Shi-Yu Wu

Structural and electronic properties of a carbon nanotorus: Effects of delocalized and localized deformations

The bending of a carbon nanotube is studied by considering the structural evolution of a carbon nanotorus from elastic deformation to the onset of the kinks and eventually to the collapse of the walls of the nanotorus. The changes in the electronic properties due to delocalized deformations are contrasted with those due to localized deformations to bring out the subtle issue underlying the reason why there is only a relatively small reduction in the electrical conductance in the former case even at large bending angles, while there is a dramatic reduction in the conductance in the latter case at relatively small bending angles.

Geometric and electronic structures of graphitic-like and tubular silicon carbides: Ab-initio studies

The structural optimization and energetics studies of SiC graphitic-like structures have been investigated theoretically in the context of formations of stable graphitic-like layer structures, single- and multi-walled nanotubes using the DFT-based Vienna ab-inito simulation package. The bonding nature of atoms in the optimized structures has been examined using a local analysis technique based on a self-consistent and environment-dependent semi-empirical Hamiltonian. Results of our studies reveal that stabilized SiC graphitic-like layer structures possess the sp2 bonding nature, different from the sp3 bonding nature in bulk SiC. Such flexibility in bonding configurations between Si and C atoms holds the possibility for a wide range of stable SiC-based structures, similar to those for carbon-based structures. In the case of SiC-based nanotubes, we have calculated quantities such as the strain energy, the degree of buckle in the cylindrical shell, and bond charges between Si and C atoms, to obtain an understanding of the optimized structures. The optimized interlayer spacing of SiC graphitic-like multilayer sheets has been found to depend on the ordering of atoms in different layers of the SiC graphitic-like structure (0.37 nm for the Si-C sequence of bilayer arrangement versus 0.48 nm for either the Si-Si or the C-C sequence of bilayer arrangement). These observations may be attributed to the Coulomb interactions due to the charge redistribution among Si and C atoms. On the other hand, the existence of two different ranges of interlayer separation in SiC double-walled nanotubes (0.38 nm for zigzag and 0.48 nm for armchair) is found to be related to whether the dominant interlayer neighbors are of the Si-C type or the Si-Si and C-C types.

Energetics, relative stabilities, and size-dependent properties of nanosized carbon clusters of different families: Fullerenes, bucky-diamond, icosahedral, and bulk-truncated structures

Structures and relative stabilities of carbon clusters belonging to different families have been investigated for diameters d < or = 5 nm based on an efficient semiempirical molecular dynamics (MD) scheme as well as a density functional theory based simulation. Carbon clusters studied include fullerenes and fullerene-derived structures (e.g., cages and onions), icosahedral structures, bucky-diamond structures, and clusters cut from the bulk diamond with spherical and facetted truncations. The reason for using a semiempirical MD is partly due to the large number of different cases (or carbon allotropes) investigated and partly due to the size of the clusters investigated in this work. The particular flavor of the semiempirical MD scheme is based on a self-consistent and environment-dependent Hamiltonian developed in the framework of linear combination of atomic orbitals. We find that (i) among the families of carbon clusters investigated, fullerene structures have the lowest energy with the relative energy ordering being E(fullerene) < E(onion) < E(icosahedral) < E(bucky-diamond) < E(bulk-truncated), (ii) a crossover between bucky-diamond and icosahedral structures is likely at d approximately 8 nm, (iii) the highest occupied molecular orbital-lowest unoccupied molecular orbital gap as a function of the diameter for the case of fullerenes shows an oscillatory behavior with the gap ranging from 2 eV to 6 meV, and the gap approaching that of gapless graphite for d > 3.5 nm, and (iv) there can be three types of phase transformations depending on the manner of heating and cooling in our simulated annealing studies: (a) a bucky-diamond structure --> an onionlike structure, (b) an onionlike --> a cage structure, and (c) a bucky-diamond --> a cage structure.

Strain relaxation mechanisms and local structural changes in Si 1 − x Ge x alloys

In this work, we address issues pertinent to the understanding of the structural and electronic properties of

Low-dimensional boron structures based on icosahedron B12

{\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}

Next generation of the self-consistent and environment-dependent Hamiltonian: Applications to various boron allotropes from zero- to three-dimensional structures

alloys, namely, (i) how does the lattice constant mismatch between bulk Si and bulk Ge manifest itself in the alloy system? and (ii) what are the relevant strain release mechanisms? To provide answers to these questions, we have carried out an in-depth study of the changes in the local geometric and electronic structures arising from the strain relaxation in

Initial stage of growth of single-walled carbon nanotubes: modeling and simulations

{\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}

Towards a Coherent Treatment of the Self-Consistency and the Environment-Dependency in a Semi-Empirical Hamiltonian for Materials Simulation

alloys. We first compute the optimized lattice constant for different compositions (x) by fully relaxing the system and by minimizing the total energy with respect to the lattice constant at each composition, using an ab initio molecular dynamics scheme. The optimized lattice constant, while exhibiting a general trend of linear dependence on the composition (Vegards law), shows a negative deviation from Vegards law in the vicinity of

Colossal Paramagnetic Moments in Metallic Carbon Nanotori

x=0.5.

Real space Green's function approach to vibrational dynamics of a Vicsek fractal.

We delineate the mechanisms responsible for each one of the above features. We show that the radial-strain relaxation through bond stretching is responsible for the overall trend of linear dependence of the lattice constant on the composition. On the other hand, the negative deviation from Vegards law is shown to arise from the angular-strain relaxation. More specifically, the combined effect of the local bond-angle deviations from the tetrahedral angle and the magnitudes of the corresponding peaks for the partial-angle distribution function determines the negative deviation from Vegards law. The electronic origin of the changes in the local geometric structure due to strain relaxation is also presented in this work. In particular, the correlation between the bond charges and the bond-lengths for Si-Si, Ge-Ge, and Si-Ge pairs in