Ab-initio transport properties of nanostructures from maximally-localized Wannier functions

We present a comprehensive first-principles study of the ballistic transport properties of low dimensional nanostructures such as linear chains of atoms (Al, C) and carbon nanotubes in presence of defects. A novel approach is introduced where quantum conductance is computed from the combination of accurate plane-wave electronic structure calculations, the evaluation of the corresponding maximally-localized Wannier functions, and the calculation of transport properties by a real-space Green's function method based on the Landauer formalism. This approach is computationally very efficient, can be straightforwardly implemented as a post-processing step in a standard electronic-structure calculation, and allows to directly link the electronic transport properties of a device to the nature of the chemical bonds, providing insight onto the mechanisms that govern electron flow at the nanoscale.Comment: to be published in Phys. Rev. B (2003

Ab initio study of semiconducting carbon nanotubes adsorbed on the Si(100) surface: diameter- and registration-dependent atomic configurations and electronic properties

We present a first-principles study of semiconducting carbon nanotubes adsorbed on the unpassivated Si(100) surface. We have found metallicity for the combined system caused by n-doping of the silicon slab representing the surface by the SWNT. We confirm this metallicity for nanotubes of different diameters and chiral angles, and find the effect to be independent of the orientation of the nanotubes on the surface. We also present adsorption energetics and configurations which show semiconducting SWNTs farther apart from the surface and transferring less charge, in comparison with metallic SWNTs of similar diameter.Comment: Replaces old (Jan 2006) version; more supporting material. 11 pages, 8 figures, 7 table

Bromophenyl functionalization of carbon nanotubes : an ab initio study

We study the thermodynamics of bromophenyl functionalization of carbon nanotubes with respect to diameter and metallic/insulating character using density-functional theory (DFT). On one hand, we show that the activation energy for the grafting of a bromophenyl molecule onto a semiconducting zigzag nanotube ranges from 0.73 eV to 0.76 eV without any clear trend with respect to diameter within numerical accuracy. On the other hand, the binding energy of a single bromophenyl molecule shows a clear diameter dependence and ranges from 1.51 eV for a (8,0) zigzag nanotube to 0.83 eV for a (20,0) zigzag nanotube. This is in part explained by the transition from sp2 to sp3 bonding occurring to a carbon atom of a nanotube when a phenyl is grafted to it and the fact that smaller nanotubes are closer to a sp3 hybridization than larger ones due to increased curvature. Since a second bromophenyl unit can attach without energy barrier next to an isolated grafted unit, they are assumed to exist in pairs. The para configuration is found to be favored for the pairs and their binding energy decreases with increasing diameter, ranging from 4.34 eV for a (7,0) nanotube to 2.27 eV for a (29,0) nanotube. An analytic form for this radius dependence is derived using a tight binding hamiltonian and first order perturbation theory. The 1/R^2 dependance obtained (where R is the nanotube radius) is verified by our DFT results within numerical accuracy. Finally, metallic nanotubes are found to be more reactive than semiconducting nanotubes, a feature that can be explained by a non-zero density of states at the Fermi level for metallic nanotubes.Comment: 7 pages, 5 figures and 3 table

Computational design of chemical nanosensors: Transition metal doped single-walled carbon nanotubes

We present a general approach to the computational design of nanostructured chemical sensors. The scheme is based on identification and calculation of microscopic descriptors (design parameters) which are used as input to a thermodynamic model to obtain the relevant macroscopic properties. In particular, we consider the functionalization of a (6,6) metallic armchair single-walled carbon nanotube (SWNT) by nine different 3d transition metal (TM) atoms occupying three types of vacancies. For six gas molecules (N_{2}, O_{2}, H_{2}O, CO, NH_{3}, H_{2}S) we calculate the binding energy and change in conductance due to adsorption on each of the 27 TM sites. For a given type of TM functionalization, this allows us to obtain the equilibrium coverage and change in conductance as a function of the partial pressure of the "target" molecule in a background of atmospheric air. Specifically, we show how Ni and Cu doped metallic (6,6) SWNTs may work as effective multifunctional sensors for both CO and NH_{3}. In this way, the scheme presented allows one to obtain macroscopic device characteristics and performance data for nanoscale (in this case SWNT) based devices.Comment: Chapter 7 in "Chemical Sensors: Simulation and Modeling", Ghenadii Korotcenkov (ed.), 47 pages, 22 figures, 10 table

