Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session V12: Focus Session: Wetting and Hard-Soft Interfaces |
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Sponsoring Units: DMP DCMP Chair: E. Kaxiras, Harvard University Room: Baltimore Convention Center 304 |
Thursday, March 16, 2006 11:15AM - 11:27AM |
V12.00001: Water Enhanced Catalysis of CO Oxidation on Free and Supported Gold Nanoclusters Angelo Bongiorno, Uzi Landman The enhancement by water molecules of the catalytic activity of gas-phase and supported gold nanoclusters toward CO oxidation is investigated with first-principles calculations. Coadsorption of water and oxygen molecules leads to formation of a complex well-bound to the gold cluster, even on a defect-free MgO(100) support. Formation of the complex involves partial proton-sharing between the adsorbates, that in certain configurations results in proton transfer leading to the appearance of a hydroperoxyl-like complex. The O-O bond is activated, leading to a weakened peroxo or superoxo-like state, and consequently the reaction with carbon oxide to form carbon dioxide occurs with a small activation barrier of about 0.5 eV. A complete catalytic cycle of the water-enhanced CO oxidation is discussed. [Preview Abstract] |
Thursday, March 16, 2006 11:27AM - 11:39AM |
V12.00002: Tuning the diamond surface from hydrophobic to superhydrophilic by submonolayer surface modification Sheng Meng, Zhenyu Zhang, Efthimios Kaxiras The extreme limits of wettability, superhydrophilic and superhydrophobic behavior, are useful in industrial applications such as anti-fogging and self-cleaning. Superhydrophilic behavior is highly desirable in biomedical applications, protecting biomolecules from damage in their interaction with biomaterials. While substantial progress has been made in designing superhydrophobic materials, relatively little effort has been devoted to the development of superhydrophilic materials, particularly biocompatible ones. Here we show, using first-principles calculations, that the water affinity of an initially highly hydrophobic H-passivated diamond (111) surface can be drastically altered with introducing a fraction of a monolayer of alkali metals. In particular, terminating the diamond surface with F and replacing 1/3ML F by Na, or introducing one third monolayer of Li and methylating half of the remaining surface sites, produces stable surfaces with superhydrophilic behavior which are perfect candidates for biomedical applications. [Preview Abstract] |
Thursday, March 16, 2006 11:39AM - 11:51AM |
V12.00003: Infrared Studies of Water Adsorption on Acetic Acid thin films Michel Malick Thiam, Maryam Ebrahimi, Kam Tong Leung Infrared reflection-absorption spectroscopy is used to investigate H$_{2}$O ice deposited onto non-crystalline (dimers [1]) and polycrystalline (infinite chains [1]) acetic acid films. The condensed water film grown at $\sim $135 K on these different substrates can be characterized as amorphous dense ice. The H$_{2}$O molecules are shown to interact mainly with the carbonyl and the carboxyl oxygens, forming hydrogen bonds. Upon water adsorption on the non-crystalline acetic acid film, saturation of the change induced in the intensity of the C=O and C-O peaks occurs at an average H$_{2}$O exposure of $\sim $ 2.52 L. The amount of H-bonding involving C=O or C-O (of acetic acid) and OH (of water) on the polycrystalline film has been reduced considerably compared to the situation on the non-annealed one, but saturation of the carbonyl oxygen even for a water exposure of 9 L has not been observed while the carboxyl oxygen saturates at $\sim $2.76 L. Thermal evolution studies for the ice film on non-crystalline and polycrystalline acetic acid films show that water co-evaporates with acetic acid likely as a water-acetic acid complex in the temperature range of 140-155 K, which continues until the entire ice film has been exhausted at 160 K. [1]: Q. Gao and K. T. Leung, J. Phys. Chem. B 109, (2005) 13263. . [Preview Abstract] |
Thursday, March 16, 2006 11:51AM - 12:27PM |
