Self-alignment of SiO₂ colloidal particles on physically and/or chemically patterned surfaces

We report effects of patterned surface structures with the modulation of hydrophobicity in the fabrication of two-dimensional (2D) self-aligned colloidal crystals. Colloidal silica particles have been synthesized by the Stöber process (near monodispersion of 280 nm and 350 nm in diameter). When a drop of the silica suspension with a given volume and concentration (typically 5 μL and 0.5 wt % of the silica particles) is placed upon the substrate, it spreads and forms a concave shape in geometric confinement. In the drop of suspension, horizontal convective flow occurs due to the concave shape and capillary forces. Silica particles are packed together hexagonally resulting in a self-aligned silica monolayer inside the patterned lines. The surfaces with chemical and/or physical patterned lines (∼3 and 5 μm in width and spacing) were fabricated on silicon substrate with silicon dioxide by photolithography combined with the modified chemically through SAMs (self-assembled monolayers) of OTS (octadecyltrichlorosilane) using a microcontact printing technique to create alternating hydrophilic/hydrophobic line patterns. Highly ordered silica 2D patterns were observed in the hydrophilic regions, while no silica particles were found in OTS-covered line patterns. Self-aligned silica particle patterns were also observed in a quite large area (at least several hundreds of micrometers) through the line patterns of the surfaces with superhydrophilic modification. This method can serve as a position selective self-alignment aggregation and can be applicable to photonics and micro/nanofabrication.