1. Graphene
Research Centre and Department of Chemistry, National University of Singapore, 6 Science Drive 2, 117546 Singapore
2. Division
of Physics and Applied Physics, School of Physical and Mathematical
Sciences, Nanyang Technological University, 21 Nanyang Link, 637371 Singapore
3. School
of Physical Sciences, CAS Key Laboratory of Vacuum Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
4. Materials
Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States