Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session K13: Atomic And Molecular Processes at Solid Surfaces |
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Sponsoring Units: DCP Chair: Eric Borguet, Temple University Room: Baltimore Convention Center 305 |
Tuesday, March 14, 2006 2:30PM - 2:42PM |
K13.00001: Thermal Chemistry of N$_{2}$, CO$_{2}$, and CH$_{4}$ on Cesiated Pt(111) Kristy DeWitt, Leticia Valadez, Ian Harrison, Merrick DeWitt Promoting transition metal surfaces with alkali metals is known to drastically reduce the surface work function. We have investigated the effect of Cs promotion on the thermal chemistry of CH$_{4}$, N$_{2}$ and CO$_{2}$ using temperature programmed desorption (TPD). At low Cs coverage the CH$_{4 }$TPD peak broadens and shifts to higher temperature. As the Cs coverage increases further three CH$_{4}$ peaks develop: one at higher temperature than on a bare Pt surface, and two at lower temperatures. With Cs coverage of one saturation monolayer (\textit{$\theta $}$_{sat}$ = 0.41 ML) or more only the lowest temperature peak remains. CH$_{4}$ sticking also varies as a function of Cs coverage. N$_{2}$ displays similar behavior to CH$_{4}$ in response to coadsorption of Cs, but does not stick to Cs multilayers. CO$_{2}$ exhibits very unusual behavior as a function of Cs coverage. At low Cs coverage the physisorbed CO$_{2}$ peak broadens, then splits into a doublet, with one peak at higher temperature than the ``normal'' CO$_{2}$ peak, and the other at lower temperature. With increasing Cs coverage CO$_{2}$ begins to dissociate, evidenced by high temperature recombinative desorption and the presence of chemisorbed CO. As the Cs coverage approaches \textit{$\theta $}$_{sat}$ the physisorbed peak disappears entirely, and only dissociative chemisorption is seen. [Preview Abstract] |
Tuesday, March 14, 2006 2:42PM - 2:54PM |
K13.00002: Observation and Manipulation of Subsurface Hydride in Pd(111) Charles Sykes, Luis Fern\'{a}ndez-Torres, Sanjini Nanayakkara, Brent Mantooth, Ryan Nevin, Paul Weiss We have observed and manipulated hydrogen atoms beneath the surface of a Pd(111) crystal using low-temperature scanning tunneling microscopy (STM). The subsurface region of Pd can be populated with hydrogen atoms from the bulk by applying voltage pulses from a STM tip. Topographic and local electronic data characterizing subsurface hydrogen in these stable sites is presented. We discuss our ability to selectively populate subsurface sites with hydrogen and present a full STM characterization of this state. This phenomenon is explained with an inelastic excitation mechanism, whereby hydrogen atoms in the bulk are excited by tunneling electrons and are promoted to more stable sites in the subsurface region. [Preview Abstract] |
Tuesday, March 14, 2006 2:54PM - 3:06PM |
K13.00003: Oxygen-Induced Faceting of Ir(210) Timo Jacob, Payam Kaghazchi, Matthias Scheffler Although rough clean metal surfaces usually have higher surface free energies than their closed-packed analogs, adsorbates are able to modify this behavior and cause changes in surface morphology through reconstructions or facet formations. Using different surface sensitive techniques (e.\,g. STM or LEED) Madey {\it et al.} [1] found that Ir(210) shows interesting surface faceting at high oxygen coverages ($\theta>0.5$\,ML). The facets were identified as three-sided nanoscale pyramids with two (311) and one (110) planes. In oder to understand this effect we used density functional theory in combination with the {\it ab initio} atomistic thermodynamics and studied the influence of an oxygen atmosphere on the structure of Ir(210). Assuming an oxygen partial pressure of 1\,atm, it turned out that below $T<1000$\,K the oxygen-covered facets are thermodynamically more stable than non-faceted O/Ir(210). Further heating reverts the substrate structure to planar O/Ir(210). While most nanoscale pyramids consist of smooth and unreconstructed planes, some (110) facets show a complex reconstructed superstructure. Thus, present studies aim on the stability and an atomistic understanding of those structures.\newline [1] I. Ermanoski {\it et al.}, {\it Surf. Sci.} {\bf 549}, 1 (2004). [Preview Abstract] |
Tuesday, March 14, 2006 3:06PM - 3:18PM |
