Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session S42: Reactions: Kinetics & Dynamics |
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Sponsoring Units: DCMP Chair: Kristen Fichthorn, Pennsylvania State University Room: Colorado Convention Center 505 |
Wednesday, March 7, 2007 2:30PM - 2:42PM |
S42.00001: Dissociative and molecular adsorption and recombinative desorption of methylchlorosilanes at the Cu~(001) surface J. Lallo, L.V. Goncharova, A.V. Ermakov, B.J. Hinch, D. Strongin The Direct Synthesis of dichlorodimethylsilane above Cu/Si containing surfaces, is central to the commercial manufacture of many silicone materials. The atomic scale mechanisms responsible for the high selectivity for dichlorodimethylsilane, vs. other chloromethylsilanes, is poorly understood. As part of a more extensive program we report here on the interactions of chloromethylsilanes with copper. Adsorption of dichlorodimethylsilane on Cu(001) is, at least in part, dissociative. Yet AES indicates non-stoichiometric surface concentrations of Cl and C. This observation, which is apparent in both low (140K) and room temperature exposures, is indicative of a facile recombinative desorption process. Subsequent TPD spectra also indicate the desorption of not only the parent adsorbate, but other species also. Indeed the combined cracking patterns, of simultaneously desorbing species, are inconsistent with solely (CH$_{3})_{x}$SiCl$_{y}$ species. We report on the TPD, AES, and high-resolution helium atom scattering studies of (i) adsorption of methylchlorosilanes, and (ii) their coadsorption with added SiH$_{4}$, CH$_{3}$ and Cl species, on Cu(001). Evidence for methyl cracking, and disilane production will discussed. [Preview Abstract] |
Wednesday, March 7, 2007 2:42PM - 2:54PM |
S42.00002: CO adsorption on a transition metal quantum well system: fcc Co/Cu(100).$^{1}$ Levan Tskipuri, Hua Yao, Robert Bartynski We have examined the unoccupied electronic structure and CO bonding strength on the n-ML fccCo/Cu(100) metallic quantum well (MQW) system using inverse photoemission (IPE) and temperature programmed desorption (TPD), respectively. As-grown Co films exhibit well-defined MQW states that disperse upward with increasing film thickness, but they do not cross the Fermi level and are less pronounce than on other similar systems owing to partial overlap of exchange spit states. Upon CO adsorption a well-defined structure centered about 3.8 eV above the Fermi level appears and is assigned to the unoccupied CO 2$\pi $* orbital. CO adsorbs molecularly at room temperature and in TPD measurements we find a desorption temperature of $\sim $ 375 K, which is about 30 K lower than what is observed for CO adsorbed on the hcp Co surfaces. When Co films are dosed at low temperatures ($\sim $ 120K), we find a second CO desorption peak around 230 K, once again similar to what is seen for hcp Co, but at a markedly lower temperature. We have observed similar desorption peak temperature shifts for CO desorption from the Ni/Cu(100) system. The CO desorption temperature varies with Co layer thickness and the possible role of quantum size effects on the molecule-surface bond will be discussed. $^{1 }$ACS-PRF Grant Number 40236-AC-5S [Preview Abstract] |
Wednesday, March 7, 2007 2:54PM - 3:06PM |
S42.00003: The influence of overlap interactions on chemical reactions in confinement Erik E. Santiso, Keith E. Gubbins, Aaron M. George, Marco Buongiorno Nardelli Chemical reactions are often carried out in nano-structured materials due to their large surface area per unit mass. It is, however, difficult to understand fully the role of the nano-structure in many reactions due to the superposition of multiple effects. Such effects include: the reduced dimensionality of the system, the heterogeneity of the pore surfaces, the selective adsorption of reactants/products, catalytic effects, and transport limitations. Experimental studies often show many of