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