Utilizing Nearest-Neighbor Interactions To Alter Charge
Transport Mechanisms in Molecular Assemblies of Porphyrins on Surfaces
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Abstract
When
tunneling is the dominant mechanism of charge transport in
a molecular junction, the conductivity of the junction is largely
insensitive to chemical and structural perturbations which do not
impact the overall length of the junction. This severely hampers the
seemingly limitless potential of molecules to modulate charge transport
at interfaces and their application in a host of device designs. This
is a particular challenge for molecules baring insulating features
like saturated hydrocarbons which decouple functional groups from
the surface. Such decoupling groups increase the energy required to
isolate charge on the molecule, pushing transport into the tunneling
regime in many cases. Herein, we demonstrate that, through enhancement
of nearest neighbor interactions, lateral delocalization of charge
states in molecular islands can be used to shift transport out of
the tunneling regime to the more efficient, and more chemically tunable,
charge-hopping regime. In a previous study, it was found that through-bond
tunneling was the dominant mechanism of charge transport through a
hydrocarbon-tethered free-base porphyrin thiol. With coordination
of zinc(II), the formation of large molecular islands in an alkanethiol
matrix on a Au(111) surface was facilitated. Bias-induced switching
and unphysical tunneling efficiencies observed by scanning tunneling
microscopy of these molecular islands, as well as Coulomb blockade
observed in low-temperature crossed-wire tunnel junction measurements,
indicate charge hopping becomes the dominant mechanism of transport
in the molecular islands, whereas transport in single molecules was
consistent with through-bond tunneling. These results elucidate the
basis for functional conductivity–structure and supramolecular
relationships that may be employed in the design of molecular junctions
in organic thin films