Nature Nanotechnology 5, 417–422 (2010); published online: 30 May 2010; corrected after print: 9 February 2011.

In the version of this Letter originally published, components of the systems illustrated in Figs 3a–d, 4a and 4d were incorrectly labelled. In the Supplementary Information, components of the systems illustrated in Figs S7a, S9a–c and S10 were also incorrectly labelled. These errors have now been corrected in the HTML and PDF versions of the text, and in the Supplementary Information.