Nature Nanotechnology 6, 511–516 (2011); published online 24 July 2011; corrected after print 22 November 2011.

In the version of this Letter originally published, in the discussion of Fig. 5c on page 514, the fitting function should have been Δ = A|cos(θθ0 ± π/2)| + B, and the offsets of θ0 should have been −30 ± 4° and −39 ± 5° for Vsg = −0.5 V and 1.0 V, respectively. These errors have been corrected in the HTML and PDF versions of the Letter.