Two different polymorphic forms of free base phthalocyanine films have been grown on glass substrates by ultrahigh vacuum organic molecular beam deposition. Postgrowth annealing of films grown at room temperature leads to transformation from the R to the β 1 phase. The effects of annealing lead to a number of transition states whose morphological, structural, and spectroscopic properties can be identified using atomic force and optical interference microscopy, X-ray diffraction, and Raman and electronic absorption spectroscopy. Detailed morphological studies indicate that the transition occurs via a discrete number of nucleations and is preceded by an elongation of the R crystallites. Quantitative analysis of the crystallites and domain size shows that the individual β 1 crystals grow but are confined to domains of similar orientation. The film thickness plays a critical role and three regimes have been identified. The R f β 1 transformation is only complete for films thicker than ∼940 Å, and thick films lead to a higher degree of orientation and larger domains.

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Phthalocyanines are a well-studied class of molecules that have a number of wide-ranging applications in fields as diverse as dyes, [1] light-emitting diodes (LEDs), [2] solar cells, and field-effect transistors. [3] They exist as a range of derivatives, where the polyaromatic ring (abbreviated Pc for the phthalocyanato anion C 32 H 16 N 8 2± ) is most commonly bound to a transition metal (MPc) or hydrogen (H 2 Pc). [4] Pcs crystallize as a variety of polymorphs, and metal-free phthalocyanines (H 2 Pc) are known to exist in two predominant phases. The a form is metastable and can be obtained as small crystallites by the acid-paste method. The b form is commercially available and is more stable than the a phase above a critical crystal size. [5] Both forms adopt a herringbone structure where molecules stack along the b-axis, but the angle between the plane of the molecule and the b-axis is different in both cases. The a phase crystallizes in the C2/c space group, [6] while the b form belongs to the P2 1 /a space group. [7] Pc films are used as molecular components in a number of electronic and optoelectronic devices. [8] The structure, morphology, electronic, and optical properties of the films are therefore crucial for their technological applications. For example, high crystallinity is known to improve the conducting properties, but smooth amorphous films allow better contacts. Growth under ultrahigh vacuum (UHV) conditions using organic molecular beam deposition (OMBD) provides high-quality films, where the incorporation of impurities is minimized by the UHV environment and the growth conditions are highly reproducible. [8] Both polymorphs of H 2 Pc have been grown by OMBD on glass substrates. The a phase is obtained by deposition at room temperature, and forms small spherical crystallites. [9] The b phase is formed after applying heat to the substrate and it can be divided further into two sub-phases; i) a b 1 phase obtained by annealing an a film, and with a morphology characterized by long, oriented crystallites, and ii) a b 2 phase, where the elongated crystallites are randomly oriented. [10] Many electronic and optoelectronic devices rely on the growth of multilayered molecular thin-film structures. This generally involves the growth of heterostructures in which one type of molecular layer is deposited on top of another.A key aspect of the device is the influence the first layer has on determining the morphology and structural properties of the second and subsequent layers, since this will have a strong influence on the electronic and optical properties of the film. In this paper, we study this effect in a simple double-layer system grown by OMBD, where each layer is a different polymorph of the same molecule, H 2 Pc. The study involves structural and morphological analysis by a variety of ex-situ techniques, and we provide unequivocal evidence that the first layer has a very strong influence on the subsequent growth, effectively acting as both a morphological and structural template.The films were gr...

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