MIT physicists have metaphorically turned graphite, or pencil lead, into gold by isolating five ultrathin flakes stacked in a specific order. The resulting material can then be tuned to exhibit three important properties never before seen in natural graphite.

“It is kind of like one-stop shopping,” says Long Ju, an assistant professor in the Department of Physics and leader of the work. “Nature has plenty of surprises. In this case, we never realized that all of these interesting things are embedded in graphite.”

Further, he says, “It is very rare material to find materials that can host this many properties.”

Graphite is composed of graphene, which is a single layer of carbon atoms arranged in hexagons resembling a honeycomb structure. Graphene, in turn, has been the focus of intense research since it was first isolated about 20 years ago. More recently, about five years ago, researchers including a team at MIT discovered that stacking individual sheets of graphene and twisting them at a slight angle to each other, can impart new properties to the material, from superconductivity to magnetism. The field of “twistronics” was born.

In the current work, “we discovered interesting properties with no twisting at all,” says Ju, who is also affiliated with the Materials Research Laboratory.

He and colleagues discovered that five layers of graphene arranged in a certain order allow the electrons moving around inside the material to talk with each other. That phenomenon, known as electron correlation, “is the magic that makes all of these new properties possible,” Ju says.

Bulk graphite — and even single sheets of graphene — are good electrical conductors, but that’s it. The material Ju and colleagues isolated, which they call pentalayer rhombohedral stacked graphene, becomes much more than the sum of its parts.

Key to isolating the material was a novel microscope Ju built at MIT in 2021 that can quickly and relatively inexpensively determine a variety of important characteristics of a material at the nanoscale. Pentalayer rhombohedral stacked graphene is only a few billionths of a meter thick.

Scientists including Ju were looking for multilayer graphene that was stacked in a very precise order, known as rhombohedral stacking. Says Ju, “there are more than 10 possible stacking orders when you go to five layers. Rhombohedral is just one of them.” The microscope Ju built, known as Scattering-type Scanning Nearfield Optical Microscopy, or s-SNOM, allowed the scientists to identify and isolate only the pentalayers in the rhombohedral stacking order they were interested in.

From there, the team attached electrodes to a tiny sandwich composed of boron nitride “bread” that protects the delicate “meat” of pentalayer rhombohedral stacked graphene. The electrodes allowed them to tune the system with different voltages, or amounts of electricity. The result: They discovered the emergence of three different phenomena depending on the number of electrons flooding the system.

“We found that the material could be insulating, magnetic, or topological,” Ju says. The latter is somewhat related to both conductors and insulators. Essentially, Ju explains, a topological material allows the unimpeded movement of electrons around the edges of a material, but not through the middle. The electrons are traveling in one direction along a “highway” at the edge of the material separated by a median that makes up the center of the material. So the edge of a topological material is a perfect conductor, while the center is an insulator.

“Our work establishes rhombohedral stacked multilayer graphene as a highly tunable platform to study these new possibilities of strongly correlated and topological physics,” Ju and his coauthors conclude.

For more information: Nature Nanotechnology

Two-dimensional (2D) materials are just a single or a few layers of atoms thick. These materials often have exotic properties that may be useful for next-generation technologies. When layers of these materials are stacked, the electronic properties that emerge can be manipulated by, for example, twisting the layers with respect to one another. To fully understand these properties and correlate them with the twist angle, scientists need advanced microscopy techniques. Researchers developed a novel operating mode for the interferometric four-dimensional scanning transmission electron microscopy (4D-STEM) technique. This special technique allows researchers to measure the atomic-scale structural distortions, twist angle, and interlayer spacings that influence the unique electronic properties of layered 2D materials.

Layered 2D materials have special properties that can advance technology beyond existing capabilities. For example, they could lead to faster and more energy-efficient computers or more reliable electricity storage. The individual layers that make up these materials may each be oriented differently. This creates challenges in fully understanding their 3D atomic structures with existing microscopy techniques. Interferometric 4D-STEM can reveal the relative positions of atoms within separate layers of stacked and twisted 2D materials. The technique opens avenues to the design and development of materials with useful properties.

Layered 2D materials have attracted considerable attention due to their interesting electronic properties, which can be modified by changing the twist angle of bilayer materials, the stacking sequence of trilayer materials, or other factors. To fully understand and control the properties of these materials, researchers need to study their atomic structures. However, visualizing the atomic structure of few-layered materials is often challenging using conventional microscopy techniques, such as when working with materials composed of light elements or when 3D information is needed. Researchers need new techniques to improve precision and locally measure distortions and interlayer spacings in twisted materials composed of two or three layers, especially when they contain light elements or high twist angles.

