Optical Properties of Gold and Silver Nanoparticles

Abstract

Nobel metals nanoparticles particularly silver (Ag) and gold (Au) exhibit unique and tuneable optical properties owing to localized surface plasmon resonance. This phenomenon occurs when noble metals have size in nanoscale interact with electromagnetic radiation. As the result, strong electric field is created near nanoparticle surface and being maximized at resonance frequency. The resonance frequency is actually depends on the type of metal nanoparticles and their size, shape as well as the dielectric function of the environment around nanoparticles. This work is focus on the investigation of the effect of particle sizes on the dielectric function and quality factor of silver and gold. The dielectric function of bulk silver and gold was taken from experimental data reported by Johnson and Christy. The theoretical model used to calculate size dependent dielectric function was based on Drude-Summerfeld model for free electrons. In addition, the obtained results were used to calculate size dependent quality factor. The obtained results showed significant effect of particle sizes between 5-100 nm on the dielectric function of both silver and gold while for sizes larger than 100 nm, the dielectric function almost matches the bulk values in agreement with definition of nanoparticles reported in literature. Moreover, this effect is found to be more pronouncing in silver compared to the gold. The quality factor results (for silver and gold) also showed significant variation when the particle size become smaller than 100 nm. In addition, silver showed higher quality factor for all sizes compared to the gold. Moreover, the maximum quality factor is found to be in visible and infrared regions for silver and gold, respectively.