Francesca Baletto

Crossover among structural motifs in transition and noble-metal clusters

The energetics of nanoclusters is investigated for five different metals (Ag, Cu, Au, Pd, and Pt) by means of quenched molecular dynamics simulations. Results are obtained for two different semiempirical potentials. Three different structural motifs are considered: icosahedra (Ih), decahedra (Dh), and truncated octahedra (TO). The crossover sizes among structural motifs are directly calculated, considering cluster up to sizes N≃40 000. For all the systems considered, it is found that icosahedra are favored at small sizes, decahedra at intermediate sizes, and truncated octahedra at large sizes. However, the crossover sizes depend strongly on the metal: in Cu, the icosahedral interval is rather large, and it is followed by a very wide decahedral window; on the contrary, in Au, the icosahedral interval is practically absent, and the decahedral window is narrow. The other metals display intermediate behaviors, Ag being close to Cu, and Pd and Pt being close to Au. A simple criterion, which is based on the rat...

Modeling free and supported metallic nanoclusters: structure and dynamics

We compare atomic structure and dynamics of free and supported metallic clusters via molecular dynamics simulations using tight-binding semiempirical potentials for metal–metal interactions and a potential fitted to ab initio calculations for the metal-supported ones, the support being essentially the MgO(100) surface in the case of a nonreactive metal–oxide interface. The structural transition for free Ni, Pd, Pt, Cu, Ag, Au clusters with noncrystalline structures (mainly icosahedral and decahedral) at small sizes to FCC truncated octahedrons for larger sizes is reported as well as the variation of the critical size of transition from 3d to 5d metals. In the case of Pd clusters on the MgO(100) surface, we analyze the substrate-induced modifications in morphology and atomic structure and follow their evolution as a function of cluster size. The mechanism of strain release by misfit interfacial dislocations in 3D clusters is described at the atomic level. Dynamics of growth and melting of free silver clusters are discussed and some effects of the oxide substrate in melting transition are pointed out, notably the delay in melting induced by the epitaxial relation with the support.

Molecular dynamics simulations of surface diffusion and growth on silver and gold clusters

Abstract We report a systematic study of the diffusion of single adatoms and of the growth of fcc silver and gold clusters (Wulff polyhedra) by molecular dynamics simulations. Both metals have been modelled by many-body tight-binding potentials. The energy barriers for adatom diffusion on the cluster facets are calculated by the nudged elastic band method. Concerning single-adatom diffusion, we have studied the mechanisms connecting different facets [(111) and (100)] that play the main role during growth. We have found that the diffusion among different facets takes place by exchange processes in both metals; however, we have found that the mobility from a (111) to a (100) facet is much easier in gold than in silver. This has important consequences for the growth modes of the clusters. In fact, our growth simulations have shown that the transition from the Wulff shape to the octahedron is possible with gold even at low temperatures and quite fast deposition rates, whereas for silver much higher temperatures are required.

Freezing of silver nanodroplets

Abstract The freezing of silver nanodroplets is studied by molecular dynamics simulations on time scales which mimic typical experimental situations. At small sizes (2–3 nm) both crystalline and non-crystalline structures (icosahedra and decahedra) are observed; while at large sizes, also around magic icosahedral numbers, we show that silver droplets solidify preferentially as non-icosahedral clusters. We compare two different ways of growing clusters: the solid-state growth from a very small seed and the freezing of a droplet. We find that the final shape of clusters depends on the growth process, and that the solid-state growth gives results in better agreement with the experiments.

