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Heteroepitaxy of Organic Nanofibers: Example of Ternaphthalene on p‑Hexaphenyl

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journal contribution
posted on 05.11.2014, 00:00 by Clemens Simbrunner, Günther Schwabegger, Roland Resel, Theo Dingemans, Francesco Quochi, Michele Saba, Andrea Mura, Giovanni Bongiovanni, Helmut Sitter
Nowadays heteroepitaxy is well understood and investigated for inorganic compounds. In contrast, the epitaxial growth of organic–organic multilayer structures is rarely reported. By a comprehensive comparison between experiments and simulations, we demonstrate that highly anisotropic, needle-shaped p-hexaphenyl (p-6P) crystallites can efficiently act as an organic template and that the epitaxial overgrowth by 2,2′:6′,2″-ternaphthalene (NNN) yields a high molecular order and optical anisotropy of the nucleated NNN crystallites. It is shown that surface corrugations formed by the p-6P template are responsible for a parallel molecular alignment and a geometrical adoption of the herringbone stacking sequence of NNN. On the basis of the obtained results, it can be concluded that, in contrast to inorganic heteroepitaxy, lattice matching plays a minor role, whereas a geometrical adoption of the molecular stacking is directly connected with an optimized adsorption energy. In that sense, polarization-dependent photoluminescence studies prove a significantly increased optical anisotropy of NNN crystallites, when a p-6P template layer is inserted between NNN and the muscovite mica substrate. The organic interlayer is also responsible for the formation of a different NNN contact plane and the suppression of island-shaped crystal morphologies which are a fingerprint for standing molecular configurations. Consequently, only highly anisotropic, lying molecular orientations are obtained, which is relevant for the design of future organic-based optoelectronic devices.