Researchers at the Massachusetts Institute of Technology (MIT) have found a way to efficiently create composite materials containing hundreds of layers that are just atoms thick but span the full width of the material. The discovery could open up possibilities for designing new, easy-to-manufacture composites for optical devices and electronic systems.

Materials such as graphene and carbon nanotubes are “some of the strongest, hardest materials we have available”—says Michael Strano, professor in chemical engineering—because their atoms are held together entirely by carbon-carbon bonds. So researchers have been searching for ways to use these nanomaterials to add strength to composite materials, in much the way steel bars are used to reinforce concrete.

One obstacle has been finding ways to embed these materials within a matrix of another material in an orderly way. These sheets and tubes have a tendency to clump, so stirring them into a batch of liquid resin before it sets doesn’t always work.

(Click to enlarge.) The process of making a stack of parallel sheets of graphene starts with a chemical vapor deposition process (I) to make a graphene sheet with a polymer coating; these layers are then stacked (II), folded and cut (III), stacked again and pressed, multiplying the number of layers. Image credit: MIT.(Click to enlarge.) The process of making a stack of parallel sheets of graphene starts with a chemical vapor deposition process (I) to make a graphene sheet with a polymer coating; these layers are then stacked (II), folded and cut (III), stacked again and pressed, multiplying the number of layers. Image credit: MIT.The MIT team’s insight was in finding a way to create large numbers of layers, stacked in an orderly way, without having to stack each layer individually. At the heart of their method is a technique in which a layer of material is spread flat, then doubled over on itself, pounded or rolled out and then doubled over again and again. With each fold, the number of layers doubles, thus producing an exponential increase in the layering.

But in this research, rather than folding the material, the team cut the whole block—itself consisting of alternating layers of graphene and the composite material—into quarters, then slid one quarter on top of another, quadrupling the number of layers, and then repeated the process.

The MIT team produced composites with up to 320 layers of graphene embedded in them and were able to demonstrate that, even though the total amount of graphene added to the material was less than 0.001% by weight it led to improvement in overall strength.

“The graphene has an effectively infinite aspect ratio,” Strano says, since it is infinitesimally thin yet can span sizes large enough to be seen and handled. It can span two dimensions of the material even though it is only nanometers thick.

The team also found a way to make structured fibers from graphene, potentially enabling the creation of yarns and fabrics with embedded electronic functions. The method uses a shearing mechanism to peel off layers of graphene in a way that causes them to roll up into a scroll-like shape.

That could overcome one of the biggest drawbacks of graphene and nanotubes, in terms of their ability to be woven into long fibers: their slipperiness. Because they are so smooth, strands slip past each other instead of sticking together in a bundle. The new scrolled strands not only overcome that problem, they are also extremely stretchy, unlike other super-strong materials such as Kevlar. That means they might lend themselves to being woven into protective materials that could “give” without breaking.

An unexpected feature of the new layered composites, Strano says, is that the electrically conductive graphene layers maintain their continuity all the way across their composite sample without any short-circuiting to the adjacent layers. So, simply inserting an electrical probe into the stack to a certain depth would make it possible to uniquely “address” any one of the hundreds of layers. This could ultimately lead to new kinds of complex multilayered electronics, he says.

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