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

• Published in: Nanotechnology Vol. 25; no. 20; p. 205402
• Main Authors: García-Suárez, Víctor M, Lambert, Colin J, Manrique, David Zs, Wandlowski, Thomas
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 23.05.2014; Institute of Physics
• Subjects: Condensed matter: structure, mechanical and thermal properties; Electrodes; Electronics; Exact sciences and technology; Figure of merit; Gold; Lattice dynamics; molecular wires; OPE; Phonons; Phonons in low-dimensional structures and small particles; Physics; quantum interference; single molecule conductance; Thermal conductivity; Thermal properties of condensed matter; Thermal properties of small particles, nanocrystals, nanotubes; Thermoelectricity; thermopower; Wire; Gold; Dithiol; Molecular wires; Phonons; Seebeck effect; Quantum interference phenomena; Thermoelectric properties; Hydroquinone; Thermal properties; Thermoelectric materials; Thermal conductivity; Triple bond; Resonance; Figure of merit
• ISSN: 0957-4484; 1361-6528
• 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 + p el), where p is the phonon contribution to the thermal conductance and el is the electronic contribution. For unstructured gold electrodes, p el 1 and therefore strategies to reduce p are needed to realize the highest possible figure of merit.