1 Hunan Institute of Advanced Sensing and Information Technology, Xiangtan University, Xiangtan 411105, China.
2 Hunan Institute of Advanced Sensing and Information Technology, Xiangtan University, Xiangtan 411105, China; Key Laboratory for the Physics and Chemistry of Nanodevices and Center for Carbon-based Electronics, Department of Electronics, Peking University, Beijing 100871, China. Electronic address: youfanhu@pku.edu.cn.