Electronic transmission in conjugated-oligomer tunnel structures: effects of lattice fluctuations

Abstract

The electronic transmission across metal/conjugated-oligomer/metal structures is discussed, emphasizing the role of lattice fluctuations in short oligomer chains. Four cases are discussed: (a) one oligomer chain, (b) two oligomer chains, (c) chains which form a two-dimensional (2D) structure, and (d) chains which form a three-dimensional (3D) structure, sandwiched between metal contacts. The lattice fluctuations are approximated by white-noise disorder. For the one-chain case, resonant tunnelling occurs when the energy of the incoming electron coincides with an electronic level of the oligomer and the corresponding peak diminishes in intensity on increasing the strength of the disorder. Due to the lattice fluctuations, there is an enhancement of the electronic transmission for energies that lie within the electronic energy gap of the oligomer. In the two-chain case the spatial mirror symmetry with respect to the middle line of the two chains is broken when fluctuations are introduced and coherence between the wave functions of the two chains is partly lost. For the 2D and 3D cases the momentum perpendicular to the oligomer chains is no longer conserved when fluctuations are considered and thus a `scattered' flux, which represents a deviation from the `specular' flux, appears. The integrated scattered flux over the energy is a measure of the strength of the fluctuations in the oligomers. If only one of the oligomer chains exhibits lattice fluctuations, the incoming electrons can optimize their path so as to tunnel through the chains with a larger transmission: when the energy of the incoming electron is larger than the gap of the ordered oligomer, the electrons avoid the disordered chain; when the energy of the incoming electron lies in the gap of the ordered oligomer, the probability of electrons being near the disordered chain is enhanced.