Nature Nanotechnology 4, 186–192 (2009); published online: 1 February 2009; corrected after print: 4 November 2010.

In the version of this Article originally published, a systematic error affected all the values of microstiffness presented in the paper (including in the figures and figure captions). This error resulted in all the values of microstiffness being too high by a factor of √π. The values for nanostiffness are not affected and this error does not affect the conclusions of the paper. This error has now been corrected in the HTML and PDF versions of the text. The Supplementary Information and corresponding e-mail address have also been updated.