Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session X19: Surfaces, Interfaces, and Colloids II |
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Sponsoring Units: DCP Chair: Steve George, University of Colorado Room: Colorado Convention Center 104 |
Friday, March 9, 2007 8:00AM - 8:12AM |
X19.00001: Low Energy Inelastic Helium Atom Scattering from a Monolayer Solid. L.W. Bruch, F.Y. Hansen A time-dependent wave packet calculation for inelastic low energy atomic scattering by a physisorbed monolayer\footnote{L. W. Bruch and F. Y. Hansen, J. Chem. Phys. {\bf 122}, 114714 (2005)} has demonstrated that $^4$He atoms incident on an incommensurate monolayer solid of Xe/Pt(111) readily excite shear horizontal monolayer phonons. Adding an absorbing potential has enabled the calculation to be extended to such long times that transient trapping has decayed and the scattering event is nearly complete. Extending the spatial grids has enabled the use of more nearly monochromatic, spatially broad, wave packets and the inelastic intensities are found to be sensitive to variations of scattering energy by 0.3 meV at incident energies of 4 to 9 meV. Trends for the inelastic intensities at low incident energies, near thresholds, are determined and correlations between the inelastic scattering results and the time development of the elastic scattering are discussed. These calculations were stimulated by the systematic body of experimental data available for $^4$He scattering by a Xe/Pt(111) monolayer\footnote{L. W. Bruch, A. P. Graham, and J. P. Toennies, J. Chem. Phys. {\bf 112},314 (2000)}. [Preview Abstract] |
Friday, March 9, 2007 8:12AM - 8:24AM |
X19.00002: Study of Oxidation of Silicon by X-ray Photoelectron Spectroscopy A.R. Chourasia, William Johnston, R.L. Miller, Ryan Jacob The oxidation of silicon has been investigated using the technique of x-ray photoelectron spectroscopy. The silicon substrates have been exposed to oxygen at a partial pressure of 5 $\times $ 10$^{-6}$Torr. The exposure was done at substrate temperatures of 100\r{ }C, 200\r{ }C, 300\r{ }C, 400\r{ }C, 500\r{ }C, and 600\r{ }C, The substrates have been analyzed by angle resolved XPS. The magnesium anode (energy = 1253.6 eV) have been used for this purpose. The silicon 2p and oxygen 1s core level regions have been investigated. The spectral data have been recorded at 10\r{ }, 20\r{ }, 40\r{ }, 60\r{ },80\r{ }, and 90\r{ } take-off angles. The QUASES software has been used to determine the thickness of the oxide layer formed on the substrates. The experimental data on the thickness of the oxide layer has been correlated with that obtained from the QUASES model. [Preview Abstract] |
Friday, March 9, 2007 8:24AM - 8:36AM |
X19.00003: Ab Initio Calculations for Br and Cl adsorption on the Ag(100) surface Tjipto Juwono, Ibrahim Abou Hamad, Per Arne Rikvold Ab-initio density-functional methods have been used to find the ground-state configurations of Br and Cl adsorbates on Ag(100) surfaces with coverages of $1/9$, $2/9$, $1/4$, $1/3$, and $1/2$ monolayers. The supercell slab method was used to calculate the electron-density distributions for each configuration. The charge-transfer function, surface dipole moments, adsorbate resident charge, and adsorption energies were calculated and compared with results from electrochemical adsorption exeperiments and Monte Carlo simulations. The lateral adsorbate-adsorbate interactions and the binding energies were extracted from the adsorption energies using a lattice-gas model. The calculated quantities are weakly dependent on the coverage, and the overall shape of the charge-transfer function is nearly coverage independent. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X19.00004: Cracks, Meltdowns and Crossover Sizes: An abrupt change in sublimation kinetics associated with the thermally-activated introduction of disclination charge in crystallites. Alex J. Levine, Moumita Das, Don Blair Recent experiments and numerical studies of the sublimation kinetics of 2d colloidal crystals show an abrupt increase in the sublimation rate at a particular crystallite size [J. R. Savage {\it et. al.