1. Department of Physics, University of California at Berkeley, Berkeley, CA 94720, USA.
2. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
3. Department of Materials Science and Engineering, KAIST, Daejeon, 305-701, South Korea.
4. Department of Chemistry, University of California at Berkeley, Berkeley, CA 94720, USA.
5. National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
6. Center of Integrated Nanomechanical Systems, University of California at Berkeley, Berkeley, CA 94720, USA.