Peer-reviewed Publications |
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Aguirregabiria, G., Marinica, D. C., Esteban, R., Kazansky, A. K., Aizpurua, J., & Borisov, A. G. (2018). Role of electron tunneling in the nonlinear response of plasmonic nanogaps. Phys. Rev. B, 97(11), 115430.
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Alyabyeva, N., Ouvrard, A., Lindfors-Vrejoiu, I., Kolomiytsev, A., Solodovnik, M., Ageev, O., & McGrouther, D. (2018). Modified cantilevers to probe unambiguously out-of-plane piezoresponse. Phys. Rev. Mater., 2(6), 064402.
Résumé: We demonstrate and investigate the coupling of contributions from both in-plane (IP) polarization and out-of-plane (OP) components in BiFeO3 (BFO) thin-film polarization probed by piezoresponse force microscopy (PFM). Such coupling leads to image artifacts which prevent the correct determination of OP polarization vector directions and the corresponding piezoelectric coefficient d33. Using material strength theory with a one-dimensional modeling of the cantilever oscillation amplitude under electrostatic and elastic forces as a function of the tip length, we have evidenced the impact of IP piezoresponse to the OP signal for tip length longer than 4 μm. The IP polarization vector induces a significant longitudinal bending of the cantilever, due to the small spring constant of long tips, which provokes a normal deviation superimposed to the OP piezoresponse. These artifacts can be reduced by increasing the longitudinal spring constant of the cantilever by shortening the tip length. Standard cantilevers with 15-μm-long tips were modified to reach the desired tip length, using focused ion-beam techniques and tested using PFM on the same BFO thin film. Tip length shortening has strongly reduced IP artifacts as expected, while the impact of nonlocal electrostatic forces, becoming predominant for tips shorter than 1 μm, has led to a non-negligible deflection offset. For shorter tips, a strong electric field from a cantilever beam can induce polarization switching as observed for a 0.5-μm-long tip. Tip length ranging from 1 to 4 μm allowed minimizing both artifacts to probe unambiguously OP piezoresponse and quantify the d33 piezoelectric coefficient.
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Alyabyeva, N., Ouvrard, A., Zakaria, A. - M., Charra, F., & Bourguignon, B. (2018). Transition from disordered to long-range ordered nanoparticles on Al 2 O 3 /Ni 3 Al(111). Appl. Surf. Sci., 444, 423–429.
Résumé: Application of preparation recipes of the literature failed to produce an ordered array of NPs on our particular Ni3Al sample. This has motivated a systematic survey of Pd NP nucleation as a function of experimental parameters. We have shown that the increase of oxidation temperature during the preparation of Al2O3 ultra-thin film on Ni3Al(111) leads to a transition from disordered to long-range ordered Pd nanoparticle (NP) nucleation. Alumina films were prepared at different temperatures ranging from 990 to 1140 K. Crystallinity, electronic structure of the alumina film and Pd nucleation and growth have been investigated using Low Energy Electron Diffraction and Scanning Tunnelling Microscopy. NP density and long-range order nucleation along the so-called “dot structure” of 4.2 nm periodicity, strongly increase for temperatures higher than a threshold value of 1070 ± 20 K. This transition relies on the alumina film improvement and suggests that the modulation of Pd adsorption energy at nucleation centres which is necessary to nucleate NPs at ordered sites, requires higher preparation temperature. Long-range ordered NPs with a high density were obtained 140 K above reported recipes in the literature. This optimized temperature has been tested on a fresh sample (issued from the same supplier) for which just a few cleanings were enough to obtain long-range ordered NPs. Presumably the variability of the optimal oxidation temperature for our samples with respect to the literature is related to fluctuations of the stoichiometry from sample to sample.
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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. J. Phys. Condens. 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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Bourguignon B. (2018). Pulse Shaping in Surface Science. Reference Module in Chemistry, Molecular Sciences and Chemical Engineering, , 604–610.
