J. L. Vialle

Evidence for icosahedral atomic shell structure in nickel and cobalt clusters. Comparison with iron clusters

Abstract For the first time, the mass distribution of nickel, cobalt and iron clusters is analyzed in a large mass range with near threshold photoionization experiments and standard time-of-flight mass spectrometry. In the case of nickel and cobalt, oscillations observed in mass spectra correspond to icosahedral atomic shell structure in the studied mass range (50–800 atoms). For iron clusters, the situation is less clear. The exact location of structures in mass spectra depends on source operating conditions. We have observed the competition between different cluster geometries.

Photolysis experiments on SiC mixed clusters: From silicon carbide clusters to silicon-doped fullerenes

Silicon carbon binary clusters are generated in a laser vaporization source from SixC1−x mixed targets (x=0 to 50%). We have first analyzed stoichiometric (SiC)n (n⩽40) clusters grown from a silicon carbide target (x=50%). Both high fluence photoionization of (SiC)n neutral clusters and photofragmentation of size-selected (SiC)n+ natural positive ions show that silicon-doped fullerenes emerge as stable photoproducts through the laser induced annealing of these clusters. They are detected as stable species as soon as a sufficient amount of silicon is eliminated through unimolecular processes involving the sequential losses of Si2C and Si3C neutral molecules in the earliest evaporation steps. This result is in favor of an efficient substitution of silicon atoms (about 12) into stable “cagelike” carbon networks. We will also show that an efficient doping of carbon fullerenes with silicon atoms can be obtained in carbon-rich mixed clusters directly grown as positive ions from nonstoichiometric targets (x<25%). Mass abundance spectroscopy gives a clear signature of cagelike structures where silicon atoms are substituted for carbon ones. The results on the favored stability of even-numbered C2n−qSiq+ clusters with q=0, 1, 2 are presented here in the size range: 2n=32–80. More largely doped species (q⩾3) cannot be evidenced in abundance mass spectroscopy because of unavoidable mass coincidences. A careful analysis of the photofragmentation behavior of selected sizes relative to the laser fluence nevertheless succeeds in indicating the contribution to the photofragmentation spectra of largely doped heterofullerenes C2n−qSiq+ (q=7 at least) that mainly dissociate by the loss of small even-numbered mixed molecules such as Si2,Si3C,… . Both approaches are consistent with the surprising capability of substituting a large number of silicon atoms into fullerenes without destabilizing their cage structure too much. In this respect, a value close to 12 seems to be an upper limit.Silicon carbon binary clusters are generated in a laser vaporization source from SixC1−x mixed targets (x=0 to 50%). We have first analyzed stoichiometric (SiC)n (n⩽40) clusters grown from a silicon carbide target (x=50%). Both high fluence photoionization of (SiC)n neutral clusters and photofragmentation of size-selected (SiC)n+ natural positive ions show that silicon-doped fullerenes emerge as stable photoproducts through the laser induced annealing of these clusters. They are detected as stable species as soon as a sufficient amount of silicon is eliminated through unimolecular processes involving the sequential losses of Si2C and Si3C neutral molecules in the earliest evaporation steps. This result is in favor of an efficient substitution of silicon atoms (about 12) into stable “cagelike” carbon networks. We will also show that an efficient doping of carbon fullerenes with silicon atoms can be obtained in carbon-rich mixed clusters directly grown as positive ions from nonstoichiometric targets (x<25%)...

Functional nanostructures from clusters

Functional cluster-assembled nanostructures with original structures and properties are prepared using the Low Energy Cluster Beam Deposition method (LECBD). This technique consists in depositing supersonic clusters produced in the gas phase using a combined laser vapourisation-inert gas condensation source. Low energy clusters with typical sizes ranging from ∼1 to a few nm are not fragmented upon impact on the substrate (soft landing regime) leading to the formation of cluster-assembled nanostructures which retain the original structures and properties of the incident free clusters. Model nanostructured systems of any kind of materials (metallic, covalent, oxides) well suited for fundamental studies in various fields (nanoelectronics, nanomagnetism, nanophotonics, catalysis or nanobiology) and for applications to very high integration-density devices (∼Tbits/in/²) are prepared using this method. After a brief review of techniques to produce, analyse, mass select, and deposit clusters in the LECBD-regime, the specific aspects of the nucleation and growth process which govern the formation of cluster-assembled nanostructures on the substrate are presented, especially the preparation of 2D-organised arrays of cluster-assembled dots by depositing low energy clusters on FIB-functionalised substrates. Characteristic examples of cluster systems prepared by LECBD are also described: i) metallic (Au, Ag, Au-Ag, Ag-Ni, Ag-Pt)) and oxide (Gd2O3, ZnO) cluster-assembled nanostructures for applications to linear and non linear nano-optics; ii) magnetic nanostructures from Co-based nanoclusters (i.e., Co-Pt) exhibiting a high magnetic anisotropy which is well suited for applications to high density data storage devices; iii) gold or Pd-Pt or Au-Ti clusters for chemical reactivity and catalysis applications. In some specific cases, we were able to perform studies from an isolated individual nanocluster up to 2D or 3D-collections of non-interacting or interacting particles leading to a rather good understanding of the intrinsic as well as the collective properties at nanoscale.

