How to Fix Silver for Plasmonics

TL;DR

This paper introduces a simple alloying method with gold to improve silver thin films for plasmonics, enhancing their stability, optical quality, and durability without complex fabrication steps.

Contribution

The study demonstrates that alloying silver with 5% gold via thermal co-evaporation significantly improves film stability and optical properties, enabling direct deposition on glass for plasmonic applications.

Findings

Ag-Au alloys show reduced surface roughness and oxidation resistance.

Ag95Au5 films outperform pure Ag in optical losses and stability.

Fabricated Ag95Au5 antennas demonstrate long-term durability.

Abstract

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 co-evaporation. 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 co-evaporated thin films (down to 0.4 nm RMS), and significantly enhance the resistance to oxidation, while maintaining a defined crystallite growth. Importantly, these improvements are achieved without the need for template stripping, metallic wetting layers, or epitaxial substrates, enabling direct deposition on glass. 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 antennas that exhibit long-term durability under ambient conditions. Our approach provides a practical solution to overcome the limitations of Ag for plasmonic device applications.