[1] Natl Phys Lab, Teddington TW11 0LW, Middx, England
[2] Queens Univ Belfast, Sch Maths & Phys, Ctr Nanostruct Media, Belfast BT7 1NN, Antrim, North Ireland
[3] Queen Mary Univ London, Sch Engn & Mat Sci, London E1 4NS, England
[4] Queen Mary Univ London, Nanoforce Technol Ltd, London E1 4NS, England