1. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
2. Mechanical Engineering Department, University of California, Berkeley, California 94720, USA
3. National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
4. Materials Science and Engineering Department, University of California, Berkeley, California 94720, USA
5. Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Kong