We investigate, by means of ab initio calculations, electronic transport in molecular junctions composed of a biphenyl molecule attached to metallic carbon nanotubes. We find that the conductance is proportional to cos 2 , with the angle between phenyl rings, when the Fermi level of the contacts lies within the frontier molecular orbitals energy gap. This result, which agrees with experiments in biphenyl junctions with nonorganic contacts, suggests that the cos 2 law has a more general applicability, irrespective of the nature of the electrodes. We calculate the geometrical degree of chirality of the junction, which only depends on the atomic positions, and demonstrate that it is not only proportional to cos 2 but also is strongly correlated with the current through the system. These results indicate that molecular conformation plays the preponderant role in determining transport properties of biphenyl-carbon nanotubes molecular junctions.

We investigate electronic molecular transport in several conjugated organic oligomers by means of ab initio calculations and nonequilibrium Green's functions method. We demonstrate that the I-V characteristics of these molecules constitute a direct manifestation of their degree of molecular chirality, which is calculated using group theory and depends exclusively on the atomic positions. This result shows that electronic current through these specific molecules is strongly correlated with their geometrical degree of chirality.

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We investigate theoretically the charge accumulated Q in a three-terminal molecular device in the presence of an external electric field. Our approach is based on ab initio Hartree-Fock and density functional theory methodology contained in Gaussian package. Our main finding is a negative differential resistance (NDR) in the charge Q as a function of an external electric field. To explain this NDR effect we apply a phenomenological capacitive model based on a quite general system composed of many localized levels (that can be LUMOs of a molecule) coupled to source and drain. The capacitance accounts for charging effects that can result in Coulomb blockade (CB) in the transport. We show that this CB effect gives rise to a NDR for a suitable set of phenomenological parameters, like tunneling rates and charging energies. The NDR profile obtained in both ab initio and phenomenological methodologies are in close agreement.

In this work is presented a theoretical investigation of the neutral and bipolaronlike ground and excited states of molecules and polymers isoelectronic composed by Polyacetylene, Polyazine and Polyazoethene. The results obtained, utilizing DFT and ab initio methodologies, reveal that a very good defects description can be important in the investigation of insulator-metal transition of quasiunidimensional polymers indicating metallic behavior around the Fermi level as mechanism of conductivity of polymers. This result is consistent with experimental data and do not anticipate by Su-Schrieffer-Heeger (SSH) methodology. Our results are consistent with significant features as a nanodevice and can be summarized as: (i) it could be used as single directional molecular rectifier with a conformational geometry with small lead coupling; (ii) our non-equilibrium green function simulation present that Polyacetylene, Polyazine and Polyazoethene could rectified without gate current; (iv) based on properties of bonds type ( / , it can be utilized to design devices with applications in molecular electronics.