Effects of anisotropy, morphology, and interparticle coupling on the far-infrared optical modes of randomly oriented ZnO nanoparticles

Abstract

Polar dielectric nanoparticles (NPs) with uniaxial anisotropy support two-surface phonon polariton modes inside a reststrahlen band. The effective permittivity of a randomly orientated ensemble of NPs is usually assumed to be isotropic due to averaging of the optical response over all orientations. In this work, we demonstrate that this assumption is not valid for ZnO nanoparticles, and the resultant absorption of a nanoparticle film can be tailored by the nanoparticle morphology. We measure distinct features in the absorption spectrum for films or ensembles of interacting polar dielectric NPs, which we attribute to the excitation of SPhP modes due to anisotropy in the dielectric permittivity of the NPs. We identify and characterize these modes in elliptical and rod-like ZnO NPs prepared by solvothermal synthesis and dispersed within an optically transparent matrix. Localized optical modes are identified using Fourier transform infrared absorption spectroscopy and confirmed by finite element simulations. The broadening and maxima of the modes are shown to be governed by the effects of anisotropy, nanoparticle morphology, and interparticle coupling within nanoparticle ensembles.