1. Institute of Nanotechnology, CNR-NANOTEC, via Orabona 4, 70126 Bari, Italy
2. Department of Electrical and Computer Engineering, Duke University, Durham, North Carolina 27708, USA
3. Departamento de Física Aplicada, Universidad de Cantabria, Avda. de los Castros s/n, 39005 Santander, Spain