Peer-reviewed Publications |
Amiaud, L., Martin, I., Milosavljevic, A. R., Michaelson, S., Hoffman, A., Azria, R., & Lafosse, A. (2011). Low-energy electron scattering on deuterated nanocrystalline diamond films-a model system for understanding the interplay between density-of- states, excitation mechanisms and surface versus lattice contributions. PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 13(24), 11495–11502.
Résumé: Electron energy loss spectrum, elastic reflectivity and selected vibrational excitation functions were measured by High Resolution Electron Energy Loss Spectroscopy (HREELS) for deuterated nanocrystalline dc GD CVD diamond films. The electron elastic reflectivity is strongly enhanced at about 13 eV, as a consequence of the second absolute band gap of diamond preserved up to the surface for D-nano-crystallites. The pure bending modes delta(CD(x)) at 88 meV and 107 meV are dominantly excited through the impact mechanism and their vibration excitation functions mimic the electron elastic reflectivity curve. Pure diamond phonon mode nu(CC) can be probed through the resolved fundamental loss located at 152 meV and through the multiple loss located at 300 meV. In addition to the well-known 8 eV resonance, two supplementary resonances located at 4.5 eV and 11.5 eV were identified and clearly resolved for the first time. A comprehensive set of data is now available on low-energy electron scattering at hydride terminated polycrystalline diamond films grown either by HF (microcrystalline) or dc GD (nanocrystalline) chemical vapour deposition. The careful comparison of the vibrational excitation functions for hydrogen/deuterium termination stretching modes nu(sp(3)-CH(x)) and nu(sp(3)-CDx), for hydrogen termination bending modes delta(CH(x)) mixed with diamond lattice modes nu(CC), for deuterium termination bending modes delta(CD(x)), and for multiple loss 2 nu(CC) demonstrates the close interplay between three characteristics: (i) the density-of-states of the substrate, (ii) the vibrational excitation mechanisms (dipolar and/or impact scattering including resonant scattering) and (iii) the surface versus lattice character of the excited vibrational modes. This work shows clearly that excitation function measurement provides a powerful and sensitive tool to clarify loss attributions, involved excitation mechanisms, and surface versus lattice characters of the excited vibrational modes.
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Azria, R., Lafosse, A., Amiaud, L., Michaelson, S., & Hoffman, A. (2011). Hydrogenated polycrystalline diamond films: Elastic and inelastic electron reflectivity. PROGRESS IN SURFACE SCIENCE, 86(5-8), 94–114.
Résumé: The microstructure and properties of carbon-based thin films depend on the deposition process and conditions used, including pressure, gas phase composition, and substrate temperature, as well as the energy of the reactive species (atoms or ions). For instance concerning diamond films, each method results in different type of films which may differ in terms of diamond grain size (from nano to micro), grain boundary nature, hydrogen content, defect density, amorphous or graphitic components, morphological properties and different chemical and physical properties. Among them, the well-known negative electron affinity, very attractive for the detection and emission of electrons, and high conductivity of diamond surfaces are properties of fully hydrogenated diamond surfaces. Similarly, diamond grain size may influence the electronic and optical properties of the films. More generally the chemical and physical characterization of the uppermost surface atomic layer of diamond films presents a great challenge. In this review we present results on hydrogen bonding configuration in hydrogenated polycrystalline diamond films of varying size (few nanometers up to micrometers) obtained by high resolution electron energy loss spectroscopy (HREELS). More precisely we will present energy loss spectra extended up to 800 meV, as well as elastic and inelastic reflectivity curves (associated to different vibrational modes of hydrogenated diamond surfaces), measured over the 3-18 eV electron energy range. We will show in particular that due to the specific features of diamond bulk electronic band structure, which is maintained up to the surface in the case of fully hydrogenated diamond, it is possible to extract from these data valuable information about the surface properties and composition such as diamond or graphitic like nature of the films, surface versus lattice nature of the vibrational modes. (C) 2011 Elsevier Ltd. All rights reserved.
