1. National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
2. Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA
3. Physics of Nanostructured Materials, Faculty of Physics, University of Vienna, 1090 Vienna, Austria
4. Institute for Advanced Research, Nagoya University, Nagoya 464-8603, Japan