Electric capacitance as nanocondensers in zigzag nanographite ribbons

Electronic states in nanographite ribbons with zigzag edges are studied using the extended Hubbard model with nearest neighbor Coulomb interactions. The nearest Coulomb interactions stabilize electronic states with the opposite electric charges separated and localized along both edges. Such states are analogous as nanocondensers. Therefore, electric capacitance, defined using a relation of polarizability, is calculated to examine nano functionalities. We find that the behavior of the capacitance is widely different depending on whether the system is in the magnetic or charge polarized phases. In the magnetic phase, the capacitance is dominated by the presence of the edge states while the ribbon width is small. As the ribbon becomes wider, the capacitance remains with large magnitudes as the system develops into metallic zigzag nanotubes. It is proportional to the inverse of the width, when the system corresponds to the semiconducting nanotubes and the system is in the charge polarized phase also. The latter behavior could be understood by the presence of an energy gap for charge excitations.Comment: 11 pages; 7 figures; accepted for publication in Japanese Journal of Applied Physic

Chemically active substitutional nitrogen impurity in carbon nanotubes

We investigate the nitrogen substitutional impurity in semiconducting zigzag and metallic armchair single-wall carbon nanotubes using ab initio density functional theory. At low concentrations (less than 1 atomic %), the defect state in a semiconducting tube becomes spatially localized and develops a flat energy level in the band gap. Such a localized state makes the impurity site chemically and electronically active. We find that if two neighboring tubes have their impurities facing one another, an intertube covalent bond forms. This finding opens an intriguing possibility for tunnel junctions, as well as the functionalization of suitably doped carbon nanotubes by selectively forming chemical bonds with ligands at the impurity site. If the intertube bond density is high enough, a highly packed bundle of interlinked single-wall nanotubes can form.Comment: 4 pages, 4 figures; major changes to the tex

Defective transport properties of three-terminal carbon nanotube junctions

We investigate the transport properties of three terminal carbon based nanojunctions within the scattering matrix approach. The stability of such junctions is subordinated to the presence of nonhexagonal arrangements in the molecular network. Such "defective" arrangements do influence the resulting quantum transport observables, as a consequence of the possibility of acting as pinning centers of the correspondent wavefunction. By investigating a fairly wide class of junctions we have found regular mutual dependencies between such localized states at the carbon network and a strikingly behavior of the conductance. In particular, we have shown that Fano resonances emerge as a natural result of the interference between defective states and the extended continuum background. As a consequence, the currents through the junctions hitting these resonant states might experience variations on a relevant scale with current modulations of up to 75%.Comment: 8 pages, 8 figure

RKKY interaction in carbon nanotubes and graphene nanoribbons

We study Rudermann-Kittel-Kasuya-Yosida (RKKY) interaction in carbon nanotubes (CNTs) and graphene nanoribbons in the presence of spin orbit interactions and magnetic fields. For this we evaluate the static spin susceptibility tensor in real space in various regimes at zero temperature. In metallic CNTs the RKKY interaction depends strongly on the sublattice and, at the Dirac point, is purely ferromagnetic (antiferromagnetic) for the localized spins on the same (different) sublattice, whereas in semiconducting CNTs the spin susceptibility depends only weakly on the sublattice and is dominantly ferromagnetic. The spin orbit interactions break the SU(2) spin symmetry of the system, leading to an anisotropic RKKY interaction of Ising and Moryia-Dzyaloshinsky form, besides the usual isotropic Heisenberg interaction. All these RKKY terms can be made of comparable magnitude by tuning the Fermi level close to the gap induced by the spin orbit interaction. We further calculate the spin susceptibility also at finite frequencies and thereby obtain the spin noise in real space via the fluctuation-dissipation theorem