1. Department of NanoEngineering; University of California; San Diego, La Jolla CA 92093 USA
2. Department of Electrical and Computer Engineering; University of California; San Diego, La Jolla CA 92093 USA
3. Department of Physics; University of California; San Diego, La Jolla CA 92093 USA
4. Materials Science and Engineering; University of California; San Diego, La Jolla CA 92093 USA