these effects at the same time, making the results difficult to interpret. In this work we present results of density functional theory calculations illustrating the influence of overlap interactions (shape-catalytic effects) on chemical reactions. In particular, we show the effect of confinement in small pores on the rates of rotational isomerizations of n-butane, 1-butene and 1,3-butadiene. We find that the rates of these transitions change as the double exponential of the pore size in the molecular sieving limit. These results are a first step towards an integrated model for the design of catalytic materials. [Preview Abstract] |
Wednesday, March 7, 2007 3:06PM - 3:18PM |
S42.00004: Reactions of water molecules at the Si/SiO2 interface I.G. Batyrev, L. Tsetseris, D.M. Fleetwood, R.D. Schrimpf, S.T. Pantelides Water molecules can be present to a varying degree at Si/SiO2 interfaces, either introduced by the oxidation process or through absorption from the atmosphere. Such water molecules may affect critically the electrical properties of metal-oxide-semiconductor (MOS) structures and devices. Here we present results of first-principles density-functional calculations of several pathways of water reactions near the Si/SiO2 interface. We found that (i) the presence of water molecules is energetically favored in the a-silica rings near the interface with an energy gain of $\sim $ 0.3 eV relative molecule in vacuum; (ii) a water molecule causes depassivation of Si-H bonds with formation of a Si dangling bond, H$_{2}$ molecule and loose OH complex; (iii) a water molecule may passivate a Si dangling bond and form a loose OH complex; (iv) a water molecule may oxidize the Si-Si bonds and form a H$_{2}$ molecule. The interplay of reactions results in changes in interface-trap density. We compare the results with radiation-response measurements of nMOSFETS right after production and after 18 years of aging and explain the observed growth of interface trap density (0.7x10$^{11}$cm$^{-2})$ just after X-ray irradiation. This work was supported in part by the AFOSR and the US Navy. [Preview Abstract] |
Wednesday, March 7, 2007 3:18PM - 3:30PM |
S42.00005: Growth kinetic of perovskite oxide monolayers on SrTiO$_{3}$(100) studied with reflection high energy electron diffraction (RHEED) and oblique-incidence optical reflectivity difference measurements. Xu Wang, Yiyan Fei, Xiangdong Zhu Using a combination of RHEED and an oblique-incidence optical reflectivity difference technique (OI-RD, a form of polarization modulated ellipsometry), we studied the growth of 5{\%} doped Nb:SrTiO$_{3}$ monolayers on SrTiO$_{3}$(100) under pulsed laser deposition condition (PLD) in a molecular oxygen ambient. By interrupting the deposition at the completion of one monolayer and continuing the RHEED and OI-RD measurement during post-deposition annealing, we can separate contributions to the optical reflectivity difference signal from kinetics of growth and oxidation. Based on a mean-field theory of optical reflectivity difference off an atomically rough and yet optically smooth film, we find that a growth-dependent part of the reflectivity difference signal is proportional to the step edge density or equivalently the root-mean-square (rms) of the roughness of a growth surface. We show that the surface roughness during the interrupted deposition of Nb:SrTiO$_{3}$ monolayers on SrTiO$_{3}$(100) can be analyzed with a 8-level growth model with two adjustable parameters. The parameters obtained by fitting the model to the OI-RD signal during the \textit{interrupted} deposition can be used subsequently to predict the growth behaviour of Nb:SrTiO$_{3}$ on SrTiO$_{3}$(100) in a \textit{continuous} pulsed laser deposition as reported earlier by Fei and co-workers. [Preview Abstract] |
Wednesday, March 7, 2007 3:30PM - 3:42PM |
