AVS1996 Session SS-TuP: Surface Science Poster Session II

Tuesday, October 15, 1996 6:30 PM in Ballroom A

Tuesday Evening

Time Period TuP Sessions | Topic SS Sessions | Time Periods | Topics | AVS1996 Schedule

SS-TuP-1 Reaction Chemistry of HCl and OClO on Sulfuric Acid Thin Films J. Roberts, E. Guldan (University of Minnesota) Sulfuric acid surfaces play an important role in atmospheric chemistry. We have generated sulfuric acid films in an ultrahigh vacuum environment via the reaction of co-condensed sulfur trioxide and water. The films were characterized using Fourier transform infrared reflection absorption spectroscopy (FTIRAS), temperature programmed desorption (TPD), and x-ray photoelectron spectroscopy (XPS). FTIRAS, TPS, and XPS spectra indicate that the sulfuric acid films prepared range in concentration from approximately 10mole% sulfuric acid to nearly pure sulfuric acid; film thickness can be varied from a few monolayers (ML) to 150 ML. The interactions of HCl and OClO with sulfuric acid films of various concentration (30mole% - nearly pure) have been investigated. While HCl does not adsorb to pure sulfuric acid, exposure to more dilute films results in one or more desorption products. Experiments using D\sub2\SO\sub4\ show H-D exchange. Exposure of sulfuric acid to OClO results in a reversibly adsorbed state with OClO evolving into the gas phase at 150K. No other H\sub2\SO\sub4\ - OClO interaction was observed. The result of coincident adsorption of both Cl-compounds on sulfuric acid films is currently under investigation.
SS-TuP-2 Row-by-Row Growth: The One-dimensional Equivalent of Layer-by-Layer Growth V. Marsico, K. Kern (EPF Lausanne, Switzerland) Binding energies for adsorbates at step sites are in general larger than on terrace sites due to the increased coordination. As a consequence, two-dimensional adsorbate islands preferentially nucleate at steps if the adatom mobility is sufficiently high. Similarly to the growth mode classification in thin film, different step growth modes can be distinguished, the occurrence of which depend on the detailed interaction of the adsorbate with the substrate step. These growth types can be studied by thermal energy helium scattering only if grazing incidence can be achieved. We have developed a novel triple-axis He- surface spectrometer allowing for the first time such grazing incidence experiments. For a regularly stepped Pt(997) surface we demonstrate that during the adsorption of rare gases in the sub-monolayer range oscillations in the reflected helium intensity occur. These intensity variations reflect the transition between incomplete and complete rows growing at the steps during deposition. The influence of different adsorbate-step interactions will be discussed and a quantitative model is presented. We believe that the row by row growth mode is not a characteristic of Xe/Pt(997) only but shall be observed for many others adsorption systems as our most recent experiments suggest.
SS-TuP-3 Kinetics of the Formation of Self-assembled Monolayers Studied In Situ by Second Harmonic Generation (SHG) O. Dannenberger, M. Buck (Universit\um a\t Heidelberg, Germany) Adsorption of alkanethiols from solution has been monitored in real time. Substitution of the methyl end group by a nonlinear-optically (nlo) active moiety allows to address the molecule/gold substrate bond and the end group separately by second harmonic spectroscopy. The coverage dependent SH signal from the end group exhibits a behavior markedly different from that originating from the Au-S bond. In contrast to n-alkanethiols the time dependence of the SH intensity cannot be described anymore by simple Langmuir kinetics. Both the susceptibility and the phase of the end group are subject to changes above coverages of \Theta\ = 0.4. The phase shift of the SH signal which is associated with a blue shift of the charge transfer resonance is interpreted by an energetically unfavorable alignment due to anchoring of the molecules to the substrate. Further SH studies reveal a pronounced effect of the solvent on the kinetics of film formation. The adsorption of thiols monitored by the Au-S bond formation is four times faster in hexane than in ethanol. In contrast, the end group exhibits changes which proceed much slower in hexane than in ethanol. This indicates that the film formation is decisively affected by the molecule solvent interaction.
SS-TuP-4 Surface Charge Density Wave - Pb/Ge(111) J. Carpinelli, H. Weitering, E. Plummer (University of Tennessee & Oak Ridge National Laboratory) The interplay of geometric and electronic structure can result in a rich variety of physical phenomena, especially in reduced dimensions where "divergent" response functions are more easily achieved. In the Ge(111)-(\sr\3 x \sr\3)R30 interface, for example, 1/3 ML of Pb adatoms order in a hexagonal array atop the bulk truncated Ge lattice. At room temperature, this interface is unbuckled (all Pb adatoms equivalent) and metallic. Upon cooling below room temperature however, this overlayer gradually and reversibly undergoes a transition (onset ~ -20 C). LEED shows a new low- temperature (3 x 3) lattice symmetry. EELS data indicate this phase is semiconducting with a 0.065 eV band gap. STM data clearly establish this new structure's honeycomb-type charge reorganization. What we observe is unambiguous: on decreasing temperature, the Pb/Ge(111)-\alpha\ interface forms a commensurate surface charge density wave concomitant with a metal-nonmetal transition. This transition is likely initiated by electron- phonon coupling enabled by a nested Fermi surface. However, electron correlation effects are responsible for the semiconducting nature (electron counting indicates a metallic surface) of this low temperature phase and consequently its stability. Measurements on the iso-structural Sn/Ge(111) interface indicated the general nature of this phenomenon.
