[1] Natl Univ Singapore, Dept Phys, Singapore 117548, Singapore
[2] Harbin Inst Technol, Dept Astronaut Sci & Mech, Harbin 150006, Peoples R China
[3] Zhejiang Univ, Dept Mat Sci & Engn, Hangzhou 310003, Zhejiang, Peoples R China
[4] Natl Univ Singapore, Dept Chem, Singapore 117548, Singapore