1. Institute of Materials Research and Engineering, A*-STAR, 2 Fusionopolis Way, Singapore 138634
2. Centre
for Advanced 2D Materials and Graphene Research, National University of Singapore, 6 Science Drive 2, Singapore 117546
3. Department
of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542
4. Institute
for Structure and Function and Department of Physics, Chongqing University, Chongqing 400044, People’s Republic of China