1. Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California 94550, United States
2. Department of Physics, University of California, Berkeley, California 94720, United States
3. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
4. Center of Integrated Nanomechanical Systems, Berkeley, California 94720, United States
5. Kavli Energy NanoSciences Institute at the University of California, Berkeley, and the Lawrence Berkeley National Laboratory, Berkeley, California 94720 United States