Atomistic Simulations For Structural Evolutions And Melting Characteristics Of Metallic Clusters

As atomistically investigated in the present thesis within the modified analytic embedded-atom method through molecular dynamics simulation, metallic nanoclusters exhibit their characteristics of structural stability, magic-number evolution, thermodynamic transition of structures, premelting and melting. The present objective is to fill some blanks in the field of intermediate- or large-scale nanoclusters, verify some experimental evidence, predict possible existence of titanic nanoclusters, and get deep insight into thermodynamics of nanoclusters. The present consequence would play a role in fabrication and applications of nanoclusters.From energetic and thermodynamic point of view, shell-periodic nickel nanoclusters evolve in the face-edge-vertex way which coincides with the experimental magic numbers and from which large icosahedral nanoclusters were constructed. By means of stability function, specific heat, radial contour, radial distribution function, coordination numbers, root-meam-square bond length fluctuation, and structural visualization, structural transition, premelting, and melting behavior of nickel and platinum nanoclusters were studies in detail. The energetically structural stability level of nanoclusters depends strongly on their atomic constituents and dimensions. Preferable configurations defer from size to size. Nanoclusters of calcium and sodium metals have a frequent transitions of structural stability when they are in small size, whereas those of transition metals, nickel and platinum, continue such transition until they are very large.The investigation of small-sized (N = 2-5) calcium and nickel clusters shows that the energetically optimized isomers seem to be independent of the cluster constituents as exemplified in the literature. Predictable premelting for calcium clusters with more than 300 atoms is due to the strong stability of their structures. Based on structurally hybrid nickel clusters, Mackay-type shells are more stable than Bergman-type shells so that the Mackay-type multiple shells could be a barrier against the structural unstability. As for the experimental evidence of magic numbers for nickel clusters, the theoretical verification was given in a detail atom-by-atom growing of clusters from 147-atom icosahedral core to 309-atom and 561-atom shell-closed configurations in an umbrella covering way. The possible size of nanoclusters was predicated to be about 260000 atoms.Through the analysis of the melting process by modified embedded-atom method within molecular dynamics for cuboctahedral and icosahedral nickel clusters, icosahedral clusters keep higher thermal stability than cuboctahedral clusters until the cluster size reaches as large as 923 atoms and the melting temperature is beyond 1380 K. The critical implication suggests that icosahedral clusters could exist at a size of about 3.2 nm in diameter, and cuboctahedral clusters, truncated out of fcc bulk, would be the last candidate for bulk nickel. For small clusters with 309 atoms or fewer, cuboctaheral-to-icosahedral structural transition appears in the melting process prior to solid-to-liquid phase transition. For sodium clusters, the irregular melting behavior takes on the stage when the clusters are small. But the large sodium clusters prefer the bcc structure. Platinum clusters show no preference to truncated decahedral structures which are always displaced by icosahedral isomers, while cuboctaheral isomers oppose the icosahedrals except in small sizes. With no exception, structural transitions of small clusters and surface premelting of larger clusters are inevitable.In addition to the energetics and thermodynamics of nanocluster structures, their geometries are derived in great detail. Being covered in fcc, bcc, hcp, and quasi crystals, icosahedron, truncated decahedron, truncated octahedron, rhombic dodecahedron, and hexagonal polygon are geometrically parametrized. Different translational and rotational periodicity of bulk crystals, nanoclusters are commonly constructed in onion-like shells or shell periodicity. Shell-by-shell construction of nanoclusters releases the standard geometry for coordination number analysis and radial distribution function pattern. Shell-by-shell energetic optimization of nanoclusters is also presented.