A door lock with two different keys: one turns easily, the other demands noticeable effort. That is how two layers of hexagonal boron nitride behave when twisted relative to each other. This material consists of boron and nitrogen atoms arranged in a hexagonal lattice, and its layers can sit in different registries, like two identical grids shifted by different angles. Scientists have found that there are two such stable registries, and the transition between them is controlled by simple rotation.
The experiment resembled a jeweler's work: tiny pillars of boron nitride, about 250 nanometers tall and with radii from 200 to 500 nanometers, were stacked on top of each other, and the top one was twisted with the tip of an atomic force microscope. This tip probes the surface like a finger, but billions of times more sensitively. Everything took place in a pure nitrogen atmosphere to keep water and oxygen from interfering. The instrument recorded how much force was needed for each degree of rotation.
Computer simulations showed why this happens. One state, called AA′, rearranges gradually, like plasticine that slowly flows from the edges toward the center. The other, AB, gives way abruptly, like a brittle ice crust that cracks all at once. Meanwhile, the force grows in proportion to the contact area, not the volume, which contradicts expectations for rigid bodies. This means that the edges play the main role, as they are especially sensitive to shear.
This result paves the way for mechanical polarization switches in nanoelectromechanical devices. By rotating the layers, one can control the electrical properties of the material, like turning a key to switch on a light. There are no household applications in sight yet, but in laboratories this approach will help precisely control friction at the atomic level and build reconfigurable systems. The door lock made of two atomic layers already clicks; now we just need to find the right door for it.
The torque required to rotate a disk of 500 nm radius is only about 10⁻¹⁸ N·m, comparable to the moment of force exerted by the weight of a single bacterium on a lever arm of one micrometer.