1. Department of Physics, University of California at Berkeley, Berkeley, California 94720, United States
2. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
3. Department of Physics, Yonsei University, Seoul 03722, Korea
4. Center for Nanomedicine, Institute for Basic Science, Seoul 03722, Korea
5. National Center for Electron Microscopy, The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
6. Kavli Energy NanoSciences Institute at the University of California at Berkeley, Berkeley, California 94720, United States