1. School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853, USA.
2. Electron Microscopy Group of Materials Science, University of Ulm, Ulm 89081, Germany.
3. Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, NY 14853, USA.
4. Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, NY 14853, USA.