1. Center for Nanoscale Characterization and Devices (CNCD), Wuhan National Laboratory for Optoelectronics (WNLO) and School of Physics, Huazhong University of Science and Technology (HUST), Luoyu Road 1037, Wuhan 430074, P.R. China
2. School of Science, Hubei University of Automotive Technology, Shiyan 442002, P.R. China
3. School of Physics and Technology, Center for Electron Microscopy, MOE Key Laboratory of Artificial Micro- and Nano-Structures, and the Institute for Advanced Studies, Wuhan University, Wuhan 430072, P.R. China