| AVS 61st International Symposium & Exhibition | |
| Thin Film | Friday Sessions |
| Session TF+AS-FrM |
| Session: | Thin Film Characterization |
| Presenter: | Roland Resel, Graz University of Technology, Austria |
| Authors: | R. Resel, Graz University of Technology, Austria C. Röthel, Graz University of Technology, Austria A. Pichler, Graz University of Technology, Austria I. Salzmann, Humboldt University, Germany R.G. DellaValle, University Bologna, Italy O. Rosconi, University Bologna, Italy T. Dingeman, Delft University of Technology, Netherlands C. Simbrunner, University Linz, Austria |
| Correspondent: | Click to Email |
A large number of organic molecules exhibit polymorphism and a well-known phenomenon are specific crystallographic phases which are present exclusively in thin films. Such crystallographic phases are often denoted as surface induced phases, since the presence of a surface during the crystallisation is of primary importance for their formation. In general, such thin-film polymorphs do not exist as macroscopic free standing single crystals, so that existing methods of crystal structure solution e.g. from single crystal diffraction or powder diffraction does not work. A number of surface induced crystal structures of conjugated molecules are solved during the last years, examples are in relevant molecules for organic electronic applications like pentacene or sexithophene.
Two different methods of structure solution from a thin films will be introduced. Both methods are based on grazing incidence x-ray diffraction. In a first step the crystallographic unit cells and the lattice constants are determined by indexing the diffraction pattern. The evaluation of the molecular packing is based on either rigid body refinement or molecular dynamics simulations. While rigid body refinement is based on test structures and a comparison of the calculated diffraction intensities with the experimental intensities, molecular dynamics work on energy minimisation of the molecular packing. It is found that the approach works best for fully rigid molecules like pentacene or parylene. Good results are also obtained for semi-rigid molecules like ternaphtalene or molecules with flexible side-chains like dioctyl-terthiophene. In both cases the crystallographic unit cell is filled by two molecules. However an increasing number of molecules per unit cell (e.g. four) makes the solution of the surface induced phases difficult. Similarities and differences in the molecular packing between known crystal structures (from single crystal solutions) and surface induced phases will be discussed.