Nature Nanotechnology 9, 1002–1006 (2014); published online 2 November 2014; corrected after print 20 May 2016.

In the version of this Letter originally published, the following sentence was not included in the Acknowledgements: 'Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under contract no. DE-AC02-05CH11231.' This has been added in the online versions of the Letter.