1. Department
of Physics, National University of Singapore, 117542 Singapore
2. Graphene
Research Center, National University of Singapore, 117542 Singapore
3. Nikolaev
Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Sciences 3, Acad. Lavrentiev prospect, Novosibirsk 630090, Russia
4. NanoCore, National University of Singapore, 117576 Singapore