K13.00004: The Nature of Metal-Oxygen Bonding at Corundum (0001) Surfaces in the Presence of Oxygen Emily Jarvis, Orkid Coskuner, Anne Chaka Metal oxides are of critical importance in a variety of applications ranging from gas sensing to catalysis to environmental containment of heavy metals. In all of these cases, it is the surface chemistry that is essential to the necessary function. The detailed surface structure is heavily dependent on the physical and chemical features of the surroundings, and its reactivity is largely dictated by the nature of the local bonding character. We use all electron density functional calculations combined with \textit{ab initio} thermodynamic predictions to explore surfaces of several corundum-structured metal oxides as a function of oxygen partial pressure, which is a crucial first step in understanding corrosion, passivation, and catalysis. Additionally, we apply quantum mechanical wavefunction-based methods with the goal of elucidating the local bonding character of these surfaces. Our calculations provide detailed insight into the nature of the chemical bonding at chromia and hematite (0001) surfaces with M=O termination, i.e., the chromyl- and ferryl-terminated surfaces, and explain the dissimilar thermodynamic predictions for the alumina (0001) surface relative to these transition metal oxide surfaces. [Preview Abstract] |
Tuesday, March 14, 2006 3:18PM - 3:30PM |
K13.00005: Theoretical treatment of excited electronic states of adsorbates on metals: electron attachment to CO$_{2}$ adsorbed on Pt(111)* Jerry Whitten, Laura Sremaniak Photochemistry involving adsorbates on metals often proceeds by photoexcitation of the metal followed by transient attachment of photoemitted electrons to the adsorbate. First principles theoretical methods suitable for describing electronic states embedded in a near continuum of metal to metal excitations are described and an application to electron attachment to CO$_{2}$ adsorbed on Pt(111) is reported. Wavefunctions are constructed by \textit{ab initio} configuration interaction methods which allow a rigorous resolution of states and differentiation between competing pathways of molecular desorption and dissociation. An embedding theory is used to achieve high accuracy in the adsorbate-surface region. The overall process can be understood as formation of an electron attached state at an energy lower than the work function of the metal, localization of the metal hole and attraction of the charged adsorbate to the metal. Optimum geometries are calculated and pathways between these geometries determine whether molecules will desorb, dissociate by bond rupture directly in the excited electronic state, or dissociate after return to the ground state potential energy surface via vibrational processes. The influence of a coadsorbed potassium electron donor atom on the energy required to form the electron attachment state is also reported. [Preview Abstract] |
Tuesday, March 14, 2006 3:30PM - 3:42PM |
K13.00006: Quantum Nature of Hydrogen on Metals: Ground-State Geometry of Vacancies Sungho Kim, Seong-Gon Kim, Steven C. Erwin When hydrogen is adsorbed on a Pd(111) surface, very simple vacancy defects -- which form quite commonly -- exhibit fundamentally quantum wavelike behavior. Conventional wisdom has it that the quantum nature of hydrogen and other light atoms is only rarely manifested, typically in specially designed experimental protocols. A great many studies have proceeded under the assumption that, for most purposes, as long as the electronic motion is handled quantum mechanically, the nuclear motion of hydrogen can be treated classically. We show that this approximation fails badly in a very simple system. Moreover, by treating the problem fully quantum mechanically, several recent experimental finding [Mitsui et al, Nature 422, 705 (2003)] can be plausibly interpreted as having a fundamentally quantum origin. In particular, we present new ground-state geometry of hydrogen vacancies on Pd(111) surface that are obtained from a full quantum treatment of the hydrogen nuclear motion. This new structures enable us to predict that all vacancy defects to have an unexpected triangular appearance in scanning tunneling microscopy; recent experiments have found precisely this behavior. [Preview Abstract] |
