1. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore
2. Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore
3. School of Physics, Southeast University, Nanjing 211189, China
4. Department of Materials Science and Engineering, National University of Singapore, Singapore 117575, Singapore
5. Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
6. NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, Centre for Life Sciences, #05-01, 28 Medical Drive, Singapore 117456 Singapore
7. NUSSNI-NanoCore, National University of Singapore, 5A Engineering Drive 1, Singapore 117411, Singapore
8. School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
9. Magnetic Spectroscopy Group, Diamond Light Source, Didcot OX11 0DE, U.K.