1. Department
of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
2. Centre
for Advanced 2D Materials, National University of Singapore, 6 Science
Drive 2, Singapore 117546, Singapore
3. Department
of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, P.R. China
4. College
of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P.R. China
5. Department
of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117551, Singapore
6. School
of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore