Preventing Corrosion of Aluminum Metal with Nanometer-Thick Films of Al₂O₃Capped with TiO₂for Ultraviolet Plasmonics

Extending plasmonics into the ultraviolet range imposes the use of aluminum to achieve the best optical performance. However, water corrosion is a major limitation for UV aluminum plasmonics, as this phenomenon occurs significantly faster in the presence of UV light, even at low laser powers of a few microwatts. Here, we assess the performance of nanometer-thick layers of various metal oxides deposited by atomic layer deposition (ALD) and plasma-enhanced chemical vapor deposition (PECVD) on top of aluminum nanoapertures to protect the metal against UV photocorrosion. The combination of a 5 nm Al₂O₃layer covered by a 5 nm TiO₂capping provides the best resistance performance, while a single 10 nm layer of SiO₂or HfO₂is a good alternative. We also report the influence of the laser wavelength, the laser operation mode, and the pH of the solution. Appropriately choosing these conditions significantly extends the range of optical powers for which the aluminum nanostructures can be used. As an application, we demonstrate the label-free detection of streptavidin proteins with an improved signal-to-noise ratio. Our approach is also beneficial to promote the long-term stability of aluminum nanostructures. Finding the appropriate nanoscale protection against aluminum corrosion is the key to enabling the development of UV plasmonic applications in chemistry and biology.