1. Electrical Engineering Division, Engineering Department, University of Cambridge, 9 JJ Thomson Avenue, CB3 0FA, Cambridge, U.K.
2. Current address: Nokia Research Centre c/o Nanoscience Centre, University of Cambridge, 11 JJ Thomson Avenue, CB3 0FF, Cambridge, U.K.
3. Contributed equally to this work.
4. Nanoscience and Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, Cambridge, U.K.