Physicists turn pencil lead into metaphorical 'gold’

MIT physicists and colleagues isolated five ultrathin wafers stacked in a specific order and turned graphite, or pencil lead, into figurative gold. The resulting material can be tuned to exhibit three key properties never before seen in natural graphite.

„It’s like one-stop shopping,” said Long Zhu, an assistant professor in the MIT Department of Physics and lead author of the work. Nature Nanotechnology. „In this case, we never realized that all these interesting things were embedded in graphite.”

And, he says, „it’s very rare to find materials that can offer so many properties.”

Graphite is made of graphene, a single layer of carbon atoms arranged in hexagons like a honeycomb structure. Graphene has been the focus of intense research since it was first isolated over 20 years ago. About five years ago researchers, including a team from MIT, found that by stacking individual sheets of graphene and twisting them at a small angle, they could give the material new properties, from superconductivity to magnetism. The field of „Twitronics” was born.

In the current work, „we discovered interesting properties without twisting,” says Zhu, who is affiliated with the Materials Research Laboratory.

He and his colleagues discovered that five layers of graphene arranged in a specific order allow electrons moving inside the material to talk to each other. That phenomenon, called electron interaction, is „the magic that makes all these new properties possible,” Zhu says.

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

Novel microscope

A novel microscope Zhu, built at MIT in 2021, can quickly and relatively cheaply determine various key properties of a nanoscale material. Pentalayer rhombohedral stacked graphene is only a few billionths of a meter thick.

The scientists, including Zhu, looked for multilayer graphene stacked in a very precise order called rhombohedral stacking. Zhu says, „There are more than 10 stacking orders when you go up to five layers. Rhombohedral is one of them.” The microscope Zu 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.

Three in one

From there, the team attached the electrodes to a small sandwich of boron nitride „bread” that protects the delicate „meat” of pentalayer rhombohedral stacked graphene. The electrodes allowed the system to be tuned to different voltages or levels of electricity. Conclusion: They discovered the appearance of three different phenomena depending on the number of electrons flooding the system.

„We found that the material can be insulating, magnetic, or topological,” Zhu says. The latter is somewhat related to both conductors and dielectrics. Basically, Zhu explains, a topological material allows the unhindered movement of electrons around the edges of a material, but not in the middle. Electrons travel in one direction along a „highway” along the edge of the material, which forms the core of the material. So the edge of a topological material is a perfect conductor, while the center is a dielectric.

„Our work establishes rhombohedral stacked multilayer graphene as a highly tunable platform for studying these new possibilities of strongly correlated and topological physics,” Zhu and his co-authors conclude.

Besides Zhu, the authors of the paper are Donggang Han and Zhengguang Lu. Han is a graduate student in the Department of Physics; Postdoctoral Assistant at Le Materials Research Laboratory. Both are co-first authors of the paper.