1. Department of Electrical and Computer Engineering; National University of Singapore; Singapore 117576 Singapore
2. Centre for Advanced 2D Materials and Graphene Research Centre; National University of Singapore; 6 Science Drive 2 Singapore 117546 Singapore
3. Faculty of Science; King Mongkut's University of Technology Thonburi; Bangkok 10140 Thailand
4. Division of Physics and Applied Physics; School of Physical and Mathematical Sciences; Nanyang Technological University; Singapore 637371 Singapore
5. State Key Laboratory of Electronic Thin Films and Integrated Devices; University of Electronic Science and Technology of China; Chengdu 610054 China