1. Department of Physics
2. Center of Integrated Nanomechanical Systems
3. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
4. Department of Chemistry, University of California at Berkeley, Berkeley, California 94720, United States
5. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
6. Condensed Matter and Materials Division, Lawrence Livermore National Laboratory, Livermore, California 94554, United States