1. Faculty of Engineering, University of Bayreuth, Bayreuth 95447, Germany
2. Faculty of Biology, Chemistry and Earth Sciences and Bavarian Polymer Institute, University of Bayreuth, Bayreuth 95447, Germany
3. Institute of Solid State Physics, University of Latvia, Kengaraga St. 8, Riga, LV-1063, Latvia
4. Faculty of Engineering Sciences and Bavarian Polymer Institute, University of Bayreuth, Bayreuth 95447, Germany