Nature Materials 11, 301–305 (2012); published online 26 February 2012; corrected after print 16 February 2015.

In the version of this Letter originally published, Khashayar Ghaffarzadeh, with affiliation at the London Centre for Nanotechnology, University College London, London WC1H 0AH, UK, had been omitted from the list of authors. Following internal and external investigations, this has now been corrected in the online versions of the Letter. Sanghun Jeon, Seung-Eon Ahn, Ihun Song, Chang Jung Kim, U-In Chung, Inkyung Yoo and Kinam Kim disagree with this correction.