1. Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
2. Department of Physics, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan
3. Physics Division, National Center for Theoretical Sciences, Taipei, 10617, Taiwan
4. Centre for Advanced 2D Materials (CA2DM), National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore