Length-dependent conductance and thermopower in single-molecule junctions of dithiolated oligophenylene derivatives

TL;DR

This study provides a theoretical analysis of how conductance decreases exponentially while thermopower increases linearly with molecular length in dithiolated oligophenylene junctions, and explores tuning these properties via side groups.

Contribution

It offers a comprehensive theoretical investigation of length-dependent transport properties and the effect of side groups in single-molecule junctions, supported by density-functional and tight-binding models.

Findings

Conductance decays exponentially with length

Thermopower increases linearly with length

Side groups can tune transport properties

Abstract

We study theoretically the length dependence of both conductance and thermopower in metal-molecule-metal junctions made up of dithiolated oligophenylenes contacted to gold electrodes. We find that while the conductance decays exponentially with increasing molecular length, the thermopower increases linearly as suggested by recent experiments. We also analyze how these transport properties can be tuned with methyl side groups. Our results can be explained by considering the level shifts due to their electron-donating character as well as the tilt-angle dependence of conductance and thermopower. Qualitative features of the substituent effects in our density-functional calculations are explained using a tight-binding model. In addition, we observe symmetry-related even-odd transmission channel degeneracies as a function of molecular length.