The team used electron beam evaporation to shoot individual boron molecules onto an atomically clean silver substrate, creating “borophene.” The resulting material’s corrugated, 2-D structure leads to strong, highly directional conductivity. [Image: Mannix et al., Science, doi: 10.1126/science.aad1080]
A multinational research team has synthesized and analyzed the properties of atomically thin sheets of boron, creating a new 2-D material the team has dubbed borophene (Science, doi: 10.1126/science.aad1080). The long-sought form of boron has some potentially intriguing electronic and physical properties, and offers another component in the growing toolkit of 2-D materials for optoelectronic applications.
Intriguing but hazardous
Since the discovery of graphene—the atomically thin carbon sheets whose isolation and characterization, more than ten years ago, captured the 2010 Nobel Prize in Physics—the family of 2-D materials has grown; graphene’s siblings now include hexagonal boron nitride, transition-metal dichalcogenides (TMDs) such as molybdenum disulfide, and other entries in the 2-D sweepstakes. Mixing and matching these materials, which have different electronic and optical characteristics, has offered the potential for a new generation of ultrathin optoelectronic devices (see “Optoelectronics in Flatland,” OPN, July/August 2015).
Researchers in the 2-D materials realm have also long been intrigued by the potential of sheets of atomically thin, pure boron. This element, a semimetal, is lodged on the periodic table between metallic beryllium and nonmetallic carbon, and chemical and theoretical analyses have suggested that it could have some unusual metallic properties if it could be fashioned into graphene-like sheets.
That’s been a big “if,” however. Whereas graphene can be created by mechanical exfoliation of sheets from bulk graphite, working with boron at the nanoscale has until now involved the use of highly toxic, expensive precursors such as diborane—a factor that has kept many labs away from the work.
New synthesis, new insights
To get around some of these difficulties, the authors of the new study—led by scientists from the Argonne National Lab and Northwestern University in the U.S. and the Skolkovo Institute of Science and Technology in Russia—tried out a very different approach to getting to 2-D boron. Starting with a 99.9999-percent-pure boron rod, and working in ultrahigh-vacuum conditions, they used electron beam evaporation to spall off individual boron atoms, carefully monitoring and controlling the process to maintain a uniform boron flux. The atoms were then deposited onto a substrate of atomically clean silver, an inert metal, which provided nonreactive base for the boron sheet.
Armed with this new capability to create boron sheets, the team next set about analyzing them. Scanning tunneling microscopy revealed that the boron sheets were not utterly flat, but instead showed tiny corrugations, like a sheet of cardboard. Further theoretical work, confirmed by scanning tunneling spectroscopy, revealed that in the transition to the 2-D structure, the semiconducting boron turns metallic—and that its conductivity is highly directional, controlled by the anisotropy imposed by the nanoscale corrugations.
Those properties, along with others such as extreme tensile strength, could, in the view of the research team, make the new borophene a material with strong potential in electronic devices, optoelectronics, photovoltaics and more. “This is a brand new material,” said lead author Andrew Mannix, a grad student at Northwestern University, “with exciting properties that we are just beginning to investigate.”