Shell structure in photoionization spectra of large aluminum clusters

Photoionization mass spectrometry experiments, performed on an extensive size range of aluminum clusters produced by laser vaporization technique, are reported. Ionization potential values are deduced from individual photoionization efficiency curves for the smaller AlN clusters (N=36–112). Our results confirm and complete those previously published. The mass spectra of larger clusters (N≊250–1400) reveal a regular signal oscillation. Several additional experiments give proof that this striking pattern originates from size‐dependent ionization threshold effects. This structure exhibits exact periodicity as a function of N1/3 or Ne1/3 (Ne the number of valence electrons).

Absolute optical extinction measurements of single nano-objects by spatial modulation spectroscopy using a white lamp

This article describes a high sensitivity spectrophotometer designed to detect the overall extinction of light by a single nanoparticle (NP) in the 10(-4)-10(-5) relative range, using a transmission measurement configuration. We focus here on the simple and low cost scheme where a white lamp is used as a light source, permitting easy and broadband extinction measurements (300-900 nm). Using a microscope, in a confocal geometry, an increased sensitivity is reached thanks to a modulation of the NP position under the light spot combined with lock-in detection. Moreover, it is shown that this technique gives access to the absolute extinction cross-sections of the single NP provided that the incident electromagnetic field distribution experienced by the NP is accurately characterized. In this respect, an experimental procedure to characterize the light spot profile in the focal plane, using a reference NP as a probe, is also laid out. The validity of this approach is discussed and confirmed by comparing experimental intensity distributions to theoretical calculations taking into account the vector character of the tightly focused beam. The calibration procedure permitting to obtain the absolute extinction cross-section of the probed NP is then fully described. Finally, the force of the present technique is illustrated through selected examples concerning spherical and slightly elongated gold and silver NPs. Absolute extinction measurements are found to be in good consistency with the NP size and shape independently obtained from transmission electron microscopy, showing that spatial modulation spectroscopy is a powerful tool to get an optical fingerprint of the NP.

Gas phase study of silicon–C60 complexes: Surface coating and polymerization

(C60)nSim+ cationic clusters are produced in a laser vaporization source by quenching the vapors from two independent C60 and silicon targets. They are analyzed in the gas phase by abundance and photofragmentation time-of-flight mass spectroscopy. For complexes containing only one C60 molecule, silicon is unlikely to wet the fullerene surface. Mass spectroscopic studies are rather in favor of a three-dimensional growth of silicon clusters weakly bound to C60. For larger systems, one can distinguish two classes of silicon atoms: most of them group in the form of compact islands (or clusters) and some others are directly involved in the linkage of C60 molecules. Particular geometric structures for the stable polymers (C60Si)n−2(C60)2+, (C60Si)n−1C60+, and (C60Si)n+ are postulated.

Shell structure of small indium clusters below N≈200 atoms

Abstract Neutral indium cluster In N ( N ≤200) are laser photoionized near threshold and analyzed by time-of-flight mass spectrometry. Strong shell effects are observed in cluster mass spectra. For In N clusters below N =125, individual ionization potentials are deduced. They are compared with theoretical predictions calculated within the framework of the spherical-jellium model. The agreement is qualitatively quite good and the observed shell effects correspond to the successive major electronic shell closures. Deviations from electronic shell model predictions are much less important for indium clusters than for aluminum ones. A tentative explanation of this difference is given.

Competition between atomic shell and electronic shell structures in aluminum clusters

Under usual experimental conditions, aluminum clusters have specific geometric arrangement. By heating the nozzle, we obtain melted aluminum clusters, and a new periodicity appears in mass spectra, which corresponds to electronic shells up to 1800 electrons.

Coating and polymerization of C60 with carbon: A gas phase photodissociation study

(C60)nCq+ cationic clusters are produced in a laser vaporization source by quenching the vapors from C60 and graphite targets. They are analyzed in the gas phase by abundance and photofragmentation time-of-flight mass spectroscopy. Among the large number of expected isomers, the present experiments give evidence for the stability of special arrangements that may result from the attachment of preformed carbon rings to the fullerene cage. The particular case of C10 will be discussed, considering the results obtained on complexes with one, two or even three C60 molecules. This study reveals that the incorporation of large molecules into carbon cages could be an essential mechanism during the growth process of larger fullerenes and that the polymerization of C60 molecules is promoted by carbon links involving a single atom or a dimer.

A CYLINDRICAL REFLECTRON TIME-OF-FLIGHT MASS SPECTROMETER

A new reflectron time-of-flight mass spectrometer with a cylindrical electrostatic mirror instead of the usual plane reflector is described. When it is mounted at right angles to the direction of an incoming molecular beam, this system automatically compensates for the transverse drift of the ions in the spectrometer due to their initial velocity parallel to the molecular beam direction. As a consequence, ions of any mass can be collected onto the detector, whatever their initial transverse velocity is. The mass range accessible in a single scan with our cylindrical reflectron is thus strongly increased as compared to the case of a usual plane reflectron. This article presents the general design of our cylindrical reflectron. A detailed description of the apparatus is given and its performances are illustrated on selected examples. Cluster mass spectra extending over a very wide size range are presented. A mass resolution of about 4000 is achieved in mass spectra of laser photoionized aluminum clusters. M...