1. Kavli Institute of Nanoscience, Delft University of Technology, P.O. Box 5046, 2600 GA Delft, The Netherlands
2. Department of Physics, Virginia Tech, Blacksburg, Virginia 24061, United States
3. Department of Chemical and Geological Sciences
and INSTM, University of Modena and Reggio Emilia, via G. Campi
183, I-41125 Modena, Italy