AVS1997 Session SS2-ThM: Facets, Phase Transitions, and Surface Reconstruction

Thursday, October 23, 1997 8:20 AM in Room A1/2-A

Thursday Morning

Start	Invited?	Item
8:20 AM		SS2-ThM-1 Surface Relaxation on the Clean and Hydrogen Covered Ru(0001) Surface Determined by X-ray Diffraction V. Jahns, A.P. Baddorf, D.M. Zehner (Oak Ridge National Laboratory); D. Gibbs (Brookhaven National Laboratory) Several previous LEED investigations have reported a small (2%) contraction of the outermost layer of the clean Ru(0001) surface. After hydrogen adsorption, this contraction is removed and a slight expansion occurs (1-2%).¹ In contrast, first principles theory predicts a much larger interlayer contraction of 4% for the clean surface and a smaller, but still inward, contraction of 1.5% for hydrogen covered Ru(0001).² Motivated by this controversy, we have measured x-ray diffraction intensities of specular and off-specular truncation rods for the clean Ru(0001) surface at temperatures ranging from 300 K to over 2000 K, and for the hydrogen saturated surface at 300 K. Truncation rod intensities for the clean surface at 300 K and 500 K are not significantly different. Since the latter temperature is above the known desorption temperature for hydrogen, contamination of the clean surface can be dismissed. At 300 K a small interlayer contraction is found for both the clean and hydrogen covered surfaces. At the highest temperatures examined, 1890 and 2040 K, evidence for anharmonicity is observed as a large decrease of off-specular intensities and by a reduction of the outermost interlayer relaxation.³ ¹G.Michalk, W.Moritz, H.Pfnür, D.Menzel, Surf. Sci. 129 (1983) 92. ²P.J.Feibelman, Surf. Sci. 360 (1996) 297. ³ORNL is managed by Lockheed Martin Energy Research Corp. under U.S. Department of Energy contract DE-AC05-96OR22464. Work at BNL is supported by DOE DE-AC0276CH00016.
8:40 AM		SS2-ThM-2 Crystallographic Steps and Chemical Adsorption: Fundamental Measurements of H/Vicinal Al(111) E.P. Go, J.E. Reutt-Robey (University of Maryland, College Park) Understanding how crystallographic steps and defects influence the chemisorption properties of surfaces is an important goal. We have studied the chemisorption of atomic hydrogen on Al(111) and Al(332) with vibrational (IRRAS) and diffraction (LEED) measurements to allow for comparison to the results of recent first principles calculations.^1,2 At lower H coverages on both Al(332) and Al(111), the vibrational spectra are dominated by sharp high frequency bands (ν > 1790 cm^-1), indicating exclusive occupation of atop sites at temperatures up to desorption. As predicted by Feibelman¹, H occupies step sites preferentially at lower coverages, with a characteristic vibrational frequency of 1793 cm^-1. Suprisingly, this H-step band is lower in frequency by 65 cm^-1 than the H-terrace band, showing that the H-Al frequency does not scale simply with the coordination of Al substrate atoms. The vibrational properties of these two surfaces change dramatically with increasing H coverage. On both surfaces, a very broad and more intense vibrational feature grows in at 1600 cm^-1, appearing together with the high frequency (atop) bands. Mixed isotope experiments identify this new feature as a monohydride species. As H saturation coverage is approached, the vibrational properties of the two substrates become indistinct, suggesting similar H-induced structures. Indeed, the high frequency of the 1600 cm^-1 band dispels occupation of regular high symmetry sites on the original substrates, supporting the recent predictions² that Al(111) is unstable with respect to H adsorption. Nonetheless, LEED measurements show only an increase in the background diffuse intensity with no apparent facet beams. This structural transformation is apparently limited to length scales of <100 Å, producing rough, but not globally ordered surfaces. A possible role for H-decorated crystallographic steps in these structural transformation is proposed. ¹P. J. Feibelman, Phys. Rev. Lett. 69, 1568, (1992). ²R. Stumpf, preprint, (1997).
