Nature Materials 9, 239–244 (2010); published online: 14 February 2010; corrected after print: 19 February 2010. In the version of this Letter originally published, the first sentence in the Acknowledgements should have been: “This work was supported by BP and in part by the Department of Energy EFRC program under grant DE-SC0001293, and made use of facilities supported by the Center for Science and Engineering of Materials, an NSF Materials Research Science and Engineering Center at Caltech.