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Complete mapping of the thermoelectric properties of a single molecule

Abstract

Theoretical studies suggest that mastering the thermocurrent through single molecules can lead to thermoelectric energy harvesters with unprecedentedly high efficiencies.1,2,3,4,5,6 This can be achieved by engineering molecule length,7 optimizing the tunnel coupling strength of molecules via chemical anchor groups8 or by creating localized states in the backbone with resulting quantum interference features.4 Empirical verification of these predictions, however, faces considerable experimental challenges and is still awaited. Here we use a novel measurement protocol that simultaneously probes the conductance and thermocurrent flow as a function of bias voltage and gate voltage. We find that the resulting thermocurrent is strongly asymmetric with respect to the gate voltage, with evidence of molecular excited states in the thermocurrent Coulomb diamond maps. These features can be reproduced by a rate-equation model only if it accounts for both the vibrational coupling and the electronic degeneracies, thus giving direct insight into the interplay of electronic and vibrational degrees of freedom, and the role of spin entropy in single molecules. Overall these results show that thermocurrent measurements can be used as a spectroscopic tool to access molecule-specific quantum transport phenomena.

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Fig. 1: The experimental system.
Fig. 2: Maps of electronic and thermoelectric properties.
Fig. 3: Comparison to theoretical model.
Fig. 4: Temperature dependence of the differential conductance.

Data availability

Data for the main text are available online at https://doi.org/10.4121/13264931. Any other supporting data can be made available upon request to the corresponding author.

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Acknowledgements

We acknowledge financial support from the EU (Marie-Skłodowska-Curie 748642-TherSpinMol and 707252-SpinReMag; ERC-StG-338258-OptoQMol, ERC-CoG-773048-MMGNRs, FET-767187-QuIET); the Glasstone Research Fellowship; the Royal Society (URF and grant funds) and the Royal Society of Edinburgh; the EPSRC (EP/T01377X/1, EP/N017188/1-QuEEN, EP/R513295/1-Doctoral Prize); and the NWO/OCW (Frontiers of Nanoscience programme and Vrij Programma-CISS). We acknowledge use of the University of Oxford Advanced Research Computing facility (https://doi.org/10.5281/zenodo.22558) and the Quest high-performance computing facility at Northwestern University, jointly supported by the Office of the Provost, the Office for Research and Northwestern University Information Technology.

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Contributions

P.G. conceived the project and performed the electrical/thermoelectric measurements. P.G. and J.K.S. evaluated the data and performed the fitting. J.K.S. developed the theoretical transport models supervised by E.M.G. J.d.B. and C.H. supported the experiments. M.v.d.S. fabricated the devices. The experimental work was supervised by P.G. and H.S.J.v.d.Z. J.J.L.R. and L.B. synthesized the compound. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Corresponding author

Correspondence to Pascal Gehring.

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The authors declare no competing interests.

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Supplementary Information

Supplementary Figs. 1–13, Sections 1–14 and Tables I–III.

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Gehring, P., Sowa, J.K., Hsu, C. et al. Complete mapping of the thermoelectric properties of a single molecule. Nat. Nanotechnol. 16, 426–430 (2021). https://doi.org/10.1038/s41565-021-00859-7

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