1. Institute of Materials Research and Engineering; Agency for Science, Technology and Research (A*STAR); 2 Fusionopolis Way Singapore 138634 Singapore
2. Department of Physics; National University of Singapore; 2 Science Drive 3 Singapore 117542 Singapore
3. Research Institute for Biomimetics and Soft Matter; Fujian Provincial Key Laboratory for Soft Functional Materials Research; Xiamen University; Xiamen 361005 China