1. Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, 1219 Zhongguan West Rd, Ningbo 315201, Zhejiang, P. R. China
2. Department of Chemistry, Ångström Laboratory Uppsala University, P.O. Box 538, Uppsala SE-751 21, Sweden
3. Max Planck Institute for Polymer Research, Ackermannweg 10, Mainz 55128, German
4. Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, 19A Yuquan Rd, Shijingshan District, Beijing 100049, P. R. China