SS-TuP-5 Collective Excitations in Thin K Films on Al(111) B. Kim (University of Tennessee and Oak Ridge National Laboratory); E. Plummer (University of Tennessee & Oak Ridge National Laboratory) The surface collective modes of thin K films deposited on Al(111) have been investigated using frequency dependent photo-yield (PY) measurements and momentum resolved inelastic electron scattering. Jellium based theoretical calculations have predicted more complex set of modes in the thin films than for the surface of a semi-infinite solid [1] resulting from interference between the substrate-film and film-vacuum interfaces. The use of an optical probe and electron scattering has allowed us to draw a more complete picture of the dynamic screening in thin films. The number, dispersion, damping and optical activity of the collective modes of the thin films have been measured as a function of K film thickness. For thin K films (from 2ML to 50ML) the PY spectra show two distinct modes, the lower energy one near the multipole mode energy of the thick film and the higher energy mode near the bulk plasmon energy. As the film thickness grows, the relative intensity of the low energy mode to the high energy mode increases but the overall intensity of both decreases dramatically. These two dipole surface collective modes originate from the interference between the two interfaces in this system. This interference decreases as the film thickness and the parallel momentum transfer increase. There are also significant changes in the dispersion and damping as a function of film thickness. In general the results are in good agreement with TDLDA calculations [1]. 1. A. Liebsch, Phys. Rev. Lett. 67, 2858, (1991).
SS-TuP-6 Testing Dynamical Steering Theories: State-Resolved Sticking Coefficients for H\sub 2\ on Pd(111) M. Gostein, G. Sitz (University of Texas, Austin) Advances in theoretical models have recently enabled high-dimensional simulations of molecule/surface encounters on ab initio potential energy surfaces. For H\sub 2\ dissociation on Pd, calculations have shown a dynamical steering effect\super 1\, in which molecules approaching the surface with low velocities and low angular momenta are preferentially "steered" to favorable dissociation geometries. To test this effect, we have used molecular beam and laser spectroscopy techniques to measure the sticking coefficients of H\sub 2\ in specific rovibrational states on a Pd(111) surface over a range of translational energies. Present data indicate that the sticking coefficient decreases significantly with increasing rotational quantum number. \super 1\A. Gross, S. Wilke, and M. Scheffler, Phys. Rev. Lett. 75, 2718 (1995). (insert figure here) Figure. Measured sticking coefficient of H\sub 2\ on Pd(111) versus initial H\sub 2\ rotational quantum number.
SS-TuP-7 Chemical Interaction of Pulsed Laser Deposited Cobalt with the MoS\sub 2\(0001) Surface J. Bulicz, L. Morales de la Garza, S. Fuentes (Instituto de F\ag i\sica, U.N.A.M., Mexico) The interaction of cobalt deposited by pulsed laser on MoS\sub 2\(0001) was studied by X-ray Photoelectron Spectroscopy, Auger Electron Spectroscopy and Reflection High Energy Electron Diffraction. The deposition of cobalt was carried out with 150 pulses of a KrF eximer laser of wavelength 248nm with fluence of 10 J/cm\super 2\ aimed at solid cobalt target. This produced cobalt crystallites randomly oriented on the MoS\sub 2\(0001) surface. The deposition of the cobalt by pulsed laser, induced the formation of metallic molybdenum and a depletion of the sulphur on the surface as shown by XPS spectra. Annealing the surface after the deposition to different temperatures give no appreciable difference on the metallic molybdenum content and the concentrations of sulphur on the surface as given by AES and XPS spectra. The relative concentrations of species calculated from XPS and AES intensities were consistent with sulphur incorporation to the cobalt overlayer film. Annealing up to 1070K caused aglomeration of the cobalt overlayer and high concentration of cobalt at the step edges.
