Computational Modelling for the Optical Properties of Dye Molecules Adsorbed onto Metallic Nanoparticles

The study of light scattering by particles has become fundamental and applied interests in the fields of chemistry, biology, and most importantly in physics. In this context, this thesis focuses on understanding the optical properties of dye layers adsorbed onto metallic nanoparticles (NP), which is essential for interpreting the results of plasmon-dye coupling experiments. To model such a system, Mie theory is often used to solve for the exact solution to Maxwell’s equations for spherical homogeneous and isotropic coated NP. The effects of the NP’s plasmon resonances on the optical properties of the adsorbed dye layer have been predicted using an effective medium model, where the dye-layer is treated as an isotropic layer with an effective dielectric function accounting for the dye resonance. However, this isotropic shell model is inadequate as it cannot account for the dye surface concentration and the anisotropy of the optical response of the dye layer.

In this thesis, we introduce anisotropic effects within Mie theory and develop microscopic models to define effective dielectric functions which explicitly include the dye-concentration effect in the shell model. Combining anisotropic Mie theory with a concentration-dependent effective shell model allows us to form new theoretical tools to model the optical properties of adsorbed dye layers on metallic NPs of spherical shape. With this new refined effective medium model, we are then able to study shell models for elongated particles beyond the quasi-static approximation. This is implemented using the finite element method (FEM) to numerically solve Maxwell’s equations. The FEM implementation is then used to investigate how the NP’s plasmon resonance can be affected by the dye’s orientation and location on the NP’s surface. We show that the orientation and location of the dye molecules on the NP determine how strongly the plasmon resonance is shifted.

The results of this work will improve our ability to accurately model the optical properties of anisotropic molecules adsorbed on metallic NPs. This is important in a number of applications including the development of localised surface plasmon resonance (LSPR) sensing and the design of plasmonic devices.