Self-organization of phthalocyanines on Al₂O₃ (1120) in aligned and ordered films

We studied the self-organization process of F₁₆CuPc films (20–30 ML) on stepped Al₂O₃ (1120) substrates. X-ray diffraction measurements revealed a highly ordered layered structure with the molecules in a nearly upright configuration. The morphology, investigated by atomic force microscopy, consisted of long (several microns) and narrow (20–100 nm) needlelike terraces unidirectionally aligned along one of the main crystallographic directions of the Al₂O₃ (1120) surface. High resolution atomic force microscopy images revealed in-plane molecular order with the molecular stacking direction parallel to the needlelike terraces. Such anisotropic morphology is the result of a self-organization process of F₁₆CuPc in elongated crystallites driven to a preferential orientation by the interaction with the substrate. Spectroscopic ellipsometry showed that these films exhibit anisotropic optical properties correlated with the molecular arrangement.