One-dimensional boron, investigated by theoretical physicists at Rice University, could be a unique material that incorporates both a semiconductor (the ribbon portion) and a metallic conductor (the single-atom chain). Because it can transform from one form to the other under stress, the material could form an adjustable Schottky junction. Courtesy of the Yakobson Research
Scientists at Rice University suggest that boron could be the most interesting new nanomaterial of the century, over graphene.
A Rice team that simulated one-dimensional forms of boron, both single-atom chains and two-atom-wide ribbons, discovered that they possess unique properties. The latest findings feature in the recent issue of the Journal of the American Chemical Society.
For instance, metallic ribbons of boron morph into antiferromagnetic semiconducting chains when they are stretched, and they fold back into ribbons when released.
A truss-like ribbon of boron atoms transforms as it stretches into a carbyne-like chain, also going from a semiconductor to metallic conductor, according to Rice University scientists. In this simulation, the one-dimensional chain is stretched to the breaking point. Courtesy of the Yakobson Group
The 1D boron materials also have mechanical stiffness on a par with the highest-performing known nanomaterials. They can also behave as nanoscale, constant-force springs.
Experimental labs are making progress in synthesizing atom-thin and fullerene-type boron, which made the Rice researcher Boris Yakobson assume that 1D boron may ultimately become real as well.
Yakobson’s lab develops atom-level computer simulations of materials that do not necessarily exist yet. Simulating and testing their energetic properties will help experimentalists working to develop real-world materials. Carbon-atom chains called carbine, two-dimensional films called borophene and boron fullerenes, all predicted by the Rice group, have since been developed by the labs.
Our work on carbyne and with planar boron got us thinking that a one-dimensional chain of boron atoms is also a possible and intriguing structure. We wanted to know if it is stable and what the properties would be. That’s where modern theoretical-computational methods are impressive, because one can do pretty realistic assessments of non-existing structures. Even if they never exist, they’re still important since we’re probing the limits of possibility, sort of the final frontier.
Boris Yakobson, Researcher, Rice University
In this simulation, the one-dimensional chain is stretched to the breaking point. Courtesy of the Yakobson Group
1D boron develops two well-defined phases, ribbons and chains, which are connected by a “reversible phase transition,” highlighting the fact that they can turn from one form to the other and back.
The team used a computer to “pull” the ends of a simulated boron ribbon with 64 atoms in order to demonstrate these interesting chemomechanics. This forced the atoms to rearrange into a single carbyne-like chain. In their simulation, the team left a piece of the ribbon to serve as a seed, and when the tension was released, the atoms from the chain carefully returned to ribbon form.
Boron is very different from carbon. It prefers to form a double row of atoms, like a truss used in bridge construction. This appears to be the most stable, lowest-energy state. If you pull on it, it starts unfolding; the atoms yield to this monatomic thread. And if you release the force, it folds back,” he said. “That’s quite fun, structurally, and at the same time it changes the electronic properties. That makes it an interesting combination: When you stretch it halfway, you may have a portion of ribbon and a portion of chain. Because one of them is metal and the other is a semiconductor, this becomes a one-dimensional, adjustable Schottky junction.
Boris Yakobson , Researcher, Rice University
A Schottky junction is considered to be a barrier to electrons at a metal-semiconductor junction and is frequently used in diodes that allow the flow of current in only one direction.
Boron, as a ribbon, is
“a true 1D metal robust to distortion of its crystalline lattice (a property known as Peierls distortion),” the researchers wrote. The material is provided with extraordinary stiffness by the truss-like construct. This stiffness is a measure of its ability to resist deformation from an applied force.
As a chain of atoms, the material is also a strain-tunable, wide-gap antiferromagnetic semiconductor. In an antiferromagnet, the atomic moments, referring to the direction of the atoms’ “up” or “down” spin states, line up in opposite directions. This coupling of electronic transport and magnetic state can be manipulated in order to develop high-performance electronic devices.
“It may be very useful because instead of charge transport, you can have spin transport. That’s considered an important direction for devices that make use of spintronics,” he said.
The springiness of 1D boron is also interesting, Yakobson said.
It’s also a special spring, a constant-force spring. The more you stretch a mechanical spring, the more the force goes up. But in the case of 1D boron, the same force is required until the spring becomes fully stretched. If you keep pulling, it will break. But if you release the force, it completely folds back into a ribbon. It’s a mechanically nice structure.
Boris Yakobson , Researcher, Rice University
That property could be useful in nanoscale sensors to gauge extremely small forces, he said.
Rice alumna Mingjie Liu, currently a research associate at Brookhaven National Laboratory, is lead author of the paper. Vasilii Artyukhov, also a Rice alumnus and currently a research scientist at Quantlab Financial, is co-author. Yakobson is the Karl F. Hasselmann Professor of Materials Science and NanoEngineering and a professor of chemistry.
The research was supported by the Office of Naval Research and the Robert Welch Foundation. Calculations were carried out on Rice’s National Science Foundation-supported DAVinCI supercomputer, which was administered by Rice’s Center for Research Computing and procured in collaboration with the Ken Kennedy Institute for Information Technology.