Thermopower of molecular junctions: Tunneling to hopping crossover in DNA

TL;DR

This paper investigates the thermopower and conductance in molecular junctions, identifying different transport mechanisms and their crossover, especially in DNA, through analytical models and numerical simulations aligned with experimental data.

Contribution

It introduces a combined analytical and numerical approach to distinguish transport mechanisms in molecular junctions, including DNA, and demonstrates the tunneling-to-hopping crossover in thermopower.

Findings

Identification of tunneling, ballistic, and hopping regimes via conductance and thermopower.

Analytical expressions for thermopower in different transport regimes.

Observation of tunneling-to-hopping crossover in DNA thermopower consistent with experiments.

Abstract

We study the electrical conductance $G$ and the thermopower $S$ of single-molecule junctions, and reveal signatures of different transport mechanisms: off-resonant tunneling, on-resonant coherent (ballistic) motion, and multi-step hopping. These mechanisms are identified by studying the behavior of $G$ and $S$ while varying molecular length and temperature. Based on a simple one-dimensional model for molecular junctions, we derive approximate expressions for the thermopower in these different regimes. Analytical results are compared to numerical simulations, performed using a variant of B\"uttiker's probe technique, the so-called voltage-temperature probe, which allows us to phenomenologically introduce environmentally-induced elastic and inelastic electron scattering effects, while applying both voltage and temperature biases across the junction. We further simulate the thermopower of GC-rich DNA molecules with mediating A:T blocks, and manifest the tunneling-to-hopping crossover in both the electrical conductance and the thermopower, in accord with measurements by Y. Li et al., Nature Comm. 7, 11294 (2016).