86,617 research outputs found
Structural properties of sub-nanometer metallic clusters
No full textAt the nanoscale, the investigation of structural features becomes fundamental as we can establish relationships between cluster geometries and their physicochemical properties. The peculiarity lies in the variety of shapes often unusual and far from any geometrical and crystallographic intuition clusters can assume. In this respect, we should treat and consider nanoparticles as a new form of matter. Nanoparticle structures depend on their size, chemical composition, ordering, as well as external conditions e.g. synthesis method, pressure, temperature, support. On top of that, at finite temperatures nanoparticles can fluctuate among different structures, opening new and exciting horizons for the design of optimal nanoparticles for advanced applications. This article aims to overview geometrical features of transition metal clusters and of their various rearrangements
Structural properties of nanoclusters: Energetic, thermodynamic, and kinetic effects
Full text linkThe structural properties of free nanoclusters are reviewed. Special attention is paid to the interplay of energetic, thermodynamic, and kinetic factors in the explanation of cluster structures that are actually observed in experiments. The review starts with a brief summary of the experimental methods for the production of free nanoclusters and then considers theoretical and simulation issues, always discussed in close connection with the experimental results. The energetic properties are treated first, along with methods for modeling elementary constituent interactions and for global optimization on the cluster potential-energy surface. After that, a section on cluster thermodynamics follows. The discussion includes the analysis of solid-solid structural transitions and of melting, with its size dependence. The last section is devoted to the growth kinetics of free nanoclusters and treats the growth of isolated clusters and their coalescence. Several specific systems are analyzed
Tailoring the structural motif of AgCo nanoalloys: Core/shell versus janus-like
No full textThis paper deals with the computational study of the growth of small silver-cobalt clusters, modeled by a semiempirical potential. Both atom-by-atom growth molecular dynamics and simulations of freezing are used to identify formation patterns of silver-cobalt nanoalloys. Different pathways leading to core/shell and bicompartmentalized configurations, reminiscent of Janus geometry, are observed in cobalt-rich and in silverrich alloys, respectively. The physical origin of the formation of asymmetric structures and their evolution toward core/shell motifs, as well as the opposite process, are discussed
Doped golden fullerene cages
Full text linkA 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
Island adsorption and adatom diffusion on 3D non-crystalline silver nanoclusters
No full textWe report a systematic study of island adsorption and single-adatom diffusion on free silver nanoclusters, and discuss the consequences on the growth. In our calculations, silver is modelled by semiempirical many-body potentials. We consider magic non-crystallographic structures at different sizes: icosahedra (Ih) at 55, 147 and 309 atoms; Marks-truncated decahedra (m-Dh) at 75, 146 and 192 atoms. We calculate the map of adsorption sites and the energy barriers for the different diffusion processes. We find that, due to purely geometrical reasons, medium-size (from 6-8 to 30-40 atoms depending on the cluster) islands on the cluster (1 1 1) facets prefer the hcp stacking on both Ih and Dh structures, while both smaller and larger islands are better placed on fcc stacking. Interfacet diffusion is easy on both Dh and Ih, indicating that large islands are easily grown; in particular, there are multi-atom diffusion processes which allow fast diffusion among the two caps of Dh clusters. For Dh clusters, island s on hcp stacking may lead to the appearance of new fivefold symmetry points, and to the transformation of the cluster into an icosahedron
Machine-learnt potential highlights melting and freezing of aluminum nanoparticles
Full text linkWe investigated the complete thermodynamic cycle of aluminum nanoparticles through classical molecular dynamics simulations, spanning a wide size range from 200 atoms to 11 000 atoms. The aluminum-aluminum interactions are modeled using a newly developed Bayesian Force Field (BFF) from the FLARE suite, a cutting-edge tool in our field. We discuss the database requirements to include melted nanodroplets to avoid unphysical behavior at the phase transition. Our study provides a comprehensive understanding of structural stability up to sizes as large as 3 x 10(5) atoms. The developed Al-BFF predicts an icosahedral stability range up to 2000 atoms, similar to 2 nm, followed by a region of stability for decahedra, up to 25 000 atoms. Beyond this size, the expected structure favors face-centered cubic shapes. At a fixed heating/cooling rate of 100 K/ns, we consistently observe a hysteresis loop, where the melting temperatures are higher than those associated with solidification. The annealing of a liquid droplet further stabilizes icosahedral structures, extending their stability range to 5000 atoms. Using a hierarchical k-means clustering, we find no evidence of surface melting but observe some mild indication of surface freezing. In any event, the liquid droplet's surface shows local structural order at all sizes
The effect of chemical ordering and lattice mismatch on structural transitions in phase segregating nanoalloys
Full text linkWe elucidate the effect of lattice mismatch and chemical ordering on structural transitions in bimetallic nanoalloys.</p
Diffusion of one-dimensional clusters on Au and Pt(110) (1×2)
No full textThe diffusion of one-dimensional clusters on Au and Pt(110) (1×2) is studied by molecular dynamics simulations. Both metals are modeled by many-body tight-binding potentials. The energy barriers for the elementary diffusion mechanism (concerted jump, leapfrog, dissociation, etc.) are calculated, for chains of up to six atoms, by the nudged elastic band method. The consistency of the results on energy barriers is checked by high-temperature simulations
Chemical order and magnetic properties in small M
No full textA systematic analysis of the chemical order, structure stability and magnetic behaviour
of small transition metal binary nanoalloys is performed employing spin-polarised
ab-initio simulations. The doping of icosahedral geometries at 13 and 19 atoms of magnetic
materials with two impurities both magnetic (Fe, Co, Ni, Pt) and non-magnetic (Ag, Cu) is
considered. In CoFe, the most favourable substitutional sites are those which maximise the
total magnetic moment of the system: Fe dopants tend to occupy surface sites while Co
atoms stay in the inner. For all the other nanoalloys, the doping sites respect a chemical
order that leads to a surface energy minimization often followed by a depression of the
total magnetization. The ferromagnetic arrangement is always the energetically most
favourable order apart from the Ag-doped case where the anti-ferromagnetic alignment is
almost degenerate to the ferromagnetic phase
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