Single-particle spectroscopy of gold nanorods beyond the quasi-static limit: Varying the width at constant aspect ratio

We have examined how the surface plasmon resonances (SPRs) of chemically grown gold nanorods with tunable widths and lengths evolve due to phase retardation. For nanorods with diameters d > 30 nm, the aspect ratio is not a sufficient parameter for determining the energy of the longitudinal SPR. To rigorously study the effects of the size, we performed correlated scanning electron microscopy and single-particle spectroscopy on broad gold nanorods that were chemically grown wider (d > 100 nm) and longer while maintaining the surface chemistry and hemispherical end-cap geometry as the slim rods we compared them to (d < 30 nm). At a low aspect ratio of 2.2, the longitudinal SPR of the broad nanorods significantly red-shifted and broadened as the width increased. In addition, broad gold nanorods with d >100 nm displayed higher order plasmon modes that were not observed for slim nanorods of similar aspect ratio. To measure the full spectrum of the largest nanorods, we implemented a new strategy for acquiring single-particle extinction spectra with an extended window of 500-1700 nm by combining a Si CCD camera and an InGaAs array detector. This experiment revealed that changing the width from 25 to 120 nm while maintaining an aspect ratio of only 3.1 caused the longitudinal dipole SPR to red shift 560 nm to 1300 nm. The spectroscopic studies were complemented by theoretical modeling using the discrete dipole approximation. While we found excellent agreement between the measured and predicted maxima of the longitudinal dipole SPR, the intensities of the multipolar plasmon modes were significantly enhanced in the single-particle spectra compared to calculations.