Pore Formation in Silicon Nanoparticle Thin Films and Its Impact on Optical Properties

Unique properties can be achieved in engineered nanoparticle films, including those related to quantum confinement, surface chemistry, and light propagation, which can be exploited for energy applications. Using cluster beam deposition techniques, we can tune these properties by controlling the size and arrangement of nanoparticle clusters during deposition. We investigate the correlation between the optical properties (effective refractive index and scattering) of silicon nanoparticle thin films and their respective porosities and pore size distributions. We vary the porosity by adjusting speed of cluster impaction with the substrate and investigate the resulting pore formation through nitrogen adsorption measurements. Nanoparticle thin films with the highest refractive index (and the smallest average pore sizes) have normalized haze values of <2%; the lower refractive indices have significantly higher haze values (similar to 20%) for approximately the same quantity of the material. By use of full-wave optical simulation, together with the synthesis of a model with random networks of NPs with different statistics, the scattering effects are associated with differences in pore size distributions related to different porosities.