This
image shows cartoons and micrographics that highlight the new technique of in
situ twistronics. Courtesy: Artem Mishchenko/The University of Manchester.
A group of
international researchers at The University of Manchester have revealed a novel
method that could fine tune the angle—"twist"—between atom-thin
layers that form exotic manmade nanodevices called van der Waals
heterostructures—and help accelerate the next generation of electronics.
The new
technique can achieve in situ dynamical rotation and manipulation of 2-D
materials layered on top of each other to form van der Waals
heterostructures—nanoscale devices that boast unusual properties and exciting
new phenomena, explained team leader Professor Mishchenko.
Tuning of
twist angle controls the topology and electron interactions in 2-D
materials—and such a process, referred to as 'twistronics', is a rising
research topic in physics in recent years. The new Manchester-led study will be
published in Science Advances today.
"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.
Yaping
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 2-D materials to fabricate materials with desired twist angles,
to fine-tune electronic and quantum properties of van der Waals
materials."
Twisting
layers of 2-D crystals with respect to each other results in the formation of a
moiré pattern, where lattices of the parent 2-D 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 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 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 2-D 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 2-D crystals on the surface of the bottom flake. Such a smooth movement
of the 2-D 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.
Normally,
for a carefully fabricated PDMS hemisphere, the contact area between the hemisphere
and a 2-D 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 2-D 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 2-D 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 2-D 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) moireacute;
pattern generated by the double moireacute; 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 systems away from commensurate
regime", said Yaping Yang, who carried out the experimental work.
Professor
Mishchenko added: "We believe our technique will open up a new strategy in
device engineering and find its applications in research of 2-D quasicrystals,
magic-angle flat bands, and other topologically nontrivial systems."