A. Masson

Adsorption of small molecules on silver clusters.

We report investigations of adsorption of N(2) and O(2) molecules on silver cluster cations. We have first revisited structures of small silver clusters based on first-principles calculations within the framework of density functional theory with hybrid functional. The 2D to 3D transition for the neutral clusters occurs from n = 6 to 7 and for cations, in agreement with experiments, from n = 4 to 5. With the refined structures, adsorption energies of N(2) and O(2) molecules have been calculated. We have identified characteristic drops in the adsorption energies of N(2) that further link our calculations and experiments, and confirm the reported 2D-3D transition for cations. We have found that perturbations caused by physisorbed molecules are small enough that the structures of most Ag clusters remain unchanged, even though physisorption stabilizes the 3D Ag(7)(+) structure slightly more than the 2D counterpart. Results for pure O(2) adsorption indicate that charge transfer from Ag(n)(+) to O(2) occurs when n > 3. Below that size oxygen essentially physisorbes such as nitrogen to the cluster. We interpret the experimentally observed mutually cooperative co-adsorption of oxygen and nitrogen using results from density functional theory with generalized gradient approximations. The key to the enhancement is N(2)-induced increase in charge transfer from Ag(n)(+) cations to O(2).

Generation of rare earth metal clusters by means of the gas-aggregation technique

Generation of rare-earth clusters is achieved using a gas aggregation technique. Varying the nucleation conditions induces a change in the relative cluster abundances. This can be understood in terms of competing cubic versus icosahedron structures.

Hydrogen peroxide and ammonia on protonated ice clusters

A temperature controlled source for protonated water clusters has been combined with high-resolution mass spectroscopy to study the stability pattern of ice clusters and compounds with ammonia and hydrogen peroxide depending on temperature. The stability pattern of pure protonated ice shows the two well known peaks at 21 and 28 molecules and also less pronounced structure up to n=55. Ammonia and hydrogen peroxide do not destroy this pattern but shift it by a number of water molecules. The additives are therefore integrated in the persisting crystalline structure of the pure protonated ice. Based on this structural information, density functional theory calculations reveal that hydrogen peroxide and ammonia occupy surface positions on a dodecahedral 21-molecule cluster and are not caged in the center.

Physisorption and Chemisorption on Silver Clusters

Adsorption and coadsorption studies on free silver clusters show that nitrogen physisorbs like rare gases, whereas oxygen chemisorbs with similarities and differences to bulk silver surfaces. Silver nanoparticles activate, or even dissociate adsorbed oxygen molecules. The global electron configurations of the adsorbent and adsorbate dominate the stability at small clusters. This is more important than geometry and site effects. Due to electronic shell effects and electron pairing, the activation of oxygen strongly varies with size. At more than 40 free electrons in the complex, such quantum effects start to blur. The size dependence becomes smoother and general trends govern the reactivity, which is driven by the interaction between the charge state of the nanoparticle and the charge transfer of the reaction.

Curvature effect on the interaction between folded graphitic surface and silver clusters

Evidence of curvature effects on the interaction and binding of silver clusters on folded graphitic surfaces has been shown from both experiment and theory. Density Functional Theory (DFT) calculations within the local density and generalized gradient approximations have been performed for the structural relaxation of both Ag and Ag

Enhancement of Ag cluster mobility on Ag surfaces by chloridation

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Shape change in nano-structures from AgnOxHy silver cluster deposition on graphite induced by their chemical composition

on curved surfaces, showing a cross-over from quantum to classical behaviour. Using Lennard-Jones potential to model the interaction between a single cluster and the graphene surface, evidence is found for the curvature effect on the binding of silver nano-particles to folding graphitic surfaces. The theoretical results are compared to SEM and AFM images of samples obtained from pre-formed silver cluster deposition on carboneous substrates exhibiting anisotropic pleat structures.

Instability driven fragmentation of nanoscale fractal islands.

To understand the role of chlorine in the stability and the observed fragmentation of Ag dendritic nanostructures, we have studied computationally two model systems using density functional theory. The first one relates to diffusion of Ag(n) and Ag(n)Cl(m) (n = 1-4) clusters on an Ag(111) surface, and the second demonstrates interaction strength of (Ag(55))(2) dimers with and without chloridation. Based on our calculated energy barriers, Ag(n)Cl(m) clusters are more mobile than Ag(n) clusters for n = 1-4. The binding energy between two Ag(55) clusters is significantly reduced by surface chloridation. Bond weakening and enhanced mobility are two important mechanisms underlying corrosion and fragmentation processes.

Coarsening and pearling instabilities in silver nanofractal aggregates

Abstract We have investigated the deposition of Ag n O x H y clusters onto a graphite surface, maintained at room temperature, as a function of the cluster chemical composition. The incident clusters were produced by a gas-aggregation silver cluster source when adding O 2 and H 2 O molecules in the carrier gas, and monitored by a time of flight mass spectrometer. The prepared samples were imaged by electron microscopy. Experimental results show that cluster diffusion and agglomeration on the surface as well as the resulting island morphology strongly depend on the cluster chemical composition. If the deposition of pure silver clusters leads to nano-fractal islands, a few of oxide/hydroxide molecules incorporated as impurities into the incident clusters prior to their deposition induce a postfractal island fragmentation, keeping constant the fractal dimension. However, the deposition of totally oxidized/hydroxided clusters induces a drastic change in island morphology associated with a decrease of the cluster diffusion on graphite. In this last case a dense nano-phase material with narrow size particle distribution agglomerated in chain-like structure is obtained.