[Ai, Wei; Jiang, Jian; Zhu, Jianhui; Yu, Ting] Nanyang Technol Univ, Sch Phys & Math Sci, Div Phys & Appl Phys, Singapore 637371, Singapore.
[Huang, Xintang; Jiang, Jian; Zhu, Jianhui] Cent China Normal Univ, Dept Phys, Inst Nanosci & Nanotechnol, Wuhan 430079, Hubei, Peoples R China.
[Fan, Zhanxi; Zhang, Hua] Nanyang Technol Univ, Sch Mat Sci & Engn, Singapore 639798, Singapore.
[Yu, Ting] Natl Univ Singapore, Graphene Res Ctr, Singapore 117546, Singapore.