Porphyrin-Metal Interface Properties
Deposition of a nominal monolayer of TET – H2 – TET porphyrin on HOPG results in islands with 2 step heights: 0.6nm and 1.55 nm, Fig. a. Layer thicknesses imply that in the islands with ~ 0.7 nm height the plane of the porphyrin ring is perpendicular to the substrate, while in the islands with ~ 1.6 nm height the ring is parallel to the substrate.
Both layers (A and B) assemble into laterally ordered structures. Fig. d and e compare molecular resolution nc-AFM contrast of both layers. Within the layer A the porphyrin ring is oriented perpendicular to the substrate, superposition of the molecular structure oriented to match the lattice dimensions and the topographic contrast is shown in d. The structure on island B is determined in similar fashion. Based on the lattice parameters of the monolayer and the fact that the porphyrin ring is parallel to the substrate from c, the arrangement of porphyrin molecules in e is proposed. The phenyl rings are situated at the highest contrast since the alkane chains extend above them.
In order to relate these structures to interface properties, KFM was performed consecutively to nc-AFM, Fig. b. Variations in surface potential are correlated with the locations of the porphyrin monolayers. The difference between the potential of step A and the graphite substrate was below the energy resolution, i.e. <5 mV. The potential on island A, Vstep A = 0.64 V, and island B, Vstep B = 0.59 V, differs by 50mV.
The difference in work function with orientation reflects a difference in the coupling between the molecule and the substrate. A graphite surface presents a satisfied p-orbital which is not chemically reactive. The fact that the porphyrin oriented perpendicular to the substrate does not alter the work function implies the absence of reaction with the surface. In this case self assembly would be dominated by van der Walls interactions. The decrease in work function that occurs when the molecule is oriented parallel is indicative of a substrate-molecule interaction. The orientation of the p-systems in the graphite and molecule in this configuration might be amenable to overlap, but the distance above the surface (0.8 nm) makes direct interaction unlikely. A time varying induced dipole across this separation would result in a work function difference.