1. Center for Nanoscale Characterization & Devices, Wuhan National Laboratory for Optoelectronics and School of Physics, Huazhong University of Science and Technology, Luoyu Road 1037, Wuhan 430074, China
2. State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing 100876, 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, China