[1] Univ Cambridge, Nanosci Ctr, Cambridge CB3 0FF, England
[2] Univ Piemonte Orientale Amedeo Avogadro, Ctr Interdisciplinare Nanosistemi, Dipartimento Sci & Tecnol Avanzate, I-15121 Alessandria, Italy
[3] Ctr Ric Fiat, I-10043 Orbassano, TO, Italy
[4] Tech Univ Dresden, D-01062 Dresden, Germany
[5] Univ Coll Cork, Tyndall Natl Inst, Cork, Ireland
[6] Catalan Inst Nanotechnol CIN2 CSIC, Bellaterra 08193, Spain