1. School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China
2. Key Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy Materials and Devices, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P. R. China
3. Center of Materials and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, P. R. China
4. Beijing Key Laboratory of Nanophotonics & Ultrafine Optoelectronic Systems, Beijing Institute of Technology, Beijing 100081, P. R. China
5. Center of Material and Optoelectronics Engineering, University of Academy of Science, Beijing 100049, P. R. China
6. The Yangtze River Delta Physics Research Center, Liyang, Jiangsu 213000, P. R. China
7. Guangxi Key Lab of Processing for Nonferrous Metals and Featured Materials and School of Resources, Environments and Materials, Guangxi University, Nanning 530004, P. R. China