1. Department of Physics, University of California, Berkeley, California 94720, United States
2. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
3. Department of Chemistry, University of California, Berkeley, California 94720, United States
4. Center of Integrated Nanomechanical Systems, Berkeley, California 94720, United States
5. Physical Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California 94550, United States