Stack and twist: Physicists accelerate the hunt for revolutionary new materials

Date:
August 11, 2020
Source:
University of Bath
Summary:
Scientists have taken an important step towards understanding the
interaction between layers of atomically thin materials arranged in
stacks. They hope their research will speed up the discovery of new,
artificial materials, leading to the design of electronic components
that are far tinier and more efficient than anything known today.



FULL STORY ========================================================================== Scientists at the University of Bath have taken an important step
towards understanding the interaction between layers of atomically
thin materials arranged in stacks. They hope their research will speed
up the discovery of new, artificial materials, leading to the design
of electronic components that are far tinier and more efficient than
anything known today.


========================================================================== Smaller is always better in the world of electronic circuitry, but
there's a limit to how far you can shrink a silicon component without
it overheating and falling apart, and we're close to reaching it. The researchers are investigating a group of atomically thin materials that
can be assembled into stacks. The properties of any final material depend
both on the choice of raw materials and on the angle at which one layer
is arranged on top of another.

Dr Marcin Mucha-Kruczynski who led the research from the Department
of Physics, said: "We've found a way to determine how strongly
atoms in different layers of a stack are coupled to each other, and
we've demonstrated the application of our idea to a structure made of
graphene layers." The Bath research, published in Nature Communications,
is based on earlier work into graphene -- a crystal characterised by
thin sheets of carbon atoms arranged in a honeycomb design. In 2018,
scientists at the Massachusetts Institute of Technology (MIT) found
that when two layers of graphene are stacked and then twisted relative
to each other by the 'magic' angle of 1.1DEG, they produce a material
with superconductive properties. This was the first time scientists had
created a super-conducting material made purely from carbon. However,
these properties disappeared with the smallest change of angle between
the two layers of graphene.

Since the MIT discovery, scientists around the world have been attempting
to apply this 'stacking and twisting' phenomenon to other ultra-thin
materials, placing together two or more atomically different structures
in the hope of forming entirely new materials with special qualities.

"In nature, you can't find materials where each atomic layer is
different," said Dr Mucha-Kruczynski. "What's more, two materials can
normally only be put together in one specific fashion because chemical
bonds need to form between layers. But for materials like graphene,
only the chemical bonds between atoms on the same plane are strong. The
forces between planes -- known as van der Waals interactions -- are weak,
and this allows for layers of material to be twisted with respect to
each other." The challenge for scientists now is to make the process of discovering new, layered materials as efficient as possible. By finding a formula that allows them to predict the outcome when two or more materials
are stacked, they will be able to streamline their research enormously.



==========================================================================
It is in this area that Dr Mucha-Kruczynski and his collaborators at
the University of Oxford, Peking University and ELETTRA Synchrotron in
Italy expect to make a difference.

"The number of combinations of materials and the number of angles at
which they can be twisted is too large to try out in the lab, so what
we can predict is important," said Dr Mucha-Kruczynski.

The researchers have shown that the interaction between two layers can
be determined by studying a three-layer structure where two layers are assembled as you might find in nature, while the third is twisted. They
used angle- resolved photoemission spectroscopy -- a process in which
powerful light ejects electrons from the sample so that the energy and
momentum from the electrons can be measured, thus providing insight
into properties of the material -- to determine how strongly two carbon
atoms at a given distance from each other are coupled. They have also demonstrated that their result can be used to predict properties of
other stacks made of the same layers, even if the twists between layers
are different.

The list of known atomically thin materials like graphene is growing all
the time. It already includes dozens of entries displaying a vast range of properties, from insulation to superconductivity, transparency to optical activity, brittleness to flexibility. The latest discovery provides a
method for experimentally determining the interaction between layers of
any of these materials. This is essential for predicting the properties
of more complicated stacks and for the efficient design of new devices.

Dr Mucha-Kruczynski believes it could be 10 years before new stacked
and twisted materials find a practical, everyday application. "It took
a decade for graphene to move from the laboratory to something useful in
the usual sense, so with a hint of optimism, I expect a similar timeline
to apply to new materials," he said.

Building on the results of his latest study, Dr Mucha-Kruczynski
and his team are now focusing on twisted stacks made from layers of
transition metal dichalcogenides (a large group of materials featuring
two very different types of atoms -- a metal and a chalcogen, such
as sulphur). Some of these stacks have shown fascinating electronic
behaviour which the scientists are not yet able to explain.

"Because we're dealing with two radically different materials, studying
these stacks is complicated," explained Dr Mucha-Kruczynski. "However,
we're hopeful that in time we'll be able to predict the properties of
various stacks, and design new multifunctional materials."

========================================================================== Story Source: Materials provided by University_of_Bath. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. J. J. P. Thompson, D. Pei, H. Peng, H. Wang, N. Channa, H. L. Peng,
A.

Barinov, N. B. M. Schro"ter, Y. Chen, M. Mucha-Kruczyński.

Determination of interatomic coupling between two-dimensional
crystals using angle-resolved photoemission spectroscopy. Nature
Communications, 2020; 11 (1) DOI: 10.1038/s41467-020-17412-0 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200811153905.htm

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