1. Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, P. R. China
2. University of Chinese Academy of Sciences, Beijing 100049, P. R. China
3. Nano Science and Technology Institute, University of Science and Technology of China, Suzhou 215123, P. R. China
4. Ningbo Sunpu-Opto Semiconductor Co. Ltd., Ningbo, 315000, P. R. China
5. College of Materials, Xiamen University, Xiamen 361005, P. R. China