SS-TuP-8 An UHV Study of Sulfur Multilayers S. Li (National University of Singapore); J. Rodriguez, D. Van Campen, J. Hrbek (Brookhaven National Laboratory) Sulfur poisoning of metals is one of the major problems in catalysis. We have studied the adsorption of S/sub 2/ deposited from an electrochemical source on several metal surfaces (Ru(001), Mo(110) and Cu (111)) and discovered that the S multilayer prepared at temperatures below 200 K is stable up to 350 K. Sulfur polymers were observed to desorb from two states below room temperature and one state at 390 K. Cyclooctasulfur was the highest polymer detected in our experiments. No preferential ordering of the S multilayer was observed with LEED. The S 2p binding energy of 163.20 eV is 1.55 eV higher than that of S chemisorbed on Mo(110) surface. The UPS spectra of the S multilayer showed a complete suppression of the metal d bands, a 2 eV wide band gap and up to five low energy peaks that are comparable with known gas phase spectra of the molecular sulfur species. IR spectra of the S multilayer confirm the presence of sulfur polymers. The reactivities of S multilayers with several metals (Fe, Co, Ni, Zn, Mo and Ag) were explored and compared with the activity of hydrodesulfurization model catalysts. This work was supported by the US Department of Energy (DE-AC02-76CH00016).
SS-TuP-9 Structure of the (100) Surface of the Icosahedral Solid YB\sub 66\ as Determined by Scanning Tunneling Microscopy C. Perkins, M. Trenary (University of Illinois, Chicago); T. Tanaka (National Institute for Research in Inorganic Materials, Japan) The boron-rich solid known as YB\sub 66\ has an exceedingly complex crystal structure with approximately 1632 atoms per unit cell. Although complex in terms of atomic positions, the structure can be more simply described in terms of B\sub 156\ super-icosahedra consisting of B\sub 12\(B\sub 12\)\sub 12\ units. The bulk structure is face centered cubic with eight B\sub 156\ units and eight non-icosahedral B\sub 48\ units per unit cell. A combination of argon ion sputtering and heating to 1200 \super o\C in UHV was found by XPS to give a clean surface without altering the bulk B:Y ratio. The (100) surface gives sharp square lattice LEED patterns consistent with an unreconstructed bulk-terminated (100) plane with a lattice constant of 23.4 \Ao\. Images obtained with STM show the surface to consist of flat (100) terraces separated by 11\+-\1 \Ao\ (half unit cell) high steps. Using a metallic (W) tip, a square lattice is observed having a lattice constant 12.3\+-\1 \Ao\. With a nonmetallic tip presumed to be boron, a square lattice of lattice constant ~8.5 \Ao\ is seen, oriented 45 \super o\ with respect to that seen with a tungsten tip. The STM images are consistent with direct observation of B\sub 156\ super-icosahedra and non-icosahedral B\sub 48\ units.
SS-TuP-10 Benzene Formation by 1-Hexene on Cu\sub 3\Pt(111) A. Teplyakov, B. Bent (Columbia University) Dehydrocyclization refers to the conversion of straight chain hydrocarbons into aromatic compounds. Even though this reaction is widely used in heterogeneous catalysis, knowledge of the mechanism of dehydrocyclization remains limited, largely because it has not previously been possible to carry out the reaction under conditions amenable to detailed mechanistic study. We wish to report here UHV studies of the dehydrocyclization of monolayer coverages of 1-hexene to benzene on a Cu\sub 3\Pt(111) single crystal surface. In these studies reflection-absorption infrared spectroscopy and temperature-programmed reaction/desorption spectroscopy have been used to investigate the mechanism of the reaction. The results obtained demonstrate that the mechanism involves two steps: dehydrogenation (T=270 K) and cyclization (T=405 K). RAIRS and TPR/D suggest that a planar intermediate with C\sub 6\H\sub 8\ stoichiometry exists on the surface between 270 and 405 K. For surface coverages of up to 15 % of monolayer saturation the selectivity to benzene formation is 70 % with the remaining 30 % of the adsorbed 1-hexene dehydrogenating irreversibly to surface carbon and H\sub 2\. For higher coverages, molecular desorption commences.
SS-TuP-11 Field-emission Barrier Height of an Alloy Tip: Possible Effect of Surface Segregation K. Sano, T. Nakane, A. Sakai (Kyoto University, Japan); T. Sakata (University of Osaka Prefecture, Japan) Field-emission from a sharply pointed tip is becoming a central part in vacuum microelectronics. Ishikawa et al.[1] have shown that a gold deposition to an emitter is quite effective in reducing the barrier height and increasing the emission stability. In this paper, we report our observation that the barrier height can also be significantly reduced by heating a gold-containing alloy tip where gold segregates to the surface. Our tip material is a Ni-0.8at%Au alloy which is known for its strong gold segregation. Barrier height was obtained from the slope of the Fowler-Nordheim plot assuming the tip radius estimated from the best image voltage in field-ion imaging. The as-prepared tip has a barrier height of 5.7eV. By annealing the tip at 500K and 700K, we find that the barrier height decreases to 5.4eV and 5.1eV, respectively. As no appreciable tip blunting by annealing was observed in the field-ion images, the change of the barrier height is likely due to the effect of gold surface segregation. Our finding is consistent with a reduction of work function by surface segregation reported on the Ti/Cu system[2]. [1] J.Ishikawa et al., J. Vac. Sci. Technol. B11, 403 (1993). [2] M.Yoshitake and K.Yoshihara, J. Vac. Sci. Technol. A13, 2407 (1995).
