'Invisible' Graphene Layer Protects Nanowires from Radiation Damage

Wrapping silver nanowires in a one-atom-thick layer of graphene protects the structures from radiation damage that has historically prevented their use on a commercial scale.

"The damage occurs in medical imaging, in space applications and just from long-term exposure to sunlight," said Suprem Das, a former Purdue University doctoral student who is now a postdoctoral researcher at Iowa State University and The Ames Laboratory, who helped lead the study.

Silver nanowires are flexible and transparent, yet electrically conductive, and could replace indium tin oxide (ITO), which is relatively expensive due to the limited availability of indium. ITO is also inflexible and degrades over time, becoming brittle and hindering performance.

After conformally wrapping single-layer graphene (SLG) on top of a silver nanowire network, the researchers irradiated the network via a UV laser beam with nanosecond pulse width and a range of intensities. Even under 2.5-MW/cm² intensity — which vaporized the unwrapped wires — the SLG layer protected the silver nanowires. Unwrapped wires were damaged with an energy intensity as low as 0.8 MW/cm².

The upper images depict how silver nanowires are damaged with UV radiation intensity as low as 0.8 MW/cm². The lower images show how graphene sheathing protects the nanowires from energy intensities up to 2.5 MW/cm². Courtesy of Purdue University.

The team found that graphene "extracts and spreads" most of the thermal energy away from the nanowires. A systematic molecular dynamics simulation confirmed the mechanism of SLG shielding.

The researchers believe SLG could enable damage- and ablation-free laser-based nanomanufacturing of hybrid nanostructures. Potential applications of the technology include use in solar cells, flexible displays for computers and consumer electronics, and optoelectronic circuits for sensors and information processing.

Study coauthor and Purdue doctoral student Qiong Nian told Photonics Media that the team did not detect any negative side effects caused by the SLG wrapping. Due to its extreme thinness — less than 1 nm — Nian described the graphene as an "invisible" protection layer.

"We found the conductivity is not decreased; [rather] the conductivity is stabilized due to graphene's excellent electrical conductivity," Nian said. "I believe our findings might inspire [commercial manufacturers] to use single-layer graphene as the protective layer."

The study, published in ACS Nano (doi: 10.1021/acsnano.5b04628), was supported by the National Science Foundation and a National Research Council.