Physicists Find Misaligned Carbon Sheets Yield Unparalleled Properties

A material composed of two oneatom- thick layers of carbon has grabbed the attention of physicists worldwide for its intriguing — and potentially exploitable — conductive properties.

Dr. Fan Zhang, assistant professor of physics in the School of Natural Sciences and Mathematics at UT Dallas, and physics alumnus Qiyue Wang PhD’21 published an article in June 2020 with Dr. Fengnian Xia’s group at Yale University in Nature Photonics that describes how the ability of twisted bilayer graphene to conduct electrical current changes in response to mid-infrared light.

Graphene is a single layer of carbon atoms arranged in a flat honeycomb pattern, where each hexagon is formed by six carbon atoms at its vertices. Since graphene’s first isolation in 2004, its unique properties have been studied by scientists for potential use in advanced computers, materials and devices.

If two sheets of graphene are stacked on top of one another, and one layer is rotated so that the layers are slightly out of alignment, the resulting physical configuration, called twisted bilayer graphene, yields electronic properties that differ significantly from those exhibited by a single layer alone or by two aligned layers.

When the graphene layers are misaligned, a new periodic design in the mesh emerges, called a moiré pattern. The moiré pattern is also a hexagon, but it can be made up of more than 10,000 carbon atoms.

“The angle at which the two layers of graphene are misaligned — the twist angle — is critically important to the material’s electronic properties,” Wang said. “The smaller the twist angle, the larger the moiré periodicity.”

The unusual effects of specific twist angles on electron behavior were first proposed in 2011. Researchers later proved that offsetting two graphene layers by 1.1 degrees, the “magic angle,” produced a two-dimensional superconductor and that, when offset by 0.93 degrees, twisted bilayer graphene exhibits both superconducting and insulating states.

“The fact that you can manipulate pure carbon to superconduct is amazing and unprecedented,” Wang said.

In their most recent research, Zhang and his collaborators investigated whether and how twisted bilayer graphene interacts with mid-infrared light, which humans can’t see but can detect as heat.

“Interactions between light and matter are useful in many devices — for example, converting sunlight into electrical power,” Wang said.

Zhang and Wang set out to determine how mid-infrared light might affect the conductance of electrons in twisted bilayer graphene. Using resources of the Texas Advanced Computing Center, Wang calculated the band structure and showed how the material absorbs light.

Dr. Joe Qiu, program manager for solid-state electronics and electromagnetics at the U.S. Army Research Office (ARO) said, “This new breakthrough will potentially enable a new class of infrared detectors based on graphene with high sensitivity. These new detectors will potentially impact applications such as night vision, which is of critical importance for the U.S. Army.”

The ARO, the National Science Foundation and the Office of Naval Research supported the study.

– Amanda Siegfried