Thermopower Measurements of Highly Conducting Single-Molecule Devices

We measure the conductance ($G)$ and thermopower ($S)$ of highly conducting single-molecule junctions with Au electrodes. The junctions are formed and measured using a scanning tunneling microscope-based break-junction technique. The target molecules are synthesized with SnMe$_{\mathrm{3}}$ terminations that cleave off \textit{in situ}, allowing for the formation of direct Au-C covalent bonds to the electrodes[1,2]. We compare the conductance and thermopower for two families of molecules: pi-conjugated polyphenyls, which have a high conductance and thermopower, and sigma-bonded alkyl systems, where we observe a significant thermopower despite the low conductance. For these measurements, we use the most probable thermopower to determine a power factor, \textit{GS}$^{\mathrm{2}}$, for each molecular junction studied. Our results show that the molecular thermopower increases systematically and non-linearly with molecular length and also that the power factor is exceptionally large for the case of the biphenyl. [1] Z. L. Cheng, R. Skouta, H. Vazquez\textit{ et al.}, Nat. Nano. \textbf{6}, 353 (2011). [2] W. Chen, J. R. Widawsky, H. V\'{a}zquez\textit{ et al.}, J. Am. Chem. Soc. \textbf{133}, 17160 (2011).