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Bundaleski, N., Soulisse, P., Momeni, A., Khemliche, H., & Roncin, P. (2011). Decoherence in fast atom diffraction from surfaces. Nucl. Instrum. Methods Phys. Res., B, 269(11), 1216–1220.
Résumé: Diffraction of fast atoms from crystal surfaces at grazing incidence (GIFAD) has now been observed on all types of materials, from wide band gap insulators to metals, including semiconductors. Since mainly the (slow) motion normal to the surface is important diffraction patterns are comparable to those obtained in thermal energies atomic diffraction (TEAS), however, the specific scattering geometry of GIFAD has a strong influence on decoherence phenomena. The contribution of atomic vibrations is much less pronounced than in TEAS but other sources of decoherence such as electronic excitations, clearly observed on metals, can participate due to the comparatively large projectile velocity parallel to the surface. We present here simple models that describe these decoherence effects. The results are in good agreement with the experimental results. (C) 2010 Elsevier B.V. All rights reserved.
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Chaudhari, V., Harish M.N.K., Srinivasan, S., & Esaulov, V. A. (2011). Substitutional Self-Assembly of Alkanethiol and Selenol SAMs from a Lying-Down Doubly Tethered Butanedithiol SAM on Gold. JOURNAL OF PHYSICAL CHEMISTRY C, 115(33), 16518–16523.
Résumé: Substitutional self-assembly of thiol and selenol SAMs from a lying-down phase of butanedithiol (C4DT) (SAM) were investigated using thiols, disulfide, and diselenide molecules. The intent was to address the question if formation of a lying-down dithiol phase is an impediment to formation of standing-up dithiol phases as it has been assumed. It is demonstrated that this is not the case, and the C4DT SAM, where both the sulfur atoms are chemisorbed on gold, is removed and replaced in all cases. Differences in substitution kinetics are observed.
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Chen, L., Shen, J., Jia, J. J., Kandasamy, T., Bobrov, K., Guillemot, L., Fuhr, J. D., Martiarena, M. L., & Esaulov, V. A. (2011). Li+-ion neutralization on metal surfaces and thin films. PHYSICAL REVIEW A, 84(5), 052901.
Résumé: Li+ ions with energies ranging from 0.3 to 2 keV are scattered from Au(110) and Pd(100) surfaces and from ultrathin Ag film grown on Au(111) in order to study electron transfer phenomena. We find that neutralization occurs quite efficiently and find an anomalous ion energy dependence of the neutral fraction for Au(110) and Pd(100) surfaces previously noted for Au(111). The dependence of the neutral fraction on the azimuthal angle of the Au(110) and Pd(100) surfaces is reported. In the case of Ag monolayer on Au(111), results are similar to the case of the Ag(111) surface. To understand the anomalous ion energy dependence, we present a theoretical study using density functional theory (DFT) and a linearized rate equation approach, which allows us to follow the Li charge state evolution for the (111) surfaces of Ag, Au, and Cu, and for the Ag-covered Au(111) surface.
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Chen, L., Shen, J., Valdes, J. E., Vargas, P., & Esaulov, V. A. (2011). Energy loss of keV fluorine ions scattered off a missing-row reconstructed Au(110) surface under grazing incidence. PHYSICAL REVIEW A, 83(3), 032901.
Résumé: A joint experimental and theoretical study of energy loss is presented for 1-to-4-keV fluorine negative ions in grazing scattering on a missing-row reconstructed Au(110) surface. Measurements of energy losses for various azimuthal orientations of the crystal have been performed by means of a time-of-flight method with a pulsed beam. The dependence of the fraction of surviving negative ions on azimuthal angles, was determined. Our energy-loss data are discussed in light of trajectory and stopping-power calculations, where the explicit inclusion of the nonuniform electron density at the surface provides good agreement with the experimental data. The simulation allows us to delineate various trajectory classes that correspond to different contributions in the energy-loss spectra for various azimuthal orientations of the surface.
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Chen, L., Valdes, J. E., Vargas, P., Shen, J., & Esaulov, V. A. (2011). Energy losses of H and F ions in grazing scattering on a missing row reconstructed Au(110) surface. PHYSICA SCRIPTA, T144, 014042.
