Scientists have been interested in two-dimensional materials for their unique characteristics, particularly involving their electronic properties. Borophene is an unusual material because it shows many metallic properties at the nanoscale even though three-dimensional, or bulk, boron is nonmetallic and semiconducting. Because borophene is both metallic and atomically thin, it holds promise for possible applications ranging from electronics to photovoltaics, said Argonne nanoscientist Nathan Guisinger, who led the experiment. “No bulk form of elemental boron has this metal-like behavior,” he said.

Like its periodic table neighbor carbon, boron has a number of allotropes (different forms of the same element). But while graphite is composed of stacks of two-dimensional sheets that can be peeled off one at a time, there is no such analogous process for making two-dimensional boron.

Borophenes are extremely intriguing because they are quite different from previously studied two-dimensional materials. And because they don’t appear in nature, the challenge involved designing an experiment to produce them synthetically in our lab.

—Nathan Guisinger

Although at least 16 bulk allotropes of boron are known, scientists had never before been able to make a whole sheet, or monolayer, of borophene.

One of boron’s most unusual features consists of its atomic configuration at the nanoscale. While other two-dimensional materials look more or less like perfectly smooth and even planes at the nanoscale, borophene looks like corrugated cardboard, buckling up and down depending on how the boron atoms bind to one another, according to Andrew Mannix, a Northwestern graduate student and first author of the study.

The “ridges” of this cardboard-like structure result in a material phenomenon known as anisotropy, in which a material’s mechanical or electronic properties—such as its electrical conductivity—become directionally dependent. “This extreme anisotropy is rare in two-dimensional materials and has not been seen before in a two-dimensional metal,” Mannix said.

Based on theoretical predictions of borophene’s characteristics, the researchers also noticed that it likely has a higher tensile strength than any other known material.

The discovery and synthesis of borophene was aided by computer simulation work led by Stony Brook researchers Xiang-Feng Zhou and Artem Oganov, who is currently affiliated with the Moscow Institute of Physics and Technology and the Skolkovo Institute of Science and Technology. Oganov and Zhou used advanced simulation methods that showed the formation of the crinkles of the corrugated surface.

The experimental work was funded by the DOE’s Office of Science and was performed at Argonne’s Center for Nanoscale Materials, a DOE Office of Science user facility, and at the Northwestern University Materials Research Center.

Resources

• Andrew J. Mannix, Xiang-Feng Zhou, Brian Kiraly, Joshua D. Wood, Diego Alducin, Benjamin D. Myers, Xiaolong Liu, Brandon L. Fisher, Ulises Santiago, Jeffrey R. Guest, Miguel Jose Yacaman, Arturo Ponce, Artem R. Oganov, Mark C. Hersam, and Nathan P. Guisinger (2105) “Synthesis of borophenes: Anisotropic, two-dimensional boron polymorphs” Science 350 (6267), 1513-1516 doi: 10.1126/science.aad1080