Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session J10: Surface and Interfaces with Metals |
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Sponsoring Units: DCMP Chair: Ted Einstein, University of Maryland Room: D221 |
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J10.00001: Ordered and disordered surface alloys in Au-Pt(111) and their effect on oxygen adsorption Wei Chen, C. Wolverton, David Schmidt, William Schneider Bimetallic surface alloys are considered a promising type of catalyst for improved activity and selectivity. Understanding surface structure and its effect on catalytic performances plays a critical role in designing catalysts from surface alloys. We have studied the surface structure and ordering of AuPt(111) using a first-principles cluster expansion based method. Even though the Au-Pt system is phase-separating in the bulk, we find a series of thermodynamically stable, laterally ordered striped structures of AuPt(111) surfaces. The formation of such ordered structures is the result of a competition between the strain relaxations from stripes and the unfavorable Au-Pt bonds at stripe interfaces. We have also investigated the oxygen adsorption on these structures. The oxygen binding energy is found to be highly correlated with the type of nearest neighbor surface atoms of oxygen. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J10.00002: Order-Disorder Transitions for Au/Mo(112) Keisuke Fukutani, Yaroslav Losovyj, Natalia Lozova, Ivan Yakovkin, Ning Wu, Peter Dowben Order-disorder overlayer phase transitions are observed at the surface of Au/Mo(112) for the nominal Au coverages of 1.66 and 1.75 monolayers. These transitions are characterized by the abrupt change in the surface Debye temperature. In the search for the detailed mechanism of this phase transition, we investigated the electron-phonon coupling (EPC), in the vicinity of the Fermi level, for the surface states of Au-covered Mo(112) surface from high- resolution angle resolved photoemission data taken parallel to the surface corrugation (i.e. $<$\underline {11}1$>)$. The changes of the widths of the surface weighted bands, induced by Au layers, are discussed in terms of electron-electron interactions, electron-impurity scattering and electron-phonon coupling. Gold overlayers suppress the mass enhancement of the Mo(112) surface band crossing the Fermi level at 0.54 {\AA}$^{-1}$. The data indicate that significant contributions from impurity and defect scattering must be considered in any serious analysis of the imaginary part of the self energy and that there interface effects can have a profound influence on the imaginary part of the self energy. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J10.00003: Ab initio study of Mg self-diffusion on Mg(0001) terraces and steps Maral Aminpour, Marisol Alcantara Ortigoza, Talat Rahman The high density of states (DOS) at the Fermi level and high density of quasi-free electrons result in a singular behavior for Mg surfaces and thin films. We find, however, that the DOS around the Fermi level, surface energy and cohesive energy converge beyond 15 layers. We also show that the Friedel charge density oscillations of Mg(0001) are more complex than depicted previously by 1D and 2D plots. These oscillations are, in fact, responsible for the stacking fault of Mg adatoms and islets on Mg(0001) and also, indirectly, for the low adatom self-diffusion barrier (20 meV) on Mg(0001), which is in agreement with effective-medium theory calculations. [1] We will compare this barrier with that of Mg adatom on a narrow terrace and across the steps on Mg(0001), as well as with predictions from Kinetic Monte Carlo simulations made to fit the growth mode observed for Mg/Mg-thin-films via scanning tunneling microscopy. \\[4pt] [1] Z.J. Tian, U. Yxklinten, B.I. Lundqvist and K.W. Jacobsen.Surf. Sci.