Abstract
Cracking of photonic crystals (PCs) has received considerable attention because of its severe limitation to PC’s applications in high-performance optics devices. Although enormous efforts have been focused on the understanding and elimination of the uncontrolled cracks in the self-assembly process, no reliable, low cost and scalable methods have been demonstrated for the fabrication of large (cm or more) crack-free single-crystalline PCs. Herein, we present a facile, reliable approach for the assembly of crack-free single-crystalline PCs on the centimeter scale by the synergistic effects of substrate deformation and monomer infiltration/polymerization. The co-assembling monomer infiltrates and polymerizes in the interstices of the colloidal spheres to form an elastic polymer network, which could lower the tensile stress generated from colloid shrinkage and strengthen the long-range interactions of the colloidal spheres. Otherwise, the timely transformation of the flexible substrate releases the residual stress. This facile, scalable and environment-friendly approach to centimeter-scale crack-free single-crystalline PCs will not only prompt the practical applications of PCs in high-performance optics devices, but also have great implications for the fabrication of crack-free thin films in other fields, such as wet clays, coating and the ceramic industry. Jingxia Wang, Yanlin Song and colleagues have prepared photonic crystals that present no cracks over large areas. Photonic crystals are nanostructures consisting of well-ordered colloidal spheres that can confine and guide light by affecting the propagation of photon, which makes them attractive components in optical and sensing devices. However, stress arising between the spheres during the assembly process often results in cracking of the crystals—a considerable hindrance to their widespread application. Approaches to limit cracking do exist but they often are inconvenient or expensive. Now, by assembling the colloidal spheres on a flexible substrate—rather than a conventional rigid one—and polymerizing a supporting species in the spaces between them, the team has released stress between the spheres giving large crack-free photonic crystals over centimeter-scale areas. This method should enable a more widespread use of photonic crystals, and may also help improve other types of coatings and ceramics where cracks are detrimental. We present a facile and reliable approach for the assembly of crack-free single-crystalline photonic crystals (PCs) with centimeter scale by the synergistic effects of substrate deformation and monomer infiltration/polymerization. The critical thickness of crack-free PCs is ∼5.6 μm, below which crack-free PCs can be fabricated on proper substrate. The co-assembling monomer infiltrates and polymerizes in the interstices of the colloidal spheres to form an elastic polymer network, which could lower the tensile stress generated from colloid shrinkage and strengthen the long range interactions of the colloidal spheres. Otherwise, the timely transformation of the flexible substrate releases the residual stress. This approach to centimeter-scale crack-free single-crystalline PCs will not only prompt the practical applications of PCs in high-performance optic devices, but also have great implications for the fabrication of crack-free thin films in other fields, such as wet clays, coating and ceramic industry.
Highlights
Enormous efforts have been focused on the understanding and elimination of the uncontrolled cracks in the self-assembly process, no reliable, low cost and scalable methods have been demonstrated for the fabrication of large crack-free single-crystalline Photonic crystals (PCs)
Self-assembly of submicrometer colloidal spheres into PC structures is a promising approach due to its simplicity and low cost, compared with top-down lithography techniques.[9,10,11]
Cracking is mainly caused by the tensile stress generated in the self-assembly process, which arises from the shrinkage of the colloidal spheres during the final drying process and the constraint of this shrinkage by the rigid substrate.[15,16]
Summary
Photonic crystals (PCs) have aroused worldwide attention because of their special light manipulation properties, which are extensively exploited in integrated optical devices,[1,2,3] full-color displays[4,5] and chemical/biological sensors.[6,7,8] Self-assembly of submicrometer colloidal spheres into PC structures is a promising approach due to its simplicity and low cost, compared with top-down lithography techniques.[9,10,11] the unintended crack formation in the selfassembly process has severely hindered the fabrication of large-area single crystalline PCs and their corresponding applications in highquality optics devices.[12,13,14] much attention has been focused on the cracking mechanism[15,16] and elimination.[17,18,19,20,21,22,23,24,25]. Elastic polymer infiltration and substrate deformation result in an integration of micrometer-sized single-crystal domains into centimeter scale crack-free single-crystalline PCs, which are the largest ever-reported. The obtained PCs show an improved optical property due to the large-area well-ordered single-crystalline structure. This facile approach to large-area single-crystalline PCs will be promising for PCs’ practical applications in high-quality optical integrated devices, optical waveguides and so on
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