SS-TuP-12 CO Photo-oxidation at Anion Vacancy Sites on TiO\sub 2\(110) A. Linsebigler (General Electric Corporate Research and Development); G. Lu (North Carolina State University); J. Yates, Jr. (University of Pittsburgh) The CO photo-oxidation reaction at anion vacancy sites on TiO\sub 2\(110) was investigated by probing the thermal behavior of CO and the distinct photo-desorption behavior of O\sub 2\ and CO\sub 2\. Molecular O\sub 2\ was observed to chemisorb at 105K at anion vacancy sites on TiO\sub 2\(110) produced from annealing to T=900K. Upon irradiation, molecular O\sub 2\ experienced excitation leading to either photo-desorption of O\sub 2\ or photo-oxidation of co-adsorbed CO to produce CO\sub 2\. The yield and kinetics of the photo-produced CO\sub 2\ were controlled by the coverage of chemisorbed O\sub 2\. The yield of photo-produced CO\sub 2\ and photo-desorbed O\sub 2\ followed the same normalized excitation curve versus photon energy with a threshold at the TiO\sub 2\ bandgap, 3.1eV. The decay time for the photo-produced CO\sub 2\ is essentially identical to the characteristic decay time for photo-desorbed O\sub 2\, corresponding to a photo-desorption cross section = 8 x 10\super -17\ cm\super 2\ for 3.94 eV photons. Compared to O\sub 2\ photo-desorption, CO\sub 2\ photo-production was a minor process. The photo-production of CO\sub 2\ involved the molecular O\sub 2\ and CO chemisorbed species and lattice oxygen from TiO\sub 2\ was not chemically involved in the photo-oxidation process. _____ Work supported by the Army Research Office-ARO
SS-TuP-13 Interpretation of Critical Point Energy Shifts with Surface Termination: Dynamic Localization of Bloch-like States Near Surfaces L. Mantese, K. Bell, U. Rossow, D. Aspnes (North Carolina State University) Vicinal (001) Si surfaces with a minimum number of dangling bonds (saturated by H, O, etc.) exhibit surface-induced optical anisotropy (SIOA) spectra that closely resemble the first energy derivative of the bulk dielectric function. These results can be described by critical point energies that are dichroic, i.e., that exhibit different values depending on incident polarization. We investigate the effects of surface chemistry on these apparent critical point energy shifts by examining spectroellipsometric (SE) data of chemically prepared hydrogen or oxygen terminated (001) and (111) Si surfaces as well as data previously reported in the literature. Differences in E\sub 1\ and E\sub\2 critical point energies, on the order of 10meV, indicate that these apparent shifts are general and not restricted to SIOA data. Such shifts can be understood in terms of lifetime-induced localization where the intermediate state of the electron after excitation is determined by a competition between the photon perturbation, which favors extended Bloch states, and the surface potential which presumably favors localization. Since apparent threshold energies are sensitive to wafer orientation and processing details, it is critical that the proper reference dielectric functions be used to model SE data in determining dielectric behavior and overlayer thicknesses. This is particularly relevant to applications involving thin oxides where thicknesses need to be measured to accuracies of 0.1\Ao\.
SS-TuP-14 Active Sites for Methanol Synthesis on a Zn/Cu(111) Model Catalyst Studied by STM J. Nakamura, T. Matsuda, Y. Kushida, T. Uchijima (University of Tsukuba, Japan); T. Fujitani (National Institute for Resources and Environment, Japan) We have previously shown that a Zn-deposited Cu(111) surface is a good model of Cu/ZnO catalysts for methanol synthesis by the hydrogenation of CO\sub 2\, where the Zn is regarded as the active site for methanol synthesis. Further, the role of Zn was suggested to stabilize formate intermediates by binding to each other. Here, we report the structure of the Zn deposited on Cu(111) and the formate species studied by scanning tunneling microscope (STM). The STM measurements were carried out using a UHV-STM apparatus including LEED-AES and a reaction cell in which catalytic reactions can be performed at atmospheric pressure. Submonolayer of Zn was deposited on a clean Cu(111) surface at room temperature by resistively heating a zinc wire, and then flashed to 523K. The coverage of Zn corresponded to 20% of the number of surface copper atoms. Atomic images for the Zn-deposited Cu(111) surfaces were obtained in a range of 30 x 30 ~ 100 x 100 \Ao\\super 2\, which indicated that surface Cu atoms are substituted with Zn atoms leading to the formation of a Cu-Zn surface alloy. The images showed a somewhat distorted 1x1 structure, which was in good agreement with the results of LEED that showed a little diffuse 1x1 pattern. We propose a model that substituted zinc atoms are the active sites for methanol formation. We also obtained STM images for the formate species on Cu(111) surfaces synthesized by the hydrogenation of CO\sub 2\ in the reaction cell. Formate species on the clean Cu(111) surface was found to adsorb dispersively without island or order structure at a low formate coverage corresponding to ~2% of the number of surface copper atoms.
