Art Courtesy of Steve Blanco.
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 differently-spaced 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 polarized because the process of reflection forces all the light waves to oscillate within the same plane. Other parameters of interest like intensity or coherence can each be measured with dedicated pieces of lab equipment. For Xia, these parameters of light pose an exciting question: What if it was possible to pick up on the wealth of information provided by light using just a single sensor?
The new moiré material has a special way of interacting with light and transmitting electric current. Just as bumps on a hill change the way water flows, moiré-induced variations in the invisible landscape of the material’s electric potential change how electrons move from one point to another. Incoming light gets absorbed by the material and starts a flow of electrons, and when that flow of electrons is recorded as a current or its corresponding voltage drop, information about the light’s source is conveyed somewhere within the data. The challenge, then, is to decode the data and reveal the secrets hidden within the material’s electric signals.
“This material is highly tunable, and it interacts with light very strongly. That would allow us to combine this reconfigurable material with the latest deep learning algorithms,” Xia said. “In another 2022 paper, we used deep learning to realize the detection of many parameters of light simultaneously.” Their approach—referred to as deep sensing—could change how scientists use light. Rather than just spectroscopy, scientists could tap into new information carried by light if they watch how all the parameters change together.
“We can relate this to image recognition,” Xia said. “This high-dimensional photoresponse contains all the information we want to know, but we don’t know how to extract this information.” In the same way that a deep learning algorithm figures out to differentiate images of cats from images of dogs, an algorithm might learn to correlate the complex electric signals from the sensor to any kind of data a scientist might be examining. “Let’s assume you want to know the concentration of a certain gas. You can do that by measuring the absorption spectrum. But you don’t have to do that. You can skip that process. You can go directly from the photoresponse to the concentration of the gas if you do enough training correctly,” Xia said.
Not Just Tungsten
Although the recent paper only covers one moiré material, the researchers behind it are hopeful that the fabrication process can be applied in other ways. “The intention of studying the growth mechanisms is to understand the fundamentals of the growth processes and then try to extrapolate to other systems,” Fortin-Deschênes said. Of special interest is graphene, a substance made of two-dimensional sheets of carbon atoms, which could be mixed with silicon in the same way that sulfur was mixed with selenium. The hope is that each new system may have its own set of unique properties that can be applied to electrical engineering challenges of the future.
“Since we have so many properties that emerge and that can be easily tuned, we can expect that we will find something that is very useful using these moiré materials,” Fortin-Deschênes said. Novel fabrication methods, such as this one, are creating possibilities for new atomic arrangements of materials. By changing the way energy and electrons dance at the quantum scale, researchers may reshape the future of semiconductor devices.