1. Department of Physics, Hong Kong University of Science and Technology, Kowloon, 999077Hong Kong, China
2. Department of Chemistry, Key Laboratory of Surface & Interface Science of Polymer Materials of Zhejiang Province, Zhejiang Sci-Tech University, Hangzhou310018, China
3. Materials Characterization and Preparation Facility (GZ), Advanced Materials Thrust, The Hong Kong University of Science and Technology (Guangzhou), Nansha, Guangzhou511400, Guangdong, China
4. Department of Chemical and Biological Engineering, Hong Kong University of Science and Technology, Kowloon, 999077Hong Kong, China
5. Department of Materials Science & Engineering, Northwestern University, Evanston, Illinois60208-3120, United States
6. William Mong Institute of Nano Science and Technology, Hong Kong University of Science and Technology, Kowloon, 999077Hong Kong, China