Oscillating Seebeck coefficients in π-stacked molecular junctions

• Published in: RSC advances Vol. 8; no. 44; pp. 24711 - 24715
• Main Authors: Al-Khaykanee, Mohsin K, Ismael, Ali K, Grace, Iain, Lambert, Colin J
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 2018; The Royal Society of Chemistry
• Subjects: Chemistry; Covalent bonds; Electrodes; Mach-Zehnder interferometers; Molecular chains; Seebeck effect; Thermoelectricity
• ISSN: 2046-2069; 2046-2069
• DOI: 10.1039/c8ra04698k

When a linear aromatic molecule within a nanogap is bound only to a source electrode, and an adjacent molecule is bound only to a drain electrode, the two molecules can interact pi-pi stacking, which allows electrons to flow from the source to the drain, pi-pi bonds. Here we investigate the thermoelectric properties of such junctions, using mono-thiol oligo-phenylene ethynylene (OPE3)-based molecules as a model system. For molecules which are -connected to the electrodes, we show that the Seebeck coefficient is an oscillatory function of the length of the pi-pi overlap region and exhibits large positive and negative values. This bi-thermoelectric behavior is a result of quantum interference within the junction, which behaves like a molecular-scale Mach-Zehnder interferometer. For junctions formed from molecular monolayers sandwiched between planar electrodes, this allows both hole-like and electron-like Seebeck coefficients to be realized, by careful control of electrode separation On the other hand for -connected molecules, the Seebeck coefficient is insensitive to , which may be helpful in designing resilient junctions with more stable and predictable thermoelectric properties.