Ag _{100– <i>x</i>} Au _<i>x</i> Alloy Films: An Approach to Fixing Silver for Durable, Low-Loss Plasmonics

Silver (Ag) is considered an ideal material for plasmonic applications in the visible wavelength regime due to its superior optical properties, but its use is limited by the poor chemical stability and structural quality of thermally evaporated thin films and resulting nanostructures. In this study, we present a simple approach to enhance the structural and optical quality as well as the chemical stability of Ag thin films by alloying with gold (Au) through thermal coevaporation. We investigate Ag 100– x Au x thin films with Au contents ranging from 5 to 20 at.% analyzing their surface morphology, crystallite structure, optical properties, and chemical stability. Our results show that low Au concentrations significantly reduce the roughness of coevaporated thin films (down to Rq = 0.4 nm) and significantly enhance the chemical stability, while maintaining a defined crystallite growth. Importantly, these improvements are achieved without the need for template stripping, metallic wetting layers, or epitaxial substrates. Among the compositions studied, Ag 95 Au 5 thin films exhibit the highest chemical stability, lowest optical losses in the visible spectral range, and excellent plasmonic properties even outcompeting pure Ag. As a proof of concept, we fabricate high-quality Ag 95 Au 5 optical nanoantennas that exhibit long-term durability over one month under ambient conditions. Our approach provides a practical solution to overcome the limitations of Ag for plasmonic device applications.