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Thermopower of amine-gold-linked aromatic molecular junctions from first principles.

Using a self-energy corrected scattering-state approach based on density functional theory (DFT), we explain recent measurements of the thermopower or the Seebeck coefficient, S, for oligophenyldiamine-gold single-molecule junctions and show that they are consistent with separate measurements of their electrical conductance, G. Our calculations with self-energy corrections to the DFT electronic states in the junction predict low-bias S and G values in good quantitative agreement with experiments. We find S varies linearly with the number of phenyls N, with a gradient β(S) of 2.1 μV/K, in excellent agreement with experiment. In contrast, DFT calculations without self-energy corrections overestimate both S and β(S) (with a DFT value for β(S) three times too large). While β(S) is found to be a robust quantity independent of junction geometry, the computed values of S show significant sensitivity to the contact atomic structure-more so than the computed values of G. This observation is consistent with the experimentally measured spreads in S and G for amine-Au junctions. Taken together with previous computations of the electrical conductance (as reported in Quek, S. Y.; et al., Nano Lett. 2009, 9, 3949), our calculations of S conclusively demonstrate, for the first time, the consistency of two complementary yet distinct measurements of charge transport through single-molecule junctions and substantiate the need for an accurate treatment of junction electronic level alignment to describe off-resonant tunneling in these junctions.