Widening the Gamut of Structural Colors of Gold via Insulator–Metal Bilayer Coatings

Tuning the color of Au has been a longstanding problem in the luxury industry. Conventional approaches, involving Au alloying, compromise purity and demand distinct alloy compositions for each hue. This study demonstrates a lithography-free method for generating structural colors on a gold surface by adjusting the thickness of titanium dioxide, a high-index dielectric. While color tuneability is limited if TiO₂ is coated directly on the Au surface, a range of vivid colors can be generated if a 50−100 nm thick AuAl₂ underlayer is used. AuAl₂, an accepted alloy for purple gold, broadens the color gamut, providing a protective coating without diminishing gold purity. The reflectance dip of the bilayer structure exhibits a significant red shift with increasing thickness of the TiO₂ layer, allowing diverse colors by TiO₂ insulator tuning. Simulation studies corroborate experimental results, affirming that coating a TiO₂ layer on the AuAl₂ underlayer yields a wide range of colors. This method, based on thin-film interference, shows promise for widespread use, offering a broad spectrum of structural colors in an industry striving for diverse Au color representation.