Self-assembled Colloidal Crystals Research Articles

Reconfigurable Antireflection Coatings Enabled by PDMS Oligomer Infusion in Templated Nanoporous Polymer Films.

The diffusion of uncured polydimethylsiloxane (PDMS) oligomers out of bulk PDMS elastomers is usually detrimental to many biomedical and microfluidic applications due to the inevitable contamination of the contacting fluids and substrates. Here, we transform this detrimental process into an enabling technology for achieving novel reconfigurable antireflection (AR) coatings, which are of great technological importance in the development of new nano-optical and optoelectronic applications. Self-assembled monolayer silica colloidal crystals are first used as sacrificial templates in fabricating nanoporous polymer AR coatings. When air in the templated nanopores is replaced with infused PDMS oligomers simply by pressing a PDMS stamp on a nanoporous AR film, the original antireflection conditions are lost, and the coating transforms from a low-reflection configuration to a high-reflection state. The original antireflection performance can be fully recovered by dissolving the infused oligomers in the appropriate solvents (e.g., hexane). This novel tuning mechanism for achieving reconfigurable AR properties has been confirmed by systematic investigations using various microscopes, optical spectroscopy, nanoindentation, thermomechanical tests, and X-ray photoelectron spectroscopy. Complex micropatterns with micrometer-scale spatial resolution and drastically different AR performances can be easily printed on nanoporous AR films by using a soft lithography-based microcontact printing process. Numerical finite-difference time-domain simulations match well with experimental antireflection measurements and reveal a linear relationship between the optical transmission and the amount of infused PDMS oligomers in nanopores.

Control of visible-range transmission and reflection haze by varying pattern size, shape and depth in flexible metasurfaces

Cost-effective soft imprint lithography technique is used to prepare flexible thin polymeric surfaces containing a periodic arrangement of nanodimples and nanobumps of sub-micron size. Using a single master mold of self-assembled colloidal crystal, metasurfaces with different depths and heights of patterns with a fixed pitch are possible, which makes the process inexpensive and simple. These metasurfaces are studied for their diffuse and total transmission and reflection spectra in the visible range. The transmission haze and reflection haze are calculated from the measurements. The surface containing nanobumps of lesser pattern height result in higher values of reflection and transmission haze than from surfaces containing nanodimples of much higher depth for the same pitch. The haze is more dependent on the pattern depth or height and less dependent on the pitch of the pattern. Far-field transmission profiles measured in the same wavelength range from the patterned surfaces show that the scattering increases with the increase of the ratio of pattern depth/height to pitch, similar to the haze measurements conducted with a closed integrating sphere. These profiles show that the angular spread of scattered light in transmission is within 10°, explaining the reason for the relatively low transmission haze in all the patterned surfaces. Simulation results confirm that the nanobump pattern gives higher transmission haze compared to nanodimple pattern. By controlling the ratio of pattern depth/height to pitch of the features on these surfaces, both an increase in optical haze and a balance between total reflection intensity and total transmission intensity can be achieved.Graphical

From Self-Assembly of Colloidal Crystals toward Ordered Porous Layer Interferometry.

Interferometry-based, reflectometric, label-free biosensors have made significant progress in the analysis of molecular interactions after years of development. The design of interference substrates is a key research topic for these biosensors, and many studies have focused on porous films prepared by top-down methods such as porous silicon and anodic aluminum oxide. Lately, more research has been conducted on ordered porous layer interferometry (OPLI), which uses ordered porous colloidal crystal films as interference substrates. These films are made using self-assembly techniques, which is the bottom-up approach. They also offer several advantages for biosensing applications, such as budget cost, adjustable porosity, and high structural consistency. This review will briefly explain the fundamental components of self-assembled materials and thoroughly discuss various self-assembly techniques in depth. We will also summarize the latest studies that used the OPLI technique for label-free biosensing applications and divide them into several aspects for further discussion. Then, we will comprehensively evaluate the strengths and weaknesses of self-assembly techniques and discuss possible future research directions. Finally, we will outlook the upcoming challenges and opportunities for label-free biosensing using the OPLI technique.

