1. Departments of Chemistry and Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
2. Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, United States
3. Department of Chemistry and Smalley Institute for Nanoscale Science and Technology, Rice University, Houston, Texas 77005, United States
4. California NanoSystems Institute and Departments of Chemistry & Biochemistry and Materials Science & Engineering, University of California, Los Angeles, Los Angeles, California 90095, United States