Conductance and transparence of long molecular wires

Electron tunneling through a molecular wire is studied as a function of the length and chemical structure of the molecule. The current intensity is calculated using the electron-scattering quantum-chemistry technique, the wire being connected at both ends to a planar metal-vacuum-metal nanojunction. The tunnel channels and the stepped $I\left(V\right)\mathrm{}$ characteristics are discussed in detail for the oligo (thiophene ethynylene) molecular wire. At low bias voltage, the conductance $G$ of a metal-molecular wire-metal junction follows a ${G=G}_0{e}^{-\gamma L}$ law with $L$ the interelectrode separation. The inverse damping length $\gamma$ depends on the internal wire electronic structure and the contact conductance $G_0$ on the electrode-wire end interactions. Both $\gamma$ and $G_0$ can be optimized by changing the chemical structure of the wire, and are given for a large number of oligomers.