1. Department of Physics, Columbia University, New York, NY 10027, USA.
2. Department of Chemistry, Columbia University, New York, NY 10027, USA.
3. Energy Frontier Research Center, Columbia University, New York, NY 10027, USA.
4. Department of Physics and Graphene Research Institute, Sejong University, Seoul 143-747, Korea.
5. Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
6. Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973–5000, USA.
7. Department of Electrical Engineering, Columbia University, New York, NY 10027, USA.
8. Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027, USA.