AVS1996 Session SS-MoP: Surface Science Poster Session I

Monday, October 14, 1996 5:30 PM in Ballroom A

Monday Afternoon

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

SS-MoP-1 A New Single-Crystal Adsorption Calorimeter: Applications in Metal Adsorption / Adhesion J. Stuckless, C. Campbell (University of Washington) Chemical binding energies at the interface, which control nucleation and growth, are often unknown, especially for the adsorption of metals into configurations which are unstable at high desorption temperatures. We have built a new calorimeter for measuring adsorption heats on clean single-crystals in UHV. The apparatus has a thin-foil pyroelectric detector with a mechanically switched contact to the back of the thin (1-10 micron) sample. It has advantages over earlier designs, including measurements down to 100 K with sample preparation up to 2400 K. Also, our very high flux, remote, chopped metal atom source allows metal adsorption / adhesion energies to be measured down to 100 K. The calorimeter was tested for Cu adsorption on Cu at 300 K, showing a precision of \+-\ 5% for points every 0.03 ML, with absolute accuracy within 4%. We have also measured the heat of adsorption of Cu on W(100) versus coverage, the first data available for temperatures below surface alloying.
SS-MoP-2 Testing a Model of Conduction Electron Scattering: Adsorbate-induced IR Reflectance Change and Resistivity Change Measurements D. Kuhl (Michigan State University, United States of America); E. Krastev (Michigan State University); R. Tobin (Tufts University) We present experimental tests of a model that explains the broadband absorption of infrared light \Delta\R/R caused by adsorption on a clean metal surface in terms of conduction electron scattering. We have measured \Delta\R/R induced by CO adsorption on a Pt(111) single crystal, and made simultaneous measurements of \Delta\R/R and the dc resistivity change \Delta rho\ induced by O adsorption on epitaxial Cu(100) films. For both systems the behavior is largely consistent with the scattering model, indicating its applicability to both Drude and non-Drude metals. For CO on Pt(111) the maximum magnitude of \Delta\R/R is 0.28%\+-\0.03% at room temperature and 2800 cm\super -1\. The frequency and temperature-dependences are consistent with the scattering model. The magnitude of \Delta\R/R, however, shows a nonmonotonic dependence on coverage, with a peak at 0.33 ML. This behavior is not explained by the scattering model which assumes randomly placed adsorbates and a coverage-independent scattering cross section. Possible explanations include coherent scattering from a partially ordered overlayer or a coverage-dependent scattering cross section. The scattering model also predicts a linear relationship between \Delta\R/R and \Delta rho\. Our measurements for O on epitaxial Cu(100) generally confirm this prediction.
SS-MoP-3 Local Work Function Measurements on Cu(111)-Au Surface with Scanning Tunneling Microscope Y. Hasegawa, J. Jia, K. Inoue, T. Sakurai (Tohoku University, Japan) We studied on local work function of Au/Cu(111) surface using scanning tunneling microscope (STM). Since the tunneling barrier between the STM tip and sample is directly related to work function of the sample, local work function or surface potential can be measured from a variation of the tunneling current with the gap distance. Using a technique of modulating the gap distance and then detecting its responce in the tunneling current, local work function images can be taken simultaneously with topographic STM images. From images taken on Cu(111) surface partially covered with Au layer, we could clearly observe a difference in local work function between Au layer and Cu substrate. The work function value measured on the first Au layer is 7\+-\3% larger than that of Cu(111) substrate. This ratio is similar with that of the intrinsic work function values of Au(111) and Cu(111) surfaces. We also found that work function at the step edges is lower than that on terrace. It can be explained with a formation of dipole moment at surface steps whose direction is opposite to surface dipole. The width and depth of the low work function region at Au steps is 13\+-\2\Ao\ and 0.9\+-\0.3 eV, respectively. These values agree quantitatively with the results of the work function measurement on the stepped Au(111) surface. This work demonstrates that STM is quite useful to get information on how work function is related with surface structures and how work function spatially changes in a nano-meter scale. It also suggests a possibility that the local work function measurement can be used to identify the element on surfaces.
