Nature Materials 12, 1137–1143 (2013); published online 17 November 2013; corrected after print 21 November 2013.

In the version of this Article originally published, the middle initials of the penultimate author were missing; the name should have read Ifan E. L. Stephens. In the Author contributions and Additional information sections 'I.S.' should have read 'I.E.L.S.' These errors have now been corrected in the online versions of the Article.