Reconfigurable Grating Diffraction Structural Color in Self-Assembled Colloidal Crystals.

Self-assembled colloidal crystals display structural colors due to light diffracted from their microscale, ordered structure. This color arises due to Bragg reflection (BR)or grating diffraction (GD); the latter mechanism is much less explored than the former. Here the design space for generating GD structural color is identified and its relative advantages are demonstrated.Electrophoretic deposition is used to self-assemble crystals with fine crystal grains from colloids of diameter 1.0µm. The structural color in transmission is tunable across the full visible spectrum. The optimum optical response-represented by both color intensity and saturation-is observed at low layer number (≤5layers). The spectral response is well predicted by Mie scattering of the crystals. Taken together, the experimental and theoretical results demonstrate that vivid grating colors with high color saturation can be produced from thin layers of micron-sized colloids. These colloidal crystals extend the potential of artificial structural color materials.

Capillary Transfer of Self-Assembled Colloidal Crystals.

Colloidal self-assembly has attracted significant interest in numerous applications including optics, electrochemistry, thermofluidics, and biomolecule templating. To meet the requirements of these applications, numerous fabrication methods have been developed. However, these are limited to narrow ranges of feature sizes, are incompatible with many substrates, and/or have low scalability, significantly limiting the use of colloidal self-assembly. In this work, we study the capillary transfer of colloidal crystals and demonstrate that this approach overcomes these limitations. Enabled by capillary transfer, we fabricate 2D colloidal crystals with nano-to-micro feature sizes spanning 2 orders of magnitude and on typically challenging substrates including those that are hydrophobic, rough, curved, or structured with microchannels. We developed and systemically validated a capillary peeling model, elucidating the underlying transfer physics. Due to its high versatility, good quality, and simplicity, this approach can expand the possibilities of colloidal self-assembly and enhance the performance of applications using colloidal crystals.

Facile Synthesis of ZnO/WO3 Nanocomposite Porous Films for High-Performance Gas Sensing of Multiple VOCs.

Volatile organic compounds (VOCs) in indoor environments have typical features of multiple components, high concentration, and long duration. The development of gas sensors with high sensitivity to multiple VOCs is of great significance to protect human health. Herein, we proposed a sensitive ZnO/WO3 composite chemi-resistive sensor facilely fabricated via a sacrificial template approach. Based on the transferable properties of self-assembled monolayer colloidal crystal (MCC) templates, two-dimensional honeycomb-like ordered porous ZnO/WO3 sensing matrixes were constructed in situ on commercial ceramic tube substrates with curved and rough surfaces. The nanocomposite thin films are about 250 nm in thickness with large-scale structural consistency and integrity, which facilitates characteristic responses with highly sensitivity and reliability. Furthermore, the nanocomposite sensor shows simultaneous responses to multiple VOCs that commonly exist in daily life with an obvious suppression sensing for traditional flammable gases. Particularly, a detection limit of 0.1 ppm with a second-level response/recovery time can be achieved, which is beneficial for real-time air quality assessments. We proposed a heterojunction-induced sensing enhancement mechanism for the ZnO/WO3 nanocomposite film in which the formation of abundant heterojunctions between ZnO and WO3 NPs significantly increases the thickness of the electron depletion layer in the bulk film and improves the formation of active oxygen species on the surface, which is conducive to enhanced responses for reducing VOC gases. This work not only provides a simple approach for the fabrication of high-performance gas sensors but also opens an achievable avenue for air quality assessment based on VOC concentration detection.

