Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session Q20: Focus Session: Engineering Interfaces for New Materials II: Surfaces |
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Sponsoring Units: DMP DCMP Chair: Ted Einstein, University of Maryland Room: Morial Convention Center 212 |
Wednesday, March 12, 2008 11:15AM - 11:27AM |
Q20.00001: Impurity Decoration for Crystal Shape Control: C$_{60}$ on Ag(111) T.J. Stasevich, C.G. Tao, W.G. Cullen, E.D. Williams, T.L. Einstein The decoration of hexagonal Ag/Ag(111) monolayer islands by chains of C$_{60}$, observed via STM at 300K, dramatically changes their shape and fluctuations. We tune coverage so that a single C$_{60}$ chain fully decorates each Ag island boundary.\footnote{C.G. Tao et al., PRB 73, 125436 (2006); Nano Letters 7, 1495 (2007).} The C$_{60}$-induced rounding appears due to competing energetic and entropic effects.\footnote{T.J. Stasevich \& TLE, (SIAM) Multiscale Model. Simul. 6, 90 (2007)} We estimate the Ag - C$_{60}$ and C$_{60}$ - C$_{60}$ attractions as $\sim$0.13 eV and $\sim$0.04 eV, respectively.\footnote{T.J. Stasevich et al., submitted.} The edge fluctuations are remarkable: 1) C$_{60}$ decoration does not notably impede the step-edge diffusion (SED) and 2) while the bare-island fluctuations are driven by SED, the decorated island has the signature of non-conserved dynamics, even though the C$_{60}$s remain at the island edge. We suggest that rapidly diffusing Ag atoms randomly attracting the nearby C$_{60}$s. Generalizations of our model show that both spherical and rectangular decorating molecules will similarly lower the energy of highly-kinked boundaries, leading to similar island shape changes. [Preview Abstract] |
Wednesday, March 12, 2008 11:27AM - 11:39AM |
Q20.00002: Optical, Structural and Electrochemical Properties of CeO$_2$--Al$_2$O$_3$--SiO$_2$ Thin Films Dursen Saygin Hinczewski, Michael Hinczewski, Idris Sorar, Esat Pehlivan, Fatma Z. Tepehan, Galip G. Tepehan CeO$_2$ thin films can be used as counter-electrodes in electrochromic devices, but have the disadvantage of slow reaction kinetics. Thus research has shifted to composite CeO$_2$ films as more promising ion-storage candidates. In this work, we examine the sol-gel coating and characterization of CeO$_2$--Al$_2$O$_3$--SiO$_2$ transparent thin films deposited onto glass microslides and indium-tin-oxide-coated conducting glass. We investigate the evolution of the surface morphology, and the optical, structural and electrochemical properties of the films with varying Si-Al-Ce mol ratios. In particular we find the formation of novel complex phase-segregated structures at the surface, which have the potential for enhancing Li ion insertion/extraction. [Preview Abstract] |
Wednesday, March 12, 2008 11:39AM - 11:51AM |
Q20.00003: Organic coverage of the silicon (100) surface: first-principles calculations Giovanni Cantele, Ivo Borriello, Domenico Ninno Interfacing semiconductor surfaces with organic molecule adsorbates is one of the most challenging aspects of the modern surface and interface engineering. Controlled and periodic surface coverage can have important implications in lots of technological applications, such as molecular sensing, molecular electronics, etc. One of the widely investigated surfaces is the silicon $<$100$>$. Such a surface shows a periodic arrangement of silicon dimers (induced by reconstruction) whose bonding has been extensively debated. It turns out that its properties are similar to those of a double carbon-carbon bond, and it is therefore suitable for attaching organic molecules, especially those containing a double bond. In this study we theoretically investigate from first principles the adsorption of ethylene, cyclopentene and a class of its derivatives on the Si $<$100$>$ surface, discussing the implications in tailoring the surface properties, such as the electron affinity and work function. Each molecular adsorbate induces a dipole layer on the surface, whose magnitude depends on the considered molecular species. Our findings demonstrate that, for this class of systems, it is not enough the knowledge of the isolated molecule properties for predicting the properties of the surface-adsorbate complex. [Preview Abstract] |
