1Institute of Physics, Poznan University of Technology, ul. Piotrowo 3, 60-965 Poznan, Poland
2London Centre for Nanotechnology, University College London, 17-19 Gordon Street, London, UK
3IHP, Im Technologiepark 25, D-15236 Frankfurt (Oder), Germany
4School of Mathematical and Physical Sciences, The University of Newcastle, Callaghan 2308, Australia
5Department of Physics and Astronomy, UCL, London, WC1E 6BT, UK
6Dipartimento di Scienze, Università degli Studi Roma Tre, I-00146 Roma, Italy
7BTU Cottbus, Konrad-Zuse Str. 1, D-03046 Cottbus, Germany
8Department of Electronic and Electrical Engineering, UCL, London, WC1E 7JE, UK