Physicists from the Massachusetts Institute of Technology turn a pencil into electronic “gold”.

Thin foil insulation that can be tuned to exhibit three important properties.

Massachusetts Institute of Technology Physicists have figuratively turned graphite, or pencil, into gold by isolating five ultrafine flakes stacked in a specific arrangement. The resulting material can then be tuned to exhibit three important properties never before seen in natural graphite.

“It’s like one-stop shopping,” says Long Guo, an assistant professor in the MIT Department of Physics and leader of the research published in the October 5 issue of the journal. Nature nanotechnology. “Nature has a lot of surprises. In this case, we never realized that all these interesting things are in graphite.

Furthermore, “it is very rare to find materials that can host this many properties,” he says.

The rise of “Twistronics”

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

In the current work, “we discovered interesting properties without any twisting at all,” says Gu, who is also affiliated with the Materials Research Laboratory.

An artistic demonstration of electron binding, or the ability of electrons to talk to each other, which can occur in a special type of graphite (pencil). Image source: Sampson Wilcox, MIT Electronics Research Laboratory

He and his colleagues discovered that five layers of graphene arranged in a specific order allow electrons moving within the material to talk to each other. This phenomenon, known as electron correlation, “is the magic that makes all these new properties possible,” says Joe.

Bulk graphite – and even single graphene sheets – are good electrical conductors, but that’s about it. The material isolated by Gu and his colleagues, which they call five-layer stacked graphene, becomes much greater than the sum of its parts.

The novel microscope and its discoveries

The key to isolating matter was A Novel microscope Joe at MIT in 2021 can determine a variety of important properties of matter quickly and relatively inexpensively. Nano scale. The stacked graphene with the pentahedral layer is only a few billionths of a meter thick.

Scientists including Gu were looking for multilayered graphene that was stacked in a very precise arrangement, known as rhombic stacking. “There are more than 10 possible stacking orders when you go down to five layers,” says Joe. “Rhombohedral is just one of them.” The microscope Joe made, known as Scattering-type Scanning Nearfield Optical Microscopy, or s-SNOM, allowed scientists to identify and isolate just the five layers. In the rhombic stacking order they were interested.

Multifaceted physical phenomena

From there, the team attached electrodes to a small sandwich made up of boron nitride “bread” that protects the thin “meat” of the stacked pentahedral graphene. The electrodes allowed them to tune the system to different voltages or different amounts. The result: They discovered that three different phenomena appear depending on the number of electrons flooding the system.

MIT postdoctoral fellow Zhengguang Lu, assistant professor Long Ju, and graduate student Tonghang Han are in the lab. The three are the authors of a paper in the journal Nature Nanotechnology about a special type of graphite (pencil lead), along with seven others. Credit: GoLab

“We found that matter can be insulating, magnetic, or topological,” says Gu. The latter is related to some extent to both conductors and insulators. Joe explains that a topological material allows unhindered movement of electrons around the edges of the material, but not through the middle. Electrons move in one direction along a “highway” at the edge of the material separated by a medium that forms 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 rhombic stacked multilayer graphene as a highly tunable platform for studying these new possibilities for topological and strongly coupled physics,” Guo and his co-authors conclude in Nature nanotechnology.

Reference: “Coherent dielectric and Chern insulators in five-layer stacked graphene” by Tonghang Han, Zhenguang Lu, Giovanni Scurri, Jihu Song, Gui Wang, Tian Yi Han, Kenji Watanabe, Takashi Taniguchi, Hongkun Park, and Long Ju, 5 October 2023, Nature nanotechnology.
doi: 10.1038/s41565-023-01520-1

In addition to Gu, the paper’s authors are Tonghang Han and Zhenguang Lu. Han is a graduate student in the Department of Physics. Lu is a postdoctoral fellow in the Materials Research Laboratory. They are the first authors of the paper.

Other authors are Giovanni Scurri, Jiho Song, Joy Wang and Hongkun Park of Harvard University; Kenji Watanabe and Takashi Taniguchi of the National Institute of Materials Science in Japan, and Tianyi Han of the Massachusetts Institute of Technology for Physics.

This work was supported by a Sloan Fellowship; US National Science Foundation; Office of the Under Secretary of Defense for Research and Engineering; Japan Society for the Promotion of Science KAKENHI; The world’s leading international research initiative in Japan; and the US Air Force Office of Scientific Research.