Nanowire-filled film makes excellent electromagnetic shield

The University of California engineers developed a film which uses a quasi-one-dimensional nanomaterial filler. The result is an excellent electromagnetic shield which - among other desirable properties – is easy to manufacture.

“These novel films are promising for high-frequency communication technologies, which require electromagnetic interference shielding films that are flexible, lightweight, corrosion resistant, inexpensive and electrically insulting,” said Professor Alexandra Balandin, an expert in electrical and computer engineering. “They couple strongly to high-frequency radio-frequency radiation while remaining electrically insulating in direct current measurements.”

As electronic devices are saturating every nook and cranny of life and work, engineers are searching for the best materials to shield humans from excessive electromagnetic radiation and prevent devices from interfering with each other. Electromagnetic interference occurs when signals from different devices cross each other, affecting their performance; for instance, the signal from a phone could cause static to appear on a television screen or disrupt aircraft navigation.

Conventional shielding materials can be sufficient for situations in which many electronic, wirelessly connected devices are in close proximity, particularly when their function is critical to essential services. This calls for new shielding materials for the next generation of electronics.

The team developed the scalable synthesis of composites with bundles of quasi-one-dimensional van der Waals materials (materials with components weakly bound together). One-dimensional van der Waals materials consist of strongly bound atomic chains weakly bound by van der Waals forces; these materials “exfoliate” into needle-like structures, hence “quasi-one-dimensional”. The composites demonstrated exceptional electromagnetic shielding in the GHz and sub-THz frequency ranges: important for current and future communication technologies.

The engineers used a chemical process that can be scaled up for mass production of these materials. They synthesised the composites by treating the transition metal with a quasi-one-dimensional crystal structure with chemicals which caused it to shed its needle-like structures. These take the form of nanowires up to 106 times longer than they are thick.

The structures can be produced inexpensively and in large quantities.

“There was no standard recipe for exfoliation of these materials,” said PhD candidate Zahra Barani. “I did many trial-and-error experiments, while checking the cleavage energy and other important parameters to exfoliate them with high yield. I knew that the key is to get bundles with high aspect ratios as I can, since electromagnetic waves couple with longer and thinner strands better. That required optical microscopy and scanning electron microscopy characterisation after each exfoliation step.”

The researchers filled a polymer matrix with bundles of the exfoliated material to produce a thin black film. These demonstrated exceptional performance in blocking electromagnetic radiation. They reached over 99.99 per cent shielding for micrometre-thick films. The engineers observed the best performances in composites with low loadings of the nanowires; the higher the aspect ratio of the nanowires, the lower the concentration necessary to provide good shielding.

Barani explained that this is beneficial, as lowering the concentration allows them to take advantage of inherent properties of polymers, such as lightness and flexibility.

“I am sure we will soon see a lot of progress with quasi-one-dimensional van der Waals materials, as happened with quasi-two-dimensional materials,” Balandin said.