1. Department of Chemistry, University of Oslo, P.O. Box 1033, N-0315, Oslo, Norway, Centre of Materials Science and Nanotechnology, P.O. Box 1162, N-0318, Oslo, Norway, and Department of Inorganic, Physical and Materials Chemistry, NIS Centre of Excellence, and Centro di Riferimento INSTM, University of Turin, Via P. Giuria 7, I-10125, Torino, Italy