Peer-reviewed Publications |
Debiossac, M., Roncin, P., & Borisov, A. G. (2020). Refraction of Fast Ne Atoms in the Attractive Well of a LiF(001) Surface. J. Phys. Chem. Lett., 11, 4564–4569.
Résumé: Ne atoms with energies of </=3 keV are diffracted under grazing angles of incidence from a LiF(001) surface. For a small momentum component of the incident beam perpendicular to the surface, we observe an increase in the elastic rainbow angle together with a broadening of the inelastic scattering profile. We interpret these two effects as the refraction of the atomic wave in the attractive part of the surface potential. We use a fast, rigorous dynamical diffraction calculation to find a projectile-surface potential model that enables a quantitative reproduction of the experimental data for </=10 diffraction orders. This allows us to extract an attractive potential well depth of 10.4 meV. Our results set a benchmark for more refined surface potential models that include the weak van der Waals region, a long-standing challenge in the study of atom-surface interactions.
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El Boujlaidi, A., Rochdi, N., Tchalala, R., Enriquez, H., Mayne, A. J., & Oughaddou, H. (2020). Growth and characterization of nickel oxide ultra-thin films. SURFACES AND INTERFACES, 18, 100433.
Résumé: The oxidation of the Ni(111) surface under ultrahigh-vacuum conditions is studied experimentally with low-energy electron diffraction and high-resolution X-ray photoelectron spectroscopy. Exposure of the clean Ni(111) surface to molecular oxygen at room temperature followed by annealing at 400 K leads to the formation of two different structures (2×2) and (3root3 × 3root3 )R30°, prior to the formation of the NiO(111) monolayer. The O 1s core levels indicate that the obtained oxide is terminated by oxygen atoms while the valence band measurements clearly reveal the band gap of NiO. The energy difference between the Fermi level and the maximum of the valance band is extracted and is found to be 0.47 eV.
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Hamoudi, H., Berdiyorov, G. R., Ariga, K., & Esaulov, V. (2020). Bottom-up fabrication of the multi-layer carbon metal nanosheets. RSC ADVANCES, 10(13), 7987–7993.
Résumé: Ordered carbon composite materials have great potential for practical applications in many areas such as energy conversion, quantum computing, biotechnologies, and electronics. In this work, we demonstrate a state-of-the-art self-assembly driven building block approach to create new layered carbon-metal composite materials with advanced properties. We fabricate molecular nanocomposites using self-assembled metal intercalated multi-layers of dithiol derivatives. The obtained structures are analysed using different characterization tools (such as X-ray photoelectron and Raman spectroscopy and atomic force microscopy) and their electronic transport properties are studied by four-point probe measurements supplemented by density functional theory calculations. This work demonstrates a new strategy for low-cost, high-yield and eco-friendly nanofabrication of hybrid organometallic membranes.
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Liu, P., Yin, L., Zhang, Z., Ding, B., Shi, Y., Li, Y., Zhang, X., Song, X., Guo, Y., Chen, L., Chen, X., Gainullin, I. K., & Esaulov, V. A. (2020). Anomalous neutralization characteristics in Na+ neutralization on Al(111) surfaces. PHYSICAL REVIEW A, 101(3), 032706.
Résumé: The jellium model of free electron gas and its extended version have been widely used to understand the neutralization of alkali-metal ions on metal surfaces. We report an unexpected deviation from its prediction that we observed in the neutralization of Na+ ions scattering from an Al(111) surface. We find that the neutralization probability decreases monotonically with increasing ion velocity for the specular scattering condition, which is consistent with the well-known parallel velocity effect. However, the neutralization probability exhibits an unexpected bell shape with the variation of outgoing angle for a given incident energy. Calculations based on the jellium model using the rate equation and including the dynamic parallel velocity effect are presented. Their results agree with the velocity dependence of the neutral fraction, but completely fail in reproducing the angle dependence. This anomalous angle dependence could be due to the appearance of inelastic processes, corresponding to inner 2p electron promotion in hard encounters with Al atoms for large incidence angles, when the interatomic distances become small. This can lead to the formation of autoionizing Na states that result in the formation of extra Na+ ions, not accounted for in the jellium model.
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Mery, M., Chen, L., Valdés, J. E., & Esaulov, V. A. (2020). On the determination of stopping cross-sections in ion scattering in solids and deviations from standard models. RADIATION EFFECTS AND DEFECTS IN SOLIDS, 175(1-2), 160–176.