Wednesday, March 12, 2008 11:51AM - 12:03PM |
Q20.00004: {\it Ab initio} calculations of BaTiO$_3$, PbTiO$_3$ and SrTiO$_3$ (001) and (011) surfaces Roberts Eglitis, David Vanderbilt We present results of calculations of surface relaxations and rumplings\footnote{R. I. Eglitis and D. Vanderbilt, Phys. Rev. B {\bf 76}, 155439 (2007).} for the (001) and (011) surfaces of ATiO$_3$ perovskites (A=Ba, Pb, and Sr) using a hybrid B3PW description of exchange and correlation. On the (001) surfaces, we consider both AO and TiO$_2$ terminations. In the former case, the surface AO layer is found to relax inward for all three materials, while outward relaxations of all atoms in the second layer are found for both kinds of (001) terminations. The surface relaxation energies of AO and TiO$_2$ terminations are found to be comparable with each other for all three materials. For (011) surfaces, we consider terminations on a TiO layer, an A layer, or an O layer. The surface relaxation energies for BaTiO$_3$, PbTiO$_3$ and SrTiO$_3$ (011) surfaces for all terminations are considerable larger than for (001) surfaces. Among the (011) surfaces, the relaxation energy is much larger for the TiO-terminated surface than for the Ba- or Pb-terminated surfaces for the BaTiO$_3$ and PbTiO$_3$ perovskites. We predict a considerable increase in the Ti-O chemical bond covalency near the (011) surfaces as compared to both the bulk and the (001) surfaces. [Preview Abstract] |
Wednesday, March 12, 2008 12:03PM - 12:15PM |
Q20.00005: Structure of the rutile TiO$_2$(011)-(2x1) surface Navid Khorshidi, Andreas Stierle, Vedran Vonk, Claus Ellinger, Helmut Dosch, Ulrike Diebold, Xueqing Gong, Annabella Selloni TiO$_2$ has various applications in technology and is one of the most investigated metal oxides. It is used in solar cells and its photocatalytic activity makes an understanding of the structure of diverse surface orientations desirable. Although there are many studies on TiO$_2$ surfaces, the (011) surface has been rarely investigated. First principal DFT calculations predict the (011)-(1x1) face to have the third lowest energy and in a Wulff Construction a large part of the surface is (011) oriented. TiO$_2$ nano particles exhibit preferentially (011) oriented facets. Therefore a structure model of this surface is required to understand the photocatalytic processes on an atomic scale. We have investigated the TiO$_2$(011)-(2x1) surface using Surface X-Ray Diffraction (SXRD), Scanning Tunneling Microscopy (STM) and Low Energy Electron Diffraction (LEED). From our data we are able to derive a novel model for the (011) surface in combination with DFT calculations. The new model has a much lower surface energy than the one suggested previously and fits the X-Ray data very well. [Preview Abstract] |
Wednesday, March 12, 2008 12:15PM - 12:27PM |
Q20.00006: Structure, Reactivity, and Lead Sorption of Hydrated Alumina and Hematite Surfaces Sara Mason, Anne Chaka It is well known that polarity and structure of oxide surfaces have significant impacts on reactivity. Experimental and theoretical studies at solid-aqueous interfaces have revealed that hydrated oxide surface morphology can vary significantly from surfaces under high vacuum conditions. We apply a combination of density functional theory simulations and {\em ab initio} thermodynamics to hydrated structures of $\alpha$-Al$_{2}$O$_{3}$ and $\alpha$-Fe$_{2}$O$_{3}$ surfaces. While geometrically isostructural in the bulk, these two oxides have sharp contrasts in electronic structure and can have thermodynamically stable surface terminations which differ in both the number and type of exposed functional groups. We use the environmentally relevant interaction of Pb(II) with the hydrated surfaces to explore relationships between reactivity and both surface structure and identity. [Preview Abstract] |
Wednesday, March 12, 2008 12:27PM - 12:39PM |
