Self-Assembly of Colloidal Particles into Amorphous Photonic Crystals

: Colloidal photonic crystals (PCs) have been extensively investigated since they can be prepared in an efficient and low-cost way. Different from the conventional PCs with highly ordered structures, amorphous photonic crystals (APCs) with isotropic photonic bandgap and non-iridescent structural colors have attracted growing interests especially in pigments, angle-independent displays, and optical coatings. This review focuses on the various strategies for the fabrication of the APCs by spraying, infiltration, layer-by-layer deposition, electrolyte-induced assembly, electrophoretic deposition, phase separation, assembly of bi-disperse-suspension, particle with rough surface, soft particles, and low-charged particles. Their potential applications are also summarized, like angle-independent displays, sensors, paintings, anti-counterfeiting labels, information storage, and so on. Finally, we present our perspectives together with the challenges of APCs. the silica particles rather than the scattering of light by the amorphous structures. Recently, our group reported that brilliant noniridescent structural colors can be fabricated by the random aggregation of large silica particles (300-500 nm). Unlike the conventional non-iridescent structural colors, the new structural color can be attributed to the significant enhancement of electromagnetic resonances by particle aggregation. This work a new strategy for the fabrication of unconventional structural


Spraying of colloidal solutions
Spraying of colloidal solutions is an efficient and productive strategy for the rapid assembly of colloidal particles into the APCs by volatilizing the solvents. When the colloidal suspension passes through the nozzle, the building blocks in the droplets rapidly assemble into APCs due to the concentration of the particles by volatilization of the solvent and the wetting of the substrate. [88][89][90][91] Takeoka et al. prepared APCs with non-iridescent structural colors by spraying the suspension on glass plates using spherical silica particles as the building blocks and methanol as the dispersion medium (Fig. 2d). 44 When the nozzle was too close to the substrate or a non-volatile solvent was used, the colloidal particles will be assembled in long-range order with iridescent structural colors. 92,93 Due to the strong incoherent scattering in the APCs array, the entire visible light is scattered rendering the APCs white. The black material can absorb the whole visible light to suppress incoherent scattering, and a small amount of black material can be added to the array of APCs in order to obtain a uniform color saturation with high structural color. [94][95][96] Takeoka et al. added CB to the silica suspension for preparing the APCs, which leaded to the decreasing of baseline of its reflectance spectra. However, its peak intensity remained almost constant, and the color saturation gradually increased with higher proportion of CB. 44 Once this APCs film is wetted, the structural color disappears due to the match of refractive index between the particles and the gap. Therefore, Yang et al. prepared non-iridescent structural color membranes with superhydrophobicity (static water contact angle of ~155° and roll-off angle of ~ 2°) by spraying monodispersed fluorosilanefunctionalized silica particles ( Fig. 2a and b). 45 In addition, Yang and co-workers demonstrated that low surface tension of particles and the solvents in moderate volatility were two critical factors in the formation of amorphous arrays in the spraying method, which was acquired by studying the coffee-ring effect on the assembly of colloidal particles. 71 Besides using hydrophobic colloidal particles as building blocks, 35, 45 spraying a hydrophobic solution on an array of APCs is also a commonly used strategy to prepare the hydrophobic APCs. 37, 73, 97 Different from above, Wang et

Infiltration-induced colloidal assembly
Infiltration-induced assembly of colloidal particles enables fast and large-scale printing of noniridescent structural color. The APCs are assembled with disordered accumulation of colloidal particles on permeable substrates by rapid penetration of solvent during the filtration process. 99,100 Anodic aluminum oxide (AAO) films with uniform and size-controlled columnar channels can be applied for quantitative analysis of solvent infiltration. Duan et al. assembled colloidal particle on AAO membranes with infiltration-induced process, which was taken as a model to show the assembling process due to a strong downward infiltration flow in the porous channel of AAO. The colloidal particles were adsorbed on the substrate with disordered arrangement for lack of a crystallization drive (Fig. 3a). 74 After completely infiltrating the solvent, the colloidal particles eventually arranged in order. Finally, non-iridescent structural color printing was achieved on the paper by inkjet printer with the colloidal suspension as an ink. Duan and co-workers fabricated the thin-film APCs which were assembled by spherical/polyhedral metal-organic framework (MOF) colloidal particles. 101,102 Similarly, Chen et al. reported a crack-free, arbitrarily bendable film of APCs with bright structural color prepared by vacuum filtration of colloidal suspension through an reduced graphene oxide-sulfopropyl methacrylate potassium salt (rGO-SPM) substrate ( Fig. 3b and c). 16 Graphene oxide composites effectively reduced the reflected light from the substrate. The strategy of infiltration-induced assembly for the preparation of APCs enables the rapid construction of short-range ordered structures, which can be conveniently printed on solvent-absorbing paper with APCs patterns by inkjet printers. However, according to its assembly principle, it needs to be assembled on substrates with high infiltration and its use is limited.
