Heteroepitaxy of Organic Nanofibers: Example of Ternaphthalene on p-Hexaphenyl
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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.