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Forget Graphene and Carbon Nanotubes, Get Ready for Diamond Nanothread

The discovery of a stable form of one-dimensional diamond has scientists racing to understand its properties. The first signs are that diamond nanothread will be more versatile than anyone expected.

Hardly a week goes by without somebody proclaiming a new application for graphene, the form of carbon that occurs in single sheets with chicken wire-like structure (see “Research Hints at Graphene’s Photovoltaic Potential”). Roll a graphene sheet into a tube and it forms a carbon nanotube, another wonder material with numerous applications. And wrap it further into a ball and, with a small rearrangement of bonds, it forms buckyballs.

Now there is a new kid on the carbon block. Last month, a team at Pennsylvania State University and elsewhere announced they had created another type of carbon that takes the form of a one-dimensional diamond crystal capped with hydrogen. They call this new material diamond nanothread.

That caused a flurry of excitement and raised some interesting questions. Materials scientists are fascinated by the potential properties of a diamond nanothread and its applications. But one fear is that such a thread would be so brittle that it would shatter like glass under any kind of load, a property that would severely limit its use.

Today, we get some new insight into diamond nanothreads thanks to the work of Haifei Zhan at Queensland University of Technology in Australia and a few pals. These guys have modeled the threads using large-scale molecular dynamics simulations. And they conclude that the material could be more versatile than anyone thought. There are tentative signs that diamond nanothread could be a new a wonder material in its own right.

The Penn State team manufactured the nanothread from benzene molecules, simple rings of carbon atoms. It’s not hard to see how a stack of these could bond in a way that forms a thread.

And that’s exactly what the Penn State team did. They stacked the molecules into a line, placed it under pressure so that the molecules polymerized and, voila, created a diamond nanothread.

That sounds simple in theory but the complexity arises from the way the carbon atoms can bond. Various configurations are possible, and the question that Zhan and co investigate is how the properties of the thread depend on these arrangements.

In particular, Zhan and co look at the two most common configurations. The first is straightforward polymerized benzene—a stack of these rings bonded together. This is a rigid molecule that becomes increasingly brittle as it gets longer. Constructing anything complex with long sections of poly-benzene would be like trying to sew with like uncooked spaghetti.

But there is another configuration of carbon atoms known as Stone-Wales defects, and these are much more malleable. Indeed, the Stone-Wales defects act like hinges connecting sections of poly-benzene.

Zhan and co simulate how the properties of the nanothread vary as the density of these defects increases. And they conclude that when the density crosses a particular threshold, the thread suddenly changes from brittle to entirely flexible—rather like the difference between uncooked and cooked spaghetti.

That’s an interesting result. It implies that the property of the nanothread can be tuned simply by controlling the density of Stone-Wales defects along its length. So some parts of the thread can be made rigid, while others are entirely flexible.

What of potential applications? “Its highly tunable ductility together with its ultra-light density and high Young’s modulus makes diamond nanothread ideal for the creation of extremely strong three-dimensional nano-architectures,” say Zhan and co.

Of course, this work is just a simulation. There are almost certainly going to be differences between its predictions and the behavior of diamond nanothreads in the real world. So the next step will be for materials scientists to create some nanothread construction kits and start measuring this material’s properties for real.

Given the huge interest in carbon architecture and the vast sums of money being poured into this area—the European Union alone has a €1 billion research project focused purely on graphene—it surely won’t be long before we see diamond nanothreads in the flesh and some of the extraordinary applications that it should make possible.

Ref: From Brittle to Ductile: A Structure Dependent Ductility of Diamond Nanothread

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Illustration by Rose Wong

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