SS-TuP-15 STM Study of Sb/Si(111)-5\sr\3 X 5\sr\3 Structure: A New Passivation Layer on Si(111) K. Park, J. Ha, W. Yun, E. Lee (Electronics and Telecommunications Research Institute, Republic of Korea); J. Yi (Korea Research Institute of Standards and Science); S. Park (Kwangju Institute of Science and Technology, Korea) We investigated the 5\sr\3 x 5\sr\3 structure of Sb-passivated Si(111) surface with LEED and STM for the first time to determine the real space atomic structure. The 5\sr\3 x 5\sr\3 structure of Sb/Si(111) was prepared by Sb deposition on Si(111) at 630 degree of C and subsequent annealing for several minutes at 700 degree of C. In the STM image taken over wide surface area, the hexagonal arrangement of adatoms, which is composed of unit cell with a center adatom and surrounding six triple adatoms, was shown. In the high resolution image, the structures of corner hole and rest layer as well as the adatoms were obtained, which provide the information on the description of surface structure in atomic scale. From these experimental results, the Sb/Si(111)-5\sr\3 x 5\sr\3 is suggested to be formed by the selective replacement of Si atoms of faulted halves and surface passivation on Si(111)-5x5. The rest layer Si atoms of the faulted halves in 5x5 unit cell are replaced by Sb and the adatoms of the unfaulted halves in 5x5 unit cell are also occupied by Sb. In particular, formation of a periodicity of 5\sr\3 x 5\sr\3 structure is considered to be caused by the selective replacement of adatoms in the faulted halves due to the lattice strain by Sb atoms. Since the Sb layer with 5\sr\3 x 5\sr\3 structure is atomically smooth over wide area and chemically stable, it can be used as a new Sb passivation layer for surfactant mediated growth and formation of nanostuctures.
SS-TuP-16 In Situ IRAS Study of NO Adsorption, Dissociation and Coadsorption with CO on Pt (100) at Elevated Temperatures N. Magtoto, H. Richardson (Ohio University) We report strikingly different behavior of NO alone and in the presence of CO on Pt(100) in the temperature regions around 420 K, 470 K and 520 K from an infrared reflection-absorption spectroscopy study carried out in a continuous flow react or. At 420 K, NO initially adsorbed at 1636 cm\super -1\. This band shifted and reached its maximum absorbance at 1659 cm\super-1\ after a continued NO exposure of 35 minutes, then its intensity decreased after 45 minutes. But the band did not disappear even after an extended exposure time of 75 minutes. Upon admitting small amount of CO into the reactor, the infrared band eventually disappeared. When the supply of CO was cut off, the band reappeared at 1659 cm\super -1\. This behavior appeared to be a reversible proces s. Infrared spectra of adsorbates taken at 470 K showed a vibrational band that initially appeared at 1640 cm-1. This band grew in, shifted to 1656 cm-1, then disappeared after leaking in NO for about 200 s. When CO was bled into the reactor, a band at 1632 cm\super -1\ appeared. This band continued to grow until a critical CO pressure was reached, where it disappeared and replaced by infrared bands that correspond to a growth of CO adlayer. Initial IR spectra obtained at 520 K were unremarkable. But when CO was leaked in, a vibrational band at 1620 cm\super -1\ appeared in the spectrum. The implications of this remarkable behavior of NO on Pt(100) will be discussed.
SS-TuP-17 Desorption of Hydrogen and Deuterium from Diamond C(111) and C(100) Surfaces H. Lu (National Taiwan University, P.R. China); J. Lin (Academia Sinica, P.R. China); Y. Wang (Academia Sinica & National Taiwan University, P.R. China); K. Song, T. Chuang (Academia Sinica, P.R. China) Low energy electron diffraction (LEED) and thermal desorption spectroscopy (TDS) are used to study the kinetics of hydrogen (deuterium) adsorption and desorption on diamond C(111) and C(100) surfaces. C(111)-(2x1) reconstructed surface structure transforms into (1x1) at a relative hydrogen coverage of ~0.4, while C(100)-(2x1) structure persists to saturation coverage.Extensive analyses of TDS results reveal that the activation energy (E\sub a\) for D\sub 2\ desorption from C(111) surface exhibits significant coverage dependence; and E\sub a\ shows a maximum of ~5.2 eV at a coverage of ~ o.4. The TDS spectrum of D\sub 2\ from diamond C(100) shows two convoluted peaks, which are believed to correspond to two different adsorption states. For this surface, E\sub a\ (~3.0 eV) does not show clear coverage dependence. These results will be discussed in conjunction with the LEED measurements.For D\sub 2\ desorption from C(111), the kinetics is found to be first order, while it can not be uniquely determined for C(100) surface. These observations will be discussed and compared with the desorption of hydrogen from Si(111) and Si(100) surfaces. *This work is supported by the National Science Council (Contract No. 85-2113-M-001-015 & 85-2112-M-001-031), Taiwan, Republic of China.
