Optimization of “twist” between 2-D materials in van der Waals heterostructures to accelerate next-generation electronics

A group of international researchers from the University of Manchester has revealed a new method that could fine-tune the angle – “torsion” – between thin layers of atoms that form exotic artificial nanodevices called van der Waals heterostructures – and help accelerate the next generation of electronics.

The new technique can achieve dynamic rotation in situ and manipulation of 2-D materials stacked on top of each other to form van der Waals heterostructures – nanoscale devices that boast unusual properties and exciting new phenomena, explained the team leader. , Professor Mishchenko.

Twist angle regulation controls the topology and electron interactions in 2-D materials, and one such process, referred to as “twistronics,” is a growing research topic in physics in recent years. The new Manchester study will be published in Advances in science today.

“Our technique allows for twisted van der Waals heterostructures with dynamically tunable optical, mechanical and electronic properties.” explained Yaping Yang, the lead author of this work.

Yaping Yang added: “This technique, for example, could be used in the autonomous robotic manipulation of two-dimensional crystals to construct van der Waals superlattices, which would allow for the accurate positioning, rotation and manipulation of 2-D materials to fabricate materials with torsion angles, to fine-tune the electronic and quantum properties of van der Waals materials “.

Twisting the 2-D crystal layers relative to each other leads to the formation of a moiré pattern, where the lattices of the main 2-D crystals form a super lattice. This super lattice can completely change the behavior of electrons in the system, leading to the observation of many new phenomena, including strong electronic correlations, the fractal quantum Hall effect and superconductivity.

The team demonstrated this technique by successfully fabricating heterostructures in which graphene is perfectly aligned with the upper and lower encapsulation layers of hexagonal boron nitride, dubbed ‘white graphene’, creating double moiré lattices on the two interfaces.

As published in Advances in science, the technique is mediated by a polymer resistant patch on target 2-D crystals and a polymer gel manipulator, which can precisely and dynamically control the rotation and positioning of 2-D materials.

“Our technique has the potential to bring twistronics into cryogenic measurement systems, for example, using micromanipulators or microelectromechanical devices,” added Artem Mishchenko.

The researchers used a slide with a drop of polydimethylsiloxane (PDMS) as a manipulator, which is cured and naturally shaped into a hemispherical geometry. Meanwhile, they intentionally deposited a polymethylmethacrylate (PMMA) epitaxial patch over a target 2-D crystal via standard electron beam lithography.

The steps to manipulate target flakes in a heterostructure are easy to follow. By lowering the handle of the polymer gel, the PDMS hemisphere is brought into contact with the PMMA patch. When touching, you can easily move or rotate the target 2-D crystals on the bottom flake surface. Such a fluid movement of the 2-D flakes is based on the superlubricity between the two crystal structures.

Superlubricity is a phenomenon in which the friction between atomically flat surfaces disappears under certain conditions.

The manipulation technique allows for continuous adjustment of the torsion angle between the layers even after the heterostructure has been assembled. The PMMA epitaxial patch can be designed into an arbitrary shape upon request, normally taking the geometry that fits the target staple. The handling technique is convenient and reproducible as the PMMA patch can be easily washed off the acetone and modified by lithography.

Normally, for an accurately fabricated PDMS hemisphere, the contact area between the hemisphere and a 2-D crystal depends on the radius of the hemisphere and is highly sensitive to contact force, making it difficult to precisely control the movement of the 2- crystal. D target.

“The PMMA epitaxial patch plays a crucial role in the manipulation technique. Our trick is that the contact area of ​​the polymer gel manipulator is precisely limited to the modeled shape of the epitaxial polymer layer. This is the key to making a polymer gel. precise handling control, allowing the application of a much greater control force. ” said Jidong Li, one of the co-authors.

Compared to other 2-D material handling techniques, such as using atomic force microscope (AFM) tips to push a crystal with a specially fabricated geometry, the in situ twistronic technique is non-destructive and can manipulate flakes independently from their thickness, while an AFM tip works best only for thick flakes and could destroy thin ones.

The perfect alignment of graphene and hexagonal boron nitride demonstrates the potential of the technique in twistronic applications.

Using the in situ technique, the researchers successfully rotated the 2-D layers in a boron nitride / graphene / boron nitride heterostructure to achieve perfect alignment between all layers. The results demonstrate the formation of double moiré lattices at the two interfaces of the heterostructure. Additionally, the researchers looked at the second-order (composite) moireacute signature; pattern generated by the double moiré effect; superlattices.

This heterostructure with perfectly aligned graphene and boron nitride demonstrates the potential of the manipulation technique in twistronics.

“The technique can be easily generalized to other 2-D material systems and allows for reversible manipulation in any 2-D system away from a commensurate regime,” said Yaping Yang, who did the experimental work.

Professor Mishchenko added: ‘We believe that our technique will open a new strategy in device engineering and find its applications in the search for 2-D quasicrystals, flat magic angle bands and other topologically nontrivial systems.’