Plasmonic Nanostructures for Enhanced Solar Cells and Colorimetric Spectroscopy Techniques
King, Nicholas Sun
Halas, Naomi J
Doctor of Philosophy
Nanostructures that support plasmon resonances, the collective oscillations of conduction electrons in metallic systems, are a driving force in nanophotonic technologies. The ability to manipulate and confine electromagnetic fields below the diffraction limit of light has resulted in an explosion of nanoscale optical devices, ranging from single molecule chemical sensors to solid state optoelectronic spectrometers. The orientation, geometry, size, dielectric materials and environment determine the wavelength dependent scattering and absorption of a plasmon resonance. Efficiently designing a nanostructure for a specific application relies entirely upon understanding the effect of each of these aspects on the overall optical response of the system. This thesis presents two different plasmonic nanostructures for enhancing two popular technologies: thin film solar cells and optical dielectric/environmental sensors. In the first half of this thesis, I use an asymmetric three-dimensional nanocup antenna to demonstrate the importance of dipole orientation for directional scattering applications. The nanocup (a gold semi-shell supported by a dielectric core particle) can redirect normal incidence near infrared light into a waveguide mode of a planar device. Tilting the orientation of a nanoantenna, and consequently the dipole moment of the plasmon mode, out of the plane of the substrate greatly increases the coupling efficacy. This process can directly improve the photon capture efficiency of planar thin film solar cells, which are weak absorbers in this wavelength regime. Both the electric and magnetic plasmon responses of the gold nanocup are shown to redirect light into the substrate and the relative contributions are quantified using a simple dipole model. The second half of this thesis focuses on aluminum nanocluster geometries for optical sensing applications. Aluminum is an increasingly popular plasmonic material that complements the properties of gold and silver, which are well established plasmonic metals. The material properties of aluminum provide a greater resistance to oxidation than silver; the tunable range of aluminum continuously spans ultraviolet and visible wavelengths whereas gold is limited by interband transitions. The sensitivity of aluminum plasmon resonances at visible wavelengths can be exploited to create highly responsive optical sensors based upon the chromaticity (hue and saturation) of the scattered light. I present Fano resonant nanoclusters capable of producing any chromaticity throughout the visible regime. Modifying the geometric scale and Dn group symmetry of the nanocluster changes the interaction of the superradiant and subradiant plasmon modes defining the scattering chromaticity. Localized surface plasmon resonance (LSPR) shifts induced by changes to the dielectric environment cause the chromaticity of the Fano lineshape to drastically change. This change can be differentiated by the naked eye or basic RGB capable cameras, providing a simple, low-cost means of monitoring the changes to the local environment (e.g., changes in solution concentration, analyte degradation, molecular binding events, etc.). These two nanostructures illustrate how flexible plasmonic geometries can be focused and refined to achieve maximum enhancements for a specific application. Together with previously defined principles of plasmonic tuning, these demonstrations will facilitate continued advancement in plasmonically enhanced technologies.
Plasmonics; Solar Cells; Chromaticity; Nanotechnology