SS-TuP-18 Structure, Composition, and Thermal Properties of Ultra-Thin Chromium-Oxide Films on Pt(111) L. Zhang, M. Kuhn, U. Diebold (Tulane University) The structure, composition, and thermal properties of ultra-thin chromium-oxide films on Pt(111) have been investigated using LEED, ISS, XPS and STM. The chromium-oxide films are prepared by vapor phase deposition of Cr on Pt(111) in the presence of oxygen at 600 K or by post-oxidation of deposited Cr overlayers, also at 600 K. Films grown by depositing Cr in an oxygen atmosphere were found to be more homogeneous and stoichiometric than films grown by post-oxidation. With LEED, a 2x2 superlattice structure is observed at low coverages equivalent to two or less monolayers of Cr. For coverages greater than the equivalent of 5 monolayers of Cr, a \sr\3 x \sr\3 R30=B0 superlattice structure is observed. Between 2 and 5 equivalent monolayers both structures co-exist on the Pt(111) surface. XPS results indicate that the Cr 2p core levels shift by ~2 eV to higher binding energy in the oxide as compared to bulk metallic Cr core level position. Analysis of ISS results indicate a composition close to that of the stable bulk-oxide phase, namely Cr\sub 2\O\sub 3\. The oxidized Cr films are stable up to 750 K at which point they decompose with Cr diffusing into the Pt lattice. This is contrast to the behavior observed for Cr alone on Pt, where interdiffusion occurs immediately upon annealing and most of the Cr disappears from the surface region upon annealing to 800 K. A subsurface Cr-Pt alloy (prepared by annealing a Cr overlayer to 800 K) was found to decompose in the presence of oxygen at 600 K. This leads to the migration of Cr to the surface with subsequent formation of a chromium-oxide phase. The implications of these results will be discussed.
SS-TuP-19 XPS and UPS Study of the Initial Growth of Ge\sub x\Si\sub 1-x\ Alloys on Si (100) by Molecular Beam Epitaxy G. Jernigan, P. Thompson, C. Silvestre (Naval Research Laboratory) The growth of Ge\sub x\Si\sub 1-x\ alloys is being studied because of their potential application in optoelectronic, high speed, and high power devices. A critical component to the use of Ge\sub x\Si\sub 1-x\ devices is control over the germanium concentration within the silicon lattice for the purpose of band gap engineering. We have investigated the initial stages of Ge\sub x\Si\sub 1-x\ (x = 0.05, 0.10, 0.20, and 0.40) alloy growth (0 - 200 \Ao\) using XPS and UPS. By measuring the Ge 2p, Ge 3d, and Si 2p\super 3/2\ XPS peaks as a function of alloy thickness, we were able to observe the surface segregation of Ge during growth. For each alloy composition, a linear increase in the Ge/Si XPS peak ratio to a steady-state plateau is observed. At steady state, no additional build up of surface Ge occurs allowing us to determine the Ge concentration within the surface layers. We found 100% Ge in the topmost layer for all alloys and an increase in Ge concentration in the second layer above the nominal alloy value. The segregation of Ge results in a thickness of alloy with changing Ge concentration that is below the nominal composition. We define this thickness as the leading edge of alloy growth. Using the inelastic mean free paths of the different XPS peaks, we were able to calculate the thickness of the leading edge and have shown that the leading edge decreases with increasing Ge nominal alloy composition. UPS was used to monitor the shift in the valence band maximum (VBM) with alloy composition. The VBM shifts to lower binding energy with increasing Ge concentration as expected. The segregation of Ge results in a "germanium like" density of states at the surface.
SS-TuP-20 Surface Defects Created by Low Energy(20-240 eV) Ion Bombardment of Ge(001) S. Chey, J. Van Nostrand, D. Cahill (University of Illinois, Urbana) The interaction of low energy ions(E\<\100 eV) with surfaces has been a topic of wide interest due to the application in ion-assisted physical vapor deposition and plasma-assisted CVD. Surface defects created on Ge(001) exposed to low energy Xe ions are characterized by in-situ scanning tunneling microscopy. The temperature of the sample during ion bombardment is 165\super o\C and ion energies range from 20 to 240 eV. The ion collisions create defects(vacancies and adatoms) which nucleate and form vacancy and adatom islands. The sputtering yield of Ge(001) as measured by STM is consistent with the experiments of Wehner[1] for polycrystalline Ge. The vacancy island density increases with increasing ion energy: the vacancy island number density is 2.4x10\super -20\ cm\super -2\ for 40 eV ion bombardment and increases to 1.1x10\super -19\ cm\super -2\ for 240 eV ion bombardment. The increased nucleation rate for vacancies is attributed to clustering of defects in a manner analogous to the increased adatom island density during ion-assisted MBE growth of Pt(111)[2]. The sputtering yield for 20 eV ion is approximately 10\super -3\ but the net yield for surface defects(sum of adatoms and vacancies) is an order of magnitude higher, 10\super -2\, due to adatom-vacancy pair creation. [1] G.K.Wehner, Phys. Rev. 112, 1120 (1958). D. Rosenberg and G.K. Wehner, J. Appl. Phys. 33, 1842 (1962). [2] S. Esch, M Bott, T. Michely, and G. Comsa, Appl. Phys. Lett. 67, 3209 (1995).
