Exponential Decay Of Conductance Research Articles

Features of superexchange nonresonant tunneling conductance in anchored molecular wires

A modified superexchange model is used to clarify the physical mechanisms for the formation of nonresonant tunneling conductance in terminated molecular wires. Due to the specific relationship between its key parameters, this model has wider areas of applicability compared to the flat-barrier model and the standard superexchange model, which are widely involved for the physical interpretation of experimental results. Moreover, the results obtained in the two latest models appear in the modified model as characteristic limiting cases. Our estimates show that the exponential decay of conductance, characterized by an attenuation factor β (per repeating unit), is limited by the conditions β ≤ 1.2 and β ≥ 3.7 for the flat-barrier and standard models, respectively. At the same time, the modified superexchange model yields β > 0, which, thus, allows us to analyze the tunneling conductance in molecular wires containing both saturated and conjugated bonds. We also show that for a small number of N repeating wire units (about 3–6 depending on the value of β), the exponential dependence of conductance on N is violated and, accordingly, contact conductance is not identical to conductance at N = 0. Formulas are found which, on the basis of experimental data, make it possible to establish the values of superexchange parameters as well as indicate the conditions of possible hybridization between the orbitals of the anchor groups and the adjacent end units belonging to the interior wire region. One example is the establishment of features in the tunneling conductance of terminated alkane chains caused by the nature of their anchor groups.

Wire-Length Dependence of the Conductance of Oligo(p-phenylene) Dithiolate Wires: A Consideration from Molecular Orbitals

The electrical transmission of a series of oligo(p-phenylene) dithiolate (PDT) wires coupled with gold electrodes is investigated using a Green function method combined with a hybrid density functional method. To investigate how the nonplanarity of the π-backbone affects the electrical transmission, two types of PDT wires with planar and nonplanar forms are compared. The exponential decay of conductance at the Fermi level, G(EF) = G0 exp(−γL), is observed for both forms. The damping factors, γ's, for the planar and nonplanar PDT wires are 0.165 and 0.256 Å-1, respectively, these values being comparable with experimental data for analogous oligophenylene wires. A possible reason for the slower decay in the planar PDT wires is analyzed in terms of molecular orbitals.