(Left) Boron atoms are evaporated directly from the crucible onto a substrate. (Right) 3D scanning tunneling microscopy image of 2D boron sheet on Ag(111) surface.The explosion of interest in two-dimensional carbon has spurred curiosity into other graphene-like materials. Could next-door neighbor to carbon in the periodic table, boron, with its intriguing mix of metallic and non-metallic properties, possess similar two-dimensional forms?
The answer appears to be yes, according to recent reports. Researchers from the Institute of Physics and the Collaborative Innovation Center of Quantum Matter in Beijing, China have succeeded in fabricating two-dimensional sheets of boron just one atom thick [Feng et al., Nat. Chem. (2016), doi: 10.1038/nchem.2491].
Like carbon, boron can adopt a bonding configuration that favors the formation of low-dimensional structures such as nanotubes, fullerenes, and sheets. With these different forms—or allotropes—could come interesting and novel properties distinct from the bulk characteristics.
Kehui Wu and his colleagues used molecular beam epitaxy (MBE) to grow two-dimensional sheets of boron (or 2DB) on a metallic Ag(111) substrate by direct evaporation of a pure boron source under ultrahigh vacuum conditions.
‘‘The boron sheets only formed at substrate temperatures of 500—800 K,’’ he explains. ‘‘When the substrate temperature is lower than 500 K only clusters are formed on the surface and when the substrate temperature is higher than 800 K boron atoms cannot adsorb on the surface.’’
The researchers observed two distinct forms of boron sheets, both consisting of a triangular lattice but with different arrangements of hexagonal holes or vacancies.
‘‘Two different kinds of 2DBs were revealed, solving the long-standing question on whether 2DBs can exist and what kind of atomic structures they should take,’’ says Wu.
In one form, which agrees with recent theoretical predictions, rows of hexagonal boron atoms are separated by chains of holes; in the other, boron atoms take up narrower zigzag positions separated by arrays of holes. In both cases, the sheets are flat, metallic in character, and quite stable against oxidation in air. Moreover, the sheets appear to be reasonably robust and only weakly bound to their substrate, indicating that it might be possible to obtain freestanding sheets. This would be an advantage for the fabrication of devices in the future.
‘‘The boron sheets are metallic, so could be used for ultra thin conducting wires,’’ says Wu. ‘‘Furthermore, theoretical works suggests that the two boron sheet structures are very likely to be superconducting at a temperature of around 10 K.’’
Adding to other recent reports of boron sheets [A. J. Mannix et al., Science 350 (2015) 1513], the findings are the first hints of a new two-dimensional material. There are still many unanswered questions about the physical properties of 2DB and its potential for electronic devices. But Wu and his colleagues are now investigating the growth of 2DB on other materials—particularly semiconductors—and how to detach the sheets from substrates.
Boris I. Yakobson of Rice University agrees that the challenge now is to place boron sheets on other substrates, such as hexagonal BN for example, for more detailed characterization. ‘‘Then many exciting things may become possible! Transparent electrodes, two-dimensional superconductors, and maybe even catalysts,’’ he says.
This article was originally published in Nano Today (2016), doi:10.1016/j.nantod.2016.05.005