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

C. Lungani Mthembu, Ryan C. Chiechi*

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

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.

Original languageEnglish
Article numberd4tc03895a
Pages (from-to)1272-1280
Number of pages9
JournalJournal of Materials Chemistry C
Volume13
Issue number3
Early online date15-Nov-2024
DOIs
Publication statusPublished - 21-Jan-2025

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