Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session X34: Surfaces and Interfaces I |
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Sponsoring Units: DCP Chair: Bruce Garrett, PNNL Room: LACC 511 A |
Friday, March 25, 2005 8:00AM - 8:12AM |
X34.00001: Pt surface structure in presence of an oxygen atmosphere Timo Jacob, Matthias Scheffler Nowadays Platinum is used to catalyze a whole variety of different reactions. Expecially for electrocatalytic processes (such as in fuel cells) Pt is still one of the most sufficient materials providing a high rate for oxygen reduction at the cathode. However, this reaction occurs in a multi-component environment and under conditions of finite temperature, pressure, and electrode potential ($p=1$\,bar, $T=80-100$\,C, $\phi\ne0$\,V). Thus, the model of an pure and perfect Pt(111) surface, which is often used to study this reaction, is clearly incomplete. Therefore, to study the cathode reaction mechanism our first investigations aim on the realistic Pt(111) surface structure, which then will be the basis of further studies. Using density functional theory (DFT) in combination with a modified {\it ab inito} atomistic thermodynamics (capable to treat the electrode potential), we started with the $V=0$ case and calculated the corresponding $p/T$-phase diagram, which led to interesting surface-oxide structures. [Preview Abstract] |
Friday, March 25, 2005 8:12AM - 8:24AM |
X34.00002: Excitation of the Shear Horizontal Mode in a Monolayer by Inelastic Helium Atom Scattering L.W. Bruch, F.Y. Hansen Inelastic low energy helium atom scattering (HAS) by a physisorbed monolayer is treated in the one-phonon approximation using a time-dependent wave packet formulation. Modes with shear horizontal (SH) polarization are excited near high symmetry azimuths of the monolayer, in agreement with recent experiments. The calculations are for the conditions of HAS experiments for triangular incommensurate monolayer solids of xenon, krypton, and argon adsorbed on the (111) face of platinum, and the results show many of the systematic experimental trends for relative excitation probability of the SH and longitudinal acoustic (LA) phonon branches. Although polarization selection rules appear to preclude the excitation of SH modes, there are large departures from expectations based on analogies to inelastic thermal neutron scattering. The calculations show that the inelastic scattering at beam energies near 8 meV is exceedingly sensitive to small misalignment between the scattering plane and the high symmetry directions of the monolayer solid. [Preview Abstract] |
Friday, March 25, 2005 8:24AM - 8:36AM |
X34.00003: The Impact of CO-CO Interactions on the Diffusive Behavior of CO/Cu(111) Ludwig Bartels, Kin Wong, Bommisetty V. Rao, Ki-Young Kwon, Erick Ulin-Avila, Greg Pawin We investigated the diffusion of CO on Cu(111) and, in particular, the impact of local coverage and nearest neighbor separation on the diffusion probability and direction. Measurement of the diffusivity of isolated CO molecules over the temperature range between 30 and 40K permits us to determine a diffusion barrier of 75 +/- 5meV at a prefactor of 5.3*10$^{7}$ Hz. An increase of the local coverage of CO can increase its diffusion probability significantly, e.g. the presence of 7 CO molecules within an 8 Cu-Cu-distance radius of a CO molecule increases the CO diffusivity by a factor of ca. 3. Experiments addressing pairs of CO molecules show that at close proximity the dominant diffusion direction is orthogonal to the intermolecular vector. At larger intermolecular distance, the diffusion direction becomes significantly more isotropic. Statistical analysis of the distribution of CO pairs reveals that intermolecular distances of n+ $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ periods (n=0,1, ...) of the substrate surface state oscillation are favorable. The absence of island formation at low coverage is surprising to us and suggests that the intermolecular attraction cannot balance the entropy gain associated with a disordered coverage. [Preview Abstract] |
Friday, March 25, 2005 8:36AM - 8:48AM |
