Determination of Energy Level Alignment and Coupling
Strength in 4,4′-Bipyridine Single-Molecule Junctions
- Publication date
- Publisher
Abstract
We measure conductance and thermopower
of single Au–4,4′-bipyridine–Au
junctions in distinct low and high conductance binding geometries
accessed by modulating the electrode separation. We use these data
to determine the electronic energy level alignment and coupling strength
for these junctions, which are known to conduct through the lowest
unoccupied molecular orbital (LUMO). Contrary to intuition, we find
that, in the high-conductance junction, the LUMO resonance energy
is further away from the Au Fermi energy than in the low-conductance
junction. However, the LUMO of the high-conducting junction is better
coupled to the electrode. These results are in good quantitative agreement
with self-energy corrected zero-bias density functional theory calculations.
Our calculations show further that measurements of conductance and
thermopower in amine-terminated oligophenyl–Au junctions, where
conduction occurs through the highest occupied molecular orbitals,
cannot be used to extract electronic parameters as their transmission
functions do not follow a simple Lorentzian form