1. Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, New Mexico 87185, USA
2. Geochemistry Department, Sandia National Laboratories, Albuquerque, New Mexico 87185, USA
3. Department of Engineering, University of Cambridge, Cambridge, Cambridgeshire CB3 0FA, United Kingdom
4. Electronic and Electrical Engineering Department, University College London, London WC1E 7JE, United Kingdom