Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session A40: Focus Session: Morphology and Evolution at Surfaces: Structure and Organics |
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Sponsoring Units: DMP DCMP Chair: Fulvio Ratto, Universite' de Que'bec Room: LACC 408A |
Monday, March 21, 2005 8:00AM - 8:12AM |
A40.00001: Reconstruction of the (0001) Surface of Graphite near Step Edges Edward Kintzel, Hong Luo We report on the surface reconstruction of the (0001) surface of graphite near step edges using scanning tunneling microscopy (STM). The surface reconstruction of the (0001) surface has the well studied triangular symmetry. Further reconstruction near point defects have been studied both experimentally and theoretically. Our studies of the graphite surface near step edges shows two types of further reconstructions of the triangular lattice. The symmetry of the first (Type I) surface reconstruction remains triangular, but rotated 30\r{ } from the original surface lattice. The periodicity of the observed pattern is 1.73 times the original surface reconstruction, and is equal to 3a (a is the bond length in graphite). The second (Type II) has hexagonal symmetry with a periodicity 1.73 times the C-C bond length. The relationship between the two structures will be discussed. The effects can be qualitatively expected based on the difference in atomic configuration near and far away from step edges. [Preview Abstract] |
Monday, March 21, 2005 8:12AM - 8:24AM |
A40.00002: Relative stability of Si surfaces: a first-principles study Guang-Hong Lu, Minghuang Huang, Feng Liu, Martin Cuma Surface energies of solid surfaces are often calculated by the supercell slab technique subtracting the bulk energy from the total energy of supercell. However, there exists a common mistake that the same bulk atom energy obtained by a separate bulk calculation is used for different surface orientations and slab sizes, which makes the surface energies divergent and the comparison of their relative stability unreliable. The more accurate way to determine the atom bulk energy is to extract the atom bulk energy and surface energy simultaneously by fitting slab total energy as a function of atom number in the slab. Here, using this method, we have calculated surface energies of Si (001), (110), (111), and (113) surfaces with different reconstructions systematically using first-principles total-energy method. The relative stability of these Si surfaces are shown in decreasing order as (111), (001) to (113) at low temperature, and (001), (113), (110) to (111) at high temperature, respectively. Si(113) is found to be a stable surface at both high and low temperature in spite of its high index. [Preview Abstract] |
Monday, March 21, 2005 8:24AM - 8:36AM |
A40.00003: Atomic structure of the GaAs(001)-$c(4\times4)$ surface Peter Kratzer, Evgeni Penev, Matthias Scheffler The atomic structure of the $c(4\times4)$ reconstruction, formed on the GaAs(001) surface under high arsenic overpressure, has recently been attracting renewed interest. This has lead to a revision of the commonly accepted $c(4\times 4)$ structural model but a definitive understanding of the driving force for the newly proposed structure was lacking. Targeting the later problem, the talk will present a state-of-the-art theoretical study of the GaAs(001)-$c(4\times4)$ surface employing \textit{ab initio} atomistic thermodynamics based on density-functional theory calculations. We shall demonstrate that in a range of stoichiometries, between those of the conventional three As-dimer and the new three Ga-As dimer model, there exists a diversity of atomic structures featuring Ga-As heterodimers, \emph{driven by surface configurational entropy}. These results fully explain the experimental scanning tunneling microscopy images and are likely to be relevant also to the $c(4\times4)$-reconstructed (001) surfaces of other III-V semiconductors. [Preview Abstract] |
Monday, March 21, 2005 8:36AM - 8:48AM |
A40.00004: In-Rich Reconstructions of the InSb(100) Surface and Chemisorption of Lithium on the c (8x2) Surface - An Ab Initio Study.* Sridevi L. Ganapathy, A. K. Ray The local density approximation to density functional theory (LDA-DFT) has been used to study the different possible relaxations and reconstructions of the In-rich InSb (100) surface. The surfaces are modeled by a three-layer surface with alternating In and Sb atoms and In atoms in the first layer. Hydrogen atoms are used to saturate the dangling bonds of In atoms in the bottom layer to simulate the semi-infinite effect of the surface. Periodic boundary conditions with pseudo-potentials have been used and all simulations have been carried out with the Gaussian 03 suite of programs.$^{1}$ We will report on various electronic and geometric structure properties of the possible (1x2), (2x1), (4x2) and c (8x2) reconstructed surfaces. We will also report our studies on adsorption of Li in the most symmetric sites of c (8x2) surface. Details of the chemisorption process, such as the adsorption energies, adatom separation distances, charge distributions, density of states, will be presented and compared with available results in the literature. Possible changes in the InSb surface due to Li adsorption will also be discussed in detail. $^{\ast }$Work supported, in part, by the Welch Foundation, Houston, Texas (Grant No. Y-1525). $^{1 }$\textit{Gaussian03}, M. J. Frisch \textit{et al}., Gaussian Inc., Pittsburgh, PA. [Preview Abstract] |