reported a multicolor conductive fiber with APCs structure prepared by depositing polystyrene colloidal particles of different sizes on the surface of conductive carbon fibers via EPD. 78 In order to solve the problem that structural color fibers cannot be fabricated on a large scale, Zhang et al.
developed a electrospinning method ( Fig. 5a and b). 33 The homogeneity of the electric field during the EPD process directly affects the assembly of colloidal particles. For example, Kang et al.
reported a method of assembling the dielectric particles in eccentric core-shell microcapsules controlled by dielectrophoresis (DEP) (Fig. 5c), in which the dielectric particles subjected to an inhomogeneous electric field move to the high-or low-field region depending on the dielectric environment. 79 In addition, Takeoka    the size contrast was increased to 0.23 (Fig. 7b). In addition, by discussing the bi-disperse colloidal particles assembled with reverse emulsion and the evaporative assembled membranes, Xiao and coworkers demonstrated that small-sized colloidal particles tended to stay on the surface of microspheres while the synthetic melanin particles preferred the microsphere surface over silica particles. 148 That work offered a new pathway to the assembly of colloidal particles with different particle sizes and different particle chemistries. The structure of APCs can be created by controlling the coefficient of variation (CV) of colloidal particle diameters. 41, 149, 150 For example, Kohri et al.
prepared APCs with bright non-iridescent structural colors by assembling the polydopamine (PDA) particles in CV around 4.4-7.8% and (Fig. 7c). 48 The structure of APCs can be acquired by introducing an additive into the monodisperse suspension. 66,151,152 Zi et al. proposed a strategy to prepare APCs by mixing monodisperse polystyrene particles and ink particles of cuttlefish for self-assembly, cuttlefish ink particles are introduced into colloidal suspensions for assembly for absorbing incoherent scattering and disrupting the long-range ordered structure of the colloidal particles. 49 Kim et al. reported a strategy for preparing the mechanochromic photonic films by co-assembly of silica particles, poly(ethylene glycol) phenyl ether acrylate and polydopamine particles (Fig. 7d). 153 In this system, the repulsive force between the neighboring silica particles caused by the solvation layer results in the formation of colloidal crystal arrays. As the concentration of polydopamine particles increases, the interaction of polydopamine on the surrounding colloidal particles inhibits the crystallization of silica particles and therefore a short-range-ordered arrangement structure is gradually formed. APCs were prepared easily and rapidly by bi-disperse-suspension assembly, and the transformation from PCs to APCs structure could be achieved by adjusting the size contrast of binary particles. 147,153 However, it is necessary to alter the size of binary particles and the ratio of each component when adjusting the structural color, which is a tedious process.

