[1] Tech Univ Denmark, DTU Phys, Orsteds Plads 345, DK-2800 Lyngby, Denmark
[2] Tech Univ Denmark, Ctr Nanostruct Graphene, Orsteds Plads 345C, DK-2800 Lyngby, Denmark
[3] Aalto Univ, Dept Elect & Nanoengn, POB 13500, FI-00076 Aalto, Finland
[4] Tech Univ Denmark, DTU Foton, Orsteds Plads 343, DK-2800 Lyngby, Denmark
[5] Aarhus Univ, Interdisciplinary Nanosci Ctr iNANO, Dept Phys & Astron, DK-8000 Aarhus, Denmark
[6] Tech Univ Denmark, DTU Phys, Fys Vej 312, DK-2800 Lyngby, Denmark
[7] Natl Univ Singapore, Ctr Adv 2D Mat, 2 Sci Dr 3, Singapore 117542, Singapore
[8] Natl Univ Singapore, Graphene Res Ctr, 2 Sci Dr 3, Singapore 117542, Singapore