Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session V20: Surface Kinetics and Dynamics |
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Sponsoring Units: DCMP Chair: Guang-Hong Lu, University of Utah Room: LACC 407 |
Thursday, March 24, 2005 11:15AM - 11:27AM |
V20.00001: Molecular dynamics simulation for dissociative adsorption of SiH4 on Si(100) surface Yukinori Sakiyama, Yoshihiko Iga, Shu Takagi, Yoichiro Matsumoto In this presentation, we introduce a new interatomic potential model optimized for the dissociative adsorption of SiH4 on Si(100) surface. We have chosen Tersoff-Murty-Brenner potential form to model Si-H interaction. The parameters were optimized to reproduce the various reaction paths and their activation energies of the dissociative adsorption of SiH4 obtained by the DFT calculations using the nudged elastic band method. The activation energies calculated from the present model successfully reproduced the DFT calculations. With this potential model, the molecular dynamics simulation was conducted to investigate the reaction dynamics and compare with the molecular beam experiments. We found the dissociation probability calculated by the molecular dynamics simulations increased exponentially with the increase of the translational energy of incoming SiH4, which is consistent of experiments. [Preview Abstract] |
Thursday, March 24, 2005 11:27AM - 11:39AM |
V20.00002: Precursor-Surface Reactions in Plasma Deposition of Silicon Thin Films Tamas Bakos, Mayur Valipa, Dimitrios Maroudas Device-quality hydrogenated amorphous silicon (a-Si:H) thin films are usually grown by plasma deposition under conditions where the SiH$_{3}$ radical is the dominant deposition precursor. In this presentation, we report results of first-principles density functional theory calculations on the interactions of the SiH$_ {3}$ radical with the crystalline Si(100)-(2$\times $1):H surface in conjunction with molecular-dynamics simulations of a-Si:H thin film growth by SiH$_{3}$ radicals, which elucidate the pathways and energetics of surface reactions that govern important film properties. In particular, we show that an SiH$_{3}$ radical can insert into strained surface Si-Si dimer bonds, abstract surface H through an Eley-Rideal mechanism, and passivate surface dangling bonds; these reactions follow exothermic and barrierless pathways that lead to a temperature-independent growth rate in agreement with experimental measurements. We also identify a thermally activated surface H abstraction process, in which the SiH$_{3}$ radical diffuses through overcoordinated surface Si atoms until it encounters a favorable site for H abstraction; the diffusion and H-abstraction steps have commensurate activation barriers. This mechanism explains partly the reduction of the film H content at elevated substrate temperatures. [Preview Abstract] |
Thursday, March 24, 2005 11:39AM - 11:51AM |
V20.00003: Surface Reconstruction of Si(111)-7x7/Ge(111)-5x5 and Magic Cluster Formation D. Psiachos, M. J. Stott The 5x5 reconstruction of Ge(111) occurs only due to strain from the underlying Si(111)-7x7 substrate. Recent studies have shown the presence ``magic clusters" on Si(111)-7x7 and we are interested in studying the mechanism of self-assembly and stability of indium clusters on Ge(111)-5x5. We report results of our first principles plane wave density functional calculations for the Ge(111)-5x5 surface, including the electronic properties of this structure and theoretical predictions of indium magic cluster formation. [Preview Abstract] |
Thursday, March 24, 2005 11:51AM - 12:03PM |
V20.00004: DFT Simulations of the Growth of Thin Films on Si and Ge Collin Mui, Ye Xu, Charles Musgrave Germanium is currently being explored as a replacement for silicon for future CMOS technology nodes. Consequently, how its surface chemistry affects thin film growth on Ge substrates is of both technological and scientific interest. We have predicted the mechanisms and energetics for several reactions for growing thin films on both the Ge and Si (100)-2x1 surfaces including ALD of Hafnium oxide, H desorption, oxidation by water, and hydrogenperoxide, nitridization by ammonia and various organic functionalizations. From these results we have determined the various characteristics of the reactivity of various species towards both Si and Ge and have begun establishing guidelines for understanding the reactivity of Ge surfaces versus Si. For example, although it is commonly suggested that processing of Ge devices can be done at lower temperatures, often higher activation barriers are found for Ge and thus processing temperatures are often higher than for Si. Furthermore, intermediates and products of reaction on Ge are less stable than their analogues on Si. Finally, we have found that Ge in general tends to be more selective in its reactivity than Si because of its typically higher activation barriers. [Preview Abstract] |
