1. College of Science, National University of Defense Technology, Changsha 410073, China
2. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542
3. College of Physics and Electronics, Central South University, Changsha, Hunan 410073, China
4. Institute for Advanced Study and Department of Physics, Nanchang University, 999 Xue Fu Da Dao, Nanchang, China
5. School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798
6. Graphene Research Centre, National University of Singapore, 2 Science Drive 3, Singapore 117542
7. Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543