Molecular Thermoelectricity in EGaIn-Based Molecular Junctions

초록

Conspectus Understanding the thermoelectric effects that convert energy between heat and electricity on a molecular scale is of great interest to the nanoscience community. As electronic devices continue to be miniaturized to nanometer scales, thermoregulation on such devices becomes increasingly critical. In addition, the study of molecular thermoelectricity provides information that cannot be accessed through conventional electrical conductance measurements. The field of molecular thermoelectrics aimsto explore thermoelectric effects in electrode-molecule-electrode tunnel junctions and draw inferences on how the (supra)molecular structure of active molecules is associated with their thermo power. In this Account, we introduce a convenient and useful junction technique that enables thermovoltage measurements of one molecule thick films, self-assembled monolayers (SAMs), with reliability, and discuss the atomic-detailedstructure-thermopower relations established by the technique. The technique relies on a microelectrode composed of non-Newtonian liquidmetal, eutectic gallium-indium (EGaIn) covered with a nativegallium oxide layer. The EGaIn electrode makes it possible to formthermoelectric contacts with the delicate structure of SAMs in a noninvasivefashion. A defined interface between SAM and the EGaIn electrode allowstime-effective collection of large amounts of thermovoltage data,with great reproducibility, efficiency, and reliable interpretationand statistical analysis of the data. We also highlight recent effortsto utilize the EGaIn technique for probing molecular thermoelectricityand structure-thermopower relations. Using the technique, it was possibleto unravel quantum-chemical mechanisms of thermoelectric functions,based on the Mott formula, in SAM-based large-area junctions, whichin turn led us to set various hypotheses to boost the Seebeck coefficient.By validating the hypotheses again with the EGaIn technique, we revealedthat the thermopower of junction increases through the reduction ofthe energy offset between accessible molecular orbital energy leveland Fermi level or the tuning of broadening of the orbital energylevel. Such alterations in the shape of energy topography of junctioncould be achieved through structural modifications in anchoring groupand molecular backbone of SAM, and the bottom electrode. Molecularthermoelectrics offers a unique opportunity to build a well-definednanoscale system and isolate an effect of interest from others, advancingfundamental understanding of charge transport across individual moleculesand molecule-electrode interfaces. In the Account, we showed our recentwork involving carefully designed molecular system that are relevantto answering the question of how thermopower differs between the tunnelingand thermal-hopping regimes. The field of molecular thermoelectricsneeds to address practical application-related issues, particularlymolecular degradation in thermal environments. In this regard, wesummarized the results highlighting the thermal instability of SAM-basedjunctions based on a traditional thiol anchor group and how to circumventthis problem. We also discussed the power factor (PF)-a practicalparameter representing the efficiency for converting heat into electricity-ofSAMs, evaluated using the EGaIn technique. In the Conclusion sectionof this Account, we present future challenges and perspectives.

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