Correction to: Nature Nanotechnology https://doi.org/10.1038/s41565-017-0026-6, published online 8 January 2018.

In the version of this Article originally published online, in Fig. 4b, in the lower-right image, the value of r was incorrect; it should have read ‘r = 0.72’. This has now been corrected in all versions of the Article.