1. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
2. Gatan, Incorporated, 5794 West Las Positas Boulevard, Pleasanton, CA 94588, USA.
3. National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
4. Department of Structural Biology, Medical School, Stanford University, Stanford, CA 94305, USA.
5. Department of Material Science and Engineering, University of California, Berkeley, CA 94720, USA.