Peer-reviewed Publications |
Bocquet, F. C., Giovanelli, L., Ksari, Y., Ovramenko, T., Mayne, A. J., Dujardin, G., Spillebout, F., Sonnet, P., Bondino, F., Magnano, E., & Themlin, J. - M. (2018). Peculiar covalent bonding of C60/6H-SiC(0001)-(3 × 3) probed by photoelectron spectroscopy. JOURNAL OF PHYSICS-CONDENSED MATTER, 30(50), 505002.
Résumé: High resolution photoemission with synchrotron radiation was used to study the interface formation of a thin layer of C60 on 6H-SiC(0 0 0 1)-(3 × 3), characterized by protruding Si-tetramers. The results show that C60 is chemisorbed by orbital hybridization between the highest-occupied molecular orbital (HOMO) and the p z orbital of Si adatom at the apex of the tetramers. The covalent nature of the bonding was inferred from core level as well as valence band spectra. The Si 2p spectra reveal that a large fraction (at least 45%) of the Si adatoms remain unbound despite the reactive character of the associated dangling bonds. This is consistent with a model in which each C60 is attached to the substrate through a single covalent C60–Si bond. A binding energy shift of the core levels associated with sub-surface Si or C atoms indicates a decrease of the SiC band bending caused by a charge transfer from the C60 molecules to the substrate via the formation of donor-like interface states.
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Bryche, J. - F., Barbillon, G., Bartenlian, B., Dujardin, G., Boer-Duchemin, E., & Le Moal, E. (2018). k-space optical microscopy of nanoparticle arrays: Opportunities and artifacts. JOURNAL OF APPLIED PHYSICS, 124(4), 043102.
Résumé: We report on the performance and inherent artifacts of k-space optical microscopy for the study of periodic arrays of nanoparticles under the various illumination configurations available on an inverted optical microscope. We focus on the origin of these artifacts and the ways to overcome or even benefit from them. In particular, a recently reported artifact, called the “condenser effect,” is demonstrated here in a new way. The consequences of this artifact (which is due to spurious reflections in the objective) on Fourier-space imaging and spectroscopic measurements are analyzed in detail. The advantages of using k-space optical microscopy to determine the optical band structure of plasmonic arrays and to perform surface plasmon resonance experiments are demonstrated. Potential applications of k-space imaging for the accurate lateral and axial positioning of the sample in optical microscopy are investigated.
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Cao, S., Le Moal, E., Jiang, Q., Drezet, A., Huant, S., Hugonin, J. - P., Dujardin, G., & Boer-Duchemin, E. (2018). Directional light beams by design from electrically driven elliptical slit antennas. Beilstein Journal of Nanotechnology, 9, 2361–2371.
Résumé: We report on the low-energy, electrical generation of light beams in specific directions from planar elliptical microstructures. The emission direction of the beam is determined by the microstructure eccentricity. A very simple, broadband, optical antenna design is used, which consists of a single elliptical slit etched into a gold film. The light beam source is driven by an electrical nanosource of surface plasmon polaritons (SPP) that is located at one focus of the ellipse. In this study, SPPs are generated through inelastic electron tunneling between a gold surface and the tip of a scanning tunneling microscope.
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Kölsch, S., Fritz, F., Fenner, M.A., Kurch, S., Wöhrl, N., Mayne, A.J., Dujardin, G. & Meyer, C. (2018). Kelvin probe force microscopy studies of the charge effects upon adsorption of carbon nanotubes and C60 fullerenes on hydrogen-terminated diamond. JOURNAL OF APPLIED PHYSICS, 123(1), 15103.
Résumé: Hydrogen-terminated diamond is known for its unusually high surface conductivity that is ascribed to its negative electron affinity. In the presence of acceptor molecules, electrons are expected to transfer from the surface to the acceptor, resulting in p-type surface conductivity. Here, we present Kelvin probe force microscopy (KPFM) measurements on carbon nanotubes and C60 adsorbed onto a hydrogen-terminated diamond(001) surface. A clear reduction in the Kelvin signal is observed at the position of the carbon nanotubes and C60 molecules as compared with the bare, air-exposed surface. This result can be explained by the high positive electron affinity of carbon nanotubes and C60, resulting in electron transfer from the surface to the adsorbates. When an oxygen-terminated diamond(001) is used instead, no reduction in the Kelvin signal is obtained. While the presence of a charged adsorbate or a difference in work function could induce a change in the KPFM signal, a charge transfer effect of the hydrogen-terminated diamond surface, by the adsorption of the carbon nanotubes and the C60 fullerenes, is consistent with previous theoretical studies.
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Ramos, P., Mankarious, M., Pavanello, M., & Riedel, D. (2018). Probing Charge Transfer Dynamics in a Single Iron Tetraphenylporphyrin Dyad Adsorbed on an Insulating Surface. NANOSCALE, 10, 17603–17616.
