Solution of surface induced crystal structures

Crystal structures of conjugated molecules which are formed within thin films are partly bulk equilibrium structures (structures of macroscopic single crystals) as well as metastable crystal phases which are induced due to the presence of a surface during the thin film growth (surface induced phases). The goal of this project is to develop a specific method of crystal structure solution of molecular crystals as they are formed within thin films. The method will consider the strong texture of the molecular crystals within thin films, since they form two- dimensional powders on isotropic surfaces. The developed method will allow the crystal structure determination of surface induced phases, which is not possible with the currently available methods. The crystal structures will be solved by experimental techniques which will be partly combined with theoretical approaches. The experimental technique is grazing incidence x-ray diffraction which will be used for the determination of the crystallographic unit cell and of the crystallographic structure factors. These structure factors will be used by a Rietveld refinement procedure to perform the full solution of the crystal structure using the chemical structure of the molecule as input parameter. The theoretical approaches will be based on force field calculations taking the experimentally determined lattice constants as input parameters. The energetic stability of the surface induced phases as well as of the equilibrium structures will be obtained and compared with each other. By in-situ experiments the influence of temperature to the crystal structures will be investigated and the temperature stability of the involved crystalline structures will be obtained. These experimental results will be compared to theoretical calculations performed by quasi-harmonic lattice dynamics. The temperature dependence of the crystal structures as well as their temperature stability will be obtained. In a first step of the project the surface induced phase of sexithiophene and the temperature stability of the pentacene surface induced phase (the so called thin film phase) will be investigated. In both cases the crystal structure within thin films are of essential importance for transistor applications in organic electronics. In a second step alkyl-substituted terthiophene and phthalocyanine molecules will be investigated. In a last step of the project it will be tested if the approach of crystal solution from thin films can be applied also to the polymer poly(3- hexylthiophene) and to thermotropic liquid crystalline states.

Molecular crystals show a strong tendency to form polymorph crystal structures within thin films. The presence of a surface during the crystallisation process causes the appearance of specific surface induced phases often also called as a thin film phase of a molecular material. The aim of the project is to develop a specific approach to solve crystal structures of molecular materials from thin films. In the present case a specific experimental technique - grazing incidence x-ray surface diffraction was used. A software is developed which allows to present the experimental data, to index diffraction peaks and to extract important quantities of the experiment like intensities and positions of Bragg peaks. Thin films of mainly rod-like conjugated molecules like pentacene, sexithiophene, terthiophene and their derivatives were investigated. Some results could be obtained on disc-like conjugated molecules. Different approaches are tested for a crystal structure solution, all of them are based on the indexation of the crystallographic lattice based on the experimental data. The final solution of the crystal structure the determination of the packing of the molecules can be performed either on theoretical basis or by using the experimentally determined peak intensities. Both approaches are applied successfully. The results of the project are important for all scientific and technological fields dealing with organic thin films, e.g. surface induced crystal structures are present in thin films for organic electronic devices.