1. Departament d’Enginyeria Electrònica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain, Department of Micro and Nanotechnology, NanoDTU, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark, and Laboratory of Physics, Helsinki University of Technology, P.O. Box 1100, FI-02014 HUT, Finland