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Y.B.Z and B.K.J contributed equally to this work. This research was supported by the Air Force Office of Scientific Research (FA9550-08-1-0349), the National Science Foundation (ECCS-0801922, ECCS-0609128, and ECCS-0609128), and the Penn State Center for Nanoscale Science (MRSEC). Components of this work were conducted at the Pennsylvania State University node of the NSF-funded National Nanotechnology Infrastructure Network. Y.B.Z. recognizes the support from KAUST Scholar Award and the Founder's Prize and Grant of the American Academy of Mechanics. The authors thank I-Kao Chiang, Aitan Lawit and Thomas R. Walker for helpful discussions. |