Refined effective-medium model for the optical properties of nanoparticles coated with anisotropic molecules

This work aims to provide a simple yet complete effective dielectric function for an anisotropic layer of polarizable molecules adsorbed on a metallic surface. This effective medium model considers the important and nontrivial case of nonvacuum embedding media and accounts for orientation effects, coverage dependence through dipole-dipole interactions, and image-dipole effects. To check the model's validity, we focus in particular on the experimentally relevant case of dyes adsorbed on metallic nanospheres. We can then use anisotropic Mie theory, together with the effective dielectric function describing the molecular coating, to calculate their optical properties. We show that this effective medium description is in very good agreement with more elaborate and computationally intensive microscopic calculations based on coupled-dipole models. The effective medium model therefore provides a simple means to investigate orientation effects and coverage dependence, including in more complex systems such as dyes adsorbed on nonspherical or ensembles of nanoparticles. This model can readily be used to further our theoretical understanding of dye-nanoparticle systems, for example in the context of dye-plasmon resonance coupling or surface-enhanced Raman and fluorescence spectroscopy.

Automated summary

This work introduces an effective dielectric function model for anisotropic layers of polarizable molecules adsorbed on metallic surfaces, taking into account orientation effects and coverage dependence. The model is validated by focusing on dyes adsorbed on metallic nanospheres and shows good agreement with more complex microscopic calculations. The effective medium model provides a simple means to study orientation effects and coverage dependence in systems such as dyes adsorbed on nonspherical nanoparticles.