Orienting Tetracene and Pentacene Thin Films onto Friction-Transferred Poly(tetrafluoroethylene) Substrate

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 s-ranges. 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.