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