1. International Iberian Nanotechnology Laboratory (INL), Av. Mestre José Veiga, 4715-330 Braga, Portugal
2. Department of Physics, Center of Physics (CF-UM-UP), University of Minho, Campus of Gualtar, 4710-057, Braga, Portugal
3. Departamento de Física Aplicada, Universidad de Alicante, 03690 Sant Vicent del Raspeig, Spain