1. Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China
2. Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
3. College of Materials Sciences and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China
4. Department of Chemical Physics and NanoLund, Lund University, Box 124, 22100 Lund, Sweden