1. Department of Electrical and Electronic Engineering, the University of Hong Kong, Hong Kong, P. R. China
2. Department of Physics, The Hong Kong University of Science and Technology, Hong Kong, P. R. China
3. Department of Chemistry, The Hong Kong University of Science and Technology, Hong Kong, P. R. China
4. School of Chemistry, Beihang University, Beijing 100191, P. R. China
5. Department of Chemistry, the University of Hong Kong, Hong Kong, P. R. China