Revealing Transmission in Metal-Molecule Junctions Using Length Dependant Thermopower Measurements

Conductance in metal-molecule junctions is known to trend with molecular endgroups, backbone, and length, but a more complete picture of the junction's transmission structure has been hitherto elusive. We now report complimentary trends in the junction's thermopower ($S)$ that reveal length dependent changes in molecular orbital alignment and coupling with contact states. Phenylenediamines, phenylenedithiols, and alkanedithiols trapped between gold contacts were examined. $S$ increases linearly with length for phenylenediames and dithiols while it decreases linearly in alkanedithiols. Comparison of this data suggests that the molecular backbone determines the length dependence of $S$, while the endgroup determines the zero-length, or contact $S$. Transport in phenylenes was dominated by the HOMO, which moves closer to the Fermi energy of the contacts as \textit{$\sim $1/L}, and broadens due to contact coupling as \textit{$\sim $e}$^{-L}$. In contrast, the decreasing trend in $S$ for alkanedithiols suggests that transmission is largely effected by gold-thiol gap states between the HOMO and LUMO.