Thursday, March 24, 2005 12:03PM - 12:15PM |
V20.00005: Oxidation induced internal exoemission in thin film Mg/Si Schottky diodes Hermann Nienhaus, Stefan Glass When thin film Mg/p-Si(111) diodes are exposed to oxygen molecules an internal reverse current is observed. Such chemicurrents are due to electron-hole pairs created by the chemical reaction at the metal surface and indicate the non-adiabatic character of chemical energy dissipation. The transient of the current represents the kinetics of the Mg oxidation process and may be explained by a nucleation-and-growth model. Two types of Mg/Si diodes with different homogeneous Schottky barrier heights of 0.7 and 0.8 eV, respectively, could be fabricated by modifying the Mg/Si interface. A dependence between the homogeneous barrier height and the sensitivity of the diode to detect the chemically induced hot charge carriers can be demonstrated. The energy distribution of the internally excited charge carriers deduced from the data may be described by a Boltzmann exponential function with an effective electron temperature of approximately 1600 K. [Preview Abstract] |
Thursday, March 24, 2005 12:15PM - 12:27PM |
V20.00006: \textit{Ab initio} molecular dynamics study of the dissosciation of H$_2$ on clean and hydrogen-covered Pd(100) Axel Gross The adsorption of H$_2$ on the clean and hydrogen-covered Pd(100) surfaces has been studied by \textit{ab initio} molecular dynamics simulations using density functional theory (DFT). In order to obtain statistically meaningful sticking probabilities as a function of the kinetic energy and the hydrogen coverage, about 150 trajectories of H$_2$ impinging on the surface have been computed per kinetic energy and hydrogen coverage. I have particularly focused on the role of the substrate atoms in the adsorption process. On the clean surface, the sticking probability is hardly influenced by the recoil of the metal substrate atoms because of the large mass mismatch. However, if the H$_2$ molecules impinge on a hydrogen-covered surface, the recoil of the preadsorbed hydrogen atom causes an significant enhancement of the sticking probability. This leads to a H$_2$ sticking probability which is much larger than what one would expect from a simple Langmuir kinetics picture. [Preview Abstract] |
Thursday, March 24, 2005 12:27PM - 12:39PM |
V20.00007: Kinetics of PdO reduction with hydrogen J.I. Avila, R.J. Matelon, U.G. Volkmann, A.L. Cabrera Kinetics of the reduction of PdO films with hydrogen was studied by changes of resistance of the PdO film, changes in photoconductivity and changes in reflectivity to visible light. The PdO films are obtained by oxidizing 50 nm Pd film- e-beam evaporated -over glass substrates. The oxidation is obtained by exposing the Pd films in 50{\%} O$_{2}$-50{\%} Ar atmospheres at 700 $^{\circ}$C for 48 hrs. The PdO films are place inside a chamber with a glass window where a controlled atmosphere is sustained and electrical feedthroughs are available. Electrical connections are made to the films in order to monitor its resistivity during reduction. A red LASER is used to measure p-reflectivity from the sample during reduction. The reduction takes place immediately when exposing the films to 4 Torr of hydrogen at room temperature. [Preview Abstract] |
Thursday, March 24, 2005 12:39PM - 12:51PM |
V20.00008: Adsorption and dissociation of water at the rutile TiO2(011)-2x1 surface Thomas Beck, Matthias Batzill, Andreas Klust, Ulrike Diebold, Cristiana Di Valentin, Antonio Tilocca, Annabella Selloni Surface science studies of rutile TiO2 are increasing in popularity. There is evidence that TiO2's reactivity has strong dependencies of the reactivity on the surface orientation. Here studies on the (011) surface are presented and we demonstrate that this surface exhibits a higher activity towards the dissociation of water than the (110) surface. We recently demonstrated that the (011) reduces its energy by forming a 1x2 reconstruction.1 This surface exhibits Ti=O (titanyl) groups that makes it unique compared to any other TiO2 surface. Variable temperature UPS and room temperature STM together with molecular dynamics (MD) and DFT calculations have been employed to show that water dissociates at the perfect (011)-1x2 surface at temperatures below $\sim $300 K, forming submonolayer coverage of hydroxyl groups. A mixed monolayer of dissociated and molecular adsorbed water is obtained below $\sim $150 K. Water dissociating on the defect free (011) surface is in stark contrast to the (110) surface where dissociation is mediated by oxygen vacancies only. MD calculations indicate that the reduced distance between the H-atom of the adsorbed water and the surface oxygen atom at the (011) surface compared to the (110) surface facilitates dissociation. 1 T.J. Beck et al. Phys. Rev. Lett. 93, 036104 (2004). [Preview Abstract] |
