R. Kofman
University of Nice Sophia Antipolis
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Featured researches published by R. Kofman.
Surface Science | 1994
R. Kofman; P. Cheyssac; A. Aouaj; Y. Lereah; G. Deutscher; T. Ben-David; J.M. Penisson; A. Bourret
Abstract Recent unambiguous experiments (melting of ultrafine particles, premelting at the surface of a bulk crystal, superheating, etc.) offer clear evidence of the key role of the surface in determining the melting of a material. In this work we concentrate our attention on spherical and non-spherical nanometric lead inclusions. We report experimental results on Pb/SiO and Pb/Al 2 O 3 systems obtained at different temperatures by two techniques: high-sensitivity optical reflectance and dark-field electron microscopy. The main result is the existence, below the melting temperature and at the surface of the inclusion, of a liquid layer whose thickness is much larger than that observed on the bulk (zero curvature). This thickness, which depends on local curvature, increases continuously with temperature until a uniform curvature of the solid core is attained; then the core melts suddenly. A phenomenological model, based on the minimization of the free energy, is proposed and reported in detail. It represents a significant improvement compared to previous theoretical approaches related to well-known thermodynamic size-effect models, particularly insofar as the agreement with the experimental results is concerned.
Soft Matter | 2011
Thierry Darmanin; Frédéric Guittard; Sonia Amigoni; Elisabeth Tafin de Givenchy; Xavier Noblin; R. Kofman; Franck Celestini
The surface construction to reach super oil non-wetting properties is very complex because of the necessary force for impeding the natural spreading of low surface tension oils. Here, a polymer, which is able to reach the superoleophobicity when it is electrodeposited on smooth surfaces, has been deposited on micro-patterned substrates made of cylindrical arrays (∅: 13 µm, H: 25 µm, distance between cylinders: 40 µm) in order to determine the effect of the pattern on the super oil-repellency properties. The surface analysis using various oils has shown that the pattern used highly decreases the time of deposition and, as a consequence, the required amount of polymer to obtain anti-oil surfaces. This work is the first step in the short term prospects for the elaboration of superoleophobic surfaces combining electropolymerization with lithography.
Applied Physics Letters | 1996
C. E. Bottani; C. Mantini; Paolo Milani; M. Manfredini; A. Stella; P. Tognini; P. Cheyssac; R. Kofman
Quantum confinement effects in Ge nanocrystals in the size range 26–130 A have been investigated both on the electronic joint density of states and Raman spectra. The high degree of crystallinity of the particles as well as the minimized interaction with the matrix allow detection of shift and broadening of the TO Raman peak, which can be compared with theoretical expectations based on phonon confinement. The evolution of Raman and absorption spectra and their dependence on the nanocrystal dimensions can be related.
Surface Science | 1996
E. Søndergård; R. Kofman; P. Cheyssac; A. Stella
In this paper we present a method for the production of arrays of nanoparticles recently developed in our laboratory. It is based on UHV-evaporation and condensation on a substrate in the Volmer-Weber mode. Basic differences with some interesting similarities with respect to other current methods are exhibited. New results concerning the growth and control of the distribution of nanoparticles are here reported both in general terms and with specific reference to a test system, which is Sn on a SiOx substrate. Details of the self-organization process and the results of a thermal post-treatment are presented.
Philosophical Magazine Part B | 2001
Y. Lereah; R. Kofman; J. M. Pénisson; G. Deutscher; P. Cheyssac; T. Ben David; A. Bourret
Abstract We review our quantitative results related to the physical properties of metallic nanoparticles that were obtained by time-resolved electron microscopy. These studies include the solid–liquid transition and structural instabilities. Surface melting has been demonstrated, quantitatively measured and analysed within the frainework of a phenomenological model. The nature of the liauid layer is discussed. Quantitative studies of the structural instabilities indicate a spontaneous appearance of twin defects inside the nanoparticles and their spontaneous disappearance. It has been found that this process is thermally activated.
Applied Physics Letters | 2004
G. B. Parravicini; A. Stella; M. C. Ungureanu; R. Kofman
A negative capacitance (NC) effect in a low-frequency range (4–8×105Hz), previously shown to take place mainly in semiconductor structures, is evidenced in a nanometric system constituted by metallic (Ga) nanoparticles embedded in an insulating (SiOxwithx~1) matrix. The dependence of the NC phenomenon on the time-dependent transient current through the system is evidenced. A remarkable enhancement of the effect with size reduction of the nanoparticles is manifested. The physical mechanism responsible for the current inertia appears to be related to the space charges located at the multiple insulator–metal interfaces.
Surface Science | 1974
R. Kofman; R. Garrigos; P. Cheyssac
Abstract Electroreflectance spectra of gold have been investigated at oblique incidence, the light wave being polarized either perpendicular or parallel to the plane of incidence. The experimental procedure is described and the electrochemical behaviour of the metal-electrolyte junction is examined in great detail. A phenomenological interpretation of the electro-reflectance at the metal-electrolyte interface is given, in which this effect is due to a surface current related to a change in the electron density at the surface of the metal. The agreement between the theoretical and experimental results is quite good.
Applied Physics Letters | 2006
G. B. Parravicini; A. Stella; P. Ghigna; Giorgio Spinolo; A. Migliori; F. d’Acapito; R. Kofman
Low-melting-point metal nanoparticles show remarkable undercoolings: exceptional values ΔT∕Tm≅0.5 were previously reported for confined Ga droplets in the 100–1000nm range. This was considered a characteristic limiting temperature for Ga. We here give evidence that Ga nanoparticles in the 3–15nm range can be undercooled at least down to 90K (ΔT∕Tm⩾0.7) without crystallization. Since computations and experiments on clusters with <50 atoms show on the contrary the signatures of melting at temperatures well above the bulk value, the result is particularly relevant in stressing the importance of interfacial and finite-size effects on the thermodynamics of the nanoscale.
Chemical Physics | 2000
S. Stagira; M. Nisoli; S. De Silvestri; A. Stella; P. Tognini; P. Cheyssac; R. Kofman
Abstract A detailed analysis of the ultrafast optical response of gallium and tin clusters, embedded in a dielectric matrix, with sizes ranging from the sub-micrometer to the nanometer region is presented. Two distinct regimes of relaxation dynamics depending upon particle radius are observed. Particles with a radius greater than the mean electronic free path present similar temporal evolution of the excitation decay with superimposed damped oscillations, interpreted in terms of acoustic waves generated inside the particle. For nanoparticles with a radius smaller than the mean electronic free path, a clear size dependence of relaxation dynamics is observed. This is a strong indication of an efficient electrons–surface phonon interaction in this regime.
Applied Physics Letters | 1999
P. Tognini; A. Stella; S. De Silvestri; M. Nisoli; S. Stagira; P. Cheyssac; R. Kofman
It is shown that the ultrafast carrier dynamics in Ge nanoparticles involving the electronic density of states well above the gap can be separated in two regimes: a faster one (∼1 ps) characterized by bleaching of the absorption around 490 nm and a slower one (up to a few hundred picoseconds) governed by band gap renormalization. There is also a clear correspondence between the spectral response in both regimes and a bulk-like band structure for sizes below the exciton Bohr radius. Quantum confinement is manifested through the measurable blueshift of E1+Δ1 spectral structure in the ultrafast optical response.