1. College of Physics, Guizhou University, Guiyang, Guizhou 550025, P. R. China
2. College of Materials, Xiamen University, Xiamen, Fujian 361005, P. R. China
3. Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen, Fujian 361005, P. R. China