1. School of Physics and Materials Science, Nanchang University, Nanchang 330031, P. R. China
2. School of Materials Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, P. R. China
3. Nanoscale Science and Technology Laboratory, Institute for Advanced Study, Nanchang University, Nanchang 330031, P. R. China
4. School of Materials and Energy, Jiangxi Science and Technology Normal University, Nanchang 330038, P. R. China