SS-TuP-21 Laser Postionization of Benzene Desorbed from C\sub 6\H\sub 6\/Ag(111) Induced by keV Ion Bombardment R. Chatterjee, D. Riederer (Pennsylvania State University); Z. Postawa (Jagellonian University, Poland); B. Garrison, N. Winograd (Pennsylvania State University) Kinetic energy and angular distributions of neutral benzene sputtered from C\sub 6\H\sub 6\/Ag(111) have been measured using multiphoton ionization. Benzene was adsorbed on a clean Ag(111) surface at 125 K for a range of exposures. The time delay between the primary ion pulse and the laser pulse was varied for energy distribution measurements. Angular information was obtained by using a position sensitive detector. The peak in the kinetic energy (K.E.) distribution shifts from 0.8 eV -0.4 eV for submonolayer to monolayer coverages.In the case of multilayers of benzene, the kinetic energy peak is significantly lower (0.04 eV). Two components are present in the energy distribution, where for sub-monolayer coverage the high K.E. peak dominates and for multilayers the low K.E. peak dominates. The low K.E. peak intensity increases with bombardment time for desorption from the multilayer system. The angular distribution of benzene from sub-monolayer coverage shows an anisotropy similar to that silver from clean Ag(111) surface. Both the benzene and silver angular distributions from multilayered C\sub 6\H\sub 6\/Ag(111) surface do not show this anisotropy but rather has a shape which approximates cos\super n\\theta\. A collision cascade in the metal surface initiates the benzene desorption. Molecular dynamics simulations have been performed to investigate the nuclear motions in the solid that lead to benzene desorption. The mechanistic implications of these results to the desorption process of molecules will be discussed.
SS-TuP-22 Electrochemical Healing of Surface and Subsurface Defects in TiO\sub 2\ J. Daschbach, L. Wang, M. Engelhard, D. Baer (Pacific Northwest National Laboratory) The role of surface and subsurface defects on the electrochemical behavior of TiO\sub 2\ is not well understood. By preparing single crystal samples in ultra high vacuum (UHV) conditions and introducing surface defects in a controlled manner prior to transfer to an electrochemical cell the electrical and electrochemical implications of such defects have been studied. We have investigated the controlled potential interaction of surface defects with an electrolyte in an effort to understand the role and fate of these defects in different electrochemical environments. Both XPS and electrical measurements have been made on samples prepared in somewhat different ways. Defects are introduced via electron or ion bombardment. The healing or migration of these defects in the space charge layer is observable in capacitance measurements of these samples well after Ti\super 3+\ states are no longer observable in XPS and UPS spectra of the emersed crystal. Defects introduced at lower energy or remaining in the subsurface after vacuum annealing are oxidized or healed irreversibly over a narrow range of applied potential. Defects introduced at higher energy have both an irreversible component manifested over a broader potential range and a smaller quasi-reversible component.
SS-TuP-23 Chemistry of Cyclopentadiene on a Cu(100) Surface: Implications for Copper CVD from CpCuP(C\sub 2\H\sub 5\)\sub 3\ D. Sun, S. Leang, B. Bent (Columbia University) The chemistry of cyclopentadiene (C\sub 5\H\sub 6\ or Cp-H) on a Cu(100) surface has been studied by temperature-programmed desorption under ultra-high vacuum conditions. When half a monolayer of cyclopentadiene is adsorbed on a Cu(100) surface, 15% of the cyclopentadiene molecules desorb molecularly while the remaining molecules undergo dehydrogenation at temperatures below 200 K. The amount of H\sub 2\ desorption at 335 K and the evolution of C\sub 5\H\sub 5\D from the reaction of the dehydrogenated species with deuterium atoms indicate that cyclopentadienyl (Cp, C\sub 5\H\sub 5\) groups and H atoms are chemisorbed on the surface after dehydrogenation. At temperatures above 600 K, the Cp groups undergo self-hydrogenation (some sacrificial Cp groups give H to other Cp groups to produce Cp-H). The sacrificial Cp groups achieve a stoichiometry of C\sub 5\H\sub 3\ and remain on the surface to even higher termperature where they further dehydrogenate to produce sur! face carbon and evolve H\sub 2\. The implications of these results for the chemical vapor deposition of copper films from CpCuP(C\sub 2\H\sub 5\)\sub 3\ (whose surface chemistry on Cu(100) has also been studied) will be discussed.
