1. Department of Applied Physics, KTH Royal Institute of Technology, Electrum 229, SE-164 40 Kista, Sweden
2. Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Campus UAB, 08193 Bellaterra, Barcelona, Spain
3. ICREA, Pg. Lluis Companys 23, 08010, Barcelona, Spain