Evaporation-driven Assembly as a Route to Photonic Materials by Robert Gene Shimmin

Evaporation-driven colloidal self-assembly is a promising family of routes to large-area, low-cost, if imperfect, photonic crystals. The simplest example occurs at the surface of a stagnant, evaporating colloidal suspension. For an evaporating colloidal suspension in which the interface falls faster than the particles sediment, particles will accumulate at the solvent-air interface. If neither diffusion nor convection can disperse this accumulation, it can grow into a colloidal multilayer many microns thick. Experimental evidence that colloidal crystallization can and does occur beneath an evaporating solvent-air interface is provided for the case of 1.0 µm diameter sulfate polystyrene microspheres in water at 45 °C. Peaks in the infrared reflectance spectrum from this surface are consistent with the formation of a colloidal crystal beneath the interface, with planes of hexagonally-packed spheres oriented parallel to the interface. This phenomenon is driven by a sufficient evaporation rate: when the same particles are placed in D 2 O at room temperature, they concentrate beneath the interface due to their buoyancy, but they do not crystallize at the reduced evaporation rate. Although readily prepared, such interfacial colloidal films lack a solid substrate; their fragility makes them difficult to study, much less use. A means of making them mechanically robust by immobilizing them in acrylamide hydrogel is demonstrated. Vertical deposition is a widely used evaporation-driven method for depositing a thin colloidal crystal film on a substrate. Based on the Fabry-Perot fringes in the reflectance spectra of colloid crystals vertically deposited from 1 µm diameter polystyrene latex suspensions of 0.002 ≤ φ ≤ 0.008, the thickness of these colloidal crystal films increases linearly with distance in the growth direction. These thickness profiles agree iv quantitatively with the expected thickness of a horizontal colloidal crystal beneath the water-air interface, but it is demonstrated that the agreement stems from a less direct mechanism than Langmuir-Blodgett like transfer of the horizontal colloidal crystal onto the substrate. A means for performing vertical deposition at growth rates slower than the evaporation rate by adding solvent to the bottom of the colloidal suspension is introduced. Halving the growth rate of vertical deposition increases both the thickness and the reflectivity of the resulting colloidal crystals, effects indistinguishable from those of doubling the concentration of the colloidal suspension. Colloidal crystals deposited using both high concentration and slowed grow can have peak IR reflectance in excess of 80%, exceeding most previously published values for polystyrene colloidal crystals on glass. …