Thin films of Au-Al2O3 for plasmonic sensing

This works reports on the development of nanoplasmonic thin films, composed of Au nanoparticles embedded in an Al2O3 matrix. The Au-Al2O3 thin films were deposited by magnetron sputtering, and then subjected to thermal treatments (in-air) to promote the growth of the Au nanoparticles. The change of the number of gold pellets placed in the erosion zone of the aluminium target, originated Au concentrations in the films from 8.9 at.% to 20.7 at.%. While the Al2O3 matrix remained roughly stoichiometric and amorphous after the thermal treatment, a progressive crystallization of the Au nanoparticles was observed when the annealing temperature increased from 400 degrees C to 700 degrees C. Nonetheless, the amorphous matrix limited the growth of Au nanoparticles up to 20 nm. Moreover, the application of an argon plasma treatment enabled the removal of superficial layers, increasing the density of Au nanoparticles partially exposed at the films' surface. Envisaging the application in localized surface plasmon resonance sensors, the thin films were tested using two dielectric (liquid) environments, showing a consistent response under different H2O/DMSO cycles, yet with low sensitivities (few nm/RIU). To enhance the sensitivity of these thin film system other strategies are discussed.