1. Institute for Advanced Materials and Technology University of Science and Technology Beijing Beijing 100083 P. R. China
2. School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue Singapore 639798 Singapore
3. School of Biological Sciences Nanyang Technological University 50 Nanyang Avenue Singapore 639798 Singapore
4. Department of Physics University of South Florida Tampa FL 33620 USA
5. College of Science Hubei University of Technology Wuhan 430068 P. R. China
6. Institute of Metal Research Chinese Academy of Sciences Shenyang 110016 P. R. China
7. Department of Chemistry National University of Singapore 3 Science Drive 3 Singapore 117543 Singapore