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Spontaneous organisation of preformed size selected platinum and gold clusters on graphite is discussed in terms of reactivity with residual gases. The difference in reactivity between gold and platinum can explain the spontaneous organisation in an high vacuum for platinum and poor vacuum for gold. Contrary to earlier assumptions (Friedel oscillations, moirés and graphite stiffness), the hypothesis of a chemical surface modification agrees with the evolution of thin film morphologies with the deposition pressure as well as with the constant edge-edge distance between two neighbouring clusters (1.2 nm). The possibility of cluster decoration by CO is discussed. [ABSTRACT FROM AUTHOR]

The extinction spectrum of single silver nanoparticles with size ranging from 20 to 80 nm is investigated with the spatial modulation spectroscopy technique using either a tunable laser or a white lamp as the broadband source. Results are in good agreement with the prediction of the Mie theory, permitting to extract the nanoparticle size from the measured absolute value of the optical extinction cross-section. In contrast, the deduced refractive index of the nanoparticle environment and the reduction of the electron mean free path show a large dependence on the precise value of the bulk silver dielectric function. [ABSTRACT FROM AUTHOR]

The optical properties of noble metal clusters are measured and compared in the size range 1.4–7 nm in diameter. The clusters of copper, silver and gold are produced by the same experimental technique, the deposition of preformed clusters in a transparent matrix. The size dependence is found to be different for the three metals. As the size decreases, the surface plasmon resonance is only slightly blue-shifted for silver, more strongly blue-shifted and damped for gold while this peak resonance vanishes for copper. We show that these results cannot be interpreted by a simple classical theory. Since ab initio calculations are not possible in this size range, we obtain a complete theoretical description of these optical properties through the same semi-quantal model for the three metals. [ABSTRACT FROM AUTHOR]

We investigate the fundamental properties of films resulting from the deposition of Eu3+ doped Gd2O3 clusters. We first report that the crystalline structure of clusters with diameter as low as 2.8 nm is still BCC (as in the bulk phase at ambient temperature and pressure). No phase transition to monoclinic structure is observed. We show that contamination from air (formation of hydroxide) is important and leads to the modification of the luminescence of the Eu3+ ions. Furthermore, films protected from this contamination have been fabricated and show a new feature (broad peak at 625 nm). It means that contamination is not the only mechanism responsible for the modification of the light emission spectrum. The crystal field symmetry breaking induced by the surface also plays a major role. Eventually, we show that the widening of the optical gap continues for these very small clusters. We discuss this effect in the frame of the quantum confinement model. [ABSTRACT FROM AUTHOR]

Effects of confinement on the electron-electron (e-e) and electron-phonon (e-ph) thermalization dynamics in noble metal clusters are calculated using simple approaches. The model predictions are compared with femtosecond pump-probe measurements which display an acceleration of the e-e and e-ph relaxation dynamics. The size-effects on the e-e relaxation dynamics are consistent with a model involving the surface-induced reduction of the screening efficiency of the Coulomb e-e interaction. With regard to the e-ph relaxation dynamics, this model yields too large time constants, pointing out deficiencies of the standard modelling of the e-ph energy exchanges in bulk metals. Analysis of these deficiencies shows that the bare e-ion interaction has to be involved in the transition matrix element describing the non-adiabatic e-ph energy exchanges. [ABSTRACT FROM AUTHOR]

(C60)mNan clusters are produced in a tandem laser vaporization source and analyzed by photoionization and photofragmentation time-of-flight mass spectroscopy. At low sodium coverage, the special behavior of (C60)m=1,2Nan clusters (n ≤ 6m) is consistent with a significant electron transfer from the first six adsorbed atoms towards each of the C60 fullerenes and an ionic-like bonding in this size range. However, the stability of the (C60)Na3+ cation is found much more pronounced than the one of (C60)Na7+ predicted to be a magic size under the hypothesis of a full charge transfer from the metal atoms to the C60 molecule. When more sodium atoms are present, metal-metal bonds tend to become preponderant and control the cluster properties. Relative to the number of sodium atoms, an odd-even alternation in their stability is explained by the high dissociation rates for even-numbered clusters. The even clusters evaporate neutral sodium atoms whereas odd ones prefer to evaporate Na2 molecules. The hypotheses for the growth of a sodium droplet that does not wet the fullerene surface or for the formation of a concentric metallic layer are discussed in the light of this study. [ABSTRACT FROM AUTHOR]

