Boron enters flatland: Borophene sheets on the way
- Published: Feb 3, 2014
- Channels: UV/Vis Spectroscopy
From C to B
Laboratory experiments and supercomputer simulations have suggested to chemists that a cluster of 36 boron atoms can form a flat disc with a hexagonal hole in the middle. The shape fits theoretical predictions for a potential new nanomaterial dubbed "borophene" which would not be dissimilar to the much-vaunted all-carbon material graphene.
Graphene has emerged as the darling of the materials world, a pencil scribble peeled from a glass microscope slide, the carbon monolayers that resemble sub-microscopic chicken wire fencing have revealed themselves to have unusual and potentially useful properties in the realms of optics, electronics, engineering and even as impermeable membranes for use as contraceptives. However, carbon's elemental near neighbour, boron, one step to the left in the Periodic Table, may muscle in on graphene's domain as the possibility of "borophene" the boron analogue of the all-carbon material emerges from supercomputer simulations and experimental hints.
Researchers at Brown University in Providence, Rhode Island, USA, have determined the unique arrangement taken up by 36 boron atoms which could form a flat disc with a hexagonal hole at centre and might act as the preferred building blocks for the construction of much grander borophene sheets. The team reports its initial findings in the journal Nature Communications.
Of course, since the first experimental demonstration of graphene, scientists have mused on the possibility of forming monolayer sheets of other elements, including boron. While carbon-carbon bonds abound throughout nature, on the chemist's laboratory bench and in the materials lab, there are many fewer boron analogues. Nevertheless, this element can form the counterpart of hydrocarbon chains and cages in which the carbons in the organic structures are substituted for boron atoms. Perhaps this element might also be nudged into a single-atom sheet arrangement. Theoretical work suggested it was possible, but the atoms would need to be in a very particular arrangement.
Bonding in boron is different from that seen with carbon because boron has one fewer electron than carbon and so it cannot form the requisite flat bonds that are possible when carbon atoms get together in a planar honeycomb lattice, as seen in graphite and graphene. For boron to form a single-atom layer, theorists suggested that the atoms must be arranged in a triangular lattice with hexagonal vacancies - holes - in the lattice.
"That was the prediction," explains Brown chemist Lai-Sheng Wang, "but nobody had made anything to show that’s the case." He concedes that he and his colleagues are yet to synthesise borophene itself, but the building block structure they have produced with its odd hexagonal hole suggests that borophene might be more than a synthetic construct. Wang and his research team are experts in carbon's next-door neighbour and now writing in Nature Communications, they report an almost flat, symmetrical 36-boron cluster symmetrical, just one atom thick with a perfect hexagonal hole at its centre.
"It's beautiful," Wang enthuses. "It has exact hexagonal symmetry with the hexagonal hole we were looking for. The hole is of real significance here. It suggests that this theoretical calculation about a boron planar structure might be right." It may be possible, he adds, to use B36 basis to form an extended planar boron sheet. In other words, B36 may well be the embryo of a new nanomaterial.
Bonding and binding
The work required a combination of laboratory experiments and computational modelling. In the lab, Wang and his student, Wei-Li Li, probe the properties of boron clusters using photoelectron spectroscopy on laser-vaporised chunks of bulk boron the debris from which is frozen by a jet of helium. The mass spectra reveal the existence of any clusters of boron atoms. The photoelectron data showed that the B36 cluster was special with an extremely low electron-binding energy when compared to other known boron clusters. The profile of the cluster's binding spectrum also hinted at its symmetry, which was corroborated by computational simulations carried out by team member Zachary Piazza. He modelled more than 3000 potential structures for B36 on the supercomputer and demonstrated that the most stable would be the planar disc with a hexagonal hole.
To ensure that they have truly found the most stable arrangement of the 36 boron atoms, they enlisted the help of Jun Li, who is a professor of chemistry at Tsinghua University in Beijing and a former senior research scientist at Pacific Northwest National Laboratory (PNNL) in Richland, Wash. Li, a long-time collaborator of Wang’s, has developed a new method of finding stable structures of clusters, which would be suitable for the job at hand. Piazza spent the summer of 2013 at PNNL working with Li and his students on the B36 project. They used the supercomputer at PNNL to examine more possible arrangements of the 36 boron atoms and compute their electron-binding spectra. They found that the planar disc with a hexagonal hole matched very closely the spectrum measured in the laboratory experiments, indicating that the structure Piazza found initially on the computer was indeed the structure of B36.
The boron-boron bond is very strong, almost as strong as the carbon-carbon bond. So borophene should be a very strong material. Its electrical properties might also be rather more interesting, being predicted as fully metallic, rather than the semi-metal status of graphene, which means it might be a better conductor. Of course, tests on borophene will only be realized once someone finds a way to synthesise it, there will be no simple pencil scribble on a glass slide for this hypothetical material after all.
"We will continue solving the structures of boron clusters in this size range and larger to see how the cluster would grow one atom at a time," Wang told SpectroscopyNOW. "This has been pretty much what we have been doing over the past decade. I'd very much like to synthesize borophene ultimately, but that would require very different experimental setups and skills, which we are not well equipped at the moment."
Nature Commun, 2014, online: "Planar hexagonal B36 as a potential basis for extended single-atom layer boron sheets"
Article by David Bradley
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.