Tuesday, March 14, 2006 3:42PM - 3:54PM |
K13.00007: Positron annihilation induced Auger electron spectroscopic study of the changes in the top layers of electrodeposited Cu$_{2}$O surfaces resulting from vacuum annealing M. Nadesalingam, J. Zhu, N. Fazleev, S. Mukherjee, N. R. de Tacconi, S. Somasundaram, C.R. Chenthamarakshan, K. Rajeshwar, A. H. Weiss Positron Annihilation induced Auger electron Spectroscopy (PAES) and electron stimulated Auger Spectroscopy (EAES) were used to measure changes in the surface of electrodeposited Cu$_{2}$O resulting from vacuum annealing. The PAES measurements show a very large increase in the intensity of the Cu MVV Auger peak after annealing at 535 K. Similar but significantly smaller changes were observed in the EAES spectra consistent with the fact that PAES is primarily sensitive to the top-most atomic layer due to the fact that the positrons are trapped just outside the surface prior to annihilation while EAES samples several atomic layers. The PAES and EAES data indicate that vacuum annealing of Cu$_{2}$O results in a surface with a high concentration of Cu in the top-most atomic layer. Research supported by the Welch Foundation Y-1100 and NSF DMR 98-12628. [Preview Abstract] |
Tuesday, March 14, 2006 3:54PM - 4:06PM |
K13.00008: Atomic-scale study of the Rh/$\gamma $-alumina catalytic system Shuping Zhuo, Yiping Peng, Stephen Pennycook, Karl Sohlberg Prerequisite to the non-empirical design and refinement of improved heterogeneous catalysts is the identification of the atomic-scale structure and properties of the catalytically active sites. We report an investigation of the Rh/$\gamma $-alumina system, which is representative of many common heterogeneous catalysts that consist of transition metals dispersed on a high surface area support. Previous atomic-resolution Z-contrast STEM observations have shown Rh-containing ``rafts'' on the (100) exposure. This finding is surprising given that the preferred exposure of $\gamma $-alumina is (110). First-principles density functional studies and image simulations suggest that these Rh-containing structures consist of the high-pressure rhodium sesquioxide (II) phase growing on the surface. The high pressure phase of rhodium sesquioxide and $\gamma $-alumina (100) exposure yield improved interface match over the regular rhodium sesquioxide phase and $\gamma $-alumina (110) exposure. [Preview Abstract] |
Tuesday, March 14, 2006 4:06PM - 4:18PM |
K13.00009: Nature of Versatile Chemisorption on TiC(111) and TiN(111) Bengt I Lundqvist, Carlo Ruberto, Aleksandra Vojvodic Extensive density-functional calculations on the polar TiX(111) (X = C, N) surfaces show (i) for the clean surfaces strong Ti3d-derived surface resonances (SR's) at the Fermi level, unlike on the stable TiX(001) surfaces, and X$2p$-derived SR's in the upper valence band, and (ii) for chemisorption of atoms in the first three periods pyramid-shaped trends in atomic adsorption energies Eads, with an extremely strong O-atom bond. The adsorption is successfully described by a by us proposed concerted-coupling model (CCM), where adsorbate states couple to both kinds of SR's in a concerted way, and which is consistent with trends in the adsorbate-induced DOS and explains the characteristic variations in Eads and in bond lengths, as well as radical differences between TiX(111) and TiX(001), and similarities and small differences between TiC and TiN. The versatility of chemisorption on TiX(111) and the general nature of CCM indicate both ramifications and predictive abilities in, e.g., growth and catalysis. [Preview Abstract] |
Tuesday, March 14, 2006 4:18PM - 4:30PM |
