1. State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering, Zhejiang University, Hangzhou 310027, China
2. Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, Singapore 117546
3. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117551
4. National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
5. Department of Physics, Zhejiang University, Hangzhou 310027, China
6. Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543