S42.00006: Grating-coupled excitation and detection of surface plasmon polariton waves (SPPW) on Cu(111) using periodic density patterns of rare gas monolayers. Yiyan Fei, Xu Wang, Xiangdong Zhu Using periodic density profiles of xenon (Xe) as thin as 1 $\sim $ 5 monolayers, we have excited and detected grating-coupled surface plasmon polariton waves (SPPW) on Cu(111) in ultrahigh vacuum. The periodic density profiles are formed by laser-induced thermal desorption with a pair of coherent laser pulses at vacuum wavelength of 0.532 $\mu $m. The periodicity of the profiles is 5.45 $\mu $m. By illuminating the xenon-density-grating-covered Cu(111) with a converging He-Ne laser covering a span of incidence angles from 66.4$^{\circ}$ to 74.4$^{\circ}$ and detecting the oblique-incidence reflectivity difference r$_{p}$/r$_{p0}$ -- r$_{s}$/r$_{s0}$ vs. incidence angle with a multiple-element photodiode array, we observed the surface-plasmon resonance (SPR) peaked at $\phi _{SPR}$ = 70.4$^{\circ}$ with a full-width at half-maximum $\delta \phi _{SPR}$ = 0.29$^{\circ}$. From the resonance angle $\phi _{SPR}$ and $\delta \phi _{SPR}$, we have determined the optical dielectric constant of single crystalline Cu at 633 nm to be $\varepsilon _{Cu}$ = -9.53 + $i$ 0.142, markedly different from the literature values for evaporated Cu films. At elevated temperatures such that a xenon density grating on Cu(111) decays in contrast, the surface plasmon resonance as measured by r$_{p}$/r$_{p0}$ -- r$_{s}$/r$_{s0}$ diminishes, reflecting the kinetic of surface diffusion of xenon on Cu(111). [Preview Abstract] |
Wednesday, March 7, 2007 3:42PM - 3:54PM |
S42.00007: STM-induced passivation of Si (100) surface from physisorbed molecular hydrogen at 5 K Amena L. T. Khan, Allan R. MacDairmid, David C. Fortin, Xiaobin Zhu, Mark R. Freeman Gas phase adsorption of hydrogen on silicon surfaces is usually achieved through reaction with molecular hydrogen at elevated temperatures and/or exposure of the surface to atomic hydrogen.[1] In the present work, we introduce molecular hydrogen to a silicon (100) surface at T = 5 K. In these conditions we observe physisorption of molecular hydrogen,[2] which can act as a precursor for the formation of hydrides on the silicon surface. It is found that scanning tunneling microscopy can be used to induce patterned chemisorption of hydrogen on silicon. Upon repetitive scanning of the same area at negative sample bias, complete passivation of the area can be achieved. The route to complete termination involves two stages, the second of which appears to proceed via nucleation and growth of completely terminated islands from within a disordered, partially terminated phase. At positive sample bias the second stage is not observed, with the induced chemisorption ending in an unreactive ``locked'' configuration of the surface still containing silicon dangling bonds. This work is supported by NSERC, iCORE and CIAR. References: [1] A. J. Mayne, D. Riedel, G. Comtet, G. Dujardin, Prog. Surf. Sci., 81, 1, 2006. [2] T. R. Govers, L. Mattera, G. Scoles, J. Phys. Chem., 72, 5446, 1980 [Preview Abstract] |
Wednesday, March 7, 2007 3:54PM - 4:06PM |
S42.00008: Quantum Size Effects on Surface Catalysis from First-Principles Studies Li Huang, Xingao Gong, Zhenyu Zhang Using first-principles calculations within density functional theory, we find that the adsorption energies and diffusion barriers of O and CO on quantum (Au+Ag) films show thickness-dependent oscillations. Such oscillations are solely determined by the total thickness of the (Au+Ag) films. The synergetic standing waves formed in the films due to quantum confinement and interference give rise to such quantum oscillation. This result points to potential tunability of the chemical reactivity on ultrathin metal films. [Preview Abstract] |
Wednesday, March 7, 2007 4:06PM - 4:18PM |
