From superflimsy to ultrastiff: transforming graphene

Graphene is an ultrathin material characterised by its ultrasmall bending modulus, known as superflimsiness. Researchers at the University of Jyväskylä have now demonstrated how an experimental technique called optical forging can make graphene ultrastiff, increasing its stiffness by several orders of magnitude.

Graphene is an atomically thin carbon material loaded with useful properties such as large charge carrier mobility, good thermal conductivity and high optical transparency. Its impermeability and high tensile strength — 200 times that of steel — make it suitable for nanomechanical applications. Unfortunately, its exceptional flimsiness makes any three-dimensional structures unstable and difficult to fabricate.

Now researchers at the Nanoscience Center of the University of Jyväskylä have shown how to make graphene ultrastiff using a specifically developed laser treatment. This stiffening, described in the journal npj 2D Materials and Applications, opens up whole new application areas for this wonder material.

The same research group had previously prepared three-dimensional graphene structures using a pulsed femtosecond laser patterning method called optical forging. The laser irradiation causes defects in the graphene lattice, which in turn expands the lattice, causing stable three-dimensional structures.

In their latest effort, the group used optical forging to modify a monolayer graphene membrane suspended like a drum skin and measured its mechanical properties using nanoindentation. The measurements revealed that the bending stiffness of graphene increased up to five orders of magnitude compared to pristine graphene — said to be a new world record.

“At first, we did not even comprehend our results. It took time to digest what optical forging had actually done for graphene,” said Dr Andreas Johansson, who led the work on characterising the properties of the optically forged graphene. “However, gradually the full gravity of the implications started to dawn on us.”

Analysis revealed that the increase in bending stiffness was induced during optical forging by strain-engineering corrugations in the graphene layer. As part of the study, thin-sheet elasticity modelling of the corrugated graphene membranes was performed, showing that the stiffening happens on both the micro and nano scales, at the level of the induced defects in the graphene lattice.

“The overall mechanism is clear, but unravelling the full atomistic details of defect-making still needs further research,” said Professor Pekka Koskinen, who performed the modelling.

Stiffened graphene opens up avenues for novel applications, such as fabrication of microelectromechanical scaffold structures or manipulating mechanical resonance frequency of graphene membrane resonators up to the GHz regime. With graphene being light, strong and impermeable, one possibility is to use optical forging on graphene flakes to make micrometre-scale cage structures for intravenous drug transport.

“The optical forging method is particularly powerful because it allows direct writing of stiffened graphene features precisely at the locations where you want them,” said Professor Mika Pettersson, who oversaw the development of the new technique. “Our next step will be to stretch our imagination, play around with optical forging and see what graphene devices we can make.”

Image caption: Top: Atomic force microscopy images of the suspended graphene drum skin before and after optical forging. Below: Presentation of how a material can become stiffer when it is corrugated. Image credit: University of Jyväskylä/Pekka Koskinen, Vesa-Matti Hiltunen.

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