1. Henan Key Laboratory of Photovoltaic Materials, Laboratory of Low-Dimensional Materials Science, School of Physics & Electronics, Henan University, Kaifeng 475004, People's Republic of China
2. Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, People's Republic of China
3. College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China