1. Department of Physics, Chalmers University of Technology, 412 96 Gothenburg, Sweden
2. National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
3. Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, 8700 Leoben, Austria
4. Department of Materials Science and Engineering, University of California, Berkeley, California 94720, United States