1. Department of Physics, University of Konstanz, Konstanz 78457, Germany
2. Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, UK
3. Nanotechnology Group, University of Salamanca, Salamanca 37008, Spain
4. Department of Physics, American University of Sharjah, Sharjah 26666, United Arab Emirates
5. Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA
6. Dipartimento di Fisica “E. R. Caianiello”, Università degli Studi di Salerno, 84084 Fisciano, SA, Italy