Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session W21: Surfaces and Interfaces II |
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Sponsoring Units: DCMP Chair: Will Castleman, Pennsylvania State University Room: Morial Convention Center 213 |
Thursday, March 13, 2008 2:30PM - 2:42PM |
W21.00001: Nano-confined water on surfaces of metal oxide nanoparticles Andrey Levchenko, Juliana Boerio-Goates, Brian Woodfield, Alexander Kolesnikov, Nancy Ross, David Wesolowski, David Cole, Alexandra Navrotsky Nanolayers of water interacting with metal oxide surfaces demonstrate physical properties that are significantly different from those of bulk water and ice. Our recent water adsorption experiments suggest that the entropy of surface water is lower than those of bulk water and ice implying restricted motion of the water on the surface. We have studied dynamics of water on nanoparticles of two oxides, TiO$_{2}$ and SnO$_{2}$ by inelastic neutron scattering. Vibrational density of states for surface phonons of water confined by oxide surfaces has been calculated. Heat capacity of confined water has been measured by adiabatic calorimetry and compared with values derived from the vibrational density of states. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W21.00002: X-ray Reflectivity Study of Thermal Capillary Waves and the Interfacial Profiles of Water-Alcohol Mixtures Yoonnam Jeon, Jaeho Sung, Doseok Kim, Wei Bu, David Vaknin The liquid/vapor interfaces of water-alcohol (methanol, ethanol, and propanol) mixtures were investigated by X-ray reflectivity. Analysis of X-ray reflectivity data shows that the interfacial widths (surface roughnesses) of all mixtures at a fixed temperature depend solely on the surface tension of the mixture, and the intrinsic surface roughness is on the order of inter-atomic distances, and within error, independent of solution constituents. The implications of our results in the regard to the origin of the intrinsic roughness and the capillary wave-vector cutoffs applicable to X-ray scattering will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W21.00003: Viscoelacticity of Water in Sub-nanometer Gaps Tai-De Li, Elisa Riedo Direct and simultaneous measurements of the normal and lateral forces encountered by a nanosize spherical silicon tip approaching a solid surface in purified water are reported. For tip-surface distances, 0$\pm $0.03nm $<$ $d <$ 2nm, experiments and grand canonical molecular-dynamics simulations find oscillatory solvation forces for hydrophilic surfaces, mica and glass, and less pronounced oscillations for a hydrophobic surface, graphite. The simulations reveal layering of the confined water density and the development of hexagonal order in layers proximal to a quartz surface. For subnanometer hydrophilic confinement, the lateral force measurements show orders of magnitude increase of the viscosity with respect to bulk water, agreeing with a simulated sharp decrease in the diffusion constant. No significant viscosity increase is observed for hydrophobic surfaces. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W21.00004: ABSTRACT WITHDRAWN |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W21.00005: Study of Hydrophobic Water Interfaces with Phase-sensitive Sum-frequency Vibrational Spectroscopy Chuanshan Tian, Y. Ron Shen Self-assembled monolayer of octadecyltrichlorosilane (OTS) on fused silica has been used extensively as a representative hydrophobic surface in study of water/hydrophobic interfaces. However, the interfacial water structure and how it changes upon solvated ions are not clear. We have carried out a study on OTS/water interface using the newly developed phase-sensitive sum-frequency vibrational spectroscopy (PS-SFVS). It allows measurement of both real and imaginary parts of the surface spectral response with the latter playing a role equivalent to absorption and emission coefficients and provides information on net polar-orientations of various interfacial water species contributing to the different parts of the spectrum. The result shows that at low pH (pH$\sim $2), water molecules in both ice-like and liquid-like region have weak net polar-orientations with H pointing towards the liquid. At high pH (pH$\sim $11), they are well aligned with H pointing to the solid substrate. It is due to that OTS/silica is negatively charged at high pH because of the adsorption of OH$^{-}$ ions on methyl groups of OTS, so that the surface field will reorient part of the interfacial water molecules. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W21.00006: Electronic Structure and adsorption of Pentacene on Cu and Ag (110). Abdelkader Kara The adsorption of pentacene (C$_{22}$H$_{14})$ at coverages of one and 0.8 monolayer on Ag(110) and Cu(110) is studied using density functional theory. The unit cells for these systems are 6x2 and 7x2 for Ag(110) and Cu(110), respectively. The pentacene molecule adsorbs nearly flat (with a structural corrugation of about 0.6 ) at a position 2.5 above the surface. On Ag(110), the adsorbed pentacene is even flatter (0.45 corrugation) and sits higher (about 2.8 ) then the case of Cu. On Cu(110), most of the carbon atoms adsorb on top of Cu atoms, which is not the case on Ag(110). The resulting changes in the electronic states and the nature of the bonding will be discussed and comparison between the two systems will be presented. