[1] Nanyang Technol Univ, Sch Mech & Aerosp Engn, Singapore 639798, Singapore
[2] Nanyang Technol Univ, Sch Phys & Math Sci, Div Phys & Appl Phys, Singapore 637371, Singapore
[3] Natl Univ Singapore, Dept Mat Sci & Engn, Singapore 119260, Singapore
[4] Univ Sci & Technol China, Hefei Natl Lab Phys Sci Microscale, Hefei 230026, Peoples R China
[5] Univ Sci & Technol China, Dept Phys, Hefei 230026, Peoples R China