Convective assembly of nanoparticles into thin structured films.

Abstract

Convective nanoparticle film assembly is a process whereby particles from dilute liquid suspension assemble onto a substrate. Assembly occurs at the suspension-substrate-air contact line, where particles are carried toward it by convective currents set up in the meniscus region due to liquid evaporation. In the past, convectively assembled nanosphere films have been shown to be highly ordered. Convective assembly is initially explored as a potential method for the fabrication of ``tiled'' (uniformly oriented) nanocrystal films for application in zeolite membrane technology. It is found that films are generally assembled into jumbled multilayers, and that they are often nonuniform in coverage, sometimes leaving large areas of bare substrate in a banded pattern. Nevertheless, particles are shown quantitatively to be preferentially oriented, and some regions of the films do exhibit the desired ``tiled'' arrangement. A convective assembly apparatus is introduced as an experimental platform for further investigation of the method as a practicable one for large-scale production of thin particle films in general, and zeolite membrane precursor films in particular. The apparatus performance and final film characteristics are explored using a model silica nanosphere system. Monolayer film assembly turns out to be possible but difficult, with discrete banded film patterns being common in both sub-monolayer and super-monolayer films. The regularity and repeatability of these banded films are, however, very high. The wavelength of the banded film patterns (specifically, inter-band spacing) are shown to be strongly dependent on particle size in sub-monolayer films. The relationship is investigated by experiment, and modeled using a simple geometric exclusion argument based on the liquid meniscus profile described by the Young-Laplace equation. The model implies that band wavelength should also be dependent on the thickness of the films in super-monolayer films. The above model makes unrealistic assumptions about the liquid meniscus during convective assembly, namely that the system is static. Thus, the final topic is to attempt an extension of the model by including liquid flow. This quantitatively refines the meniscus geometry and allows a wider range of predictions of band spacing, although it seems to be far from the last word on modeling the banding phenomenon.