Researchers developed a new interferometric 4D-STEM modality that can provide information about local structural deformations within layers, twist direction and magnitude between layers, and interlayer distances for few-layered 2D materials. This new operating mode of 4D-STEM is still based on Bragg interferometry but uses a defocused electron probe to directly provide information about the relative positions of atoms within separate layers, as demonstrated in this study in bilayer and trilayer graphene. The technique sheds new light on the interplay between electronic properties and the precise structural arrangements of few-layer 2D materials.

For more information: U.S. Department of Energy Office of Scientific and Technical Information

Image: A convergent electron beam in a scanning transmission electron microscope interacts with a twisted bilayer of graphene (carbon), generating intricate disk-shaped intensity patterns that encode the precise local atomic arrangement.

The way light interacts with naturally occurring materials is well-understood in physics and materials science. But in recent decades, researchers have fabricated metamaterials that interact with light in new ways that go beyond the physical limits imposed on naturally occurring materials.

A metamaterial is composed of arrays of “meta-atoms,” which have been fabricated into desirable structures on the scale of about a hundred nanometers. The structure of arrays of meta-atoms facilitate precise light-matter interactions. However, the large size of meta-atoms relative to regular atoms, which are smaller than a nanometer, has limited the performance of metamaterials for practical applications.

Now, a collaborative research team led by Bo Zhen of the University of Pennsylvania has unveiled a new approach that directly engineers atomic structures of material by stacking the two-dimensional arrays in spiral formations to tap into novel light-matter interaction. This approach enables metamaterials to overcome the current technical limitations and paves the way for next-generation lasers, imaging, and quantum technologies.

“It’s similar to stacking a deck of cards but twisting each card slightly before adding it to the pile,” says Zhen, a senior author of the paper and an assistant professor in the School of Arts & Sciences at Penn. “This twist changes how the entire ‘deck’ responds to light, enabling it to exhibit new properties that individual layers, or traditional stacks, do not possess.”

Bumho Kim, postdoctoral researcher in the Zhen Lab and first author of the paper, explains that by stacking layers of a material called tungsten disulfide (WS₂) and twisting them at certain angles, they introduced what’s known as screw symmetries.

“The magic lies in controlling the twist,” Kim explains. “When you twist the layers at specific angles, you change the symmetry of the stack. Symmetry, in this context, refers to how certain properties of materials—like how they interact with light—are constrained by their spatial arrangement.”

By tweaking this arrangement at the atomic scale, the researchers have bent the rules of what these materials can do, and by controlling the twist across multiple layers of WS₂, they created what’s known as 3D nonlinear optical materials.

Kim explains that a single layer of WS₂ has particular symmetries, which allow certain types of interactions with light, where two photons at a given frequency can interact with the material to produce a new photon at double the frequency, a process known as second-harmonic generation (SHG).

“But, when two layers of WS₂ are stacked with a twist angle different from the conventional 0° or 180°, all the mirror symmetries that were present in the single layer are broken,” says Kim. “This broken mirror symmetry is crucial because it leads to a chiral response—something entirely new and not seen in the individual layers.”

The researchers explain that the chiral response is significant because it is a cooperative effect resulting from the coupling between the electronic wavefunctions of the two layers, a phenomenon that can only arise in twisted interfaces.

An interesting property, Zhen adds, is that the sign of the chiral nonlinear response flips when the twist angle is reversed. This demonstrates direct control over the nonlinear properties by simply changing the twist angle between layers—a level of tunability that could be revolutionary for designing optical materials with custom responses.

Moving from bilayers to trilayers and beyond, the researchers observed how the interfacial SHG responses can constructively or destructively interfere depending on the twist angles between the layers.

In a stack with layers in multiples of four, “the chiral responses from all interfaces add up, while the in-plane responses cancel out,” says Kim. “This leads to a new material that exhibits only chiral nonlinear susceptibilities. This result could not be achieved without the precise stacking and twisting of the layers.”

The researchers found that screw symmetry enables new selectivity for the light’s electric field in the material, a part of light that determines its direction and intensity  Kim notes how they found that screw symmetry enables a new kind of light generation in twisted four- and eight-layer stacks, counter-circularly polarized third harmonic generation, wherein light travels in the opposite spiral direction—a quality not seen in constituent WS2 monolayers.

“Adding an artificial screw symmetry allows us to control nonlinear optical circular selectivity at the nanoscale,” Kim says.

In testing this technique experimentally, the researchers verified the predicted nonlinearities inherent in various configurations of twisted WS₂ stacks. The team observed new nonlinear responses and circular selectivity in twisted WS₂ stacks that cannot be found in naturally occurring WS₂, a revelation that could have profound implications in the field of nonlinear optics.

For more information: Nature Photonics