[1] Nanjing Normal Univ, Sch Phys Sci & Technol, Nanjing 210023, Peoples R China
[2] Southeast Univ, Sch Phys, Minist Educ, Nanjing 211189, Peoples R China
[3] Southeast Univ, Key Lab MEMS, Minist Educ, Nanjing 211189, Peoples R China
[4] ASTAR, Inst Mat Res & Engn IMRE, 2 Fusionopolis Way,Innovis 08-03, Singapore 138634, Singapore
[5] Nanyang Technol Univ, Sch Phys & Math Sci, Singapore 637371, Singapore
[6] Natl Univ Singapore, Dept Mat Sci & Engn, 9 Engn Dr 1, Singapore 117575, Singapore
[7] Nanyang Technol Univ, Sch Elect & Elect Engn, Singapore 639798, Singapore