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

Abstract

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.

Phase separation in vacuum codeposited pentacene/6,13-pentacenequinone thin films

“…We found the onset of the lowest-energy transition, attributed to the P energy gap, at 1.79 eV with its peak maximum at 1.85 and 1.94 eV ͑the two components of the Davydov 0-0 band doublet 60 ͒, as well as the the 0-1 band ͑at 2.12 and 2.28 eV, respectively͒. This result is in good agreement with previously reported results for P thin films on oriented poly͑tetrafluoroethylene͒, 61 Al 2 O 3 ͑sapphire͒, 62 and quartz. 60,63 For the pure PQ film on quartz the first peak maximum, assigned to the fundamental absorption, appears at 2.92 eV ͑low-energy onset at 2.81 eV͒ and at 3.09 eV, respectively, which is in good agreement with the results found for PQ on sapphire.…”

Characterization of Phase Purity in Organic Semiconductors by Lattice‐Phonon Confocal Raman Mapping: Application to Pentacene

“…1, left side), labeled as the C phase. [10] For a long time this structure was assumed to be the only pentacene crystalline modification. In 1999 Holmes et al, [11] interested in the planarity of N-phenylenes, obtained an X-ray structure at 180 K, and noticed that, although still triclinic with two molecules per unit cell, the structure was different from the previously reported C phase.…”

Weak Epitaxy Growth Affording High‐Mobility Thin Films of Disk‐Like Organic Semiconductors

“…[3][4][5][15][16][17] A known epitaxial growth technology [6][7][8]18] was used to fabricate phthalocyanine thin films, as used in the inorganic semiconductor industry. [19] But the organic molecule usually showed the epitaxy growth on a single-crystal substrate [20,21] or oriented substrate [22][23][24] with molecules tending to adhere flat to their substrate due to the strong interaction between the molecules and the substrate. As a consequence, the higher conductive direction between the molecules cannot be parallel to the film plane, leading to low in-plane mobility.…”

“…We call this method a weak epitaxy growth (WEG) to distinguish it from other oriented epitaxy growth on single-crystal substrates [20,21] or oriented substrates. [22][23][24]27] The WEG approach produces highly oriented and continuous organic thin films of disk-like phthalocyanine compounds with the molecule p-p interaction direction parallel to the substrate, leading to a significant improvement of the carrier transportation in organic electronic devices. Exceptional electronic performance is especially expected in an organic field-effect transistor (OFET), where carriers are accumulated at the interface of the organic semiconductor near the gate insulator, with the transport direction parallel to the substrate.…”