X34.00004: First Principles Studies of Adhesion and Adhesive Transfer at Aluminum-Graphite and Aluminum-Diamond Interfaces Yue Qi, Louis Hector, Jr. Density functional theory was used to investigate adhesion and adhesive transfer at Al/diamond (Al(111)/C(111)-1x1, Al(111)/C(111)-2x1 and Al(111)/C(111)-1x1:H) and Al(111)/graphite(0001) interfaces. The work of separation, W$_{sep}$, was computed for each system and compared with that for other interfaces. Bond character was explored by plotting contours of the electron localization function (ELF). Adhesive transfer at the Al/diamond interfaces was investigated by subjecting each interface to a series of tensile strain increments up to fracture. For the Al/graphite interface, W$_{sep}$, was 0.11J/m$^{2}$, and the work of decohesion for a single layer of graphite transferring to aluminum, W$_{dec}$, was 0.077J/m$^{2}$. Although our theoretical framework does not include van der Waals forces that mitigate interlayer bonding between graphite sheets, the fact that W$_{dec} \quad <$ W$_{sep}$ suggests that it may be energetically favorable for one graphite layer to transfer to Al instead of separating right at the interface. [Preview Abstract] |
Friday, March 25, 2005 8:48AM - 9:00AM |
X34.00005: Improving the Quality Factor of Micromechanical Resonators with Surface Chemistry Debodhonyaa Sengupta, Joshua Henry, Yu Wang, Melissa Hines The sensitivity of a micromechanical resonator to an applied force or mass is determined, in part, by the resonator's quality factor, or Q, which is a measure of mechanical energy dissipation. By changing a single monolayer of molecules on the surface of a 5-$\mu$m-wide, 250-nm-thick, MHz-range Si resonator - less than 0.07\% of the total mass - the quality factor of the resonator can be improved by at least 70\%. We report on experiments to determine the chemical origin of this effect and to improve the Q of silicon devices. Resonators terminated by a suitably prepared methyl monolayer have higher Q's and better stability than similar resonators coated with the best termination previously demonstrated - a single monolayer of H atoms. Equally importantly, our experiments show that surface-induced mechanical losses are relatively insensitive to the mechanical properties of the monolayer itself. Our working hypothesis for the chemical origins of this effect will be discussed. [Preview Abstract] |
Friday, March 25, 2005 9:00AM - 9:12AM |
X34.00006: Structure, bonding, and adhesion of MoSi$_{2}$/Fe and SiO$_{2}$/MoSi$_{2}$ from first principles De-en Jiang, Emily Carter The high-melting-point compound MoSi$_{2}$ is a promising candidate for a high temperature coating on iron steels and refractory metals. A coherent silica scale that forms leads to the high temperature oxidation and corrosion resistance of MoSi$_{2}$. Using periodic density functional theory techniques we examine the adhesion strength, interfacial geometry, and bonding characteristics of MoSi$_{2}$/Fe and SiO$_{2}$/MoSi$_{2}$ interfaces. We predict that MoSi$_{2}$/Fe interfaces have intrinsic adhesion energies of $\sim $3.85 J/m$^{2}$, significantly stronger than the adhesion between iron and other ceramic coating materials such as ZrC and TiC. We find that the bonding at the interface is local, with covalent character exhibited between Fe-Si and Fe-Mo across the interfaces. Experiment shows that amorphous (a) silica forms on MoSi$_{2}$. We use $\beta $-cristobalite to model a-silica, since they have similar local structure. We find that Si-O covalent bonding dominates the interfacial adhesion of SiO$_{2}$/MoSi$_{2}$, yielding strong adhesion energy of 5.75 J/m$^{2}$. These very high interfacial adhesion energies suggest that MoSi$_{2}$ indeed should be a quite thermally stable coating for steels. [Preview Abstract] |
Friday, March 25, 2005 9:12AM - 9:24AM |
X34.00007: The relevance of sub-nanometer thin oxide films in the CO oxidation on Pd(100) Jutta Rogal, Karsten Reuter, Matthias Scheffler There is an increasing awareness that oxides may form at the surface of transition metal (TM) catalysts under the oxygen-rich environmental conditions of oxidation catalysis. What is, however, still an open question is, if the metal is fully oxidized or if only nanometer thin surface oxide films are formed and which of these different phases are crucial for the catalytic activity of the material. On Pd(100) the oxidation of the surface proceeds through several different stages, including a $(\sqrt{5}\times \sqrt{5})R27^{\textrm{o}}$ surface oxide structure. The stability of this surface oxide and other oxidation states of Pd(100) in an O$_2$ gas phase up to ambient pressures was recently analyzed in a combined experimental and theoretical study [1]. We now use density-functional theory and atomistic thermodynamics to also account for the other reactant in CO oxidation catalysis by considering the surface structure and composition in a \textit{constrained} equilibrium with an O$_2$ and CO gas phase. Under gas phase conditions typical of technological CO oxidation the surface oxide results then as the most stable structure. However, this may be significantly affected by the on-going reactions, which continuously drive the system away from equilibrium. Such kinetic effects under steady-state conditions are discussed on the basis of preliminary kinetic Monte Carlo simulations. [1] E. Lundgren \textit{et al.}, Phys. Rev. Lett. \textbf{92}, 046101 (2004) [Preview Abstract] |