SS-MoP-4 Orientation and Bonding of Molecular Ferrocene Adsorbed on Ag(100) and Cu(100) C. Hutchings, D. Welipitiya, C. Waldfried, P. Dowben (University of Nebraska, Lincoln) While the preferential bonding orientations of diatomic molecules (N\sub 2\, CO, NO) has been widely investigated by angle resolved photoemission spectroscopy (ARUPS) few such studies exist for large molecules. Adsorption of ferrocene on Ag(100) and Cu(100) has been studied with ARUPS. It is seen that the substrate to ligand interactions play an important role in determining the bonding orientation to these substrates. Photoemission spectra obtained for different light polarizations show that the cyclopentadienyl (Cp) ligands bond parallel to the Ag(100) surface, but perpendicular to the Cu(100) surface. Higher coverages do not change the orientation of the molecules but do alter the bonding energies of the orbitals, indicating the existence of weak lateral interactions. The results for different orientation of the ferrocene molecules on Ag(100) and Cu(100) will be compared with other spectroscopies.
SS-MoP-5 Force Interaction between Tip and Sample in Noncontact Atomic Force Microscopy M. Ohta, H. Ueyama (Osaka University, Japan); Y. Sugawara (Hiroshima University, Japan); S. Morita (Osaka University, Japan) Recently, we succeeded in true atomic resolution imaging with a noncontact mode UHV- AFM on InP(110) surface.[1] However, the measurement condition to achieve the atomic resolution imaging in attractive region is not fully understood. So, we investigate the force gradient as a function of the distance between the tip and InP(110) surface to discuss the dominant tip-sample force interaction in attractive region. The distance dependence of the force gradient in attractive region can be divided into two regions with respect to the distance; the weak distance dependence when the tip is far from the surface (4 nm < z) and the strong distance dependence close to the surface (z < 4 nm). The force gradient in individual region decays at a rate proportional to exp(-z/a). The decay lengths were estimated be 1.6 nm in the region 1 (z < 4 nm) and 10.6 nm in the region 2 (4 nm < z). Therefore, the tip-sample force interaction concentrates at the topmost atom of the tip in the region 1. This result suggest that the atomic resolution imaging in attractive region should be performed under the condition of z < 4 nm. [1] H.Ueyama et al.: Jpn.J.Appl.Phys.34(1995) L1086.
SS-MoP-6 Dimer Flip-Flop Motion and Related Structural Change on the Si(100) Surface Studied by Low-temperature Scanning Tunneling Microscopy H. Shigekawa, K. Miyake, M. Ishida, K. Hata (University of Tsukuba, Japan); T. Nagamura (Unisoku Co., Ltd., Japan) Dimer flip-flop motion and related structural change on the Si(100) surface was studied by scanning tunneling microscopy at 80K and 6K At 80K, the observed surface structure of Si(100) was mainly c(4x2), and symmetric dimers were preferably induced at the boundaries instead of p(2x2) structure, similarly as the previous results obtained at 120 $B!A (B140K. Phase bo undaries on a dimer row, with a similar structure as the type-C defect, which we call type-P defect, formed a pair with another phase boundary on its neighboring dimer row, and fluctuated in a position. Fluctuation between c(4x2) and symmetric structures was observed partially on the surface. At 6K, the apparently symmetric dimers observed at higher temperature were frozen, thereby, p(2x2) structure appered at the boundaries. Therefore, change in the dimer buckling direction described above induced apparent fluctuation between c(4x2) and p(2x2) arragements. Creation and annihilation of the anti-phase domain caused by creation and annihilation of two type-P defects were also observed on an atomic scale.
SS-MoP-7 Nucleation and Growth of C\sub 60\ on Al(111) Studied with STM M. Johansson (Lund University, Sweden); A. Maxwell (Uppsala University, Sweden); S. Gray (Lund University, Sweden); P. Br\um u\hwiler (Uppsala University, Sweden); L. Johansson (Lund University, Sweden); N. Martensson (Uppsala University, Sweden) We have employed scanning tunnelling microscopy and spectroscopy to study the adsorption and interaction of C\sub 60\ nucleate on the lower terrace of the step edges indicating a high mobility of the molecules. The diffusion barrier at the lower step edge is sufficient to hinder any further motion except along the steps as is indicated by the formation of chains of molecules. Before the step edges are saturated the growth of islands begins, extending out on terraces on both sides of a step. The C\sub 60\ (2\sr\3x2\sr\3)R30 symmetry with respect to the substrate surface which is compressed by 1% in all directions compared to the (111) plane of fullerite. Voltage dependent intramolecular structure is observed which indicates that strong adsorbate-substrate bonding occurs, which prevents the molecules from rotating and also shows that the C\sub 60\ six-membered ring facing the surface. Annealing of the adsorbed monolayer results in a thermally stable phase. The C\sub 60\ molecules are arranged into a close-packed hexagonal layer of (2\sr\3x2\sr\3)R30 periodicity with the symmetry reduced to (6x6) as a result of a substrate reconstruction which raises one third of the fullerenes.Molecularly resolved tunnelling spectroscopy on this surface provides experimental evidence for the geometrical displacement in height and shows dramatic differences in the electronic structure between the fullerenes in the two inequivalent adsorption sites.
