Nature 412, 169–172 (2001).

We inadvertently omitted to cite an earlier reference alongside ref. 8 (G. Che, B. Lakshmi, E. R. Fisher and C. R. Martin Nature 393, 346–349 (1998), which was published in 1995 (and not 2000 as printed). Also, our suggestion that using the pores in a microporous material as templates could be a way in which to produce nanoscale materials has been discussed before (see, for example, C. R. Martin Science 266, 1961–1966 (1994) and J. C. Hulteen & C. R. Martin J. Mater. Chem. 7, 1075–1087 (1997)).