Résumé: When atomic particles traverse solids they suffer energy losses due to elastic scattering from nuclei and excitation of electron-hole pairs. These are referred to as nuclear and electronic stoppings respectively. In this paper we discuss methods of determining energy losses and 'stopping' cross-sections in ion transmission through thin films, as well as in large angle and grazing backscattering from surfaces. This is done on the basis of deterministic simulations of ion scattering by following ion trajectories as they pass through the solid and sample regions of different electron densities depending on the distance from atomic nuclei. The ab initio calculated electron densities in the crystal are used to determine the stopping power, as predicted by the free electron gas model, and including a threshold value for d electron excitation. We discuss some aspects that are not included in standard descriptions based on the use of free electron models and averaged effective electron densities. In this context, we point out the possibility of inelastic processes involving inner-shell excitations, and briefly summarise main findings.
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Roncin, P. (2020). Revisiting atomic collisions physics with highly charged ions, a tribute to Michel Barat. Journal of Physics B: Atomic, Molecular and Optical Physics, 53, 202001.
Résumé: Michel Barat passed away in November 2018 at the age of 80 after a rich career in atomic and molecular collisions. He had participated actively in formalizing the electron promotion model, contribuing to low energy reactive collisions at the frontier of chemistry. He investigated electron capture mechanisms by highly charged ions (HCI), switched to collision induced cluster dissociation and finally to UV laser induced fragmentation mechanisms of biological molecules. During this highly active time he created a laboratory, organized ICPEAC and participated actively in the administration of research. This paper covers the 10 years when he mentored my scientific activity in the blossoming field of electron capture by
HCI. In spite of an impressive number of open channels, Michel found a way to capture the important parameters and to simplify the description of several electron capture processes; orientation propensity, electron promotion, true double electron capture, transfer ionisation, transfer excitation, formation of Rydberg states, and electron capture by metastable states. Each time Michel established fruitful collaborations with other groups.
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Tchalala, M. R., Enriquez, H., Bendounan, A., Mayne, A. J., Dujardin, G., Kara, A., Ali, M. A., & Oughaddou, H. (2020). Tip-induced oxidation of silicene nano-ribbons. NANOSCALE ADVANCES, 2, 2309–2314.
Résumé: We report on the oxidation of self-assembled silicene nanoribbons grown on the Ag (110) surface using scanning tunneling microscopy and high-resolution photoemission spectroscopy. The results show that silicene nanoribbons present a strong resistance towards oxidation using molecular oxygen. This can be overcome by increasing the electric field in the STM tunnel junction above a threshold of +2.6 V to induce oxygen dissociation and reaction. The higher reactivity of the silicene nanoribbons towards atomic oxygen is observed as expected. The HR-PES confirm these observations: even at high exposures of molecular oxygen, the Si 2p core-level peaks corresponding to pristine silicene remain dominant, reflecting a very low reactivity to molecular oxygen. Complete oxidation is obtained following exposure to high doses of atomic oxygen; the Si 2p core level peak corresponding to pristine silicene disappears.
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Tiouitchi, G., Ali, M. A., Benyoussef, A., Hamedoun, M., Lachgar, A., Kara, A., Ennaoui, A., Mahmoud, A., Boschini, F., Oughaddou, H., El Moutaouakil, A., El Kenz, A., & Mounkachi, O. (2020). Efficient production of few-layer black phosphorus by liquid-phase exfoliation. Royal Society Open Source, 7(10), 201210.
Résumé: Phosphorene is a new two-dimensional material that has recently attracted much attention owing to its fascinating electrical, optical, thermal and chemical properties. Here, we report on high-quality exfoliation of black phosphorus nanosheets, with controllable size produced in large quantities by liquid-phase exfoliation using N-methyl-2-pyrrolidone (NMP) as a solvent under ambient conditions. The as-synthesized few layers show a great potential for solar energy conversion based on the optical results shown in this work.
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Tiouitchi, G., ElManjli, E., Mounkachi, O., Mahmoud, A., Boschini, F., Kara, A., Oughaddou, H., Hamedoun, M., Benyoussef, A., & Ait Ali, M. (2020). From Amorphous Red Phosphorus to a Few Layers of Black Phosphorus: A Low-cost and Efficient Preparation Process. JORDAN JOURNAL OF PHYSICS, 13(2), 149–155.