SS-MoP-8 Adhesion and Plasticity in Clean W/Au Contacts G. Franklin, J. Houston, T. Michalske (Sandia National Laboratories) We have used the recently developed ultra-high vacuum interfacial force microscope (UHV/IFM) to investigate the detailed character of the adhesive bond between clean interfaces consisting of a W tip and a single-crystal Au(100) surface. The IFM is a scanning force microscopy distinguished by its use of a self-balancing force sensor, which has the advantage of eliminating the "snap to contact" present in displacement force-sensor measurements and represents a zero-compliance device. Both interfacial surfaces were cleaned by inert-ion sputtering. The experiments involve bringing the two surfaces into contact (at a constant rate) up to a pre-selected load and then separating them at the same rate. During this process the force, displacement and the conductance of the contact are monitored. The conductance permits a measure of the contact area. We have monitored initial clean contact as well as repeated contacts at the same sample position. In the abscence of the sensor snap to contact, we observe a similar avalanche instability involving the materials themselves. Under the influence of the strong adhesive interaction, the Au surface distorts in order to make contact with the tip. The resulting jump in force is followed by a further rise to a peak attractive force and various forms of hysteretic behavior depending on the rate of approach, etc. We discuss the details of this overall behavior in terms of the surface energies, surface roughness, and Au plasticity, while contrasting the behavior with predictions from standard adhesion models. This work was supported by the DOE under Contract No. DE-AC04-94AL85000.
SS-MoP-9 Nanoindentation Studies of "Staircase" Yielding on Gold Single Crystals S. Corcoran, R. Colton (Naval Research Laboratory); E. Lilleodden (Stanford University); W. Gerberich (University of Minnesota) Nanoindentation studies often show an instantaneous displacement-excursion in the load-displacement curve. This anomaly is generally associated with a surface contamination effect, dislocation emission, a phase transition, or an oxide break-through event. The determination of which effect is operative is often difficult when investigating oxide covered surfaces. We have performed a detailed nanoindentation study on clean, flame annealed single-crystal Au thus eliminating the effects of a surface oxide or contamination layer. We find that the three low index faces of Au exhibit a reproducible displacement excursion in agreement with a dislocation nucleation event. In fact, a new phenomenon of "staircase" yielding was observed in which the plasticity was confined to a series of excursions separated by elastic deformation. Owing to the fact that our radius of contact is more than an order of magnitude smaller than the average dislocation spacing expected for well annealed Au, the excursions are explained in terms of multiple dislocation nucleation events on parallel slip bands and compared to a recently developed model. The dependency of dislocation emission is discussed and compared to indentation data on Au (111), (110), and (100) single crystal surfaces. The effects of adsorbed organic monolayers on dislocation nucleation is also discussed. AFM images of the surface prior and post-indentation were obtained by direct imaging of the surface with the Hysitron, Inc. Picoindenter in conjunction with a Digital Instruments Nanoscope III SPM.
SS-MoP-10 VUV Photoemission Studies of Rare-Earth Thin Films N. Tucker, R. White, R. Blyth, C. Searle, M. Lee, S. Barrett (University of Liverpool, United Kingdom) As a prerequisite to growing multilayers the mechanisms of thin film growth and interface formation should be understood. We have been studying the early stages of rare-earth thin film growth using UV photoemission core-level shift spectroscopy. This is part of the groups investigations into the geometric, electonic and magnetic structure of the rare earths. Three rare earths (Gd, Y and Yb) have been deposited onto a W(110) substrate. Room temperature growth of Gd and Y on the W(110) surface has shown to be of the layer-by-layer type with no alloying occuring at the interface. The films were grown using tungsten a wire basket evaporator (Gd & Y) or knudsen cells (Yb) at a rate of one monolayer every 20 minutes. The film thickness was determined from the relative intensities of photoemission features. For Gd, the growth rate and mode was determined by measuring the Gd 4f photemission signal at resonance as a function of time during deposition. The cleanliness and order of the films was monitored using LEED and valence band photoemission spectroscopy. The W 4f photoemission peaks have been studied during deposition at the Daresbury Synchrotron Radiation Source,in the U.K.. Our spectra show components for the bulk, surface and for the buried interface. In addition, for Gd and Y, another component is seen at a lower binding energy which we attribute to partially co-ordinated W atoms at the surface during the initial stages of the Gd growth. It arises from submonolayer structures formed during the deposition of the first monolayer. This feature is not seen for Yb. The spectra have been fitted and the relative core-level shifts extracted.
