1. International Iberian Nanotechnology Laboratory, Av. Mestre José Veiga, 4715-330 Braga, Portugal
2. Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre s/n, 4169-007 Porto, Portugal
3. Nanotechnology Group, Department of Fundamental Physics, University of Salamanca, 37008 Salamanca, Spain
4. Centre of Physics of Minho and Porto Universities (CF-UM-UP), Azurém Campus, 4800-058 Guimarães, Portugal