B. Prével

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.

Nanostructured silicon films obtained by neutral cluster depositions

Nanosize neutral silicon clusters produced using a laser vaporization source were analyzed in the gas phase and deposited on various substrates at room temperature in ultrahigh vacuum. Nanostructured thin films with thickness around 100 nm resulting from this nearly ballistic deposition process were subsequently characterized by several complementary electron spectroscopy techniques to investigate the electronic structure. The film properties are comparable to those of a disordered phase but different from the properties of conventional amorphous or nanoporous silicon. The specific features observed in the Si-cluster assembled films cannot be simply interpreted on the basis of quantum confinement effects and are rather attributed to the presence of odd-membered rings in the incident-free cluster structure. Some Si-cluster geometries in the subnanometric size range are proposed and discussed on the basis of the experimental results and a tight binding scheme.

STRUCTURAL, VIBRATIONAL, AND OPTICAL PROPERTIES OF SILICON CLUSTER ASSEMBLED FILMS

Distributions of neutral Si-clusters centered around mean sizes of 50 and 200 atoms have been produced using a laser vaporization source and deposited on various substrates at room temperature in ultrahigh vacuum. The Si-cluster assembled films obtained, resulting from the random stacking of incident free nanosize clusters, were subsequently coated by appropriate protective layers before removing in air to perform ex situ infrared, visible, Raman, and photoluminescence spectrometry measurements, as well as transmission electron microscopy observations. The main characteristics of the cluster films are comparable to those observed for amorphous hydrogenated silicon and quite different to those of conventional nanoporous structures or clusters larger than 2–3 nm. The observed intense photoluminescence signal and band gap suggest the presence of a low number of dangling bonds probably due to surface reconstruction effects, connections between adjacent clusters, and oxygen contamination. As for the oxygen con...

Deposition of preformed gold clusters on HOPG and gold substrates : influence of the substrate on the thin film morphology

Nanometric gold clusters have been produced in a laser vaporisation source and deposited on HOPG and gold (111) surfaces. We have found that coalescence and diffusion of the incident clusters depend on the nature of the substrate. The influence of these processes on thin film morphologies is evidenced experimentally. Among those two substrates, the HOPG appears as a suitable candidate for gold cluster ordering in order to make devices at nanoscale level.

Cobalt and nickel cluster-assembled thin films obtained by low-energy cluster beam deposition

Abstract Nanostructured films of cobalt and nickel have been prepared using the low-energy neutral cluster beam deposition technique on various substrates. The nucleation-and-growth process of the films has been studied by in-situ electrical conductivity measurements and transmission electron microscopy observations. Complementary results obtained from X-ray diffraction and absorption measurements on films 100 nm thick confirm the nanostructured morphologies of both Co and Ni films consisting of a random stacking of well crystallized fcc grains having a diameter of around 3–4 nm. In terms of magnetic properties the granular structure leads to films which roughly behave like amorphous ferromagnets but with local order of the extent of a grain. From magnetization measurements and ferromagnetic resonance experiments which showed spin-wave oscillations, the magnetic parameters characteristic of the cluster-assembled films (i.e. magnetocrystalline anisotropy and intergrain exchange coupling energies) have been...

From the superparamagnetic to the magnetically ordered state in systems of transition metal clusters embedded in matrices

Abstract One of the interests in systems of ultrafine particles embedded in matrices is to adjust the size of the nanoparticles and the mean distance between them separately to study cluster interactions in a controlled environment. The synthesis of iron and cobalt clusters embedded in an insulating or metallic matrix has been realized by the co-deposition of both beams arriving at the same time on a 45°-tilted substrate. The nanoparticles were produced from an intense cluster beam of selected size (centered around 300 atoms per cluster) produced from a laser vaporization source. We used a Knudsen cell to evaporate the matrix. From the ratio of the deposition rates of both beams, we were able to continuously adjust the atomic concentrations of the clusters in the matrix from 3 to 70%. In situ electrical conductivity measurements confirmed a 3D percolation threshold of around 25%. The typical size distribution of embedded clusters determined from HRTEM observations revealed nanocrystallized grains with a very narrow dispersion in size. Two local environments emerge from EXAFS simulations: core atoms in the cluster with the density of bulk and surface atoms with a dilated parameter. In agreement with structural considerations, we clearly observed by magnetoresistance and magnetization measurements versus temperature and concentration a magnetic percolation threshold of around 25% for Co clusters in a Ag matrix (maximum of 12% GMR) corresponding to the transition from the superparamagnetic to the magnetically ordered state.

