Nature Nanotechnology 12, 914–919 (2017); published online 19 June 2017; corrected after print 26 September 2017

In the version of this Article originally published, the label in Fig. 1d that said 'σ = π' was incorrect, and should have said 'α = π'. This has been corrected in the online version and the correct panel is shown here. This change does not affect the results of the paper.