1. School of Physics, Huazhong University of Science and Technology, Wuhan 430074, P. R. China
2. Key Laboratory of Structure and Functional Regulation of Hybrid Materials of Ministry of Education, Institutes of Physical Science and Information Technology, Anhui University, Hefei 230601, P. R. China
3. Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P. R. China
4. Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing 100871, P. R. China
5. School of Physics and Information Technology, Shaanxi Normal University, Xi’an 710119, China
6. School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, P. R. China
7. Shenzhen Geim Graphene Center, Tsinghua-Berkeley Shenzhen Institute (TBSI), Tsinghua University, Shenzhen 518055, P. R. China