V12.00004: Ab-initio simulations of H2O interaction with b-SiC surfaces. Invited Speaker: We report on the interaction of gas phase water molecules with polar and non polar stoichiometric surfaces of cubic silicon carbide, a leading candidate semiconductor for bio-compatible devices. Finite temperature ab-initio molecular dynamics shows that the specific geometric arrangement of atoms on the outermost surface layer is responsible for water orientation and coordination and thus plays a major role in determining the surface reactivity. In particular, irrespective of coverage, water spontaneously dissociates on both polar[1] Si-terminated (001) and non-polar[2] (110) surfaces, while the C-terminated1 (001) surface is non-reactive and hydrophobic. Based on our results, we propose that STM images and photoemission experiments may detect specific changes induced by water on both the structural and electronic properties of SiC surfaces. We completed our investigation by studying a realistic solid/liquid interface[3] for the case of Si-SiC(001), after full hydroxylation takes place. We observed the formation of a thin (~ 3 /AA) interfacial layer, which presents a higher density than bulk water. The liquid does not uniformly ‘wet’ the surface, rather molecules preferably bind along directions parallel to the Si dimer rows. Our calculations also predict that at ~ 1 nm, the structural and electronic properties of liquid water are weakly affected by one-dimensional confinement between hydrophilic, solid substrates. [1] G. Cicero, A. Catellani and G. Galli Phys Rev. Lett. 93 (2004), 0161102; G. Cicero, A. Catellani and G. Galli J. Phys. Chem. B 108 (2004), 16518. [2] A. Catellani, G. Cicero and G. Galli J. Chem. Phys., in press. [3] G. Cicero, J. Grossman, A. Catellani and G. Galli J. Am. Chem. Soc. 127 (2005), 6830. [Preview Abstract] |
Thursday, March 16, 2006 12:27PM - 12:39PM |
V12.00005: Physics of solid and liquid alkali halide surfaces near the melting point Tatyana Zykova-Timan, Davide Ceresoli, Ugo Tartaglino, Erio Tosatti NaCl (and other alkali halide) crystal surfaces have the peculiar property of repelling their own melt. As a result they let themselves be wetted only partially by their own liquid at the melting point T$_{M}$. We recently investigated the physical reasons for this unusual behavior. We found them through theory and molecular dynamics simulation to stem from the conspiracy of three factors. First, the solid NaCl(100) surface is exceptionally anharmonic,but also exceptionally stable. It can in fact survive even well above the melting point, for unlike most other surfaces it does not spontaneously melt. Second, the solid-liquid interface is very costly, due to a 27$\%$ density difference between solid and liquid. Third, the surface tension of liquid NaCl is relatively high. This last feature is due to an unexpected entropy deficit, that can in turn be traced to incipient molecular charge order in the outermost regions of the molten salt surface[1,2].\newline [1] T. Zykova-Timan, D. Ceresoli, U. Tartaglino, E. Tosatti, Phys. Rev. Lett. 94, 176105 (2005) \newline [2] T. Zykova-Timan, D. Ceresoli, U. Tartaglino, E. Tosatti, J. Chem. Phys. 123, 164701 (2005) [Preview Abstract] |
Thursday, March 16, 2006 12:39PM - 12:51PM |
V12.00006: Nanoscale Observation of Alkane Dewetting Mengjun Bai, Klaus Knorr, Matthew Simpson, Sven Trogisch, Haskell Taub, Ulrich Volkmann, Flemming Hansen To investigate the structure, morphology, and wetting of both solid and liquid films of an intermediate-length alkane, we have conducted Atomic Force Microscopy measurements in the noncontact mode on dotriacontane ($n$-C$_{32}$H$_{66}$ or C32) films deposited from a heptane solution onto SiO$_{2}$-coated Si(100) wafers. At low coverages after annealing, the C32 film consists at room temperature of isolated bulk particles situated above one to two layers of molecules oriented with their long axis parallel to the surface. For such samples, we find a narrow temperature range close to the bulk C32 melting point $T_{b}$ in which a monolayer with C32 molecules oriented perpendicular to surface is stable. This perpendicular monolayer phase undergoes a dewetting transition to a three-dimensional (3D) fluid phase on heating to just above $T_{b}$ and to a 3D crystalline phase on cooling to a few degrees below $T_{b}$. We summarize our results in terms of an equilibrium phase diagram that provides a useful framework for interpreting the unusual spreading and receding behavior that we observe for the perpendicular monolayer phase.. [Preview Abstract] |
Thursday, March 16, 2006 12:51PM - 1:03PM |
