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Institute of Solid State Physics
The solution of crystal structures from thin films is essential in organic thin film technology, since new types of polymorphs arise when a substrate surface is present during the crystallisation process. Such surface induced crystal structures play an important role in organic electronics and in pharmaceutical research, where a considerable enhancement of material performance has been observed. The crystal structure solution from thin films is performed by a combination of experimental and theoretical techniques, combining grazing incidence X-ray diffraction (GIXD) with modelling of the molecular packing. This project will solve two open problems associated with crystal structure solution from thin films. The first problem is associated with the assignment of Laue indices to the Bragg peaks observed by grazing incidence diffraction (the so-called indexation) to determine the lattice constants. There, two components of the scattering vector – the in-plane part, qxy, and the out-of-plane part of the scattering vector, qz – have to be used. Mathematical equations for the fundamentals have to be derived, a numerical solution method for the indexation has to be developed and a refinement process for the lattice constants must be determined. The second problem is associated with biaxially oriented crystals at surfaces, such structures are obtained by epitaxial thin film growth. The collection of a complete diffraction pattern will be performed by a novel experimental approach using GIXD. Rotating the sample around the surface normal and collecting the diffracted intensity during a whole rotation of 360° enables the recording of a complete diffraction pattern even for epitaxially grown crystallites; the diffraction patterns will be collected from large volumes of reciprocal space. Enhancement in the quality of the experimental data in terms of peak position and peak intensities is expected. Crystal structure solution from thin films will be applied to thin films of the molecule 2-decyl-7-phenyl-benzothieno[3,2-b]benzothiophene, a molecule with outstanding performance in organic electronic devices. The non-symmetric chemical structure of the molecule makes it prone to form numerous polymorphic phases. A systematic variation of crystallisation conditions by solution processing and physical vapour deposition using biaxial crystallisation by directed nucleation methods and by uniaxial and single crystalline surfaces will allow the exploration of the ability of the molecule to form new polymorphic phases. This project will be a considerable contribution to the field of thin film crystallography, since a defined methodology for crystal structure solution from thin films is not fully developed so far.