1. Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas 66045, United States
2. Department of Materials Science and Engineering, University of Shanghai JiaoTong University, Shanghai 200240, China
3. Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing JiaoTong University, Beijing 100044, China
4. School of Microelectronics, Xi’an Jiaotong University, Xi’an 710049, Shanxi, China