'Hopelessly attached': Scientists discover new 2D material that sticks the landing

Unlike systems such as graphene on copper, where atoms may diffuse into the substrate without forming a distinct alloy, the boron atoms in this case formed a defined 2D copper boride -- a new compoundwith a distinct atomic structure. The finding, published in Science Advances by researchers from Rice and Northwestern University, sets the stage for further exploration of a relatively untapped class of 2D materials.

"Borophene is still a material at the brink of existence, and that makes any new fact about it important by pushing the envelope of our knowledge in materials, physics and electronics," said Yakobson, Rice's Karl F. Hasselmann Professor of Engineering and professor of materials science and nanoengineering and chemistry. "Our very first theoretical analysis warned that on copper, boron would bond too strongly. Now, more than a decade later, it turns out we were right -- and the result is not borophene, but something else entirely."

Previous studies successfully synthesized borophene on metals like silver and gold, but copper remained an open -- and contested -- case. Some experiments suggested boron might form polymorphic borophene on copper, while others suggested it could phase-separate into borides or even nucleate into bulk crystals. Resolving these possibilities required a uniquely detailed investigation combining high-resolution imaging, spectroscopy and theoretical modeling.

"What my experimentalist colleagues first saw were these rich patterns of atomic resolution images and spectroscopy signatures, which required a lot of hard work of interpretation," Yakobson said.

These efforts revealed a periodic zigzag superstructure and distinct electronic signatures, both of which deviated significantly from known borophene phases. A strong match between experimental data and theoretical simulations helped resolve a debate about the nature of the material that forms at the interface between the copper substrate and the near-vacuum environment of the growth chamber.

Although copper boride was not the material researchers set out to make, its discovery offers important insight into how boron interacts with different metal substrates in two-dimensional environments. The work expands the knowledge on the formation of atomically thin metal boride materials -- an area that could inform future studies of related compounds, including those with known technological relevance, such as metal borides among ultra-high temperature ceramics, which are of great interest for extreme environments and hypersonic systems.

"2D copper boride is likely to be just one of many 2D metal borides that can be experimentally realized. We look forward to exploring this new family of 2D materials that have broad potential use in applications ranging from electrochemical energy storage to quantum information technology," said Mark Hersam , Walter P. Murphy Professor of Materials Science and Engineering at Northwestern University, who is a co-corresponding author on the study.

The discovery comes shortly after another boron-related breakthrough by the same Rice theory team. In a separate study published in ACS Nano , researchers showed that borophene can form high-quality lateral, edge-to-edge junctions with graphene and other 2D materials, offering better electrical contact than even "bulky" gold. The juxtaposition of the two findings highlights both the promise and the challenge of working with boron at the atomic scale: its versatility allows for surprising structures but also makes it difficult to control.

"Those images we initially saw in the experimental data looked quite mysterious," Yakobson said. "But in the end, it all fell into place and provided a logical answer -- metal boride, bingo! This was unexpected at first, but now, it is settled -- and the science can move forward."

The research was supported by the Office of Naval Research (N00014-21-1-2679), the National Science Foundation (DMR-2308691) and the United States Department of Energy (2801SC0012547). The content herein is solely the responsibility of the authors and does not necessarily represent the official views of the funding organizations and institutions.