Features of superexchange nonresonant tunneling conductance in anchored molecular wires

Elmar G Petrov,Vladislav Snitsarev,Yevgen V Shevchenko,Victor V Gorbach,Svetlana Lyubchik,Andrey V Ragulya

doi:10.1063/1.5124386

Abstract

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 &gt; 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.

Highlights

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
As an example of applying the theory to the analysis of experimental data, we considered nonresonant tunneling conductance in the terminated n-alkane chains
This is supported by the fact that in the highest occupied molecular orbital (HOMO) the electron density is concentrated mainly on N(= n − 1) C–Cσ bonds.[8,59,60] (In the lowest unoccupied molecular orbital (LUMO), the electron density is largely located at the C atoms.8) As to the terminal units, they are anchored to the electrodes through the groups SH, NH2, and COOH

Summary

Introduction

One of the fundamental processes that manifest themselves in nanoscale physics is the tunneling of charges (electrons/holes) through individual molecular wires. A typical molecular wire is a regular chain of repeating monomers, where the end monomers are attached to terminal units. The latter play an important role in the formation of stable self-assemble monolayers and the specific circuits for molecular devices.[3,7,13,17,18] Numerous experiments on the study of the tunneling processes in molecular wires embedded between the metal electrodes show an exponential decrease in conductance g with the number N of repeating chain units,[19–21] i.e., g ≃ A exp(−βN). As for the attenuation factor β, it represents the features of the formation of coherent charge transfer through a regular chain (interior range of the wire)

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Conclusion