1. Department of Materials Science; Fudan University; Shanghai 200433 P. R. China
2. Department of Physics; Faculty of Science; King Abdulaziz University; Jeddah 21589 Saudi Arabia
3. Key Laboratory of Functional Polymer Materials and the Centre of Nanoscale Science and Technology; Institute of Polymer Chemistry; College of Chemistry; Nankai University; Tianjin 300071 P. R. China