1. Department of Materials, ETH Zürich, Wolfgang-Pauli-Strasse 10, 8093 Zürich, Switzerland, Department of Physics and Fribourg Center for Nanomaterials, University of Fribourg, Ch. du Musée 3, CH-1700 Fribourg, Switzerland, and Nestlé Research Center, Vers-Chez-les-Blancs, CH-1000 Lausanne 26, Switzerland