We thank J. C. Weaver for advice with the SEM imaging, S. K. Y. Tang and R. Sadza for the microfluidic experiments, L. Hendriks for growing the structures in Fig. 5F, and A. J. Aizenberg for help with the manuscript. This work was supported by the NSF Materials Research Science and Engineering Centers under award no. DMR-0820484. W.L.N. thanks the Netherlands Organization for Scientific Research for financial support. EM was performed at Harvard’s Center for Nanoscale Systems, supported by the NSF under award no. ECS-0335765.