SS-MoP-11 Low Energy Photoelectron Diffraction from the Ta(001) Surface: Experimental Measurements and Single Scattering Calculations. W. Siu, B. Utter, R. Bartynski (Rutgers University) Photoemission intensities from both the bulk and surface shifted 4f levels of clean and H-covered Ta(001) were measured as a function of photon energy at normal emission. We have also simulated the intensity modulations using a single scattering cluster (SSC) calculation [D.J. Friedman and C.S. Fadley, Physica Scripta \ul 41\, 948 (1990)]. The SSC calculations indicate that the interference is dominated by 180\deg\ backscattering and thus the intensity profile of surface emission is highly sensitive to the first interlayer spacing, d\sub 12\, while emission from deeper layers is almost independent of this parameter. For photoelectron kinetic energies below about 80 eV, there is surprisingly good agreement between the SSC calculations and previously published multiple scattering (MS) simulations. Comparison between the experimental surface/bulk ratios from the clean surface and the SSC calculations for different values of d\sub 12\, show the best agreement with a 12.5\+-\3\\%\ contraction relative to the bulk values, consistent with previous result. Similar agreement between experimental data and SSC simulations is found for the W(001) surface. Simulation of the H/Ta(001) system indicates that structural changes more extensive than modification of d\sub 12\ are present. These results show that the relatively simple SSC calculations can be used to extract structural parameters of clean metal surfaces from low energy photoelectron diffraction data.
SS-MoP-12 Direct Observation and Quantification of a Double Auger Decay M. Hindi (Tennessee Technical University); Z. Lin (TOSOH SMD, Inc.); R. Avci (Montana State University); P. Miocinovic, R. Kozub (Tennessee Technical University); G. Lapeyre (Montana State University) The double Auger (DA) decay of a K hole has been observed directly by detecting the two emitted electrons in coincidence. The hole was created in \super 37\Cl following the electron capture decay of \super 37\Ar. The probability of DA decay, per Auger decay, with the two electrons both having an energy > 250 eV was found to be 3.7 \+-\ 0.2%. The DA probability was found to decrease exponentially as the energy partitioning between the electrons changed from the asymmetric case (E\sub 1\ >> E\sub 2\) to the symmetric case (E\sub 1\ ~ E\sub 2\). The DA probability accounts for the bulk of the intensity of high charge states previously measured in \super 37\Cl.
SS-MoP-13 Surface Diffusion Effects in Si\sub 1-x\ Ge\sub x\ Film Growth R. Ditchfield, C. Allen, E. Seebauer (University of Illinois, Urbana) # Surface diffusion is believed to influence some aspects of Si\sub1-x\Ge\sub x\ film growth. We have examined surface diffusion of Ge on Si(111) by both experimental and computational means. Experimentally, we have measured by second harmonic microscopy an activation energy of 56 kcal/mole and a pre-exponential factor of 2 x 10\sup 2\ cm\sup 2\/s. No coverage dependence appears in these numbers up to at least 0.5 monolayers. Computationally, we have employed molecular dynamics simulations with a Stillinger-Weber potential to model the atomic motion. Although many complex phenomena appear in the simulations, including island formation and exchange diffusion, the overall diffusion parameters match almost precisely the experimental numbers. We discuss recent film growth results from other laboratories, and the diffusion-based models they have spawned, in terms of the numbers we obtain.
