1. Department of Chemistry, University of California, Berkeley, California 94720, United States
2. Molecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
3. Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States
4. Center for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois 60439, United States