1. The Institute for Advanced Studies Wuhan University Wuhan 430072 P. R. China
2. Hunan Institute of Advanced Sensing and Information Technology Xiangtan University Xiangtan 411105 P. R. China
3. Key Laboratory of Artificial Micro‐ and Nano‐structures of Ministry of Education School of Physics and Technology Wuhan University Wuhan 430072 P. R. China
4. Key Laboratory for the Physics and Chemistry of Nanodevices and Center for Carbon‐based Electronics School of Electronics Peking University Beijing 100871 P. R. China
5. School of Physics and Electronics Hunan University Changsha 410082 P. R. China
6. School of Materials Science and Engineering Peking University Beijing 100871 P. R. China