1. MIND, Departament d'Enginyeria Electrònica i Biomèdica, Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain
2. Institute of Nanoscience and Nanotechnology (IN2UB), Universitat de Barcelona, Av. Diagonal 645, 08028 Barcelona, Spain
3. Laboratory of Nanotechnology, IMTEK, Faculty of Engineering, Albert-Ludwigs Universität Freiburg, Georges-Köhler-Allee 103, 79110 Freiburg, Germany
4. Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 121 16 Prague 2, Czech Republic