Conversion of Heat to Electricity in Single Molecules Explored

A schematic model of a single-molecule device.

A schematic model of a single-molecule device. Credit: Osaka University

A Japanese team of researchers has investigated how geometry can affect electricity generation from heat by single-molecule devices. The groundbreaking research is premised on the direct conversion of a temperature difference into electricity, which is known as the thermoelectric effect. This approach is environmentally friendly and can directly harvest electricity from heat.

How is the ability of a material to convert heat to electricity measured? It is measured by its thermoelectric figure of merit. So, materials which have a high thermoelectric figure of merit are more widely used in energy harvesting. There could be an increase in the thermoelectric figure of merit with the quantum confinement effects in nanomaterials arising from the discrete electronic states. The bridging of two electrodes by a single molecule can display quantum confinement. A large thermoelectric effect can be yielded by the optimisation of the electronic states of a single molecule bridging electrodes. The thermoelectric behaviour would be further influenced by the contact between the molecule and the electrodes.

The researchers at Osaka University have made groundbreaking investigations on how the thermoelectric behaviour of the molecule could be influenced by the geometry of single molecule-electrode contacts. It was recently reported that they concurrently measured the electrical conductance and the thermovoltage of molecules with diverse groups anchoring the molecules to the electrodes in vacuum at room temperature.

The team started with fabricating structures consisting of gold electrodes bridged by single molecules. The distance between electrodes which were held under a temperature gradient was continually decreased and increased with the intent to measure each structure’s electrical conductance and thermovoltage.

Makusu Tsutsui, a corresponding author said that they investigated the thermoelectric characteristics of various single benzene-based molecules with an emphasis on influence on their junction structures. Tsutsui added that the molecules displayed different behaviour depending on their electrode-anchoring groups and all molecule types displayed thermovoltage states.

The multiple thermovoltage states of the molecules were further investigated by thermoelectric measurements and theoretical analysis. The structures which contained a stretched thiol linkage with the gold electrode were observed with the largest thermoelectric effect. The increased thermovoltage of the structures with a stretched gold-thiol bond was attributed to the configuration.

The research team’s results contribute to how the geometry of a single-molecule device could influence the thermoelectric figure of merit. The findings further contribute to the development of single-molecule thermoelectric devices that can resourcefully derive electricity from heat.

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