(20c) Nanoscale Structural Engineering of Metal Nanoparticle Lattices for Ultra-Narrow Plasmon Resonances

Plasmonic nanostructures are emerging as platforms for various studies ranging from near-field spectroscopy to biological sensing because of their ability to confine light into sub-wavelength volumes with enhanced local electromagnetic fields. A major hindrance to realizing practical applications of plasmonic nanostructures is large losses that reduce plasmon lifetime and local field strength. These losses are correlated with the spectral linewidth of the plasmon resonance; narrow resonances are preferable but challenging to realize. This presentation describes a thermal annealing method that can narrow the linewidths to the theoretical limits of plasmon resonances from metal nanoparticle (NP) lattices. The heating process improved the nanoscale morphology, surface roughness, and crystallinity of Au, Ag, and Al NPs in lattices and achieved surface lattice resonances (SLRs) with linewidths down to 4 nm. By taking advantage of the catalytic properties of Cu, we also demonstrated a chemical vapor deposition process to grow graphene on Cu NPs surfaces to prevent oxidation. The graphene-encapsulated Cu NP lattices showed the narrowest SLR linewidths (2 nm) and remained stable for months. Our results demonstrate that nanoscale structural engineering can significantly improve the plasmonic properties of metal NP lattices and provide design guidelines for high-quality plasmonic nanostructures.