Correction to: Nature Nanotechnology https://doi.org/10.1038/nnano.2016.280, published online 9 January 2017.
In the version of this Article originally published, incorrect images were used for columns 4–6 in Fig. 4a. Also, the fluorescent scale of columns 1–3 was inconsistent with the others, and the scale bar was missing. The original and corrected panels of Fig. 4a are shown below. This does not affect the text or the conclusions of the paper.