1. Department of Materials Science and Engineering; National University of Singapore; 9 Engineering Drive 1 117575 Singapore
2. Department of Chemistry; National University of Singapore; 3 Science Drive 3 117543 Singapore
3. State Key Laboratory of Advanced Technology for Materials Synthesis and Processing; Wuhan University of Technology; Wuhan 430070 P. R. China
4. Department of Physics; National University of Singapore; 2 Science Drive 3 117542 Singapore