1. Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, P. R. China
2. Department of Materials Science and Engineering, CAS Key Laboratory of Materials for Energy Conversion, University of Science and Technology of China, Hefei 230026, P. R. China
3. Anhui Laboratory of Advanced Photon Sciences and Technology, University of Science and Technology of China, Hefei 230026, P. R. China
4. Department of Physics, University of Science and Technology of China, Hefei 230026, P. R. China