Unusual Conductance of Polyyne-Based Molecular Wires

Abstract

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...

Cumulene molecular wire conductance from first principles

“…12 For the transport properties one can expect the LDA to perform quite well due to the metallic character of the junction. 10 Core electrons were modeled with Troullier-Martins 13 soft norm-conserving pseudopotentials and the valence electrons were expanded in a basis set of local orbitals. 14 The transmission spectrum, which gives the probability for electrons with incident energy E to be transferred from the left to the right electrode, is calculated from 15 …”

Solvent Dependence of the Single Molecule Conductance of Oligoyne-Based Molecular Wires

“…Single molecule wires of oligoynes have been experimentally investigated up to octa-1,3,5,7-tetraynes (n = 4), 3,4 while theoretical evaluations have been made for hypothetical infinitely long chains as well as finite chains. 1 However, it would be wrong to assume that oligoynes offer rigid-rod linear geometries; on the contrary they can be considerably curved and flexible offering a surprising low bending force. 9 Nevertheless, this structural flexibility does not detract from the impressive electronic properties of oligoynes, which offer considerable interest for their electronic, opto-electronic and electrical charge transport properties.…”

Communication: Stable carbon nanoarches in the initial stages of epitaxial growth of graphene on Cu(111)

“…The monatomic 1D carbon chains have been long pursued due to their predicted exceptional physical and chemical properties. [17][18][19] However, little has been experimentally demonstrated mainly due to the lack of a reliable and effective fabrication method. [20][21][22][23][24] The present work suggests that such 1D carbon chains can be formed via self-assembly on Cu(111) substrates.…”