1. Department
of Photonics Engineering, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark
2. Center
for Nanostructured Graphene (CNG), Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark
3. Department
of Micro and Nanotechnology, DK-2800 Kongens Lyngby, Denmark
4. Department
of Physics and Nanotechnology, Aalborg University, DK-9220 Aalborg, Denmark
5. Department
of Physics, Key Laboratory of Micro- and Nano-Photonic Structures
(Ministry of Education) and State Key Laboratory of Surface Physics, Fudan University, Shanghai 200433, China