1. Department of Physics, Central Washington University, Ellensburg, Washington 98926, USA
2. The University of Tennessee, Knoxville, Tennessee 37996, USA
3. Salk Institute for Biological Studies, 10010 N Torrey Pines Road, La Jolla, California 92037, USA
4. Materials Science and Engineering Program, University of California, San Diego, La Jolla, California 92093, USA
5. Center for Advanced Nanoscience, University of California, San Diego, La Jolla, California 92093, USA
6. Department of Physics, University of California, San Diego, La Jolla, California 92093, USA