Length-dependent thermopower of self-assembled monolayers of alkanethiolates depends on direction of temperature gradient

Abstract

Self-assembled monolayers of alkanethiols on gold are often used as a benchmark for molecular junctions, particularly as a reference for length-dependence. However, their thermopower decreases with molecular length and is bifurcated. While the bifurcation can be explained by the presence of gateway states, the length-dependence directly contradicts theoretical predictions. We reproduced both experimentally using conical tips of eutectic Ga–In (EGaIn) to form junctions. We then show that this unusual length-dependence arrises from the direction of the applied thermal gradient; when the gold substrate is cooled instead of heated, thermopower increases with length monotonically as predicted by theory. This unprecedented relationship between length-dependence and direction of thermal gradient persists over a very large range of absolute temperature. We therefore ascribe it to shifts in level-alignment induced by the electric field in the leads, which is present experimentally, neglected in theoretical modeling and is determined by the direction of the thermal gradient.