Gold Nanotubes, Boron Nanotubes, And Metal-encapsulted Silicon Fullerenes: Ab Initio Predictions Of Configurations, Properties, And Performances

Since the discovery of C60 and carbon nanotubes, these molecules have have attracted wide attention due to their novel electronic properties and great potential of applications. Especilly, the unique structures and novel mechanical, themal, electronic and chemical properties have revealed potential applicantions on field emission, nanoelectronics, sensing probe, superconductivity, and chemistry. Recently, great attentions have addressed on the studies of nanocages and nanotubes.In this project, the density functional theory is employed to investigate the stabilities, properties and performances of the transition metal-encapsulated silicon fullerenes, gold nanotubes, and boron nanotubes, as well as the growth behaviors and electronic properties of Ni-doped and Au-doped silicon clusters.Unlike fullerene cages, a hollow Si cage is unstable because sp2 hybridization is highly unfavorable in silicon. By introducing metal atoms, the silicon clusters can be dramatically stabilized, and the choice of the central metal atom becomes a key point in the design of cage clusters and in their resultant chemical behavior. In this work, we have focused on Si15 and Si16 cage clusters with encapsulated 3d TM atoms and systematically investigated the effects of the encapsulated metal atoms on the configurations, stability, and magnetic properties of the clusters at two different all-electron DFT levels. The results show that Ti@Si16 and Ti@Si15 have the largest embedding energies and relative large HOMO-LUMO gaps in this series. It suggests that titanium atom is an ideal guest for Sin (n=15, 16) cages as far as stability is concerned.Furthermore, a metal-encapsulated silicon fullerene, Eu@Si20, has been predicted by density function theory to be by far the most stable fullerene-like silicon structure. The Eu@Si20 structure is a regular dodecahedron with Ih symmetry in which the europium atom occupies the center site. The calculated results show that the europium atom has a large magnetic moment of nearly 7.0 Bohr magnetons. In addition, it was found that a stable"pearl necklace"nanowire, constructed by concatenating a series of Ih-Eu@Si20 units, each with a central europium atom retains the high spin moment.The configurations, stability, and electronic structure of NiSin (n=1-14) and AuSin (n=1-16) clusters have been investigated within the framework of the density functional theory. The calculated results of NiSin clusters reveal that the Ni atom prefers to occupy the surface site when n < 9 and for the clusters with n≥9, the Ni atom starts to encapsulate in the cage. The results of AuSin clusters show that the Au atom begins to occupy the interior site from Si11 and for Si12 the Au atom completely falls into the interior site forming Au@Si12 cage. Furthermore, the doping of the Ni atom enhances the stability of silicon clusters while the doping of the Au atom does not enhance the stability of pure Sin clusters, similar to the case of Ag-doped silicon clusters. Relatively large embedding energy and small HOMO-LUMO gap are also found for this Au@Si12 structure indicating the enhanced chemical activity and good electronic transfer property. In addition, stable Au-cored Si nanotube is also found to be formed by Au@Si12 unit and it has relatively small energy gap making it attractive for electron transport device.A series of Aun (n=37, 42, 47, 52,…, ?n =5) hollow tubelike structures have been constructed by capping the nanotube with halves of the icosahedral Au32 cage at both ends of the (5,5) single-wall gold nanotube, rolled from the (1 1 1) Au crystal plane, repeated unit cell. Based on the scalar relativistic density functional theory, the stablity and electronic properties of tubelike Aun structures have been investigated. The results indicate that tubelike structures of Au37,Au42,Au47 are relatively stable. At the same time, a large hollow tubelike ground state Au42 is predicted as a new ground-state configuration.The recent prediction of the boron buckyball B80 cage has stimulated intensive studies on boron sheets and boron nanotubes. Here, two new classes of flat stable boron sheets have been constructed. Rolled from them, two series of boron nanotubes are obtained. Within the framework of density functional theory, the configurations, stability and electronic structures of new classes of boron sheet and related boron nanotubes have been predicted. The theoretic results show that within the scope of our research, the nanotubes rolled from the stable sheets with various diameters and chiral vectors are stable, and they are all metallic except for the thin (4, 0) tube with a band gap of 0.441 eV.Boron is known to be an important additive to magnetic materials. It can affect the magnetic properties substantially and improve the magnetostrictive characteristics of transition-metal thin films. Within the framework of all-electron density functional theory, the configurations, electronic structure and magnetic properties of MnB (M = Fe, Co, Ni; n=1-12, 14, 18) clusters have been calculated. The ground-state structures of the MnB clusters were determined. The results reveal that the doping of a B atom enhances the stabilities significantly and has a slight effect on the spin magnetic moments at the scope of our study. In addition, the size dependence of the spin moments in NinB clusters exists one terrace from n = 5 to n = 13, which is mostly due to the quantum confinement.