Van der Waals (VdW) heterostructures are materials which are composed of 2D material layers stacked on top of each other. In these materials, intermolecular interactions (van der Waals forces) hold the layers together. The rotational alignment of the layers of these heterostructures is thought to have a big impact on the electronic properties of the material. However, controlling the rotation of a 2D material flake is a challenge.

Shuze Zhu, Harley T. Johnson, University of Illinois at Urbana-Champaign, USA, Pascal Pochet, Université Grenoble Alpes, France, and colleagues have used molecular dynamics (MD) simulations to investigate the internal rotation of the layers in VdW heterostructures. The team simulated a 2D bilayer system in which the top layer takes the shape of an equilateral triangle. They evaluated the relationship between the preferred rotational orientation and the flake size.

The simulation results showed that the interfacial energy landscape continually changes with the flake size. The team also found that the moiré patterns formed by the layers are rotation dependent. Moiré patterns are interference patterns that occur, for example, when two transparent fabrics are layered. According to the simulations, rotation is fundamentally driven by this interface lattice moiré. The team also found that strain engineering can be used to change the energy landscape of the moiré in a finite region, and in turn, change the interlayer rotation within different VdW structures. This could help with the design of rotation-tunable electronics.

• Controlling Rotation of Two-Dimensional Material Flakes,
  Shuze Zhu, Pascal Pochet, Harley T. Johnson,
  ACS Nano 2019.
  https://doi.org/10.1021/acsnano.9b01794