Dependence of Interface Properties on Molecular Orientation at Porphyrin – Metal Contacts
Maxim Nikiforov, Ulrich Zerwek, Christian Loppacher*, Tae-Hong Park, Michael J. Therein, Lukas Eng*, Dawn Bonnell1
1University of Pennsylvania, Philadelphia, USA. *Technical University of Dresden, Germany
Developments in molecular electronics over the last 5 years emphasize that the contact between molecules and electric contacts often define the behavior of the device. Controlling the structure and consequent properties of these junctions is of paramount importance. To date, it has not been possible to probe the relation between molecular structure and properties at the nm scale. The combination of molecular structure and local property measurement is demonstrated here.
Vapor deposition of TET – H2 – TET porphyrin on HOPG results in islands that self assemble into 2 structures; one with the molecules oriented perpendicular to the film, one with a parallel orientation. The molecular structures are determined by nc-AFM. Variations in surface potential determined by Kelvin Force Microscopy (Scanning Surface Potential Microscopy) are correlated with the orientation of the porphyrin monolayers. The difference between the potential of the two structures is 50mV.
The difference in work function with orientation reflects a difference in the coupling between the molecule and the substrate. Perpendicular porphyrin does not alter the work function implying the absence of reaction and self assembly is dominated by van der Walls interactions. The decrease in work function with parallel orientation is indicative of a substrate-molecule interaction.
The is a direct measure of the effect of molecular orientation on the electronic properties of the junction.