Résumé: In this work, we present an efficiency synthesis method of phosphorene from red phosphorus by means of black phosphorus. The latter was synthesized by using copper, tin, tin iodide and red phosphorus as precursor at low pressure-temperature. Characterizations with powder X-ray diffraction, scanning electron microscopy (SEM), energy dispersive spectrometry (EDX), high-resolution TEM (HR-TEM) and Raman spectroscopy were performed to confirm the high quality and purity of black phosphorus crystal. Liquid phase method was used to exfoliate black phosphorus to phosphorene in N-methyl-2-pyrrolidone (NMP) as solvent. Atomic force microscopy and STEM were used to confirm the exfoliation of black phophorus in a few layers of phophorene.
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Tong, Y., Bouaziz, M., Oughaddou, H., Enriquez, H., Chaouchi, K., Nicolas, F., Kubsky, S., Esaulov, V., & Bendounan, A. (2020). Phase transition and thermal stability of epitaxial PtSe2 nanolayer on Pt(111). RSC ADVANCES, 10(51), 30934–30943.
Résumé: This work relates to direct synthesis of the two-dimensional (2D) transition metal dichalchogenide (TMD) PtSe2 using an original method based on chemical deposition during immersion of a Pt(111) surface into aqueous Na2Se solution. Annealing of the sample induces significant modifications in the structural and electronic properties of the resulting PtSe2 film. We report systematic investigations of temperature dependent phase transitions by combining synchrotron based high-resolution X-ray photoemission (XPS), low temperature scanning tunnelling microscopy (LT-STM) and low energy electron diffraction (LEED). From the STM images, a phase transition from TMD 2H-PtSe2 to Pt2Se alloy monolayer structure is observed, in agreement with the LEED patterns showing a transition from (4 × 4) to (√3 × √3)R30° and then to a (2 × 2) superstructure. This progressive evolution of the surface reconstruction has been monitored by XPS through systematic de-convolution of the Pt4f and Se3d core level peaks at different temperatures. The present work provides an alternative method for the large scale fabrication of 2D transition metal dichalchogenide films.
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Tong, Y., Bouaziz, M., Zhang, W., Obeid, B., Loncle, A., Oughaddou, H., Enriquez, H., Chaouchi, K., Esaulov, V., Chen, Z., Xiong, H., Cheng, Y., & Bendounan, A. (2020). Evidence of new 2D material: Cu2Te. 2D MATERIALS, 7(3), 035010.
Résumé: The number of two-dimensional (2D) materials has grown steadily since the discovery of graphene. Each new 2D material demonstrated unusual physical properties offering a large flexibility in their tailoring for high-tech applications. Here, we report on the formation and characterization of an uncharted 2D material: 'Cu2Te alloy monolayer on Cu(111) surface'. We have successfully grown a 2D binary Te-Cu alloy using a straightforward approach based on chemical deposition method. Low electron energy diffraction (LEED) and scanning tunneling microscopy (STM) results reveal the existence of a well-ordered alloy monolayer characterized by (√3 × √3)R30° superstructure, while the x-ray photoemission spectroscopy (XPS) measurements indicate the presence of single chemical environment of the Te atoms associated with the Te-Cu bonding. Analysis of the valence band properties by angle resolved photoemission spectroscopy (ARPES); in particular the electronic states close to the Fermi level suggests a strong hybridization between Te and Cu electronic states leading to an appearance of new dispersive bands localized at the surface alloy, which is confirmed by first-principles calculations. These bands are strongly influenced by the surface reconstruction and undergo a back-folding at the boundaries of the reduced surface Brillouin zone (SBZ). More interesting, a band gap of about 0.91 eV and a Rashba splitting in the conduction band are obtained. These findings taken together clearly prove the presence of 2D-type electron system within the Cu2Te alloy layer, which is promising for spintronic application.
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Wakamura, T., Wu, N. J., Chepelianskii, A. D., Gueron, S., Och, M., Ferrier, M., Taniguchi, T., Watanabe, K., Mattevi, C., & Bouchiat, H. (2020). Spin-Orbit-Enhanced Robustness of Supercurrent in Graphene/WS2 Josephson Junctions. PHYSICAL REVIEW LETTERS, 125(26), 266801.
