1. Key Laboratory of Functional Polymer Materials (Ministry of Education), Institute of Polymer Chemistry, College of Chemistry, Nankai University, Tianjin 300071, P. R. China
2. Department of Materials and Environmental Chemistry, Stockholm University, Stockholm 10691, Sweden
3. College of Chemistry, Nankai University, Tianjin 300071, P. R. China
4. Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon 999077, Hong Kong, P. R. China