Voltage Controlled Nanoparticle Plasmon Resonance Tuning through Anodization

Frequency control of plasmon resonances is important for optical sensing applications such as Surface Enhanced Raman Spectroscopy. Prior studies that investigated substrate-based control of noble metal nanoparticle plasmon resonances mostly relied on metal substrates with organic or oxide spacer layers that provided a fixed resonance frequency after particle deposition. Here we present a new approach enabling continuous resonance tuning through controlled substrate anodization. Localized Surface Plasmon tuning of single gold nanoparticles on an Al film is observed in single-particle microscopy and spectroscopy experiments. Au nanoparticles (diameter 60 nm) are deposited on 100 nm thick Al films on silicon. Dark field microscopy reveals Au nanoparticles with a dipole moment perpendicular to the aluminum surface. Subsequently an Al2O3 film is formed with voltage controlled thickness through anodization of the particle coated sample. Spectroscopy on the same particles before and after various anodization steps reveal a consistent blue shift as the oxide thickness is increased. The observed trends in the scattering peak position are explained as a voltage controlled interaction between the nanoparticles and the substrate. The experimental findings are found to closely match numerical simulations. The effects of particle size variation and spacer layer dielectric functions are investigated numerically. The presented approach could provide a post-fabrication frequency tuning step in a wide range of plasmonic devices, could enable the investigation of the optical response of metal nanostructures in a precisely controlled local environment, and could form the basis of chemically stable frequency optimized sensors.