Refine the “twist” between 2D materials in van der Waals heterostructures to help 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 – “twist” – between thin layers of atoms that form man-made exotic nanodevices called van der Waals heterostructures – and help speed up the next generation of electronics.

The new technique allows for dynamic in situ rotation and manipulation of superimposed 2D materials to form van der Waals heterostructures – nanoscale devices that offer unusual properties and exciting new phenomena, the chef explained. team, Professor Mishchenko.

Adjusting the torsion angle controls the topology and interactions of electrons in 2D materials – and such a process, called “twistronics,” has been a growing topic of research in physics in recent years. The new Manchester-led study will be published in Scientific advances today.

“Our technique enables twisted van der Waals heterostructures with dynamically adjustable optical, mechanical and electronic properties. explained Yaping Yang, the main author of this book.

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 precise positioning, rotation and manipulation of 2D materials for fabricate materials with torsion angles, to refine the electronic and quantum properties of van der Waals materials. “

The twisting of the layers of 2-D crystals relative to each other leads to the formation of a moiré pattern, where the lattices of the parent 2-D crystals form a superlattice. This superlattice 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 where the graphene is perfectly aligned with the top and bottom encapsulation layers of hexagonal boron nitride – dubbed “white graphene” – creating double moiré superlattices at both. interfaces.

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

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

The researchers used a glass slide with a droplet of polydimethylsiloxane (PDMS) as a manipulator, which is hardened and naturally shaped into a hemispherical geometry. In the meantime, they intentionally deposited a patch of epitaxial polymethyl methacrylate (PMMA) on top of a 2-D target crystal by standard electron beam lithography.

The steps for handling target flakes in a heterostructure are easy to follow. By lowering the polymer gel handle, the PDMS hemisphere is brought into contact with the PMMA patch. When they touch each other, the target 2-D crystals can be easily moved or rotated on the surface of the lower snowflake. Such a smooth movement of the 2-D flakes is based on the superlubricity between the two crystal structures.

Superlubricity is a phenomenon where friction between atomically flat surfaces disappears depending on certain conditions.

The manipulation technique allows a continuous adjustment of the angle of twist between the layers even after the assembly of the heterostructure. The epitaxial PMMA patch can be designed in an arbitrary form on demand, normally taking the geometry which corresponds to the target flake. The manipulation technique is practical and reproducible since the PMMA patch can be easily washed with acetone and re-modeled by lithography.

Normally, for a carefully crafted PDMS hemisphere, the area of ​​contact between the hemisphere and a 2-D crystal depends on the radius of the hemisphere and is very sensitive to the contact force, making it difficult to precisely control the movement of the hemisphere. 2-D crystal target.

“The epitaxial PMMA 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 patterned shape of the epitaxial polymer layer. This is the key to achieving precise handling control, allowing a much greater control force to be applied. “said Jidong Li, one of the co-authors.

Compared to other 2D material manipulation techniques, such as using atomic force microscope (AFM) tips to push a crystal with a specifically fabricated geometry, the in situ twistronic technique is non-destructive and can manipulate flakes regardless of their thickness, whereas an AFM tip works best only for thick flakes and can 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 succeeded in rotating 2D layers in a boron nitride / graphene / boron nitride heterostructure to achieve perfect alignment between all the layers. The results demonstrate the formation of double moiré superlattices at the two interfaces of the heterostructure. In addition, the researchers observed the signature of the second-order (composite) moireacute; pattern generated by the double moireacute; super-networks.

This perfectly aligned graphene and boron nitride heterostructure demonstrates the potential of the twistronic manipulation technique.

“The technique can be easily generalized to other 2D hardware systems and allows reversible manipulation in all 2D systems away from the corresponding regime,” said Yaping Yang, who carried out 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 2D quasicrystals, magical angle flat bands and other topologically non-trivial systems. ”