From silica colloidal particles to photonic crystals: Progress in fabrication and application of structurally colored materials

Photonic crystals have attracted extensive attention because of their wide application in optics and photonics. These structures can be made by “top-down” or “bottom-up” self-assembly methods. Self-assembly methods have received particular attention due to their low cost, simple manufacturing process, relatively easy scaling, and ease of creating complex structures with nanometer precision. Self-assembled colloidal crystals play an essential role in the formation of photonic crystals. In this article, the assembly of colloidal crystals-silica particles, the synthesis of silica particles, and the assembly of three-dimensional photonic crystals based on inorganic/inorganic and inorganic/organic composites are introduced in detail. The formation of silica based bionic structure material can produce different optical properties of conventional materials. It provides unprecedented opportunities for textiles, pigments, sensing, information encryption, stealth materials, anti-counterfeit marks, energy collection, environmental remediation, and a variety of other application fields.

PH-sensitive colloidal crystals built from monodisperse poly(St-MMA-SPMAP-AA) microspheres via air-liquid interface assembly

By changing the amount of acrylic acid, different monodisperse poly(styrene-co-methyl methacrylate-co-3-sulfopropyl methacrylate potassium salt-co-acrylic acid) (P(St-MMA-SPMAP-AA)) microspheres are synthesized by soap-free emulsion polymerization. Two-dimensional or three-dimensional PSMSSA colloidal crystals are successfully prepared by the gas-liquid interface assembly method. A noticeable shift of the bandgap of the PSMSSA colloidal crystals is observed at different pH, after the addition of diluted HCl aqueous, a structure color of amaranth is found for the PSMSSA colloidal crystals, and a structure color of blue-green is formed under neutral conditions. By using PSMSSA colloidal crystals on the liquid interface, the colloidal voids are filled with liquid, and its optical and pH-responsive properties can be directly measured without re-transfer, realizing the simple and efficient preparation of pH-responsive colloidal crystals sensors. Compared with the traditional pH sensor, the gas-liquid interface self-assembled colloidal crystals sensor is not only economical and economical, but also a reversible sensor visible to the naked eye.

Biomineral-Inspired Colloidal Liquid Crystals: From Assembly of Hybrids Comprising Inorganic Nanocrystals and Organic Polymer Components to Their Functionalization.

ConspectusBioinspired organic/inorganic synthetic composites have been studied as high-performance and functional materials. In nature, biominerals such as pearls, teeth, and bones are self-organized organic/inorganic composites. The inorganic components are composed of calcium carbonate (CaCO3) and hydroxyapatite (HAp), while the organic components consist of peptides and polysaccharides. These composites are used as structural materials in hard biological tissues. Biominerals do not show significantly higher performances than synthetic composites such as glass-fiber- or carbon-fiber-reinforced plastics. However, biominerals consist of environmentally friendly and biocompatible components that are prepared under mild conditions. Moreover, they form elaborate nanostructures and self-organized hierarchical structures. Much can be learned about material design from these biomineral-based hierarchical and nanostructured composites to assist in the preparation of functional materials.Inspired by these biological hard tissues, we developed nanostructured thin films and bulk hybrid crystals through the self-organization of organic polymers and inorganic crystals of CaCO3 or HAp. In biomineralization, the combination of insoluble components and soluble acidic macromolecules controls the crystallization process. We have shown that poly(acrylic acid) (PAA) or acidic peptides called polymer additives induce the formation of thin film crystals of CaCO3 or HAp by cooperation with insoluble organic templates such as chitin and synthetic polymers bearing the OH group. Moreover, we recently developed CaCO3- and HAp-based nanostructured particles with rod and disk shapes. These were obtained in aqueous media using a macromolecular acidic additive, PAA, without using insoluble polymer templates. At appropriate concentrations, the anisotropic particles self-assembled and formed colloidal liquid-crystalline (LC) phases.LC materials are generally composed of organic molecules. They show ordered and mobile states. The addition of stimuli-responsive properties to organic rod-like LC molecules led to the successful development of informational displays, which are now widely used. On the other hand, colloidal liquid crystals are colloidal self-assembled dispersions of anisotropic organic and inorganic nano- and micro-objects. For example, polysaccharide whiskers, clay nanosheets, gibbsite plate-shaped particles, and silica rod-shaped particles exhibit colloidal LC states.In this Account, we focused on the material design and hierarchical aspects of biomineral-based colloidal LC polymer/inorganic composites. We describe the design and preparation, nanostructures, and self-assembled behavior of these new bioinspired and biocompatible self-organized materials. The characterization results for these self-assembled nanostructured colloidal liquid crystals found using high-resolution transmission electron microscopy, small-angle X-ray scattering, and neutron scattering and rheological measurements are also reported. The functions of these biomineral-inspired liquid crystals are presented. Because these biomineral-based LC colloidal liquid crystals can be prepared under mild and aqueous conditions and they consist of environmentally friendly and biocompatible components, new functions are expected for these materials.