S42.00009: Theoretical investigation of CO oxidation on AuAg (110) alloy surface. Jyh Pin Chou, Ching-Ming Wei Carbon monoxide (CO) oxidation on the gold-silver (110) binary alloy surface has been investigated by using density functional theory. For clean metal (110) surface, it was found that CO prefers to adsorb on atop and short bridge site of Au(110) surface [1] and O$_{2}$ prefers to adsorb on fourfold hollow (FFH) site of Ag(110) surface [2]. In this work, we present the results of CO, O$_{2}$ adsorption, and CO+O$_{2}$ coadsorption on AuAg(110) surface. The results indicate that the adsorption energies are mainly determined by the type of atoms on the bonding site. For CO, adsorption on Au atop and Au$_{2}$ short bridge site are favored and have an adsorption energy range of 0.43$\sim $0.63 eV. For O$_{2}$, adsorption on FFH site with Au$_{5-n}$Ag$_{n}$ (n$\mathbin{\lower.3ex\hbox{$\buildrel>\over {\smash{\scriptstyle=}\vphantom{_x}}$}} $3) configuration is possible and has an adsorption energy range of 0.12$\sim $0.53 eV. For CO+O$_{2}$ coadsorption, the coadsorption energy on Au$_{5-n}$Ag$_{n}$ (n$\mathbin{\lower.3ex\hbox{$\buildrel>\over {\smash{\scriptstyle=}\vphantom{_x}}$}} $3) FFH site is slightly less than the sum of CO and O$_{2}$ adsorption energy by only 0.05$\sim $0.1 eV. The oxidation of adsorbed CO with O$_{2}$ is found to proceed via formation of an intermediate, OCOO, and has a surprising low energy barrier of 0.2$\sim $0.3 eV. [1] Tatyana E. Shubina, Christoph Hartnig and Marc T. M. Koper, Phys. Chem. Chem. Phys. 6, 4215,(2004) [2] P. A. Gravil and D. M. Bird, Phys. Rev. Lett. 77, 3993(1996) [Preview Abstract] |
Wednesday, March 7, 2007 4:18PM - 4:30PM |
S42.00010: What is Special in Silver for Ethylene Epoxidation Paola Gava, Anton Kokalj, Stefano de Gironcoli, Stefano Baroni We present a first-principles study of Ethylene Oxide (EO) synthesis on different transition and noble metal surfaces. Recently Linic et al. have shown that on silver ethylene oxametallacycle (OMC) is a common intermediate for EO as well as for acetaldehyde (Ac) formation, the latter leading to undesired total combustion [1]. Our results provide a rationale of these findings which stem from the mild reactivity of silver that hinders H--C and C--C bond breaking both in ethylene and in the OMC intermediate. By analyzing the transition state geometries toward EO and Ac we identify an indicator that strongly correlates with selectivity for EO formation and could be an useful tool in the rational search for an improved catalyst. \newline [1] S. Linic et al., J. Am. Chem. Soc. {\bf 125}, 4034 (2003) [Preview Abstract] |
Wednesday, March 7, 2007 4:30PM - 4:42PM |
S42.00011: CS radical formation in the Hot Filament CVD of diamond thin film by the CRDS Madalina Buzaianu, Vladimir Makarov, Arturo Hidalgo, Brad Weiner, Gerardo Morell In the present study, the CS radical was detected using Cavity Ringdown Spectroscopic (CRDS) during the Hot Filament CVD growth of diamond thin film for the CH$_{4}$ / H$_{2}$ mixture doped with H$_{2}$S. The absolute absorption optical density of the CS radical was obtained, and the concentration of this radical was estimated as function of CH$_{4}$ and H$_{2}$S concentrations. It was found that the yield of the CS radical depends on the presence of the substrate. The experimental results show that the heterogeneous sources of the CS radical are more significant in the presence of the substrate than in experiments without substrate. The relationship between the homogeneous and heterogeneous channels of the CS radical generation was estimated for both cases with and without substrate. The translational (Doppler analysis of the line contour) and rotational (fitting of the rotationally resolved CS spectrum) temperatures were estimated. [Preview Abstract] |
Wednesday, March 7, 2007 4:42PM - 4:54PM |
