1. Materials Science and Engineering Program, University of California, San Diego, La Jolla, California 92093, USA
2. Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, California 92093, USA
3. Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, New Mexico 87185, USA
4. Department of NanoEngineering, University of California, San Diego, La Jolla, California 92093, USA