange into ordered 2-D W-oxide nanolayer phases with well-defined atomic structures. These 2-D structures are investigated experimentally with STM and LEED and theoretically by DFT calculations.

Supported by the ERC Advanced Grant SEPON

[1] O. Bondarchuk, X. Huang, J. Kim, B.D. Kay, L.-S. Wang, J.M. White, Z. Dohnalek, Angew. Chem. Int. Ed. 45 (2006) 4786

We have used low energy electron microscopy (LEEM) to study in real time the growth of iron oxides on the (001) surface of yttria‑stabilized zirconia - YSZ(001). The FeO_x-YSZ system is currently used as a working material for solar thermochemical splitting of H₂O and CO₂ [1], but little fundamental information is available concerning the structure and composition of the mixed oxides and their surfaces. Upon Fe deposition in ~10^-6 Torr of O₂ background pressure, iron oxides grow on the surface. Observation of low energy electron diffraction (LEED) patterns during growth shows the development of 3 distinct patterns with 12‑fold symmetry, and one pattern with 4‑fold symmetry. Dark field LEEM imaging shows a complicated domain structure where each of the 12‑fold patterns corresponds to 2 or 4 rotationally equivalent domains. Some of the domains overlap laterally, pointing to a layered arrangement of different oxide structures. We use LEEM I‑V and LEED to determine the oxide composition (hematite, magnetite, wüstite) and surface structure, both of which depend on the layer thickness, substrate temperature, and background oxygen pressure.

Sandia National Laboratories is a multi‑program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE‑AC0494AL85000. Funding for this work was provided through Sandia’s LDRD Office.

[1] Diver, R.B., Miller, J.E., Allendorf, M.D., Siegel, N.P., Hogan, R.E., “Solar thermochemical water‑splitting ferrite‑cycle heat engines”, Journal of Solar Energy Engineering, 130 (2008) 041001

Catalysts based on WO₃ have been found to be active in a number of reactions including isomerization of alkanes and alkenes, partial oxidation of alcohols, selective reduction of nitric oxide, and metathesis of alkenes. In the present study we explore the growth of novel ordered WO₃ thin films on Pt(111) via direct sublimation of monodispersed (WO₃)₃ clusters. The prepared films are characterized using x-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), low energy electron diffraction (LEED), reflection-adsorption infrared spectroscopy (RAIRS), and scanning tunneling microscopy (STM). The factors that affect the WO_x structure such as WO₃ exposure and substrate temperature were explored. At submonolayer coverages, the as-deposited (WO₃)₃ clusters are partially reduced leading to chainlike tungsten oxide structures with W in (6+) and (5+) oxidation states . Higher substrate temperatures (< 800 K) lead to further reduction of deposited (WO₃)₃. At higher coverages, an ordered (3 × 3) structure composed of (WO₃)₃ trimers is observed upon 700 K deposition. The experimental findings are complemented by DFT calculations that provide further insight into the observed WO₃ structures.

This research was performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.

It is demonstrated that, thanks to the high energy resolution used, it is possible to discriminate between the metallic and oxidised aluminium on the basis of the Al^₀ KL_2,3L_2,3 and Al³⁺ KL_2,3L_2,3 Auger electron peaks recorded within one and the same spectrum. Oxygen electron-stimulated desorption is observed for both aluminium oxides. Effective cross-sections for this oxygen desorption are estimated for the thin and thick oxide layers. It is shown that when an Al₂O₃ layer is irradiated with an electron beam surface charging occurs. These phenomena remain limited thanks to the presence of the underlying metallic aluminium substrate. The electron beam-induced effects are compensated by setting the angle of incidence of the primary electron beam to 15 °. Finally, it is shown that inhomogeneities at the surface of the analysed oxides result in significant chemical shifts of the O KLL and Al KLL Auger electron transitions. It is investigated whether these inhomogeneities can be linked with lateral differences in hydroxyl fraction at the surface.