1. Department of Physics and Institute of Advanced Materials, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Kowloon 224, Hong Kong, P. R. China
2. Institute of Molecular Functional Materials, Department of Chemistry and Institute of Advanced Materials, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Kowloon 224, Hong Kong, P. R. China