Chakram S. Jayanthi

Reversible electromechanical characteristics of carbon nanotubes underlocal-probe manipulation

The effects of mechanical deformation on the electrical properties of carbonnanotubes are of interest given the practical potential of nanotubes in electromechanicaldevices, and they have been studied using both theoreticaland experimental approaches. One recent experiment used the tip of an atomic force microscope (AFM) to manipulate multi-wallednanotubes, revealing that changes in the sample resistance were small unlessthe nanotubes fractured or the metal–tube contacts were perturbed. Butit remains unclear how mechanical deformation affects the intrinsic electricalproperties of nanotubes. Here we report an experimental and theoretical elucidationof the electromechanical characteristics of individual single-walled carbonnanotubes (SWNTs) under local-probe manipulation. We use AFM tips to deflectsuspended SWNTs reversibly, without changing the contact resistance; insitu electrical measurements reveal that the conductance of an SWNT samplecan be reduced by two orders of magnitude when deformed by an AFM tip. Ourtight-binding simulations indicate that this effect is owing to the formationof local sp3 bonds caused by the mechanical pushingaction of the tip.

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.

Adsorption of oxygen molecules on individual single-wall carbon nanotubes

Our study of the adsorption of oxygen molecules on individual semiconductiong single-walled carbon nanotubes at ambient conditions reveals that the adsorption is physisorption, the resistance without O2 increases by approximately two orders of magnitude as compared to that with O2, and the sensitive response is due to the pinning of the Fermi level near the top of the valence band of the tube, resulting from impurity states of O2 appearing above the valence band.

Electrostatic deposition of graphene in a gaseous environment: a deterministic route for synthesizing rolled graphenes?

The synthesis of single-wall carbon nanotubes of desired diameters and chiralities is critical to the design of nanoscale electronic devices with desired properties. The existing methods are based on self-assembly, therefore lacking control over the diameters and chiralities. The present work reports a direct route for rolling graphene. Specifically, we found that the electrostatic deposition of graphene yielded: (i) flat graphene layers under high vacuum (10(-7) Torr), (ii) completely scrolled graphene under hydrogen atmosphere, (iii) partially scrolled graphene under nitrogen atmosphere, and (iv) no scrolling for helium atmospheres. Our study shows that the application of the electrostatic field facilitates the rolling of graphene sheets exposed to appropriate gases and allows the rolling of any size of graphene. The technique proposed here, in conjunction with a technique that produces graphene nanoribbons of uniform widths, will have significant impact on the development of carbon nanotube based devices. Furthermore, the present technique may be applied to obtain tubes/scrolls of other layered materials.

Factors responsible for the stability and the existence of a clean energy gap of a silicon nanocluster

We present a critical theoretical study of electronic properties of silicon nanoclusters, in particular the roles played by symmetry, relaxation, and hydrogen passivation on the stability, the gap states and the energy gap of the system using the order N [O(N)] nonorthogonal tight-binding molecular dynamics and the local analysis of electronic structure. We find that for an unrelaxed cluster with its atoms occupying the regular tetrahedral network, the presence of undistorted local bonding configuration is sufficient for the appearance of a small clean energy gap. However, the energy gap of the unrelaxed cluster does not start at the highest occupied molecular orbital (HOMO). In fact, between the HOMO and the lower edge of the energy gap, localized dangling bond states are found. With hydrogen passivation, the localized dangling bond states are eliminated, resulting in a wider and clean energy gap. Relaxation of these hydrogen passivated clusters does not alter either the structure or the energy gap appre...

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

Broken symmetry, boundary conditions, and band-gap oscillations in finite single-wall carbon nanotubes

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

MULTILAYER RELAXATION AND MELTING OF A METAL-SURFACE

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