1. Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore.
2. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore.
3. Laboratory for Topological Quantum Matter and Spectroscopy (B7), Department of Physics, Princeton University, Princeton, NJ 08544, USA.
4. Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan.
5. Institute of Physics, Academia Sinica, Nankang, Taipei 11529, Taiwan.
6. Department of Physics, Northeastern University, Boston, MA 02115, USA.