Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session H12: Thin Film Growth and Interfaces |
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Sponsoring Units: DCMP DMP Chair: Michael Scarpulla, University of California, Berkeley Room: Baltimore Convention Center 304 |
Tuesday, March 14, 2006 11:15AM - 11:27AM |
H12.00001: Imaging of ALD Grown W Atoms on $\alpha $-TiO2(110) by X-ray Standing Wave Fourier Inversion Chang-Yong Kim, Jeffrey Elam, Michael Pellin, Dipak Goswami, Steven Christensen, Mark Hersam, Peter Stair, Michael Bedzyk Supported metal oxides are among the most important of catalytic materials systems. However, there is a lack of experimental atomic-scale structural information for describing the relevant interfaces. We combine atomic layer deposition (ALD) and x-ray standing wave (XSW) atomic-imaging to address this challenge. As a first case, we determine the precise registry of W atoms on a rutile TiO2(110) surface. The ALD W layer was formed by first exposing the TiO$_{2}$ surface to disilane (Si$_{2}$H$_{6})$ and then to tungsten hexafluoride (WF$_{6})$. The direct-space image reveals that W occupies the Ti-site that would be occupied by Ti if the bulk structure were extended above surface. The tungsten atoms are vertically shifted upward from the ideal Ti-site location by +0.23 {\AA}. These XSW measurements of ALD heterolayer growth clearly show for the first time that ALD conformal layers can be highly coherent with the substrate lattice. The ALD method for supported catalyst material preparation provides a bridge over the ``materials preparation gap'' that typically separates practical, high-surface-area and single crystal model catalytic materials. [Preview Abstract] |
Tuesday, March 14, 2006 11:27AM - 11:39AM |
H12.00002: Beating Effects on X-ray Reflectivity for Pulsed Laser Deposition of TiN Yen-Ru Lee, Aaron Gray, Peter Czoschke, Jon Tischler, Hawoong Hong, Tai-Chang Chiang We grew TiN on sapphire $(0001)$ substrates by pulsed laser deposition (PLD). The growth was monitored, in situ, in real time by x-ray diffraction at the $(0, 0, \frac{1}{2})$ anti-Bragg position using a synchrotron beam. The experiment was carried out at the UNICAT undulator beamline at the Advanced Photon Source. The measured x-ray intensity shows atomic-layer-resolved oscillations superimposed on a beating pattern with changing wavelength. Numerical modeling shows that the results can be well described in terms of an evolving island height distribution. Implications in regard to film structure and morphology will be discussed. [Preview Abstract] |
Tuesday, March 14, 2006 11:39AM - 11:51AM |
H12.00003: Characterization of Graphene Films and Ribbons on Silicon Carbide Tianbo Li, Asmerom Ogbazghi, Xuebin Li, Zhimin Song, Claire Berger, Walt De Heer, Phillip First Ultrathin graphite films, 1-6 graphene layers thick, were grown on both the Si-terminated (0001) and C-terminated ($000\bar {1})$ faces of 4H- and 6H-SiC via thermal desorption of silicon in UHV. Through investigations of the thinnest films (monolayer graphene) by LEED and STM, we show the existence of a complex $\sqrt 3 \times \sqrt 3 R30^{\circ}$reconstructed layer between the lowest graphene layer and the SiC substrate. The $6\sqrt 3 \times 6\sqrt 3 R30^{\circ}$ LEED pattern obtained after annealing above $1250^{\circ}C$ can be explained predominantly by kinematic scattering from the slightly $6\times 6$ corrugated graphene, from the reconstruction, and from the SiC substrate. Low-order double-scattering is required to explain all of the diffraction features. LEED and STM on graphene ribbons patterned from these films (80-400 nm width) show that graphene layers remain intact after photolithography and plasma etching. [Preview Abstract] |
Tuesday, March 14, 2006 11:51AM - 12:03PM |
H12.00004: A new work function measurement technique allowing simultaneous material deposition, and reflectivity measurements. B. J. Hinch, A.V. Ermakov, E. Z. Ciftlikli, S. E. Sysoev, I. G. Shuttleworth We report on the application of a novel technique for relative work function measurements, of single crystal surfaces, employing grazing incidence electron reflection. This technique cannot suffer from photo or e- beam induced surface damage effects, and yet does enable free access to the surfaces for simultaneous deposition/evaporation, and/or application of other surface characterization methods. We discuss the optimization of experimental parameters in this method, and demonstrate its application in concurrent measurements of helium atom reflectivity and work function changes, taking place during oxygen exposure of a Cu(001) surface. Observed work function changes reproduce well the trends seen in earlier polycrystalline film measurements. In addition, the work function changes indicate structural changes that are not apparent in helium atom scattering at oxygen exposures in excess of $\sim $50L. [Preview Abstract] |
Tuesday, March 14, 2006 12:03PM - 12:15PM |
H12.00005: Si/Cu Interface Structure and Adhesion Xiao-Gang Wang, John Smith An $ab$ $initio$ investigation of the Si(111)/Cu(111) interfacial atomic structure and adhesion is reported [1]. Misfit dislocations appear naturally, as do hcp interfacial silicide phases that vary with temperature. The silicides form in the interface even at relatively low temperatures. These results are consistent with available experimental data. \newline \newline [1] Xiao-Gang Wang, John Smith, Physical Review Letters 95, 156102 (2005). [Preview Abstract] |
Tuesday, March 14, 2006 12:15PM - 12:27PM |
H12.00006: Modeling the formation of nanoscale pores in metal oxides during anodization Christine Sample, Alexander Golovin A theory of the formation of nanoporous metal oxides grown by anodization is developed. Linear stability analysis of an oxide layer is performed for the case of field-dependent conductivity, and parameter regions are determined corresponding to the onset of instability and the growth of pores. It is shown that competition between the destabilizing effect of field-assisted dissolution and the stabilizing effect of surface energy provides the selection mechanism of the pore diameter. The weakly nonlinear evolution of the oxide layer is shown to be governed by the Kuramoto-Sivashinsky equation whose solutions exhibit spatio-temporal chaos. A system of strongly nonlinear long-wave equations describing the evolution far from the instability threshold is described. It's numerical solution illustrates the formation of deep pores. [Preview Abstract] |
Tuesday, March 14, 2006 12:27PM - 12:39PM |
H12.00007: Local field distribution near corrugated interfaces: Green function formalism versus effective medium theory Chun Wing Choy, Jun Jun Xiao, Kin Wah Yu The Green function formalism (GFF) has been established recently [1,2] to study the local field electric field distribution near a periodic interface separating two homogeneous media of different dielectric constants. In GFF, the integral equations can be solved conveniently because of the existence of an analytic expression for the kernel (Greenian). However, due to a severe singularity in the kernel, the formalism was formerly applied to compute the electric fields away from the interface region [1,2]. In this work, we have been succeeded in extending GFF to compute the electric field inside the interface region, by taking advantage of a sum rule. To our surprise, the strengths of the electric fields are quite similar in both media across the interface, despite of the large difference in the dielectric constants. Moreover, we propose a simple effective medium approximation (EMA) to compute the electric field inside the interface region. We show that EMA can indeed give an excellent description of the electric field except near a geometric resonance. \newline \newline [1] K. W. Yu and Jones T. K. Wan, Comput. Phys. Commun. 142, 368 (2001). \newline [2] H. F. Tam and K. W. Yu, Annual APS March Meeting, abstract Q31.009 (2002). [Preview Abstract] |
Tuesday, March 14, 2006 12:39PM - 12:51PM |
H12.00008: Textbook adsorption at non-textbook adsorption sites: alkali atoms on alkali halide surfaces Bo Li, Angelos Michaelides, Matthias Scheffler Density-functional theory (DFT) and second order M{\o}ller-Plesset perturbation theory (MP2) calculations for F, Cl and Br adatoms on the (100) surfaces of NaF, NaCl and NaBr indicate that on these surfaces the halide adatoms adsorb preferentially above halide substrate sites, rather than above Na substrate sites as might be anticipated. Analysis of the electronic structures in each system reveals that this hitherto unheard of adsorption mode is stabilized by the formation of perfectly conventional covalent bonds between the halide adatoms and the halide substrate atoms. This finding has implications to, for example, alkali-halide crystal growth, atomic manipulation on insulator surfaces, and the design of accurate empirical potentials to describe insulator-adatom interactions. [Preview Abstract] |
Tuesday, March 14, 2006 12:51PM - 1:03PM |
H12.00009: Adsorption and desorption of hydrogen on Cu(111) and Xe-covered Cu(111). Yiyan Fei, Xiangdong Zhu Using an optical ellipsometry technique to monitor adsorbate coverage in-situ, we studied activated adsorption of filament-heated molecular hydrogen on Cu(111) and the isothermal desorption of hydrogen adatoms from Cu(111). The adsorption follows a zeroth order kinetics such that the net sticking probability, though small, is independent of the hydrogen adatom coverage until a saturation coverage is reached. The desorption follows a second-order kinetics with an activation energy of 0.63 eV and a pre-exponential factor of 1$\times $10$^{9}$/s. A pre-adsorbed monolayer of Xe atoms on Cu(111) prohibits subsequent adsorption of filament-heated molecular hydrogen, making physisorbed Xe useful templates for spatial patterning of hydrogen adatom density on Cu(111). [Preview Abstract] |
Tuesday, March 14, 2006 1:03PM - 1:15PM |
H12.00010: Pattern formation of hydrogen adatom density on Cu(111) using patterns of xenon adatoms as templates. Xiangdong Zhu, Yiyan Fei We formed grating-like patterns of hydrogen adatom density on Cu(111) by using complementary patterns of xenon adatom density as the template. The template was pre-formed by laser-induced thermal desorption of a Xe monolayer on Cu(111) using the interference pattern of two coherent laser pulses. The ``patterned'' surface was subsequently exposed to hot hydrogen molecules which dissociate and adsorb preferentially on the uncovered part of the surface. We removed the Xe template by laser-induced thermal desorption which left behind a grating-like hydrogen density pattern intact on Cu(111). By following the evolution of hydrogen density gratings on Cu(111) from 153 to 183 K with linear optical diffraction, we found that the diffusion of hydrogen atoms on Cu(111) in this temperature range was characterized by an activation energy barrier E$_{diff}$ = 6.4 kcal/mol (or 279 meV) and a pre-exponential factor D$_{0}$ = 2.0 $\times $ 10$^{-3}$ cm$^{2}$/sec. [Preview Abstract] |
Tuesday, March 14, 2006 1:15PM - 1:27PM |
H12.00011: CRDS Detection of HS Radical in HFCVD Diamond Thin Film Madalina Buzainau, Vladimir Makarov, Gerardo Morell, Brad Weiner In the present study, the HS radical was detected using the Cavity Ring Down Spectroscopic (CRDS) method in hot filament diamond CVD for the CH$_{4}$(0.4{\%})/H$_{2}$ mixture doped by H$_{2}$S (400 ppm). The HS radical was detected both by the 0 - 0(A ---X) and 1 - 0(A --- X) transitions. The absolute absorption density spectrum of the HS radical was obtained as well as the absolute HS concentration estimation. This study proofs that the HS radical is an intermediate produced during the diamond CVD synthesis. [Preview Abstract] |
Tuesday, March 14, 2006 1:27PM - 1:39PM |
H12.00012: The CRDS method application for study of the gas-phase processes in the hot CVD diamond thin film. Vladimir BuzaianuMakarov, Arturo Hidalgo, Gerardo Morell, Brad Weiner, Madalina Buzaianu For detailed analysis of problem related to the hot CVD carbon-containing nano-material growing, we have to detect different intermediate species forming during the growing process as well as investigate dependences of concentrations of these species on different experimental parameters (concentrations of the CJH$_{4}$, H$_{2}$S stable chemical compounds and distance from the filament system to the substrate surface). In the present study, the HS and CS radicals were detected using the Cavity Ring Down Spectroscopic (CRDS) method in the hot CVD diamond thin film for the CH$_{4}$(0.4 {\%}) + H$_{2}$ mixture doped by H$_{2}$S (400 ppm). The absolute absorption density spectra of the HS and CS radicals were obtained as a function of different experimental parameters. This study proofs that the HS and CS radicals are an intermediate, which forms during the hot filament CVD process. The kinetics approach was developed for detailed analysis of the experimental data obtained. The kinetics scheme includes homogenous and heterogenous processes as well as processes of the chemical species transport in the CVD chamber. [Preview Abstract] |