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W21.00007: Plasmonic Coupling of a Gold Colloid and a Gold Film Albert Chang, Fei Le, Felicia Tam, Naomi Halas, Peter Nordlander, Kevin Kelly Engineering of plasmon resonances is important for a variety of applications, including but not limited to surface enhanced Raman spectroscopy (SERS) and near field scanning optical microscopy (NSOM). Of special interest are systems where the plasmons of a nanoparticle and a thin gold film are coupled.~ This coupling allows for a greater degree of control of the plasmon resonance of the system, as well as a strong, localized, enhancement of the incident electric field.~ We demonstrate that this coupling, and the resulting enhancement, can be used for SERS applications, and explore its impact on potential NSOM applications. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W21.00008: Dipole interactions between dielectric spheres in AC electric fields Manish Mittal, Eric Furst Rheological properties of Electrorheological (ER) suspensions change dramatically on application of electric field. One of the key issues in the study of ER fluids is the nature and strength of the forces between suspended particles. Micron-sized dielectric spheres aggregate to form linear chains on the application of an AC electric field. The dipole-dipole attraction is the dominant force in this process. The dipole moment has contributions from Maxwell-Wagner charge distribution and the double layer polarization. Using optical tweezers the force between a pair of polystyrene spheres has been measured by observing the displacement of particle held in a static optical trap, of known trap stiffness, from its equilibrium position. At a fixed salt concentration, frequency and electric field strength the radial and tangential force have been measured as a function of the center-to-center separation (r) and angle ($\theta )$ with the electric field to create a 2-dimensional force map. Such a complete 2-dimensional interaction profile of micron-sized particles has never been measured before. Subsequently the effect of field, frequency and background salt concentration was studied. It was found that adding salt and increasing field frequency suppressed dipolar interactions. This effect can be explained qualitatively by the double layer polarization theory. Finally the effect of particle geometry was studied by measuring interactions between particles of different size and shape. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W21.00009: Gap-mode enhanced Raman scattering of organic monolayers on flat Au(111) surface Katsuyoshi Ikeda, Norihiro Fujimoto, Kohei Uosaki A metal-molecular junction is recognized as a fundamental issue of molecular-based nano-devices. Charge transport through organic layers is strongly affected by chemical and physical properties of metal-molecular interfaces. In order to investigate molecular structures adsorbed on metal surfaces, surface-enhanced Raman scattering (SERS) is widely utilized as a powerful spectroscopic tool. Because of the electromagnetic origin of SERS, however, conventional SERS spectroscopy is applicable only for ``rough'' metal surfaces. Since various adsorption sites are exposed on rough metal surfaces, it is difficult to obtain intrinsic information of metal-molecular junctions. Here, we provide a simple method of enhanced Raman spectroscopy for organic monolayers on ``flat'' metal surfaces based on gap-mode plasmon excitation. The gap-mode enhanced Raman spectra measured on ``flat'' Au(111) single crystal facets were compared with conventional SERS spectra, and adsorption site dependence of molecular structures was discussed. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W21.00010: Cooperative plasmon-mediated fluorescence of molecules near a metal nanoparticle V. N. Pustovit, T. V. Shahbazyan We study radiative and nonradiative decays of an ensemble of molecules attached to a metal nanoparticle. We show that when the system size is smaller than the radiation wavelength, the excited molecular dipoles are hybridized with each other via the nanoparticle surface plasmon, leading to cooperative plasmon-mediated emission similar to Dicke superradiance. In particular, an ensemble of N molecules located at random positions but at the same distance from nanoparticle surface has only 3 bright (superradiant) eigenstates each characterized by the single-molecule plasmon-enhanced radiative decay rate multiplied by approximately N/3, while the remaining N-3 states are optically dark (subradiant). The fluorescence quenching by the nanoparticle exhibits a similar behavior, with bright states having single-molecule nonradiative decay rate multiplied by the same factor and dark states having much longer but still finite non-radiative lifetime due to contribution of higher angular momenta. As a result, the radiation power of an ensemble is thrice that of a single molecule near a nanoparticle irrespective to total number of molecules. Calculations were performed for both perpendicular and parallel dipole orientations with respect to the nanoparticle surface. