Influence of Spike Geometry on Electromagnetic Field Enhancement and the Linear and Nonlinear Plasmonic Properties of Gold Nanourchins

Wet-chemistry-synthesized gold nanourchins (Au NUs), characterized by spiky morphologies with spherical cores, exhibit complex and geometry-dependent plasmonic field enhancement properties distinct from those of symmetrical nanostructures. While plasmon hybridization and mode coupling in branched nanostructures have been broadly studied, the specific optical behavior of Au NUs─particularly regarding spike length distribution and ultrafast dynamics─remains underexplored. This study investigates the steady-state and transient absorption spectra of Au NUs with 50–80 nm cores and 5–20 nm spikes, revealing multiple resonance bands. Transient absorption spectroscopy at various excitation wavelengths confirmed the presence of distinct resonances. Electromagnetic simulations based on TEM tomography-inspired models identified two key extinction bands: a green-wavelength dark mode resonance and a red/nIR spike-induced resonance attributed to the lightning rod effect. Simulations further showed that short, uniformly distributed spikes (aspect ratio ≤1) weakly excite dark resonances, while longer spikes (aspect ratio >1) induce hybridized longitudinal resonances and significant redshifts. Broad spike length distributions result in multiple coexisting resonances, aligning with experimental extinction spectra. A preliminary surface-enhanced Raman scattering study using 2-naphthalene thiol confirmed stronger enhancement for long-spiked Au NUs under 785 nm excitation, validating the field enhancement potential of the identified resonances.