Highly ordered titanyl phthalocyanine films grown by directional crystallization on oriented poly(tetrafluoroethylene) substrate

Highly oriented films of titanyl phthalocyanine (TiOPc) were obtained by high-vacuum sublimation onto an oriented poly(tetrafluoroethylene) (PTFE) substrate. The dependence of the structure and morphology on deposition parameters (substrate temperature T-s, deposition time t, and deposition rate tau) was followed by X-ray diffraction, transmission electron microscopy (TEM), and atomic force microscopy (AFM) to uncover the origin of the oriented growth process. At T-8 = 100 degreesC, an original growth mechanism was observed whereby alpha-TiOPc microcrystallites were formed by the static coalescence and reorganization of small amorphous aggregates in close contact. Nucleation of alpha-TiOPc is initiated at the PTFE macrosteps from which oriented crystallization propagates, leading to uniform a-TiOPc films with a twinned texture and a dense (0 1 0) contact plane. The molecules are in an edge-on orientation, with their molecular plane oriented parallel to the PTFE chains. At the mesoscale, the nucleation of crystalline a-TiOPc involves the alignment and oriented coalescence of amorphous prenucleation aggregates along the PTFE macrosteps, i.e., a graphoepitaxial process. At the molecular scale, it is proposed that the TiOPc orientation and the preferential nucleation of the a polymorph are enforced by the topography and structure of the PTFE macrosteps in conjunction with the requirement for a minimal nucleation free energy. The optical absorption of the films in the near infrared is found to be strongly correlated with the structural and morphological modifications in the films. The oriented character of the alpha-TiOPc films results in a strong polarization of the 850-nm band perpendicular to the PTFE chain axis direction.