Since the discovery of graphene, materials scientists have had the dream of making atomically thin, two-dimensional sheets of boron—theoreticians have long predicted that the planar boron layers could exist. In a pair of experimental breakthroughs, two research teams have independently synthesized versions of the dream material for the first time.
Two-dimensional materials such as graphene, silicene, phosphorene, and transition-metal sulfides such as MoS2 are well-known. With their exceptional conductivity and mechanical properties, they are fast becoming attractive components for fabricating smaller, faster electronics and more powerful energy-storage devices.
Boron is a latecomer to the 2-D materials scene, however, in part because it’s intrinsically a 3-D element that is hard to get to lay flat. With only three valence electrons, boron must compensate for being electron-deficient by forming framework structures in which it can more readily share electrons. The result is that boron has at least 16 structurally diverse 3-D polymorphs. Researchers have made a few examples of planar boron clusters, but extended planar networks of pure boron have remained elusive for experimentalists until now.
In one of the breakthroughs, a team led by Artem R. Oganov of Stony Brook University, SUNY; Mark C. Hersam of Northwestern University; and Nathan P. Guisinger of Argonne National Laboratory used an electron-beam evaporator to ablate solid boron under ultrahigh vacuum to prepare a one-atom-thick sheet of boron, referred to as borophene, on a silver surface (Science 2015, DOI: 10.1126/science.aad1080). The material, composed of nearly planar B7 clusters, is a metal-like conductor. With the exception of borophene, all known boron polymorphs are semiconductors, though they may become metallic only under extreme pressure.
In the other breakthrough, a team led by Guoan Tai of Nanjing University of Aeronautics & Astronautics fired up mixtures of boron and boron oxide (B2O3) powders to 1,100 °C in a chemical vapor deposition furnace, then reduced the boron with hydrogen at 1,000 °C and passed the low-pressure vapor over copper foil to obtain a polycrystalline boron monolayer (Angew. Chem. Int. Ed. 2015, DOI: 10.1002/anie.201509285). The researchers found that the semiconducting material, characterized as the γ-B28 polymorph, consists of icosahedral B12 units linked together into a network by B2 dumbbells. Although the material is not strictly a single-atom layer like borophene, it is a 2-D material by definition.
“These materials show us that we may expect more 2-D polymorphs of boron in the future with other outlandish electronic and magnetic properties,” says Alexander I. Boldyrev of Utah State University, a computational chemist who studies planar boron clusters. “Indeed the experimental realization of 2-D boron, which was a grand challenge for many years, proves that we can make 2-D materials composed of almost all the elements.”