1. State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Qianjin Street 2699, Changchun 130012, China
2. Electron Microscopy Laboratory, School of Physics, Peking University, Beijing 100871, China
3. College of Physics, Jilin University, Changchun 130012, China
4. State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China