BORON THINS DOWN MATERIALS: Researchers prepare
first atomically thin sheets of element number five
S
INCE THE DISCOVERY of graphene, materi-
als 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, and transition-metal sulfides such as MoS2 are well-known (See page 24). With their exceptional conductivity and mechanical properties, they are fast becoming attractive components for making 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 lie flat. With only three valence electrons, boron must compensate for being electrondeficient 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 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.”—STEVE RITTER
ARTEM OGANOV (TOP) AND GUOAN TAI (BOTTOM)
NEWS OF THE WEEK
These structural models depict the 2-D layer of B7 borophene (top) and a top and side view of a 2-D layer of γ-B28 (bottom).
LEGISLATION Science funding sees surge in U.S. government spending bill A massive federal spending bill unveiled on Dec. 16 would provide increases for most U.S. science funding agencies in fiscal 2016, which began on Oct. 1. The long-awaited agreement between congressional Republicans and Democrats would include a $2 billion increase for the National Institutes of Health, up to $32 billion in fiscal 2016. The Department of Energy’s Office of Science would receive $5.4 billion, up $279 million from 2015. The National Science Foundation would go up $119 million to $7.3 billion. In an important development for the chemical enterprise, a tax reform plan
that accompanies the bill would permanently enact the research and development tax credit for companies. The credit, which gives businesses a tax break for up to 20% of qualifying research expenses, was first passed in 1981. The credit is popular with chemical manufacturers, pharmaceutical companies, and other research-based industries. Congress has failed to make the tax credit permanent despite years of wrangling over whether to do so. Another provision would restrict the Food & Drug Administration from approving drugs or biological products that come out of inheritable changes to hu-
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man embryos. And a recently approved genetically modified salmon will not be able to be sold until FDA finalizes its labeling guidelines (See page 40); its draft guidelines made labeling voluntary. The bill also would hold the Environmental Protection Agency’s budget flat and keep the agency’s staffing levels below those last seen in 1989. As C&EN went to press, the bill was scheduled for a vote in the House of Representatives on Dec. 18 with the Senate expected to weigh in soon afterward. Congress was scheduled for a holiday recess starting Dec. 18.—ANDREA WIDENER
