1. Centre for Advanced 2D Materials, National University of Singapore, 6 Science Drive 2, 117546 Singapore.
2. Department of Physics, Faculty of Science, National University of Singapore, 2 Science Drive 3, 117542 Singapore.
3. Department of Physics, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada.
4. School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 637371 Singapore.
5. Institut für Theoretische Physik und Astrophysik, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany.
6. Yale-NUS College, 16 College Avenue West, 138527 Singapore.