Long-term stability of plasmonic resonances sustained by evaporated aluminum nanostructures
Résumé
Aluminum is now regarded as one of the best metals for pushing plasmonics towards ultraviolet. When exposed to air, a 3-5 nm alumina shell is formed rapidly around aluminum, preventing further oxygen penetration. This natural oxidation layer is known to chemically stabilize Al. Nevertheless, due to the large surface to volume ratio of Al nanoparticles, their long-term stability is an issue, especially when they are polycrystalline. This critical point has to be developed as the optical properties of conventionally evaporated Al nanostructures may evolve over time. In this article, the evolution of the plasmonic properties sustained by Al nanodisks with a varying oxidation layer is studied by numerical calculations. Their stability over time is also experimentally monitored over 250 days. When exposed to ambient air, their optical properties are preserved for 90 days whatever their diameter, due to a very slight oxidation. Beyond this period, the nanodisks lose their optical properties more or less rapidly depending, this time, on their diameter. A competition between oxidation and self-annealing is proposed in order to explain these results. Nanodisks with a particular diameter of 100 nm are surprisingly stable, exhibiting plasmonic resonances lasting over 250 days. Additionally, when Al nanodisks are exposed to a water environment, a strong corrosion effect shortens their lifetime to 5 days. The obtained results are of importance for further use of conventionally evaporated Al nanostructures for optical applications, as they should remain stable over a long period of time.