Conductance of Conjugated Molecular Wires: Length Dependence, Anchoring Groups, and Band Alignment

The conductance of pi-conjugated Molecular wires bonded to gold electrodes at zero bias is studied using density functional theory combined with nonequilibrium Green's function method. For all systems considered, we find that the conductance length dependence follows the simple exponential law characteristic of tunneling through a barrier, G = G(c) exp(-beta L). For thiophene, pyrrole, and phenyl wires with thiol end-groups, we calculate decay constants (beta) of 0.211, 0.257, and 0.264 angstrom(-1), respectively, and contact conductances (G(c)) of 1.25, 2.90, and 1.22G(0), where G(0) = 2e(2)/h is the conductance quantum. In comparison, the corresponding values for amine-terminated thiophene are calculated to be beta = 0.160 angstrom(-1) and G(c) = 0.038G(0). These results show that (1) the contact resistance is mainly determined by the anchoring group and (2) the decay constant, which determines the conductance in the long wire limit, is not solely determined by the intrinsic band gap of the molecular wire but also depends on the anchoring group. This is because the alignment of the metal Fermi level wih respect to the molecular levels is controlled by charge transfer and interface dipoles which in turn are determined by the local chemistry at the interface. Analysis of the charge transfer at the interface shows that the thiol-bonded Molecules receive electrons from the Au electrodes while the amine-bonded molecules donate electrons to the Au electrodes.