Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session A10: Structure and Morphology of Oxide Surfaces and Interfaces |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Shao-Chun Li, Tulane University Room: D221 |
Monday, March 21, 2011 8:00AM - 8:12AM |
A10.00001: The interactions of bridging oxygen vacancies on the rutile (110) surface Cristian Ciobanu, Branden Kappes, William Maddox, Danda Acharya, Peter Sutter Using density functional theory calculations at the level of Hubbard-corrected generalized gradient approximation (GGA+U), we calculate the formation and interaction energies of oxygen vacancies on the (110) surface of rutile for neutral and positively charged slabs for different values of the Hubbard parameter U. We find that the interaction of vacancies is elastically repulsive at long range, and that there is a short-range attraction between nearest neighbor vacancies (or oxygen vacancy pairs). With this physical description of the interactions, we derive a closed formula for the surface energy of reduced (110) rutile surface with two same-row vacancies within a given spatial periodicity along the bridge oxygen row, as well as a simple statistical mechanics description of the probability of finding two vacancies at a given distance d. The results of our theoretical model are consistent with our scanning tunneling microscopy determination of the distribution of inter-vacancy separations, and provide a framework for interpreting previous works in the literature. [Preview Abstract] |
Monday, March 21, 2011 8:12AM - 8:24AM |
A10.00002: Revisiting Low-Temperature Reconstruction of TiO$_{2}$(001) N.-H. Yu, K.T. Park, V.B. Nascimento, Z. Liao, G. Li, X. He, J. Teng, J. Zhang, E.W. Plummer TiO$_{2}$(001) has been investigated by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). After cycles of Ar sputtering and surface annealing at moderate temperatures (up to 600\r{ }C for 15 minutes), TiO$_{2}$(001) reveals the so-called latticework reconstruction: row-like linear structures running along [110] and [1-10] directions. Each row further consists of bright spots separated by 6.5 {\AA}. In some areas, the rows are separated by 13 {\AA} consistent with the lattice domains of (2$\surd $2\textbf{$\times $}$\surd $2) R45 observed by LEED. In other areas, the rows are distributed in a more random fashion. Thus various nearest neighbor distances and relative heights of the rows form different microfacets. From the LEED and STM data, the surface reconstruction is modeled by added rows of stoichiometric TiO$_{2}$, aligned along [110] and [1-10] directions. [Preview Abstract] |
Monday, March 21, 2011 8:24AM - 8:36AM |
A10.00003: Cu/Cu oxide growth on ZnO and TiO2 for CO2 reduction Fei Wang, Ziyu Zhang, Richard Kurtz, Phillip Sprunger Monolayer copper growth on ZnO(10-10) and TiO2(110) have been studied with STM, EELS and LEED. These systems are attractive due to their photochemical and electrochemical reduction of CO2. However, determination of the particular reaction pathway(s) has been elusive because final reduction products strongly depend on the coverage and size of Cu clusters. In order to detangle substrate effects, single crystal ZnO(10-10) and TiO2(110) have been chosen as supports for Cu growth. STM is employed to investigate the nucleation and growth of Cu on both substrates. Cu tends to grow nanoclusters on both substrates with preferred nucleation sites and directions. Upon annealing, Cu clusters ripening have been seen on ZnO substrate but not on TiO2. Subsequent oxidation of Cu clusters is also studied with STM. CO2 vibrational modes on both substrates will be studied with EELS. [Preview Abstract] |
Monday, March 21, 2011 8:36AM - 8:48AM |
A10.00004: First-Principles Calculations of Palladium Nanostructures Formed on $\gamma $-Alumina Xin Liu, Sanwu Wang Palladium clusters supported on the $\gamma $-alumina surface serve as a catalyst for a variety of important chemical reactions. We report results of our first-principles quantum mechanical calculations for the bonding configurations of palladium atoms and clusters that are supported on the $\gamma $-Al$_{2}$O$_{3}$(110) surface. In particular, our results show that while a single Pd atom prefers to be bonded on the bridge sites of two surface aluminum atoms, a chain nanostructure and a ring-like nanostructure may be formed when more Pd atoms are adsorbed on the surface. [Preview Abstract] |
Monday, March 21, 2011 8:48AM - 9:00AM |
A10.00005: Atomic structure and interfacial energy of copper and cuprous oxide forming heterojunctions with the ZnO(0001) surface Oliver Warschkow, Katawut Chuasiripattana, Matthew Lyle, Bernard Delley, Cathy Stampfl The system Cu/ZnO is industrially important as a catalyst for methanol synthesis and water-gas-shift reactions. The pairing of copper and zinc oxide is crucial to catalytic efficacy; however, the atomic-scale interactions between the two phases are far from resolved. This presentation will focus on three heterojunctions of relevance to catalytic action, namely, Cu(111):ZnO(0001), Cu$_{2}$O(110):ZnO(0001), and Cu$_{2}$O(111):ZnO(0001). We use density functional theory to characterize these interfaces in terms of their environment-dependent structure and energetics. This allows us to assess the relative stability of competing structures, and discuss their possible roles in an active catalyst. [Preview Abstract] |
