Structural Order and Plasmonic Response of Nanoparticle Monolayers

Collective plasmon resonances in superlattice assemblies of metallic nanoparticles are influenced by nanoparticle attributes and assembly structure. Although grain boundaries and other structural defects are inherent to superlattices assembled in the laboratory, their impact on near- and far-field optical properties remains poorly understood. Here, we study variations in structural and optical properties of model two-dimensional superlattices of spherical metallic nanoparticles, focusing on large-scale monolayers of approximately constant area fraction formed by different assembly rates. Our modeling addresses how disorder generates distributions of hot spot intensities and nanoparticle-level resonant frequencies. The highest intensities occur in narrow gaps between particle pairs aligned parallel to field polarization. Despite the structural diversity of the monolayers investigated, we find the highest hot-spot intensity adjacent to a nanoparticle strongly correlates with the particle's resonant frequency, though the introduction of realistic variability in nanoparticle dielectric parameters weakens the link. These trends hold for different nanoparticle compositions, and predictions are compared with experimentally prepared monolayers of tin-doped indium oxide nanocrystals. Though defectivity only modestly influences ensemble extinction peak frequency and surface-enhanced infrared absorption, increasing structural disorder broadens spectral lineshapes and boosts surface-enhanced Raman scattering enhancement factors. The magnitude of spectroscopic enhancement and the sensitivity of optical properties to local structural order depend primarily on dipole polarizability contrast. These results can help inform tolerances and trade-offs relevant to designing materials and assembly protocols to achieve desired optical properties for applications, including sensitive molecular detection.