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We investigate the transport properties (IxV curves and zero bias transmittance) of pristine graphene nanoribbons (GNRs) as well as doped with boron and nitrogen using an approach that combines nonequilibrium Green's functions and density functional theory (DFT) [NEGF-DFT]. Even for a pristine nanoribbon we verify a spin-filter effect under finite bias voltage when the leads have an antiparallel magnetization. The presence of the impurities at the edges of monohydrogenated zigzag GNRs changes dramatically the charge transport properties inducing a spin-polarized conductance. The IxV curves for these systems show that depending on the bias voltage the spin polarization can be inverted. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

We performed self-consistent multiple-scattering Xα cluster calculations on the electronic structure of self-interstitial impurities (Ga and As) and 3d transition-metal (TM) atoms at three different interstitial sites in GaAs. Our results show that Ga at interstitial sites induces a t2 state resonant in the conduction band, occupied by one electron, and it is electrically inactive. Contrarily, As at interstitial sites may introduce three donor levels in the gap. For the TM impurities we also include many-electron effects. Our results show that the TMs act mostly as donors. We provide important information about stable charge states as a function of chemical potential and the possibility of an enhanced migration barrier via carrier capture.

The dependence of the band structure of cerium hydrides on hydrogen concentration is investigated by considering an approximate model Hamiltonian that takes into account the hybridization of the d and hydrogen-derived s bands. A Green's function technique is used to obtain the hybrid bands in the Hartree–Fock approximation. The model is used to discuss the role of H concentration, the inter-d exchange interaction, the s–d interaction, and the electrical conductivity on the observed metal–semiconductor phase transition in CeH(x).

The recently developed variational cellular method (VCM) is used to investigate the ground and excited electronic structures of F2 and Ne2 molecules. This work reports our first attempt to apply the VCM to the study of molecular excited states. It is concluded that VCM yields potential energy curves for these molecules with a good degree of physical realism.

A multiple-scattering-Xα cluster model of tetrahedrally coordinated covalent semiconductors is discussed. In order to simulate the rest of the crystal at the cluster boundaries, all electrons filling dangling bonds are eliminated from the self-consistent calculations and transferred to a Watson sphere. This manner of simulating the “saturating effects” of the rest of the crystal is more practical than the usual procedure of terminating the atoms at the surface of the cluster with hydrogen atoms. The model is applied to perform the first self-consistent calculations of the electronic structure of vacancies and substitutional impurities in GaAs. The efficiency and accuracy of the model is demonstrated by calculating the well-studied states of an ideal vacancy in Si. The obtained results are in good agreement with other self-consistent calculations.

We propose the use of the Si atom in the C 5 9 Si molecule as a possible way to controllably anchor fullerene molecules on an Si surface, due to the formation of a strong bond to one of the Si surface atoms. We considered a monohydride Si(100) surface with the H removed from one of the Si surface atoms, and the C 5 9 Si was adsorbed at this site. Through ab initio calculations based on Density Functional Theory, we obtained a final Si s u r f a c e -Si C 5 9 S i bond distance of 2.37 A, and a binding energy of -3.0 eV. Considering the reaction H s u r f a c e + C 5 9 SiH → (C 5 9 Si) a d s o r b e d + (H 2 ) g a s , we find this process to be exothermic by 0.41 eV. Upon adsorption of an isolated molecule, we find four levels in the gap, three empty and one singly occupied, all of them being basically localized at the C 5 9 Si molecule.

The authors developed an empirical potential for interactions between Si and N to describe silicon nitride systems using the Tersoff functional form. With this model, they explored the structural properties of amorphous silicon nitride through the Monte Carlo simulations and compared them to available experimental data. The empirical model provided a very good description of such properties for a-SiN{sub x} (0 < x < 1.5). Electronic structure of amorphous and point defects in crystalline silicon nitride were then studied using first-principles calculations. For such calculations, the configurations were created by the empirical model, with the relaxed structures used as input for the first-principles calculations. Atomic relaxation was later allowed in the first-principles calculations.

