Nature Materials 9, 205–213 (2010); published online 19 February 2010; corrected online 1 September 2010. In Fig. 1a of the version of this Review originally published, the graph labelled '2-μm-thick Si wafer' is that for a 10-μm-thick Si wafer. The corrected figure is shown below. The original figure caption and descriptions in the text are correct.