A. Zahab
University of Montpellier
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Featured researches published by A. Zahab.
Solid State Communications | 1992
Takashi Matsuo; Hiroshi Suga; William I. F. David; R.M. Ibberson; P. Bernier; A. Zahab; Claude Fabre; André Rassat; A. Dworkin
Abstract High resolution heat capacity measurements of the prototypic fullerene, C60, are presented between 13 and 300 K. The well-documented first-order phase transition is clearly observed at 257.6 K and is associated with enthalpy and entropy changes of 7.54 and 30.0 J K−1 mol−1, respectively. A more subtle transition is observed at 86.8 K. This transition is attributable to the onset of orientational glass behaviour related to the kinetics of molecular reorientation. The rate of the enthalpy relaxation at the glass transition is accurately reproduced by a simple exponential function. The Arrhenius parameters describing the temperature dependence of the relaxation time are Ea = 22.2 ± 1.0 kJ mol−1 and τ0 = 4×10−11±1s. At low temperatures, solid C60 is a glassy crystal in which molecular orientational disorder is frozen in as a consequence of the long relaxation time of the molecular reorientation.
Nano Letters | 2011
Dmitry I. Levshov; T. X. Than; Raul Arenal; V. N. Popov; R. Parret; Matthieu Paillet; Vincent Jourdain; A. Zahab; Thierry Michel; Yu. I. Yuzyuk; Jean-Louis Sauvajol
We perform transmission electron microscopy, electron diffraction, and Raman scattering experiments on an individual suspended double-walled carbon nanotube (DWCNT). The first two techniques allow the unambiguous determination of the DWCNT structure: (12,8)@(16,14). However, the low-frequency features in the Raman spectra cannot be connected to the derived layer diameters d by means of the 1/d power law, widely used for the diameter dependence of the radial-breathing mode of single-walled nanotubes. We discuss this disagreement in terms of mechanical coupling between the layers of the DWCNT, which results in collective vibrational modes. Theoretical predictions for the breathing-like modes of the DWCNT, originating from the radial-breathing modes of the layers, are in a very good agreement with the observed Raman spectra. Moreover, the mechanical coupling qualitatively explains the observation of Raman lines of breathing-like modes, whenever only one of the layers is in resonance with the laser energy.
Molecular Physics | 1992
Daniel André; A. Dworkin; Henri Szwarc; René Céolin; Viatcheslav Agafonov; Claude Fabre; André Rassat; Leo Straver; P. Bernier; A. Zahab
A single crystal of C60 grown from a toluene solution has been studied by X-ray diffraction at room temperature. The lattice is face-centered cubic, with space group Fm3m and Z = 4, which agrees with previous powder diffraction measurements. It is shown that, contrary to what is obtained in other plastic crystals, the Pauling-Fowler model (the so-called free rotation one), which implies an isotropic molecular disorder, gives a better description of the molecular packing than the site model does. It is concluded that the molecules undergo a rotational diffusion as previous molecular dynamics simulations have described.
MRS Proceedings | 1999
C. Marliere; P. Poncharal; L. Vaccarini; A. Zahab
Single wall nanotubes have been made by arc-discharge method. Residual impurities (fullerenes, amorphous carbon, catalyst metals…) have been removed by tangential filtration process followed by high temperature annealing under vacuum (1200 °C). In this work we present results on the influence of the surrounding gas nature (N 2 , H 2 , CO 2 , H 2 O…) on the electrical resistivity of carefully outgassed mat of such samples. In particular, we have observed that the sample resistance exhibits a strong dependence on water contamination during the transfer to the measurement reactor.
EPL | 1994
R. Kerkoud; Pascale Auban-Senzier; D. Jérome; J. M. Lambert; A. Zahab; P. Bernier
We report the measurement of the 13C NMR frequency in K3C60 under pressure. Comparing pressure dependences of the hyperfine spin lattice relaxation rate and of the resonance frequency provides the first determination of the 13C Knight shift ((61 ± 4) p.p.m.) and of the chemical shift of the C603- molecule (125 p.m.m./TMS). NMR data under pressure can be undestood with a weak-Coulomb-interaction model.
Journal of The Chemical Society, Chemical Communications | 1992
Nicole Coustel; P. Bernier; R. Aznar; A. Zahab; J. M. Lambert; Patrick Lyard
C60(98% pure) crystallizes during toluene Soxhlet extraction of fullerenes from soot—recrystallization of this enriched material gives 99.5% pure C60.
Journal of Crystal Growth | 1992
V. Agafonov; R. Ceolin; R. Moret; Daniel André; A. Dworkin; Henri Szwarc; Claude Fabre; André Rassat; J.M. Cense; A. Zahab; P. Bernier
Abstract Ten-sided prismatic crystals of C 60 grown by evaporation of n-hexane solutions have been characterized by scanning electron microscopy and X-ray diffraction. Various unusual morphologies have been observed. X-ray precession photographs confirm the decagonal symmetry and they are analyzed by means of twinning of monoclinic variants related by π/5 rotations. Possible twinning mechanisms are discussed.
Synthetic Metals | 1996
A. Zahab; L. Firlej; F. Brocard; N. Kirova
Abstract We report the evolution in time of d.c. conductivity of polycrystalline thin films of C 60 at different temperatures and under various applied voltages. All films show a clear semiconducting behaviour. The initial resistivity (at T = 140 °C) and the activation energy are ρ ~ 10 5 Ω cm and E a ~ 0.5 eV, respectively. For various types of metallic electrodes M (M = Au, Ag or In), a field-induced conductivity increase by several orders of magnitude and an activation energy decrease are observed. The results are interpreted in terms of electrode metal diffusion into the film.
Synthetic Metals | 1993
S. Lefrant; E. Faulques; C. Godon; J.P. Buisson; Pascale Auban-Senzier; D. Jérome; Claude Fabre; André Rassat; A. Zahab; J.M. Lambert; P. Bernier
Abstract Recent studies carried out on an organic superconductor, s-(BEDT-TTF) 2 I 3 , have shown the important role played by the 13 C isotope on the T c transition, when substitution is made on the central CC bond of the molecule. Simultaneously, Raman studies determined the frequency shifts observed in the CC stretching vibrations in the substituted samples and consequently, put in evidence the role of the intramolecular Ag vibrations. We have extended these studies to compounds of the new carbon family, the fullerenes, in which 82% of the carbon atoms were substituted with 13 C in C 60 . The superconducting transition measured on the same sample doped with potassium is lowered by 1.5 to 2K as compared to the standard material doped in similar conditions. In the Raman spectrum, the main Ag band measured at 1467 cm −1 in 12 C 60 is shifted to 1420 cm −1 in enriched 13 C 60 . Taking into account the ratio between the molecular weight of the enriched 13 C 60 and of the standard material, one predicts a theoretical value for this mode at 1418 cm −1 in the enriched sample. A preliminary analysis of the Raman spectrum will be presented.
Synthetic Metals | 1997
L. Firlej; A. Zahab; F. Brocard; N. Kirova
Abstract The metal intercalation ( M Au, Ag, In ) from the electrode into the polycrystalline C 60 film is studied by electrical conductivity measurements under controlled oxygen free atmosphere at different applied voltages. The resistivity changes are interpreted on the basis of field induced metal diffusion and local phase M x C 60 formations.