The optical properties of mixed (Ni0.5Ag0.5)n and Pt/Ag clusters are investigated in the size range 2-5 nm. Low Energy Ion Spectroscopy (LEIS) experiments show that the cluster surface is entirely covered by silver atoms for the two systems. The optical spectra of Ni/Ag clusters exhibit a large Surface Plasmon Resonance (SPR), damped and widened when the cluster size decreases, in agreement with a classical model assuming a core-shell geometry and including the reduction of the conduction electron mean-free path in the silver shell. For Pt/Ag clusters, no SPR emerges in the size range 2-5 nm, although it is predicted within a classical model, a pronounced SPR appearing only for clusters larger than 10 nm in diameter. [ABSTRACT FROM AUTHOR]

In order to resolve problems concerning the understanding and the control of laser-induced damage of silica optical elements, a collaboration between the CEA and different university laboratories has been undertaken. Ultra-pure silica model samples, seeded with gold nanoparticles whose diameter did not exceed 5 nm, were prepared. The aim in using these samples was to observe the mechanism of damage initiation that could be attributed to inclusions of nanometric size. This paper presents the different steps encountered during this study: preparation of the samples, the laser-induced damage tests, the Nomarski and atomic-force microscope observations of this damage and a series of experiments using a time-of-flight mass spectrometer at Argonne National Laboratory. The experimental data are then interpreted, and, in particular, compared to numerical simulations. A very encouraging result is the existence of a pre-damage phase at very low fluences that is not detectable by classical optical devices. The experimental means developed for such model samples should be transposable to the analysis of industrial glasses. [ABSTRACT FROM AUTHOR]

The effects of surface-induced lattice contraction on the size evolution of the surface plasmon resonance and of the electronic thermalization time in small silver clusters have been investigated in the framework of a mixed classical/quantum model. The increase of the conduction-electron density results in a blue-shift trend for decreasing cluster size. However this effect is counterbalanced by the increase of the dielectric function associated to the ionic-core background. Agreement with the blue-shift trend observed in experiment is recovered by introducing an inner surface skin of vanishing ionic-core polarizability having a thickness practically unchanged as compared to previous estimations. The influence of the lattice contraction on the electron dynamics is also discussed. It is shown that this influence is negligible as compared to the surface effects arising from the spillout and the inner skin of reduced ionic-core polarizability which are both responsible for a decrease of the electron thermalization time as the particle size decreases. [ABSTRACT FROM AUTHOR]

This paper deals with a new type of SiC bonding where silicon atom seems to bridge C60 molecules. We have studied films obtained by deposition of (C60)nSim clusters prepared in a laser vaporization source. Prior deposition, free ionized clusters were studied in a time-of-flight mass spectrometer. Mixed clusters (C60)nSim were clearly observed. Abundance and photofragmentation mass spectroscopies revealed the relatively high stability of the (C60)nSi n +, (C60)nSi n - 1 + and (C60)nSi n - 2 + species. This observation is in favor of the arrangement of these complexes as polymers where the C60 cages may be bridged by a silicon atom. Free neutral clusters are then deposited onto substrate making up a nanogranular thin film ( ≃ 100 nm). The film is probed by Auger and X-ray photoemission spectroscopies, but above all by surface enhanced Raman scattering. The results suggest an unusual chemical bonding between silicon and carbon and the environment of the silicon atom is expected to be totally different from the sp3 lattice: ten or twelve carbon neighbors might surround silicon atom. The bonding is discussed to the light of the so-called fullerene polymerization as observed for pure fullerite upon laser irradiation. This opens a new route for bridging C60 molecules together with an appreciable energy bonding, since the usual van der Waals bonding in fullerite could be replaced by an ionocovalent bond. Such an assumption must be checked in the future by XAS and EXAFS experiments. [ABSTRACT FROM AUTHOR]

Optical properties of mixed clusters (AuxAg 1 - x ) n and (NixAg 1 - x ) n, produced by laser vaporization and embedded in an alumina matrix, are reported. The size effects are investigated for different concentrations ( x = 0.25, 0.5 and 0.75) in the diameter range 2-4 nm. For alloyed clusters (AuxAg 1 - x ) n of a given size an almost linear evolution of the surface plasmon frequency ω s with the concentration is observed (between those of pure gold and pure silver clusters). Moreover the blue-shift and the damping of the resonance with decreasing size is all the more important as the gold concentration in the particles increases. Such results are in agreement with theoretical calculations carried out in the frame of the time-dependent local-density-approximation (TDLDA) including an inner skin of ineffective screening and the porosity of the matrix. The optical response of (NixAg 1 - x ) n clusters exhibits a surface plasmon resonance in the same spectral range as the one observed for pure silver clusters, but considerably damped and broadened. For a given mean cluster size 3.0 nm, a blue-shift of the resonance is observed when increasing the nickel concentration (between x = 0.25 and x = 0.75). The results are in good qualitative agreement with classical predictions in the dipolar approximation, assuming a core-shell geometry. [ABSTRACT FROM AUTHOR]

