1. Institute of Solid State Electronics and Center for Micro- and Nanostructures, TU Wien, 1040 Vienna, Austria
2. Austrian Academy of Sciences, 1010 Vienna, Austria
3. Technische Physik, Physikalisches Institut and Wilhelm-Conrad-Röntgen Research Center for Complex Material Systems, University of Würzburg, 97074 Würzburg, Germany
4. nanoplus Nanosystems and Technologies GmbH, 97218 Gerbrunn, Germany
5. SUPA, School of Physics and Astronomy, University of St Andrews, St Andrews KY16 9SS, United Kingdom