SS-MoP-14 Surface Structure and Dynamics of KTaO\sub3\ by High Resolution Helium Atom Scattering J. Li, J. Baker, E. Akhadov, J. Skofronick, S. Safron (Florida State University); L. Boatner (Oak Ridge National Laboratory) The (001) surface of the perovskite KTaO\sub3\, produced by cleaving in situ, has been examined by high-resolution helium-atom scattering (HAS) over the temperature range from about 50 K to 220 K. Drift spectra (specular beam intensity as a function of incident helium atom wavevector) yield oscillatory patterns consistent with a surface structure having terraces separated by steps of differing heights. Fourier analysis of these spectra indicates that the step heights are of both integral and half-integral lattice spacings and hence that the cleavage produces both KO and TaO\sub2\ planes. The diffraction spots in both the <100> and <110> high-symmetry directions correspond to the lattice spacing expected from the bulk crystal (about 4\Ao\). However, for a fixed incident helium atom wavevector, the relative diffraction intensities appear to vary with the surface temperature suggesting that the surface corrugation changes markedly with temperature. Additionally, at temperatures below about 140 K small, broad half-order spots can be observed. As phase changes have not been seen in the bulk structure of KTaO\sub3\, these observations suggest that the surface behavior of this material is a much more sensitive indicator of its dynamical properties. To this end, studies of the surface phonon dispersion are being carried out in both high symmetry directions. This work is funded by U.S. DOE grant No. DE-FG05-85ER45208 and (LAB) contract No. DE-AC05-84OR21400.
SS-MoP-15 STM Study of the Initial Stages of Growth of Cu/Si(111) - The Formation and Thermal Evolution of Cu\sub 3\Si and the "5x5" Incommensurable Structure R. Sim\t a\o, C. Achete (Universidade Federal do Rio de Janeiro, Brazil); H. Niehus (Humboldt Universit\um a\t, Germany) The epitaxial growth and the structure of Cu on Si(111)7x7 was investigated by Scanning Tunneling Microscopy (STM) and Auger Electron Spectroscopy (AES). The so called quasi-"5x5" incommensurable structure of Cu/Si(111) was observed by STM with atomic resolution only probing the surface empty states with 2,5V. STM showed the "5x5" structure as made up of rounded shaped clusters with periodicity between (5,3x5,3) and (5,7x5,7) times the Si bulk unit. The "5x5" structure grows layer-by-layer and is composed of both silicon and copper in a two dimensional arrangement. Films of 2ML of Cu/Si(111) heated up to less than 500oC presents a very interesting and uncommon arrangement. A first, complete layer of the incommensurable structure is formed and over it a second layer of bidimensional crystals with irregular interfaces begins to nucleate. Over these bidimensional crystals, more bidimensional crystals can grow. All the layers are made up of the same clusters and the periodicity of the arrangement differs from layer to layer. There are many different rotated domains on all the layers. The "5x5" domains grow upon annealing to 550oC for 1 minute and the angle of rotation between the domains is only 7o+/- 1o on the bidimensional crystals. Further annealing leads to pure silicon migration to the surface where it can be found reconstructed Si(111)-5x5. The formation of tridimensional crystals made out of Cu3Si was also observed on 1 to 2ML Cu/Si(111) films annealed to temperatures above 700oC and their electronic structure is completely different from the one observed for the incommensurable "5x5" structure.
SS-MoP-16 Lead-Deposition-Induced Interface-Stress Measurements on Microfabricated Cantilevers in an Electrochemical Environment P. Oden, T. Thundat, R. Warmack (Oak Ridge National Laboratory) Electrochemical cyclic voltammetry for solution-based metallic-deposit characterization is a standard method in solid-liquid interface science. We have carried out cyclic voltammetry on a microcantilever coupled with cantilever-deflection monitoring for investigating the surface stresses introduced by the deposited species. Specifically, we have investigated the deposition of Pb onto polycrystalline Au-coated microcantilevers with simultaneous surface stress and electrochemical-current measurements for both continuous and stepped electrochemical potentials. This technique can be used as a compliment to other more standard electrochemical methods for investigations of solid-liquid interface properties.
SS-MoP-17 Calibration of Frictional Forces in Atomic Force Microscopy R. Carpick (Lawrence Berkeley National Laboratory and University of California, Berkeley); D. Ogletree, M. Salmeron (Lawrence Berkeley National Laboratory) Although some techniques to calibrate normal forces measured with atomic force microscopy (AFM) have been described or proposed, there has not yet been a general method presented to calibrate lateral (friction) forces. Here we present an in-situ method, called the "wedge calibration technique"[1], which calibrates lateral forces in terms of normal forces. We show that this technique can be used to reliably calibrate the lateral force response of the AFM, including those using the common commercial V-shaped cantilevers. The method also provides information on the AFM tip shape at the nanometer-scale. The measured calibration compares well with theoretical calculations of the lateral force response of V-shaped cantilevers. [1] D.F. Ogletree, R.W. Carpick, M. Salmeron, Rev. Sci. Instrum., submitted.