Monday, March 21, 2005 8:48AM - 9:00AM |
A40.00005: SIESTA study of c-GaN(001)-4x1 surface reconstruction: Tetramers andtheir STM images. Nancy Sandler, Hamad Al-Brithen, Muhammad Haider, Arthur Smith, Pablo Ordejon Recent STM, STS and RHEED studies [1] on cubic GaN(001), grown using rf MBE under Ga-rich conditions, have revealed a surface structure consistent with predicted tetramer formation [2,3]. STM images reveal a surface consisting of rows aligned along the [110] direction with a periodicity along the $[1{\underline 1}0]$ direction of about $12.8 \AA$. STS measurements indicate the semiconducting nature of the surface and RHEED patterns of the surface provide further evidence of a periodicity consistent with tetramer formation. We report on a first principle study of this particular surface reconstruction using the SIESTA code [4], a self-consistent density functional method using standard norm-conserving pseudo-potentials and a flexible numerical linear combination of atomic orbitals basis set. Band structure calculations are in good agreement with previously reported results and the STM images obtained reproduce experimental observations. [1] H. Al-Brithen, M. Haider, A. Smith, N. Sandler and P. Ordejon. Submitted to PRL. [2] Neugebauer et al. Phys. Rev. Lett. 80, 3097 (1998) [3] Feuillet et al. Appl. Phys. Lett. 70, 24 (1997) [4] D. Sanchez-Portal, P. Ordejon, E. Artacho, and J.~M. Soler, Int. Journ. of Quant. Chem. 65, 453 (1999). [Preview Abstract] |
Monday, March 21, 2005 9:00AM - 9:12AM |
A40.00006: First Principles Study of Dihydride Chains on H-Terminated Si(100)-2$\times$1 Surface Yuji Suwa, Masaaki Fujimori, Seiji Heike, Yasuhiko Terada, Tomihiro Hashizume STM observation of H-terminated Si(100)-2$\times$1 surface often shows the existence of a single dihydride-chain structure parallel to a step. That chain is located near the S$_{\rm B}$ step, but away from it more than one Si-dimer's distance. In order to discuss the mechanism of the formation of such a structure, we have performed first principles calculations. As a result, we have found that the ``rebonded'' step edges of the clean Si surface turns into one dihydride chain and a ``non-rebonded'' step edge at the time of hydrogen termination. We have also found that the chain adjacent to the step is energetically less stable than in a distant position from the step. We will show these results and discuss how the dihydride chain changes its position beyond a monohydride-Si-dimer row. This study was performed through Special Coordination Funds for Promoting Science and Technology of the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government. [Preview Abstract] |
Monday, March 21, 2005 9:12AM - 9:24AM |
A40.00007: Application of a direct method to solve the structure of ($\surd $3x$\surd $3)Sb/Au (110) from surface x-ray diffraction measurements Russell Fung, Valentin Shneerson, Somendra Parihar, Harry Johnson-Steigelman, Paul Lyman, Dilano Saldin Lack of phase information in typical x-ray diffraction measurements makes it very difficult to recover the atomic-scale structure of a crystal by direct inversion of the measured amplitudes. We have developed a direct method for surface x-ray diffraction (SXRD) where the aim is to recover the part of the surface structure that is different from the truncated bulk. The iterative algorithm we have developed employs prior knowledge of the truncated bulk structure and alternately satisfies constraints in real and reciprocal space. Here we report on an application of the method to determine the unknown structure of ($\surd $3x$\surd $3)Sb/Au (110) from experimental data. In this application, the direct method is adapted to deal with the presence of four symmetry-related domains. [Preview Abstract] |
Monday, March 21, 2005 9:24AM - 9:36AM |
A40.00008: STM study of azobenzene self-assembly at clean metal surfaces Jongweon Cho, Matthew J. Comstock, Niv Levy, Armen Kirakosian, Michael F. Crommie Azobenzene derivatives form a unique class of photoactive molecules that have potential for nanoscale optical applications. We have examined the self-assembly behavior of azobenzene molecules on the Au(111) and NiAl(110) surfaces using a variable temperature UHV STM. We observe a variety of low-dimensional molecular configurations, some of which can be manipulated with the tip of the STM. These structures are highly temperature dependent and typically require cryogenic operation for stable imaging. [Preview Abstract] |
Monday, March 21, 2005 9:36AM - 9:48AM |
A40.00009: Density-functional study of adsorption of isocyanides on the gold (111) surface. Yulia Gilman, Philip B. Allen Density functional theory (DFT) is used to study how the isocyanides HNC and CH$_{3}$NC attach to the gold (111) surface. Slab calculations are performed for monolayers with 1 molecule per 3 gold atoms coverage. For both molecules a weak binding of about 0.2 eV is found at the top site. No binding is found at other sites. [Preview Abstract] |
