Papers

AVS2001 Session SS1-TuM: Adsorption on Oxide Surfaces

Tuesday, October 30, 2001 8:20 AM in Room 121

Tuesday Morning

Start	Invited?	Item
8:20 AM		SS1-TuM-1 Microstructure of the Al₂O₃(11-20) Surface and Ultrathin Nb Films G. Witte, Th. Becker, A. Birkner, Ch. Woöll (Ruhr-Universität Bochum, Germany) Despite the technological importance of sapphire such as a support for devices or catalysts the microstructure of its surfaces have yet not been investigated in great detail. Here we report on a combined AFM, LEED, HAS and XPS study of the microstructure of the Al₂O₃(11-20) surface and the initial growth of ultrathin (110) oriented Nb-films. The samples were first heated in air which leads to atomically flat but carbon coated surfaces as indicated by the AFM and XPS measurements and requires a further preparation by sputtering and annealing to produce clean and well ordered surfaces. The resulting surface structure was found to depend sensitively on the annealing temperature. Heating above 1300K causes a substantial oxygen reduction at the surface which is accompanied by a long ranging highly periodic surface reconstruction without facetting. The subsequent growth of ultrathin Nb-films reveals the well known epitaxial relation between Nb and sapphire but also indicates that the films are not continuos. Instead flat islands are formed in a growth mode similar to that obtained for Ni on diamond.¹ ¹J.Braun, J.P.Toennies and Ch.Wöll, Phys. Rev. B60, 11707 (1999).
8:40 AM		SS1-TuM-2 Growth and Structure of Ultrathin Silver and Silver Oxide Films on Sapphire Surfaces F.X. Bock (University of Maine); S.B. Rivers (Rhode Island College); R.J. Lad (University of Maine) Silver oxide is a p-type semiconductor that has potential applications as a sensing film or optical recording element. Silver is relatively inert towards oxygen and silver oxide is rather unstable. We studied the interactions of Ag with r-cut sapphire (α-Al₂O₃) surfaces at ambient temperature resulting from (i) thermal evaporation of Ag in UHV, (ii) evaporation of Ag in molecular O₂, and (iii) evaporation of Ag in the presence of an electron cyclotron resonance (ECR) oxygen plasma source. Pure Ag deposited in UHV grows with a random polycrystalline structure. However, when Ag is evaporated in a 10^-4 Torr molecular O₂ background, epitaxial Ag growth with (110) orientation is observed as determined by RHEED and XRD. No oxygen is incorporated in the film as measured by XPS and only a slight lattice mismatch at the Ag/sapphire interface is found suggesting that the adsorbed oxygen acts as a surfactant in influencing the Ag nucleation and surface transport. Using an ECR oxygen plasma, Ag₂O is formed with a structure being dependent on deposition rate; below 0.2 Å/s, (111) orientation is formed while at faster rates the oxide is amorphous. The Ag₂O films are unstable above ~180°C. AFM observations of the evaporation process following vacuum annealing treatments indicate that the decomposition occurs through a nucleation process leaving behind metallic Ag clusters on the bare sapphire.
9:00 AM		SS1-TuM-3 STM Study of Metal Growth on ZnO Surfaces O. Dulub (Tulane University); L.A. Boatner (Oak Ridge National Laboratory); U. Diebold (Tulane University) Metals on ZnO surfaces are widely applied in catalysis, gas sensing, and microelectronic fabrication. The morphology of the polar zinc-terminated (0001) and oxygen-terminated (000-1), as well as the non-polar (10-10) and (11-20) prism faces of ZnO was investigated with STM. Images of polar surfaces, prepared by sputtering and annealing at 500-750°C, show two types of terraces rotated by 60° with respect to each other. Mono-atomic steps alternate between straight and saw-toothed profiles. Increasing the annealing temperature to 800°C smoothes the surfaces but creates a higher density of small holes on the stepped terraces. A (1x3) reconstruction was observed on the ZnO (000-1) surface after annealing at 750°C. After sputtering and annealing the ZnO (10-10) surface at 550â€“700°C, the terraces are separated by steps running along either [000-1] (type A) or [11-20] (type B) directions. The same treatment of the ZnO (11-20) surface leads to a â€œwavyâ€ surface morphology, i.e., hills (consisting of small terraces) with an average height of 80Å and a separation of 55Å. Near-atomic resolution was achieved on both non-polar faces. Cu nucleation is strongly correlated with surface defects. Cu deposited on a "freshly-annealed" ZnO (10-10) surface shows preferential nucleation of exclusively 3D islands oriented perpendicular to the atomic row direction at the step edges. When the density of impurity atoms on the terraces is high, both 2D and 3D islands are randomly distributed across the terraces. These results are compared with those obtained for growth of Cu and Pt on the other faces of ZnO. Work performed at Tulane was supported by a NSF-CAREER grant. ORNL is supported by the U.S. DOE-BES under contract No. DE-AC05-96OR22464.
