1. SinBeRISE CREATE, National Research Foundation, CREATE Tower, 1 Create Way, Singapore 138602, Singapore
2. Department
of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
3. School
of Physical Sciences, CAS Key Laboratory of Vacuum Physics, University of Chinese Academy of Sciences, Beijing 100049, China
4. Department
of Physics and Astronomy and Department of Electrical Engineering
and Computer Science, Vanderbilt University, Nashville, Tennessee 37235, United States
5. Centre
for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore
6. Materials
Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States