1. School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue Singapore 639798 Singapore
2. Department of Chemistry Northwestern University Evanston Illinois 60208 USA
3. Department of Materials Science and Engineering Northwestern University Evanston Illinois 60208 USA
4. Department of Physics University of Michigan Ann Arbor Michigan 48109 USA
5. Institute of Materials Research and Engineering A*STAR (Agency for Science, Technology and Research) 2 Fusionopolis Way, Innovis #08-03 Singapore 138634 Singapore