1. Key Laboratory of Micro-systems and Micro-structures Manufacturing of Ministry of Education; Harbin Institute of Technology; Harbin 150080 P. R. China
2. School of Materials Science and Engineering; Harbin Institute of Technology; Harbin 150080 P. R. China
3. State Key Laboratory of Superlattices and Microstructures; Institute of Semiconductors; Chinese Academy of Sciences; Beijing 100083 P. R. China
4. Cambridge Graphene Centre; University of Cambridge; Cambridge CB3 0FA UK