Assembly of particle with rough surface
The rough-surface colloidal particles assist in the assembly of the APCs structures. Yang et al.
grafted polypyrrole (PPy) particles onto the surface of polystyrene nanoparticles by in situ polymerization, which led to the formation of amorphous arrays due to the bumpy surface of the core-shell nanoparticles ( Fig. 8a and b). The black PPy shells provided strong contrast in the APCs by suppression of incoherent scattering and multiple light scattering. 154 In addition, Yang et al. View Article Online the tensile stress caused by the shrinkage of the colloidal suspension during drying due to the generation of covalent bonds between PDA and APTES, addressing the cracking problem during the preparation of APCs (Fig. 8d). 157,158 The utilization of surface roughness of the assembled particle can simplify the preparation process to obtain APCs in high quality. [159][160][161][162][163] High-quality APCs can be prepared with surface rough particles, which are not constrained by substrates and solvents, and the rough black viscous shell improves the light absorption efficiency and adhesion properties of the particles to the substrate. 89,157 However, the modification of a certain thickness of rough shell on smooth colloidal particles is not easily controlled, which limits their large-scale synthesis. assembled from core-shell particles reversibly depend on temperature. Manoharan et al. synthesized the colloidal particles consisted of polystyrene core and poly(N-isopropylacrylamide-co-acrylicacid) shell in a similar way and assembled them as the APCs membranes (Fig. 9a). 42 The noniridescent photonic pigments in the visible range were prepared by Manoharan et al. The pigments can weaken multiple scattering by using microfluidic techniques to encapsulate the colloidal particles in spherical capsules. 122 The APCs assembled by colloidal particles coated with soft polymer shells exhibit weak incoherent scattering and bright structural colors. And the assembly of particles with temperature-sensitive polymer shells can be used to obtain APCs with temperature response.

Assembly of low-charged particles
Colloidal particles with surface-grafted polymer tend to form short-range-ordered arrangements during assembly due to reduced electrostatic repulsive forces. 18 (PMMA)-grafted monodispersed silica colloidal particles in ionic liquids (Fig. 9d). 40 Then, they detailly investigated the relationships among colloidal glass transition, ion transport and structural color in this system. They demonstrated that the soft glassy colloidal arrays underwent glass transition in higher volume fraction than the one in the hard sphere systems and the APCs preformed higher ionic conductivity in the high concentration region. 172 Moreover, they also reported the assembly of thermosensitive poly(benzyl methacrylate) (PBnMA)-modified silica particles in ionic liquids, and showed phase separation at lower temperatures over a wider temperature range (Fig.   9c). 167 In the colloidal system assembled by ionic liquid, the presence of ions reduces the electrostatic repulsion between colloidal particles, while the steric forces and solvation effectively stabilize the colloidal particles. 173 Based on that, Watanabe and co-workers prepared colloidal glasses and gels by assembling grafted polymeric silica particles and unstable silica particles in an ionic liquid. 173 Colloidal glass with APC structure shows uniform non-iridescent structural color and fluidic properties that are expected to be used in wide viewing angles for color displays.
When the polymer is incorporated directly into the colloidal suspension, the absorbed polymer on the colloidal particles results in the closely packed short-range ordered structure. 86 and Eu-undoped Y 2 O 3 particles. In addition, we reported a novel method to prepare APCs with noniridescent structural colors by assembling colloidal particles in less-polar dispersant. 179 Based on the sensitivity of the APCs to the refractive index contrast, their structural color disappears once the APCs is wetted. Accordingly, we developed an information storage system for water recognition.
Our works have important implications towards the simple and fast large-area preparation of APCs in non-iridescent structural colors and understanding the assembly behavior of colloidal particles in different pH and diverse polar solutions. Generally, there are two ways to obtain low potential colloidal particles, one is by grafting on the surface of the particles and the other is by adding polymers to the colloidal suspension. Both strategies can prepare APCs with uniform structural color, but the former requires further modification of the particles synthetically is time-consuming, while the latter can simply and rapidly prepare APCs by merely introducing polymers to coassemble with colloidal particles.