Quantum-dot systems prepared by 2D organization of nanoclusters preformed in the gas phase on functionalized substrates

The low-energy cluster beam deposition (LECBD) technique is used to deposit gold nanoclusters preformed in the gas phase on functionalized graphite substrates (highly oriented pyrolitic graphite (HOPG)), to prepare 2D- organized arrays of cluster assembled dots. Functionalized HOPG substrates are obtained using the focused ion beam (FIB) nanoengraving technique to pattern 2D-organized arrays of defects (nanoholes, nanobumps) which act as traps for the diffusing clusters. Depending on the deposition conditions (nature, size and fluence of the deposited clusters) and the functionalized substrates (nature and size of the FIB-induced defects, geometry of the 2D array of defects and temperature during deposition) high-quality quantum-dot arrays can be obtained with well controlled and reproducible morphologies. Kinetic Monte Carlo simulations of the cluster deposition experiments on functionalized substrates allow us to obtain quite good fits of the experimental images performed by tapping mode atomic force microscopy (TMAFM), leading to systematic investigations of the best conditions to realize high-quality quantum dots systems. This combined top-down-bottom-up approach (LECBD-FIB) seems a promising method for preparing high-integration-density devices (∼Tbit cm −2 ) well suited for future applications to data storage, nanoelectronics, nano-optics, nanomagnetic systems.

Optical properties of nanostructured thin films containing noble metal clusters: AuN, (Au0.5Ag0.5)N and AgN

The optical properties of nanocomposite thin films of gold, silver and bimetallic silver-gold clusters embedded in a porous alumina matrix have been investigated in the size range 2–6.7 nm. The metallic particles are produced by laser vaporization of either an Au0.5Ag0.5 alloy or a pure metal target whereas the dielectric matrix is evaporated by an electron gun. Samples involving a low metal concentration have been characterized by several complementary techniques in order to determine their composition, morphology and cluster size distribution. The mixed particles have the same stoichiometry as the target rod. Optical absorption spectra exhibit a surface plasmon resonance whose position is shifting with cluster mean size, giving evidence of finite size effects. Theoretical calculations in the framework of Time-Dependent-Local-Density-Approximation (TDLDA), taking into account an inner skin of ineffective screening and the porosity of the matrix, are consistent with observed size evolutions of the Mie frequency in each type of sample.

Low‐energy excitations in glass‐forming aqueous lithium–chloride

Low frequency Raman and inelastic neutron scattering from glass‐forming aqueous lithium chloride (LiCl,4H2O; LiCl,6H2O) are compared. The measurements were carried out from the vitreous to liquid states. The scattering from anharmonic oscillations (light scattering excess), which was separated from the one from harmonic vibrations, is zero in the vitreous state, and increases rapidly in the supercooled state. The frequency dependence of light‐vibration coupling coefficient is linear for low‐energy harmonic vibrations. It is different for anharmonic oscillations. From the frequency dependence of the density of vibrational states and the absence of light scattering excess in the glassy state, it is deduced that the structure is compact in the glassy state.

Comparative analysis of optical properties of gold and silver clusters embedded in an alumina matrix

Abstract The optical properties of nanocomposite thin films of gold and silver clusters embedded in an alumina matrix have been investigated. The samples have been characterized by several techniques in order to determine their composition, morphology and cluster size distribution. Ellipsometry and photoabsorption measurements have been performed. The differences in the optical properties of the various samples reflect finite size effects. Time Dependent Local-Density-Approximation (TDLDA) calculations, taking into account the d-electrons and matrix screening effects, are consistent with observed size evolutions of the Mie frequency. Low frequency Roman scattering spectra exhibit a strong size-dependent band related to quadrupolar vibration modes of the clusters.