Screen-Printed Carbon Electrodes with Macroporous Copper Film for Enhanced Amperometric Sensing of Saccharides.

A porous layer of copper was formed on the surface of screen-printed carbon electrodes via the colloidal crystal templating technique. An aqueous suspension of monodisperse polystyrene spheres of 500 nm particle diameter was drop-casted on the carbon tracks printed on the substrate made of alumina ceramic. After evaporation, the electrode was carefully dipped in copper plating solution for a certain time to achieve a sufficient penetration of solution within the polystyrene spheres. The metal was then electrodeposited galvanostatically over the self-assembled colloidal crystal. Finally, the polystyrene template was dissolved in toluene to expose the porous structure of copper deposit. The morphology of porous structures was investigated using scanning electron microscopy. Electroanalytical properties of porous copper film electrodes were evaluated in amperometric detection of selected saccharides, namely glucose, fructose, sucrose, and galactose. Using hydrodynamic amperometry in stirred alkaline solution, a current response at +0.6 V vs. Ag/AgCl was recorded after addition of the selected saccharide. These saccharides could be quantified in two linear ranges (0.2–1.0 μmol L−1 and 4.0–100 μmol L−1) with detection limits of 0.1 μmol L−1 glucose, 0.03 μmol L−1 fructose, and 0.05 μmol L−1 sucrose or galactose. In addition, analytical performance of porous copper electrodes was ascertained and compared to that of copper film screen-printed carbon electrodes, prepared ex-situ by the galvanostatic deposition of metal in the plating solution. After calculating the current densities with respect to the geometric area of working electrodes, the porous electrodes exhibited much higher sensitivity to changes in concentration of analytes, presumably due to the larger surface of the porous copper deposit. In the future, they could be incorporated in detectors of flow injection systems due to their long-term mechanical stability.

Analytical model for transmission dips in self-assembled two-dimensional colloidal crystals.

Self-assembled two-dimensional (2D) colloidal crystals (CCs) are utilized in various optical devices, lasers, biosensors, and light harvesting applications. Optical design tuning capabilities, in terms of sphere refractive index and diameter size, can influence the optical characteristics for the close-packed single-layer or multilayer structures. Often transmission dips in 2D CCs are observed, which cannot be explained by Bragg diffraction as it does for 3D photonic crystals. In this work, an analytical attempt to accurately model the transmission dips observed in the 2D CCs optical spectra is presented, aiming to explain the origin of these dips. The formation of a broad dip was studied experimentally as well. A less than 1% mismatching error was found between experiment and theory for the two blaze peak positions as well as for the transmission intensity ratio. Finally, the 2D CCs were integrated in mesostructured solar cells as light trapping structures.

Effect of Particles of Irregular Size on the Microstructure and Structural Color of Self-Assembled Colloidal Crystals.

