Selective absorption of solar energy in ultrafine metal particles: Model calculations

We compute spectral reflectance from metal surfaces with coatings of ultrafine metal particles dispersed in an insulating medium. This model is appropriate to several kinds of selective absorbers for efficient photothermal conversion of solar energy. Our main interest lies in the parameters which govern λc—the wavelength below which the surfaces are good absorbers and above which they are good reflectors. We analyze the roles of coating thickness, substrate metal, particle shape and orientation, possible dielectric cores in the particles, and dielectric permeability (real and imaginary parts) of the embedding medium and of graded volume fractions of metal. A particularly interesting result is that an enhanced eccentricity of the particles is, under most conditions, highly effective for shifting λc towards longer wavelengths. A similar shift is also found for spherical metallic shells surrounding dielectric cores of increasing size.