Low-frequency Raman scattering experiments have been performed on thin films consisting of pure gold or gold-silver alloy clusters embedded in alumina matrix. It is clearly shown that the quadrupolar vibrational modes are observed by Raman scattering because of the effect of resonance with the excitation of the electronic surface dipolar plasmon. This is due to the strong coupling between the collective electronic dipolar excitation and the quadrupolar vibrational modes. This effect of resonance does not exist with the core electron excitations. The mixing of the conduction electron dipolar excitation (surface plasmon) with the core electrons leads to the quenching of the resonant Raman scattering. [ABSTRACT FROM AUTHOR]

We have investigated SiC films, obtained by the low-energy cluster beam deposition (LECBD) technique, and using AFM Raman and XPS spectroscopies. We produce these films at room temperature and in an ultrahigh vacuum environment to protect them from the pollution. The inner morphology of the clusters is close to an amorphous-like structure. However, most of the theoretical models predict an sp3 hybridization, in disagreement with our experimental results. We find that the films are composed mainly by free components formed by rich silicon, rich carbon, and rich SiC regions, respectively. In our case, the mean hybridization of the rich carbon region is mainly sp n-like with 2< n<3. [ABSTRACT FROM AUTHOR]

Absorption and low-frequency Raman scattering experiments have been performed on thin films consisting of small silver clusters embedded in a porous alumina matrix. When the Raman excitation wavelength is close to the maximum (≈420 nm) of the Mie band (dipolar surface plasmon resonance) the Raman spectra exhibit a strong band located around 10 cm-1, the maximum of which depends on the mean cluster diameter 〈D〉 in the sample according to the approximate law ωvib∝〈D〉-1. The Raman band corresponds to the excitation of the quadrupolar vibration mode of the clusters. Moreover, the maximum of the Raman band shifts towards lower frequencies when the excitation light is shifted to the red. This feature, as well as the rather large Mie-band width, is thought to reflect the ellipsoidal shape distribution of part of the embedded clusters. [ABSTRACT FROM AUTHOR]

Silicon–carbon binary clusters with various mean compositions are generated in a laser vaporization source from targets processed as mixtures of graphite and silicon powders. Their size distribution is first analyzed by time-of-flight mass spectroscopy, which shows the stability of carbon fullerenes doped with silicon atoms in substitutional sites. Further investigations on the level of silicon doping are carried out by means of the laser-induced fragmentation of selected sizes. The photoproduct size distributions give evidence for at least nine silicon atoms substituted into still stable fullerene networks. The synthesis of heterofullerenes is mainly assisted by the nucleation mechanisms from Si-C mixed atomic vapors. Just as in the case of externally doped fullerene precursors, the laser-induced annealing of stoichiometric silicon-carbide clusters appears as an alternative route to produce heterofullerenes in the gas phase. [ABSTRACT FROM AUTHOR]

We synthesized Co-Sm mixed clusters from a SmCo5 solid target mounted in a laser vaporization source. We studied both pure nanostructured SmCo5-cluster-assembled films and the same isolated clusters embedded in a matrix. The supported cluster size distribution centered around 3 nm was determined from transmission electron microscopy observations. Mass spectra obtained on the photoionized clusters in the gas phase and Rutherford backscattering spectrometry analyses performed on 100-nm-thick films confirm the conservation of the compound-target stoicheiometry in both free and deposited clusters. However, X-ray photoemission spectroscopy measurements seem to indicate a segregation of samarium at the cluster surface. This segregation effect could explain the low values of the magnetization and coercive fields obtained from magnetometry measurements with respect to those in the bulk SmCo5 crystallized phase. [ABSTRACT FROM AUTHOR]