9:00 AM		SS2-ThM-3 C₆₀ Adsorption on Clean and Au Covered Ni(110): Evidence for Interfacial Restructuring and One Dimensional Molecular Nanoscale Templating P.W. Murray, I.M. Brookes, S.A. Haycock, G. Thornton (University of Manchester, United Kingdom); M.O. Pedersen, E. Laegsgaard, I. Stensgaard, F. Besenbacher (University of Aarhus, Denmark) By means of Scanning Tunneling Microscopy (STM) we have studied the growth of C₆₀ on the clean and Au covered Ni(110) surfaces. C₆₀ induces a novel restructuring of the Ni(110) surface. Instead of the usual hexagonal overlayers observed on metal substrates the Ni(110) surface is restructured to form a corrugated C₆₀ termination which is tentatively explained within a simple model, based on the interaction between the molecular orbitals of C₆₀ and the narrow d-bands of the surface. A completely different structure is observed for C₆₀ deposition on Au covered Ni(110). Au is known to form alloy chain structures along the [001] direction for coverages greater than 0.4 ML, and the separation of these chains can be controlled as a function of coverage. C₆₀ molecules are found to adsorb along the chains forming one dimensional rows of fullerene molecules in contrast to the clean substrate. The results demonstrate that by creating such templates to modify both the morphology and reactivity of surfaces, it is possible to create novel molecular structures on a surface.
9:20 AM		SS2-ThM-4 Reconstruction of the W(211) Surface As Induced by Ultrathin Films of Rh, Pt and Pd K. Pelhos, I.M. Abdelrehim, T.E. Madey (Rutgers, The State University of New Jersey) The (111) surface of tungsten has been shown to undergo massive surface restructuring when a thin film of Rh, Pt, Pd or Au is deposited onto the surface and the substrate is annealed above a threshold temperature of 750K. The morphology of the resulting system consists of triangular pyramids and pits, with (211) faces having nanometer dimensions, as shown by Low Energy Electron Diffraction (LEED) and atomic resolution Scanning Tunneling Microscopy observations. LEED experiments also suggest that the (211) faces of these pyramids may undergo further reconstruction. Therefore, in the present study we investigate the behavior of the planar W(211) surface when covered by ultrathin films of Rh, Pt and Pd using Temperature Programmed Desorption (TPD) at very high temperatures (up to 2400 K), Auger Electron Spectroscopy (AES) and LEED. We have found that upon annealing above a threshold temperature of 900 K, both Rh and Pt cause a 3x1 reconstruction of the W(211) surface. Correlation with TPD and AES data, however, indicates that reconstruction is limited only to a narrow coverage range: between one and two monolayers. Furthermore, our analysis of the multiple peaks in the TPD spectra shows that the lowest temperature peak corresponds to zero order desorption from multilayer coverages, with desorption energies that are in excellent agreement with known sublimation energies of the overlayer metals. High temperature peaks correspond to fractional monolayer coverages, and the analysis yields binding energies of overlayer atoms to the substrate. The differences between fractional monolayer binding energies for the metal/W(211) and metal/W(111) systems may provide insights into the large surface free energy anisotropy of this system, which is believed to be the main driving force behind the large scale faceting phenomena on the thin film covered W(111) surface.
