1. Institute of Advanced Ceramics; Hamburg University of Technology; Denickestrasse 15 21073 Hamburg Germany
2. Institute of Materials Research, Materials Mechanics; Helmholtz-Zentrum Geesthacht; Max-Planck-Strasse 1 D-21502 Geesthacht Germany
3. Institute of Nanostructure and Solid State Physics; Universität Hamburg; Jungiusstrasse 11 20355 Hamburg Germany
4. Institute of Optical and Electronic Materials; Hamburg University of Technology; Eissendorfer Strasse 38 21073 Hamburg Germany