Playing with the spontaneous curvature of nano-objects to control their properties

Semiconductor crystal nano wafers are extremely thin, flat objects belonging to the quantum dots family. These materials are so small (1000 times thinner than a human hair) that their properties are governed by quantum effects due to electron confinement. Awarded the Nobel Prize in Chemistry in 2023, quantum dots have exceptional optical properties that illuminate TV screens, LEDs and many other optoelectronic devices. But the application potential of these nanomaterials is far from explored. In particular, playing with their shape and geometry, even during use, could lead to new properties such as selective light reflection, new ways of conducting electricity and so on. In this context, flat and ultra-thin crystals are particularly attractive because of their ability to wrap around themselves like a ribbon. 

This is what scientists from the Laboratoire de Chimie (CNRS/ENS Lyon), the Institut Lumière Matter (CNRS/Université Claude Bernard Lyon 1), LadHyx (CNRS/Ecole Polytechnique/Institut Polytechnique de Paris) and the University of Sydney in Australia have set out to do in a controlled manner. The inspiration for this research came from the observation of numerous natural phenomena in which helical structures predominate, from the structuring of DNA to the spontaneous twisting of seed pods and the formation of galaxies.

Researchers have discovered that when cadmium selenide nanoplatelets are coated with a layer of small organic molecules, they spontaneously bend into complex shapes - tubes, spirals or helices. These transformations are driven by the forces exerted by the organic molecules capping the top and bottom faces of the nano-crystals. Properly controlled, this coiling can generate highly interesting chiral* objects. The spontaneous curvature, and hence the final structure of the crystal, is governed by several parameters, including the interaction between the ligands and the surface, as well as the geometry (width, thickness and symmetry) of the crystal.

This work, published in the Proceedings of the US National Academy of Sciences, provides a conceptual framework for understanding and controlling the shape of nanoplatelets with exceptional optical properties. This provides scientists with a new tool for designing materials with precisely tuned properties for use in technologies ranging from advanced electronics to smart, reactive materials.

*Like a right hand and a left hand, chiral objects have two structures that are symmetrical but not superimposable.