A two-dimensional sheet of boron, a material known as borophene, has been developed by researchers at the US Department of Energy.
Two-dimensional materials have intrigued scientists 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.
Since borophene is both metallic and atomically thin, it holds promise for possible applications ranging from electronics to photovoltaics, according to Argonne National Laboratory nanoscientist Nathan Guisinger, who led the experiment. No bulk form of elemental boron has this metal-like behavior, he says.
“Borophenes are extremely intriguing because they are quite different from previously studied two-dimensional materials,” Guisinger said. “And because they don’t appear in nature, the challenge involved designing an experiment to produce them synthetically in our lab.”
Like its neighbor on the periodic table, carbon, which appears in nature in forms ranging from humble graphite to precious diamond, boron wears a number of different faces, called allotropes. But that’s where the similarities end.
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.
Brand New Material
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.
“It’s only in the recent past that researchers have been able to make tiny bits of boron at the nanoscale,” said Andrew Mannix, first author of the study. “This is a brand new material with exciting properties that we are just beginning to investigate.”
One of boron’s most novel features is 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 Mannix.
The “ridges” of this cardboard-like structure result in a material phenomenon known as anisotropy, in which a material’s mechanical or electronic properties—like 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. Tensile strength refers to the ability of a material to resist breaking when it is pulled apart.
Synthesis of borophenes: Anisotropic, two-dimensional boron polymorphs, Science, DOI: 10.1126/science.aad1080
Illustration: Tetraborate-xtal-3D-balls by Ben Mills Licensed under Public Domain via Commons