Scientists have been interested in two-dimensional materials for their electronic properties. Borophene shows many metallic properties at the nanoscale even though three-dimensional, or bulk, boron is non-metallic and semiconducting.
Because borophene is both metallic and atomically thin, it holds promise for possible applications ranging from electronics to photovoltaics, according to the scientists.
"Borophenes are intriguing because they are quite different from previously studied two-dimensional materials," Argonne nanoscientist, Nathan Guisinger said. "And because they don't appear in nature, the challenge involved designing an experiment to produce them synthetically in our lab."
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," added Andrew Mannix, Northwestern graduate student. "This is a brand new material with exciting properties that we are just beginning to investigate."
One of boron's most unusual features consists of its atomic configuration at the nanoscale. While other two-dimensional materials look perfectly smooth at the nanoscale, borophene is corrugated, buckling up and down depending on how the boron atoms bind to one another.
The ‘ridges’ of this corrugated 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.
Unlike previous experiments that used highly toxic gases in the production of nanoscale boron-based materials, this experiment involved a non-toxic technique called electron-beam evaporation, which essentially vaporises a source material and then condenses a thin film on a substrate - in this case, boron on silver.
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. "Other two-dimensional materials have been known to have high tensile strength, but this could be the strongest material we've found yet," Guisinger concluded.
