Localized Surface Plasmon Resonance Spectroscopy of Single Silver Triangular Nanoprisms

TLDR

The study investigates the plasmonic properties of single silver triangular nanoprisms using dark‑field optical microscopy and spectroscopy. The authors employ dark‑field optical microscopy and spectroscopy combined with discrete dipole approximation modeling to identify dipolar and quadrupolar plasmon modes and to measure sensing‑relevant parameters of individual nanoprisms. Two localized surface plasmon resonances—a highly intense dipolar mode and a weaker quadrupolar mode—were observed; the dipole resonance shows extreme sensitivity to particle geometry and to the surrounding refractive index (205 nm RIU⁻¹, 0.17 eV linewidth, FOM 3.3) and a linear shift of 4.4 nm per CH₂ unit, representing the highest short‑range refractive index sensitivity reported for a nanoparticle.

Abstract

The plasmonic properties of single silver triangular nanoprisms are investigated using dark-field optical microscopy and spectroscopy. Two distinct localized surface plasmon resonances (LSPR) are observed. These are assigned as in-plane dipolar and quadrupolar plasmon excitations using electrodynamic modeling based on the discrete dipole approximation (DDA). The dipole resonance is found to be very intense, and its peak wavelength is extremely sensitive to the height, edge length, and tip sharpness of the triangular nanoprism. In contrast, the intensity of the quadrupole resonance is much weaker relative to the dipole resonance in the single particle spectra than in the ensemble averaged spectrum. Several parameters relevant to the chemical sensing properties of these nanoprisms have been measured. The dependence of the dipole plasmon resonance on the refractive index of the external medium is found to be as high as 205 nm RIU-1 and the plasmon line width as narrow as ∼0.17 eV. These data lead to a sensing figure of merit (FOM), the slope of refractive index sensitivity in eV RIU-1/line width (eV), as high as 3.3. In addition, the LSPR shift response to alkanethiol chain length was found to be linear with a slope of 4.4 nm per CH2 unit. This is the highest short-range refractive index sensitivity yet measured for a nanoparticle.