Mapping the Transmission Functions of Single-Molecule Junctions

Charge transport phenomena in single-molecule junctions
are often dominated by tunneling, with a transmission function dictating
the probability that electrons or holes tunnel through the junction. Here,
we present a new and simple technique for measuring the transmission
functions of molecular junctions in the coherent tunneling limit, over an
energy range of 1.5 eV around the Fermi energy. We create molecular
junctions in an ionic environment with electrodes having different exposed
areas, which results in the formation of electric double layers of dissimilar
density on the two electrodes. This allows us to electrostatically shift the molecular resonance relative to the junction Fermi levels
in a manner that depends on the sign of the applied bias, enabling us to map out the junction’s transmission function and
determine the dominant orbital for charge transport in the molecular junction. We demonstrate this technique using two groups
of molecules: one group having molecular resonance energies relatively far from EF and one group having molecular resonance
energies within the accessible bias window. Our results compare well with previous electrochemical gating data and with
transmission functions computed from first principles. Furthermore, with the second group of molecules, we are able to examine
the behavior of a molecular junction as a resonance shifts into the bias window. This work provides a new, experimentally simple
route for exploring the fundamentals of charge transport at the nanoscale