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

Type of publication Peer-reviewed
Publikationsform Original article (peer-reviewed)
Author García-Suárez Víctor M, Lambert Colin J, Manrique David Zs, Wandlowski Thomas,
Project Electron Transport at the Nanoscale - An Electrochemical Approach II
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Original article (peer-reviewed)

Journal Nanotechnology
Volume (Issue) 25(20)
Page(s) 205402 - 205402
Title of proceedings Nanotechnology
DOI 10.1088/0957-4484/25/20/205402


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 Z el T 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 Z el T are unprecedented. We also evaluate the phonon contribution to the thermal conductance, which allows us to compute the full figure of merit ZT = Z el T/(1 + κ p/κ el), where κ p is the phonon contribution to the thermal conductance and κ el is the electronic contribution. For unstructured gold electrodes, κ p/κ el ≫ 1 and therefore strategies to reduce κ p are needed to realize the highest possible figure of merit.