Moiré patterns are interference fringes which appear when two identical gratings are placed on top of each other with a slight offset. In the case of 2D materials, if two layers are placed on top of each other with a small relative rotation, moiré interference can modify the electronic band structure of the stack so that, depending on the angle, it displays new properties such as superconductivity. Usually, twisted structures are created by mechanically picking up one layer, rotating it slightly and placing it on another, which makes it not only tedious but difficult to control and reproduce the angles. Now, writing in Science, Yuzhou Zhao and colleagues present a method to directly grow multiple layers of twisting spirals of 2D materials on curved substrates.
Crystal growers usually use flat substrates to ensure high quality, lattice-matched, layer-by-layer crystal growth, but sometimes screw dislocations can emerge and drive the crystal growth process. In 2D materials, a screw dislocation forms when there is shear which causes the atoms in one layer to slide vertically up and join the layer above. As the growth continues, a structure like a spiral staircase — or a screw — begins to appear. On a flat surface, the 2D crystal lattice remains oriented in the same direction; for example, in a triangular dislocation spiral, the edges of the triangles all line up (pictured; left). However, when a screw dislocation spiral grows on a cone-shaped surface, a relative angle appears between each successive layer. In this case, the edges of the triangle no longer line up (pictured; right). Each triangle is rotated from the one above and below it by a slight angle – perfect for observing a moiré interference pattern.
Zhao, the lead author of the Science paper has been working on screw dislocations in nanostructures for a long time and began to notice occasional interesting twisted structures in some samples. This happened “due to the combination of the screw dislocation spirals and curved surfaces” says Song Jin, the principal investigator. “Once we understood the underlying mechanism, we purposefully designed a more reproducible growth of these twisting spirals”. To create curved substrates in their experiment, Jin’s team melted nanoparticles onto the substrates to form cone shapes before growth, which led to the more consistent formation of twisted spirals. A simple mathematical model describes how curvature of the substrate could be used to control the relative twist angle between each layer.
“The translation from theory to experiment was surprisingly quite simple” Jin notes. “However, this is not yet a deterministic process. Not every single object is a spiral, not every spiral twists, and the twisting varies from one object to the next.” He hopes to develop more precise control of the twist angle, expand to more 2D materials, and study the electronic and optical properties of these twisted 2D structures.
Zhao, Y. et al. Supertwisted spirals of layered materials enabled by growth on non-Euclidean surfaces. Science 370, 442–445 (2020)