Tunneling Diodes under Environmental Effects

We examine the robustness of single-molecule tunneling diodes to thermal-environmental effects. The diode is comprised of three fragments: two different conjugated chemical groups at the boundaries, and a saturated moiety in between, breaking conjugation. In this setup, molecular electronic levels localized on the conjugated groups independently shift with applied bias. While in the forward polarity a resonance condition is met, enhancing conductance, in the reversed direction molecular electronic states shift away from each other, resulting in small tunneling currents. In the absence of interactions with a thermal environment (consisting, e.g., of internal vibrations, solvent), rectification ratios reach 3 orders of magnitude. We introduce decoherence and inelastic-dissipative effects phenomenologically, by using the voltage probe approach. We find that when gamma(d) less than or similar to v, with gamma(d) as the interaction energy of electrons with the environment and v as the tunneling energy across the saturated link, the diode is still highly effective, although rectification ratios are cut down by a factor of 2-4 as compared to the coherent limit. To further enhance rectification ratios in molecular diodes, we suggest a refined design involving four orbitals, with a pair of closely spaced states at each conjugated moiety.