1. Beijing Computational Science Research Center, Haidian District, Beijing 100193, China
2. School of Physical Science and Technology, Southwest Jiaotong University, Chengdu 610031, China
3. Department of Physics and Centre for Advanced Two-Dimensional Materials, National University of Singapore, Singapore 117551, Singapore
4. Institute of Materials Research & Engineering, A*STAR (Agency for Science Technology and Research), 2 Fusionopolis Way, Innovis, Singapore 138634, Singapore
5. Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Hong Kong 999077, P. R. China
6. Asian School of the Environment and School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore
7. Department of Chemistry and Biochemistry, California State University, Northridge, Northridge, California 91330, United States