\textbf{258} (1991), p. 427 [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J10.00004: Theoretical aspects of studies of high coverage oxidation of the Cu(100) surface using low energy positrons N.G. Fazleev, W.B. Maddox, J.A. Reed The study of adsorption of oxygen on transition metal surface is important for the understanding of oxidation, heterogeneous catalysis, and metal corrosion. The structures formed on transition metal surfaces vary from simple adlayers of chemisorbed oxygen to more complex structures which results from diffusion of oxygen into the sub-surface regions. In this work we present the results of an ab-initio investigation of positron surface and bulk states and annihilation probabilities of surface-trapped positrons with relevant core electrons at the Cu(100) missing row reconstructed surface under conditions of high oxygen coverage. Calculations are performed for various surface and subsurface oxygen coverages ranging from 0.50 to 1.50 monolayers. Calculations are also performed for the on-surface adsorption of oxygen on the unreconstructed Cu(001) surface for coverages up to one monolayer to use for comparison. Estimates of the positron binding energy, positron work function, and annihilation characteristics reveal their sensitivity to atomic structure of the topmost layers of the surface and charge transfer. Theoretical results are compared with experimental data obtained from studies of oxidation of the Cu(100) surface using positron annihilation induced Auger electron spectroscopy. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J10.00005: Deposition of metal onto a sulfur loaded substrate Daeho Kim, Dezheng Sun, Wenhao Lu, Eric Chu, Jon Wyrick, Zhihai Cheng, Ludwig Bartels A Cu(111) surface can be loaded with sulfur to form a variety of surface patterns. In this work, we study the deposition of copper and molybdenum on a Cu(111) surface and the resultant film morphology as a function of the sulfur pre-loading of the substrate. For copper deposition, we find the formation of adstructures of different geometry depending on the sulfur decoration of the substrate. A 0.143 ML S coverage leads to rectangular structure consisting of 6 lobes while a 0.118 ML S coverage leads to 7$\times $7 structure. Notably, annealing allows the sulfur to float up decorating the newly deposited layer. Deposition of molybdenum shows a similar pattern, with ordered MoS2 forming as a result of annealing. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J10.00006: Oxides on Nanoscale Platinum Surfaces Daniel Hennessy, Vladimir Komanicky, Michael S. Pierce, Kee-Chul Chang, Hoydoo You We demonstrate the existence of oxide layers on nanoscale Pt interfaces annealed in an oxygen environment. The sample is a Pt single crystal cut at the midpoint between the 100 and 111 crystal directions; annealing in Ar produces a smooth surface, while annealing in air produces $\sim $10 nm-sized 100 and 111 facets. Synchrotron x-ray crystal truncation rod (CTR) measurements indicate a bilayer Pt oxide structure on the nanofacets. Fitted Pt occupancies are consistent with a nearest-neighbor avoidance structure of the surface oxygen atoms. Electrochemical cycling of the faceted surface in CO-saturated solution removes the oxide and leaves clean, ordered facets. Pt single crystals of 100 and 111 surface orientations prepared the same way did not support an oxide layer. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J10.00007: The amazing kinetic stability of the high temperature ($\sqrt{3}$ $\times$ 6)rect. striped structure of decanethiol SAMs on Au(111) and other interesting properties. A scanning tunneling microscopy study. Lloyd Bumm, Daminda Dahanayaka, Abhijit Biswas, Ronald Halterman We present an STM study of the properties of the high temperature ($\sqrt{3}$ $\times$ 6)rect. phase of decanethiol SAMs on Au (111). Although this phase is known, it has not been extensively studied. We show a simple reliable way to grow the ($\sqrt{3}$ $\times$ 6) rect. phase and show that its coverage is 75\% of the normal (2 $\sqrt{3}$ $\times$ 3)rect. phase. Although it has lower density compared to the normal alkanethiol SAM structure, it shows a remarkable kinetic stability with respect to uptake of additional alkanethiol molecules and reversion to the normal (2$\sqrt{3}$ $\times$ 3) rect. phase. Other properties of the ($\sqrt{3}$ $\times$ 6)rect. phase will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J10.00008: Epitaxial orientations of para-sexiphenyl platelets grown on alkali halide (001) surfaces Edward Kintzel, Detlef Smilgies Thin film growth of simple aromatic molecules has been researched intensely in recent years in the burgeoning field of organic electronics. Film growth for simple rodlike molecules on the atomically well-defined and nonreactive alkali halide (001) surfaces also constitutes an archetypical model system for the study of molecular epitaxy. We have observed a surprising variety of preferential orientations of para-sexiphenyl platelets on a series of alkali halide surfaces with lattice constants ranging from 4.6 to 6.6 Angstroms. We present a metric that helps to classify the dominant epitaxial orientations and allows us to predict epitaxial orientations on other rocksalt-type substrates, and we identified surface corrugation as the driving force for these preferred relative orientations. