1. Hunan Institute of Advanced Sensing and Information Technology, Xiangtan University, Xiangtan 411105, P. R. China
2. School of Physics and Optoelectronics, Xiangtan University, Xiangtan 411105, P. R. China
3. Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing 100871, P. R. China