1. MIND, Electronic and Biomedical Engineering Department, Universitat de Barcelona, 08028 ,Barcelona Spain
2. Electronic Engineering Department, Universitat Autònoma de Barcelona, 08193 Barcelona, Spain
3. IMTEK, Faculty of Engineering, Albert-Ludwigs-Universität Freiburg, 79110 Freiburg, Germany
4. Institute of Nanoscience and Nanotechnology (IN2UB), Universitat de Barcelona, 08028 Barcelona, Spain