2H-NbO₂—a novel van der Waals oxide synthesized by researchers from Japan—exhibits strongly correlated electronic properties with two-dimensional flexibility. By chemically extracting lithium ions from the layered sheets of LiNbO₂, the researchers transformed a three-dimensional oxide into a two-dimensional material—unlocking unique properties like Mott insulating states and superconductivity. Bridging transition metal oxides and 2D materials, the discovery paves the way for realizing advanced quantum materials in next-generation electronic devices.
Two-dimensional (2D) materials have become a cornerstone of next-generation electronic research. These materials—with their layers held together by weak van der Waals (vdW) forces—are celebrated for their unique quantum properties and promising applications in electronics. However, despite significant progress in 2D materials like graphene and transition metal dichalcogenides, one attractive family of materials called "transition metal oxides" or TMOs, remains unexplored for 2D application.
TMOs are a versatile class of materials known for their complex electronic properties like superconductivity, magnetism, and metal-insulator transitions. But due to their inherently strong ionic bonding, these oxides do not typically form vdW structures and therefore remain absent from 2D materials basically.
In a pioneering study to bridge this gap, a research team led by Assistant Professor Takuto Soma, Associate Professor Kohei Yoshimatsu, Professor Akira Ohtomo, and graduate student Ms. Aya Sato from the Department of Chemical Science and Engineering, School of Materials and Chemical Technology at Institute of Science Tokyo (Science Tokyo), Japan, in collaboration with Professor Hiroshi Kumigashira from Tohoku University, Japan, synthesized 2H-NbO₂, a novel 2D material with unique electronic properties of strongly correlated TMOs. The findings of the study were made available online on July 18, 2025, and were published in Volume 19, Issue 29 of the journal ACS Nano on July 29, 2025.
"By synthesizing 2H-NbO₂, we achieved the strongly correlated vdW oxide that exhibits the characteristics of both TMOs and 2D materials," highlights Soma.
To achieve the synthesis of 2H-NbO2, the researchers used a unique chemical strategy. They used nano-scale thin films of LiNbO₂, which has a layered oxide structure. By using a high-temperature strong oxidation reaction, they achieved selective removal of lithium ions (Li) from LiNbO₂ via a process called Li deintercalation. This deintercalation resulted in a "2H-type layered structure" with atoms arranged in a hexagonal honeycomb-like pattern stacked in two repeating layers, which was similar to classic 2D materials but with strongly correlated electrons.
Advanced analysis of 2H-NbO₂ confirmed that it becomes a correlated insulator primarily due to half-filled, single-electron bands. This specific electronic configuration leads to strong electron-electron repulsion, resulting in an insulating behavior, despite the presence of Nb 4d electrons. These properties are closely linked to key phenomena in modern physics, including metal-insulator transitions and unconventional superconductivity, both of which are of significant interest due to their complex underlying physics and potential technological applications.
Notably, the researchers observed metal-insulator transition, superconductivity, and non-Fermi liquid behavior in partial Li deintercalation. All of these are hallmark properties of strongly correlated systems, which is very similar to both of high-temperature copper oxide superconductors and Moiré superlattices of 2D materials.
"The significance lies in bridging two research domains—correlated oxides and 2D materials—that have so far evolved separately," explains Soma. "Our findings unlock a new class of 2D quantum materials that combine strong electronic correlations with the structural flexibility of vdW compounds."
This breakthrough is expected to have wide applications in the fields of quantum materials, next-generation electronic devices, and sustainable materials science.
