This electron microscopy image shows nanostructures with different proportions of gold (Au) and silver (Ag) as different colors. Image: 2022 EPFL.
This electron microscopy image shows nanostructures with different proportions of gold (Au) and silver (Ag) as different colors. Image: 2022 EPFL.

Engineers at the Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland have developed a low-temperature annealing method that maintains the structure of gold and silver when the two metals are combined in an alloy. These new alloys, reported in a paper in Advanced Materials, can reflect the full spectral range and so will prove useful for the manufacture of contact lenses, holographic optical elements and other optical components.

Gold, silver, copper and aluminum are widely used in the manufacture of optical components because of their reflective properties. Gold, for instance, reflects red light, while silver reflects blue light. Furthermore, nanomaterials made from these metals have completely different optical properties to the bulk metals. At the nanoscale, light interacts differently than it would with the same metal in a larger quantity, such as in a gold bar. With this in mind, engineers at the Nanophotonics and Metrology Laboratory (NAM), part of EPFL’s School of Engineering (STI), set themselves the challenge of developing a material that reflects every color in the spectrum.

“We realized that, by creating an alloy of gold and silver, we could combine the optical effects of both metals in a single material,” says Olivier Martin, who heads NAM.

Conventional gold and silver alloys are fabricated at high temperatures of 800–1,000°C, but this process alters the form of the nanostructures. “Current annealing methods don’t maintain the structure of the two metals,” explains Martin. To get around this problem, the engineers set about developing a low-temperature annealing method that would work for any alloy mixture.

In the lab, Martin’s team first demonstrated the feasibility of using a low-temperature annealing method to fabricate a gold and silver alloy. The engineers heated both metals to 300°C for eight hours, and then to 450° for a further 30 minutes, successfully producing an alloyed gold-silver thin film.

“We use nanoscale layers in our process,” says Jeonghyeon Kim, a PhD student and member of the team. “They’re incredibly thin.”

The engineers discovered that their method maintains the structures of the two metals – and that the new material reflects the full spectral range, depending on its composition. “Low-temperature annealing produces well-alloyed materials but doesn’t alter the form of the particles,” explains Kim. “It’s as if we’ve combined the optical properties of gold and silver. Our alloy reflects new colors.”

The research team also experimented with different alloy ratios. “The optical effects change as we add more gold or silver to the mixture,” says Martin. He adds that their method, which could be used to manufacture new optical instruments, also has more everyday applications: “Our material could find its way onto watch and clock dials, for instance.”

This story is adapted from material from EPFL, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.