Ultrathin Metals on a Transparent Seed and Application to Infrared Reflectors

Ultrathin metal films (UTMFs) are widely used in optoelectronic applications, from transparent conductors to photovoltaic cells, low emissivity windows, and plasmonic metasurfaces. During the initial deposition phase, many metals tend to form islands on the receiving substrate rather than a physically connected (percolated) network, which eventually evolves into continuous films as the thickness increases. For example, in the case of Ag and Au on dielectric surfaces, percolation begins when the thickness of the metal film is at least about 5 nm. It is known that the type of growth can be changed when a proper seed layer is used. Here, we show that a CuO layer directly deposited on a substrate can dramatically influence surface wetting and promote early percolation of polycrystalline Ag and Au UTMFs. We demonstrate that the proposed seed is effective even when its thickness is sub-nanometric, in this way maintaining the full transparency of the receiving substrate. The Ag and Au films seeded with CuO showed a percolation thickness close to 1 nm and were morphologically and optically characterized from an ultraviolet (λ = 300 nm) to a midinfrared (λ = 15 μm) wavelength. Infrared reflectors, a mirror and a resonant plasmonic structure, were also demonstrated and uniquely tuned by electrical gating, this being possible owing to the small thickness of the constituting Au UTMF.