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The fabrication of large-area crack-free single-crystalline PCs is shown in Scheme 1. Monodisperse colloidal spheres of poly(St-MMA-AA) are co-assembled with a water-soluble monomer (NIPAm for instance) on metal foils. While the colloidal spheres self-assemble into well-ordered periodic structures, the monomer infiltrates and polymerizes in the interstices of the colloidal spheres to form a crosslinked elastic polymer network. The crosslinked polymer network can fix the perfectly-ordered latex arrangement in the saturated dispersion, favoring the achievement of large-area crack-free single crystalline PCs. Meanwhile, the used flexible substrate of metal foils can be deformed to release the residual stress. The synergetic effects of elastic polymer infiltration and the substrate deformation allow the achievement of large-area crack-free composite opal and inverse opal PCs.

The colloidal spheres of poly(St-MMA-AA) synthesized in our lab were chosen because they are favorable for the fabrication of large area crack-free single crystalline PCs. Firstly, the colloidal sphere of poly(St-MMA-AA) has good monodispersity with a polydispersity index of 0.005. The excellent monodispersity is beneficial for the large-scale assembly of uniform PCs. Secondly, the colloidal spheres have a special core-shell structure with a hydrophilic PAA shell and a hydrophobic PS core. The hydrogen bonding among carboxyl groups of the hydrophilic shell is beneficial for the well-ordered colloidal assembly. Thirdly, the hydrophilic shell is helpful for the infiltration of the hydrophilic co-assembling monomer, such as NIPAm or acrylamide, into the interstices of the colloidal particles, favoring their homogeneous infiltration.

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Fabrication process for crack-free photonic crystals (PCs) by polymerization-assisted assembly on aluminium foil. In the assembly process, the monomer polymerizes and forms an elastic polymer in the interstices of the colloidal spheres. The elastic deformation of the as-formed polymer counteracts the volume change resulted from latex shrinkage and decreases the tensile stress generated. Meanwhile, the substrate deformation releases the residual stress. Both contribute to the achievement of crack-free single-crystalline PCs.

Self-assembly of colloidal particles into colloidal films has many actual and potential applications. While various strategies have been developed to direct the assembly of colloidal particles, fabrication of crack-free and transferrable colloidal film with controllable crystal structures still remains a major challenge. Here we show a centrifugation-assisted assembly of colloidal silica spheres into free-standing colloidal film by using the liquid/liquid interfaces of three immiscible phases. Through independent control of centrifugal force and interparticle electrostatic repulsion, polycrystalline, single-crystalline and quasi-amorphous structures can be readily obtained. More importantly, by dehydration of silica particles during centrifugation, the spontaneous formation of capillary water bridges between particles enables the binding and pre-shrinkage of the assembled array at the fluid interface. Thus the assembled colloidal films are not only crack-free, but also robust and flexible enough to be easily transferred on various planar and curved substrates.

Self-assembly on a solid substrate is the most common strategy to fabricate colloidal films. While the solid substrate provides a support to colloidal crystallization, it may also hinder the assembly into the most energetically favorable position. For example, in gravity or centrifugal sedimentation method13,14,15,16, the rearrangement of packed particles appears to be severely restricted by their low mobility on the bottom substrate when the stabilizing factor (e.g. gravity or centrifugal force) exceeds the destabilizing factor (e.g. Brownian motion or interparticle electrostatic repulsion). Thus, the products are considerably less ordered, unless a slow attainment of equilibrium (typically hours to days) is achieved between two competing factors, causing poor assembly efficiency. Alternatively, the vertical deposition by lateral capillary attraction during solvent evaporation is effective in inducing oriented crystallization17. However, cracks often occur upon drying, due to the transverse tensile stress arising from the capillary-force-induced shrinkage of the pre-assembled array against a rigid substrate18. To avoid undesirable cracks, sol-gel precursors and monomers have been introduced to reduce the capillary effect18,19. Nevertheless, this approach is limited to some specific material systems. Spin-coating offers another method to produce crack-free colloidal films, but also requires use of refractive-index matching monomers and produces non-close-packed structures20. Additionally, because the hydrophilic/hydrophobic nature and surface roughness of a substrate are critical to the success of crystal growth17,20, a limited choice of substrates (e.g. silicone wafer or glass) are often used to deposit colloids. However, once the crystal film is formed on such substrates, it is not easy to transfer the film to another substrate (e.g. III-V device, lens or polymer substrate) for device integration or flexible applications.