Tuesday, March 14, 2006 1:39PM - 1:51PM |
H12.00013: A Density Functional Study of the Americium (001) Surface Da Gao, Asok Kumar Ray Electronic structure properties of the fcc americium (001) surface, modeled by periodically repeated fcc Am (001) surface slabs, have been studied with the full-potential linearized augmented plane wave (FP-LAPW) method. The alternative basis set APW+lo is used inside the atomic spheres for chemically important orbitals that are difficult to converge, whereas LAPW is used for others. The core states are treated fully relativistically and for valence states, two levels of treatments are implemented: (1) a scalar relativistic scheme including the mass-velocity correction and the Darwin s-shift and (2) a fully relativistic scheme with spin-orbit coupling included in a second variational treatment using the scalar-relativistic eigenfunctions as basis. Our results indicate that the ground state of Am (001) surface is anti-ferromagnetic. The quantum size effects in the surface energies and the work functions of the (001) fcc americium ultra thin films (UTF) have been examined up to seven layers. In addition, the Am (001) surface properties are compared with our earlier study of the Am (111) surface. [Preview Abstract] |
Tuesday, March 14, 2006 1:51PM - 2:03PM |
H12.00014: A Density Functional Study of Carbon Monoxide Adsorption on (100) Surface of $\gamma $-Uranium Asok Ray, Pratik Dholabhai Adsorption of the CO molecule on the $\gamma $-U (100) surface has been studied at both non-spin-polarized and spin-polarized levels using the generalized gradient approximation to density functional theory (GGA-DFT) with Perdew and Wang (PW) functionals. The bridge position with the vert2 approach is found to be most favorable site with a chemisorption energy of 2.93eV for the non-spin-polarized case, and 3.19eV for the spin-polarized case. The distances of the carbon atom from the uranium surface are found to be 1.59{\AA} and 1.72{\AA} for the non-spin-polarized and spin-polarized cases, respectively. The inter-atomic distances between the carbon and oxygen atoms for the most favorable position are found to be 1.13{\AA} and 1.21{\AA} for the non-spin-polarized and spin-polarized cases, respectively. The magnetic moment for the most favorable site is found to be 0.04$\mu _{B}$ per atom. A significant charge transfer from the first layer of the uranium surface to the carbon and oxygen atoms is found to occur, implying that the bonding is partly ionic. Significant hybridization between the U $5f $and CO 2$p$ orbitals are also observed. This work is supported by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U. S. Department of Energy (Grant No. DE-FG02-03ER15409) and the Welch Foundation, Houston, Texas (Grant No. Y-1525). [Preview Abstract] |
Tuesday, March 14, 2006 2:03PM - 2:15PM |
H12.00015: A Density Functional Study of Atomic Oxygen and Carbon Adsorptions on the (100) Surface of $\gamma $-Uranium Pratik Dholabhai, Asok Ray Atomic oxygen and carbon adsorptions on the $\gamma $-U (100) surface have been studied using the generalized gradient approximation to density functional theory (GGA-DFT) with Perdew and Wang (PW) functionals. Different chemisorption sites at both non-spin-polarized and spin-polarized levels of theory have been thoroughly investigated. For O adsorption, the bridge position is the most favorable site with chemisorption energies of 7.887eV and 7.965eV for the non-spin-polarized and spin-polarized cases, respectively. The distances of the O adatom from the U surface are found to be 1.19{\AA} and 1.22{\AA}, respectively. For C adsorption, the center position is the most favorable site with a chemisorption energy of 7.816eV for the non-spin-polarized case, and 7.895eV for the spin-polarized case. The distances of the C adatom from the U surface are 0.62{\AA} for the non-spin-polarized and 0.52{\AA} for the spin-polarized cases. A study of the density-of-states (DOS) for O and C adsorbed uranium surfaces shows that the hybridization between U 5$f$ orbitals and the O and C 2$p$ orbitals is weak and the bonding is primarily ionic. Comparisons with published results will be presented. [Preview Abstract] |
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