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'Nano' plates, 100nm thick, can be manipulated by hand

04 December 2015

Researchers at the University of Pennsylvania claim to have created the thinnest plates that can be picked up and manipulated by hand.

Even though they are less than 100 nanometres thick, the researchers' plates are strong enough to be picked up by hand and retain their shape after being bent and squeezed (photo: University of Pennsylvania)

Despite being thousands of times thinner than a sheet of paper and hundreds of times thinner than household cling wrap or aluminium foil, the corrugated plates of aluminium oxide spring back to their original shape after being bent and twisted.

Like cling wrap, comparably thin materials immediately curl up on themselves and get stuck in deformed shapes if they are not stretched on a frame or backed by another material.

Being able to stay in shape without additional support would allow this material, and others designed on its principles, to be used in aviation and other structural applications where low weight is at a premium.

"Materials on the nanoscale are often much stronger than you'd expect, but they can be hard to use on the macroscale" says Penn's Professor Igor Bargatin. "We've essentially created a free-standing plate that has nanoscale thickness but is big enough to be handled by hand. That hasn't been done before."

Graphene is prized for its electrical properties, but its mechanical strength is also very appealing, especially if it could stand on its own. However, graphene and other atomically thin films typically need to be stretched like a canvas in a frame, or even mounted on a backing, to prevent them from curling or clumping up on their own.

"The problem is that frames are heavy, making it impossible to use the intrinsically low weight of these ultra-thin films," Bargatin says. "Our idea was to use corrugation instead of a frame. That means the structures we make are no longer completely planar, instead, they have a three-dimensional shape that looks like a honeycomb, but they are flat and contiguous and completely free-standing.

The researchers' plates are between 25 and 100 nanometres thick and are made of aluminium oxide, which is deposited one atomic layer at a time to achieve precise control of thickness and their distinctive honeycomb shape.

"Aluminium oxide is actually a ceramic, so something that is ordinarily pretty brittle," says Bargatin. "You would expect it, from daily experience, to crack very easily. But the plates bend, twist, deform and recover their shape in such a way that you would think they are made out of plastic. The first time we saw it, I could hardly believe it."

The hexagonal corrugation of the plates is responsible for their stiffness and strength (photo: University of Pennsylvania)

Once finished, the plates' corrugation provides enhanced stiffness. When held from one end, similarly thin films would readily bend or sag, while the honeycomb plates remain rigid. This guards against the common flaw in un-patterned thin films, where they curl up on themselves.

This ease of deformation is tied to another behaviour that makes ultra-thin films hard to use outside controlled conditions: they have the tendency to conform to the shape of any surface and stick to it due to Van der Waals forces. Once stuck, they are hard to remove without damaging them.

Totally flat films are also particularly susceptible to tears or cracks, which can quickly propagate across the entire material. If a crack appears in Penn's plates, however, it doesn't go all the way through the structure; instead, it stops when it gets to one of the vertical walls of the corrugation.

The corrugated pattern of the plates is an example of a relatively new field of research: mechanical metamaterials. Like their electromagnetic counterparts, mechanical metamaterials achieve otherwise impossible properties from the careful arrangement of nanoscale features. In mechanical metamaterials' case, these properties are things like stiffness and strength, rather than their ability to manipulate electromagnetic waves.

Other existing examples of mechanical metamaterials include 'nanotrusses', which are exceptionally lightweight and robust three-dimensional scaffolds made from nanoscale tubes. The Penn researchers' plates take the concept of mechanical metamaterials a step further, using corrugation to achieve similar robustness in a plate form and without the holes found in lattice structures.

That combination of traits could be used to make wings for insect-inspired flying robots, or in other applications where the combination of ultra-low thickness and mechanical robustness is critical.

An article describing this work is published in the journal, Nature Communications.

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