1. Department of Chemistry and Centre for Advanced 2D Materials; National University of Singapore; 3 Science Drive 3 17543 Singapore
2. Department of Physics and Astronomy; Vanderbilt University; Nashville TN 37235 USA
3. Materials Science and Technology Division; Oak Ridge National Lab; Oak Ridge TN 37831 USA
4. School of Physical Sciences; CAS Key Laboratory of Vacuum Physics; University of Chinese Academy of Sciences; Beijing 100049 China