Charge transport in molecular junctions: From tunneling to hopping with the probe technique

TL;DR

This paper introduces a phenomenological probe technique to simulate charge transport in molecular wires, capturing various mechanisms like tunneling, hopping, and ballistic conduction, aligning well with experimental observations.

Contribution

The study demonstrates that the Landauer-Büttiker's probe technique can effectively model different charge transfer mechanisms and environmental effects in molecular junctions.

Findings

Short wire conductance decays exponentially with length.

Hopping dominates in long wires, showing ohmic behavior.

Thermal activation enhances current at high temperatures.

Abstract

We demonstrate that a simple phenomenological approach can be used to simulate electronic conduction in molecular wires under thermal effects induced by the surrounding environment. This "Landauer-B\"uttiker's probe technique" can properly replicate different transport mechanisms: phase coherent nonresonant tunneling, ballistic behavior, and hopping conduction, to provide results consistent with experiments. Specifically, our simulations with the probe method recover the following central characteristics of charge transfer in molecular wires: (i) The electrical conductance of short wires falls off exponentially with molecular length, a manifestation of the tunneling (superexchange) mechanism. Hopping dynamics overtakes superexchange in long wires demonstrating an ohmic-like behavior. (ii) In off-resonance situations, weak dephasing effects facilitate charge transfer. Under large dephasing the electrical conductance is suppressed. (iii) At high enough temperatures, $k_BT/\epsilon_B>1/25$, with $\epsilon_B$ as the molecular-barrier height, the current is enhanced by a thermal activation (Arrhenius) factor. However, this enhancement takes place for both coherent and incoherent electrons and it does not readily indicate the underlying mechanism. (iv) At finite-bias, dephasing effects impede conduction in resonant situations. We further show that memory (non-Markovian) effects can be implemented within the Landauer-B\"uttiker's probe technique to model the interaction of electrons with a structured environment. Finally, we examine experimental results of electron transfer in conjugated molecular wires and show that our computational approach can reasonably reproduce reported values to provide mechanistic information.