The team found that optimizing the electronic states of a single molecule bridging electrodes could yield a large thermoelectric effect. The contact between the molecule and electrodes also influences the thermoelectric behaviour.
The team first fabricated structures consisting of gold electrodes bridged by various single molecules. The distance between the electrodes, which were held under a temperature gradient, was repeatedly increased and decreased while the electrical conductance and thermovoltage of each structure was measured. The team simultaneously measured the electrical conductance and thermovoltage of molecules with different groups, anchoring the molecules to the electrodes at room temperature in vacuum.
"We investigated the thermoelectric characteristics of various single benzene-based molecules with an emphasis on influence of their junction structures," said researcher Makusu Tsutsui. "The molecules displayed different behaviour depending on their electrode-anchoring groups, and all molecule types displayed multiple thermovoltage states."
The multiple thermovoltage states of the molecules were investigated by thermoelectric measurements and theoretical analysis. The largest thermoelectric effect was observed for structures containing a stretched thiol linkage with the gold electrode. The increased thermovoltage of the structures with a stretched gold-thiol bond was attributed to this configuration shifting the energy level of the molecule involved in electron transport to a more favorable position.
"The observed dependence of thermovoltage on the anchoring group in the junction structures reveals a way to modulate the thermoelectric performance of single-molecule devices," he said.
The group's results expand our understanding of how the geometry of a single-molecule device can influence its thermoelectric figure of merit. These findings should contribute to the development of single-molecule thermoelectric devices that can efficiently derive electricity from heat.
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