US researchers use light to weld meshes of nanowires

A team from Stanford University relied on ‘plasmonics’ to weld electrically conductive meshes made of metal nanowires. Plasmonics is the interaction of light and metal in which the light flows across the surface of the metal in waves, like water on the beach.

The researchers say that, in addition to making it easier to produce stronger and better performing nanowire meshes, the technique could open the possibility of mesh electrodes bound to flexible or transparent plastics and polymers

‘When two nanowires lay crisscrossed, we know that light will generate plasmon waves at the place where the two nanowires meet — creating a hot spot,’ explained Mark Brongersma, an associate professor of materials science engineering at Stanford and an expert in plasmonics.

’The beauty is that the hot spots exist only when the nanowires touch, not after they have fused. The welding stops itself. It’s self-limiting.’

Electron-microscope images can be used to show that the individual nanowires are visually distinct prior to the illumination process. When illuminated, the nanowire lying on top acts like an antenna, directing the plasmon waves of light into the bottom wire and creating heat that welds the wires together. Post-illumination images show crisscrossed nanowires lying flat against the substrate with fused joints.

To demonstrate the possibilities, the researchers applied their mesh on cling film. They sprayed a solution containing silver nanowires in suspension on the plastic and dried it. After illumination, an ultrathin layer of welded nanowires was left.

‘Then we balled it up like a piece of paper. When we unfurled the wrap, it maintained its electrical properties,’ said co-author Yi Cui, an associate professor materials science and engineering. ‘And when you hold it up, it’s virtually transparent.’

This could lead to inexpensive window coatings that generate solar power while reducing glare for those inside, the researchers said.

’There are many possible applications that would not even be possible in older annealing [heat-treatment] techniques,’ said Brongersma. ’This opens some interesting, simple and large-area processing schemes for electronic devices — solar, LEDs and touch-screen displays, especially.’

Former nanowire welding techniques required high temperatures and pressures, which used to damage the meshes in the process.