C. Mottet

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...

Alloy surfaces: segregation, reconstruction and phase transitions

Abstract Surface segregation in alloys, i.e., concentration modulation in the surface selvedge at thermodynamical equilibrium, can be viewed as resulting from two kinds of competition or synergy. The first one is between surface and bulk interactions, i.e., surface energy versus bulk alloying interactions, whereas the second one is between these chemical forces and the atomic size-mismatch. Modelling the phenomenon then requires to account for all these forces on the same level. This leads to use both realistic energetic models derived from electronic structure and efficient statistical tools, from mean–field approximation to Monte Carlo simulations. A particular attention must be payed to the possible multisolution character of the problem. Actually, the possible coexistence of stable and metastable solutions may be at the origin of superficial phase transitions, in which the surface acts as a precursor of bulk order–disorder transitions: layering transitions, concentration profile transitions, surface induced order or disorder. All these phenomena are illustrated here on some specific systems and analyzed with tools derived from tight-binding formalism. Finally, the strong coupling between surface segregation and atomic reconstructions is illustrated in the case of strong size-mismatch by means of molecular dynamics calculations.

Structure and chemical ordering in CoPt nanoalloys

The structure and chemical ordering of CoPt nanoclusters in the size range of 1 to 3 nm are investigated by global optimization methods and Monte Carlo simulations using a many body potential derived from the tight binding model. For the smaller systems (number of atoms N < 100), the optimized clusters display a polyicosahedral-like atomic structure with a little core-shell chemical ordering characterized by a particular surface chemical configuration: some pentagonal or hexagonal Pt rings centered, respectively on a Co atom or a Co dimer. A transition to the decahedral symmetry occurs at about N = 100 atoms, with a pseudo L1(0) ordered phase in each tetrahedral unit. For larger cluster sizes, 201 < N < 1289, the L1(0)-ordered/disordered transition on the face centered cubic truncated octahedron is studied by canonical Monte Carlo simulations showing that the critical disordering temperature decreases with the cluster size. We also notice a Co surface segregation especially at edges and, possibly, (100) facets, depending on the cluster size, on both cubic and fivefold symmetry structures.

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.

Growth and structural properties of CuAg and CoPt bimetallic nanoparticles

Core/shell CuAg and alloyed CoPt have been synthesized using two vapor phase deposition techniques. For CuAg prepared by Thermal Evaporation (TE), the size and the morphology of the Cu cores are the key parameters to promote the formation of the core/shell arrangement. For CoPt synthesized by Pulsed Laser Deposition (PLD), the growth kinetics of nanoparticles, depending on the deposition rate, the substrate nature and the temperature, controls the nanoparticle morphology. The competition between the growth and the ordering kinetics governs the nanoparticle structure. By reducing the growth kinetics, as-grown L1(0) ordered nanoparticles are obtained according to the bulk phase diagram.

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.

New magic numbers in metallic clusters: an unexpected metal dependence

Using a many-body tight-binding potential within the second moment approximation in a quenched molecular dynamics simulation, we calculate the internal energy of free Cu, Ag and Au clusters of various sizes and morphologies. We find that the icosahedral structure, which is the equilibrium shape for small sizes at least for Cu and Ag, adopts a very inhomogeneous atomic relaxation. More surprisingly, introducing a vacancy at the center lowers the mean energy per atom for sufficiently large size icosahedra. This means that above a critical size, which decreases from Cu to Au, the icosahedron admits a constitutional vacancy. Taking into account the stability domain of the icosahedron relative to the fcc structure (namely the Wulff polyhedron), we find that there is a stability range of size for Cu and Ag icosahedra with a central vacancy, but not for Au icosahedra. This trend along the noble metal column is discussed in view of tight-binding potential parameters.

Structures of metal nanoparticles adsorbed on MgO"001…. II. Pt and Pd

The structure of metal clusters supported on a MgO(001) substrate is investigated by a computational approach, with the aim to locate stable structural motifs and possible transition sizes between different epitaxies. Metal-metal interactions are modeled by a second-moment approximation tight-binding potential, while metal-oxide interactions are modeled by an analytic function fitted to first-principles calculations. Global optimization techniques are used to search for the most stable structural motifs at small sizes (N < or = 200), while at larger sizes different structural motifs are compared at geometric magic numbers for clusters up to several thousand atoms. Metals studied are Ag, Au, Pd, and Pt. They are grouped according to their mismatch to the oxide substrate (lattice constant of the metal versus oxygen-oxygen distance on the surface). Ag and Au, which have a smaller mismatch with MgO, are studied in Paper I, while Pd and Pt, with a larger mismatch, are investigated in Paper II. For Ag the cube-on-cube (001) epitaxy is favored in the whole size range studied, while for Au a transition from the (001) to the (111) epitaxy is located at N=1200. The reliability of the model is discussed in the light of the available experimental data.

A Monte Carlo simulation of submonolayer homoepitaxial growth on Ag(110) and Cu(110)

Abstract The low-coverage (θ≤0.05) growth of Ag/Ag(110) and Cu/Cu(110) is studied by kinetic Monte Carlo simulations. Our model includes deposition, diffusion and fully reversible aggregation on a 2D rectangular lattice with both anisotropic diffusion barriers and anisotropic attachment. The barriers for the diffusion processes are calculated by many-body RGL potentials, and compared to the available data with good agreement. Depending on T, in both metals the model shows morphology transitions to 1D in-channel strips and then to 2D compact islands. The latter transition is due to the activation of in-channel detachment followed by terrace-mediated reshaping and not to direct corner rounding. Even if the transition takes place in the same temperature range for both metals, the 1D strips obtained in Cu are much longer. At the transition, a break in the slope of the island density versus 1/T is obtained, due to the onset of ripening during growth. There, the island density presents a maximum at very low coverages, followed by a decrease due to island dissolution and not to coalescence. The possibility of extracting the diffusion barriers by measuring the island density is discussed, finding that it is possible to extract a reliable value for the in-channel diffusion barrier in Ag, where the diffusion anisotropy is stronger. In Cu, an average between the in-channel and the cross-channel barriers is extracted.

Electronic structure of Pd clusters in the tight-binding approximation : influence of spd-hybridization

We show that including spd-hybridization in tight-binding model allows an accurate description of the local electronic density of states of Pd clusters in a large range of size and gives reliable site energies. We put in evidence a narrowing of the band with decreasing coordination number accompanied by a significant variation of the local density of states near the Fermi level according to the type of site (vertex, edge, facets or core). From the energetical point of view, we find that a quasi-linear dependence of the attractive part of the potential as a function of the coordination number is better suited than the usual square root dependence. Moreover, we point out the influence of cluster symmetry by comparing the cuboctahedral structure (fcc type) with the icosahedral one (five-fold symmetry) which is adopted by the small fcc clusters.