Correction to: Nature Nanotechnology https://doi.org/10.1038/s41565-017-0023-9, published online 18 December 2017.

In the version of this Article originally published, the second affiliation for Walter A. de Heer had not been included; it should be ‘TICNN, Tianjin University, Tianjin, China’. This has now been added and the numbering of subsequent affiliations amended accordingly in all versions of the Article.