Self-assembled colloidal crystals can exhibit structural colors, a phenomenon of intense reflection within a range of wavelengths caused by constructive interference. Such diffraction effects are most intense for highly uniform crystals; however, in practice, colloidal crystals may include particles of irregular size, which can reduce the quality of the crystal. Despite its importance in realizing high-quality structural colors, a quantitative relationship between particles of irregular size, crystal quality, and the resultant structural color response remains unclear. This study systematically and quantitatively investigates the effect of adding particles of irregular size on the microstructural quality and structural color reflectivity of colloidal crystals formed by evaporative self-assembly via experiment and simulation. We examine two sizes of irregular particles─those which are 1.9 times larger and 0.4 times smaller than the host crystal. We find that small irregular particles are more detrimental to surface crystal quality and structural color reflectivity than large irregular particles. When incorporated with 10% volume fraction of irregularly sized particles, the reflectivity of crystal films with large (small) irregularly sized particles decreases by 18.4 ± 5.6% (27.5 ± 5.8%), and a measure of surface crystal quality derived from Fourier analysis of scanning electron microscopy images reduces by 40.0 ± 4.5% (48.8 ± 6.0%). By modeling colloidal films incorporated with irregular particles via molecular dynamics simulation and computing the reflection spectra of the modeled crystals via the finite-difference time-domain method, we find that the peak reflectivity of the assembled structures increases monotonically with overall crystallinity, and that overall crystallinity is correlated with the volume fraction of incorporated irregular particles. The quantitative relationships developed in this study can be applied to predict the level of irregularly sized particles that can be tolerated in colloidal films before significant degradation in crystal quality and reflectivity occurs.

Suction-Cup-Inspired Adhesive Micromotors for Drug Delivery.

Micromotors have opened novel avenues for drug delivery due to their capacity for self‐propelling. Attempts in this field trend towards ameliorating their functions to promote their clinical applications. In this paper, an ingenious suction‐cup‐inspired micromotor is presented with adhesive properties for drug delivery in the stomach. The micromotors are fabricated by using hydrogel replicating the structure of suction‐cup‐like microparticles, which derive from self‐assembly of colloidal crystals under rapid solvent extraction, followed by loading magnesium (Mg) in the bottom spherical surface. The Mg‐loaded micromotors can realize spontaneous movement due to the continual generation of hydrogen bubbles in gastric juice. The combination of unique suction‐cup‐like structure with excellent motion performance makes the micromotor an ideal carrier for drug delivery as they can efficiently adhere to the tissue. Moreover, benefiting from the porous structure, the hydrogel micromotors exhibit a high volume‐surface ratio, which enables efficient drug loading. It is demonstrated that the suction‐cup‐inspired micromotors can adhere efficiently to the ulcer‐region in the stomach and release drugs due to their distinctive architecture and spontaneous motion, exhibiting desirable curative effect of gastric ulcer. Thus, the suction‐cup‐inspired micromotors with adhesive properties are expected to advance the development of micromotor in clinical applications.

Unexpected Dependence of Photonic Band Gap Size on Randomness in Self-Assembled Colloidal Crystals.

Using computer simulations, we explore how thermal noise-induced randomness in a self-assembled photonic crystal affects its photonic band gaps (PBGs). We consider a two-dimensional photonic crystal composed of a self-assembled array of parallel dielectric hard rods of infinite length with circular or square cross section. We find that PBGs can exist over a large range of intermediate packing densities and the largest band gap does not always appear at the highest packing density studied. Remarkably, for rods with square cross section at intermediate packing densities, the transverse magnetic (TM) band gap of the self-assembled (i.e., thermal) system can be larger than that of identical rods arranged in a perfect square lattice. By considering hollow rods, we find the band gap of transverse electric modes can be substantially increased while that of TM modes show no obvious improvement over solid rods. Our study suggests that particle shape and internal structure can be used to engineer the PBG of a self-assembled system despite the positional and orientational randomness arising from thermal noise.

From colloidal particles to photonic crystals: advances in self-assembly and their emerging applications.