} Science {\bf 314}, 795(2006)]. Motivated by this observation, we propose that the abrupt change in the sublimation kinetics is due to the thermally activated introduction of a disclination charge leading to large internal stresses. These stresses are then relaxed by a fission event precipitating the break-up of the remaining crystallite. We use our numerical simulations to show that the average disclination charge indeed increases to one at the `crossover size' corresponding to the observed change in sublimation rate. Using the Griffith criterion for the spontaneous propagation of microscopic cracks, we see that the effect should depend sensitively upon the range of the attractive interparticle potential. We test this prediction using numerical simulations of the sublimating system. Where that potential is short-ranged, the crystal is brittle allowing for the proposed mechanism. For longer-ranged potentials, however, the material is more ductile preventing this abrupt fission event. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X19.00005: The structure of a C$_{60}$ monolayer on Ag(111) Renee Diehl, Hsin Li, Kelly Hanna, Wolfgang Moritz The structure of a monolayer of C$_{60}$ on Ag(111) was studied using dynamical LEED, the first such study for C$_{60}$ molecules. The C$_{60}$ monolayer adopts a commensurate hexagonal (2$\surd $3x2$\surd $3)R30\r{ } structure with a nearest-neighbor separation of 10.0 {\AA}. LEED intensities were measured at a sample temperature of 32 K using incident beam energies up to 600 eV. The LEED analysis was performed using conventional methods adapted for large molecular adsorbates, with up to 15 phase shifts to describe the scattering potential. The structure of the monolayer consists of C$_{60}$ molecules occupying the top sites with their hexagonal faces down, at a distance of 2.27 {\AA} above the Ag(111) surface. There is an accompanying deformation of the Ag surface that involves the downward displacement of the Ag atoms nearest to the C$_{60}$ molecules, consistent with a charge transfer between the surface and the molecule. The C$_{60}$ molecules have a significant librational motion about their vertical axes, even at 32 K. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X19.00006: Accelerated Molecular Dynamics Simulation of Thermal Desorption Kelly Becker, Kristen Fichthorn Thermal desorption has been the focus of much surface science research recently. Alkane desorption experiments on graphite [1] show a prefactor that is constant with chain length, while experiments on magnesium oxide [2] show a prefactor that increases with chain length. We utilize an all-atom model to study alkane desorption from graphite. Transition state theory is used to obtain rate constants from the simulation. Accelerated molecular dynamics techniques are used to extend the simulations to experimentally relevant temperatures. Our results provide an explanation [3] for this seemingly contradictory functionality of the prefactor. We also examine the effect that film structure has on the rate of desorption and the shape of the desorption profile through varying coverage. [1] K.R. Paserba and A.J. Gellman, \textit{J. Chem. Phys. }\textbf{115}, 6737 (2001). [2] S.L. Tait et al., \textit{J. Chem. Phys. }\textbf{122}, 164707 (2005). [3] K.E. Becker and K.A. Fichthorn, \textit{J. Chem. Phys.} \textbf{125, }184706 (2006). [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X19.00007: Irreversible nanoparticle adsorption on patterned substrates A. Cadilhe, N. A. M. Araujo, V. Privman The adsorption of nanoparticles on a surface has interest in fields like heterogeneous catalysts and quantum dots. We simulate the monolayer adsorption of nanoparticles on patterned substrate. We adopted a pattern consisting of equal squares of size $\alpha$ and a distance $\beta$ apart from each other, and characterize the system, by reckoning the mean value and variance of the distance between the nanoparticles and the radial distribution function of their distances. Proper control of $\alpha$ and $\beta$ parameters leads to morphologies range from lattice to homogeneous, with interesting non-trivial behaviors in between. Our study shows the relevance of geometrical constraints to obtain different morphologies of colloidal monolayer films with potential for practical applications. [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X19.00008: Subsurface oxygen stabilization by a third species: Carbonates on Ag(210) Pushpa Raghani, Letizia Savio, Andrea Gerbi, Luca Vattuone, Mario Rocca, Nicola Bonini, Stefano de Gironcoli Subsurface species have often been invoked to explain the activation of catalytic surfaces for specific reactions. In particular, subsurface oxygen is thought to be important for the chemistry of Ag catalysts. Here we show by high resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS) combined with density functional theory (DFT) and density functional perturbation theory (DFPT) that on Ag(210) surface, the subsurface oxygen is stabilised more efficiently by carbonates than by oxygen adatoms or when there is no supersurface oxygen present. Experimentally a maximum of six subsurface oxygens are found to be stabilised by each carbonate. These results could have an importance in catalytic reactions where subsurface oxygen is known to play a crucial role. