1. Departments of Chemistry and Physics, University of California, Irvine, California 92697, United States
2. Nano-Science Center and Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen Ø, Denmark
3. Department of Physics, University of Arizona, 1118 East Fourth Street, Tucson, Arizona 85721, United States
4. Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States