S42.00012: High Pressure and High Temperature Decomposition Studies of PETN and TATB Michael Pravica, Hubertus Giefers, Edward Romano, Brian Yulga, Zachary Quine, Wenge Yang, Hans Peter Liermann We present a variety of high pressure and high temperature studies investigating radiation-induced decomposition of the high explosives PETN and TATB using white beam synchrotron radiation at the Advanced Photon Source. Diffraction line intensities were measured as a function of time using energy-dispersive techniques. By measuring the decomposition rate as a function of pressure and temperature, kinetic and other constants associated with the decomposition reaction were extracted. [Preview Abstract] |
Wednesday, March 7, 2007 4:54PM - 5:06PM |
S42.00013: Investigation of the SH formation mechanism in Hot Filament CVD of diamond thin film by CRDS Vladimir Makarov, Madalina Buzaianu, Arturo Hidalgo, Brad Weiner, Gerardo Morell The SH radical formation mechanism during the Hot Filament CVD (HFCVD) of diamond thin film was studied using Cavity Ringdown Spectroscopy (CRDS) for the CH$_{4}$/ H$_{2}$ mixture upon addition of traces amounts of H$_{2}$S. The absorption spectrum of the SH radical as function of different parameters (filament material, distance between filament and probing laser area of CRD cell, CH$_{4}$ and H$_{2}$S concentrations, presence and absence of substrate) was studied. The gas temperature and the SH concentration profiles were obtained. The SH radical yield saturates for CH$_{4}$ concentrations higher than 4 {\%}. From the analysis of the experimental data we expect to understand the sources and the decay channels related to the mechanism of the SH radical formation during the HFCVD of the diamond thin film. The SH translational (Doppler analysis of the line countur), rotational (rotationally resolved spectrum fitting) and vibrational (measurement of the relative populations of SH on the $v''$ = 0 and $v''$ = 1 vibronic states) temperatures were estimated. [Preview Abstract] |
Wednesday, March 7, 2007 5:06PM - 5:18PM |
S42.00014: Investigation of the Role of Surface Oxides in Catalysis by Gold Nanoparticles Hongqing Shi, Catherine Stampfl In contrast to the long held view that gold is catalytically inert, it is now well known that supported gold nanoparticles are notably more active than other transition metals for low temperature catalytic oxidation of CO, as well as promoting several other catalytic reactions [1]. This has stimulated huge efforts in an attempt to understand the mechanisms responsible for the high activity, including investigations into the nature of oxygen on gold surfaces [2]. Through density-functional theory and the approach of \textit{ab initio} thermodynamics [3] we have found that on the gold (111) surface, thin oxide-like structures are significantly more stable for the pressure and temperature conditions of CO oxidation. The energetic preference for such partially oxidized gold, is in accord with very recent experimental results [4]. For the identified lowest energy surface oxide-like structure, we investigate the adsorption of CO on the surface and determine the reaction pathways for CO oxidation. \newline [1] Haruta, Catal. J. New. Mater. Electro. Sys. \textbf{7,} 163 (2004). \newline [2] R. Meyer \textit{et al}., Gold Bull. \textbf{37}, 72 (2004), and references therein. \newline [3] K. Reuter, C. Stampfl and M. Scheffler, in Handbook of Materials Modeling, Volume 1, Fundamental Models and Methods, Sidney Yip (Ed) 2005, 149-194. \newline [4] L. Fu \textit{et al.} J. Phys. Chem. B 109, 3704 (2005). [Preview Abstract] |
Wednesday, March 7, 2007 5:18PM - 5:30PM |
S42.00015: Modeling the effect of adsorbates on the surface segregation of binary alloy surfaces Ole M. L{\O}vvik, Susanne M. Opalka The effect of adsorbed species on surface segregation in binary alloys has been investigated using band-structure density-functional theory. Particular emphasis is given to hydrogen adsorption on the Pd-Ag and Pd-Cu systems, which are of relevance for hydrogen selective dense metal membranes. It is demonstrated how adsorption can significantly alter the atomic-scale surface segregation in such binary alloy surfaces. [Preview Abstract] |
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