1. Fibre and Particle Engineering Research Unit, University of Oulu, Pentti Kaiteran katu 1, 90014 Oulu, Finland
2. Nano and Molecular Systems Research Unit, University of Oulu, Pentti Kaiteran katu 1, 90014 Oulu, Finland
3. Departament of Condensed Matter Physics, University of the Basque Country (UPV/EHU), Barrio Sarriena, s/n, 48940 Leioa, Spain