1. Analytical and Testing Center, Chongqing University, Chongqing 401331, P. R. China
2. College of Physics, Chongqing University, Chongqing 401331, P. R. China
3. School of Science, Chongqing University of Posts and Telecommunications, Chongqing 400065, P. R. China
4. College of Materials Science and Engineering, Chongqing University, Chongqing 400044, P. R. China