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 observation 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” added Artem 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 is easy to follow. By lowering down the polymer gel handle, 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.
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 realize 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 realize 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.