1. Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California 94720, USA
2. National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
3. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
4. Accelerator Technology and Applied Physics Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA