Microstructure of Silica Particle Monolayer Films Formed by Capillary immersion Force

We have investigated the effect of inter-particle repulsion and particle–substrate interaction on the microstructure of silica particle monolayer films fabricated by an evaporation-induced self-assembly method as a function of surface coverage. Suspensions of mono-dispersed colloidal silica particles (106 nm diameter) without binders cast on PET substrates by bar coating were evaporated under a fixed condition to give two-dimensional particle films. The inter-particle repulsion was controlled by zeta potential of the particle suspensions. The particle–substrate interaction was varied by the surface treatment of the substrates by argon radio frequency plasma. The self-assembled microstructure was observed by atomic force microscopy (AFM) and quantitatively evaluated by Voronoi analysis in terms of particle order and domain quality. In all the conditions attempted, the microstructure improved with the increase of surface coverage (φC). However, little difference in the correlation between φC and the microstructure due to the experimental conditions was observed until φC reached about 0.6, where both particle order and domain quality of more inter-particle repulsion (higher absolute value of zeta potential) began to exceed those of less inter-particle repulsion only when the substrates with less particle–substrate attraction (substrates with no surface treatment) were used. These results were consistent with the AFM observation. By considering a feasible model for the process, it could be inferred that the microstructure dependence on the inter-particle repulsion originated from a leap in the frequency of inter-particle collision at a critical value of φC (=49/75≈0.65 in the model). We found that stronger inter-particle repulsion and weaker particle–substrate attraction were preferable for the better microstructure above the critical φC of about 0.6.