Highly crystalline organic heteroepitaxial layers with controlled molecular orientations and morphologies are one of the keys for optimum organic device performance. With studies of molecular orientation, structure, and morphology, we have investigated the ability of oriented organic films to act as substrate templates for the growth of a second organic layer. Depending on the molecular orientation in the sexiphenyl substrate, crystalline sexithiophene nanostructures of either pyramidal or needlelike morphology, with either near vertical or parallel molecular orientations, respectively, grow.
The epitaxial order of sexiphenyl crystals on muscovite(001) is investigated by x-ray diffraction, lattice misfit calculations and atomic force microscopy. Depending on the substrate temperature during the thin film growth process, different epitaxial orientations are formed. Sexiphenyl thin films prepared at 370 K preferentially form crystals with the crystallographic (11-1) planes parallel to the substrate surface while at 434 K a strong fraction of crystals with (11-2) orientations is grown. The epitaxial orders of sexiphenyl crystals are compared with lattice misfit calculations. The in-plane order of the {11-1} crystals can be explained by a point-on-line coincidence I, which reveals that the interface is formed by undisturbed crystal surfaces. The epitaxial order of the {11-2} oriented crystals is characterised by the experimental observation that low indexed crystal directions in the sexiphenyl(11-2) plane and the muscovite(001) surface coincide with each other, forming a near-coincidence case. Corrugations of the substrate surface are responsible for this second type of epitaxial order. Characteristic features in the thin film morphology could be correlated to the two observed epitaxial orientations of the sexiphenyl crystals.
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