Electrostatic control of thermoelectricity in molecular junctions

Abstract

Molecular junctions hold significant promise for efficient and high-power-output thermoelectric energy conversion1,2,3. Recent experiments have probed the thermoelectric properties of molecular junctions4,5,6,7. However, electrostatic control of thermoelectric properties via a gate electrode has not been possible due to technical challenges in creating temperature differentials in three-terminal devices. Here, we show that extremely large temperature gradients (exceeding 1 × 109K m−1) can be established in nanoscale gaps bridged by molecules, while simultaneously controlling their electronic structure via a gate electrode. Using this platform, we study prototypical Au–biphenyl-4,4′-dithiol–Au and Au–fullerene–Au junctions to demonstrate that the Seebeck coefficient and the electrical conductance of molecular junctions can be simultaneously increased by electrostatic control. Moreover, from our studies of fullerene junctions, we show that thermoelectric properties can be significantly enhanced when the dominant transport orbital is located close to the chemical potential (Fermi level) of the electrodes. These results illustrate the intimate relationship between the thermoelectric properties and charge transmission characteristics of molecular junctions and should enable systematic exploration of the recent computational predictions1,2,3 that promise extremely efficient thermoelectric energy conversion in molecular junctions.

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Figure 1: Experimental platform for tuning the thermoelectric properties of molecular junctions.
Figure 2: Effect of tuning the electronic structure on the thermoelectric properties of Au–BPDT–Au junctions.
Figure 3: Effect of tuning the electronic structure on the thermoelectric properties of Au-C60-Au junctions.

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Acknowledgements

P.R. acknowledges support from the Office of Naval Research (award no. N00014-13-1-0320; nanofabrication of devices), the Department of Energy–Basic Energy Sciences (a grant from the Scanning Probe Microscopy Division, award no. DE-SC0004871; scanning thermal microscopy), the Air Force Office of Scientific Research (award no. FA9550-12-1-0058; instrumentation) and the University of Michigan–Ben Gurion University of the Negev Joint Research Collaboration (device modelling). All authors acknowledge the Lurie Nanofabrication Facility (LNF) for facilitating the nanofabrication of devices.

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The project was conceived by P.R. Thermopower gating and other electrical measurements were performed by Y.K. and W.J. Nanoscale thermal imaging was performed by K.K. and W.L. Finite-element thermal modelling was performed by W.J. EBJIHs were designed and nanofabricated by W.J., Y.K. and K.K. The manuscript was written by P.R., Y.K. and W.J., with comments and input from all authors.

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Correspondence to Pramod Reddy.

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The authors declare no competing financial interests.

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Kim, Y., Jeong, W., Kim, K. et al. Electrostatic control of thermoelectricity in molecular junctions. Nature Nanotech 9, 881–885 (2014). https://doi.org/10.1038/nnano.2014.209

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