Assembly of Crack-Free Photonic Crystals: Fundamentals, Emerging Strategies, and Perspectives

Photonic crystals (PCs) with a periodically arranged structure have aroused enormous interest in the regulation of photon motion for their unique property of a photonic band gap (PBG), which can block the propagation of specific electromagnetic waves. The PBG is generated by the periodic modulation of the refractive indices between the building blocks and surrounding medium, which could lead to a vivid structural color when PBG is located in the visible spectra. Because of the special properties of maneuvering and controlling photons in the visible range, considerable attention has been devoted to the PC in relation to various applications in color signage, display, biological and chemical sensors, detection, optoelectronic devices, etc. Notably, PCs have long existed in nature, such as gem opals, which are natural silica gel particle aggregations. Many creatures also comprise the PC nanostructures to adapt to nature, for example, butterfly, peacock, chameleon, and so forth. Inspired by nature, the bottom-up self-assembly of colloidal nanoparticles has been manifested to be a convenient manmade method to construct PC nanostructures. Similar to the synthesis of new compound molecules by the chemical bonding of atoms, colloidal nanoparticles can be driven to form aggregates with a periodic ordered structure by physical or chemical driving forces, such as capillary forces and surface tension, hydrogen bonds, van der Waals forces, etc. Typically, such nanoparticles consist of SiO₂, ZnO, Fe₃O₄, or organic polymers (polystyrene (PS), poly(methyl methacrylate) (PMMA), poly(acrylic acid) (PAA), etc.). The nanoparticle assembly process is governed by preferential thermodynamic states to stack together in a minimized free energy. However, the self-assembly of colloidal nanoparticles is easily susceptible to various external factors (solvent, substrate, temperature, concentration, zeta potential, pH, etc.), accidentally leading to the formation of unfavorable defects. Large-scale preparation of crack-free PCs is the critical limit for real-world application of PCs industrialization. Recently, the research on the mechanism and eliminating methods of defect creation in the colloidal PC assembly process has become an important research hotspot. This Account reviews the research progress on the crack-free PCs assembly methods, including the fundamental theory of PCs assembly, the formation mechanisms and elimination methods of assembly defects based on the assembly driving force manipulation, and developing high-quality colloidal nanoparticles. We outline three main mechanisms of crack generation during PC self-assembly, in which the assembly driving forces that are influenced by external factors to break the dynamic balance of colloidal particle assembly are discussed in detail. Subsequently, a series of crack elimination strategies, like novel high-performance assembly unit preparation (acrylic ester, tertiary-carbon, and fluorinated colloidal particles) and various assembly driving forces introduction, including hydrophobic force driving assembly (HFDA), molecular surface force-assisted assembly (MSFA), soft substrate-induced assembly (SSA), “colloid skin” enhanced assembly (CSE), template-assisted method (TA), spin-coating, layer-by-layer scooping transfer (LST) technique, inkjet printing, centrifugation-assisted assembly (CA), microfluidic technique, and modified vertical deposition method, are summarized. Eventually, we provide an outlook on more efficient techniques that can accomplish large-area and rapid construction of PCs with high crystallinity, no cracks, and vivid structure color to promote the industrialization of PC materials.