  We report the first concurrent determination of conductance (G) and
thermopower (S) 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 1000's of junctions, we use the
most probable Seebeck coefficient to determine a power factor, GS^2, for each
junction studied, and find that GS^2 increases with G. Finally, we find that
conductance and Seebeck coefficient values are in good quantitative agreement
with our self-energy corrected density functional theory calculations.