Thursday, March 24, 2005 12:51PM - 1:03PM |
V20.00009: Exclusively Linear Diffusion of 9,10-Dithioanthracene on an Isotropic Cu(111) Surface Ki-Young Kwon, Robert Perry, Greg Pawin, Erick Ulin-Avila, Kin Wong, Ludwig Bartels One-dimensional diffusion of adsorbates is a common feature of anisotropic surfaces such as the (110) and (211) cuts of an fcc crystal. The technologically-relevant lowest energy (111) surfaces of coinage metals have sixfold symmetry in the top layer and, hence, generally allow diffusion of adsorbates along more than one direction. Here, we report on the diffusion of individual 9,10-dithioanthracene (DTA) molecules on Cu(111). DTA adsorbs with the aromatic system lying flat on the substrate and with both sulfur atoms anchored at hollow sites of the substrate. In variable-temperature STM studies, we find that it diffuses exclusively in the direction in which it aromatic moiety happened to adsorb. We neither find rotation of the molecule in the surface plane nor diffusion perpendicular to the aromatic axis of the molecule. We investigated the dynamics of the one-dimensional diffusion of DTA and find an energy barrier of 114 meV. DTA molecules form well-ordered molecular rows. By temperature dependent measurements of the spontaneous abstraction of DTA molecules from such row, we could determine that the intermolecular attraction is 40 meV in addition to the diffusion barrier of the isolated molecule. At higher coverage, DTA forms square islands of (3,-1,3,4) symmetry on Cu(111). [Preview Abstract] |
Thursday, March 24, 2005 1:03PM - 1:15PM |
V20.00010: The initial oxidation mechanisms of Cu and Cu-Au by in situ UHV-TEM Judith Yang, Guangwen Zhou, Liang Wang, Jeff Eastman A surprising paucity of information concerning the transient oxidation stages, from the formation of the initial oxide to the growth of the thermodynamically stable oxide, still exists. As engineered materials approach the nanometer regime, understanding environmental stability at this scale will become crucial. To bridge this gap, we are visualizing the initial oxidation of copper by \textit{in situ} ultra-high vacuum (UHV) transmission electron microscopy (TEM). The nucleation and growth of Cu$_{2} $O due to oxidation of single crystal Cu and Cu-Au films were monitored at various temperatures and pressures. Oxidation potentially involves surface diffusion, nucleation and growth, which is strikingly similar to epitaxial deposition. Heteroepitaxial concepts are surprisingly synergistic with the nucleation and growth of Cu$_{2}$O. For pure Cu films, oxygen surface diffusion is the dominant mechanism, whereas for Cu-Au alloys, initially surface diffusion is the dominant mechanism but then an entirely different mechanism of Cu diffusion through an Au-rich layer becomes the rate-controlling step for Cu2O growth. [Preview Abstract] |
Thursday, March 24, 2005 1:15PM - 1:27PM |
V20.00011: Probing of Charging and Reactivity of Supported Gold Nanoclusters: Bonding of CO to Gold Octamers Adsorbed on Magnesia Bokwon Yoon, Hannu H\"akkinen, Uzi Landman, A. W\"{o}rz, J.-M. Antonietti, S. Abbet, K. Judai, U. Heiz Gold octamers supported on Mg(001) are the smallest clusters to catalyze the low-temperature oxidation of CO to carbon dioxide, while clusters deposited on close-to-perfect magnesia surfaces remain chemically inert. Charging of the supported clusters plays a key role in promoting their chemical activity. Direct evidence for enhanced electron transfer from the F-centers to the anchored clusters is provided here through infra-red measurements, under reaction conditions, of the stretch vibration of CO adsorbed on mass-selected gold octamers soft- landed on close-to-perfect or defect-rich MgO(001). The experiments agree with quantum ab-initio calculations that predict a red-shift of the C–O vibration when the molecules are adsorbed on gold octamers anchored to F-centers. This results from electron back-donation to the CO anti-bonding orbital that weakens the intra-molecular bond. [Preview Abstract] |
