Manchester group helps fine-tune ‘twistronics’ of 2D crystals
An international research team led by The University of Manchester has revealed a breakthrough method that could lead to autonomous robotic control and therefore precise fine tuning of the ‘twist’ between atom-thin 2D materials layers stacked in a superlattice structure – a pioneering device that could help transform technology and achieve superconductive electronics.
The group, led by Professor Artem Mishchenko at The University of Manchester, developed a novel technique for achieving in situ dynamical rotation and manipulation of 2D materials layered in van der Waals heterostructures – nanoscale devices that boast exciting and unusual physical properties.
Tuning of twist angle controls the topology and electron interactions in 2D materials – and such a process, referred to as ‘twistronics’, is a rising research topic in physics in recent years. The new Manchester-led study was published in Science Advances on Friday, 4 December.
“Our technique enables twisted van der Waals heterostructures with dynamically tuneable optical, mechanical, and electronic properties.” explained Yaping Yang, the main author of this work.
Yang added: “This technique, for example, could be used in autonomous robotic manipulation of two-dimensional crystals to build van der Waals superlattices, which would allow accurate positioning, rotation, and manipulation of 2D materials to fabricate materials with desired twist angles, to fine-tune electronic and quantum properties of van der Waals materials.”
Our technique enables twisted van der Waals heterostructures with dynamically tuneable optical, mechanical, and electronic properties. This technique, for example, could be used in autonomous robotic manipulation of two-dimensional crystals to build van der Waals superlattices, which would allow accurate positioning, rotation, and manipulation of 2D materials to fabricate materials with desired twist angles, to fine-tune electronic and quantum properties of van der Waals materials.”
Twisting layers of 2D crystals with respect to each other results in the formation of a moiré pattern, where lattices of the parent 2D crystals form a superlattice. This superlattice can completely change the behaviour of electrons in the system, leading to the observation of many novel phenomena, including strong electron correlations, fractal quantum Hall effect and superconductivity.
The team demonstrated this technique by successfully fabricating heterostructures where graphene is perfectly aligned with both top and bottom encapsulating layers of hexagonal boron nitride - dubbed “white graphene” - creating double moiré superlattices at the two interfaces.
As published in Science Advances, the technique is mediated by a polymer resist 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 inside cryogenic measurement systems, for instance, by using micromanipulators or micro-electro-mechanical devices,” said Professor Mishchenko.
The researchers used a glass slide with a droplet of polydimethylsiloxane (PDMS) as a manipulator, which is cured and naturally shaped into a hemisphere geometry. In the meantime, they intentionally deposited an epitaxial polymethyl methacrylate (PMMA) patch on top of a target 2D crystal through a standard electron-beam lithography.
The steps to manipulate target flakes in a heterostructure are as follows. By lowering the polymer gel handle, the PDMS hemisphere is brought in contact with the PMMA patch. When they touch each other, one can easily move or rotate the target 2D crystals on the surface of the bottom flake. Such a smooth movement of the 2D flakes is based on the superlubricity between the two crystalline structures. Superlubricity is a phenomenon where the friction between atomically flat surfaces disappears depending on certain conditions.
The manipulation technique enables continuous tuning of the twist angle between the layers even after the heterostructure assembly. One can design the epitaxial PMMA patch into an arbitrary shape on demand, normally taking the geometry that fits the target flake. The manipulation technique is convenient and reproducible since the PMMA patch can be easily washed away by acetone and re-patterned by lithography.
The polymer resist patch patterned onto target flakes is the key. Normally, for a carefully fabricated PDMS hemisphere, the contact area between the hemisphere and a 2D crystal depends on the hemisphere radius and is highly sensitive to the contact force, making it difficult to precisely control the motion of the target 2D crystal.
“The epitaxial PMMA patch plays a crucial role in the manipulation technique. Our trick lies in that the contact area of the polymer gel manipulator is limited precisely to the patterned shape of the epitaxial polymer layer. This is the key to realise precise control of the manipulation, allowing a much larger controlling force to be applied.” said Jidong Li, one of the co-authors.
Compared to other manipulation techniques of 2D materials, such as using atomic force microscope (AFM) tips to push a crystal with a specifically fabricated geometry, the in situ twistronics technique is non-destructive and can manipulate flakes regardless of their thickness, whereas an AFM tip works better only for thick flakes and might destroy thin ones.
Perfect alignment of graphene and hexagonal boron nitride demonstrates the potential of the technique in twistronics applications.
Using the in-situ technique, the researchers successfully rotated 2D layers in a boron nitride/graphene/boron nitride heterostructure to realise a 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) moiré pattern generated by the double moiré superlattices.
This heterostructure with perfectly aligned graphene and boron nitride demonstrates the potential of the manipulation technique in twistronics.