[1] Natl Univ Singapore, Dept Elect & Comp Engn, Singapore 117583, Singapore
[2] Southern Univ Sci & Technol, Dept Phys, Shenzhen 518055, Peoples R China
[3] Natl Univ Singapore, Dept Mat Sci & Engn, Singapore 117575, Singapore
[4] ASTAR, Inst Mat Res & Engn, Singapore 117602, Singapore