1. ICMAB and Centro de Investigacíon en Nanociencia y Nanotecnología (CSIC), Campus UAB, 08193 Bellaterra, Spain, EMAT, University of Antwerp, Groenenborgerlaan, B-2020 Antwerp, Belgium, and Group de Nanomaterials i Microsistemes, Departament de Física, UAB, 08193 Bellaterra, Spain