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W21.00011: Anisotropic mass enhancement factors of $\bar {\Gamma }$ state on Be(0001) surface TeYu Chien, Emile Rienks, Maria Jensen, Asier Eiguren, Eugene Chulkov, Philip Hofmann, Ward Plummer It is controversial on the values of mass enhancement factors, $\lambda $, of Be(0001) $\bar {\Gamma }$ state. Three possible explanations are: (1) $\lambda $ is anisotropic along the Fermi circle; (2) method-based difference for extracting $\lambda $; (3) failure of theoretical model for capturing EPC features. We demonstrate a systematically survey of $\lambda $ along the Fermi circle of Be(0001) $\bar {\Gamma }$ state. By adopting different methods to extract $\lambda $, the possibility of method-based inconsistency was ruled out. The trend of the anisotropic $\lambda $ is clear and is confirmed by theoretical calculations, though the values are inconsistent with experiment. With model data simulation, we are confident about the extracted $\lambda $ while we have noisy data. The possible explanation is that the DFT-LDA calculation can not catch the features of EPC on Be(0001) $\bar {\Gamma }$ state correctly. This work is financially supported by National Science Foundation (grants NSF-DMR-0451163). [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W21.00012: Electron-phonon induced complex quasiparticles in the 1x1 H/W(110) surface. Asier Eiguren, Claudia Ambrosch-Draxl We show that the solution of the complex Dyson equation for the electron-phonon problem induces several quasiparticle states for a given wave vector. The Dyson equation is considered in the full complex plane and it is solved without considering the imaginary part of the self-energy as an small parameter. By a first principle application of the formalism to the 1x1 H covered W(110) surface, we show that some aspects of the surface band splitting [Rotenberg et. al., Phys. Rev. Lett. \textbf{84}, 2925 (2000)] can be traced back to electron-phonon coupling, where we present the energy and lifetimes of each quasiparticle. Despite this breakdown of the single quasi-particle picture, it is remarkable that the spectral functions are very well Represented by the predicted multiple quasi-particles. From these results, we can deduce that some of the features that previously where prescribed in ARPES spectra as \textit{incoherent structure} could eventually be re-interpreted as contributions from additional quasi-particle states. Our method could also help to understand similar phenomena observed in high T$_{c}$ cuprates and various other surfaces. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W21.00013: Light-enhanced diffusion at the solid-liquid interface Sung Chul Bae, Janet Wong, Steve Granick Positively-charged rhodamine 6G molecules were allowed to adsorb onto quartz and mica surfaces and their translational and rotational diffusion was studied simultaneously by combined fluorescence correlation spectroscopy and time-correlated single photon counting. Surprisingly, the surface translational diffusion coefficient increased in direct proportion to the laser power used to excite these dye molecules. To elucidate the diffusion mechanism, we have investigated the excitation wavelength dependence, the spatial position dependence of diffusion coefficients, and the correlations between rotation and translation motion. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W21.00014: Epitaxial Growth of Quinacridone Derivative on Ag(110) Xiaobo He, Jinming Cai, Dondxia Shi, Werner A. Hofer, E. Ward Pluumer, Hongjun Gao The growth behavior of quinacridone derivative (QA16C) molecules on Ag(110) surface is studied using low-temperature scanning tunneling microscopy and low energy electron diffraction. At low coverage two distinct molecular orientations can be observed on silver terraces. At higher coverage up to 1 monolayer, depending on the growth temperature we observe two different structures on Ag(110) surface. At 100K the molecules organize in a complete monolayer on the surface, with a network-like structure. At 300K, the molecular orientation on the surface gives rise to row-like ordering, with a substantially higher molecular density. A theoretical analysis reveals that the structure of the molecular layer is controlled by the competition between molecular deposition rates and molecular diffusion along the surface. [Preview Abstract] |
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