SS-MoP-18 Growth and Structural Characterization of Magnesium Chloride Thin Films on Pd(111) D. Fairbrother, J. Roberts, G. Somorjai (University of California, Berkeley) Magnesium chloride provides the support for the active phase in the Ziegler-Natta polymerization process. However, very little work has been carried out regarding its structural characterization. To this end gas phase deposition of magnesium chloride onto a Pd(111) surface under ultra-high vacuum conditions has been found to provide a successful route to prepare and characterize well ordered structures. At about 1 monolayer a 13 x 13 R(13.9) LEED pattern is observed which subsequently evolves into a well resolved (1 x 1) structure by 4 layers. The crystallographic orientation of this overlayer is observed to lie parallel to that of the underlying Pd(111) substrate. The apparent role of the surface in controlling the crystallographic orientation of the overlayer is supported by desorption measurements which reveal the presence of both monolayer and multilayer states. Structural calculations, based upon intensity-voltage measurements, will also be presented.
SS-MoP-19 The Parallel Dimer Structure of Sn on Si(001) Surfaces J. Glueckstein, M. Evans, J. Nogami (University of Wisconsin, Milwaukee) We have studied the growth of Sn on Si(001) surfaces at low coverages using scanning tunneling microscopy (STM). The metal self assembles into chains running perpendicular to the Si dimer rows underneath. High resolution STM images provide strong support for the "parallel dimer" model for the Sn in these chains, with the adatom dimerization parallel to that of the Si dimers. There is also evidence that these dimers buckle. The buckling is seen in STM as a displacement of the Sn dimer with respect to the most symmetric site above the Si dimer rows. This displacement occurs along the Sn chains, and is enhanced when parallel Sn chains lie two unit cells apart, the minimum allowed distance. As the coverage approaches 0.5 monolayers, the Sn rows entirely cover the surface, forming a 2x2 reconstruction. The 2x2 is disordered due to the displacement of the Sn dimers.
SS-MoP-20 Temperature, Flux and Time Dependent Investigations of the Interaction of Ag with Si(111)7x7 using Variable Temperature STM A. Feltz, H. Wengelnik, M. Sander (Omicron Vakuumphysik GmbH, Germany); F. Henn (Omicron Associates); T. Berghaus (Omicron Vakuumphysik GmbH, Germany) The interaction of Ag with Si(111)7x7 was studied in the temperature range 50 K to 950 K using variable temperature STM during in-situ adsorption/desorption of Ag. In contrast to ex-situ experiments the ability of the VT STM to adjust the temperature, in conjunction with precise in-situ dosing of the adsorbate allows to follow and characterise the dynamic behaviour, i.e. structural transformations as well as the thermodynamics of the interaction processes. Three stages of structural changes have been observed upon temperature increase during Ag adsorption: growth of clusters, thermally activated formation of Si(111)/Ag(root3xroot3)R30, and reappearance of the Si(111)7x7 during the desorption of Ag. The temperature range from RT to 550 K can be described by Stranski-Kranstanov (SK) growth; at temperatures above 550 K this growth mode ceases and formation of the Si(111)/Ag(root3xroot3)R30 structure is observed. Desorption of Ag is accompanied by the reappearance of Si(111)7x7 above 900 K. SK growth on top of the reconstructed Si(111)/Ag(root3xroot3)R30 was found only after cooling to below RT. At T = 200 K large clusters were visible indicating a considerable diffusion length of Ag; smaller clusters formed at 50 K, but even here indications were found that Ag is still mobile on the surface. It can be concluded, that for the same temperature the diffusion length of Ag on Si(111)/Ag(root3xroot3)R30 is strongly increased compared to Si(111)7x7. An outlock will be given on a new variable temperature UHV AFM. VT AFM becomes feasable by the new needle sensor for AFM detection; it overcomes limitation of standard cantilevers because of its insensitivity against temperature changes.
