1. Department of Engineering Physics and Center for New Materials, Helsinki University of Technology, P.O Box 5100, FI-02015 TKK, Finland; Department of Chemistry, University of Helsinki, P.O. Box 55, 00014 Helsinki, Finland; Department of Physics and Fribourg Center for Nanomaterials, University of Fribourg, Perolles Fribourg, CH-1700 Switzerland; and Nestlé Research Center, Vers-Chez-les-Blanc, 1000 Lausanne 26, Switzerland