[1] Nanyang Technol Univ, Sch Math & Phys Sci, Div Phys & Appl Phys, Singapore 637371, Singapore
[2] Nanchang Univ, Educ Minister Engn Res Ctr Luminescence Mat & Dev, Nanchang 330047, Jiangxi, Peoples R China
[3] Nanyang Technol Univ, Sch Elect & Elect Engn, Singapore 639798, Singapore