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 τ) 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
s = 100 °C, an original growth mechanism was observed
whereby α-TiOPc microcrystallites were formed by the static coalescence and reorganization
of small amorphous aggregates in close contact. Nucleation of α-TiOPc is initiated at the
PTFE macrosteps from which oriented crystallization propagates, leading to uniform α-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 α-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 α 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 α-TiOPc films results in a strong polarization of the 850-nm band
perpendicular to the PTFE chain axis direction.
Oriented films of tetracene and pentacene have been obtained by high vacuum sublimation onto oriented poly(tetrafluoroethylene) (PTFE) substrates. Polymorphism, orientation, and morphology of the pentacene and tetracene films are studied as a function of deposition parameters [substrate temperature (T s ), deposition time (t), and deposition rate (τ)] using X-ray and electron diffraction, transmission and scanning electron microscopies and atomic force microscopy (AFM). Oriented films of the triclinic structures reported by Campbell et al. (Acta Crystallogr. 1961, 14, 705; 1962, 15, 289) have been obtained in well-defined T sranges. Oriented acene films of 50 nm consist of a continuous texture of flat-lying and terraced domains with a dense (0 0 1) contact plane and a high degree of in-plane orientation. Specific nucleation sites of pentacene on the PTFE substrate have been identified by AFM in the early stage of deposition. The similarity found in the growth phenomenology of tetracene and pentacene allows for identification of the mechanism responsible for oriented growth which involves (i) ledge-directed nucleation and (ii) confined growth of nanocrystals by the PTFE meso-scale relief, e.g. nanoscopic grooves between successive PTFE fibrils. In addition, various crystal defects, e.g. edge dislocations, have been identified by high-resolution TEM.
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