Molecular wires: Charge transport, mechanisms, and control

By molecular wires, one generally means molecular structures that transmit a signal between two termini. We discuss some theoretical models and analysis for electronically conductive molecular wires in which a single molecule conducts charge between two electrodes. This situation resembles both intramolecular non-adiabatic electron transfer, in which electronic tunneling between donor and acceptor is seen, and mesoscopic quantum transport. We discuss formal methods for predicting conductance in molecular wire circuits. The critical component that differs from the usual conductivity is the interface between electrode continuum and the discrete levels of the molecule. This can be described in several ways. We present an analysis based on the Bardeen tunneling formula. Specific problems (electron polarization, disorder, nuclear scattering, charge distribution) are discussed. Finally, the differing mechanisms expected for the conductance, ranging from ballistic tunneling to gated transfer, are outlined.