[1] Natl Univ Singapore, Dept Phys, Singapore 117551, Singapore
[2] Natl Univ Singapore, Dept Biol Sci, Ctr BioImaging Sci, Singapore 117543, Singapore
[3] Natl Univ Singapore, Ctr Adv Mat 2D, Singapore 117546, Singapore
[4] Natl Univ Singapore, Graphene Res Ctr, Singapore 117546, Singapore
[5] Natl Univ Singapore, Fac Engn, NUSNNI NanoCore, Singapore 117576, Singapore