Probing the correlation between morphology and optical anisotropy in ZnTPP films grown at different temperatures

Films of tetraphenylporphyrins (TPP) have been recently investigated for their ability in protecting graphite electrode surfaces from degradation. The effectiveness of the protection depends on both the molecule–substrate and molecule–molecule interactions, which determine the type of film growth and its morphology. For instance, meso-tetraphenyl porphyrin-Zn(II) (ZnTPP) arranges differently depending on the substrate, conditions used for deposition and post-growth treatments. Since many parameters influence film morphology and strategies to reach an efficient protective coverage, here we investigate the role of the substrate temperature in determining morphological variations in thick ZnTPP films. ZnTPP molecules were sublimated by an organic molecular beam epitaxy (OMBE) system on a highly oriented pyrolytic graphite (HOPG) substrate, controlled in temperature by a variable temperature cryostat. The film morphology was characterized by atomic force microscopy (AFM), whereas the type of growth by reflectance anisotropy spectroscopy (RAS), which is highly sensitive to the orientation and type of arrangement of molecules on the substrate; changes in the film valence electronic structure were monitored in situ by UV photoemission spectroscopy (UPS). The comparison between AFM and RAS investigations clearly correlates a ZnTPP film morphology to the characteristic optical signal, both influenced by temperature conditions and diffusivity of molecules on the substrate.