Ihsan Boustani

New boron based nanostructured materials

Based on a series of ab initio studies we have pointed out the remarkable structural stability of nanotubular and quasiplanar boron clusters, and postulated the existence of novel layered, tubular, and quasicrystalline boron solids built from elemental subunits. The present study illustrates and predicts qualitative structural and electronic properties for various models of nanotubular and layered boron solids, and compares them to well-known tubular and layered forms of pure carbon and mixed boron compounds.

Ab initio study of B32 clusters: competition between spherical, quasiplanar and tubular isomers

Abstract Using ab initio quantum-chemical methods, different novel structures of B 32 clusters have been investigated. The most stable isomers have quasiplanar or tubular structures often containing dove-tailed hexagonal pyramids. In contrast, hollow spheres are less stable. The stability can be understood as a competition between a curvature strain (favoring quasiplanar clusters) and elimination of dangling bonds (favoring tubular and cage structures). Atomic coordination is larger than in carbon clusters.

STRUCTURE AND STABILITY OF SMALL BORON CLUSTERS. A DENSITY FUNCTIONAL THEORETICAL STUDY

Abstract The structure and stability of small boron clusters Bn (n = 2–14) have been investigated employing density functional theory. The search for minima was performed using gradient methods at the local spin density level. Most of the final structures prefer planar or quasi-planar forms and can be considered to be fragments of a planar surface or segments of the surface of a sphere. From the curvature and the size of the convex clusters, we predict that this spherical cluster should consist of 90 boron atoms, and we propose that there may exist a group of elemental spherical boron clusters.

A comparative study of ab initio SCF-CI and DFT. Example of small boron clusters

Abstract A linear search for minima on potential energy surfaces based on analytical gradient methods and the determination of binding energies of small boron clusters B n ( n = 2–14) have been made using the ab initio Hartree-Fock and configuration interaction quantum chemical methods as well as by means of density functional theory at the local spin density and non-local corrections to the exchange-correlation levels of theory. The final Hartree-Fock optimized topologies of the neutral boron clusters are identical with those derived with the local spin density approximation. The most stable boron clusters have convex or quasi-planar structures. The convex clusters seem to be segments of the surface of a sphere. The capabilities of both methods for such cluster systems are illustrated.

Tight binding molecular dynamics studies of boron assisted nanotube growth

In this paper we report a theoretical study of the effects of the presence of boron in growing carbon nanotubes. We employ a well established tight binding model to describe the interactions responsible for the energetics of these systems, combined with the molecular dynamics simulation technique and structural relaxation calculations. We find, in agreement with the previous theoretical/experimental work of Blase et al. [Phys. Rev. Lett. 83, 5078 (1999)], that boron favors (n,0) (zig-zag) tubular structures over (n,n) (arm-chair) ones by stabilizing the zig-zag edge. Furthermore, it is shown that boron has the effect of delaying the tube closure process, a fact which could explain the improved aspect ratio experimentally observed in nanotubes synthesized in the presence of boron. Our dynamical simulations lead us to propose a mechanism through which this extension of the closure time can be explained.

An approach to control the radius and the chirality of nanotubes

The success of future nanotechnologies will strongly depend on our ability to control the structure of materials on the atomic scale. For carbon nanotubes it turns out that one of their structural parameters—the chirality—may not be controlled during synthesis. We explain the basic reason for this defect and show that novel classes of nanotubes like boron nanotubes, which are related to sheets with anisotropic in-plane mechanical properties, could actually overcome these problems. Our results further suggest that extended searches for nanomaterials similar to pure boron might allow for one of the simplest and most direct ways to achieve structural control within nanotechnology.

Use of relativistic core potentials to compute potential curves and lifetimes of low-lying states of arsenic fluoride

Abstract The effects of spin—orbit coupling and other relativistic interactions on the electronic structure of the arsenic fluoride molecule are analyzed with the aid of ab initio configuration interaction calculations based on effective core potentials. The computed results for potential curves. vibrational wavefunctions and radiative transition probabilities between various low-lying states of AsF are compared with analogous data for the fluorides of the heavier Group VA atoms, antimony and bismuth. Measured spin— orbit splittings for the X 3Σ− and A 3Π λ-s states are accurately reproduced in the relativistic Cl and computed trends in the parallel and perpendicular b0+-X transition probabilities for this group of fluorides are found to agree with the observations of Colin, Herman and Prevot. Inclusion of a semidiffuse s orbital on the fluorine atom is important in describing the σ* MO occupied in the A 3Π and c 1Π λ-s states. Computed electric dipole moments indicate that the latter states are more strongly polarized (negative F) than their lower-energy counterparts with π*2 electronic configurations.