9:40 AM	Invited	SS2-ThM-5 Equilibrium Crystal Shapes in Three and Two Dimensions H.P. Bonzel (Forschungszentrum Jülich IGV, Germany) Equilibrium crystal shapes (ECS) of crystalline solids are an important source of fundamental relative surface energetic parameters, such as the anisotropic surface and step energies as well as step interaction and kink energies. Even the temperature dependence of these quantities is in principle accessable. The accuracy of the numbers extracted from the ECS is directly related to the resolution with which an undistorted image of the shape is obtained. Here scanning tunneling microscopy of truncated 3d particles and 2d islands (vacancies or atoms), both supported on a substrate, appears to be a technique which is capable of generating accurate data. Recent experimental results for such 3d and 2d metal particles imaged by STM will be reviewed. Concerning 3d particles, particular attention will be given to the step structure of facets and the curved portions near facets. The size of facets can be determined with atomic resolution. Hence a more accurate shape analysis of the ECS near facets is possible leading to shape exponents. Expected universal exponents of 3/2 or 2 are not found for Pb particles prepared and equilibrated on Cu(111) under UHV conditions. The ECS near the (111) facet of these particles exhibits an azimuthal dependence giving rise to even azimuthally dependent shape exponents ranging between 1.5 and 1.8. This finding suggests a structure dependent step interaction behavior. Non-universal shape exponents are in agreement with previous investigations and indicate a long-range 1/d step interaction for those orientations where the exponent is greater than 1.5, in contrast to current theory. The ECS of 2d islands on (111)fcc surfaces yields orientation dependent relative step energies as well as the kink energy ratio under certain assumptions. These data supplement those obtained from 3d particle shapes and from dynamic experiments.
10:20 AM		SS2-ThM-7 High-Temperature STM of the Phase Transitions of Au(110) and Pt(110) R. Koch, M. Sturmat (Freie Universität Berlin, Germany) It is well known that Au(110)(1x2) and Pt(110)(1x2), which both are missing-row reconstructed at 300 K, consecutively undergo two phase transitions upon heating: an Ising transition, at which the surfaces deconstruct, and a 3D roughening transition. Our real-space investigation with atomic scale resolution by high-temperature scanning tunneling microscopy reveals, that - contrary to present theoretical models - the Ising transition of Au(110)(1x2) is due to antiphase domains developing during the 2D roughening of (1x3) steps. The missing-row configuration of interior terrace regions remains completely stable until the 3D roughening temperature (at about 700 K), thus ruling out an order-disorder transition via lattice gas formation. The Ising-transition of Pt(110)(1x2), on the other hand, proceeds by the simultaneous formation of 2D island/hole units at the originally flat terraces. The phase transition therefore is in accordance with the step-pairing model of Vilfan and Villain [PRL 65 (1990) 1830] and the disordered flat (DOF) phase proposed by den Nijs [PRL 66 (1991) 907], which both are characterized by a long-range three layer up-down order.
10:40 AM		SS2-ThM-8 Structure of the Ga-rich GaAs(001) (3x2) Reconstruction L. Li, S. Gan, B.K. Han, R.F. Hicks (University of California, Los Angeles) Our STM studies of GaAs(001) have revealed a new phase with a (3x2) lattice. The surface was prepared by GaAs MOVPE after which it was transferred to a UHV system without air exposure. In the UHV chamber, the (3x2) structure was obtained by annealing the sample at 550° C. This surface consists of single rows of Ga dimers oriented along the [110] direction. The rows are spaced 12 angstroms apart. They vary in length, being separated by line defects with an average spacing of 20 angstroms. A (3x2) reconstruction free of line defects was also observed after incorporation of carbon from the decomposition of carbon tetrachloride. A structural model consisting of one Ga dimer in the top layer within the (3x2) unit cell is proposed. Electron counting reveals a deficit of one electron per (3x2) unit cell for this model. Charge neutrality on the surface is provided by the line defects which expose As dimers, or by substituting one carbon atom per (3x2) cell into an As lattice site.
