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14. Redox control of thermopower and figure of merit in phase-coherent molecular wires.

Author

Garc�a-Su�rez, V�ctor M, Lambert, Colin J, Manrique, David Zs, and Wandlowski, Thomas

Subjects
*NANOWIRES, *OXIDATION-reduction reaction, *THERMOELECTRIC power, *HYDROQUINONE, *THERMAL conductivity, *PHONONS
Abstract

We demonstrate how redox control of intra-molecular quantum interference in phase-coherent molecular wires can be used to enhance the thermopower (Seebeck coefficient) S and thermoelectric figure of merit ZT of single molecules attached to nanogap electrodes. Using first principles theory, we study the thermoelectric properties of a family of nine molecules, which consist of dithiol-terminated oligo (phenylene-ethynylenes) (OPEs) containing various central units. Uniquely, one molecule of this family possesses a conjugated acene-based central backbone attached via triple bonds to terminal sulfur atoms bound to gold electrodes and incorporates a fully conjugated hydroquinonecentral unit. We demonstrate that both S and the electronic contribution ZelT to the figure of merit ZT can be dramatically enhanced by oxidizing the hydroquinone to yield a second molecule, which possesses a cross-conjugated anthraquinone central unit. This enhancement originates from the conversion of the pi-conjugation in the former to cross-conjugation in the latter, which promotes the appearance of a sharp anti-resonance at the Fermi energy. Comparison with thermoelectric properties of the remaining seven conjugated molecules demonstrates that such large values of S and ZelT are unprecedented. We also evaluate the phonon contribution to the thermal conductance, which allows us to compute the full figure of merit ZT�=�ZelT/(1�+ κp/κel), where κp is the phonon contribution to the thermal conductance and κel is the electronic contribution. For unstructured gold electrodes, κp/κel�≫⃒�1 and therefore strategies to reduce κp are needed to realize the highest possible figure of merit. [ABSTRACT FROM AUTHOR]

Published
2014
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