Plasmonic Properties of Icosahedral-Seeded Gold Nanostars

Gold nanostars (AuNSs) exhibit rich plasmonic responses that are highly sensitive to the number, orientation, and relative length of their legs. Here, we use the boundary element method to investigate the optical properties of experimentally realistic AuNSs grown from icosahedral seeds. The simulated extinction spectra reveal three main plasmonic resonances: a high-energy, radial-like mode with charge oscillations from the core to the leg tips (mode #1, 700-900 nm), an intermediate mode (mode #2, ∼1000-1200 nm) that emerges when a shorter leg is present, and a low-energy dipolar mode (mode #3, ∼1300 nm) dominated by coupling between oppositely oriented legs relative to the core. Charge-density maps show that modes #1 and #2 arise from hybridization between the single-short-leg plasmon and the collective resonance of the legs originated from the same plane of the icosahedral core. Electron energy-loss spectroscopy simulations confirmed the radial-like nature of this collective mode. Near-field calculations show tip-localized SERS enhancement factors up to ∼10⁸, with modes #2 and #3 producing the strongest hot spots. These results establish structure-property relationships for applications such as ultrasensitive detection of a given analyte.