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X19.00009: Computer simulations of the Adsorption of Xenon onto a C$_{60}$ monolayer on Ag (111) Silvina Gatica, Milton Cole, Renee Diehl We performed Grand Canonical Monte Carlo simulations to study the adsorption of Xenon on a substrate composed of C$_{60 }$molecules placed on top of a Ag(111) surface. The C$_{60}$ molecules form a commensurate structure at a distance of 0.227 nm above the Ag surface. The interaction potential between the Xe atoms and the substrate has two contributions: from the C$_{60}$ molecules and from the Ag atoms. In the simulations, the interaction with the Ag surface was computed using an \textit{ab initio} van der Waals potential, varying as 1/d$^{3}$. The interaction between the Xe atoms and each C$_{60}$ molecule was computed using a potential previously developed by Hernandez et.al. (E. S. Hernandez, M. W. Cole and M. Boninsegni, ``\textit{Wetting of spherical surfaces by quantum fluids}'', J. Low Temp. Phys. 134, 309-314 (2004)), who integrated the Lennard Jones interaction over the surface of a spherical buckyball. The total potential has especially attractive 3-fold sites, positioned~ 0.4 nm above the point between each three buckyballs. ~The low coverage uptake populates those sites, and then continues forming a monolayer. The adsorption isotherms show several steps, typical of substrates that have distinct adsorption sites. We compare the results with the experimental data. ~ [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X19.00010: First-principles study of polar molecule adsorption on hydrogenated diamond (001) Yong-Hyun Kim, S. B. Zhang, Yang Yu, L. F. Xu, C. Z. Gu Density functional theory calculations reveal that adsorption of small polar molecules such as H$_{2}$O, HF, and HCl on hydrogenated diamond (001) surfaces can result in unexpected dihydrogen bonding. This implies that in a C-H bond, H may be more electronegative than C, thus deviating from the widely-accepted Pauling's electronegativity scale, C(2.55) \textit{vs} H (2.20). Detailed analysis of the self-consistent charge densities confirms that electrons are indeed accumulated more at the H site than at the C site with respect to the free atoms. It further explains why dihydrogen bond can form for H$_{2}$O and HF on the surfaces, but not for NH$_{3}$. The true physical origin for the well-known reduction of the work function due to surface hydrogenation is now attributed to the replacement of loosely bonded surface C $\pi $ electrons by more tightly bonded C-H $\sigma $ electrons. We also propose that the favorable formation of the dihydrogen bonds may contribute to the observed p-type conductivity of diamond surfaces in acidic conditions. [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X19.00011: STM/S study of polycyclic aromatic hydrocarbons on Co (0001) Daejin Eom, Michael Lefenfeld, Kwang Taeg Rim, Li Liu, Shengxiong Xiao, Colin Nuckolls, Tony Heinz, George Flynn The fascinating physical properties of carbon nanotubes (CNT) have attracted attention for more than a decade. Both practical and scientific uses of CNTs have, however, been hindered by the incomplete synthetic control of nanotube structure (diameter and chiral angle). Understanding of growth at the microscopic level may advance our ability to control nanotube chirality. We have consequently explored the interaction of a cobalt substrate, a common catalyst for the growth of CNTs, with polycyclic aromatic hydrocarbon molecules, such as hexabenzocoronene (HBC). Using ultra-high vacuum, low-temperature scanning tunneling microscopy (STM), we have examined the changes in HBC topographic features and vibrational spectra that are induced by thermal annealing of the adsorbed molecules. The potential of hydrocarbon molecules like HBC as end caps for seeded