Oriented Colloidal‐Crystal Thin Films by Spin‐Coating Microspheres Dispersed in Volatile Media

Abstract

One of the main obstacles for putting into practice the interesting optical and structural properties of colloidal crystals in actual devices is the incompatibility of the time-consuming and unclean self-assembly crystallization techniques commonly used to make colloidal crystals with the fast and dirtfree technology required to fabricate devices. With this in mind, different approaches have been taken and outstanding improvements affecting both the crystalline quality and the ease of fabrication have been achieved. However, most techniques developed to date are very sensitive to small variations of ambient humidity or temperature, which affect the lattice thickness and degree of crystallinity, and make it difficult to tailor the properties of the material. Also, it takes a few days for crystals to deposit on the substrate, or at least a few hours if precise temperature control is achieved. Finally, increasing the area on which the crystal is deposited up to at least the size of a wafer would require the use of a large amount of spheres and large baths, which, in turn, would increase the length of the process and complicate the fine control of the different parameters involved. Recently, a new approach to colloidal crystallization of submicrometer diameter spheres that overcomes some of the obstacles mentioned above has been proposed: Jiang et al. developed a procedure to prepare thin colloidal silica–polymer composite films based on spin-coating. In brief, silica colloidal spheres are first dispersed in a mixture of a viscous triacrylate monomer and a photoinitiator. The non-volatile dispersion is then spin-coated on a silicon wafer to obtain a thin layer. As the dispersion smears on the substrate, shearing induces 3D ordering of the particles in the monomer matrix, which is photopolymerized later on, providing mechanical stability for the structure. Selective removal of either the polymer matrix or the particles results in the formation of a direct silica or inverse polymer colloidal-crystal structure, respectively. Although it constitutes a formidable step forward in terms of colloidal-crystal processing, this technique still presents a series of drawbacks that may limit its application. First, it employs a viscous, dense monomer solution as the dispersion medium, which implies that lengthy and thorough stirring is needed to ensure the suspension is aggregate-free before spin-coating. Second, this monomer solution must be photopolymerized to be stabilized; thus, the resulting composite presents no porosity and a very low refractive-index contrast, which makes it useless for most foreseen applications. Only after further selective etching is porosity recovered, and the dielectric contrast is high enough to present an intense diffraction peak. Finally, this method cannot be easily extended to the crystallization of the different types of monodisperse latex particles usually employed in the field, since the common organic nature of both particles and matrix would make almost impossible the final selective elimination of one of them by plasma, thermal, or most organic liquid etchings. In this communication, we present a simple and reliable method to crystallize submicrometer monodisperse silica and latex colloids using a mixture of volatile solvents as dispersion media, allowing one to attain a strongly diffracting opal-like structure within minutes without further processing. A thorough study of the influence of the different relevant parameters, namely particle concentration and relative concentration of each solvent in the dispersion medium, was carried out. In the course of our investigations, we found that it was possible not only to attain planarized colloidal crystals with controlled thickness and good optical quality, but also to determine the crystal growth direction with respect to the substrate, which implies a major qualitative improvement with respect to previous techniques. Evidence of such control is obtained from both electron microscopy and optical spectroscopy. Colloidal suspensions of two types (Stober SiO2 and sulfonated polystyrene) were prepared by dispersing monodisperse sediments in different mixtures of ethanol, distilled water, and ethylene glycol (EG). These compounds were chosen attending to the ease of dispersion of the colloidal particles in them, their viscosity, and their volatility. Colloidal particles were first suspended in ethanol, distilled water, or mixtures of both, and then EG was added to decrease the vapor pressure of the suspension media. By doing so, our intention was to obtain a medium that required a long time to evaporate allowing particles to order by shearing, as reported in the literature, but volatile enough so that it would practically disappear after a few minutes. C O M M U N IC A TI O N S