YSM Issue 96.3

FOCUS
Electrical Engineering
WHY TINY PATTERNS MEAN BIG THINGS
FOR THE FUTURE OF SEMICONDUCTORS
BY WILLIAM ARCHACKI
The moiré effect is a phenomenon
you can witness with just a marker
and paper. First, take your marker
and draw a honeycomb pattern of
hexagons on two sheets of paper. Now lay
them atop one another askew, rotating the
top sheet slightly. By combining these two
lattices, you should see regular, repeating
patterns much larger than any individual
hexagon. This is the moiré effect in action:
from a distance, the overlapping hexagons
make a larger tessellation that seems to
alternate between light and dark regions.
Now imagine if atoms stood at the
vertices of every hexagon on the paper,
connected to their neighbors by chemical
bonds. That’s the structure of a moiré
material. At an atomic scale, the repeated
patterns of the moiré effect change how
light interacts with a material and, in
turn, how the material transmits electrical
signals resulting from light.
In a recent Nature Materials publication
led by Fengnian Xia, professor of electrical
engineering at Yale, the team innovated
upon moiré materials. By finding a
more controllable way to produce the
moiré effect at an atomic scale, they have
made a material that has a wide range
of useful physical properties that may
pave the way for a new generation of
optical sensors.
Scientists vs. Thermodynamics
The new moiré material recipe by Xia
and his colleagues starts with three simple
ingredients: tungsten, sulfur, and selenium.
When heated in a furnace through a
process referred to as chemical vapor
deposition, these three elements combine
into flat, hexagonal lattices. The vertices
are occupied by atoms of tungsten, sulfur,
and selenium. After heating for a second
time with a supply of the same elements in
different ratios, an additional layer forms
on top of the existing hexagonal lattice,
this time with a slightly different spacing
between its atoms—a different lattice
constant. The alignment of differentlyspaced
layers signals success: the moiré
effect is present. Now, it’s a matter of lattice
size rather than rotation.
It has historically been a challenge for
researchers to fabricate moiré materials
because of the natural way that layers
form. The most stable way for two
identical layers to stack results in a
perfect alignment that never produces
the moiré effect. So, rather than using
the conventional ‘twistronics’ approach
to moiré material fabrication, which
fights against thermodynamics to force
the layers to rotate, this new approach
from Xia’s group relies on variations in
the spacing of atoms. In their recipe,
the moiré effect is created by stacking
hexagons of different sizes, rather than
different orientations.
“Twisting two layers at a specific twist
angle is not the most stable form of
matter,” said Matthieu Fortin-Deschênes, a
postdoctoral fellow in Xia’s research group
and first author on the paper. “Basically,
we came up with an approach to directly
grow these moiré patterns with tunable
spacing. Instead of twisting, we grow them
with different lattice parameters to tune
the moiré periodicity.”
By precisely varying the concentrations
of sulfur and selenium relative to tungsten,
the researchers saw that the pattern they
form has a “tunable period”. In other words,
they can control how large the patterns
appear. With a tunable period, there is a
new world of possibilities. “If you’re able
to tune the periodicity, you’re able to tune
the properties of the material,” Fortin-
Deschênes said. Tuning properties is a big
deal for electrical engineers. The next step
is figuring out how to leverage these tunable
properties for use in real technologies.
Tiny Materials, Big Implications
Working on these materials has gotten
Xia and his colleagues thinking a lot
about light. What kind of information
can we glean from light? For one answer,
look to the astronomers. When studying
exoplanets, they often examine the spectra
of light that passes through the planets’
atmospheres. By using spectroscopy, a
crucial analytical technique that works
like forensics for light, they deduce which
gases are floating around in a breath’s
worth of air many millions of miles away.
And waves of light have more parameters
than just their spectra. Measuring light’s
polarization can give insights into what
substances the light has interacted with.
For example, light that reflects off water is
12 Yale Scientific Magazine September 2023 www.yalescientific.org