Improving the Plasmonic Response of Silver Nanoparticle Arrays via Atomic Layer Deposition Coating and Annealing above the Melting Point

Optical performance of surface plasmon resonance critically depends on the material loss of the metals, which deteriorates the quality factor of the resonance. In addition to intrinsic loss that comes from interband electric transitions, the metal possesses extrinsic loss due to crystal imperfections, such as grain boundaries and point and line defects. Thermal annealing is a useful method to reduce these imperfections via recrystallization. However, the annealing treatment has to reconcile with the deformation of the shape of nanostructures at high temperatures, which limits recrystallization. Here, we boosted the annealing effect notably for silver (Ag) nanoparticle arrays by increasing the temperature higher than the melting point of Ag (=962 degrees C). To prevent the change of the shape and damage to the nanoparticles during the annealing, we coated the nanoparticles with a conformal layer of aluminum oxide (10 nm) by atomic layer deposition. The heat treatment at 1000 degrees C for 30 s in a lamp furnace successfully reduces the material loss while retaining the shape. Consequently, the quality factor of the optical resonance of the sample increases. This work opens the way to increase the performance of the plasmonic structures up to the material limit by reducing the extrinsic loss sources of crystalline imperfections.