Nature Materials 15, 48–53 (2016); published online 9 November 2015; corrected online 18 November 2015; corrected after print 19 January 2016.

In the version of this Letter originally published, the name of one of the co-authors was misspelled and should have been 'Hyun Soo Han'. This has been corrected in the online versions after print 19 January 2016.