Correction to: Nature Materials https://doi.org/10.1038/s41563-018-0109-2, published online 18 June 2018.

In the version of this Perspective originally published, the x-axis label of Fig. 1d was missing; it should have read ‘Wavelength (nm)’. The citations to Fig. 1g and Fig. 1h in the main text were incorrect; they should have been to Fig. 1f and Fig. 1g, respectively. The units of the y axis of Fig. 2c were missing; they should have read nm. The values under the x-axis tick marks for Fig. 2e were missing, they should have read 1,355, 1,370 and 1,385. The units of the y axis of Fig. 3b were incorrect; they should have been meV. The first citation to Fig. 3b in the main text was incorrect; it should have been to Fig. 3a. The citation of Fig. 3c in the main text was incorrect; it should have been to Fig. 3b. And the two instances in the 'Outlook' section of the units eV were incorrect; they should have been µeV. These issues have now been corrected.