Nanocomposite Au-ZnO thin films: Influence of gold concentration and thermal annealing on the microstructure and plasmonic response

Abstract

Nanoplasmonic thin films, composed of Au nanoparticles dispersed in a ZnO matrix were produced and characterized for Localized Surface Plasmon Resonance (LSPR) applications. The thin films were deposited by reactive magnetron sputtering, with different gold pellets area placed within the erosion zone of the target, followed by post-deposition thermal annealing to promote the nanoparticles' morphological evolution, necessary to obtain the desired LSPR bands. Four sets of thin films were prepared, containing Au atomic concentrations of 0, 9.3, 12.4 and 18.4 at.%. The Au nanoparticles were formed in a nearly stoichiometric and polycrystalline ZnO matrix, and observed in different stages of their growth, in size and shape, depending on the thermal annealing. As both annealing temperature and gold concentration were raised, large and irregular nanoparticles were formed, due to coalescence processes. For the highest concentration, there was a clear Au segregation, with the formation Au crystals (average sizes in the order of 1 μm) at the thin film's surface. Well-defined LSPR bands appeared in the films with Au concentrations of 9.3 and 12.4 at.%, but only at higher annealing temperatures (400 and 600 °C), with resonance peaks in the range from 570 nm to 615 nm. The increase of the annealing temperature also improved the LSPR properties of the Au-ZnO thin films, namely a two-fold increment of the refractive index sensitivity, showing promising responses to be tested in plasmonic applications.