1. Department of Electrical and Computer Engineering; National University of Singapore; 4 Engineering Drive 3 Singapore 117583 Singapore
2. Lab of Advanced Materials; Department of Chemistry; Fudan University; Shanghai 200433 China
3. Engineering Science Programme; National University of Singapore; 9 Engineering Drive 1 Singapore 117575 Singapore
4. Institute of Materials Research and Engineering; A*STAR (Agency for Science, Technology and Research); 3 Research Link Singapore 117602 Singapore