Preventing the Degradation of Ag Nanoparticles Using an Ultrathin a-Al2O3 Layer as Protective Barrier

We compare the morphology and optical response of plasmonic nanostructures produced by pulsed laser deposition, consisting of a 2D distribution of Ag nanoparticles exposed to air or buried under an amorphous Al2O3 layer whose thickness is tuned in the 0.5 to 14 nm range. We observe that the covering process leads to drastic changes in Ag content, which are interpreted in terms of sputtering of Ag atoms promoted by the incoming Al ions. This Ag sputtering process is avoided as soon as the nanoparticles are embedded under a subnanometer-thick layer of amorphous Al2O3. Meanwhile, the spectral position of the nanoparticles' characteristic surface plasmon resonance, measured immediately after the film growth, is not significantly affected by the deposition of the covering layer. Nevertheless, the resonance band associated with uncovered Ag nanopartides has vanished after 12 months, as a result of their oxidation. Embedding the nanopartides under a subnanometer-thick layer of amorphous Al2O3 is enough to avoid the observed atmospheric aging processes as well as to preserve the features of their surface plasmon resonance. The results presented here are therefore promising in view of the pulsed laser deposition-based elaboration, at the wafer scale, of robust and stable tailor-made plasmonic substrates that may potentially present high electromagnetic coupling with their environment due to the very small distance to the nanostructure surface.