1. Applied Physics Department and the Center for Nano-Science and Nano-Technology The Hebrew University of Jerusalem Jerusalem 91904 Israel
2. Department of Chemical and Biological Physics Weizmann Institute of Science Rehovot 76100 Israel
3. Department of Chemistry University of Pittsburgh Pittsburgh PA 15260 USA
4. Chemical Research Support Weizmann Institute Rehovot 76100 Israel
5. Department of Engineering “Enzo Ferrari”, DIEF University of Modena and Reggio Emilia 41125 Modena Italy