Correction to: Nature https://www.nature.com/articles/s41586-019-0976-y, published online 25 February 2019.

In this Letter, the Acknowledgements section should have included the following text: “the scanning transmission electron microscopy measurements at the Molecular Foundry were supported by the Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under contract number DE-AC02-05CH11231”. This omission has been corrected online.