Résumé: A joint experimental and theoretical study of low-keV H and F ions in grazing scattering on a missing row reconstructed Au(110) surface is presented. We show the influence of surface electronic corrugation and trajectory effects on energy-loss spectra. Measurements of energy losses for grazing angles scattering in surface channeling conditions for various azimuthal orientations of the crystal have been performed and discussed in semi-classical simulations, which allow us to delineate various trajectory classes that correspond to different contributions in the energy-loss spectra for various azimuthal orientations of the surface.
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Flores, M., Esaulov, V., & Yamazaki, Y. (2011). Molecular ion emission from alkanethiol-SAMs by HCI bombardment. PHYSICA SCRIPTA, T144, 014045.
Résumé: In this work, we employ highly charged ions to study the sputtering of positive molecular fragments from two different alkanethiol self-assembled monolayers (SAMs) on gold surfaces: undecanethiol and dodecanethiol. The SAMs are bombarded with a pulsed Arq+ beam (3 < q < 12) with kinetic energies ranging from 1 to 15 keV. The desorbed positive molecular ions were detected and analyzed from time-of-flight spectra, and thereby the masses and yields of secondary ions were obtained. The proton yields are dependent on the charge state of the incident ion. On the other hand, the positive molecular ion yields, such as CnHm+, are charge state independent. The positive molecular ion yields decay with the molecular size n.
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Guillemot, L., & Bobrov, K. (2011). Formation of a Chemisorbed Water-Hydroxyl Phase on Cu(110) Mediated by Surface Transport. JOURNAL OF PHYSICAL CHEMISTRY C, 115(45), 22387–22392.
Résumé: Under ambient conditions, most of the solid surfaces are commonly covered with water and oxygen both playing a crucial role in catalysis, corrosion, and electrochemistry. We present an STM study on water interaction with the oxygen preadsorbed Cu(110)-(2 x 1)-O surface. Under water adsorption at similar to 200 K, the (2 x 1) added-on Cu-O chains are gradually transformed into mobile copper-hydroxyls complexes. Long-range transport and condensation of the complexes induce nucleation and growth of 2D copper islands. Simultaneously the associated hydroxyls self-assemble with oncoming water molecules into a well-ordered 2D water-hydroxyl chemisorbed phase thermally stable up to a room temperature. We found that the assembly of the water-hydroxyl phase could be consistently described in terms of H bonds formation between the adsorbate species. We anticipate that the understanding of the forces driving the chemisorption will stimulate studies on water chemisorption on other metal surfaces, which are still far from being completed.
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Guillemot, L., & Bobrov, K. (2011). Morphological instability of the Cu(110)-(2 x 1)-O surface under thermal annealing. PHYSICAL REVIEW B, 83(7), 075409.
Résumé: We present a scanning tunneling microscope study on reactivity of chemisorbed oxygen on the Cu(110)-(2 x 1)-O surface. We have found that the Cu(110)-(2 x 1)-O surface is intrinsically unstable under thermal annealing in the 400-900 K range. In the 455-570 K range, the surface undergoes faceting. The orientational transition of the adsorbed oxygen phase displays wide [110] terraces, covered by (2 x 1)-O bands self-assembled into a superstructure, as well as bunches of oxygen-free narrow terraces. We found that the wide [110] terraces are intrinsically unstable against further restructuring at their edges. The restructuration is driven by reversible thermal dissociation of the (2 x 1)-O bands. The slightly uneven oxygen band density between terraces, consequently differing in reactivity with respect to Cu-O fragments, induces Cu atom transport between their edges. The interplay between thermal dissociation of the (2 x 1)-O bands and long-range elastic relaxation of the strained surface is suggested to be the origin of the observed inhomogeneous oxygen distribution. In the 570-810 K range the Cu atom transport reveals continuous growth of the oxygenated [110] terraces. We discuss in detail the mechanism of the Cu transport, which results in a rapid propagation of the oxygenated terraces as well as a strain development on the surface.