Using a molecular cluster with 71 atoms, we studied the electronic and structural properties of nitrogen vacancy, boron antisite, and oxygen in cubic boron nitride through an ab initio Hartree–Fock calculation. We found that all defects introduce a deep state above the middle-energy gap. These defects present a C3v local symmetry. In the case of nitrogen vacancy, the possibility of the F-center formation is discussed. All the calculations were performed using the program code GAMESS. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 941–946, 1997

Ab initio calculation of periodically Si-δ-doped GaAs was performed in the regime of high-donor concentration. The electronic confinement exhibit a quasi-2D behavior even for small periods (p = 45 A). The electronic properties of the clustering SiGaSAs also was studied. © 1995 John Wiley & Sons, Inc.

Self-consistent one-electron state calculations are carried out, for the first time, for a gallium vacancy in the GaAs1−xPx alloy. The cluster model, within the framework of the multiple-scattering Xα theory, is used to calculate several charge states of the vacancy. Suitable clusters have been considered in order to simulate the phosporous concentration x in the alloy and how the nearest-neghbors configurations affect the vacancy-related levels. It is found that the defect introduces a deep energy level into the band gap which follows the host-valence band edge as x varies. The same trend in donor and acceptor levels is observed, with a very small Mott–Hubbard potential energy. © 1993 John Wiley & Sons, Inc.

The electronic and structural properties of the single N and the pair N2 complex substitutional impurity systems in type-IV semiconductors (C, Si, and Ge) are investigated through an ab initioLCAO cluster model. We find that the stable position for N atoms (N and N2) is off center with dislocation in a 〈111〉 direction to form sp2 bonds with the host atoms. Particularly for ionized complex N there is a symmetry break lowering the point group symmetry of the system from D3d to C3v. We present also the results for hyperfine interactions over the impurity and its nearest neighbor which for isotropic terms are in fair agreement with the experiment. © 1992 John Wiley & Sons, Inc.

We present results of selfconsistent, first-principles calculations of total energies for AsGa and SbGa in GaAs. We confirm that both impurities in the substitutional Td site behave as double donors, and the first internal excitation appears at around 1 eV. For the neutral systems we obtain a metastable minimum in the total energy surface in a configuration with the impurity atom displaced toward the interstitial site; the transformation to this metastable configuration, however, is not expected to be operative for the SbGa defect.

This contribution is concerned with the occurrence of metallic character in K-doped systems and the non-occurrence in Na-doped systems. An N-fold degenerate Hubbard model in the Gutzwiller approximation, previously presented by Jian Ping Lu, is utilized to discuss the metal–insulator transitions observed in these alkali-doped fullerides. The parameters used are derived from INDO calculations in the molecular cluster approach. © 1995 John Wiley & Sons, Inc.

An exchange-correlation potential model to be used in connection with the multiple scattering method is presented. Retaining the main advantages of the multiple scattering method with the Xα potential, particularly its low computational requirements, this new formalism does not require any adjustable parameter. Test calculations on the NiF64− system are reported and compared with experimental and ab initio results.

We consider the effects of a non-muffin-tin charge density in the multiple-scattering method. The study is carried out by means of a parametrized electronic density, which is used in a variational expression for the energy of the molecular system. We show that, at least in two situations, the parametrized density reproduces the true density very well. In a numerical study of the electronic configuration of the CH4 molecule, we show that the non-muffin-tin density may shift the one-electron energies by as much as 1 eV and the total energy by as much as 1 Ry.

Using the Haldane–Anderson formalism we study the possibility of obtaining a negative U system for transition metal impurities in semiconductors. We propose, even without including electron-lattice interaction, the occurrence of negative-U as a consequence of the impurity-host hybridization to explain why some oxidation states would be missing in these systems.