growth of CNT of specific chirality will be discussed. [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X19.00012: Electrocatalytic property of PtBi and PtPb line compounds via DFT Lin-Lin Wang, D. D. Johnson A major obstacle to practical, mass market fuel cell (e.g. hydrogen and direct methanol) technology is CO poisoning of Pt anode. Pt alloys, such as disordered Pt$_{x}$Ru$_{(1-x)}$, are known to have an increased CO-tolerance. There has been significant effort to understand the mechanism for increased CO- tolerance and to design better catalyst via alloyed nanoparticles and surface alloys. Alternatively, Pt intermetallic compounds, such as with Bi and Pb, have been observed to improve dramatically the CO-tolerance. [E. Casado- Rivera et al. ChemPhysChem 4, 193 (2003) and J. Am. Chem. Soc. 126, 4043 (2004)] Here we use density functional theory to study the adsorptions of CO, H and OH on these materials. We find that (100) and (110) surfaces of PtBi and PtPb line compounds have lower cleavage energy than (001) surface. Adsorption energies and electronic structure are examined to explain the increased CO-tolerance. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X19.00013: Surface adsorbates and mechanical dissipation in micromechanical resonators Thomas Metcalf, Bradford Pate, Jeffrey Baldwin, Brian Houston, Maxim Zalalutdinov Temperature dependence measurements of the mechanical quality factor of a silicon micromechanical resonator found a pronounced dissipation peak in the neighborhood of 160 K, the magnitude of which reduced dramatically upon in-situ annealing\footnote{Haucke, \textit{et al.}, Appl. Phys. Lett. \textbf{86}, 181903 (2005)}. Present in all of the resonator's normal modes, the peak is sufficiently broad so that the mechanical dissipation was observed to decrease with increasing temperature near room temperature, indicating that this loss mechanism contributes significantly to the room-temperature dissipation value. The leading candidates for the origin of the dissipation are surface adsorbates (e.g. water). We report an investigation of the dissipation of micromechanical resonators (between $\sim100$ K and room temperature) with carefully prepared and characterized surfaces as a function of adsorbate and of adsorbate coverage. [Preview Abstract] |
Friday, March 9, 2007 10:36AM - 10:48AM |
X19.00014: An STM Study of the Interaction of hexabenzocorone on a Ru(0001) Surface Kwang Taeg Rim, Daejin Eom, Li Liu, Shengxiong Xiao, Michael Steigerwald, Mark Hybertsen, Colin Nuckolls, George Flynn The interaction of hexabenzocorone (hbc) with Ruthenium surface has been investigated using Scanning Tunneling Microscopy in ultrahigh vacuum. The images obtained at 77K and 4.5K, after hbc molecules were vacuum deposited at 325$^{o}$C onto a pristine Ru(0001) surface, exhibit surface bound molecules with off-centered bonding sites. After annealing the hbc-Ru complex at 600$^{o}$C for 15min, hbc molecules appear to be dehydrogenated to form the bowl-shaped C$_{48}$. The nature of the interaction and the dehydrogenation of the hbc on Ru(0001) surface will be discussed with DFT calculation, the supplemental IR absorption, and Temperature Programmed Desorption studies. The possibility of growing carbon nanotubes on C$_{48}$ end-cap upon dosing with C$_{2}$H$_{2}$ will also be discussed. [Preview Abstract] |
Friday, March 9, 2007 10:48AM - 11:00AM |
X19.00015: Commensurate ground states of monolayers on surfaces Alexandre Tkatchenko We present a procedure for generation of \textit{all} commensurate monolayer-surface structures of a given symmetry, up to a certain number of adsorbate particles (atoms or molecules), $N_{ads}$, in the unit cell. It is shown that each cell is related to a well-defined sequence of Fourier terms of the single particle-surface potential. Most importantly, the knowledge of Fourier amplitudes alone is sufficient to exactly predict the ground states of commensurate structures in multi-adsorbate unit cells. The impact of the developed theory for theoretical (i.e. DFT simulations) and experimental (LEED studies) determination of commensurate monolayer ground states is briefly discussed. Furthermore, the experimental results for epitaxy of alkali atoms on the Ag(111) surface and iodine on the Pt(111) surface can be described by this approach, in contrast to previous epitaxy theories. [Preview Abstract] |
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