9:20 AM		SS1-TuM-4 Metal - Support Interactions between Pt and Thin Film Cerium Oxide¹ D.R. Mullins, K. Zhang (Oak Ridge National Laboratory) We have examined the interaction between Pt and reduced or oxidized cerium oxide (CeO_X). Reduced CeO_X strongly modifies the chemisorption of CO on the supported Pt. Two different modifications are evident depending on how the sample is annealed. If the sample is annealed to greater than 800 K, the amount of CO that adsorbs on the Pt at 150 K is dramatically reduced. In addition, annealing at elevated temperatures decreases the Pt XPS intensity significantly. These results suggest that, similar to Pt on reduced TiO_X, the Pt is covered by the reduced CeO_X at elevated temperatures. If the sample is annealed to 700 K or less before CO is adsorbed at 150 K, there is no decrease in the amount of CO that adsorbs. However, the CO desorption temperature is shifted to a much lower temperature compared to CO adsorbed on a Pt single crystal. This shift in the CO desorption temperature is ascribed to a weakening of the CO - Pt bond that results from an electronic interaction between the Pt and the reduced ceria. The behavior of Pt on CeO_X is very different from Rh on CeO_X. The CO - Rh bond is strengthened on CeO_X compared to Rh single crystals and there is an increase in the CO dissociation activity. Rh also shows no evidence of encapsulation when annealed to higher temperatures. CO desorption from Pt on oxidized CeO₂ resembles that observed from Pt on Al₂O₃ and from Pt single crystals and therefore indicates no significant modifications of the Pt chemisorption properties by the CeO₂. ¹Research sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy, under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC.
9:40 AM		SS1-TuM-5 Electronic Structure of Epitaxial Thin NiO(100) Films Grown on Ag(100): Towards a Firm Experimental Basis L. Duò, M. Portalupi, G. Isella, R. Bertacco, M. Marcon, F. Ciccacci (Politecnico di Milano, Italy) In the field of electron correlations in solids NiO definitely plays a key role. A large amount of work on the electronic and magnetic properties of NiO has been done, but the longstanding problem of the full description and understanding of its electron states is still open. While for photoemission (PE) results on NiO a large set of data is available in literature, with a general consensus on the lineshapes, concerning inverse PE (IPE) only few pioneering works have been carried out in the early eighties. They gave rise to significant discrepancies, due both to the quality of the sample and to charging problems. This has relevant consequences concerning the application of model systems in terms, e.g., of the magnitude of the correlation energy and the charge transfer energy . On the other hand, very recently the possibility of growing well characterized NiO(100) monocrystalline thin films has shown up. This is achieved by evaporation of Ni in an O₂ atmosphere onto Ag(100), whose lattice parameter agrees with that of NiO. We have therefore measured IPE spectra of NiO(100) thin films (up to about 50 monolayers) grown by such a method. By studying the dispersion behavior of the various IPE features we were able to classify them as d- or sp-derived states and a d-like ligand hole structure at high energy is clearly shown. By combining these results to X-ray photoemission spectroscopy taken on the same surfaces we find that the magnitude of the gap is similar to what previously found. A new aspect is instead related to the position of the Fermi level (E_F) which is near the middle of the gap. This is at variance with previous results which showed a strong pinning of E_F at the top of the valence band and were interpreted in detail as an intrinsic effect of "pure" NiO.
