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May 2012

Probing Energy Conversion at the Nanoscale, One Molecule at a Time

Figures: Left, computationally optimized geometries of pyridine and amine-based molecules in gold junctions. Right, schematic of scanning electron microscope-based setup used to trap and measure individual molecules.

Molecular Foundry user Latha Venkataraman at Columbia University and colleagues have simultaneously probed two disparate electrical properties – conductance and thermopower – in some of the smallest circuits imaginable, individual molecular junctions. This work advances our understanding of charge and energy transport at the molecular level, a new frontier in nanoscale engineering.

Venkataraman's team at Colombia used a scanning tunneling microscope to trap individual amine and pyridine molecules between a sharp gold tip and a substrate. Using a novel setup to vary temperature and electrical biases while the molecule remained trapped, the researchers simultaneously measured conductivity and thermopower, the electrical bias produced by thermal gradient.

Foundry staff scientist Jeff Neaton and colleagues compared the measurements to state-of-the-art first-principles calculations, revealing an unexpectedly complex relationship between conductance and thermopower arising from chemical details of the metal-molecule contact, not the simple relation usually assumed. These findings critically advance knowledge of molecular-level charge transport, laying the groundwork for molecular-scale engineering of thermoelectric and other energy conversion materials.

J.R. Widawsky, P. Darancet, J.B. Neaton, L. Venkataraman, 'Simultaneous Determination of Conductance and Thermopower of Single Molecule Junctions', Nano Letters, 12, 354−358, (2012).
This work was supported in part by the EFRC program of the U.S. Department of Energy (DOE) under Award No. DE-SC0001085 and the ACS-PRF program. L.V. thanks the Packard Foundation for support. Portions of this work were performed at the Molecular Foundry and within the Helios Solar Energy Research Center, and both were supported by the Office of Basic Energy Sciences of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. We acknowledge NERSC for computing resources.