Ultrathin colloidal nanoplatelets (NPL) are ligand-coated sheet-like crystalline objects with atomic scale thickness and possessing immense potential for optoelectronic applications. In solutions NPLs can adopt a variety of shapes such as cylinders, helicoids, and helical ribbons which play an important role in determining their optical properties. However the fundamental physical reasons behind the origin of multiple shapes remain poorly understood, hindering our ability to engineer NPL shapes and subsequently control their optical properties.
Here we use a combination of experiments, simulation, and theory to show that the interaction of the adsorbed ligands with the crystal surface leads to surface stresses that differ between the top and bottom surfaces and eventually forces the NPL to its ultimate deformed shape. The deformed shape is dictated by the NPL aspect ratio, thickness, orientation of the crystal with respect to the edges, and the ligand-crystal interaction, though most of the molecular details can be lumped into the spontaneous curvature. Furthermore, NPLs display a smooth transition between helicoids and helical ribbons beyond a certain critical width, a key feature of geometrically frustrated assemblies.