Monday, March 21, 2005 9:48AM - 10:00AM |
A40.00010: Synchrotron X-ray Specular Reflectivity Measurements of Dotriacontane Films Adsorbed on a Ag(111) Surface M. Bai, S. Trogisch, H. Mo, H. Taub, S.N. Ehrlich, D. Wermeille, U.G. Volkmann, F.Y. Hansen Alkane films adsorbed on metal surfaces are of interest as model lubricants. To investigate the structure of both solid and liquid films of intermediate-length alkanes, we have conducted a series of x-ray specular reflectivity measurements as a function of temperature on dotriacontane ($n$-C$_{32}$H$_{66}$ or C32) films vapor-deposited on a single-crystal Ag(111) surface in UHV. The initial C32 coverage was sufficient to observe coexistence of a multilayer film with preferentially oriented bulk C32 particles. After heating the samples to remove the bulk particles, we obtained specular reflectivity curves at room temperature consistent with one complete C32 layer followed by partial second and third layers of progressively smaller occupancy. In each layer, the molecules are oriented with their long axis parallel to the surface. We find no evidence of a perpendicular monolayer structure as observed for C32 films deposited from solution.$^{2}$ From heating studies, we determine the melting and desorption temperatures of the first and second C32 layers.$^{ 2}$H. Mo \textit{et al}., Chem. Phys. Lett. \textbf{377}, 99 (2003). [Preview Abstract] |
Monday, March 21, 2005 10:00AM - 10:12AM |
A40.00011: Atomic Force Microscopy Measurements of Topography and Friction in Dotriacontane Films Adsorbed on SiO$_2$ M. Simpson, S. Trogisch, H. Taub, U.G. Volkmann, M. Pino, S.N. Ehrlich, F.Y. Hansen We are continuing Atomic Force Microscopy (AFM) studies of the structure, morphology, and lateral frictional force of dotriacontane ($n$-C$_{32}$H$_{66}$ or C32) films deposited from a heptane solution onto SiO$_{2}$-coated Si(100) wafers. Step heights observed in the topographic images are consistent with layer thicknesses inferred from synchrotron x-ray$^{2}$ reflectivity scans; i.e., the first one or two layers of the film grow with the long molecular axis parallel to the surface followed by a partial layer containing perpendicular molecules. Our AFM results extend our x-ray studies in several ways: besides single perpendicular layers, we observe islands consisting of multiple perpendicular layers. We are also able to determine the spatial extent of the perpendicular layers and the location with respect to them of preferentially oriented bulk particles that nucleate at higher coverages. The frictional forces measured correlate with topographic features: we measure the same force on top of the perpendicular layers as on top of the bulk particles. This force is smaller than that on the parallel layers and the bare SiO$_{2 }$surface. $^{2}$H. Mo \textit{et al}., Chem. Phys. Lett. \textbf{377}, 99 (2003). [Preview Abstract] |
Monday, March 21, 2005 10:12AM - 10:24AM |
A40.00012: Furan Decomposition Mechanism on Vicinal Pd(111) Studied by STM and DFT A. Loui, C. Y. Fong, S. Chiang We have used scanning tunneling microscopy to investigate the behavior of furan (C$_{4}$H$_{4}$O) adsorbed on stepped Pd(111) at 199 and 225 K, as well as aspects of its decomposition after heating to a maximum temperature of 415 K. Studies conducted on two substrates with relatively narrow and wide terraces show that furan preferentially adsorbs at step edge sites on both surfaces, while evidence of molecular diffusion is seen only on the narrower vicinal planes. After heating to 288 K, 300 K, and 415 K, evidence of reaction can be observed in occupied-states STM images. Our observations support a furan decomposition mechanism wherein the heterocycle preferentially adsorbs and reacts at upper step edge sites. Ab initio calculations based on Hohenberg-Kohn density functional theory (DFT) have been performed for several high-symmetry adsorption sites of furan on flat Pd(111) and show distortions of both the furan and the top layer of Pd upon adsorption. [Preview Abstract] |
Monday, March 21, 2005 10:24AM - 10:36AM |
A40.00013: Evidence of surface reconstruction during inorganic crystal nucleation under Langmuir monolayers Sumit Kewalramani, Guennadi Evmenenko, Chungjong Yu, Kyungil Kim, Jan Kmetko, Pulak Dutta When a carboxylic acid Langmuir monolayer is spread on aqueous solutions of lead and carbonate ions, an inorganic film nucleates at the organic surface. \textit{In situ} synchrotron x-ray diffraction studies show that the structure is that of hydrocerrusite (2Pb(CO$_{3})\cdot $Pb(OH)$_{2})$, oriented with the hexagonal basal planes parallel to the water surface. In addition, there are peaks from a $\surd $7 $\times \quad \surd $7 superstructure of the hydrocerrusite surface lattice. The organic monolayer unit cell contracts so as to form an epitaxial match with the superstructure. The Bragg rods of the supercell reflections reveal that the surface layer is $\sim $40 {\AA} (5-6 layers) thick, with vertical layer spacing close to that of hydrocerussite. Surface reconstruction phenomena have been observed previously only on very clean surfaces under ultrahigh vacuum [Preview Abstract] |
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