Nature Nanotechnology 8, 452–458 (2013); published online 05 May 2013; corrected after print 11 July 2013 and 28 August 2013.

Editorial note: significant concerns have been raised about the validity of the data reported in this work. After an internal inquiry, we contacted the authors' institution, the National Chiao Tung University, and asked them to launch a formal investigation into the matter. This investigation is now underway.