1. Department of Materials Science and Engineering, Cornell University , Ithaca, NY , USA
2. Department of Physics, Cornell University , Ithaca, NY , USA
3. Department of Chemistry, Princeton University , Princeton, NJ , USA
4. Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory , Upton, NY , USA
5. School of Applied and Engineering Physics, Cornell University , Ithaca, NY , USA
6. Department of Chemistry, Cornell University , Ithaca, NY , USA