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An international research team led by the University of Manchester has revealed a revolutionary method that could lead to autonomous robotic control and thus to a precise fine adjustment of the “ twist ” between layers of atom-thin 2D materials stacked in a structure. lattice: a pioneering device that could help transform technology and achieve superconducting electronics.
A group of international researchers led by Professor Artem Mishchenko at the University of Manchester has revealed a new method that could fine-tune the angle – “twist” – 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 in situ rotation and manipulation of 2D materials layered on top of each other to form van der Waals heterostructures, nanoscale devices that boast unusual properties and exciting new phenomena, explained Professor Mishchenko.
Twist angle adjustment controls topology and electronic interactions in 2D 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 today in Science Advances (Friday 4 December).
“Our technique enables 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 build van der Waals superlattices, which would allow for accurate positioning, rotation and manipulation of 2D materials to fabricate materials with angles of torsion desired, to fine-tune the electronic and quantum properties of van der Waals materials. “
The twisting of the 2D crystal layers relative to each other leads to the formation of a moiré pattern, where the lattices of the parent 2D 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 both the upper and lower encapsulating layers of hexagonal boron nitride – dubbed “white graphene” – creating double moiré lattices at the two interfaces.
As published in Science Advances, the technique is mediated by a polymer resistant patch on target 2D 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 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. In the meantime, they intentionally deposited a polymethylmethacrylate (PMMA) epitaxial patch over a target 2D crystal through 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 tapped, you can easily move or rotate the target 2D crystals on the bottom flake surface. Such a smooth movement of the 2D 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.
The key is the durable polymer patch molded to the target flakes. Normally, for a carefully fabricated PDMS hemisphere, the contact area between the hemisphere and a 2D 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 target 2D crystal.
“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 achieving precise manipulation control, allowing the application of a much greater control force. “said Jidong Li, one of the co-authors.
Compared to other 2D material handling techniques, such as using atomic force microscope (AFM) tips to push a crystal with a specially fabricated geometry, the twistronic in situ technique is non-destructive and can manipulate flakes regardless of their thick, while a tip AFM 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 2D 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. In addition, the researchers observed the signature of the second-order (composite) moiré pattern generated by the double moiré lattices.
This heterostructure with perfectly aligned graphene and boron nitride demonstrates the potential of the manipulation technique in twistronics.
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