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

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 a function of the chain length as well as positive and negative values for the contact thermopower. Also deviation from the linear behavior 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 not only do determine the contact thermopower but also can 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 our approach to more sophisticated methods like self-energy corrections and the GW theory.