Nature Materials 7, 588–595 (2008); published online 25 May 2008; corrected after print 28 February 2013.

In the version of this Article originally published, in the caption for Fig. 1 the following statement should have been included “Right-hand STM image in panel a reproduced with permission from ref. 30, © 2008 RSC.” This error has been corrected in the PDF and HTML versions of the Article.