Refractive Index Sensitivities of Noble Metal Nanocrystals: The Effects of Multipolar Plasmon Resonances and the Metal Type

Abstract

The refractive index change-based plasmonic sensing with noble metal nanocrystals has been demonstrated unambiguously in a number of studies. The refractive index sensitivity and figure of merit of noble metal nanocrystals are two key parameters for this type of plasmonic sensing. Here, we report our studies of the effects of multipolar plasmon resonances and the metal type on the refractive index sensitivity and figure of merit of noble metal nanocrystals. Ag nanocubes of 84 nm in edge length are found to exhibit well-defined dipolar, quadrupolar, and octupolar resonances. The dipolar resonance has the highest index sensitivity of 336 nm/RIU (refractive index unit), while the quadrupolar resonance possesses the largest figure of merit of 4.55. A comparison between the Ag and Au nanocubes that have comparable particle sizes and dipolar plasmon resonances in a similar spectral region shows that the index sensitivity of the Ag nanocubes is about twice that of the Au nanocubes. The effect of the metal type on the index sensitivity is further confirmed by coating a Ag shell around Au nanorods, where the index sensitivity of the Au−Ag core−shell nanobars is determined to be larger than that of the uncoated Au nanorods that have a longitudinal dipolar plasmon wavelength close to that of the core−shell nanobars. The experimentally observed overall trend in the index sensitivity is also in agreement with that revealed by electrodynamic calculations.