SS-MoP-21 Electronic Structure of the Si(111)\sr 3\X\sr 3\R\30 DEG\-Bi Surface J. Kim, J. Lee, C. Hwang, R. Park, K. An, C. Park (Sung Kyun Kwan University, Korea) Si(111)\sr 3\X\sr 3\R\30 DEG\-Bi surface was studied by angle resolved ultraviolet photoelectron spectroscopy (ARUPS) with He I discharge lamp and low energy electron diffraction (LEED). It was proposed that two phases for this surface existed at the different coverage. One is a single atom adsorption (T\sub 4\ site or H\sub 3\ site) phase with coverage of 1/3ML(1ML is the same atomic density as the ideal Si(111)1X1 surface.), the other is a trimer (or milk-stool) phase with coverage of 1ML. We measured the surface state dispersion along \bar GAMMA\-\bar MU\-\bar GAMMA\ and \bar GAMMA\-\bar KAPPA\-\bar MU\ directions of the \sr 3\X\sr 3\R\30 DEG\ surface Brillouin zone (SBZ) for the two phases, respectively. In case of the phase with 1ML coverage, it has been found that four surface states in the spectra exist on the \sr 3\X\sr 3\R\30 DEG\-Bi surface and this surface is semiconducting within the experimental error. For the other 1/3ML phase, a metallic surface state was observed near the Fermi level in the range of the small polar angle, which is very similar to those of the Si(111)\sr 3\X\sr 3\R\30 DEG\-Al surface.\super 1\ Except for this metallic surface state near the Fermi level, the dispersion of the three surface state bands of the 1/3ML phase is in accordance with the results of the theoretical calculation for the Si(111)\sr 3\X\sr 3\R\30 DEG\-column V metal surfaces with the coverage of 1/3ML.\super 2\ 1. T. Kinoshita, S. Kono, and T. Sagawa, Phys., Rev., B32, 2714(19895). 2. H. Nagayoshi, Surf., Sci., 242, 237(1991).
SS-MoP-22 Electronic Structure and Orientation of Self-assembled Monolayers of Oligo(p-phenylene)s on Gold and Copper Investigated with NEXAFS D. Fischer, G. H\um a\hner, N. Spencer, S. Brunner, W. Caseri, U. Suter (ETH Zurich, Switzerland) Near-Edge X-Ray Fine Structure Spectroscopy (NEXAFS) was used to investigate the adsorption of self-assembled substituted oligo(p-phenylene)s on gold and copper surfaces from solution. These compounds are in between flexible and rod-like molecules and may display a liquid crystalline character. The oligomers were monodisperse and contained 3-9 p-phenylene units, one of which was substituted by flexible alkyl side chains with various lengths. The thickness of the formed layers was below 1 nm on both gold and copper substrates. NEXAFS spectra, studied at the carbon K-edge, indicate that the interaction of the oligomers with the substrate occurs via the arene moieties. Depending on the substrate, strong differences in the spectra can be observed. The interaction with the gold substrate is weaker than that with the copper substrate. For the latter case the spectra show a strongly diminished \pi\\super *\ resonance at 285.4 eV. This is consistent with an earlier assumption that \eta\\super 2\- or \eta\\super 4\-coordinated arene moieties are involved in the interaction. Results from IR spectroscopy also agree with this hypothesis. Analysis of the polarization dependence of the NEXAFS spectra reveals a preferential orientation of the adsorbates, with the tilt angle depending on the substrate and the length of the main and side chains. Findings from NEXAFS are supported and complemented by ellipsometry, IR spectroscopy at grazing incidence reflection, XPS, ToF-SIMS and contact angle measurements.
SS-MoP-23 An Optical Submonolayer Surface Coverage Probe: Linear Differential Reflectometry Applied to Adsorption and Desorption Kinetics on a Metal Surface J. Dvorak, H. Dai (University of Pennsylvania) A simple optical differential reflection technique has been applied to study kinetic processes occurring on a metal surface. When an adsorbate binds to a metal surface, the reflectivity is perturbed sufficiently to allow for submonolayer detection of adsorbed species. This phenomenon has been used to monitor the adsorption of carbon monoxide, acetylene, and oxygen on Cu(100). It is found that for carbon monoxide, the adsorption induced reflectivity change is linear with coverage, and a sensitivity of 0.01 monolayer is achieved. In addition, for (?) carbon monoxide it is found that changes in the adsorption and desorption kinetics correlate with structural changes that occur in the monolayer as a function of coverage. Furthermore, the chemical nature of the adsorption process is found to influence the strength of the induced reflectivity changes, with chemisorbed monolayers causing approximately 1% reflectivity change at saturation coverage, while for physisorbed monolayers the induced reflectivity change is negligible (<0.1%).

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