Graphene stiffened using ultrafast lasers

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Top: Atomic force microscopy images of a suspended graphene drum skin before and after optical forging. Bottom: How a material can become stiffer when corrugated. (Image: University of Jyväskylä)

Scientists have developed a technique using ultrafast lasers that can dramatically increase the stiffness of graphene, which could open up a range of novel applications for the material.

Graphene is an atomically thin carbon material known for its range of excellent properties. It offers large charge carrier mobility, superb thermal conductivity, high optical transparency, impermeability and a tensile strength 200 times that of steel.

However, the exceptional flimsiness of graphene makes any three-dimensional structures notoriously unstable and difficult to fabricate. 

Researchers at the University of Jyväskylä's Nanoscience Center are addressing this issue by using a specifically developed ultrafast laser treatment to make graphene ultrastiff. According to the researchers, this stiffening will open up whole new application areas for the material.

The technique, known as optical forging, uses a pulsed femtosecond laser to induce defects in the graphene lattice, causing it to expand and produce stable three-dimensional structures. 

The group used optical forging to modify a monolayer graphene membrane suspended like a drum skin and then measured its mechanical properties using nanoindentation.

The measurements revealed that the bending stiffness of graphene increased by up to five orders of magnitude compared to pristine graphene, which according to the researchers is a new world record.

'At first, we did not even comprehend our results,’ said Dr Andreas Johansson, who led the work now published in Nature Partner Journals 2D Materials and Applications. ‘It took time to digest what optical forging had actually done for 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 modeling of the corrugated graphene membranes was performed, showing that the stiffening happens on both the micro- and nanoscales, at the level of the induced defects in the graphene lattice.

'The overall mechanism is clear but unraveling the full atomistic details of defect-making still needs further research,' said Professor Pekka Koskinen, who performed the modeling.

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 gigahertz regime. With graphene being light, strong and impermeable, one potential 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,' added 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.'

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