[1] Southeast Univ, Dept Phys, Nanjing 211189, Jiangsu, Peoples R China
[2] Nanyang Technol Univ, Sch Phys & Math Sci, Div Phys & Appl Phys, Singapore 637371, Singapore
[3] Univ New S Wales, Sch Mat Sci & Engn, Sydney, NSW 2052, Australia
[4] Natl Univ Singapore, Dept Mat Sci & Engn, Singapore 119260, Singapore