Friday, March 25, 2005 9:24AM - 9:36AM |
X34.00008: Why CO + $\frac{1}{2}$O2 $\rightarrow$ CO2 is so Active under One and a Third Gold Layer on Titania Mo(112) Shizhong Yang, James Phillips, Lizhi Ouyang Carbon monoxide oxidization on two types of substrates, one layer and one and a third layer gold, gold fully wetted surface supported with titania Mo(112) system[1] was studied using first-principles density-functional theory method. The geometry feature of active adsorption sites, adsorption energy and electronic properties were calculated. The active CO oxidation mechanism based on the energetic diagram and electronic properties was discussed. \textbf{References}: [1]. M. S. Chen and D. W. Goodman, \textit{Science}, \textbf {306}, 252 (2004). [Preview Abstract] |
Friday, March 25, 2005 9:36AM - 9:48AM |
X34.00009: Vibrational energy loss structure in very low energy photoelectron spectra of CO/Cu(001) surface Kei Hayashi, Ryuichi Arafune, Shigenori Ueda, Youichi Uehara, Sukekatsu Ushioda We have measured the very low energy photoelectron spectra of the CO/Cu(001) surface by using low energy photons (4.821 eV) as the excitation light source. The spectra for $^{12}$C$^{16}$O show a step in the photoelectron intensity at 254 meV below the Fermi level. An isotropic shift to 240 meV was observed for $^{13}$C$^{18}$O. These energies agrees with the vibrational energies of the C-O stretching mode. Furthermore such spectral feature did not appear for the clean Cu(001) surface. Thus we conclude that the observed step in the photoelectron spectra arises from the energy loss of photoelectrons through excitation of the vibrational mode. [Preview Abstract] |
Friday, March 25, 2005 9:48AM - 10:00AM |
X34.00010: Self Consistent Tight Binding Study of Titanium Dioxide Nanoclusters Serkan Erdin Here, we report our recent results on TiO$_{2 }$nanoclusters in vacuum and water based on self consistent tight binding method (SCTB). We calculate the ground state energies and structures of several anatase and rutile nanoclusters of size 2-8 nm, and determine the line energy of clusters with a model described by the surface energies and Wulf-constructed shapes. Our calculations show that SCTB method describes crystallographic rutile-to-anatase phase transition in vacuum well. SCTB molecular dynamics simulation of anatase cluster in water shows the structural change of the cluster. [Preview Abstract] |
Friday, March 25, 2005 10:00AM - 10:12AM |
X34.00011: Co-Adsorption of CF$_4$ and SF$_6$ on Graphite George Hess, Todd Hopkins, Yu Xia We have used infrared absorption (IRRAS) and ellipsometry concurrently to study adsorption of CF$_{4}$ on graphite (HOPG) precoated with a monolayer of SF$_{6}$. Over the temperature range studied, 50 to 120 K, multiple layers of CF$_{4}$ adsorb on top of the SF$_{6}$. At the higher temperatures, $>$85 K, partial but not complete displacement of the SF$_{6}$ is observed. At the lower temperatures, $<$75 K, no displacement is seen, including cases starting with partial monolayers of SF$_{6}$. Data on spectral shifts will be interpreted to give information on compression and mixing of components. [Preview Abstract] |
Friday, March 25, 2005 10:12AM - 10:24AM |
X34.00012: On the question of the stability of water layer on Ru(0001): electron-activated dissociation N.S. Faradzhev, T.E. Madey, K.L. Kostov, P. Feulner, D. Menzel There exist diverse and conflicting views on the stability of first molecular layer of water on the Ru(0001). Here we report the effect of electron irradiation on the rate of H$_{2}$O and D$_{2}$O dissociations when they adsorbed on ruthenium surface at low temperature (T$\le $100K). Our results show clearly that a molecular D$_{2}$O layer wets the surface and is thermally stable up to desorption. H$_{2}$O dissociates partly at elevated temperature to an extent which depends strongly on the exact heating procedures. Electron impact leads to partial dissociation of both H$_{2}$O and D$_{2}$O with extremely high cross section (e.g. $\sim $10$^{-15}$ cm$^{2}$ at 90 eV), and even electrons of very low energy (down to even 1 eV) are effective. We conclude that many reports on the system such as the LEED $I-V$ analysis of D$_{2}$O geometry by Held and Menzel and others have been influenced by the partial dissociation induced by slow electrons, and/or impurities in the layers. The results of DFT calculations concerning the stability of the adsorbed H$_{2}$O layer have to be viewed and interpreted with caution. [Preview Abstract] |
