1. Department of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing, P. R. China
2. Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing, P. R. China
3. College of Materials Science and Engineering, Tianjin University, Tianjin, P. R. China
4. Chemical Physics Interdisciplinary Program and Liquid Crystal Institute, Kent State University, Kent, OH, USA
5. Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Peking University, Beijing, P.R. China