In this study, we report a novel and simple centrifugation-assisted assembly method to create free-standing multilayer film from colloidal silica spheres at the liquid/liquid interfaces of three immiscible liquid phases. Our approach has several advantages: i) The present method can facilitate the binding and pre-shrinkage of the assembled array at the fluid substrate by utilizing the spontaneous formation of capillary water bridges between silica particles. This not only surmounts the cracking problem during drying of the film, but also produces a reasonably compact and robust, free-standing film, which can be easily transferred to any planar and curved substrates; ii) Centrifugal field can relax the constraint on particle hydrophobicity required for adsorption of colloidal particles to an interface without affecting the properties of interface and particles; iii) The low resistance fluid interface provides little barrier to the final equilibrium between the interfacial adsorbed particles, thus various crystal morphologies, including polycrystalline, single-crystalline and quasi-amorphous structures, can be readily obtained by independently adjusting the centrifugal force and interparticle electrostatic repulsion.

We have presented an effective centrifugation-assisted assembly method to create free-standing colloidal films by utilizing the liquid/liquid interfaces of three immiscible phases, including fluorinated oil/water/hydrocarbon solvent mixture. The perfluorinated oil phase provides a low resistance fluid substrate, allowing for the free movement of adsorbed particles towards equilibrium positions. The centrifugal field flattens the perfluorinated oil/water interface, participates in the construction of colloidal multilayer and also causes the dehydration of the assembled colloidal film by displacing the aqueous phase and hydrocarbon mixture. During assembly process, the hydration layer on silica surface can act as a steric barrier against particle coalescence, while after dehydration, the spontaneous formation of capillary water bridges between particles may lead to a binding and shrinkage toward the center of the dehydrated film. Thus, the assembled films are crack-free and robust enough to be transferred on any planar or curved substrates. Through independent control of the centrifugal force and interparticle electrostatic repulsion, polycrystalline, single-crystalline and quasi-amorphous structures can be readily obtained. This method may also be used for assembly of other colloidal particles into free-standing, crack-free and transferrable films with various structures and potential applications. It may also serve as an easily accessible model for studying the crystallization and phase transformation behaviors of colloidal systems.

To observe the internal crystal structure of the colloidal film, the free-standing film was transferred to a thin glass coverslip and covered by a drop of silicone oil to suppress the volatilization of void-filling fluid, then covered with another glass coverslip. The colloidal film was observed under a Hirox KH-7700 digital microscope equipped with a MX-10C co-axial vertical lighting zoom lens, an OL-700II objective lens and an AD-10S Directional Lighting Adapter, at a magnification of 700.

In order to reduce and ideally eliminate crack formation during the fabrication of inverse opal porous films, a co-assembly method was first developed by Meng et al.15 in order to fabricate ordered porous colloidal crystal films via a mixture of colloidal sphere and ultra-small particles in one step. Using an evaporation induced assembly by particles being driven to the meniscus and then ordering, the ultra-small particles infiltrated into the voids of the colloidal crystal formed by the larger particles during the co-assembly. Other examples of binary colloidal crystals also have been reported.16 Going beyond the co-assembly of binary particles, studies of the cracking mechanism and development of crack free colloidal films as large as square centimeters have been reported by the co-assembly of colloidal spheres and precursors, including polymer precursors6,17,18 or inorganic material precursors.1,17,19 The spheres used to form the colloidal crystal are a sacrificial template material that is later removed, and a large area inverse opal colloidal crystal film can be achieved. Low crack inorganic films can be achieved this way, but of low film thickness.20 However, utilizing 2D materials or particles with high aspect ratio as the infiltrant into the voids of the colloidal crystal remains challenging, not only to fabricate large area and thick crack free films, but in ensuring that the infiltrant does not disrupt the nearly close-packed assembly of the larger spheres, retaining order and connectivity.

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