Over the last three decades, photonic crystals (PhCs) have attracted intense interests thanks to their broad potential applications in optics and photonics. Generally, these structures can be fabricated via either "top-down" lithographic or "bottom-up" self-assembly approaches. The self-assembly approaches have attracted particular attention due to their low cost, simple fabrication processes, relative convenience of scaling up, and the ease of creating complex structures with nanometer precision. The self-assembled colloidal crystals (CCs), which are good candidates for PhCs, have offered unprecedented opportunities for photonics, optics, optoelectronics, sensing, energy harvesting, environmental remediation, pigments, and many other applications. The creation of high-quality CCs and their mass fabrication over large areas are the critical limiting factors for real-world applications. This paper reviews the state-of-the-art techniques in the self-assembly of colloidal particles for the fabrication of large-area high-quality CCs and CCs with unique symmetries. The first part of this review summarizes the types of defects commonly encountered in the fabrication process and their effects on the optical properties of the resultant CCs. Next, the mechanisms of the formation of cracks/defects are discussed, and a range of versatile fabrication methods to create large-area crack/defect-free two-dimensional and three-dimensional CCs are described. Meanwhile, we also shed light on both the advantages and limitations of these advanced approaches developed to fabricate high-quality CCs. The self-assembly routes and achievements in the fabrication of CCs with the ability to open a complete photonic bandgap, such as cubic diamond and pyrochlore structure CCs, are discussed as well. Then emerging applications of large-area high-quality CCs and unique photonic structures enabled by the advanced self-assembly methods are illustrated. At the end of this review, we outlook the future approaches in the fabrication of perfect CCs and highlight their novel real-world applications.

Dynamic Colloidal Photonic Crystal Hydrogels with Self-Recovery and Injectability

Simulation of self-recovery and diversity of natural photonic crystal (PC) structures remain great challenges for artificial PC materials. Motivated by the dynamic characteristics of PC nanostructures, here, we present a new strategy for the design of hydrogel-based artificial PC materials with reversible interactions in the periodic nanostructures. The dynamic PC hydrogels, derived from self-assembled microgel colloidal crystals, were tactfully constructed by reversible crosslinking of adjacent microgels in the ordered structure via phenylboronate covalent chemistry. As proof of concept, three types of dynamic colloidal PC hydrogels with different structural colors were prepared. All the hydrogels showed perfect self-healing ability against physical damage. Moreover, dynamic crosslinking within the microgel crystals enabled shear-thinning injection of the PC hydrogels through a syringe (indicating injectability or printability), followed by rapid recovery of the structural colors. In short, in addition to the great significance in biomimicry of self-healing function of natural PC materials, our work provides a facile strategy for the construction of diversified artificial PC materials for different applications such as chem-/biosensing, counterfeit prevention, optical display, and energy conversion.

Modeling disorder in two-dimensional colloidal crystals based on electron microscope measurements.

Self-assembled two-dimensional colloidal crystals (CCs) are critical components in many optical and optoelectronic devices. Such structures usually exhibit various types of disorder, which sometimes can be beneficial for the desired applications. However, disorder poses challenges to the modeling of two-dimensional structures. In this work, two-dimensional CCs employed in optoelectronic devices, especially dye-sensitized solar cells, are investigated. scanning electron microscope (SEM) images were used to quantify the disorder in the studied structures. As a basis for simulations, disordered model patterns were generated with properties extracted from the SEM images of prepared samples. Optical modeling was performed with a finite-difference time-domain simulator. The simulated transmission data are consistent with the experimentally measured spectra.