Q20.00007: Development of the EAM Potential for Fe-C Alloy Systems Bohumir Jelinek, Jeff Houze, Sungho Kim, Amitava Moitra, Laalitha Liyagne, Mark Horstemeyer, Seong-Gon Kim The ab-initio calculations based on density functional theory (DFT) are performed for Fe and C in their ground state crystal structures. Heats of formation are then calculated for different Fe-C alloy compounds. The lattice constant (volume), bulk modulus and shear moduli for cementite are determined from the total energy calculations. These material parameters are then used to construct the Spline-based Embedded-Atom Method (Spline EAM) potentials for Fe-C alloy systems. The results of the new potential are compared with the results of ab-initio calculations. [Preview Abstract] |
Wednesday, March 12, 2008 12:39PM - 12:51PM |
Q20.00008: Embedded Atom Method (EAM) interatomic potential for Zinc (Zn) Amitava Moitra, Sungho Kim, Jeffry Houze, Bohumir Jelinek, Laalitha Liyanage, Mark F. Horstemeyer, Seong-Gon Kim We developed a new spline-based embedded-atom method (EAM) interatomic potential for Zinc by matching forces to those of ab-initio calculations. The material parameters such as cohesive energy, equilibrium atomic volume, and bulk modulus were used to optimize the potential. The applicability of the new potential was demonstrated by performing atomistic simulations for different surfaces. The formation energies, and various point defects were also calculated. The applicability of this EAM potential to the stability analysis of small clusters was also tested. [Preview Abstract] |
Wednesday, March 12, 2008 12:51PM - 1:03PM |
Q20.00009: Calculation of Absorption Energies Using EAM Potential for Al-Mg alloy systems Laalitha Liyanage, Bohumir Jelinek, Sungho Kim, Mark F. Horstemeyer, Seong Gon Kim Spline-based embedded-atom method (EAM) interatomic potentials for Al-Mg alloy systems are developed using existing EAM potentials. The lattice constant, bulk modulus and shear modulus for the alloy are determined to demonstrate the validity of the new potential. The absorption energies of Mg atoms on Al surfaces are also calculated and compared with the results of ab-initio calculations. [Preview Abstract] |
Wednesday, March 12, 2008 1:03PM - 1:15PM |
Q20.00010: Ab initio study of the plutonium dioxide surfaces: role of electronic correlations Gerald Jomard, Francois Bottin This {\it ab initio} study is performed in the framework of density functional theory (DFT) using the projector augmented wave method. Introducing the on-site Coulomb repulsion term U in the calculations, we found equilibrium properties of both PuO$_2$ and Pu$_2$O$_3$ in good agreement with experiments. At odds with conventional DFT calculations, these two compounds are no more metallic and recover their insulating behavior with an antiferromagnetic order. As concerns the surface properties of the plutonium dioxide PuO$_2$, we perform an extensive study of eleven (1$\times$1) (110), (100) and (111) terminations then compare their thermodynamic stability by computing their surface Grand potential. Whereas conventional DFT calculations predict that a few uncompensated polar terminations can be stable, in the stability domain of the PuO$_2$ compound, the inclusion of the electronic correlations modify significantly these conclusions. We discuss this result by connecting the surface electronic structure to the polar or non-polar character of the termination. [Preview Abstract] |
Wednesday, March 12, 2008 1:15PM - 1:27PM |
Q20.00011: Effect of Mg doping on the Structure and Reflectivity of Alumina surfaces Timothy Pennycook, Juan C. Idrobo, Kalman Varga, Sokrates T. Pantelides Mg is used in the fabrication of Al alloys to increase the strength of the material. In typical applications, a layer of alumina is present on the surface. The high diffusivity and chemical reactivity of Mg means that Mg can migrate from the bulk alloy to the alumina film and the surface, where it can affect the structural and optical properties of the material. The doping of Al alloys with Mg is known to cause ``darkening'' and affect the coloration of the material. We will report results of first principles density functional theory calculations that explore the segregation modes of Mg in the near-surface region of alumina and the corresponding effect on optical properties, \textit{i.e.}, reflectivity. This work is supported in part by NSF grant DMR-0513048 and ALCOA Inc. [Preview Abstract] |
