A New Methodology for the Reproducible Elaboration of SiO₂-Based Colloid Crystals as a Template for Inverse Opals Structures

Photonic crystals (PCs) are nanomaterials with photonic properties made up of periodically modulated dielectric materials that reflect light between a wavelength range located in the photonic band gap. Colloidal PCs (C-PC) made from monodisperse nanoparticles have been employed in coatings, films for surface-enhanced Raman scattering, chromatographic columns packing, and reflectometric interference spectroscopy. Additionally, C-PC have been studied as sacrifice templates to create inverse opals (IO) employing inorganic, organic, metallic, and ceramic materials, which are exact inverse replicas of C-PC structures and therefore maintain their optical properties. Several manufacture techniques have been applied for the fabrication of C-PC that involve the self-assembly of monodisperse colloidal particles by spin and spray coating, interfacial assembly and inkjet printing, sedimentation, or vertical deposition. In practice, due to its simplicity, C-PC crystals based on convective particle aggregation are elaborated either by the method of convective assembly by solvent volatilization or vertical lifting deposition. Although capable of producing good quality crystals, these methods have low uniformity between deposits in terms of thickness, and reflectance spectrum characteristics, most likely due to slight variations in temperature, humidity, arrangement geometry and particle suspension properties that influence the outcome.<br/>In this work, a robust protocol for the elaboration of photonic crystals based on SiO₂particles (SP) deposition using vertical lifting method was studied. A wide range of lifting speed and particles suspensions concentration were investigated by evaluating C-PC reflectance spectrum key parameters such as maximum reflectance (% R max), wavelength at % R max, and reflectance peak full width at half maximum (FWHM). The experimental conditions that led to the best performance were selected and employed for reproducibility tests.<br/>Thinner and higher reflectance peaks were obtained with a decrease in the lifting speed and an increase in the SP concentrations up to certain values. Seven batches of 12 C-PC employing a SP 3 % suspension and a lifting speed of 0.28 µm/s were prepared to test the reproducibility of this method. Every C-PC fabricated in this assay have a wavelength peak in a range between 528 and 538 nm and a FWHM lower than 90 nm. IO polymeric films with a highly porous and interconnected morphology were obtained using the developed C-PC as a template. In the current work, we found the optimized conditions that allows to fabricate reproducible colloidal crystals that could be prepared in a large scale and a short period of time employing a simple device. These findings provide a step forward to scale-up to the fabrication of C-PC, 3D microporous structures, and IOs as those employed for the elaboration of photonic polymeric sensors.