Peer-reviewed Publications |
Baffou, G., Mayne, A. J., Comtet, G., Dujardin, G., Stauffer, L., & Sonnet, P. (2009). SiC(0001) 3 x 3 Heterochirality Revealed by Single-Molecule STM Imaging. J. Am. Chem. Soc., 131(9), 3210–3215.
Résumé: We report a description of the SiC(0001) 3 x 3 silicon carbide reconstruction based on single-molecule scanning tunneling microscopy (STM) observations and density functional theory calculations. We show that the SiC(0001) 3 x 3 reconstruction can be described as contiguous domains of right and left chirality distributed at the nanoscale, which breaks the to date supposed translational invariance of the surface. While this surface heterochirality remains invisible in STM topographies of clean surfaces, individual metal-free phthalocyanine molecules chemisorbed on the surface act as molecular lenses to reveal the surface chirality in the STM topographies. This original method exemplifies the ability of STM to probe atomic-scale structures in detail and provides a more complete vision of a frequently studied SiC reconstruction.
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Chiaravalloti, F., Riedel, D., Dujardin, G., Pinto, H. P., & Foster, A. S. (2009). STM topography and manipulation of single Au atoms on Si(100). Phys. Rev. B, 79(24), 245431.
Résumé: The low-temperature (12 K) adsorption of single Au atoms on Si(100) is studied by scanning tunneling microscopy (STM). Comparison between experimental and calculated STM topographies as well as density-functional-theory calculations of the adsorption energies enable us to identify two adsorption configurations of Au atoms between Si-dimer rows (BDRs) and on top of Si-dimer rows (TDRs). In both adsorption configurations, the Au atoms are covalently bound to two Si atoms through a partial electron transfer from Si to Au. STM manipulation confirms that the TDR adsorption configuration is metastable, whereas the BDR one is the most stable configuration.
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Cranney, M., Chalopin, Y., Mayne, A. J., & Dujardin, G. (2009). Adsorption and organisation of para-hexaphenyl molecules on Si(100). Appl. Phys. A, 94(4), 767–773.
Résumé: Para-hexaphenyl molecules (p-6P: C(36)H(26)) can be grown as nanofibres on various surfaces having optical properties of technological relevance. We report here the first observations of the initial stages of adsorption of individual p-6P molecules on Si(100)-(2x1) using room temperature Scanning Tunnelling Microscopy (STM). Depending on the substrate temperature, the hexaphenyl molecules adsorb in two configurations; indicating both weak and strong chemisorption. All six phenyl rings are clearly visible and the molecules are found to adsorb with characteristic angles between the long axis of the molecule and the silicon dimer rows. A statistical analysis of the spatial distribution of the molecules suggests that the clustering required for crystallographic nanofibre growth does not occur at the atomic scale under the experimental conditions used here.
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Kara, A., Leandri, C., Davila, M., Padova, P., Ealet, B., Oughaddou, H., Aufray, B., & Lay, G. (2009). Physics of Silicene Stripes. JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 22(3), 259–263.
Résumé: Silicene, a monolayer of silicon atoms tightly packed into a two-dimensional honeycomb lattice, is the challenging hypothetical reflection in the silicon realm of graphene, a one-atom thick graphite sheet, presently the hottest material in condensed matter physics. If existing, it would also reveal a cornucopia of new physics and potential applications. Here, we reveal the epitaxial growth of silicene stripes self-aligned in a massively parallel array on the anisotropic silver (110) surface. This crucial step in the silicene “gold rush” could give a new kick to silicon on the electronics road-map and open the most promising route towards wide-ranging applications. A hint of superconductivity in these silicene stripes poses intriguing questions related to the delicate interplay between paired correlated fermions, massless Dirac fermions and bosonic quasiparticles in low dimensions.
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Le Lay, G., Aufray, B., Leandri, C., Oughaddou, H., Biberian, J. P., De Padova, P., Davila, M. E., Ealet, B., & Kara, A. (2009). Physics and chemistry of silicene nano-ribbons. APPLIED SURFACE SCIENCE, 256(2), 524–529.
Résumé: We review our recent discovery of silicene in the form of silicon nano-ribbons epitaxially grown on silver (1 1 0) or (1 0 0) surfaces, which paves the way for the growth of graphene-like sheets. We further draw some perspectives for this unique novel material upon mild hydrogenation. (C) 2009 Elsevier B.V. All rights reserved.
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Riedel, D., Bocquet, M. L., Lesnard, H., Lastapis, M., Lorente, N., Sonnet, P., & Dujardin, G. (2009). Selective Scanning Tunnelling Microscope Electron-induced Reactions of Single Biphenyl Molecules on a Si(100) Surface. J. Am. Chem. Soc., 131(21), 7344–7352.
