Abstract
Theoretical studies suggest that mastering the thermocurrent through single molecules can lead to thermoelectric energy harvesters with unprecedentedly high efficiencies.1-6 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.
Highlights
The single set of roughly equidistant peaks observed in the conductance maps (Figures 2, and Supplementary Fig.2 and 4) suggest that the electronic degrees of freedom are predominantly coupled to a single low-frequency vibrational mode of frequency ω[3]
We can compare the experimental and theoretical power factors
Since we have demonstrated the importance of electronic degeneracies above, we will describe this device using a rate equation model discussed above
Summary
For a weakly coupled molecule with large charging energy Γ kBT e2/C and a doubly degenerate ground state the conductance is given by:[1]. Where f ( ) is the Fermi-Dirac distribution, and SN and SN−1 are the entropies of the molecular quantum dot occupied with N and N − 1 electrons, respectively. To derive this expression, the following assumptions were made. Where SN − SN−1 is the difference in entropy between the N and N − 1 charge ground states. The position of the conductance peak changes linearly with temperature with a slope given by the change in entropy of the molecule when one extra electron is added.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
