“The technique … allows one to effectively investigate the evolution of physical properties of 2D materials with respect to twist angles, including low-frequency interlayer modes, band structure, optical and electrical properties”Yaping Yang
An international team from the University of Manchester have demonstrated a new way to fine tune the angle, or “twist”, between atom-thin materials in van der Waals heterostructures, a breakthrough that helps control the interlayer twist angle to offer a range of possible applications. The method was shown to exhibit in situ dynamical rotation and manipulation of 2D materials that were located on top of each other to form van der Waals heterostructures, nanoscale devices that offer unusual properties and interesting phenomena.
With the tuning of the twist angle to control the topology and electron interactions in 2D materials being increasingly investigated, this technique, reported in Science Advances [Yang et al. Sci Adv. (2020) DOI: 10.1126/sciadv.abd3655], enables twisted van der Waals heterostructures with dynamically tuneable optical, mechanical and electronic properties. Twisting together layers of 2D crystals results in a moiré pattern where lattices of the parent 2D crystals form a superlattice, which would bring precise positioning, rotation and manipulation, and also changes in the behavior of electrons in the system.
The team managed to fabricate heterostructures where graphene is perfectly aligned with both top and bottom encapsulating layers of hexagonal boron nitride, producing double moiré superlattices at the two interfaces. A glass slide with a droplet of polydimethylsiloxane (PDMS) was used as a manipulator, which is cured and naturally shaped into a hemisphere geometry, while also depositing an epitaxial polymethyl methacrylate (PMMA) patch on top of a target 2D crystal.
Manipulating the target flakes involved lowering the polymer gel handle to bring the PDMS hemisphere into contact with the PMMA patch. It was then straightforward to move or rotate the target 2D crystals on the surface of the bottom flake, made possible by the superlubricity between the two crystalline structures.
The method allows for continuous tuning of the twist angle between the layers even after heterostructure assembly. It is possible to design the patch into any shape, typically taking the geometry that fits the target flake. This patch has a key role in the manipulation, with the contact area of the polymer gel manipulator being limited to the patterned shape of the epitaxial polymer layer, allowing precise control of the manipulation and a much greater controlling force to be applied.
This approach is non-destructive and can manipulate flakes irrespective of their thickness. As main author Yaping Yang said, “The technique … allows one to effectively investigate the evolution of physical properties of 2D materials with respect to twist angles, including low-frequency interlayer modes, band structure, optical and electrical properties”. The team are now exploring how in twistronics the topology and electron interactions in twisted 2D material system are highly dependent on the twist angles.
Fine tuning the “twist” between 2D materials in van der Waals heterostructures