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J10.00009: Pentacene thin films on vicinal Ag(111) surfaces Fatih Danisman, Ilker Demiroglu, Ersen Mete, Sinasi Ellialtioglu Here we present a structural study of pentacene thin films on different vicinal Ag(111) surfaces by helium atom diffraction measurements and density functional theory (DFT) calculations. Our helium atom diffraction results suggest a step flow growth mechanism evidenced by initial slow specular reflection intensity decay rate as a function of pentacene deposition time. This is in agreement with our previous helium diffraction results on flat Ag(111) surfaces with a small miscut angle. In parallel with the experimental findings, our DFT calculations predict the step edges as the most stable adsorption site on the surface. Isolated pentacene molecules adsorb on the step edges in a tilted configuration with a binding energy of 0.615 eV. In addition a complete monolayer with tilted pentacene on the step edges is found to be more stable than one with all lying flat molecules. Hence our results suggest, in agreement with prvious predictions, that step edges can trap the pentacene molecules and act as nucleation sites for the growth of ordered thin films with a crystal structure similar to that of bulk pentacene. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J10.00010: Scaled Shell-like Pattern Formation of Selenium-Based Anthracene Derivatives at a Metal Surface Ludwig Bartels, Zhihai Cheng, Jonathan Wyrick, Dezheng Sun, Daeho Kim, Yeming Zhu, Miaomiao Luo, Robert Carp, Michael Marsella We investigated the behavior of selenium-substituted anthracene molecules at a Cu(111) surface. In our previous work, the sulfur and oxygen counterparts of this molecule exhibited controlled diffusion on Cu(111) violating the substrate's symmetry. In contrast Diseleno-ateanthracene shows an isotropic and very high mobility suggesting very non-local substrate interactions. However, we observe pronounced sensitivity of the diffusion to the oscillation of the Cu(111) substrate surface state. In this talk, we will focus on the coverage-dependent pattern formation of this species: at coverages close to 1 ML, two kinds of hexagonal patterns with large unit cells are formed. Both of them show a shell superstructure with an identical central empty hole. The smaller of the features one molecular shell of 6 molecules the larger a double shells of 6 and 12 molecules. Another kind of rectangular pattern is also observed, which could be an intermediate superstructure between the small and large hexagonal patterns. We will compare these patterns to prior work on sulfur and oxygen based molecules. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J10.00011: Stable Carbon Nanoarches in the Nucleation of Graphene on Cu(111) Robert Van Wesep, Hua Chen, Wenguang Zhu, Zhenyu Zhang To fully exploit the device potential of graphene, reliable production of large-area, high-quality samples is required. Epitaxial growth on transition metal surfaces have shown promise in this regard, but further improvement would be facilitated by a more complete understanding of the nanoscale processes involved. Using density functional theory calculations, we have investigated the energetics and kinetics of graphene nucleation and growth on a Cu(111) surface. Our calculations have revealed an energetic preference for the formation of stable 1D carbon nanoarches consisting of 3-13 atoms when compared to 2D compact islands. Our findings may provide the structural link between nucleated carbon dimers [1] and larger carbon nanodomes [2], and may also explain some recent experimental observations. We will also present results on estimating the critical cluster size that marks the transition from nanoarch dominance to island dominance in the growth sequence. \\[4pt] [1] Hua Chen, et al., Phys. Rev. Lett. 104, 186101 (2010). \\[0pt] [2] Paolo Lacovig, et al., Phys. Rev. Lett. 103, 166101 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J10.00012: An \textit{Ab Initio} Study of Bulk $\gamma $-U and the (100) Surface Dayla Morrison, Asok