Using the pseudo-Hamiltonian technique we investigate, through complete self-consistent ground-state calculations, nonlocal ion-pseudopotential effects on the dipole strength distribution and the electronic shell structure of alkali clusters. The results obtained for sodium clusters are compared to the predictions of the standard jellium model (JM) and to experiments. Some wrong JM-predictions - shift of some low magic sizes, insufficient redshift of the surface plasmon frequency relative to the Mie frequency - are corrected, but a noticeable discrepancy remains with regard to the supershell structure. In the second part of this paper we restore the reliability of the pseudo-Hamiltonian approximation, on which some doubts have been recently cast, because of incorrect predictions concerning the bulk properties of lithium. From a careful analysis of the nonlocal effects in a smoothed ionic background, we show that a specific pseudo-Hamiltonian parametrization has to be systematically selected for ensuring maximum accuracy. We successfully test the suitability of this parametrization in the case of lithium clusters, through a comparison with recently published results obtained using accurate norm-conserving nonlocal pseudopotentials. [ABSTRACT FROM AUTHOR]

Unimolecular dissociation of aluminum, gallium and indium clusters is investigated. Small sizes dissociate into two channels: either the evaporation of a neutral or a charged monomer. Above a given size n, only dissociation of a neutral atom subsists. The evaporation of a charged monomer is characteristic of trivalent metal clusters and is consistent with the size evolution of the ionization potential towards the atomic value. The experiments are interpreted in the framework of the statistical R.R.K. model. For smaller sizes ( n < n), as two evaporation processes are in competition, we have evaluated cluster relative dissociation energies and ionization potentials. The competition between the two evaporation channels is well mirrored by the evolution of the ionization potentials independently measured by near-threshold photoionization experiments. For gallium, our measurements have revealed that the covalent to metal transition occurs for larger sizes ( n = 30-50 atoms) than for aluminum clusters. [ABSTRACT FROM AUTHOR]

Cluster assembled thin films of covalent materials are studied by Raman spectroscopy. The main properties are discussed in terms of frustration through several examples (carbon, silicon, and boron nitride). Some applications such as luminescence are discussed. [ABSTRACT FROM AUTHOR]

The optical response of free and matrix-embedded gold metal clusters AuN is investigated in the framework of the time-dependent local-density-approximation (TDLDA). The characteristics of the surface plasmon resonance are carefully analyzed as a function of the model parameters and the particle radius. The strong influence of the frequency-dependence of the 5 d core-electron dielectric function in the vicinity of the interband threshold is emphasized. The size evolution of the Mie-frequency in free gold clusters exhibits a noticeable blue-shift trend as the particle size decreases, much stronger than in silver clusters. The width and shape of the resonance, essentially ruled by the decay via the interband transitions, are found closely correlated to the imaginary component of the core-electron dielectric function. In presence of a surrounding matrix the blue-shift trend is largely rubbed out. Agreement with recent experimental results on size-selected gold clusters embedded in an alumina matrix may be achieved by taking into account the porosity effects at the metal/matrix interface. The comparison with the predictions of classical models is also provided. [ABSTRACT FROM AUTHOR]

1,14-Bis-(ethyl)-amino-5,10-diazatetradecane N1,N11-bis(ethyl)norspermine (BE-4-4-4) and 1,19-bis-(ethylamino)-5,10,15 triazanonadecane (BE-4-4-4-4) are two relatively new polyamine analogs synthesized for use as antineoplastic agents. In human brain tumor cell lines U-251 MG and SF-767, both agents inhibited cell growth, were cytotoxic, induced a variable G1/S block, and depleted intracellular polyamines. Since intracellular polyamine depletion did not always correlate with growth inhibition, cell survival, or cell cycle progression, it cannot completely explain the effects of these agents on growth, survival, and cell cycle progression in U-251 MG and SF-767 cells. [ABSTRACT FROM AUTHOR]

Neutral aluminum clusters have been produced by laser vaporization technique, ionized by a low-power near-threshold laser light and detected using standard TOF spectrometric methods. Ionization potentials have been deduced in the low size range. In the large size range 250

Laser photoionization experiments have been performed on a large size range of indium and thallium clusters. The metal clusters were produced by laser vaporization technique and analyzed after laser ionization by standard time-of-flight mass spectrometry. For the indium clusters, individual ionization potentials (IP) are deduced for N≤132. Abrupt decreases in the IP values are observed, which correspond to the openings of new electronic shell as predicted by the spherical jellium model. For larger indium clusters, the unresolved mass spectra present small but reproducible oscillations. Interpretations in terms of either electronic shell structure or cluster geometry remain undecided for the moment. Our results on thallium clusters are less abundant and only qualitative. Nevertheless, they show that thallium behaves more like a monovalent element than like a trivalent one. [ABSTRACT FROM AUTHOR]