Graphene is a carbon film exactly one atom thick. Two such sheets placed on top of each other with a slight twist behave very differently from a single sheet. At an angle of about 1.1 degrees, electrons slow down so much that the material can become a superconductor or an insulator — depending on the conditions. This angle is called the "magic angle."
The problem is that stacking two sheets with a precise twist by hand is nearly impossible. Scientists in China have found a way to grow twisted graphene directly on a platinum substrate. They noticed that different grains of platinum (regions with different atomic orientations) have different catalytic activity — graphene grows faster on some and slower on others. By choosing neighboring grains with strongly differing activity, the growing graphene layer folds or tears by itself at the right spot, forming a twist with a predetermined angle.
The researchers used chemical vapor deposition (CVD)—a method where carbon from gaseous methane settles on hot platinum and forms a graphene film. By observing the growth in real time, they found that the orientation of graphene depends on the microrelief of the platinum grain's surface. Atomic steps on the surface act as guides: graphene grows along them, and when transitioning to another grain, it can change direction and fold.
In this way, they obtained not only twisted layers with arbitrary angles but also with the magic angle, where electronic bands become flat. This means electrons stop accelerating and move at the same speed, greatly enhancing interactions between them and giving rise to unusual quantum states.
The method is suitable for mass production: there's no need to manually transfer and align layers; everything happens in a single growth process. This brings us closer to creating electronic devices based on twisted graphene — from ultra-sensitive sensors to elements of quantum computers.
Platinum is not just a substrate but a catalyst. It decomposes methane into carbon and hydrogen, and it is the carbon that assembles into the graphene lattice. Different facets of the platinum crystal work at different rates, which allows controlling the growth.
Science fiction often features materials with programmable properties that change with the twist of layers. For example, Neal Stephenson's novel "The Diamond Age" describes smart materials that respond to structure. Twisted graphene is a real step toward such technologies.