Résumé: We demonstrate the enhanced robustness of the supercurrent through graphene-based Josephson junctions in which strong spin-orbit interactions (SOIs) are induced. We compare the persistence of a supercurrent at high out-of-plane magnetic fields between Josephson junctions with graphene on hexagonal boron-nitride and graphene on WS{2}, where strong SOIs are induced via the proximity effect. We find that in the shortest junctions both systems display signatures of induced superconductivity, characterized by a suppressed differential resistance at a low current, in magnetic fields up to 1 T. In longer junctions, however, only graphene on WS{2} exhibits induced superconductivity features in such high magnetic fields, and they even persist up to 7 T. We argue that these robust superconducting signatures arise from quasiballistic edge states stabilized by the strong SOIs induced in graphene by WS_{2}.
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Wang, J., Ding, B., Song, X., Shi, Y., Guo, X., Liu, X., Wang, L., Wei, M., Liu, P., Liu, Y., Hu, B., Valdés, J. E., Esaulov, V. A., Chen, L., Guo, Y., & Chen, X. (2020). Nuclear versus electronic energy loss in slow Ar ion scattering on a Cu (100) surface: Experiment and simulations. PHYSICAL REVIEW A, 102(1), 012805.
Résumé: In the scattering and stopping of heavy ions on a few surface layers in solids, contributions of the electronic and nuclear energy losses can become comparable. In this work we present a study of 0.6-5-keV Ar ion scattering on a Cu (100) surface. Energy-loss spectra were measured and displayed some changes as the incident ion energy changes. Thus, a structured spectrum is observed at the higher energies. We analyze the characteristics of the spectra and contributions from the nuclear and electronic energy-loss components using both a Monte Carlo kinetics simulation assuming an averaged electron density and a semiclassical deterministic simulation with an inhomogeneous electron density in the solid. The general features of the experimental spectra were well reproduced, and semiclassical simulations allow us to identify contributions from trajectories coming from below and on top of the surface atomic layer. The relative contribution of nuclear energy loss and electronic energy loss (EEL) were delineated and a more refined analysis of the EEL for different trajectories is presented.
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Zhang, W., Enriquez, H., Zhang, X., Mayne, A. J., Bendounan, A., Dappe, Y. J., Kara, A., Dujardin, G., & Oughaddou, H. (2020). Blue phosphorene reactivity on the Au(111) surface. NANOTECHNOLOGY, 31(49), 495602.
Résumé: The synthesis of blue phosphorene by molecular beam epitaxy (MBE) has recently come under the spotlight due to its potential applications in electronic and optoelectronic devices. However, this synthesis remains a significant challenge. The surface reactivity between the P atoms and the Au atoms should be considered for the P/Au(111) system. In the MBE process, the temperature of the substrate is a key parameter for the growth of blue phosphorene. During the initial growth stage, irregularly shaped Phosphorus clusters grow on top of Au(111) surface at room temperature. When the substrate temperature is increased, these clusters transform into a phosphorene-like structure with a honeycomb lattice. An atom exchange reaction is observed between the P and first layer Au atoms under thermal activation at higher temperature, where the P atoms replace Au atoms to form a blue phosphorene structure within the top Au layer and at the step edges.
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Zhang, W., Enriquez, H., Tong, Y., Mayne, A. J., Bendounan, A., Dappe, Y. J., Kara, A., Dujardin, G., & Oughaddou, H. (2020). Phosphorus Pentamers: Floating Nanoflowers form a 2D network. ADVANCED FUNCTIONAL MATERIALS, 30, 2004531.
Résumé: An experimental investigation of a new polymorphic 2D single layer of phosphorus on Ag(111) is presented. The atomically-resolved scanning tunneling microscopy images show a new 2D material composed of freely-floating phosphorus pentamers organized into a 2D layer, where the pentamers are aligned in close-packed rows. The scanning tunneling spectroscopy measurements reveal a semiconducting character with a band gap of 1.20 eV. This work presents the formation at low temperature of a new polymorphic 2D phosphorus layer composed of a floating 2D pentamer structure. The smooth curved terrace edges and a lack of any clear crystallographic orientation with respect to the Ag(111) substrate at room temperature indicates a smooth potential energy surface that is reminiscent of a liquid-like growth phase. This is confirmed by density functional theory calculations that find a small energy barrier of only 0.17 eV to surface diffusion of the pentamers (see Supporting Information). The formation of extended, homogeneous domains is a key ingredient to opening a new avenue to integrate this new 2D material into electronic devices.
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