Oxygen adsorption on small PtNi nanoalloys

The adsorption of an oxygen molecule on nanoclusters of Pt and PtNi, in the size range between 13 and 55 atoms, has been studied using first-principle simulations. The structures have been obtained as a function of size and chemical composition of the clusters by means of the parallel excitable-walkers basin hopping method. O(2) preferentially adsorbs along the edge between two (111) facets due to a massive distortion of the Pt-Pt bond length. This bond elongation favours the adsorption in such a way that the binding energy of oxygen on a pure 55-atom cluster is still twice the value on the clean Pt(111). On the other hand, on 55-Pt(shell)Ni(core) nanoparticles, the O(2) binding energy is slightly lower than on Pt(111), because nickel core inhibits the stretching of the Pt-bond because of their size mismatch. However, as soon as its concentration is increased, Ni appears at the surface and its oxyphilic nature contributes to bind the oxygen molecule stronger.

Far-infrared absorption of water clusters by first-principles molecular dynamics.

Based on first-principle molecular dynamic simulations, we calculate the far-infrared spectra of small water clusters (H(2)O)(n) (n = 2, 4, 6) at frequencies below 1000 cm(-1) and at 80 K and at atmospheric temperature (T>200 K). We find that cluster size and temperature affect the spectra significantly. The effect of the cluster size is similar to the one reported for confined water. Temperature changes not only the shape of the spectra but also the total strength of the absorption, a consequence of the complete anharmonic nature of the classical dynamics at high temperature. In particular, we find that in the frequency region up to 320 cm(-1), the absorption strength per molecule of the water dimer at 220 K is significantly larger than that of bulk liquid water, while tetramer and hexamer show bulklike strengths. However, the absorption strength of the dimer throughout the far-infrared region is too small to explain the measured vapor absorption continuum, which must therefore be dominated by other mechanisms.

Geometrical Effects on the Magnetic Properties of Nanoparticles

Elucidating the connection between shape and properties is a challenging but essential task for a rational design of nanoparticles at the atomic level. As a paradigmatic example we investigate how geometry can influence the magnetic properties of nanoparticles, focusing in particular on platinum clusters of 1-2 nm in size. Through first-principle calculations, we have found that the total magnetization depends strongly on the local atomic arrangements. This is due to a contraction of the nearest neighbor distance together with an elongation of the second nearest neighbor distance, resulting in an interatomic partial charge transfer from the atoms lying on the subsurface layer (donors) toward the vertexes (acceptors).

Metallic nanoparticles meet metadynamics

Metadynamics coupled with classical molecular dynamics has been successfully applied to sample the configuration space of metallic and bimetallic nanoclusters. We implement a new set of collective variables related to the pair distance distribution function of the nanoparticle to achieve an exhaustive isomer sampling. As paradigmatic examples, we apply our methodology to Ag147, Pt147, and their alloy Ag(shell)Pt(core) at 2:1 and 1:1 chemical compositions. The proposed scheme is able to reproduce the known solid-solid structural transformation pathways, based on the Lipscombs diamond-square-diamond mechanisms, both in mono and bimetallic nanoparticles. A discussion of the free energy barriers involved in these processes is provided.

CCl4 dissociation on the ice Ih surface: an excess electron mediated process

Dissociation of chlorofluorocarbons in the atmosphere is a heterogeneous process that takes place mainly on the surface of ice particles. Recently an enhancement of the dissociation rate due to excess electrons has been shown theoretically and correspondingly measured experimentally. Our density functional theory calculations show that CCl(4) dissociates due to an excess electron with an energy gain of 0.8 eV on the ice surface as opposed to in the gas phase. Through the use of ab initio molecular dynamics, an atomistic pathway for this dissociation has been elucidated, this pathway shows the capture of Cl(-) by the ice surface through a partial solvation mechanism, in agreement with recent experimental findings.

Doped golden fullerene cages

A first-principles investigation of the effect of the doping of golden cages of 32 atoms is proposed. It is shown that Ag and Cu doping affects the geometrical stability of the icosahedral fullerene Au32 cage, where Ag-doping leads to a new, low symmetric, and prolate motif while Cu-doping leads to a lump, incomplete decahedral shape. Most significantly, the HOMO-LUMO gap depends strongly on the cluster geometry while its dependence on the cluster chemical composition seems to be weaker.