1. Hunan Provincial Key Laboratory of Low-Dimensional, Structural Physics & Devices, School of Physics and Electronics, Hunan University, Changsha 410082, P. R. China
2. School of New Energy, Hunan Vocational, Institute of Technology, Xiangtan 411104, P. R China
3. College of Computer Science and Electronic, Engineering, Hunan University, Changsha 410082, P. R. China