V12.00007: Energetics of benzene and phenol adsorption on a TiO$_{2}$(110) surface S. Dag, J. Zhou, S. Kalinin, S. Overbury, D. Mullins, A. Baddorf, V. Meunier Using large-scale density functional theory calculations, we have studied the structural and electronic~ properties of benzene and phenol adsorbed on a TiO$_{2}$ (110) surface.~~ We found that at low coverage,~ a benzene molecule impinging to the TiO$_{2}$ surface is most likely to be adsorbed flat on the surface 3.3 {\AA} above the Ti row, such that the $C_{2^'}$ symmetry axis of the molecule is oriented along the top Ti row, between consecutive bridging oxygen rows. Because of the high repulsive energy between benzene and bridging oxygens, benzene molecules diffuse in quasi one-dimensional channels along a Ti row. At high~ coverage, benzene molecules organize in a chain-like fashion with two of their C-C bonds being oriented alternatively parallel and perpendicular to a Ti row.~ The interaction of phenol molecules on the same surface has also been studied. In this case, the adsorption is much stronger and involves interaction between multiple phenol molecules and bridging oxygen rows.~ Results of our calculations are in excellent agreement with experimental STM data. [Preview Abstract] |
Thursday, March 16, 2006 1:03PM - 1:15PM |
V12.00008: Adsorption and reaction of benzene and phenol with Pd nanoparticles supported on TiO$_{2}$(110) J. Zhou, S. V. Kalinin, S. Dag, V. Meunier, S. H. Overbury, D. R. Mullins, A. P. Baddorf Adsorption and reaction of benzene and phenol molecules on a TiO$_{2}$(110)(1$\times $1) surface and with titania-supported Pd particles were studied under ultrahigh vacuum conditions using scanning tunneling microscopy (STM) and temperature programmed desorption (TPD). At 300 K, benzene could not be imaged with STM due to its high mobility. At 20 K, images show benzene in ordered rows on top of substrate Ti. Adsorption of phenol differs due to the presence of the OH functional group. At 300 K, adsorption of phenoxy adjacent to surface defects is indicated in STM images. Low temperature studies indicate that phenol adsorbs on the titania surface with a different configuration than benzene, with tunneling spanning two Ti rows. No obvious modifications to benzene or phenol adsorption were observed adjacent to Pd particles by STM, however catalytic effects were explored by TPD. Our results are in excellent agreement with calculations obtained using first-principles density functional theory. [Preview Abstract] |
Thursday, March 16, 2006 1:15PM - 1:27PM |
V12.00009: LEEM study of the growth of 6,13-pentacenequinone thin films Jerzy Sadowski, Abdullah Al-Mahboob, Tsuguki Nishihara, Yasunori Fujikawa, Toshio Sakurai The morphology and dynamics of the growth of 6,13-pentacenequinone (PnQ -- C$_{22}$H$_{12}$O$_{2})$ films on Si(111)-7x7 surface have been investigated \textit{in-situ} by the low-energy electron microscopy (LEEM), in order to evaluate the possibility of using PnQ molecules as an organic dopant in pentacene (Pn) films. The results of the experiments show that the nucleation and growth mechanisms dramatically differ between Pn and PnQ films, respectively. At the initial stage of the PnQ growth, after the saturation of the Si dangling bonds by the formation of a disordered wetting layer, the amorphous islands (type-I) having compact shapes were formed. After they have reached a critical size (few hundreds to several thousands nanometers, depending on the molecular flux), a second stage nucleation of the crystalline, highly anisotropic, needle-shaped islands (type-II) occurred. A gradual rotation of the in-plane (\textbf{\textit{ab}}) unit cell along the type-II islands have been observed, with the direction of the 2D `chirality' dependent on the direction of the \textbf{\textit{c}} axis of the PnQ crystal lattice and associated with selective mass incorporation at the side of the island having edge parallel to long diagonal (direction of minimum strain and a favorable direction of a kink formation) of the \textbf{\textit{ab}} unit cell. [Preview Abstract] |
Thursday, March 16, 2006 1:27PM - 1:39PM |
V12.00010: Growth of divalent metal carbonates at soft organic templates. Sumit Kewalramani, Geoffrey Dommett, Kyungil Kim, Guennadi Evmenenko, Haiding Mo, Pulak Dutta Soft template mediated inorganic mineral nucleation and growth proceeds via specific interactions between the ordered organic lattice headgroups and inorganic constituents at the organic-inorganic interface. We are studying the growth of divalent metal carbonates of R-3 (167) space group from supersaturated solutions under fatty acid, alcohol and glutamic acid rich poypeptide monolayers via in-situ grazing incidence X-ray diffraction and scanning electron microscopy. Previous studies$^{1}$ show that the presence of Cd$^{2+}$, Mn$^{2+}$, Mg$^{2+}$, even in small quantities produce highly ordered fatty acid monolayer phases and result in the formation of ionic inorganic superlattices under them. For Ba$^{2+}$, Ca$^{2+}$ and Co$^{2+}$ the extent of organic monolayer order is lower and no ionic lattice is observed. We now find that the orientation and morphology of bulk crystals grown from supersaturated solutions is strongly correlated to the order induced in organic monolayers by dilute subphases. Headgroup- ion interactions in the dilute phase can thus serve as a better guideline for choosing the monolayer for face specific nucleation as opposed to the proposed$^{2}$ geometric and stereochemical match mechanism. 