3.Applications of APCs
The short-range-ordered structure of APCs generates non-iridescent structural colors and stronger incoherent scattering compared with PCs in well-ordered arrangement, which provides a unique advantage for APCs in potential applications.

Sensors
APCs offer an effective solution for sensors due to their unique and designable optical properties coordination. 29 As the tensile strain increases from 0 to 120%, the reflection peak of the APCs elastomer shifts from 660 nm to 550 nm along with the shift of the structural color from red to green. 29 They concurrently developed a recyclable APCs elastomer prepared by combining shortrange-ordered SiO 2 arrays with a water-soluble ureidopyrimidinone (UPy) which is cross-linked with poly(ethylene glycol) (PEG) polymeric matrix. 181 The reversible cross-linking and water solubility of PEG-UPy polymers confer excellent recyclability of APCs elastomers. 181 Recently, they proposed a wide-spectrum responsive APCs elastomer assembled with carbon-coated Fe 3 O 4 as building blocks followed by embedding in Amino-PDMS, which exhibits a wavelength difference of ≈ 223 nm during reversible stretching (Fig. 10a). 180  Owing to the large proportion of PNIPA in this system, the porous PNIPA hydrogels exhibited rapid  Fig. 10d). 25 Xiao et al. reported a colorimetric humidity sensor assembled by melanin nanoparticles that produced continuous structural color changes when the relative humidity changed from 10% to 90%.
The sensor exhibited excellent cycling performance and high sensitivity at a given humidity level. 23 Song et al. infiltrated amide monomers into the templates of APCs to obtain the humidity sensor by UV curing. 23 Its structural color showed a reversible change from blue to red when the relative humidity varied between 45% and 100%. The sensor exhibited structural color changes in different humidity ranges with colloidal particles in different sizes (Fig. 10b). 23  View Article Online photoelectrochemical reaction of Ag/Ag + to modulate the intensity of noncoherent scattered light and the variation of saturation in the APCs array by light (Fig. 10e). 75  (TLC) based on mesoporous silica particle was assembled with high separation efficiency and convenient recognition ( Fig. 10f and g). 182 In  (Fig. 11a). 44 Duan et al. developed a large-scale full-spectrum structural color printing technique by infiltration-induced colloidal assembly. The patterns were printed by the colloidal inks with strong solvent absorption capacity. 74 The structural color can be easily tuned with a CMYK mode of a domestic inkjet printer (Fig. 11b). The strong permeability of the paper to solvents ensures the colloidal particles to form APCs on the paper. 74 Gu et al. painted robust noniridescent structural colors of Peking Opera character faces by spraying SiO 2 @PDA on different substrates (Fig. 11e). 89 Zhang et al. painted the patterns of APCs by spraying a mixed emulsion of thiodiphenol-formaldehyde particles and carbon black on a thermoplastic polymeric film followed by sandwiching the structure to obtain an integrated composite film by hot pressing (Fig. 11d). 39 The APCs composite film fully compensates for the lack of mechanical properties of the printed APCs.  (Fig.10c). 38 Zhang et al. printed non-iridescent structural colors and structurally stable patterns of APCs on fabric with a rapid screen printing technique. 174 In addition, our group printed structural color patterns of APCs with multicolor vivid non-iridescent colors by hand-painting and spraying methods (Fig .10g). 111,150,177 Page 24 of 45 Materials Advances

Materials Advances Accepted Manuscript
Open co-workers. 192 In the dry state, the APCs presents white color consistent with the middle layer due to incoherent scattering; in the wet state, the middle layer turns to transparent and the APCs pattern appears due to the absorption of incoherent scattering by the black background which improves the color saturation (Fig. 13e). 192  reported a polyvinylidene fluoride structural color film consisting of the inverse opal structure with ordered array layers and amorphous array layers (Fig. 13c). 52 At the specular reflection angle, the structural color of the amorphous array layer is suppressed by Bragg diffraction and the film displays iridescent color. Therefore, the patterns on the film are switched between hiding and displaying by changing the observing angles. 