1. Laboratory of Atomic and Solid-State Physics, Cornell University, Ithaca, NY 14853, USA.
2. Center for Nanoscale Systems, Cornell University, Ithaca, NY 14853, USA.
3. National Institute for Nanotechnology, National Research Council of Canada, Edmonton, AB T6G 2M9, Canada.
4. Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14853, USA.