Gold, silver, copper, and aluminum have reflective properties and are very useful in the manufacture of optical components. These metals are also being studied at the nanoscale because nanostructures have a completely different optical response than bulk materials.
Engineers at the Nanophotonics and Metrology Laboratory (NAM), part of EPFL‘s School of Engineering (STI), challenged themselves: To develop a material that reflects every color in the spectrum.
Creating an alloy of gold and silver allowed scientists to combine the optical effects of both metals in a single material. However, this process alters the form of the nanostructures. To address this problem, engineers developed a low-temperature annealing method that would work for any alloy mixture.
Engineers tested the feasibility of their method by fabricating a gold and silver alloy. They healted 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.
Jeonghyeon Kim, a Ph.D. student and member of the team, said, “We use nanoscale layers in our process. They’re incredibly thin. The engineers discovered that their method maintains the structures of the two metals – and that the new material reflects the entire spectral range, depending on its composition. Low-temperature annealing produces well-alloyed materials but doesn’t alter the form of the particles. It’s as if we’ve combined the optical properties of gold and silver. Our alloy reflects new colors.”
Professor Olivier Martin, who heads the laboratory, said, “The research team experimented with different alloy ratios. The optical effects change as we add more gold or silver to the mixture. Their method, which could manufacture new optical instruments, also has more everyday applications. For instance, our material could find its way onto watch and clock dials.”
- Debdatta Ray et al. A Low-Temperature Annealing Method for Alloy Nanostructures and Metasurfaces: Unlocking a Novel Degree of Freedom. DOI: 10.1002/adma.202108225