Nature Nanotechnology 7, 459–464 (2012); published online 17 June 2012; corrected after print 22 October 2012; corrected after print 27 November 2012.

Editorial note: Following an investigation by the Chinese Academy of Sciences under the directive of Professor Chunli Bai, the investigating committee consisting of Professor Tao Xu, Professor Rixiang Zhu, Professor Jinghui Guo and Professor Ruiming Xu concluded that, owing to lack of communication between the authors, the original paper published online on 17 June 2012 contained errors but the central findings of the paper remain valid. The authors have reconciled their opinions and relevant sections of the paper have now been corrected through a Corrigendum.