11:00 AM		SS2-ThM-9 High-Temperature Evolution of the Si(111) Surface Studied via Synchrotron Radiation A.D. Laine, S. Vandré, A. Goldoni, S. Prato (INFM, Italy); V.R. Dhanak (University of Liverpool, United Kingdom); A. Santoni (ENEA, Italy); M. Sancrotti (INFM, Italy) Various scenarios have been depicted for the evolution of semiconductor surfaces as a function of temperature. These include, for example, relaxations, reconstructions, order-order transitions, and surface metallization. In the high temperature range, precursive processes of the bulk melting might occur at temperatures much lower than T_m. This is the case of surface and incomplete melting. In this context, the Si(111) surface is an intriguing test-bench for both experiments and theory. The (7x7) to (1x1) transition (about 1140 K) of the Si(111) surface has been qualified as an order-disorder one. Here, we report on an experimental systematic study of the Si(111) surface as a function of temperature up to the bulk melting temperature (about 1688 K). The experiment has been performed at the TGM5 Wiggler/Undulaor and TGM3 beam lines of the BESSY (Berlin) synchrotron radiation facility. Si 2p core level and angle-integrated valence band photoemission spectra have been measured over a wide set of selected temperatures. The unoccupied conduction states have been probed by means of Si 2p X-ray absorption spectroscopy. Bulk versus surface electronic structure have been discriminated by exploiting the synchrotron radiation tunability. Beyond the (7x7) to (1x1) tranistion occurring at about 1150 K a further surface phase transition is found at about 1550 K and is indicated by a steep rise in emission intensity at the Fermi level along with a narrowing of and a further shift of the core level spectra. This transition is interpreted in terms of incomplete surface melting.
11:20 AM		SS2-ThM-10 The Solid-Liquid Phase Transition in a Two Dimensional Lattices of Flexible Rod Shaped Molecules D. Fuhrmann, Ch. Wöll (Ruhr-Universität Bochum, Germany) Here we present results obtained with high resolution elastic and inelastic He atom scattering on the melting transition in molecular monolayers adsorbed on Cu(111) and Cu(110) substrates. The investigation of such 2d-systems is of interest since Kosterlitz and Thouless¹ have predicted that the reduced dimensionality should give rise to an increase of the order of the phase-transition. For the present systems additional changes are expected because of the anisotropic, rod-like shape of the alkane molecules and the possibility of creating intramolecular (or conformational) defects². At low temperatures (< 160 K) the alkanes (CnH2n+2) form an ordered, well defined two-dimensional lattice, which is commensurate for octane. At higher temperatures a phase transition to a disordered, liquid-like state is observed; with the precise transition temperature depending on the length of the hydrocarbon chain. These structural changes are accompanied by shifts of the frequencies of the external (librations, frustrated translations) and internal vibrations of the adsorbed molecules, indicating the creation of a large amount of conformational defects. In addition an analysis of the energetic width of the quasielastic peak reveals a strong increase of the diffusion coefficient above the transition temperature. ¹J.M.Kosterlitz and D.J.Thouless, J.Phys.C 6, 1181 (1979) ²F.Y.Hansen and H.Taub, Phys.Rev.Lett. 69, 652 (1992)
11:40 AM		SS2-ThM-11 Structure and Dynamics of Monolayer Films of KCN Vapor-Deposited onto KBr(001) S.A. Safron (Florida State University); J.R. Baker (Florida State University (Presently at National Research Lab , Denmark)); J.A. Li, E.A. Akhadov, T.W. Trelenberg, J.G. Skofronick (Florida State University) Helium atom scattering experiments have been carried out on monolayer films of KCN vapor-deposited onto KBr(001) cleaved in situ over the surface temperature range of ~40 K to 200 K. The substrate has very nearly the same surface lattice spacing as the film: 4.66 Å for KBr and 4.62 Å for KCN in the rocksalt-structured phase (the stable solid phase ≥ 168 K). Unlike the cleaved single crystal KCN(001) surface which appears disordered in helium atom diffraction experiments, the KCN monolayer film yields small but sharp diffraction spots at positions expected for the high temperature phase. For temperatures below ~95 K additional fractional order spots are found, which grow in as the temperature is lowered. The implied structure of this phase consists of (1x2) and (2x1) domains with the cyanide dipoles aligned anti-ferroelectrically. However, the surface phonon dispersion curves measured in the <100> and <110> azimuths show very little change in going from the high temperature phase to the low temperature phase. This work was funded by U.S. DOE grant No. DE-FG05-85ER45208.