1. International Iberian Nanotechnology Laboratory, Braga 4715-330, Portugal
2. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge Massachusetts 02139, United States
3. Department
of Applied Physics, University of Santiago de Compostela, Santiago de Compostela 15782, Spain
4. Center of
Physics, University of Minho, Braga 4710-057, Portugal
5. Department of Chemistry, University of California, Davis, Davis, California 95616, United States
6. CRISMAT, UMR 6508, CNRS-ENSICAEN, Caen 14050, France