Ray The properties of bcc $\gamma $-U have been studied using the formalisms of the generalized gradient approximation to density functional theory (GGA-DFT) and hybrid density functional theory. The computational formalism is the full potential linearized augmented plane wave method as implemented in the suite of software WIEN2k. Computations have been performed both without and with spin orbit coupling (SOC). Results indicate that GGA-DFT with SOC performs comparatively better in the description of the properties of $\gamma $-U, such as the non-magnetic ground state, lattice constant, and the bulk modulus. The predicted lattice constant and bulk modulus are 3.463 A and 114 GPa, respectively, to be compared with the experimental values of 3.467 A and 113 GPa, respectively. For the (100) surface, the monolayer exhibited significant contraction but the lattice constants tend to converge after 5 layers. Based on the results of the five layers, we predict the surface energy and the work function to be 1.46 J/m$^{2}$ and 3.24eV, respectively. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J10.00013: Electronic Structures of Hydrogen and Oxygen Adsorbed Tungsten (3, 2, 0) and Tungsten (8, 7, 0) Surfaces Zhuo Bao, Aaron Bostwick, Eli Rotenberg, Stephen Kevan The Valence band electronic structues of Hydrogen adsorbed and Oxygen adsorbed Tungsten stepped surfaces, Tungsten (3, 2, 0) and (8, 7, 0) surface are investigated using angular-resolved photoemission techniques and ab-initio electronic structure calculation methods. The band features of surface states at different Hydrogen and Oxygen coverages are experimentally distinguished by using photon-energy scanning method. Quasi-one- dimensional band features are found in the surface states with saturated Oxygen coverages of both stepped surfaces. The effects of adsorbate coverages on dimensionalities of surface electronic states are studied using high-resolution band mapping methods and ab-initio calculation methods. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J10.00014: The suppression surface state near a monostep of Au(111) surface studied by low temperature scanning tunneling microscopy Qing Li, Peter Maksymovych, Sergi Kalinin, Minghu Pan The dynamics of electronic states on metal surfaces is a fundamental probe of electron transport and electronic interactions with practical relevance for nanodevices and reactions. In our study, series of scanning tunneling spectroscopy near a monostep of Au(111) surface are used to investigate the behavior of surface state. We found that the Shockley surface state of Au (111) was suppressed near the step. By carefully analyzing each dI/dV spectroscopy, we determined the lateral tip-step distance dependence of the lifetime of the surface electrons. The lifetime broadening of surface state shows linear decay close to the monostep, possibly due to the electron-electron interactions. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J10.00015: Low Energy Hot Electron Scattering in Nanometer Scale Metal Films Using Ballistic Electron Emission Microscopy John Garramone, Joseph Abel, Vincent LaBella Inelastic and elastic scattering lengths of hot electrons have been measured at low energies ($<$2~eV) in nanometer thick silver films utilizing ballistic electron emission microscopy (BEEM). BEEM is a scanning tunneling microscopy (STM) based technique that is capable of injecting electrons a few eV above the Fermi level and utilizes a third collector contact on the semiconductor of a Schottky diode\footnote{L. D. Bell, et al., Phys. Rev. Lett. 61 2368 (1988)}. Electrons tunnel from the STM tip into the metal base layer and a small fraction of these electrons travel ballistically to the metal/semiconductor interface. Electrons with energy greater than the Schottky barrier height (SBH) are collected as BEEM current. The silver attenuation length is extracted by measuring the BEEM current as a function of the Ag overlayer thickness over a series of samples for both electron and hole injection. The relative contribution of inelastic and elastic scattering is extracted by modeling the change in attenuation length with respect to the tip bias. A drastic increase in the attenuation length is observed as energies approach the SBH, which we attribute to the ballistic nature of the electrons and holes that are collected at these energies. [Preview Abstract] |
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