1. Department of Physics, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China
2. Department of Chemistry and Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration & Reconstruction, Hong Kong University of Science and Technology (HKUST), Clear Water Bay, Kowloon, Hong Kong, China
3. Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China
4. Department of Materials Science and Engineering, College of Engineering, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Peking University, Beijing 100871, China
5. Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States