1. Department of Applied Physics, Chang’an University, 710064 Xi’an, China
2. Department of Physics, National University of Singapore, 2 Science Drive 3, 117551 Singapore, Singapore
3. Institute of Materials Research and Engineering, A*-STAR, 2 Fusionopolis Way, 138634 Singapore, Singapore
4. Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, 117575 Singapore, Singapore
5. Centre for Advanced 2D Materials, National University of Singapore, 6 Science Drive 2, 117546 Singapore, Singapore