Résumé: Although the dynamics of charge transfer (CT) processes can be probed with ultimate lifetime resolution, the helplessness to control CT at the nanoscale constitutes one of the most important road-blocks to revealing some of its deep fundamental aspects. In this work, we present an investigation of CT dynamics in a single iron tetraphenylporphyrin (Fe-TPP) donor/acceptor dyad adsorbed on a CaF2/Si(100) insulating surface. The tip of a scanning tunneling microscope (STM) is used to create local ionic states in one fragment of the dyad. The CT process is monitored by imaging subsequent changes in the neighbor acceptor molecule and its efficiency is mapped revealing the influence of the initial excited state in the donor molecule. In validation of the experiments, simulations based on density functional theory show that holes have a higher donor acceptor CT rate compared to electrons and highlight a noticeable initial state dependence on the CT process. We leverage the unprecedented spatial resolution achieved in our experiments to show that the CT process in the dyad is governed via molecule-molecule coherent tunneling with negligible surface-mediated character.
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Tchalala, M. R., Kara, A., Lachgar, A., Yagoubi, S., Foy, E., Vega, E., Nitsche, S., Chaudanson, D., Aufray, B., EL Firdoussi, L., Ali, M. A., & Oughaddou, H. (2018). Silicon nanoparticles synthesis from calcium disilicide by Redox assisted chemical exfoliation. Materials Today Communications, 16, 281–284.
Résumé: We report the preparation of single-crystal silicon nanoparticles with 15 to 22 nm diameter from calcium disilicide (CaSi2) by redox assisted chemical exfoliation using solution phase synthesis route. Silicon nanoparticles are found to be highly oriented with a predominant size of 18 nm. X-Ray diffraction, as well as transmission electron microscopy studies, confirm that the silicon nanoparticles are a diamond type and highly crystalline.
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Tchalala, R. M., Enriquez, H., Mayne, A. J., Kara, A., Dujardin, G., & Oughaddou, H. (2018). First steps of silicene growth on Ag(111). Journal of Physics: Conference Series, 1081, 012005.
Résumé: In this paper we report on the first steps of silicene growth on Ag(111) using scanning tunneling microscopy. We show that the topmost atomic layer is composed of both silicon and silver. The STM observations are consistent with an exchange process between the silicon and silver atoms preferentially taking place at the step edges of the Ag substrate. In addition, silicon stripes are observed as precursors of the formation of the silicene sheet.
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Zhang, W., Enriquez, H., Tong, Y., Bendounan, A., Kara, A., Seitsonen, A. P., Mayne, A. J., Dujardin, G., & Oughaddou, H. (2018). Epitaxial Synthesis of Blue Phosphorene. SMALL, 14, 1804066.
Résumé: Phosphorene is a new 2D material composed of a single or few atomic layers of black phosphorus. Phosphorene has both an intrinsic tunable direct bandgap and high carrier mobility values, which make it suitable for a large variety of optical and electronic devices. However, the synthesis of single-layer phosphorene is a major challenge. The standard procedure to obtain phosphorene is by exfoliation. More recently, the epitaxial growth of single-layer phosphorene on Au(111) was investigated by molecular beam epitaxy and the obtained structure described as a blue phosphorene sheet. In the present study, large areas of high-quality monolayer phosphorene, with a bandgap value equal to at least 0.8 eV, are synthesized on Au(111). The experimental investigations, coupled with density functional theory calculations, give evidence of two distinct phases of blue phosphorene on Au(111), instead of one as previously reported, and their atomic structures are determined.
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Zhao, M., Almarzouqi, F., Duverger, E., Sonnet, P., Dujardin, G., & Mayne, A. J. (2018). Sub-molecular spectroscopy and temporary molecular charging of Ni-phthalocyanine on graphene with STM. PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 20(29), 19507–19514.
Résumé: In this study, the self-assembled molecular network and electronic properties of Ni-phthalocyanine (NiPc) molecules on monolayer graphene (MLG)/6H-SiC(0001) were studied by room temperature Scanning Tunnelling Microscopy (STM) and Density Functional Theory (DFT) calculations. In this study, a very weak electronic coupling between the graphene and the NiPc molecules is found. This is due to the very small charge transfer of only 0.035e- per molecule. The weak molecule-graphene interaction has two observable consequences: sub-molecular resolution was obtained in the STM spectroscopy at room-temperature with the molecules adsorbed directly on the graphene, and the occupied and unoccupied molecular resonance peaks were observed to shift their position in energy as a function of the tip-surface distance. This is due to the temporary local charging (either positive or negative) that is achieved by decreasing the surface voltage under the STM tip. This may have important consequences for future studies of the opto-electronic properties of such hybrid graphene-molecule systems.
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