Ni Clusters And The Structure And Properties Of Ni Nanowires

Recently, the study of clusters and nanowires got focus attention because of the increasing interesting on the evolution of materials from atoms to bulk materials, as well as the application potential on a broad territory like magnetic materials, electronic nanodevices and conductance nanowires. The theoretical study is very important in this field because many new phenomena appear, clusters and nanowires are hard to measure directly and the rapid development of the computer and calculation technology.Most theoretical studies of clusters and nanowires are based on numerical simulation but the traditional analytical method. The theoretical studies of clusters and nanowires include the calculations of the structure optimization, thermal properties, electronic properties, magnetic properties, optical properties, conductance properties and so on. The calculation methods include the first principle method, monte carlo method, molecular dynamics method and so on. For all these theoretical studies, the optimization of the structure of clusters and nanowires is an inevitable first step of the calculation.In experiments, the number of atoms in the clusters or nanowires ranges from several to several tens of thousands and even more. However in theoretical studies, the most accurate method, the first principle method, is hardly used in the calculation the clusters and nanowires contain more than several tens atoms. The semi-empirical methods and the methods (such as molecular dynamics method and generalized simulated annealing method) based on empirical potential can conquer these deficiencies.Firstly, we studied the structure, symmetry properties and stability of NiN (N=3~150) cluster by using Quantum Sutton-Chen potential and the steepest descend method. The results of the research indicate that the structures of Ni clusters are varied and disordered. The most stable form include triangle, tetrahedron, hexahedron, octahedron, decahedron, icosahedron, pentagonal bipyramid, close-packed fcc, hcp and disordered structure. In this range, except for the magic clusters Ni13, Ni55, Ni147 have icosahedral symmetry, Ni134 and Ni135 also have icosahedral symmetry, however Ni134 cluster is not a magic cluster. The icosahedral and decahedral structures compete with each other in the range from 64 to 126 for Ni clusters. We also compared our results with the experimental results, most of them are in good agreement with each other.Then, the melting behaviors and thermal properties of NiN clusters (N=29, 50-150) were investigated by using molecular dynamics (MD) simulations with quantum Sutton-Chen (Q-SC) potential. Direct melting, surface melting, glass transition and coexistence of solid and liquid states have been found for different Ni clusters. The relationship between the latent heat of fusion and entropy of fusion, and both of them influence on the melting temperatures of the Ni clusters are discussed, the calculated results show that latent heat of fusion is the dominate effect on the melting temperatures(Tm). Further, in order to know many body effect on the energetics and thermal properties of the clusters, both Sutton-Chen-eff potential and Sutton-Chen potential are used. Decreasing the many-body interaction of the Sutton-Chen potential will decrease the binding energy, increase the melting temperatures, latent heat of fusion, and entropy of fusion of the clusters.Finally, atomic structures and stabilities of Ni nanowires are studied by using the generalized simulated annealing method with Sutton–Chen potential. The initial structure is face-centered cubic [111] structure. The result shows that the length of the supercell strongly affects the structures and stabilities of Ni nanowires. fcc [111] structure, (6,0) , (6,3) nanowires ,fcc[110], transition structure, defect structure and fracture are found for different wire lengths by compression and extension. And from the analyses of the binding energy, it is found that (6,0) nanowires is the most stable form.