Preparation, self-assembly of SiO2/PNIPAm complex microgels and their manipulation of fluorescence emission for organic dyes

In this study, a novel strategy for manipulating fluorescence emission of organic dyes is reported, which is based on coupling optical properties of colloidal crystals (CCs) with responsive microgels. Firstly, SiO2 microspheres doped with three kinds of organic dyes (Rhodamine B, RhB; fluorescein isothiocyanate, FITC; commercial cationic Fluorescent Yellow, X-10GFF) were prepared and they were used as the cores to synthesize SiO2/PNIPAm complex microgels doped with organic dyes by free radical polymerization. Then 3D self-assembled CCs of the complex microgels were fabricated by centrifugation. The microstructure of the complex microgel was characterized by Fourier transform infrared (FTIR), transmission electron microscopy (TEM), thermogravimetric (TG), dynamic light scattering (DLS), and differential scanning calorimeter (DSC). Moreover, the effect of doping organic dyes and temperature on self-assembly of the microgels were investigated. Manipulation of fluorescence emission for organic dyes by CCs was studied by photoluminescence (PL) and UV-visible spectrophotometer (UV-Vis). The results indicate that, due to the soft and thermosensitive property of PNIPAm shell, continuous adjustment to the photonic bandgap (PBG) of the CCs can be achieved conveniently by varying concentrations of the complex microgels or ambient temperatures, and when PBG of the CCs overlaps with the fluorescence emission peak of the organic dye, fluorescence emission of the organic dye can be manipulated.

Crucial Impact of Hydrophilicity on the Self-Assembled 2D Colloidal Crystals Using Langmuir-Blodgett Method.

Large-scale close-packed two-dimensional (2D) colloidal crystal with high coverage is indispensable for various promising applications. The Langmuir-Blodgett (LB) method is a powerful technique to prepare 2D colloidal crystals. However, the self-assembly and movement of microspheres during the whole LB process are less analyzed. In this study, we clarify the crucial impact of hydrophilicity of the microspheres on their self-assembly in the LB process and on the properties of the prepared 2D colloidal crystals. The characteristic surface pressure-area isotherms of the microspheres have been analyzed and adjusted by only counting the quantity of the microspheres on the water surface, which leads to more accurate results. The critical surface pressures for hydrophilic and hydrophobic microspheres are about 61 and 46 mN/m, respectively. The decrease of the surface hydrophilicity of microspheres facilitates their self-assembly on the water surface, which further leads to higher coverage and less defects of the 2D colloidal crystals. A coverage of as high as 97% was obtained using hydrophobic microspheres. Entropy and intersphere capillary forces drive the self-assembly and transportation of the microspheres, respectively. Caused by the diffraction of visible light, opposite contrasts at local adjacent regions on the surface of the 2D colloidal crystals have been observed. The understanding of self-assembly of the microspheres during the LB process paves the way to fabricate the high-quality 2D colloidal crystals for various applications such as photonic papers and inks, stealth materials, biomimetic coatings, and related nanostructures.

Self-templating assembly of soft microparticles into complex tessellations.

Encoding Archimedean and non-regular tessellations in self-assembled colloidal crystals promises unprecedented structure-dependent properties for applications ranging from low-friction coatings to optoelectronic metamaterials1-7. Yet, despite numerous computational studies predicting exotic structures even from simple interparticle interactions8-12, the realization of complex non-hexagonal crystals remains experimentally challenging13-18. Here we show that two hexagonally packed monolayers of identical spherical soft microparticles adsorbed at a liquid-liquid interface can assemble into a vast array of two-dimensional micropatterns, provided that they are immobilized onto a solid substrate one after the other. The first monolayer retains its lowest-energy hexagonal structure and acts as a template onto which the particles of the second monolayer are forced to rearrange. The frustration between the two lattices elicits symmetries that would not otherwise emerge if all the particles were assembled in a single step. Simply by varying the packing fraction of the two monolayers, we obtain not only low-coordinated structures such as rectangular and honeycomb lattices, but also rhomboidal, hexagonal and herringbone superlattices encoding non-regular tessellations. This is achieved without directional bonding, and the structures formed are equilibrium structures: molecular dynamics simulations show that these structures are thermodynamically stable and develop from short-range repulsive interactions, making them easy to predict, and thus suggesting avenues towards the rational design of complex micropatterns.