1. School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, P. R. China
2. Key Lab of Materials Physics, Anhui Key Lab of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Science, Hefei 230031, P. R. China
3. University of Science and Technology of China, Hefei 230026, P. R. China
4. Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China