1. School of Materials Science and Engineering; Nanyang Technological University; 50 Nanyang Avenue Singapore 639798 Singapore
2. Centre for Biomimetic Sensor Science; Nanyang Technological University; 50 Nanyang Drive Singapore 637553 Singapore
3. State Key Laboratory on Integrated Optoelectronics; College of Electronic Science and Engineering; Jilin University; Changchun 130012 P. R. China
4. School of Chemical and Biomedical Engineering; Nanyang Technological University; 62 Nanyang Drive Singapore 637459 Singapore