This work was partially supported by the Materials Research Science and Engineering Center under NSF Award No. DMR-0820484. We acknowledge the use of the facili- ties at the Harvard Center for Nanoscale Systems supported by NSF Award No. ECS-0335765. We thank Dr. Alison Grinthal for her help to prepare the manuscript. S.H.K. thanks Drs. Ning Wu and Alison Grinthal for helpful discussions.