This work was supported by the AFOSR under Grant No. FA9550–09-1–0669-DOD35CAP and by the Materials Research Science and Engineering Center under NSF Grant No. DMR-0820484. We acknowledge the use of the facilities at the Harvard Center for Nanoscale Systems supported by NSF Grant No. ECS-0335765. We thank Professor L. Mahadevan and Dr. Philseok Kim for helpful discussions.