Friday, March 25, 2005 10:24AM - 10:36AM |
X34.00013: Photocatalytic Reactions on Graphite/Ice Interface Dinko Chakarov, Johan Bergeldt, Bengt Kasemo Photocatalytic splitting of water was investigated as function of photon energy and structure and thickness of ice films grown on graphite basal plane in UHV at temperatures between 35 and 150 K. While the water ice films are transparent for photons with wavelength longer than 190 nm, the strong absorption in the uppermost layers of graphite substrate generates energetic charge carriers that drive photoreactions at the graphite/ice interface. Similar schemes have been demonstrated and investigated before in the monolayer regime (ML)[1]. New elements here are (i) confinement of the precursors and the reaction products at the interface by relatively thick ice ``cap'' and (ii) possibility to control ice reactivity and permeability for reaction products by altering its structure. The most striking difference is formation of molecular oxygen, a product not observed in monolayer regime. For ice films with polycrystalline structure and thicker than $\sim $ 35 ML only hydrogen defuses from the interface into vacuum, while the oxygen and CO/CO$_{2}$ remain trapped under the ice. Release of these products takes place at elevated temperatures or if the ice films remain amorphous and porous (as deposited at low temperatures). [1] M. Gleeson, B. Kasemo, and D. Chakarov, \textit{J. Chem. Phys}., \textbf{115}, 9477; \textbf{119}, 6753. . [Preview Abstract] |
Friday, March 25, 2005 10:36AM - 10:48AM |
X34.00014: Fundamental measure density functional theory study of hard spheres solid-liquid interface Vadim Warshavsky, Xueyu Song Interfacial free energy is an important characteristic of solid-liquid interface as it is one of the crucial parameters in many formula of interface thermodynamics such the nucleation theory. Previously different aspects of crystal-melt interfaces were intensively studied with simulations [1,2,3], but theoretical studies with Density Functional Theories (DFT) are inconclusive [4,5]. In this report the structure of hard spheres fcc crystal-melt interfaces and the anisotropy of the interfacial free energies are studied using the Rosenfeld's Fundamental Measure DFT as such a functional leads to reliable coexistence results not only for the hard sphere system but also for the Lennard-Jones systems [6]. The parameters of interfacial density profile were calculated by a proper minimization procedure. For the equilibrium density profile the interfacial free energies were compared with simulation results. 1. R.L.Davidchak and B.B.Laird, Phys.Rev.Lett., 85, 4751(2000). 2. J.J. Hoyt, M. Asta and A. Karma, Phys.Rev.Lett., 86, 5530 (2001). 3. J.R.Morris and X.Song, J.Chem.Phys., 119, 3920 (2003). 4. W.A.Curtin, Phys.Rev.B, 39, 6775(1989). 5. R.Ohnesorge, H.Lowen, and H.Wagner, Phys.Rev.E, 50, 4801 (1994). 6. V.Warshavsky and X.Song, Phys.Rev.E, 69, 061113 (2004). [Preview Abstract] |
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X34.00015: Consistent picture for the wetting structure of water/Ru(0001) Sheng Meng, E.G. Wang, Christian Frischkorn, Martin Wolf, Shiwu Gao The wetting layer of D2O on Ru(0001) has been reanalyzed with ab-initio molecular dynamics and full-potential calculation. Comparison of vibrational spectra and work function with experiments favors the picture of molecular wetting rather than partial dissociation. A mixture structure consisting of both hydrogen-up and hydrogen-down bilayers is found to be consistent with experiments. The barrier and minimum energy path for bilayer dissociation have been determined. The dissociation barrier is larger than that for desorption, suggesting that dissociation is kinetically forbidden at low temperatures. These finding resolve the current controversy between previous calculations and experimental observations. [Preview Abstract] |
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