1Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore
2SEU-FEI Nano-Pico Center, Key Laboratory of MEMS of Ministry of Education, School of Electrical Science and Engineering, Southeast University, Nanjing 210096, China
3Department of Physics, Faculty of Science, National University of Singapore, 117542, Singapore
4Graphene Research Center, National University of Singapore, 117546 Singapore