Rational Fabrication of Nano-to-Microsphere Polycrystalline Opals Using Slope Self-Assembly

Self-assembly of artificial opals has garnered significant interest as a facile nanofabrication technique capable of producing highly ordered structures for optical, electrochemical, biomolecular, and thermal applications. In these applications, the optimum opal particle diameter can vary by several orders of magnitude because the properties of the resultant structures depend strongly on the feature size. However, current opal fabrication techniques only produce high-quality structures over a limited range of sphere sizes or require complex processes and equipment. In this work, the rational and simple fabrication of polycrystalline opals with diameters between 500 nm and 10 μm was demonstrated using slope self-assembly of colloids suspended in ethanol-water. The role of the various process parameters was elucidated through a scaling-based model that accurately captures the variations of opal substrate coverage for spheres of size 2 μm or smaller. For spheres of 10 μm and larger, capillary forces were shown to play a key role in the process dynamics. Based on these insights, millimeter-scale monolayered opals were successfully fabricated, while centimeter-scale opals were possible with sparse sphere stacking or small uncovered areas. These insights provide a guide for the simple and fast fabrication of opals that can be used as optical coatings, templates for high power density electrodes, molecule templates, and high-performance thermo-fluidic devices.