1. Shenzhen Geim Graphene Center Tsinghua–Berkeley Shenzhen Institute & Tsinghua Shenzhen International Graduate School Tsinghua University Shenzhen 518055 P. R. China
2. Department of Physics Southern University of Science and Technology Shenzhen 518055 P. R. China
3. Shenyang National Laboratory for Materials Science Institute of Metal Research Chinese Academy of Sciences Shenyang 110016 P. R. China