2, University of Minnesota, Minneapolis, Minnesota, United States
Nanoparticle films can have a range of applications due to their tunable properties by changing material, structure, or functionalization. Nanoparticle films are especially interesting optically when their refractive index approaches 1 from the high porosity of the film. A major optical challenge is keeping the haze low when nanoparticles aggregate into films upwards of 100 µm. To inform the synthesis and deposition of nanoparticles, electron microscopy can be used not only to determine the size of nanoparticles and the thickness of thin films, but also to analyze the shape of nanoparticles and necking between particles with transmission electron microscopy. Gas phase synthesis was performed with both PECVD and gas pyrolysis to produce 5 nm diameter, spherical, silicon nanoclusters, 10-15 nm silica nanoparticles, or nanoclusters and larger fluorine-doped tin oxide nanoparticles. The freshly synthesized nanoparticles in the gas stream were then fed into a custom spray coating tool for high throughput up to 5 µm/min of 10-15 nm nanoparticles on up to 5 in2 surfaces. The shapes and necking of nanoparticles before and after deposition were obtained and compared to determine the relationship with haze as determined by UV-Vis-NIR spectroscopy. We found that the shape and necking were also related to porosity of the film as determined by ellipsometry, affecting the refractive index. We outline ways to vary the synthesis and deposition parameters to vary porosity and minimize haze to 5% at 550 nm for a 100 µm film.