1. State Key Laboratory for Mesoscopic Physics and Frontiers, Science Center for Nano-optoelectronics, School of Physics, Peking University, Beijing 100871, China
2. Electron Microscopy Laboratory, School of Physics, Peking University, Beijing 100871, China
3. Peking University Yangtze Delta Institute of Optoelectronics, Nantong, Jiangsu 226010, China