Nature Nanotechnology 9, 768–779 (2014); published online 6 October 2014; corrected after print 5 November 2014.

In the version of this Review Article originally published, in Fig. 4c, the y-axis values were incorrect; they should have been '100, 102, 104 and 106' (from bottom to top). This error has now been corrected in the online versions of the Review Article.