We thank Prof. Huajian Gao for helpful discussions.
This work was supported by the National Science Foundation through Materials
Research Science and Engineering Centers Program DMR-0520651 at
Brown University, Materials Research Science and Engineering Centers Program
DMR-1420570 at Harvard University, and the Korean Institute of Machinery
and Materials–Brown Nano and Micromechanics for Disaster
Mitigation and Technological Reliability project.