Thursday, March 24, 2005 1:27PM - 1:39PM |
V20.00012: Adsorption of Argon on a surface with nano-scaled pores. Silvina Gatica, Nathan Urban, Milton Cole We have performed Grand Canonical Monte Carlo simulations of the adsorption of Argon on a flat surface which is pierced by a periodic array of perpendicular, cylindrical pores, which are several nanometers wide and deep. We have studied various substrates and geometrical arrangements of the pores. The adsorption isotherms show a strong dependence on strength of the substrate attraction and the depth of the pores. For example, in the case of strongly attractive substrates, a film grows on the internal surface of the hole and outer surface before capillary condensation occurs. On the contrary, if the substrate is weakly attractive, no layer growth was observed prior to capillary condensation, if it occurs. [Preview Abstract] |
Thursday, March 24, 2005 1:39PM - 1:51PM |
V20.00013: Adhesion Enhancement of Amino-functional Organosilanes for Polystyrene Thin Films Sung-Hwan Choi, Bi-min Zhang Newby Amino-functional organosilanes, such as 3-amino-propyltriethoxylsilane (APTES), have been utilized widely as adhesion promoters. Recently, APTES was also found to have the ability to prevent polymer thin films from separating, or dewetting, from substrates. Common reasons, such as forming chemical bonds with --NH2 or electrostatic interactions between APTES molecules and the materials of interest, cannot be applied to explain the strong adhesion enhance of polymeric materials observed on the APTES surfaces. Since APTES contains the active terminal --NH2 group, which can interact with the multiple active head groups and the substrate, the APTES molecules likely link together and form multi-layered networks when grafted to a substrate. The multi-layered networks may result in large enough loops for the polymer chains to slide into and create entanglement/interlocking, thus enhancing the adhesion or stability of the polymer films upon subjecting to a separation force. The hypothesis was verified using dewetting studies of polystyrene thin films from APTES surfaces. [Preview Abstract] |
Thursday, March 24, 2005 1:51PM - 2:03PM |
V20.00014: Fractal growth of ultrathin films of p-sexiphenyl on alkali halide substrates Edward Kintzel, Kenneth Herwig, Peter Pfeifer An initial study of the fractal growth of ultrathin films of $p$-sexiphenyl molecules as incipient infinite percolation clusters has been carried out using fluorescence microscopy. Individual films of nearly equal mass thickness were grown onto KBr(001), KCl(001), and NaCl(001) substrates by vapor deposition. The structure of percolation clusters can be well described by fractal concepts. In particular, we can determine the fractal dimension d$_{f}$. Analysis of the fluorescence images provides evidence that as the lattice constant of the alkali halide substrate decreases, the fractal dimension of the adsorbed film increases. [Preview Abstract] |
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V20.00015: Hydrogen Molecule Adsorption and Dissociation on Plutonium (111) Surface M. N. Huda, A. K. Ray Hydrogen molecule adsorption on plutonium (111) surface has been studied in detail using the generalized gradient approximation to density functional theory. All calculations have been performed at the non-spin-polarized and spin-polarized levels of theory using the DMol3 suite of programs [1]. Weak molecular adsorptions with a layer by layer alternate spin arrangement of the plutonium atoms were observed. Horizontal approaches on the top site both with and without spin polarization were found to be the most favorable molecular adsorption sites. For dissociative adsorption, the most favorable dissociation channel needs activation energies of 0.216eV and 0.305eV at non-spin-polarized and spin-polarized levels, respectively, with considerably higher adsorption energies than those of the molecular cases. In general, adsorption of hydrogen molecule pushes the Pu 5f band away from the Fermi level. Charge transfer to the hydrogen atoms for dissociative adsorption is larger than that of molecular adsorption and the ionic part of H-Pu bonding contributes along with the covalent part due to Pu 5f - H 1s hybridization. [1] B. Delley, J. Chem. Phys. \textbf{92}, 508 (1990); J. Chem. Phys. \textbf{113}, 7756 (2000). [Preview Abstract] |
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