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Boron may become nanomaterial of century

Monday, 30 January 2017
PTI

Washington: Move over graphene! Boron may become the nanomaterial of the century as scientists have found that two-atom-wide ribbons and single-atom chains of the element possess unique properties.

For example, if metallic ribbons of boron are stretched, they morph into antiferromagnetic semiconducting chains, and when released they fold back into ribbons. Experimental labs are making progress in synthesising atom-thin and fullerene-type boron, which led Boris Yakobson, researcher at Rice University in the US to think 1-D boron may eventually become real as well.

Yakobson's lab creates atom-level computer simulations of materials that do not necessarily exist yet.

Simulating and testing their energetic properties helps guide experimentalists working to create real-world materials. Carbon-atom chains known as carbyne, boron fullerenes and two-dimensional films called borophene, all predicted by the Rice group, have since been created by 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," said Yakobson.

One-dimensional boron forms two well-defined phases - chains and ribbons - which are linked by a 'reversible phase transition,' meaning they can turn from one form to the other and back.

To demonstrate these interesting chemomechanics, the researchers used a computer to 'pull' the ends of a simulated boron ribbon with 64 atoms. This forced the atoms to rearrange into a single carbyne-like chain. In the simulation, researchers left a fragment of the ribbon to serve as a seed, and when they released the tension, the atoms from the chain neatly 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," Yakobson said.

"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 makes it an interesting combination: When you stretch it halfway, you may have a portion of ribbon and a portion of chain," he said.

"Because one of them is metal and the other is a semiconductor, this becomes a one-dimensional, adjustable Schottky junction," he added. A Schottky junction is a barrier to electrons at a metal-semiconductor junction and is commonly used in diodes that allow current to flow in only one direction. 

The study appears in the Journal of the American Chemical Society. 

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