From Chemistry to Functionality: Trends for the Length Dependence of the Thermopower in Molecular Junctions

TL;DR

This study systematically investigates how the thermopower in molecular junctions varies with chain length, revealing complex behaviors influenced by transmission resonances and gateway states, challenging simple transport models.

Contribution

It provides a comprehensive ab-initio analysis of length-dependent thermopower in molecular junctions, highlighting the role of gateway states and limitations of standard DFT methods.

Findings

Thermopower can increase, decrease, or change sign with chain length.

Gateway states significantly influence thermopower and its length dependence.

DFT-based predictions are limited; advanced methods like GW offer improvements.

Abstract

We present a systematic ab-initio study of the length dependence of the thermopower in molecular junctions. The systems under consideration are small saturated and conjugated molecular chains of varying length attached to gold electrodes via a number of different binding groups. Different scenarios are observed: linearly increasing and decreasing thermopower as function of the chain length as well as positive and negative values for the contact thermopower. Also deviation from the linear behaviour is found. The trends can be explained by details of the transmission, in particular the presence, position and shape of resonances from gateway states. We find that these gateway states do not only determine the contact thermopower, but can also have a large influence on the length-dependence itself. This demonstrates that simple models for electron transport do not apply in general and that chemical trends are hard to predict. Furthermore, we discuss the limits of our approach based on Density Functional Theory and compare to more sophisticated methods like self-energy corrections and the GW theory.