Interparticle Distance Controls Ultrafast Photodynamics in Crystalline Gold Superstructures

The assembly of plasmonic nanoparticles into superstructures represents a promising strategy toward the development of tunable optical materials. So far, however, the ultrafast dynamics of charge carriers in dense plasmonic superstructures is underexplored. Here, superparticles are used as a model system to study the optical behavior of gold nanoparticles under precise structural control. Gold nanoparticles are assembled into face-centered cubic crystals through slow emulsion densification, decreasing the distance between nearest neighbors via ligand exchange. Ultrafast transient absorption microscopy is used to investigate the influence of the inter-nanoparticle distance on the ultrafast photodynamics of carriers at the single superparticle level. The measurements show that reducing the distance between neighboring nanoparticles induces a blueshift of the plasmonic resonance and slows the electron-phonon scattering process. The blueshift is attributed to a long-range cross-talk between nanoparticles within the crystalline superlattice, while electromagnetic simulations suggest that the dynamics slowdown is due to enhanced near-field coupling between nearest neighbors, increasing the local population of hot carriers and delaying thermalization. This combination of experiments and numerical modeling demonstrates the sensitivity of the collective response of dense plasmonic superstructures to the internanoparticle distance, paving the way to tailorable plasmonic superstructures for applications in nanophotonics, sensing, and energy conversion.