1. Key Lab of Materials Physics, Anhui Key Lab of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, P. R. China
2. University of Science and Technology of China, Hefei 230026, P. R. China
3. College of Materials and Mineral Resources, Xi’an University of Architecture and Technology, Xi’an 710055, P. R. China
4. College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, P. R. China