1. Center for Nanostructured Graphene (CNG), Department of Micro- and Nanotechnology, Technical University of Denmark, 2800 Kongens Lyngby, Denmark
2. L-NESS Laboratory, Department of Physics, Politecnico di Milano, Via Anzani 42, 22100 Como, Italy
3. Centro de Física and Departamento de Física, Universidade do Minho, P-4710-057 Braga, Portugal
4. Dipartimento di Fisica e Astronomia, Università di Catania, 64 Via Santa Sofia, 95123 Catania, Italy