[1] Nanyang Technol Univ, Sch Mat Sci & Engn, Singapore 639798, Singapore
[2] Nanyang Technol Univ, Energy Res Inst ERI N, Singapore 637553, Singapore
[3] Natl Univ Singapore, NUS Nanosci & Nanotechnol Initiat, Singapore 117576, Singapore
[4] Natl Univ Singapore, Ctr Nanofibers & Nanotechnol, Singapore 117576, Singapore