Simultaneous Determination
of Conductance and Thermopower
of Single Molecule Junctions
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Abstract
We report the first concurrent determination of conductance
(<i>G</i>) and thermopower (<i>S</i>) of single-molecule
junctions via direct measurement of electrical and thermoelectric
currents using a scanning tunneling microscope-based break-junction
technique. We explore several amine-Au and pyridine-Au linked molecules
that are predicted to conduct through either the highest occupied
molecular orbital (HOMO) or the lowest unoccupied molecular orbital
(LUMO), respectively. We find that the Seebeck coefficient is negative
for pyridine-Au linked LUMO-conducting junctions and positive for
amine-Au linked HOMO-conducting junctions. Within the accessible temperature
gradients (<30 K), we do not observe a strong dependence of the
junction Seebeck coefficient on temperature. From histograms of thousands
of junctions, we use the most probable Seebeck coefficient to determine
a power factor, <i>GS</i><sup>2</sup>, for each junction
studied, and find that <i>GS</i><sup>2</sup> increases with <i>G</i>. Finally, we find that conductance and Seebeck coefficient
values are in good quantitative agreement with our self-energy corrected
density functional theory calculations