1. School of Materials Science and Engineering; Nanyang Technological University; Singapore 639798 Singapore
2. School of Electrical and Electronic Engineering; Nanyang Technological University; Singapore 639798 Singapore
3. Department of Polymer Science and Engineering; Zhejiang University; Hangzhou 310027 P. R. China
4. Department of Electrical and Electronic Engineering; College of Engineering; South University of Science and Technology of China; Shenzhen 518055 P.R. China
5. Division of Chemistry and Biological Chemistry; School of Physical and Mathematical Sciences; Nanyang Technological University; Singapore 637371 Singapore