B. Masenelli

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.

Quantum confinement effect on Gd2O3 clusters

The evolution of the gap of a nanoscaled insulator material, namely, Gd(2)O(3), has been observed by means of vacuum ultraviolet excitation spectra of a dopant (Eu(3+)). The nanoparticles have been synthesized by the low energy cluster beam deposition technique and grown afterward by different annealing steps. A gap shift towards the blue is observed, similar to what is observed in semiconductor nanoparticles. Despite the strong ionic character of the material, the evolution exhibits a behavior similar to covalent materials. The evolution of the gap for Gd(2)O(3) follows the same empiric rule that has been derived for semiconductors (ZnO, CuBr, Si, and CdS). It shows that, in spite of the strong ionic character of the material (0.9 on the scale of Phillips), the amount of covalency is important enough for creating a significant delocalization of the electron with regard to its hole.

Fluorescent oxide nanoparticles adapted to active tips for near-field optics

We present a new kind of fluorescent oxide nanoparticle (NP) with properties well suited to active-tip based near-field optics. These particles with an average diameter in the 5-10 nm range are produced by low energy cluster beam deposition (LECBD) from a YAG:Ce3+ target. They are studied by transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS), cathodoluminescence, near-field scanning optical microscopy (NSOM) and fluorescence in the photon-counting mode. Particles of extreme photo-stability as small as 10 nm in size are observed. These emitters are validated as building blocks of active NSOM tips by coating a standard optical tip with a 10 nm thick layer of YAG:Ce3+ particles directly in the LECBD reactor and by subsequently performing NSOM imaging of test surfaces.

Ab initio study of C60–silicon clusters

We report on the energetics of C60–Si clusters. By means of ab initio calculations based on the local density approximation to the density functional theory, we have investigated stable and metastable structures of C60–Si, C60–Si–C60, and (C60–Si)2 clusters. In each case, we show that silicon preferentially binds to C60 over a carbon–carbon double bond, in accordance with calculations on the interaction of C60 with silicon surfaces. This bonding is characterized by a partial charge transfer from silicon to C60. We show that the interaction between C60 and silicon is local and not perturbed by the addition of more C60–Si clusters or C60 molecules. The binding energy for stable and metastable (C60–Si)n⩽2 systems is high enough (several eV) to open the prospect of synthesizing nanostructured films from the C60–Si unit. Furthermore, in all three cases, the silicon position on a fivefold symmetry axis is found to be a metastable position. The nature and structure of nanostructured films resulting from the depo...

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.

Covalent clusters-based materials

Abstract We review the properties of covalent clusters-based materials in relation to free cluster properties, namely carbon, silicon and mixed carbon clusters. These properties are understood in terms of quantum size especially the so called rehybridization effect. We show that low energy cluster beam deposition is a powerful technique to prepare unusual bonding. To cite this article: P. Melinon et al., C.xa0R. Physique 3 (2002) 273–288.

Si–C60 bond in cluster-based materials

We investigate the binding of Si and C 60 theoretically (by ab initio calculations within the local-density approximation to the density functional theory) and confront the predictions to experimental X-ray absorption results from Si-C 60 films synthesized by the cluster beam deposition technique. The calculations predict that Si preferentially binds to hexagon-hexagon edges. The geometry with Si bound to a pentagonal face is metastable. The binding energy is in each case higher than the typical van der Waals binding energy. Extended X-ray absorption fine structure measurements reveal that in C 60 -Si obtained from (C 60 ) m Si n clusters deposition, Si is bound to the pentagonal face of two C 60 molecules. Both theoretical and experimental investigations go to show that the polymerisation of C 60 -Si clusters is possible, leading to nanostructured C 60 -based materials with high binding energy.

Thin film growth using hetero embryo: demonstration on pyrochlore phase.

The paper reports the possible use of nanoparticles embedded in amorphous host as hetero embryos in order to grow complex crystalline phases as thin film. Demonstration is performed in the prototypical case of pyrochlore phase Gd(2)Ti(2)O(7) grown from Gd(2)O(3) nanoparticles embedded in TiO(2) matrix at low temperature. As embryos, two kinds of nanoparticles are compared: clusters deposited by low energy cluster beam deposition (LECBD) and nanostructured films elaborated by sol-gel process. The growth has been analyzed by X-ray diffraction and transmission electron microscopy. Furthermore, the nanoparticles have been doped with Eu(3+) luminescence probes in order to follow the nucleation mechanisms at the atomic scale. It is shown that the size, shape, and composition of hetero embryos and as well their interfaces are of paramount importance to enhance the formation of complex materials, such as pyrochlore. By this mean, the first step in classical nucleation science, controlling the height of the energetic barrier, is skipped and the synthesis conditions can be eased.