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Hamoudi, H., Dablemont, C., & Esaulov, V. A. (2011). Disorder, solvent effects and substitutional self-assembly of alkane dithiols from alkane thiol SAMs. SURFACE SCIENCE, 605(1-2), 116–120.
Résumé: Substitutional self-assembly of nonanedithiol from a hexanethiol self-assembled monolayer and the effects of use of ethanol and n-hexane as solvents were investigated. It was found that presence of original solvents in the HT SAM allowed easy replacement, while dried SAMs were more resilient to substitution and much longer times are necessary for substitution to occur. In general substitution in ethanol leads to production of a disordered dithiol SAM, with existence of extra molecules bonded to the SAM. Well ordered substituted SAMs were obtained with degassed n-hexane solutions and in absence of ambient light. (C) 2010 Elsevier B.V. All rights reserved.
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Kojima, T. M., Ikeda, T., Kanai, Y., Yamazaki, Y., & Esaulov, V. A. (2011). Ion beam guiding with straight and curved Teflon tubes. JOURNAL OF PHYSICS D-APPLIED PHYSICS, 44(35), 355201.
Résumé: In an effort to develop a flexible ion beam guiding scheme, the guiding capabilities of straight and curved Teflon tubes were tested with 8 keV Ar(8+) ions. The tubes used were about 50 mm long and of 1 mm/2mm inner and outer diameters. One was straight, and the others were bent with different radii of curvature corresponding to bending angles from 9.6 degrees to 26.7 degrees. Transmission of several tens of per cent of the injected beam was observed for the curved tubes, while transmission through the tilted straight tube vanished when the tilt was more than 7 degrees. This demonstrates the possibility of efficient ion beam guiding with flexible insulator tubes.
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Kristensen, L. E., Amiaud, L., Fillion, J. - H., Dulieu, F., & Lemaire, J. - L. (2011). H-2, HD, and D-2 abundances on ice-covered dust grains in dark clouds. ASTRONOMY & ASTROPHYSICS, 527.
Résumé: Aims. We seek to study the abundances of H-2, HD, and D-2 adsorbed onto ice-covered dust grains in dark molecular clouds in the interstellar medium. Methods. We use our previously developed detailed model describing temperature-programmed desorption (TPD) experiments of H-2 and its isotopologues on water ice. We here extrapolate these model results from laboratory conditions to conditions similar to those found in dark molecular clouds. Results. By means of our model we are able to infer three important results. (i) The time scale for H-2 and isotopologues to accrete onto dust grains is less than 10(4) yrs. (ii) Due to the higher binding energy of D-2 with respect to HD, D-2 becomes the most abundant deuterated species on grains by similar to 50% with respect to HD (a few times 10(-5) with respect to H-2). (iii) The surface coverage of D-2 as a function of temperature shows that at very low temperatures (i. e., less than 10 K), D-2 may be two orders of magnitude more abundant than HD. Possible implications for deuteration of water on grain surfaces are discussed when it forms through reactions between OH and H-2.
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Pasquali, L., Terzi, F., Seeber, R., Nannarone, S., Datta, D., Dablemont, C., Hamoudi, H., Canepa, M., & Esaulov, V. A. (2011). UPS, XPS, and NEXAFS Study of Self-Assembly of Standing 1,4-Benzenedimethanethiol SAMs on Gold. LANGMUIR, 27(8), 4713–4720.
Résumé: We report a study of the self-assembly of 1,4-benzenedimethanethiol monolayers on gold formed in n-hexane solution held at 60 degrees C for 30 mm and in dark conditions. The valence band characteristics, the thickness of the layer, and the orientation of the molecules were analyzed at a synchrotron using high resolution photoelectron spectroscopy and near edge X-ray adsorption spectroscopy. These measurements unambiguously attest the formation of a single layer with molecules arranged in the upright position and presenting a free -SH group at the outer interface. Near edge X-ray absorption fine structure (NEXAFS) measurements suggest that the molecular axis is oriented at 24 degrees with respect to the surface normal. In addition, valence band features could be successfully associated to specific molecular orbital contributions thanks to the comparison with theoretically calculated density of states projected on the different molecular units.
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