[1] Univ Cambridge, Nanosci Ctr, Cambridge CB3 0FF, England
[2] Univ Cambridge, Dept Mat Sci & Met, Cambridge CB2 3QZ, England
[3] Natl Univ Singapore, Dept Phys, Singapore 117542, Singapore
[4] Natl Univ Singapore, Nanosci & Nanotechnol Initiat, Singapore 117542, Singapore