10:00 AM		SS1-TuM-6 Surface Structures of Ultrathin Vanadium Oxide Films on Pd(111)^* S. Surnev (Karl-Franzens-Universität Graz, Austria); G. Kresse (Universität Wien, Austria); M.G. Ramsey, F.P. Netzer (Karl-Franzens-Universität Graz, Austria) The growth and the atomic structure of epitaxial vanadium oxide thin films on Pd(111) have been investigated by scanning tunnelling microscopy (STM) and low-energy electron diffraction (LEED), combined with ab-initio density-functional theory (DFT) calculations. At submonolayer coverage a well ordered (4x4) oxide overlayer forms which transforms into a porous oxide network with an internal (2x2) periodicity upon exposure with H₂ at room temperature. The reactivity of the (4x4) phase towards H₂ is very high, so that small amounts of H₂ from the residual atmosphere are often sufficient to promote this transformation. The (2x2) phase represents an interface-stabilised surface-V₂O₃ layer, which becomes compact upon mild annealing in vacuum exhibiting a (2x2) honeycomb structure. Between 0.5 and 1.0 monolayer equivalents (MLE) the growth of oxide islands with a zigzag stripe structure is observed along with the (2x2) layer. At 1 MLE several VO₂-like phases are coexistent at the surface in the form of islands with rectangular and hexagonal structures, which are distinguished from the known bulk-type rutile VO₂ lattice. The detailed atomic structure and energetic stability of these monolayer V-oxide phases have been revealed by the DFT calculations. Above 2 MLE three-dimensional crystallites grow epitaxially on Pd(111) with the corundum structure, which is typical of the bulk-type V₂O₃. Two stable V₂O₃(0001) terminations have been found in the STM images, which are due to bulk-type oxygen planes and terminal vanadyl species, as suggested by the DFT calculations and confirmed by HREELS. ^* Supported by the Austrian Science Foundation.
11:00 AM		SS1-TuM-9 Dissociative Adsorption of NO on TiO₂ (110) Surfaces J. Abad, O. Böhme, E. Roman (Institute of Materials Science of Madrid, Spain) The interaction of NO with stoichiometric and defective TiO₂ (110) surfaces has been studied by x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS) and low energy electron diffraction (LEED). Surfaces with different degree of defects have been characterized monitoring the evolution of the electronic surface structure, with the aim to study the influence of the surface defects on the interaction with NO. The interaction was studied for exposures up to 500L. However, the main effects occur already in the first 2L. The exposure of the surfaces to NO resulted in healing of defect sites without nitrogen adsorption.
11:40 AM		SS1-TuM-11 Investigation of Internal Interfaces: Characterization of Structure, Chemistry, and Relative Adhesion at Metal-Ceramic Interfaces E.A.A. Jarvis, E.A. Carter (University of California, Los Angeles) Reliable, detailed characterization of internal interfaces has become a topic of increasing interest over the past decade. Naturally, both fundamental scientific interest and a wide variety of applications may benefit from improved understanding of heterogeneous interfaces. We employ pseudopotential, planewave density functional theory to investigate local structure and chemistry at several metal-ceramic interfaces. The particular systems under investigation may hold implications for technological advancement of thermal barrier coatings for jet engine turbines. Specifically, we study interfaces between nickel "alloys" and zirconia as well as the nickel-silica interface. By selectively varying the composition of the metal alloy we compare relative adhesion at these interfaces. Our results indicate that inclusion of early transition metals (Group III and IV) at the nickel-zirconia interface may dramatically increase the interface adhesion strength. The potential application of silicon additions designed to form protective oxides on nickel alloys is discussed in light of our predicted behavior of such interfaces under high temperature conditions similar to those anticipated in jet engine turbine applications. For all systems studied, we analyze the geometric structure and the behavior of the valence electron density to provide insight into the bonding character as well as the trends in adhesion at these interfaces.