[1] Linkoping Univ, Dept Phys Chem & Biol, SE-58183 Linkoping, Sweden
[2] Natl Nano Device Labs, Hsinchu 300, Taiwan
[3] Heriot Watt Univ, Dept Phys, Edinburgh EH14 4AS, Midlothian, Scotland
[4] Univ Surrey, Dept Phys, Guildford GU2 7XH, Surrey, England
[5] Johannes Kepler Univ Linz, Inst Halbleiter & Festkorperphys, A-4040 Linz, Austria
[6] Univ Cambridge, Cavendish Lab, Cambridge CB3 0HE, England