Polymorphism in phthalocyanine thin films: Mechanism of the alpha-beta transition

Two different polymorphic forms of free base phthalocyanine films have been grown on glass substrates by ultrahigh vacuum organic molecular beam depos ition. Postgrowth annealing of films grown at room temperature leads to tra nsformation from the alpha to the beta(1) phase. The effects of annealing l ead to a number of transition states whose morphological, structural, and s pectroscopic properties can be identified using atomic force and optical in terference microscopy, X-ray diffraction, and Raman and electronic absorpti on spectroscopy. Detailed morphological studies indicate that the transitio n occurs via a discrete number of nucleations and is preceded by an elongat ion of the alpha crystallites. Quantitative analysis of the crystallites an d domain size shows that the individual beta(1) crystals grow but are confi ned to domains of similar orientation. The film thickness plays a critical role and three regimes have been identified. The alpha --> beta(1) transfor mation is only complete for films thicker than similar to 940 Angstrom, and thick films lead to a higher degree of orientation and larger domains.