Electromagnetic Response of Metal-Semiconductor-Oxide Particles in the Near-Infrared Regime

Abstract

Plasmonics is a study of how electromagnetic radiation is coupled to free-carrier systems within the bulk of the material or at boundaries between different materials. Localized surface plasmon resonances (LSPRs) are caused by coherent oscillations of conduction electrons near the surface of metals which lead to an increase in the optical response. In this thesis, we study the electromagnetic response of spherical and spheroidal metal, dielectric, and semiconducting microparticles and nanoparticles. The aim is to predict the optical properties of single particles and coating containing the particles at low volume fraction in the visible and infrared regime using computational methods. The methods that we use include Lorenz-Mie theory, the quasistatic approximation, surface integral equation (SIE) technique, and a Monte Carlo method. In the first part, we study the optical properties of spherical core@shell particles consisting of metallic and semiconductor layers. The results of a particle with a metallic core and a semiconductor shell show shifting and merging of the LSPR from the metallic core and Mie resonances from the semiconductor shell for different particle sizes. We also consider the reverse configuration in which the shell is a metal, and the core is a semiconductor. The plasmon modes from the metallic shell are influenced by the particle sizes, the permittivity of the surrounding core and the medium. These core@shell nanoparticles show sufficiently high and robust efficiencies for photovoltaic applications and catalysis.In the second part, we study the optical properties of unconventional semiconductor particles. We choose copper antimony disulfide, CuSbS2, as a material that can improve the optical properties as compared to conventional semiconductors. Our results demonstrate that as the spherical CuSbS2 particle size increases, the absorption and the total scattering efficiencies broaden and shift to longer wavelengths. CuSbS2 exhibits high absorption coefficients and a band gap compatible with solar radiation, making it an excellent candidate for use in nanocrystalline solar cells and other NIR devices. The results for non-spherical particles for different shapes and orientations exhibit significant differences in their electromagnetic response. Interestingly, thin spheroids exhibit a strong plasmonic resonance in some cases. Decreasing the length of the short axis of the spheroids leads to Fano resonances in the near-infrared regimes. The presence and spectral position of plasmonic and Fano peaks are tunable as a function of the orientation and size of the spheroid. As a final step, we examine the optical properties of a layer containing CuSbS2 particles using Monte Carlo method. Monte Carlo method is in conjunction with Lorenz-Mie theory or SIE technique for modelling radiation transport in a layer. Anisotropic shapes can exhibit significant changes in optical properties depending on their particle orientation within a layer. Our study confirms that the optical properties of the layer such as absorbance, transmittance and reflectance of layer containing anisotropic particles can be tuned by the orientation of the particle.