1. Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, 315201 Ningbo, P.R. China
2. University of Chinese Academy of Sciences, 100049 Beijing, P.R. China
3. 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, 100190 Beijing, P.R. China