1. Key Lab of Materials Physics, Anhui Key Lab of Nanomaterials and Nanotechnology, Institute of Solid State Physics, HFIPS, Chinese Academy of Sciences, Hefei 230031, P. R. China
2. School of Chemistry and Chemical Engineering, Anqing Normal University, Anqing 246011, P. R. China
3. School of Physics and Materials Engineering, Hefei Normal University, Hefei, Anhui 230601, P. R. China
4. University of Science and Technology of China, Hefei 230026, P. R. China