Nature Nanotechnology 5, 321–325 (2010); published online: 4 April 2010; corrected after print: 12 January 2011.

In the version of this Letter originally published, the symbols for 'Lithography (ref. 27)' and 'Sonochemical (ref. 1)' in the legend of Figure 4d were the wrong way round. This error has now been corrected in the HTML and PDF versions of the text.