Nature Materials http://dx.doi.org/10.1038/nmat4492 (2015); published online 7 December 2015; corrected online 15 December 2015.

In the original version of this Letter published online, the x axis of Fig. 2a was labelled incorrectly. In addition, tick labels have been added to the x axes of Fig. 2b and Fig. 2d. This has been corrected in all versions of the Letter.