Two-dimensional Colloidal Crystals Research Articles

Regulating two-dimensional colloidal crystal assembly through contactless acoustic annealing

Two-dimensional colloidal crystals assembled from polystyrene nanospheres have emerged as a pivotal foundation for fabricating large-area nano-functional surfaces. These assemblies, defined by their hexagonal close-packed configuration and interlaced with grain boundaries, have garnered significant attention for applications in plasmonic structures, catalysts, photonic crystals, and inverse opals. Nonetheless, achieving consistent large-scale regularity has proven challenging due to unpredictable crystal growth and the introduction of defects. Utilizing acoustic waves excited from the airside, our experiments demonstrate the significant effects of such waves on the self-assembly process, leading to larger crystal domains and reduced defects. In comparison to the extensively studied water-end excitation techniques, our air-end excitation method introduces a novel dynamic in regulating colloidal monolayer crystallization and presents a comprehensive analysis of varying acoustic parameters, frequency, amplitude, and waveform. These findings reveal the potential of airside acoustic annealing in refining the structure of two-dimensional colloidal arrays. To elucidate our experimental observations, we delve into the theoretical underpinnings of particle dynamics, driven by classical hydromechanical constraints like surface tension and gravity. Using a qualitative estimate, we shed light on the resonant excitations and their potential role in optimizing the self-assembly process, especially focusing on resonances pertinent for enhancing cluster enlargements. Conclusively, our research, steeped in robust theoretical frameworks and groundbreaking experimental techniques, offers a multifaceted solution for perfecting two-dimensional colloidal arrays. This combined approach not only broadens the scope of acoustically induced crystallization but also charts a path for its adoption across diverse environments, signaling transformative prospects for nanomanufacturing and optical research.

Formation of two-dimensional diamond-like colloidal crystals using layer-by-layer electrostatic self-assembly.

We report here that a two-dimensional (2D) diamond-like structure of micron-sized colloidal particles can be obtained by layer-by-layer self-assembly. Positively and negatively charged silica particles, 1 μm in diameter, were used in the experiments. On a positively charged, flat glass substrate, the first layer of negatively charged particles was prepared to form a non-close-packed 2D crystal. Then the second and third layers were fabricated using electrostatic adsorption. The positions of adsorbed particles were controllable by tuning the zeta-potential of the particles and the salt concentration of the medium. The FDTD calculations show that the 2D diamond structures of particles with higher refractive index (titania) have an absorption band in the wavelength range corresponding to the photonic band gap of the 3D bulk crystal. We expect these findings to be useful for the fabrication of novel photonic materials.

Field-Pulse-Induced Annealing of 2D Colloidal Polycrystals.

Two-dimensional colloidal crystals are of considerable fundamental and practical importance. However, their quality is often low due to the widespread presence of domain walls and defects. In this work, we explored the annealing process undergone by monolayers of superparamagnetic colloids adsorbed onto fluid interfaces in the presence of magnetic field pulses. These systems present the extraordinary peculiarity that both the extent and the character of interparticle interactions can be adjusted at will by simply varying the strength and orientation of the applied field so that the application of field pulses results in a sudden input of energy. