Direct deposition of complex polycrystalline colloidal monolayers on solid substrates from the evaporation of hanging drops

Microfabrication strategies designed to produce ordered arrangements of colloidal particles on solid supports are of great interest in materials science and technology due to their wide range of applications, from plasmonic and biomimetic surfaces to tags for anti-counterfeiting and goods identification. Existing methods allow excellent control over particle organization yet share a degree of complexity hampering their facile implementation. Here we present a simple methodology for creating ordered particle arrays on solid substrates only requiring the deposition on the target substrate of a colloidal suspension drop and flipping it upside-down for particle sedimentation, self-organization at air/water interface and evaporation-driven deposition. Particle adsorption at the air/water interface, here enabled by the addition of small amount of cationic surfactants, is critical to ensure two-dimensional confinement of repulsive particles and their self-assembly into extended ordered arrays prior to complete solvent evaporation and particle transfer to the substrate. We demonstrate the versatility of the method by the creation of arrays of various kinds of particles (silica, polystyrene) with diameters in the range 300 nm – 5 µm on different target substrates (polystyrene, polydimethylsiloxane, glass) as well as the generation of a variety of patterns. The size and morphology of the deposit, from single-crystals to multi-crystalline patches, can be controlled by the curvature of the drop and the total number of particles, while more complex colloidal structures, like colloidal crystal “alloys” and “irises” can be achieved by successive depositions, co-crystallization or double flipping.