1. Departamento de Física, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais 30123-970, Brazil
2. Department of Physics, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany
3. Department of Physics and Materials Science, University of Luxembourg, L-4422 Belvaux, Luxembourg
4. Department of Chemistry and Center for NanoScience (CeNS), Ludwig-Maximilians-Universität München, Butenandtstraße 5-13 (E), 81377 Munich, Germany
5. Institute of Nanotechnology, Karlsruhe Institute of Technology, 76201 Eggenstein-Leopoldshafen, Germany