Monday, March 21, 2011 9:00AM - 9:12AM |
A10.00006: Hydrogen Adsorption on polar ZnO$(0001)$-Zn - extending equilibrium surface phase diagrams to kinetically stabilised structures Mira Todorova, Markus Valtiner, Joerg Neugebauer Hydrogen adsorption on the Zn-terminated polar ZnO$(0001)$ surface is studied by a combination of density-functional theory calculations and {\it atomistic thermodynamics}. Going beyond the thermodynamic limit and constructing meta-stable phase diagrams we extend the concept of equilibrium surface phase diagrams to include kinetically stabilised surface reconstructions. Using this approach we were able to identify new and hitherto not reported structures that become stable under non-equilibrium extreme H-rich conditions. Experimental situations that realise such conditions will be discussed. \\[4pt] M. Valtiner, M. Todorova, and J. Neugebauer, Phys. Rev. B {\bf 82}, 165418 (2010). [Preview Abstract] |
Monday, March 21, 2011 9:12AM - 9:24AM |
A10.00007: DFT Study on ZnO Nanoplate Towards Magnetic Property Seung Soon Jang, Ji-Il Choi, Jung-Il Hong, Zhong Lin Wang, Robert Snyder Using a GGA+U method and Density Functional Theory, we present a theoretical study for the existence of a magnetic moment in ZnO nanoplate without any extrinsic doping of magnetic impurities. Nanoplate are configured with a Zn-terminated $\left( {0001} \right)$ surface and O-terminated $\left( {000\bar {1}} \right)$ surfaces. The surface reconstruction was considered by optimizing the structures. Using GGA PBE, we calculated the spin density of states for both spin states and individual density of states for each orbital to clarify the degree of contributions. Compared to the electronic configuration of bulk wurtzite ZnO, net spins are observed in ZnO nanoplates depending on the plate thickness, which is thought to be due to large changes in the degree of hybridization throughout the plate. As the electronic configuration of a ZnO nanoplate is converged to that of bulk ZnO with increasing plate thickness, its net spin disappears. Specifically, It is found that the net spin of the ZnO nanoplate disappears when its thickness increases beyond $\sim $ 6 nm. In our presentation, we will discuss the change of the electronic configurations as a function of the plate thickness with a rationalization of this change. [Preview Abstract] |
Monday, March 21, 2011 9:24AM - 9:36AM |
A10.00008: Ultrathin film growth of iron oxides on YSZ(001) and (111) Gary Kellogg, Ivan Ermanoski We use low energy electron microscopy (LEEM) and low energy electron diffraction (LEED) to study in real time the growth of iron oxides on the (001) and (111) surfaces of yttria{\-}stabilized zirconia (YSZ). Investigations of the FeO$_{x}$-YSZ system are motivated by its use as a working oxide for thermochemical fuel production via splitting of H$_{2}$O and CO$_{2}$. LEED patterns obtained from YSZ(001) during Fe deposition in $\sim $10$^{-6}$~Torr O$_{2}$ at 600\r{ }C and above indicate first-layer growth of FeO(111) and second-layer growth of Fe$_{2}$O$_{3}$(0001). LEEM imaging shows highly anisotropic first-layer growth into four non-equivalent domains (two rotations and two stacking orientations). Distinct LEEM-IV (intensity-voltage) spectra are obtained for the two stoichiometries providing unique fingerprints of the observed oxide phases. On YSZ(111), growth $>$800\r{ }C in O$_{2}$ is similar to (001) in that FeO is observed in the first layer and Fe$_{3}$O$_{4}$ in the second. Sandia is a multi{\-}program laboratory operated by Sandia Corporation, a subsidiary of Lockheed Martin, for the U.S. DOE's NNSA under contract DE{\-}AC0494AL85000. Funding was provided through Sandia's LDRD Office. [Preview Abstract] |
Monday, March 21, 2011 9:36AM - 9:48AM |
A10.00009: \emph{In Situ} Synchrotron Studies of a Model Catalyst: WO$_x$/$\alpha$-Fe$_2$O$_3$ Martin McBriarty, Zhenxing Feng, Joseph Libera, Jeffrey Elam, Donald Ellis, Michael Bedzyk Statistically averaging surface-sensitive X-ray techniques are employed to elucidate the surface morphology of a model oxide-supported heterogeneous catalyst, tungsten oxide (WO$_x$) on hematite ($\alpha$-Fe$_2$O$_3$). Atomically flat $\alpha$-Fe$_2$O$_3$ (0001) single crystals were coated with sub-monolayer WO$_x$ by atomic layer deposition (ALD). \emph{In situ} X-ray standing wave (XSW) imaging with X-ray fluorescence (XRF) was used to determine W position relative to bulk-like cation lattice sites under nominally reducing and oxidizing chemical conditions. X-ray absorption fine structure (XAFS) reveals details of W coordination, bond length, and chemical state on WO$_x$-coated hematite single crystals and nanopowders. Synchrotron characterization results are compared with morphologies predicted by density functional theory (DFT) calculations for clean WO$_x$/$\alpha$-Fe$_2$O$_3$ surfaces. Thermodynamics and atomic configurations for H$_2$O and CO adsorption are also predicted. Excited-state self-consistent field (SCF) calculations are used to model X-ray photoelectron spectroscopy (XPS) results. [Preview Abstract] |
