News from the NNI Community - Research Advances Funded by Agencies Participating in the NNI

The discovery in 2018 of superconductivity in two single-atom-thick layers of graphene stacked at a precise angle of 1.1 degrees (called “magic”-angle twisted bilayer graphene) came as a big surprise to the scientific community. Since the discovery, physicists have asked whether magic graphene's superconductivity can be understood using existing theory (conventional superconductors), or whether fundamentally new approaches are required (unconventional superconductors). Now, researchers at Princeton University have settled this debate by showing an uncanny resemblance between the superconductivity of magic graphene and that of high-temperature (or unconventional) superconductors. 

This article reviews research on twisted multilayer (bilayer, trilayer, and double bilayer) graphene by researchers at the University of Maryland’s Joint Quantum Institute and Condensed Matter Theory Center. The article provides historical context for this research and discusses theories developed by the researchers to explain the occurrence of superconductivity and magnetism in multilayer graphene.

The absence of a property called a band gap in graphene restricts its ability to function as a semiconductor. The dilemma has led scientists to explore ways to produce a band gap in graphene. One popular method has been to chemically modify the surface of graphene with hydrogen, a process called "hydrogenation." But the conventional way of doing this can seriously damage the surface of graphene within seconds or minutes. Now, scientists at Princeton University and the U.S. Department of Energy's Princeton Plasma Physics Laboratory have developed a novel method for hydrogenating graphene that uses low-temperature hydrogen plasma.

Researchers at Rice University have created nanostructures of silica with a sophisticated 3D printer, demonstrating a method to make electronic, mechanical, and photonic micro-scale devices from the bottom up. The printing process required the researchers to develop a unique ink by creating resins that contain nanospheres of silicon dioxide doped with polyethylene glycol to make them soluble.

Researchers at Harvard University, MIT, and the National Institute for Materials Science in Japan have carried out a study aimed at investigating Chern insulator ground states in twisted bilayer graphene. In Chern insulator ground states, the bulk of the material is insulating, yet electrons can propagate along the edges without dissipating heat. The researchers provide evidence of the existence of a sequence of incompressible states with unpredicted Chern numbers in twisted bilayer graphene.

A new generation of electronics and optoelectronics may soon be possible by controlling twist angles in a particular type of bilayer 2D material used in these devices, strengthening the intrinsic electric charge that exists between the two layers. Researchers from Penn State, Harvard University, MIT, and Rutgers University have worked with 2D materials called regular transition metal dichalcogenides (TMDs) and Janus TMDs. In the case of Janus TMDs, the atoms on each side of these materials are different, leading to varied charge transfer when each side is in contact with other 2D materials.

Metasurfaces are nanoscale structures that interact with light. Today, most metasurfaces use monolith-like nanopillars to focus, shape, and control light. Researchers at Harvard University have developed a metasurface that uses deep and narrow holes, rather than tall nanopillars, to focus light to a single spot. The diameter of these long, thin holes is only a few hundred nanometers, making the aspect ratio – the ratio of the height to width – nearly 30:1. The metalenses were fabricated using conventional semiconductor-industry processes and standard materials, which could allow them to be manufactured at scale in the future.

Researchers at Northwestern University have developed a versatile composite nanomaterial that can deactivate both biological threats, such as the novel coronavirus that causes COVID-19, and chemical threats, such as those used in chemical warfare. The composite nanomaterial, which can be easily coated on textile fibers, builds on an earlier study in which the researchers created a nanomaterial that deactivates toxic nerve agents. With some manipulations, the researchers were able to also incorporate antiviral and antibacterial agents into the material.

At Pacific Northwest National Laboratory, researchers are looking into how to design metal oxide thin films that can be harnessed to produce clean energy. They discovered that varying the composition of lanthanum nickel iron oxide (LNFO) thin films affects their ability to convert water into oxygen. This reaction is important for clean energy production. LNFO has the potential to reduce the need for or replace expensive precious metal-based catalysts.

A team of scientists at Stanford University grew 2D layers of perovskites, interleaved with thin layers of other materials in large crystals that assembled themselves. The self-assembly takes place in vials in which the chemical ingredients for the layers tumble around in water, along with barbell-shaped molecules that direct the action. This simpler and faster method lays the foundation for making a wide array of complex semiconductors in a more deliberate way, including combinations of materials that have not been known previously to pair up in crystals.