Fig. 2. Geometry of curved CdSe NPLs.

a, b STEM images of 5 ML helicoidal NPL⁴⁶. c Representation of the principal local curvatures of a surface. Left: the two principal curvatures have opposite signs leading to a negative Gaussian curvature κ₂ = c₁ × c₂. Right: the Gaussian curvature is zero. d Different conformations of a ribbon. The shadows below the different shapes correspond to their projections on a surface and should thus resemble the TEM images of the corresponding particles. e, g CdSe 3 ML NPLs folded into tubular structures. Reprinted with permission from refs. ^22,29. Copyright 2019 and 2013 American Chemical Society. f 3 ML NPLs adopting a spiral ribbon geometry. Reprinted with permission from ref. ⁴⁹. Copyright 2019 American Chemical Society. h–j Spiral ribbons coated with a silica shell. Reprinted with permission from ref. ⁴⁷. Copyright 2014 American Chemical Society. Scale bars correspond to 5 nm.