1.Kmetko et al. J. Phys. Chem. B 105, 10818 (2001). 2. Mann et al. Science, 261, 1286 (1993). [Preview Abstract] |
Thursday, March 16, 2006 1:39PM - 1:51PM |
V12.00011: Structural characterization and molecular dynamics within low-coverage films of $p$-terphenyl adsorbed onto graphite Edward Kintzel, Kenneth Herwig Neutron scattering experiments have been carried out on low-coverage films of $p$-terphenyl molecules physisorbed onto the surface of graphite. The molecular arrangement within these films has been determined as a function of temperature and coverage. Analysis of the diffraction data reveals two-dimensional film structures where the molecules prefer a parallel orientation relative to the underlying substrate. Further examination provides evidence for an order-disorder phase transition within a monolayer film, temperature-dependent structural evolution within bilayer films, and structural variations based on film coverage. Initial dynamics results indicate a structural phase change in the temperature range 275-325 K. Below this temperature range, the scattering is consistent with phenyl ring reorientations around the long molecular axis. At elevated temperatures, the quasielastic signal is stronger and consistent with translational motion in combination with the phenyl ring reorientations. [Preview Abstract] |
Thursday, March 16, 2006 1:51PM - 2:03PM |
V12.00012: Comparative STM study of azobenzene derivatives on bare and insulator coated metal surfaces Niv Levy, Matthew J. Comstock, Jongweon Cho, Armen Kirakosian, Carine Edder, Jean M. J. Frechet, Frank Lauterwasser, Jessica Harvey, Dirk Trauner, M. F. Crommie Photoactive molecules, such as azobenzene and its derivatives, have great potential for nanoscale opto-mechanical applications. However, one of the main difficulties in optical actuation of singe molecules at a surface is decoupling them from the electronic states of the substrate. One technique to accomplish this is to coat the substrate with a thin insulating layer, while another is to functionalize the molecule with ``spacer legs'' to lift it off the surface. We have conducted a comparative study of adsorption and self-assembly behavior of bare azobenzene, functionalized bis-tert-butyl-azobenzene, and functionalized tetra-tert-butyl-azobenzene molecules on bare and insulator coated metal surfaces using a variable temperature UHV STM. We observe a variety of temperature dependent molecular configurations, from which we infer the degree of molecule/surface decoupling due to surface modification and molecular functionalization. [Preview Abstract] |
Thursday, March 16, 2006 2:03PM - 2:15PM |
V12.00013: Unoccupied electronic states at the interface of alkanethiol SAMs on Au(111) Matthias Muntwiler, Chad Lindstrom, Xiaoyang Zhu Alkanethiol self-assembled monolayers (SAMs) on Au(111) are model systems for metal-molecule contacts in molecular electronics. Using time-resolved two-photon photoemission (TR- 2PPE) we probe the unoccupied electronic structure of the alkanethiolate/Au interface. Two distinct peaks appear in the spectra: The first, non-dispersive peak is attributed to the antibonding $\sigma^{\star}$ orbital of the Au-S chemisorption bond, and the second, dispersive peak to a laterally delocalized interfacial resonance induced by the image potential at the metal surface. Both peaks show lifetimes shorter than 30 fs. For the $\sigma^{\star}$ resonance such a short lifetime is expected due to the wavefunction overlap with the metal, whereas for the second resonance it is an effect of scattering at S atoms inside the molecular layer. In fact, the insensitivity of the energy level, dispersion, and lifetime to layer thickness suggests that the electron wavefunction is concentrated inside the dielectric layer close to the metal-molecule interface -- in contrast to physisorbed alkanes where it is pushed out of the layer. [Preview Abstract] |
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