Résumé: Selective electron-induced reactions of individual biphenyl molecules adsorbed in their weakly chemisorbed configuration on a Si(100) surface are investigated by using the tip of a low-temperature (5 K) scanning tunnelling microscope (STM) as an atomic size source of electrons. Selected types of molecular reactions are produced, depending on the polarity of the surface voltage during STM excitation. At negative surface voltages, the biphenyl molecule diffuses across the surface in its weakly chemisorbed configuration. At positive surface voltages, different types of molecular reactions are activated, which involve the change of adsorption configuration from the weakly chemisorbed to the strongly chemisorbed bistable and quadristable configurations. Calculated reaction pathways of the molecular reactions on the silicon surface, using the nudge elastic band method, provide evidence that the observed selectivity as a function of the surface voltage polarity cannot be ascribed to different activation energies. These results, together with the measured threshold surface voltages and the calculated molecular electronic structures via density functional theory, suggest that the electron-induced molecular reactions are driven by selective electron detachment (oxidation) or attachment (reduction) processes.
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Riedel, D., Cranney, M., Martin, M., Guillory, R., Dujardin, G., Dubois, M., & Sonnet, P. (2009). Surface-Isomerization Dynamics of trans-Stilbene Molecules Adsorbed on Si(100)-2x1. J. Am. Chem. Soc., 131(15), 5414–5423.
Résumé: Photoinduced trans-cis isomerization studies of stilbene molecules in the gas phase have led to a precise understanding of the corresponding molecular dynamics. Yet, when such molecules are adsorbed on surfaces, these reactions are expected to be strongly modified as compared to what is know in the gas phase. In this work, a low temperature (5 K) scanning tunneling microscope (STM) is used to image the trans-stilbene molecules deposited on a Si(100)-2x1 surface at 12 K. trans-Stilbene undergoes conformational changes during the adsorption process such that four different stilbene conformers are observed: trans-stilbene (TS), cis-stilbene (CS), and two new conformers I(1) and I(2). Furthermore, electronic excitation of individual stilbene molecules, by means of tunnel electrons, is shown to activate specific reversible molecular surface isomerization (TS <-> I(1) and CS <-> I(2)) combined with diffusion across the surface. Calculated STM topographies, using the tight binding method, indicate that the CS and TS molecules are physisorbed. The molecular conformations of the surface isomers I(1) and I(2) are suggested to be analogous to transient states conformations of the stilbene molecule when stabilized by the silicon surface. The measurements of the molecular surface isomerization and diffusion reaction yields are used to build a qualitative potential energy surface of the various stilbene reactions. The molecular surface-isomerization dynamics is shown to be influenced by the type of dopant (n or p). This is related to surface charging, which reveals modifications in the stilbene ionization potential.
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Riedel, D., Delacour, C., Mayne, A. J., & Dujardin, G. (2009). Very low thermally induced tip expansion by vacuum ultraviolet irradiation in a scanning tunneling microscope junction. Phys. Rev. B, 80(15), 155451.
Résumé: The thermal and photoelectronic processes induced when a vacuum ultraviolet (VUV) laser irradiates the junction of a scanning tunneling microscope (STM) are studied. This is performed by synchronizing the VUV laser shots with the STM scan signal. Compared to other wavelengths, the photoinduced thermal STM-tip expansion is not observed when the VUV radiation is freed from spurious emissions. Furthermore, we demonstrate that the purified VUV photoinduced transient signal detected in the tunnel current is entirely due to photoelectronic emission and not combined with thermionic processes. The ensuing photoelectron emission is shown to be independent of the tip-surface distance while varying linearly with the pure VUV laser intensity. These results illustrate a strong decoupling between phonons and photoelectrons which allows a very weak STM-tip expansion.
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Vizzini, S., Oughaddou, H., Hoarau, J. Y., Biberian, J. P., & Aufray, B. (2009). Y Growth of ultrathin film aluminum oxide on Ag(111). APPLIED PHYSICS LETTERS, 95(17), 173111.
Résumé: Scanning tunneling microscopy (STM) and spectroscopy and electron energy loss spectroscopy were used to characterize ultrathin epitaxial aluminum oxide on Ag(111) which was prepared via a specific deposition and oxidation process. Atomically resolved STM images show self-organized oxide stripes of 4 nm width with excellent homogeneity in depth and chemical composition properties which are crucial for applications of this oxide to magnetic tunnel junctions. We also found that electrons tunnel easily through the oxide film. This oxide has a different spectroscopic signature from that of alumina and has a wide bandgap close to 6.6 eV. (C) 2009 American Institute of Physics. [doi:10.1063/1.3251778]
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