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(V) 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.