1. School of Physics and Electronic Engineering, Chongqing Normal University, Chongqing 401331, People's Republic of China
2. School of Physics, Chongqing University, Chongqing 401331, People's Republic of China
3. State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People's Republic of China