The Effect of Molecular Orientation on the Potential of Porphyrin−Metal Contacts

The effect of molecular orientation at metal contacts on interface properties was determined by examining the local work function of porphyrin
on atomically smooth graphite. The orientation was varied by self-assembly from the vapor phase, and the local potential was quantified by
Kelvin force microscopy (scanning surface potential microscopy). When the porphyrin ring is oriented parallel to the substrate, the surface
work function is 50 mV less than that of the highly ordered pyrolytic graphite; in contrast, when the porphyrin molecular plane is oriented
perpendicular to the substrate, the surface work function is unchanged. The orientation dependence of the surface work function is determined
by the geometric relationships between the delocalized charge densities in the molecule and substrate and possible interface bonding. The
differences in interface properties with molecular configuration have important design implications to molecular electronic and organic electronic
devices.