Arta Sadrzadeh

Polymorphism of Two-Dimensional Boron

The structural stability and diversity of elemental boron layers are evaluated by treating them as pseudoalloy B(1-x)[hexagon](x), where [hexagon] is a vacancy in the close-packed triangular B lattice. This approach allows for an elegant use of the cluster expansion method in combination with first-principles density-functional theory calculations, leading to a thorough exploration of the configurational space. A finite range of compositions x is found where the ground-state energy is essentially independent of x, uncovering a variety of stable B-layer phases (all metallic) and suggesting polymorphism, in stark contrast to graphene or hexagonal BN.

Probing Properties of Boron α-Tubes by Ab Initio Calculations

We investigate the properties of nanotubes obtained from recently described boron alpha-sheet, using density functional theory. Computations confirm their high stability and identify mechanical stiffness parameters. This allows one to further analyze the basic vibrations, including the radial breathing mode Raman frequency, fRBM = 210(nm/ d) cm (-1). Careful relaxation reveals the curvature-induced buckling of certain atoms off the original plane. This distortion changes the overlap of the orbitals near the Fermi level and opens up the gap in narrow tubes, rendering them semiconducting. Wider tubes with the diameter d greater, similar 1.7 nm retain original metallic character of the alpha-sheet. This combination of properties could make boron alpha-tubes (BT) an important material for electronic, bio- and chemical sensing, and optical applications.

Metallacarboranes: Toward Promising Hydrogen Storage Metal Organic Frameworks

Using first principles calculations, we show the high hydrogen storage capacity of metallacarboranes, where the transition metal (TM) atoms can bind up to 5 H(2)-molecules. The average binding energy of ∼0.3 eV/H favorably lies within the reversible adsorption range. Among the first row TM atoms, Sc and Ti are found to be the optimum in maximizing the H(2) storage (∼8 wt %) on the metallacarborane cluster. Being an integral part of the cage, TMs do not suffer from the aggregation problem, which has been the biggest hurdle for the success of TM-decorated graphitic materials for hydrogen storage. Furthermore, the presence of carbon atom in the cages permits linking the metallacarboranes to form metal organic frameworks, which are thus able to adsorb hydrogen via Kubas interaction, in addition to van der Waals physisorption.

The Boron Buckyball and Its Precursors: An Electronic Structure Study

Using ab initio calculations, we analyze electronic structure and vibrational modes of the boron fullerene B(80), a stable, spherical cage similar in shape to the well-known C(60). There exist several isomers, lying close in structure and energy, with total energy difference within approximately 30 meV. We present detailed analysis of their electronic structure and geometry. Calculated radial breathing mode frequency turns out to be 474 cm(-1), which can be a characteristic of B(80) in Raman spectroscopy. Since the B(80) structure is made of interwoven double-ring clusters, we also investigate double-rings with various diameters. We present their structure and HOMO-LUMO dependence on the diameter, and find out that the gap alternates for different sizes and closes its value for infinite double-ring.

Electronic properties of twisted armchair graphene nanoribbons

We study the effect of twist on the electronic structure of H-terminated armchair graphene nanoribbons, for both relaxed and unrelaxed unit cell size. We investigate the band gap change as a function of the twist angle for different ribbon widths. In the case of unrelaxed unit cell size, the band gap closes for smaller twist angles as opposed to relaxed unit cell size. We calculate strain energy as a function of twist angle and show its direct correlation with the reduction of the band gap. Furthermore, the conductance is calculated at arbitrary degree of torsion.

Electron transport of nanotube-based gas sensors: An ab initio study

The effect of physisorption of NO2 gas molecules on quantum transport properties of semiconducting carbon nanotubes is studied using ab initio calculations and Green function formalism. The results show that the conductance change is mainly due to the electric dipole moment of NO2. It is also shown that upon exposure of nanotube to different concentrations of gas, the common feature is the shift in conductance toward lower energies. This suggests that physisorption of NO2 will result in a decrease (increase) in conductance of p-type (n-type) nanotubes with Fermi energies close to the edge of valence and conduction band.

Riemann Surfaces of Carbon as Graphene Nanosolenoids.

Traditional inductors in modern electronics consume excessive areas in the integrated circuits. Carbon nanostructures can offer efficient alternatives if the recognized high electrical conductivity of graphene can be properly organized in space to yield a current-generated magnetic field that is both strong and confined. Here we report on an extraordinary inductor nanostructure naturally occurring as a screw dislocation in graphitic carbons. Its elegant helicoid topology, resembling a Riemann surface, ensures full covalent connectivity of all graphene layers, joined in a single layer wound around the dislocation line. If voltage is applied, electrical currents flow helically and thus give rise to a very large (∼1 T at normal operational voltage) magnetic field and bring about superior (per mass or volume) inductance, both owing to unique winding density. Such a solenoid of small diameter behaves as a quantum conductor whose current distribution between the core and exterior varies with applied voltage, resulting in nonlinear inductance.