K13.00010: Depth Profiling and Interface Analysis by Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS): Potential, Performance and Limitations. Corey Foster, Sven Kayser, Thomas Grehl, Rudolf Moellers, Ewald Niehuis TOF-SIMS has always been a very sensitive tool for surface spectroscopy and imaging. In recent years TOF-SIMS has improved significantly for traditional dynamic SIMS applications. Operated in the so-called dual-beam mode, where the sample is sputtered with a low energy sputter beam, while the analysis is performed in the center of the sputter crater using a high energy beam, TOF-SIMS provides high-sensitivity depth profiling capabilities comparable to quadrupole or magnetic sector mass spectrometers. Providing parallel mass detection, high mass resolution, advanced charge compensation and 3D imaging capabilities, TOF-SIMS has developed considerable advantages for the characterization of technologically advanced materials. In this study we investigated the depth resolution, detection limits, quantification and reproducibility now available with TOF-SIMS. During this talk we will show examples and applications of TOF-SIMS depth profiling in the field of inorganic materials and compare the depth profiling capabilities of TOF-SIMS instruments with other dynamic SIMS machines. [Preview Abstract] |
Tuesday, March 14, 2006 4:30PM - 4:42PM |
K13.00011: Insight into the mechanisms of metal ion binding in hexagonal tungsten bronze Shane Kennedy, Suzanne Smith, Maxim Avdeev, Alex Fuchs Tungsten trioxides and their alkali-metal-intercalated products M$_{z}$WO$_{3+}$ show potential for application in electrodes, as ion exchangers, catalysts and the treatment of radioactive waste. Hexagonal tungsten bronze (HTB) was selected for the present work because its structure features hexagonal channels, of diameter $\sim $0.54 nm, that may be useful for selective and reversible binding of metal ions. X-ray and neutron powder diffraction were used to provide an insight into the mechanisms of metal ion binding of the Mo doped HTB's. Combined Rietveld refinement of the X-ray and neutron diffraction shows that doping with Mo degrades the crystallinity of HTB, in particular by creating a high degree of disorder in the a-b planes. Structural information combined with solution chemistry indicates several potential mechanisms of binding and metal ion exchange sites. This type of investigation provides invaluable information for new strategies in the design of inorganic sorbents and their optimization for metal ion separation. [Preview Abstract] |
Tuesday, March 14, 2006 4:42PM - 4:54PM |
K13.00012: Adsorption of NO on Rh(111) studied by STM. J.H.A. Hagelaar, C.F.J. Flipse The ability of Rhodium to efficiently dissociate NO makes it the most suitable metal for the removal of nitrogen oxides from automotive exhaust gasses in catalytic converters. Using a STM, the adsorption of NO on Rh(111) was studied in ultra high vacuum. First, the NO is adsorbed at T=200 K, giving rise to two structures with coverages of 0.5 ML and 0.75 ML. These structures differ from the 0.5 ML (4x2)-2NO and 0.75 ML (2x2)-3NO structures found in literature. Once a structure was obtained, the sample was cooled down to 4.8K to perform inelastic electron tunneling spectroscopy (IETS) on the individual molecules within the structure. The molecules in the 0.5 ML structure showed a significant shift in the NO-stretch frequency compared to the expected frequency of 1600 cm$^{-1}$. These results will be presented and discussed in detail. [Preview Abstract] |
Tuesday, March 14, 2006 4:54PM - 5:06PM |
K13.00013: Picosecond real time observation of low energy electron scattering from large molecules on surfaces Claudio Cirelli, Matthias Hengsberger, Andrei Dolocan, Juerg Osterwalder, Thomas Greber, Herbert Over In order to observe in real time structural changes of molecules, it is necessary to develop a technique providing both nanometer and picosecond resolution. This task is tackled with a time resolved Low Energy Electron Diffraction (LEED) experiment in a pump-probe scheme. The pump pulses are 800 nm 100 fs laser light pulses and the probe 60eV energy electron pulses. Using a novel electron gun with a measured energy resolution of 0.7eV at 100eV, we observe transient changes in the differential scattering cross section and an energy shift of the diffracted LEED spot up to about 250meV. As this energy shift is pump fluence dependent, we can assign it to the onset of a space charge effect created by the pump beam on the sample surface. Conclusions about surface space charge and molecular dynamics can be drawn by tracking this effect on the picosecond timescale. Results on the response of the bare Cu(111) surface and of C$_{60}$/Cu(111) will be presented. [1] A. Dolocan et al., Jap. J. Appl. Phys. 45 (2006) [Preview Abstract] |
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