Wednesday, March 12, 2008 1:27PM - 1:39PM |
Q20.00012: Down the primrose path of dalliance: how iterative structural determination routines for thin films may lead to partial or false solutions Naji Husseini, Codrin Cionca, Yizhak Yacoby, Roy Clarke Iterative methods are frequently used to solve thin film structures. Convergence, however, may terminate at partial or even incorrect solutions. Coherent Bragg Rod Analysis (COBRA), a direct phase retrieval method with minimal iterations, has had recent success with buried interfaces in perovskite oxides and semiconductors. Here, we investigate the role of iterations in COBRA by means of simulations on a model lattice-matched system of PbTiO3 on SrTiO3 with realistic stochastic noise. Out-of-plane atomic displacements were added in various monolayers of the PbTiO3 film. With increasing iterations, the positions of the heaviest element (Pb) became more accurate at the expense of the lighter elements -- particularly the oxygen sublattices. In addition, the error in the central monolayers of the film decreased while the error at both the film-substrate and film-air interface increased. Our results provide new insights on the influence of uncertainties in measuring subtle structural details at interfaces. [Preview Abstract] |
Wednesday, March 12, 2008 1:39PM - 1:51PM |
Q20.00013: Effect of surface nanostructure on temperature programmed reaction spectroscopy Michael Rieger, Jutta Rogal, Karsten Reuter Using the catalytic CO oxidation at RuO$_2$(110) as a showcase, we employ first-principles kinetic Monte Carlo simulations to illustrate the intricate effects on temperature programmed reaction (TPR) spectroscopy data brought about by the mere correlations between the locations of the active sites at a nanostructured surface. Even in the absence of lateral interactions, this nanostructure alone can cause inhomogeneities that cannot be grasped by prevalent mean-field data analysis procedures, which thus lead to wrong conclusions on the reactivity of the different surface species. The RuO$_2$(110) surface studied here exhibits only two prominent active sites, arranged in simple alternating rows. Yet, the mere neglection of this still quite trivial nanostructure leads mean-field TPR data analysis [1] to extract kinetic parameters that are in error by several orders of magnitude and that do not even reflect the relative reactivity of the different surface species correctly [2].\newline [1] S. Wendt, M. Knapp, and H. Over, JACS 126, 1537 (2004).\newline [2] M. Rieger, J. Rogal, and K. Reuter, Phys. Rev. Lett (in press). [Preview Abstract] |
Wednesday, March 12, 2008 1:51PM - 2:03PM |
Q20.00014: NMR Evidence of Cage-to-Cage Diffusion of H$_{2}$ in H$_{2}$-Clathrates Lasitha Senadheera, Mark Conradi H$_{2}$ and heavy-ice at P$>$1 kbar and T $\sim $250 K form H$_{2}$-D$_{2}$O clathrate; four and one H$_{2}$ may occupy each large (L) and small (S) cage, respectively. In H$_{2}$-THF-H$_{2}$O clathrate, H$_{2}$ occupies singly and only S cages. Previous electronic-structure calculations estimate the barriers for H$_{2}$ passage though hexagonal and pentagonal faces of cages as $\sim $6 and $\sim $25 kcal/mol, respectively. Our H$_{2}$ NMR linewidth data reflect random crystal fields from frozen cage-wall D$_{2}$O orientations. We find dramatic reductions in linewidth starting at 120 K (175 K) for H$_{2}$-D$_{2}$O (H$_{2}$-TDF-D$_{2}$O) indicating time-averaging of the crystal fields. Assuming Arrhenius behavior, our data imply energies for escape from L (S) cages of about $\sim $4 ($\sim $6) kcal/mol. For L cages, the agreement with the calculated (cages were treated as rigid) barrier is reasonable. For H$_{2}$ in S cages, in H$_{2}$-TDF-D$_{2}$O, the extreme disagreement with theory points to another mechanism of time-averaging, reorientations of the cage-wall D$_{2}$O molecules, as suggested by previous work in TDH-H$_{2}$O clathrate. Our limited NMR spectra at high T $\sim $145 K in H$_{2}$-D$_{2}$O show evidence of distinct resonances from diffusionally mobile and immobile H$_{2}$ molecules, as expected. [Preview Abstract] |
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