1. Department of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
2. Centre
for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, Singapore 117546, Singapore
3. Institute
of Mathematical Sciences and Physics, University of The Philippines Los Baños, Los Baños 4031, Laguna, Philippines
4. Department of Physics, Northeastern University, Boston, Massachusetts 02115, United States