1. Division of Physics and Applied Physics; School of Physical and Mathematical Sciences; Nanyang Technological University; 637371 Singapore
2. School of Materials Science and Engineering; Nanyang Technological University; 639798 Singapore
3. Institute of Nanoscience and Nanotechnology; Department of Physics; Central China Normal University; Wuhan Hubei 430079 China
4. Department of Physics; Faculty of Science; National University of Singapore, 117542 Singapore Graphene Research Centre, National University of Singapore; 117546 Singapore