Résumé: Pulse shaping consists in adjusting the spectral and temporal shapes of a laser pulse to enhance the efficiency of specific laser-induced processes. Only ultrafast lasers have a suitable (broad enough) spectrum. Spatial pulse shaping is another type of shaping which consists in controlling the wave front profile to keep the beam spatially uniform and allow precise control of the energy distribution around the focus point. Pulse shaping has been developed to optimize the intrinsic quality of lasers, in particular to make ultrahigh intensity lasers, and to optimize specific light–matter interactions. In surface science, pulse shaping is mainly used for optimization of laser ablation and for Broad Band Sum Frequency Generation vibrational spectroscopy.
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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. J. Appl. Phys., 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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Ghalgaoui, A., Horchani, R., Wang, J., Ouvrard, A., Carrez, S., & Bourguignon, B. (2018). Identification of active sites in oxidation reaction from real-time probing of adsorbate motion over Pd nanoparticles. J. Phys. Chem. Lett., 9(18), 5202–5206.
Résumé: Obtaining insight into the type of surface sites involved in a reaction is essential to understand catalytic mechanisms at the atomic level and a key for understanding selectivity in surface-catalyzed reactions. Here we use ultrafast broad-band vibrational spectroscopy to follow in real-time diffusion of CO molecules over a palladium nanoparticle surface toward an active site. Site-to-site hopping is triggered by laser excitation of electrons and followed in real-time from subpicosecond changes in the vibrational spectra. CO photoexcitation occurs in 400 fs and hopping from NP facets to edges follows within ∼1 ps. Kinetic modeling allows to quantify the contribution of different facet sites to the catalytic reaction. These results provide useful insights for understanding the mechanism of chemical reactions catalyzed by metal NPs.
Supporting Information
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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. J. Appl. Phys., 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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Marinica, D. C., Silkin, V. M., Kazansky, A. K., & Borisov, A. G. (2018). Controlling gap plasmons with quantum resonances. Phys. Rev. B, 98(15), 155426.
Résumé: We use classical electrodynamics, time-dependent density functional theory, and random-phase approximation to study the gap plasmons propagating in the nm-wide gap between metal surfaces. Particular emphasis is given to the quantum effects emerging when the junction is functionalized with a nanostructure supporting unoccupied gap localized electronic states. With the example of a quantum well (QW) introduced in the junction we show that the optically assisted electron transport across the junction via the gateway QW localized electronic states might strongly affect the lifetime and the propagation length of the gap plasmon. The coupling to the single-particle electron-hole excitations from occupied electronic states at metal surfaces into the QW-localized electronic states provides an efficient decay channel of the gap plasmon mode. Different from the through-gap electron tunneling discussed in the plasmonics literature, the electron transport involving the gateway electronic state is characterized by the threshold behavior with plasmon frequency. As a consequence, the dynamics of the gap plasmon can be controlled by varying the binding energy of the QW-localized electronic state. In more general terms, our results demonstrate strong sensitivity of the gap plasmons to the optically assisted electron transport properties of the junction which opens further perspectives in design of nanosensors and integrated active optical devices.
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Momeni, A., Staicu Casagrande, E. M., Dechaux, A., & Khemliche, H. (2018). Ultrafast Crystallization Dynamics at an Organic-Inorganic Interface Revealed in Real Time by Grazing Incidence Fast Atom Diffraction. J. Phys. Chem. Lett., 9(4), 908–913.
Résumé: The poor structural properties of organic-inorganic interfaces and their variability represent the main cause of device under-performance. Understanding and controlling the development of these properties in real time has been a difficult experimental challenge. Using a recent technique based on grazing incidence fast atom diffraction (GIFAD), we were able to directly observe during deposition structural transitions in a perylene monolayer on Ag(110). Crystallization from the liquid phase occurs into two distinct structures with drastically different dynamics. Transition to the most compact packing occurs by self-organization only after a second layer has started to build up; subsequent incorporation of molecules from second to first layer triggers an ultrafast crystallization on a macroscopic sale. The final compact crystalline structure shows a long-range order and superior stability, which opens good perspectives for producing in a controlled manner highly ordered hybrid interfaces for photovoltaics and molecular electronics.
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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(37), 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, 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. Phys. Chem. Chem. Phys., 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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