Ph. Cahuzac’s research while affiliated with Laboratoire Aimé Cotton and other places

The interaction between neutral silver clusters and graphite surfaces is studied by cluster deposition and aggregation experiments. It is shown that the cluster diffusion is strongly influenced by the surface curvature. Convexly bent areas act as repulsive barriers for the cluster diffusion. Bent areas can therefore be used as constraints and guides, in which clusters aggregate quasi-one-dimensionally to organized, elongated silver structures with diameters below 20 nm.

Condensation of nano-droplets in a supersaturating vapor decomposes in two steps: the formation of a nucleation center, also called critical nuclei or nucleation seed, and the growth sequence, by accretion of further atoms on the nucleation center. These two steps have been investigated separately through the clustering of homogeneous particles Nan and heterogeneous particles NanX in a helium buffer gas (X = (Na2O)2 or (NaOH)2). The growth sequence is analyzed with preformed molecules X injected in a supersaturating sodium vapor and driving production of NanX clusters. Cluster distribution mean sizes are controlled by sodium concentration and by the condensation cell effective length. The signal intensities observed for homogeneous and heterogeneous clusters are proportional to the homogeneous and heterogeneous nucleation center numbers respectively. We can measure the efficiency for the homogeneous nucleation center production versus sodium concentration. This process is the onset of the condensation phase transition.

Preformed clusters carrying surfactant are used as primary blocks for the building of nano structures. Self assembly of silver atom based clusters, soft landed on a HOPG surface, generates a large variety of new architectures depending on the nature and on the concentration of the impurities. Fractal shapes fragmented into multiple compact like islands, and chain like structures might be formed. A strong local enhancement of the silver atom mobility at the surface of islands is responsible for those morphology changes.

Both gas phase Nan and Nan(NaOH)2 nanoparticle ensembles (n=50–250) are produced in a gas aggregation source and studied by using laser-ionization time-of-flight mass spectrometry. Heterogeneous Nan(NaOH)2 clusters are shown to be produced by accretion of sodium atoms on nucleation centers initially formed by a chemical reaction between H2O and Na or Na2 molecules. Mass distributions are compared for Nan grown on an homogeneous nucleation center and for Nan(NaOH)2 developing on an heterogeneous seed in the same condensation source, allowing the comparison between homogeneous and heterogeneous clustering efficiency.

We investigate the morphological changes of 3D supported fractal aggregates generated through the deposition of silver clusters on graphite. The fractal relaxation, activated after their formation by perturbing them either by thermal annealing or by using a surfactant, as oxide molecules, carried by silver clusters in a subsequent deposition, shows evidence of two distinct fragmentation patterns. The post coarsening, driven by thermal heating, gives a broad asymmetrical distribution of fragments in agreement with a random process, whereas the entire silver fractal pearling fragmentation is driven by chemical adjunction of the surfactant.

The deposition of preformed clusters on surfaces offers new possibilities to build complex artificial nanostructures, the shape of which depends on the cluster size. We describe routes for generating unusual polymorphic nanoislands, which constitute unique platforms for exploring instabilities. As coverage increases, the constraints accumulated in such nanostructures induce spectacular flattening collapse processes, which are not observed when the constraints are imposed by the substrate.

We present combined experimental and theoretical studies of cationic sodium hydroxide clusters. The stability of Na+(NaOH)n, n=4–26, is investigated observing the unimolecular dissociation channels and measuring the unimolecular decays. Ground electronic state geometries and binding energies are obtained using Born–Oppenheimer local spin density molecular dynamics calculations for n=1–7. The observed dissociation channels are discussed in the light of the calculated dissociation energies, showing that dynamics effects should play a role in the fragmentation process.

We present a combined theoretical and experimental study of cationic sodium fluoride clusters stability. The dissociation channels of Na+(NaF)(n), n = 3-11, have been measured from unimolecular decay experiments. Binding and dissociation energies have been calculated using a simple potential. The discussion of the observed dissociation channels in the light of their relative binding energy reveals that entropic and kinetic effects play an important role in the fragmentation process. (c) 2005 Elsevier B.V. All rights reserved.

The synthesis of silver rich molybdate clusters is achieved by laser induced chemical reaction of coadsorbed MoO(3) and O(2) molecules on free silver clusters. The reactants MoO(3) and/or O(2) molecules condensed at low temperature (77 K-175 K) on free silver clusters. Then, the silver clusters together with their adsorbed molecules are flashed either ionized with a discharge or ionized and heated by a laser. Then they are cooled down by evaporation. The synthesized chemical compounds are analyzed by a high-resolution time-of-flight mass spectrometer. If only one type of reactant is adsorbed on the cluster, only one oxide molecule is stabilized on the metallic core after the heating and cooling cycle. On the contrary, the coadsorption of the two types of molecules MoO(3) and O(2) on Ag(n) (+), at 77 K, leads to complex aggregates that transform, after laser heating, into a molybdate rich metal clusters. These synthesized species cool down by evaporating silver atoms showing evidence of a binary oxide that is more stable than the metallic core. Moreover we demonstrate that for small size molybdate clusters, a stoichiometric composition may differ from the bulk one.