1. Istituto Nanoscienze - CNR , via G. Campi 213/A, Modena 41125 , Italy
2. FIM Department, University of Modena and Reggio Emilia , via G. Campi 213/A, Modena 41125 , Italy
3. Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich , Jülich 52425 , Germany
4. Thermo Fisher Scientific , PO Box 80066, KA Eindhoven 5600 , The Netherlands