Tunable optical and plasmonic response of Au nanoparticles embedded in Ta-doped TiO2 transparent conducting films

Localized surface plasmon resonances (LSPRs) are fascinating optical phenomena occurring in metal nanostructures, like gold nanoparticles (NPs). Plasmonic excitation can be tailored efficiently in the visible range by acting on size, shape, and NP surroundings, whereas carrier density is fixed, thus restricting the LSPR modulation. Transparent conductive oxides (TCOs), on the other hand, are gaining increasing interest for their transparency, charge carrier tunability, and plasmonic features in the infrared. The combination of these two materials into a metal-TCO nanocomposite can give access to unique electrical and optical characteristics, to be tailored in view of the desired optoelectronic application. In this paper, Au NPs and Ta-doped TiO2 TCO films have been merged with the aim to master the Au plasmon resonance by acting on the dielectric properties of the surrounding TCO. Morphology, structure, and electrical properties have been investigated as well to understand the optical response of the nanosystems. The role of the embedding geometry has been explored, revealing that the largest LSPR shift (550-760 nm) occurs when the NPs are sandwiched in the middle of the film and not at the bottom of the film (substrate-film interface). Ta doping in the TCO has been varied (5-10% at. and bare TiO2) to induce a permittivity change of the matrix. As a result, Au LSPR is clearly blueshifted when decreasing the dielectric permittivity at higher Ta content in the sandwich configuration. Despite the nonoptimal electrical performance caused by defectivity of the films, Au-Ta:TiO2 multifunctional nanocomposites are promising candidates for their optical behavior as highly tunable plasmonic conductive metamaterials for advanced light management.