1. Miller Institute for Basic Research in Science, University of California , Berkeley, CA 94720 , USA
2. National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory , Berkeley, CA , USA
3. Department of Materials Science and Engineering, Northwestern University , Evanston, IL , USA
4. Department of Materials Science and Engineering, University of California , Berkeley, CA , USA
5. Department of Physics, University of California , Berkeley, CA , USA
6. Materials Sciences Division, Lawrence Berkeley National Laboratory , Berkeley, CA , USA