1. State Key Laboratory of Polymer Physics and Chemistry; Institute of Chemistry; Chinese Academy of Sciences; Beijing 100190 China
2. Laboratory of Advanced Optoelectronic Materials; College of Chemistry; Chemical Engineering and Materials Science; Soochow University; Suzhou 215123 China
3. State Key Laboratory for Mechanical Behavior of Materials; Xi'an Jiaotong University; Xi'an 710049 P. R. China
4. Department of Physics; North Carolina State University; Raleigh NC 27695 USA