1. State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China
2. University of Science and Technology of China, Chinese Academy of Sciences, Hefei 230026, P. R. China
3. Department of Physics, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong 999077, P. R. China
4. Shenzhen Research Institute, City University of Hong Kong, Shenzhen 518057, P. R. China