1. School of Physics and Electronic Information Engineering, Henan Polytechnic University, Jiaozuo, P. R. China;
2. Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi'an Jiaotong University, Xi'an, P. R. China;
3. School of Materials Science and Engineering, Northeastern University, Shenyang, P. R. China