1. School of Metallurgy and Environment Central South University Changsha 410083 China
2. Department of Chemistry City University of Hong Kong Kowloon Hong Kong 999077 China
3. Department of Applied Physics Hong Kong Polytechnic University Kowloon Hong Kong 999077 China
4. School of Physics and Electronics Hunan University Changsha 410083 P. R. China
5. Key Laboratory of Materials Processing and Mold (Zhengzhou University) Ministry of Education Zhengzhou University Zhengzhou 450002 China