1. Graduate Program of Materials Science and Engineering, University of California, San Diego, La Jolla, California 92037, USA
2. Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, California 92037, USA
3. Department of Nanoengineering, University of California, San Diego, La Jolla, California 92037, USA
4. Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, New Mexico 87185, USA