1. College of Physics, Qingdao University, Qingdao 266071, China
2. College of Electronic and Information Engineering, Qingdao University, Qingdao 266071, China
3. Lab of New Fiber Materials and Modern Textile, Growing Base for State Key Laboratory, Qingdao University, Qingdao 266071, China
4. Electronic Ceramics Center, DongEui University, Busan 614-714, Korea