1. Contribution from the Department of Materials & Interfaces, The Weizmann Institute of Science, 76100 Rehovot, Israel, Department of Solid State Physics, Risø National Laboratory, DK 4000, Roskilde, Denmark, Niels Bohr Institute, H. C. Ørsted Laboratory, DK 2100, Copenhagen, Denmark, and Laboratoire de Chimie Supramoléculaire Institut Le Bel, Université Louis Pasteur, F-67000 Strasbourg, France