We report a full self-consistent ab initio calculation of the current-voltage curve and the conductance of thiolate capped polyynes in contact with gold electrodes. We find the conductance of polyynes an order of magnitude larger compared with other conjugated oligomers. The reason lies in the position of the Fermi level deep in the HOMO related resonance. With the conductance weakly dependent on the applied bias and almost independent of the length of the molecular chain, polyynes appear as nearly perfect molecular wires. The study of transport properties of single molecules have attracted a significant attention because of their potential use in molecular electronic devices.One of the major classes of molecules considered in conductivity studies, primarily for their molecular wire behavior, 1,2,3,4,5 is conjugated oligomers. They have shown a number of useful nonlinear properties such as conductance switching and negative differential resistance.6,7 However, inspite of a number of interesting experiments 4,8 a molecule with good molecular wire properties has not yet been spotted.A useful molecular wire should provide a high and stable conductance over a wide bias region and for various lengths of the molecules. A linear chain of carbon atoms with double bonds between neighboring atoms, usually referred to as cumulene, was proposed as an ideal molecular wire 1 and the calculations of the conductance of cumulene connected to gold electrodes were reported. 9,10Lang and Avouris 9 showed that the conductance of cumulene did not stay constant in the ballistic regime, but rather oscillated between the constant values characteristics of the odd and even number of atoms in the chain.In this paper we show an entirely different behavior of polyynes, another form of the carbon atom chain. Polyynes are simple and yet most intriguing of conjugated organic oligomers. Only recently, have they been assembled up to decayne.11 Formed as a linear chain of carbon atom pairs (CC) n , with alternating single and triple bonds, they are a unique, truly one-dimensional, molecular system. Two π-electron systems of the sphybridized structure provide polyyne with approximately cylindrical electronic delocalization along the conjugated backbone. The electronic transport is therefore independent of the rotation around the single bond, which is a limitation often present in other organic oligomers. 7We have obtained the electronic structure and transport properties of a series of polyynes up to octayne, connected to gold electrodes. The stability of polyyne with respect to single-and triple-bond alternations was achieved by fixing the molecule at the ends with thiol bonds. In addition, the thiol capped polyynes make a strong chemisorption bond onto the metallic electrodes. We found that they had more than an order of magnitude higher conductance when compared with other conjugated oligomers. In contrast to the cumulenes they were not prone to oscillations in conductance with the length of the molecule. We also found that their conductance wa...

We theoretically study the electronic transport in the monolayer of dithiolated phenylene vinylene oligomeres coupled to the (111) surfaces of gold electrodes. We use non-equilibrium Green functions (NEGF) and density functional theory(DFT) implemented in the TranSIESTA package to obtain a full ab initio self-consistent description of the transport current through the molecular nanostructure with different electrochemical bias potentials. The calculated current-voltage characteristics (IVC) of the systems for the same contact geometry have shown a systematic decrease of the conductivity with the increased length of the molecules. We analyze the results in terms of transmission eigenchannels and find that besides the delocalization of molecular orbitals the distance between gold electrodes also determines the transport properties.

We have theoretically studied the stability and reconstruction of ͑111͒ surfaces of Au, Pt, and Cu. We have calculated the surface energy, surface stress, interatomic force constants, and other relevant quantities by ab initio electronic structure calculations using the density functional theory in a slab geometry with periodic boundary conditions. We have estimated the stability towards a quasi-one-dimensional reconstruction by using the calculated quantities as parameters in a one-dimensional Frenkel-Kontorova model. On all surfaces we have found an intrinsic tensile stress. This stress is large enough on Au and Pt surfaces to lead to a reconstruction in which a denser surface layer is formed, in agreement with experiment. The experimentally observed differences between the dense reconstruction pattern on Au͑111͒ and a sparse structure of stripes on Pt͑111͒ are attributed to the details of the interaction potential between the first layer of atoms and the substrate.

We consider some recently developed schemes for treating van der Waals interactions within the density functional theory ͑DFT͒ on the widely discussed example of adsorption of Xe on Cu͑111͒ and Pt͑111͒ surfaces. Consistent with the overall weakness of the Xe surface and Xe-Xe interactions we assess the performance of the schemes that are appropriate to systems consisting of nearly isolated fragments in which the coefficients of the van der Waals expansion are deduced from DFT calculations. Such generalized DFT calculations of potential energy surfaces yield the structure of Xe adlayers in good agreement with experiments and retrieve the dilation of commensurate monolayer phase in which the intralayer Xe-Xe radial force constants are strongly reduced. This provides a first principles interpretation of the observed vibrational properties of adlayers, in general, and the much debated dispersion of in-plane polarized vibrations, in particular.

We study the variation of electron transmission through Au-S-benzene-S-Au junctions and related systems as a function of the structure of the Au:S contacts. For junctions with semi-infinite flat Au(111) electrodes, the highly coordinated in-hollow and bridge positions are connected with broad transmission peaks around the Fermi level, due to a broad range of transmission angles from transverse motion, resulting in high conductivity and weak dependence on geometrical variations. In contrast, for (unstable) S-adsorption on top of an Au atom, or in the hollow of a 3-Au-atom island, the transmission peaks narrow up due to suppression of large transmission angles. Such more one-dimensional situations may describe more common types of contacts and junctions, resulting in large variations in conductivity and sensitivity to bonding sites, tilting and gating. In particular, if S is adsorbed in an Au vacancy, sharp spectral features appear near the Fermi level due to essential changes of the level structure and hybridization in the contacts, admitting order-of-magnitude variations of the conductivity. Possibly such a system, can it be fabricated, will show extremely strong non-linear effects and might work as uni-or bi-directional voltage-controlled 2-terminal switches and non-linear mixing elements. Finally, density-functional-theory (DFT) based transport calculations seem relevant, being capable of describing a wide range of transmission peak structures and conductivities. Prediction and interpretation of experimental results probably require more precise modeling of realistic experimental situations.