O.C. acknowledges support from the Brown University Startup Fund, the Salomon Award Fund, the IMNI Seed Fund and the UAC grant from the Xerox foundation. The Cornell High Energy Synchrotron Source was supported by the NSF award DMR-1332208. This work was performed, in part, at the Center for Nanoscale Materials, a US Department of Energy Office of Science User Facility, and supported by the US Department of Energy, Office of Science, under contract number DE-AC02-06CH11357. The TEM and SEM measurements were performed at the Electron Microscopy Facility in the Institute for Molecular and Nanoscale Innovation (IMNI) at Brown University.

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Nature thanks J. Fang, A. Petukhov and the other anonymous reviewer(s) for their contribution to the peer review of this work.