52  Eu particles are easy to assemble into APCs with non-iridescent structural colors due to their instability and high polydispersity. Thus, patterns assembled with these two particles in the same diameter present the same structural color observed by naked eyes. 178 Here, we handwrote the patterning letters with Y 2 O 3 :Eu particles and the rest letters with the Y 2 O 3 particles in the same diameter as background. Irradiated by natural light, these letters show a consistent structural color, while they immediately present red fluorescence under UV illumination. Thus, we successfully constructed a fully reversible hiding-showing patterns by UV irradiation. (Fig. 14a and b). This invisible printing method has the following advantages: (1) the invisible photonic printing is realized based on high contrast of luminous intensity between the pattern and the background other than the structural color and (2) multicolor display can increase the difficulty of identification. In the work on modulating the assembly behavior of colloidal particles by pH value, we developed a new method construct two photonic microstructures by assembly of one type of silica particles. 168 More importantly, a coding/decoding system based on the two photonic crystals with different optical properties has been developed by combining APCs with PCs. APCs and PCs with the same stop band were prepared by the solution of aminated silica particles with different pH values. The two films present the same structure color with the same reflective wavelength but different reflection intensity at the viewing angle of 0°. Therefore, by shifting the reflected wavelength to the near infrared region, the color contrast is nearly the same because the human eye is insensitive to near infrared light. As proof-of-concept, we construct a 3 × 3 circular grid with the combination of APCs and PCs at the wavelength of 668 nm. At a viewing angle of 0°, APCs and PCs cannot be identified because of their similar structural colors and low contrast. At other viewing angles, the structural color of APCs remains constant while the structural color of PCs changes, and therefore the APCs are identified (Fig. 14c and d). The coding/decoding system is superior in easily hiding the signal of structural color and combining multiple codes. In our work on assembling the APCs by less polar solvents, a novel system for information storage and identification was developed based on the wetting character of APCs. Their structural colors disappear by wetting due to the reduction of the internal refractive index contrast. 179 A 6-circular-grids system was constructed with APCs whose displaying region were hydrophobic after treated with fluoroalkylsilane and the background remained hydrophilic. The APCs in the hydrophilic background are easily wetted water and along amorphous arrangement by external technical means. The assembly methods include spraying, infiltration-induced colloidal assembly, layer-by-layer deposition and electrophoretic deposition.
The other is to weaken the surface potential of the building blocks or change their surface roughness by surface modification or shell coating. 156,166,173 For example, assembly of rough-surface particles, assembly of soft particles and assembly of polymer-grafted particles. Subsequently, we detailly listed several methods to eliminate the strong incoherent scattering of APCs.  49,75,85 In addition, the incoherent scattering of APCs can be absorbed with black substrates. Finally, various applications of APCs are described, including applications in sensors based on external stimulus response, structural color printings, wide-view displays, and anti-counterfeit labels.
Although great progress has been made in the preparation of APCs, much efforts still required for the application of APCs. First, the broad peaks and faint structural colors of APCs limit their application in sensors. For example, the APCs-based mechanochromic sensors show blurry change of structural color during the deformation of APCs elastomers. In contrast, PCs sensors have vivid and sensitive change in structural color. Second, the applications of APCs in display and printing usually require consideration of durability, compatibility and cost, but most APCs have poor mechanical properties and are out of use after recycles. Third, anti-counterfeiting labels based on APCs have excellent mechanical properties and stability, 183,191 and can simply identify the security pattern by stretching or rotating a certain angle. 51,153,168 The solutions to the present challenges of APCs will boost their development and lead to the ideal optical devices with unique features and wide applications.
It should be noted that noniridescent structural colors also can be obtained by particle aggregations that originates from the electromagnetic resonances of the silica particles rather than