Enhancing the optical anisotropy of ZnTPP films by increasing the heterogenous nucleation density.

Vacuum-deposited films of tetraphenylporphyrins (TPP) have been recently investigated for their ability in protecting the electrode surfaces from corrosion. The protective action of porphyrin films is related to both the molecule-substrate and molecule-molecule interactions, determining the type of film growth and its morphology. For example, ZnTPP shows different types of molecular aggregation, depending on the deposition and post-growth procedures. Finding the best strategy to reach a well-defined target for the film morphology plays a key role for the realization of effective protective coatings for electrodes. Here, we investigate the nucleation density of crystals in thin ZnTPP films, as a function of the substrate temperature. ZnTPP molecules were sublimated by an organic molecular beam epitaxy (OMBE) system onto a graphite substrate, controlled in temperature by using a cryostat. The morphology and type of growth were characterized by atomic force microscopy (AFM) and reflectance anisotropy spectroscopy (RAS), the latter being highly sensitive to the orientation and type of aggregation of ZnTPP on the substrate, which originate the optical anisotropy of the molecular film.