1. State Key Laboratory for Mesoscopic Physics and Frontiers Science Center for Nano-optoelectronics, School of Physics, Peking University, Beijing 100871, China
2. Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China
3. Electron Microscopy Laboratory, School of Physics, Peking University, Beijing 100871, China
4. Key Laboratory of Optoelectronic Materials and Devices, Chinese Academy of Sciences, Beijing 100083, China
5. Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China