Preparation Of Large-area And Crack-free Photonic Crystal Films On Superhydrophobic Substrates

During the last several decades,photonic crystals(PCs)have aroused tremendous interests for controlling and manipulating the flow of light with periodic structure.Due to their unique features,PCs hold great promise for applications in many fields,such as biosensors,solar cells,displays,and optical devices.However,owing to the unavoidable shrinkage of the colloidal spheres and tensile stress generated during an assembly process,cracks are spontaneously produced and often inevitable during the formation of PCs films,which greatly degrade their optic quality and strength.Therefore,it is highly desired to fabricate large-scale self-assembled structures with no defects and structurally controllable.On the other hand,superhydrophobic surfaces,which possess water contact angles higher than 150° and their roll angles less than 5°,have also attracted great attention in recent years for their appealing performances in colloidal assembly,water repellency,anti-fog and self-cleaning.Up to now,various methods have been developed to construct superhydrophobic surfaces.The principle of constructing superhydrophobic material is very simple which surface roughness and surface energy are the crucial factors.There are two strategies.Firstly,we make micro-nano structure on the low surface energy material surface.The other one is that micro-nano structure was prepared on the high surface energy materials(metal,etc.),and then the previously prepared structure was modificated using the low surface energy material.In this paper,we develop an effective strategy for preparing centmeterscale crack-free photonic crystals films by the combined effects of soft assembly and superhydrophobic nanopin surfaces.The different grafted PS colloid spheres were prepared by soap-free emulsion polymerization.The self-assembly experiments were carried out on a superhydrophobic substrate prepared by low temperature hydrothermal reaction and a hydrophilic substrate.When the colloidal suspension was self-assembled on a high-adhesive hydrophilic substrate,colloidal spheres had a tendency to crack due to the loss of solvent(water)molecules.In contrast,owing to its large contact angle and low adhesive force on the superhydrophobic substrate,the colloidal suspension exhibits a continuous retraction of the solid-liquid-gas three-phase contact line(TCL)during water evaporation.The retracted TCL and deformed colloidal spheres maintain the free shrinkage of the colloidal spheres without limitation.This can timely close the gaps due to the loss of solvent molecules.As a result,close-packed and well-ordered assembly structures could be easily obtained.In summary,we demonstrated an interesting example that soft colloid spheres and superhydrophobic nanopin surface could serve as an excellent basis for preparing large-area crack-free PCs films.