1. School of Materials Science and Engineering, Beihang University, Beijing 100191, P. R. China
2. Institute of Physics & University of Chinese Academy of Sciences, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, P. R. China
3. Shenzhen Geim Graphene Center and Low-Dimensional Materials and Devices Laboratory, Tsinghua-Berkeley Shenzhen Institute (TBSI), Tsinghua University, Shenzhen 518055, P. R. China