Sensitivity of metal nanoparticle plasmon resonance band position to the dielectric environment as observed in scattering

Abstract

Electrodynamic simulations of gold nanoparticle scattering spectra were used to investigate the sensitivity of localized surface plasmon band position to the refractive index, n, of the medium for moderately high aspect ratio nanoparticles of various shapes and hollow nanoshells of various sizes. For the sub-150 nm particles investigated here, the sensitivity of the peak scattering wavelength, λ*, to refractive index, n, is found to increase linearly with peak wavelength. The refractive index sensitivities of scattering peak location, thus, are found to have the same dependence upon peak position as was previously observed to characterize the sensitivities of extinction peak position. Whereas the plasmon bands of solid nanospheres display peak scattering at longer wavelengths than the wavelengths of peak extinction, the scattering and extinction peaks of high aspect ratio particles of comparable sizes have scattering and extinction spectra that are more closely overlapping. Thus, for these particles, band position sensitivities are independent both of the nature of the spectral measurement and the particle shape, except through shape control of the location of the resonance. Using an analytical expression for the sensitivity of peak wavelength to index of refraction of the medium that captures the dependence of the sensitivity upon the slope of the real part, e', of the particle dielectric function as a function of wavelength, the peak wavelength dependences of the sensitivities were characterized in terms of parameters derived from both linear and quadratic fits to e' in the visible wavelength range. Comparison of sensitivities determined by electrodynamic simulation with the predictions based upon these two dielectric function models indicates that sensitivity estimates based upon linear fit parameters provided better estimates of the sensitivities of plasmon bands in the visible than do estimates based upon quadratic fits appropriate to true Drude metals. The bulk refractive index sensitivities presented here serve as upper bounds to sensitivities of nanoparticles on dielectric substrates and sensitivities of nanoparticles to local refractive index changes, such as those associated with biomolecule sensing.