SS-TuP-24 H and H\sub 2\O Interaction with the (111) Surface of Gold G. Gvalani, A. Hanbicki, E. Plummer (University of Tennessee & Oak Ridge National Laboratory); D. Zehner (Oak Ridge National Laboratory) We have investigated the interaction of atomic hydrogen with the Au(111) surface. The dissociative adsorption of molecular hydrogen on this surface is activated. Recent theoretical calculations of this system [1], and experimental reports that H goes subsurface on Au(001) [2], motivated this study. Initial adsorption of H creates a reconstructed (2x2) phase leading to an unstable (3x3) at higher H-exposure. H\sub 2\ desorbs from Au(111) near 135 K. Coverage measurements using nuclear reaction analysis (NRA) indicate H-saturation of ~1/3 ML. NRA measurements also yield coverages of several monolayers with prolonged exposure, but these measurements are a consequence of the presence of H\sub 2\O. The co-adsorption of water is facilitated by the low sticking coefficient and low temperature needed for adsorption of H on Au(111). The interaction of H\sub 2\O with the Au surface exhibits interesting chemistry in its own right. Water desorbs at 161 K. At the surface, ~10 percent of the water undergoes an autocatalytic reaction producing HO and atomic H, the latter recombining on the surface to desorb as H\sub 2\. We are investigating the bonding sites of H and H\sub 2\O as well as isotopic mixing, using ELS. 1. B. Hammer and J.K. Norskov, Nature 376 (1995) 238. 2. H. Iwai, K. Fukutani, and Y. Murata, Private Communication.
SS-TuP-25 Structure and Chemistry of CH\sub 3\Br on Ru(001) and Cu/Ru(001) T. Livneh, M. Asscher (The Hebrew University, Israel) Methyl bromide was found, using TPD, to adsorb on Ru(001) at 82K with a saturation coverage of \theta\=0.23. Work function change (\Delta\\Phi\) measured during adsorption reveals a decrease of 2.17 V, for the first layer, indicating adsorption with the bromine towards the surface. Modeling of the data indicates considerable polarizability and dipole moment increase, relative to the gas phase, and confirms the strong interaction with the Ru(001) surface. Combining TPD and work function measurements enables one to follow dissociation pathways and multilayer desorption in detail, due to the sensitivity of work function to structural changes, not observable in TPD. At submonolayer coverages, a shift of the onset for work function increase to lower temperatures is observed as coverage increases. It suggests a mechanism by which a flipping of the molecule to the opposite configuration (methyl down) occurs due to strong dipole-dipole repulsion between adjacent molecules. This mechanism is operative at temperatures below the onset for desorption. Upon continued adsorption the second and higher layers populate simultaneously, indicating mostly "methyl down" configuration for the second layer ((\Delta\\Phi\) increase) and "bromine down" for the third layer. 60% of the molecules in the first saturated monolayer dissociate when heating the sample to above 200K. Preadsorbed Cu modifies the reactivity of the Ru substrate. At \theta\\sub Cu\=0.65 temperature programmed reaction shows a change from H\sub 2\ on the bare metal to CH\sub 4\, which arises from an abrupt change in the surface reaction mechanism. At coverages higher than 2ML Cu the decomposition of the CH\sub 3\Br was negligible.
SS-TuP-26 Temperature Effect on the Scattering Process of Molecule-Surface Interaction X. Shen (Tsinghua University, P.R. China); C. Feng (Boston University) In recent years there has been considerable interest in theories of the dynamics of inelastic gas-surface scattering with phonon participation. A new theory is presented to study the temperature effect on the scattering process of molecule-surface interaction, in which some of the degrees of freedom are treated classically while the other modes involved are described by quantum theory. The effective potential energy surface that depends on surface temperature and governs the entire collision dynamics has been established by combining the mean field theory and the modified LEPS formulation to include surface phonons. The detailed analysis of the properties and functions of the potential parameters in the model, compared with the existing theoretical and experimental data, has shown that the new model potential has the advantages of high adjustability, relatively less computation consumption and reasonably good reliability for dynamic studies. Quasiclassical trajectory calculations have been carried out to investigate the dynamics of molecule-surface interaction, such as nitrogen/tungsten, based on the temperature-dependent potential surface. The dependence of the dissociative adsorption probability on incident energy and angles, and on surface temperature has been analyzed. Nice agreement is found between the calculated results and recent experimental studies. As a main point the mechanism of energy transfer in different degrees of freedom under various conditions has been investigated in detail. The results show that phonon exchange plays a significant role and should be included in the dynamic study of inelastic molecule-surface scattering.

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