Silver nanowires offer transparency

Researchers from King Abdullah University of Science and Technology (KAUST) have developed a submicrometre-thin mesh of silver nanowires that are transparent to light, highly electrically conductive, flexible, stretchable and simple to make — and have the potential to be used in future applications including flexible electronic displays, sensors, solar cells and even incubators for newborn babies.

“The idea came to me when I was in the hospital with my son, who was in an incubator,” said KAUST researcher Atif Shamim. “I worried about the exposure of the babies to all the electromagnetic radiation in the room.”

While opaque metals are well known for their electrical conductivity, serving as blocks to electromagnetic radiation, they are generally not transparent or stretchable. Shamim said, “We didn’t want to protect the incubators against the radiation by using metal because then we wouldn’t be able to see the infants.”

There are printed films of metal nanowires in which the conductive metal nanowires are so thin that light can pass between them. However, it has been challenging to directly create nanowire sheets that simultaneously have high conductivity, high transparency, flexibility and stretchability. The KAUST researchers have now found a solution to this problem, with the results published in the journal npj Flexible Electronics.

First, the team refined the method for making silver nanowires, adapting the previous protocol to make larger quantities of longer nanowires. The nanowires could be used directly in a new ink formulation that resulted in a uniform layer of silver when applied by screen printing onto a flexible substrate.

Thanks to the length of the nanowires, the team could achieve high conductivity with a relatively sparse covering of nanowires that in turn improved optical transparency. The team also used laser sintering to weld together adjacent nanowires at points of contact: this improved electrical conductivity further while also reducing the thickness of the silver layer and letting even more light pass through.

This scanning electron microscope (SEM) image shows that the Ag nanowires are completely connected after laser treatment. Image ©2019 Li et al.

The material maintained its electrical performance even after 1000 stretch-release cycles and 1000 bending cycles. The team tested their material’s performance by printing the conductive ink in patterns that enabled it to absorb predetermined wavelengths of electromagnetic radiation.

“With the huge number of wireless devices, we are all exposed to electromagnetic radiation all the time,” said postdoctoral researcher Weiwei Li. “We designed special electromagnetic absorbers using our transparent ink which can absorb more than 90% of the electromagnetic signals in a specific frequency band.”

Shamim concluded, “We believe that this work will benefit the development of future flexible, transparent and stretchable conductive electronics at low cost and at large scale.” Other potential applications include radar-absorbing coatings to cover the curved surface of fighter planes, rendering them invisible to enemy radar systems.

Top image: Weiwei Li and Atif Shamim inspect their transparent electronic devices. Image ©2019 KAUST.

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