1. Department of Polymer Science and Engineering; School of Chemistry and Biological Engineering; University of Science and Technology Beijing; Beijing 100083 P. R. China
2. Department of Physics and Materials Science; City University of Hong Kong; 83 Tat Chee Avenue Kowloon Hong Kong P. R. China
3. State Key Laboratory of Electrical System; Department of Electrical Engineering; Tsinghua University; Beijing 100084 P. R. China