We report an ab-initio investigation of several possible Si and Ge pristine nanowires with diameters between 0.5 and 1.2 nm. We considered nanowires based on the diamond structure, high-density bulk structures, and fullerene-like structures. We find that the diamond structure nanowires are unstable for diameters smaller than 1 nm, and undergo considerable structural transformations towards amorphous-like wires. Such instability is consistent with a continuum model that predicts, for both Si and Ge, a stability crossover between diamond and high-density-structure nanowires for diameters smaller than 1 nm. For diameters between 0.8 nm and 1 nm, filled-fullerene wires are the most stable ones. For even smaller diameters (d ∼ 0.5 nm), we find that a simple hexagonal structure is particularly stable for both Si and Ge. . These nanowires usually depict a crystalline core surrounded by an oxide outer layer. Further removal of the oxide layer by acid treatment may lead to hydrogenpassivated silicon nanowires as thin as one nanometer [4]. Pristine (non-passivated) silicon wires with diameters of a few nanometers have also been produced from Si vapor deposited on graphite [5]. The elongated shape of silicon and germanium clusters of up to a few tens of atoms, determined by mobility measurements [6,7], indicates that even thinner pristine structures, with diameters smaller than 1 nm, can been produced.The growth of such small-diameter structures raises the question of the limit of a bulk-like description of bonding in these nanowires, since for small enough diameters the predominance of surface atoms over inner (bulklike) atoms will eventually lead to bonds (and structures) distinct from those of the bulk system. In the present work, we use first principles calculations to investigate several periodic structures of silicon and germanium pristine nanowires of infinite length, with diameters ranging from 0.45 to 1.25 nm. The nanowire structures considered are based on the diamond structure, fullerene-like structures, and the high-density bulk structures β-tin, simple cubic (sc), and simple hexagonal (sh).Our calculations are performed in the framework of the density functional theory [8], within the generalizedgradient approximation (GGA) [9] for the exchangecorrelation energy functional, and the soft normconserving pseudopotentials of Troullier-Martins [10] in the Kleinman-Bylander factorized form [11]. We use a method [12] in which the one electron wavefunctions are expressed as linear combinations of pseudo-atomic numerical orbitals of finite range. A double-zeta basis set is employed, with polarization orbitals included for all atoms. For the nanowire calculations, we employ supercells that are periodic along the wire axis, and that are wide enough in the perpendicular directions to avoid interaction between periodic images. All the geometries were optimized until residual forces were less than 0.04 eV/Å. Total-energy differences were converged to within 4 meV/atom with respect to orbital range and k-point sampling.Most ...

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We have investigated by means of first principles calculations the structural and electronic properties of hydrogenated graphene structures with distinct grain boundary defects. Our total energy results reveal that the adsorption of a single H is more stable at grain boundary defect. The electronic structure of the grains boundaries upon hydrogen adsorption have been examined. Further total energy calculations indicate that the adsorption of two H on two neighbor carbons, forming a basic unit of graphane, is more stable at the defect region. Therefore, we expect that these extended defects would work as a nucleation region for the formation of a narrow graphane strip embedded in graphene region. [7] have been theoretically proposed, and can be produced by removing H atoms from graphane [8] or adsorbing them on graphene [9,10]. These structures can present a suitable band gap energy [9,10] for pratical applicantions in the new nanoelectronic devices.Grain boundary (GB) defects have been observed in graphene in very recent experiments [11,12]. Theoretical works [13] have also proposed structural models for GB defects, which resemble to those of Refs. [11,14]. Tight-binding calculations suggest that the H adsorption is more stable at the GB defects than in perfect graphene [15]. However, there is no ab-initio investigations on the properties of H adsorption at those observed GB defects, H/GB. Thus, in this work we have investigated, by means of first-principles calculations, the effect of H adsorption on the properties of graphene with grain boundary defects. Our results indicate that the adsorption of a single H is more stable at GB defect. Also, the adsorption of two H on two neighbor carbons, forming a basic unit of graphane, is more stable at the defect region. Thus, we can infer that these extended defects would work as a nucleation region for the formation of a narrow graphane strip embedded in graphene region. The modifications in the electronic structure due to H adsorption at GB defects are also investigated.Spin-polarized density functional theory (DFT) [16] calculations, within the generalized-gradient approximation (GGA) [17], were performed using the SIESTA code [18], where core states were replaced by normconserving pseudopotentials [19] in the factorized form [20]. We have employed a double-zeta plus polarization (DZP) basis set with an energy shift of 100 meV [21], and an energy cutoff of 200 Ry for the real-space mesh. 10 (210) special k-points for geometry (band structure) calculations were used. All atomic positions were fully relaxed until the residual forces were converged to within 10 meV/Å. We have employed a distance of 15Å between graphene layers and supercells with (60) 60, 96, and 50 carbons atoms to represent the (perfect) defective graphene structures.We first describe the atomic structure of GB defects in graphene. Here, we have considered three distinct structural models for the GB defects, shown in Fig. 1. These extended defects can be built from a suitable regular arrangement of 5-...

We report an ab initio study of the electronic properties of surface dangling-bond (SDB) states in hydrogen-terminated Si and Ge nanowires with diameters between 1 and 2 nm, Ge/Si nanowire heterostructures, and Si and Ge (111) surfaces. We find that the charge transition levels ε(+/−) of SDB states behave as a common energy reference among Si and Ge wires and Si/Ge heterostructures, at 4.3 ± 0.1 eV below the vacuum level. Calculations of ε(+/−) for isolated atoms indicate that this nearly constant value is a periodic-table atomic property.PACS numbers: 73.20.Hb, When a junction of two distinct, undoped semiconductor materials (a heterojunction) is formed, one can define the band-lineup using the valence-and conduction-band edge energies on each side of the junction. The measurement or calculation of such lineup can be made either directly from the band discontinuities at the interface, or indirectly through a common bulk energy reference on both sides of the interface. From the theoretical side, several such common energy references have been proposed. Tersoff suggests [1] that an effective midgap energy E B (which can be calculated from the bulk electronic structure) can be used to predict lineups within an accuracy of 0.2 eV. Electronic states of transition metal impurities [2-4] and charge transition levels ε(+/−) of hydrogen interstitial impurities [5,6] have also been shown to work as common energy references.

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