Nanoparticle thin film coatings provide an opportunity for a wider range of designer-specified optical, electrical, and catalytic properties compared to conventional thin films synthesized by methods such as sputtering, evaporation, and thermal oxidation. In cluster beam deposition techniques, the physical and chemical properties of resulting thin films can be tuned by controlling the size and arrangement of nanoparticle clusters during deposition. To attain the prescribed characteristics, extensive studies of their tunable parameters are often required.
In this study, silicon (Si) nanoparticle thin films are deposited onto a glass substrate via supersonic impaction following a plasma synthesis of Si nanocrystals. The custom-designed deposition tool has evolved from previous work presented in refs. [1-2]. The film porosity is tuned by adjusting the distance between the exit nozzle and the substrate during deposition, which alters the speed (and therefore the cluster size) upon impact with the substrate. Using this technique, we have previously demonstrated the tunability of the effective refractive index (neff) for nanoparticle thin films between that of air and their constituent material . However, tuning the effective refractive index through porosity control in NP thin films introduces additional optical effects that have not yet been studied in detail.
This work explores how tuning the porosity of Si nanoparticle thin films alters their scattering behavior. The porosity of Si nanoparticle thin films depends on the size and packing density of Si nanoparticle clusters that compose it, resulting in varying pore size distributions. Through experimental and simulation methods, we study the link between porosity, pore size distribution, and scattering properties of the Si nanoparticle thin films. We start with three Si nanoparticle samples of varying porosities (70%, 80% and 90%) and similar effective thicknesses (teff). The pore size distribution of the three NP thin film samples is determined using the nitrogen adsorption technique based on the Brunauer–Emmett–Teller (BET) theory. Their scattering properties are determined by measuring the angular intensity distribution of transmitted light at normal incidence. These experimental results serve as a basis for developing a finite-difference time-domain (FDTD) model with pore size distribution as input, allowing us to expand the analysis beyond what we can measure experimentally.
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 Z. C. Holman and U. R. Kortshagen. A flexible method for depositing dense nanocrystal thin films: Impaction of germanium nanocrystals, Nanotechnology 21, 335302 (2010).
 M. Boccard et. al. Low-refractive-index nanoparticle interlayers to reduce parasitic absorption in metallic rear reflectors of solar cells, Phys. Status Solidi A, 1700179 (2017).