1. Department of Physics and Center
of Integrated Nanomechanical Systems, University of California at Berkeley, Berkeley, California 94720, United States
2. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California
94720, United States
3. Department
of Mechanical Engineering
and Materials Science, Department of Chemistry, and the Richard E.
Smalley Institute for Nanoscale Science and Technology, Rice University, Houston, Texas 77005, United States