1. Institute of Applied Physics and Materials Engineering, University of Macau, Avenida da Universidade, Taipa, Macau SAR 999078, China
2. Key Laboratory of Artificial Micro- and Nano-structures of the Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan, 430072, China
3. National Graphene Institute, University of Manchester, Oxford Road, Manchester M13 9PL, UK
4. School of Materials Science and Engineering, Peking University, Beijing, 100871, P. R. China