The influence of void space on antireflection coatings of silica nanoparticle self-assembled films

This study investigates the deposition by ionic self-assembly of alternating silica nanoparticle and poly(allyamine hydrochloride) layers with the goal to create a single-material antireflection coating. The condition that the optical thickness of the film be equal to lambda/4 can be satisfied by depositing the requisite number of bilayers to obtain minimum reflectivity at the chosen wavelength. The second condition for antireflection, that the index of refraction of the film be equal to n(c)=root n(1)n(2), where n(1) and n(2) are the refractive indices of the media on each side of the film, requires that n(c)=1.22 for a film with air on one surface and glass (assuming n=1.50) on the other. Such a low index of refraction can be created in films consisting of nanoparticles if the proper volume fraction of void space exists in the film. In the wavelength range lambda=350-700 nm, minimum reflectivities of >= 2.0%, <= 0.2%, and <= 0.2% were obtained with films created on both sides of a glass slide using 15, 45, and 85 nm average diameter silica nanoparticles, respectively. The maximum transmittances for the corresponding films were 97%, >= 98%, and >= 97%. The minimum reflectance of films prepared with 15 nm average diameter silica nanoparticles was limited by insufficient void volume in the films. The maximum transmittances of films prepared with 45 and 85 nm average diameter silica nanoparticles were limited by diffuse scattering arising from the inhomogeneous morphology of the films. The extinction of normal incident light (=1-R-T, where R and T are the reflectance and transmittance, respectively) provides a measure of diffuse scattering for light with wavelength longer than the absorption edge of the film. It was found that the extinction is proportional to 1/lambda(4) for lambda > 450 nm suggesting that the mechanism for extinction at long wavelengths is Rayleigh scattering. The Rayleigh slope (diffuse scattering intensity versus 1/lambda(4)) increased with increasing diameter silica nanoparticles. For a given average diameter silica nanoparticle, the Rayleigh slope increased with increasing film thickness for films less than approximately 150 nm thick, but did not depend on film thickness, within experimental scatter, for films that were thicker than 150 nm. The results suggest that the source of Rayleigh scattering was not in the bulk of the film (such as, fluctuations in the index of refraction), but rather was primarily associated with surface roughness. (c) 2006 American Institute of Physics.