Optical properties of Ag nanoparticle layers deposited on silicon substrates

Silver nanoparticles of various sizes, shapes and modified distances between them were prepared on silicon substrates using thermally evaporated metal thin films of varying thicknesses followed by annealing. The ∼4 nm silver thin film annealed around ∼300 °C showed considerable reflectance reduction from the silicon substrate in the entire polychromatic spectrum. The effects of dipolar and quadrupolar resonances of silver nanoparticles on the reflectance reduction from the silicon substrate are discussed. The quadrupolar resonances of silver nanoparticles lead to reduced reflectance from the silicon substrate in the near UV–visible region (∼350–600 nm) due to the enhanced forward scattering. The reflectance reduction in the Vis and NIR regions (∼600–1300 nm range) is explained by the interaction of the surface plasmons of the metal nanoparticles, which is very sensitive to the size and shape of the particles, and the distances between them. Some of the waveguide modes existing at the interface between the silicon and the metal nanoparticles also couple the excited surface plasmons, which helps in trapping the light near the NIR region. With proper tuning of the metal particle sizes, shapes and distances between the particles in the layers, one can reduce the total reflectance from the silicon substrate in the entire polychromatic solar spectrum.