1. Department of Chemistry and Institute of Advanced Materials, Hong Kong Baptist University, Kowloon 224, Hong Kong, P. R. China
2. Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong 999077, P. R. China
3. State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, P. R. China