1. Shenzhen Geim Graphene Center Tsinghua–Berkeley Shenzhen Institute and Institute of Materials Research Shenzhen International Graduate School Tsinghua University Shenzhen 518055 P. R. China
2. State Key Laboratory of Structural Chemistry Fujian Institute of Research on the Structure of Matter Chinese Academy of Sciences Fuzhou 350002 P. R. China
3. Department of Physics Southern University of Science and Technology Shenzhen 518055 P. R. China