1. Center for Integrated Nanotechnologies, Sandia National Lab, Albuquerque, New Mexico 87185, United States
2. Sandia National Laboratories, Albuquerque, New Mexico 87185, United States
3. University College London, Electronic and Electrical Engineering, London WC1E 7JE, U.K.
4. Center for High Technology Materials, University of New Mexico, Albuquerque, New Mexico 87185, United States