Monday, March 21, 2011 9:48AM - 10:00AM |
A10.00010: First-Principles Investigations of Oxygen Vacancies on SnO2 Nanofilms Daniel Cellucci, Steven Lewis The n-type semiconductor tin dioxide (SnO2) has long been used as the working material for robust, inexpensive oxidizable-gas sensors. In recent years, advances in nanofabrication have made possible the well-controlled formation of SnO2 nanocrystals. Since gas sensing in SnO2 involves changes in surface resistivity as a function of gas concentration, nanocrystalline SnO2 holds great promise for high-sensitivity gas sensors, due to the high surface-to-volume ratio. A key feature of the sensing mechanism is the facile formation and destruction of oxygen vacancies at (or near) the surface. In this talk I will discuss our ongoing first-principles investigations of surface oxygen vacancies in SnO2 nanofilms. We have focused on vacancy formation among the so-called bridging oxygen atoms on the (110) surface of rutile SnO2, as a function of vacancy concentration and film thickness, studying the effect on local atomic and electronic structure. This work is the first phase of a longer-term investigation of surface vacancy phases on SnO2 (110) as a function of temperature and oxygen vapor pressure. [Preview Abstract] |
Monday, March 21, 2011 10:00AM - 10:12AM |
A10.00011: Microstructural relaxation phenomena on laser-modified fused silica surfaces Manyalibo Matthews, Thomas Soules, James Stolken, Ryan Vignes, Steven Yang, Selim Elhadj Laser-driven phase transformations and associated morphological deformations on vitreous SiO$_{2 }$surfaces are presented. Direct imaging of Si-O-Si asymmetric stretch transverse-optic (TO) mode shifts using a combination of scanning Infrared and Raman spectromicroscopy revealed the creation of the high pressure phase stishovite through the nonlinear absorption of ultraviolet laser pulses. Structural relaxation at $\sim $1900 K of modified surfaces back to the amorphous state could be correlated with Si-O bond angle shifts and used to describe the thermally-driven transformation kinetics. Kohlrausch relaxation functions are applied through finite element modeling of the calculated sub-surface thermal histories to extract reasonable values for the activation enthalpy and annealing point relaxation time of laser-modified silica. Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy, National Nuclear Security Administration under Contract DE-AC52-07NA27344. [Preview Abstract] |
Monday, March 21, 2011 10:12AM - 10:24AM |
A10.00012: Polarizing-Depolarizing fields competition on PbTiO$_3$ nanocapacitors Miguel Angel Mendez Polanco, Ilya Grinberg, Andrew Rappe We analyzed the stability of various interfacial atomic arrangements in PbTiO$_3$ (PTO) based nanocapacitors, using density functional theory (DFT). We observed that particular constructions induce a large polarization enhancement via a net field depolarizing-to-polarizing swap within the PTO layers, as revealed by analysis of electrostatic potential profiles. In contrast to those with a dominant depolarizing field, possessing a polarization below that of the bulk, the polar structures are stable in the thin-film regime. Interface atomic relaxation is also observed to be a key factor in determining the overall stability of the different capacitor configurations. This boosted charge screening capacity along with appropriate engineering of the interface chemistry, are potential how-to pointers to alleviate the critical thickness in ferroelectric-based nanocapacitors. [Preview Abstract] |
Monday, March 21, 2011 10:24AM - 10:36AM |
A10.00013: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 10:36AM - 10:48AM |
A10.00014: Growth of vanadium dioxide thin films using magnetron sputtering Fangfang Song, B.E. White The unique electronic properties of vanadium dioxide have been a focus of intense experimental and theoretical investigation. Although the origin of the metal-insulator transition in this material is still under investigation, the magnitude of the resistivity change at the metal-insulator transition and closeness of the transition temperature to room temperature suggest this material has high potential for future electronic devices. However, the existence of a large number of distinct stable vanadium oxide phases offers a particular challenge to the growth of thin films of this material. In this work, we present our experimental investigation of vanadium dioxide thin film deposition. RF and DC Magnetron sputtering are used for thin film deposition and the effect of oxygen partial pressure, substrate material, and deposition temperature are studied. The impact of deposition conditions on the structural and morphological properties of the thin films, as determined by x-ray diffraction and scanning electron microscopy, will be discussed. Results indicate that on the technologically relevant silicon dioxide surface, the transitional phase of vanadium dioxide can be stabilized with an appropriate post deposition anneal. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700