Utilizing Nearest-Neighbor Interactions To Alter Charge Transport Mechanisms in Molecular Assemblies of Porphyrins on Surfaces
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2015 American Chemical Society. 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 lowerature 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.
