Atomic Scale Simulation Of The Thermal Stability Of Isolated And Confined Copper Clusters

As a bridge between individual atoms and bulk materials, nanoclusters containing tens to thousands of atoms or molecular attracted great attention duo to their unique physicsal and chemical properties, as well as their possible applications in nano-catalysts, surface nanostructure, new materials and nanoscale electronic devices. It has been a particularly popular subject to investigate the structure, properties, thermal stability and structural evolution of clusters. But it has been difficult to study the detailed structural evolution and melting and freezing mechanism experimentally because of widen nanoclusters region and more momentary melting and crystallization process. Computer simulations based on (semi)empirical potentials provide very important tool to investigate the structural evolution involving in many interesting dynamical phenomena in clusters at the atomic scale.In this work, Molecular dynamics (MD) simulations in conjunction with the embedded-atom method (EAM) potential models have been used to study the melting and freezing behavior, and detailed structural evolution of isolated and confined copper clusters at the atomic level. In those simulations, we divide the clusters into different shells determined by atom dendisy profile calculations, and calculate the average energy of the atoms, the radial density distribution function(RDDF), the pair distribution function (PDF), mean square displacement (MSD),and pair analysis indices (HA) of the cluster.First, the melting behavior and detailed structural evolution of isolated FCC Cun (n=531,603,683) and truncated icosahedral Cu135, Cu297 and Cu549 clusters upon heating have been studied. The simulation results show that the melting behaviors of those clusters present quite different patterns. The FCC Cun (n=531,603,683) clusters have similar melting behavior, the melting process can be described as initially pre-melting of the surface and then the overall melting of the whole clusters on heating. The drastic changes of microstructure of clusters occurred at two temperature ranges of 700K-800K and 1000K-1100K. The melting behaviors of truncated icosahedral Cu135, Cu297 and Cu549 show similar to the bulk, there is a temperature of energy drastic change, especially the cluster Cu135 have definitive melting point (791K).Second, the structural changes of the molten Cu55 andCu135 during slowly freezing, as well as the structural evolution of a molten Cu555 cluster during freezing at two different cooling processes have been investigated. The influence of cooling ways and sizes of clusters on final structures and the structural evolution has been analysed also. The simulation results show that the structural evolution of molten Cu55 and Cu135 cluster during slowly freezing is different from big size clusters, which is a process of the formation of the ordered structures from inner shells to outer shells. The structural change of a molten Cu55 cluster involves three stages owing to continuously interchanging positions among atoms. In the first stage, a three-shell structure was formed in this cluster. Then, atoms in this cluster distribute in four-shell, and meanwhile 13 atoms in the inner part form an icosahedron. Finally, the icosahedral structure with 55 atoms was constructed. The structural changes of a molten Cu135 cluster present more steps:one atom moved into the center at the initial stage first; then the packing in interior atoms was changed into an ordered icosahedron structure, while outer atoms are becoming locally ordered with a fivefold symmetry; subsequently, nanocrystallization at lower temperatures propagated outward from the interior icosahedron structure, leading to form a icosahedron-like cluster. The final structures and local structural-change processes of the molten Cu555 cluster have different patterns in the two cooling ways. In the quenched way, the final geometrial configuration was based on an icosahedron. The formation of the ordered structures started at the innermost and outermost shells and, in the following time, many atoms moved from outermost shell into inner shells leading to a dramatic increase of the number of atoms in the inner shells. Finally, the icosahedral-like structure was formed. In the cooling way, the final crystallization structure is the FCC structure. The crystallization starts at the innermost, and then proceeds smoothly throughout the whole cluster. The cooling ways have important effects on the final structures and structural change of melten Cu555 cluster as well as structural change processes of Cu55 andCu135 clusters, but nothing to do with the final structures of melten Cu55 andCu135 clusters.Third, the structual changes of molten Cu55 clusters embedded in three restriction systems at different quenched temperatures, as well as the influence of restriction systems, quenched temperatures and confined extent on the final structures and local structural-change of molten CU55 clusters have been studied. The simulated results show that final structure of the CU55 cluster embedded in FCC bulk mainly presents local faced center cubic (FCC) structure at the quenched temperatures. During solidification, the atoms continuously interchange their positions, and the rearrangement of atom positions is sensitive to the temperature change. The final structures of the confined molten Cu55 clusters in Y direction have a direct bearing on the quenched temperatures and confined extent. For molten Cu55 clusters embedded in a furrow, The final structures have nothing to do with the width and depth of the furrow as well as the position of CU55 clusters in the furrows, but the width and depth of the furrows as well as position of Cu55 clusters in the furrows produce a direct impact on detailed structural evolution processes and distribution of atoms in furrows at final phases.