1. State Key Laboratory of Superhard Material, and College of Physics, Jilin University, Changchun 130012, P. R. China
2. Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun 130012, P. R. China
3. Institute of Semiconductors, South China Normal University, Guangzhou 510631, P. R. China
4. School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, P. R. China