ELECTRONIC TRANSMISSION IN CONJUGATED-OLIGOMER TUNNEL STRUCTURES - EFFECTS OF LATTICE FLUCTUATIONS

The electronic transmission across metal/conjugated-oligomer/metal str uctures is discussed, emphasizing the role of lattice fluctuations in short oligomer chains. Four cases are discussed: (a) one oligomer chai n, (b) two oligomer chains, (c) chains which form a two-dimensional (2 D) structure, and (d) chains which form a three-dimensional (3D) struc ture, 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 di minishes 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 o f the oligomer. In the two-chain case the spatial mirror symmetry with respect to the middle line of the two chains is broken when fluctuati ons are introduced and coherence between the wave functions of the two chains is partly lost. For the 2D and 3D cases the momentum perpendic ular to the oligomer chains is no longer conserved when fluctuations a re considered and thus a 'scattered' flux, which represents a deviatio n from the 'specular' flux, appears. The integrated scattered flux ove r the energy is a measure of the strength of the fluctuations in the o ligomers. If only one of the oligomer chains exhibits lattice fluctuat ions, the incoming electrons can optimize their path so as to tunnel t hrough the chains with a larger transmission: when the energy of the i ncoming electron is larger than the gap of the ordered oligomer, the e lectrons avoid the disordered chain; when the energy of the incoming e lectron lies in the gap of the ordered oligomer, the probability of el ectrons being near the disordered chain is enhanced.