1. Hefei National Research Center for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China
2. Institute of Energy, Hefei Comprehensive National Science Center, Hefei, Anhui 230031, P. R. China
3. Hefei National Laboratory, University of Science and Technology of China, Hefei, Anhui 230088, P. R. China