1. Technische Physik and Wilhelm-Conrad-Röntgen Research Center for Complex Material Sytems, University of Würzburg, 97074 Würzburg, Germany
2. nanoplus Nanosystems and Technologies GmbH, Oberer Kirschberg 4, 97218 Gerbrunn, Germany
3. SUPA, School of Physics and Astronomy, University of St Andrews, St Andrews KY16 9SS, United Kingdom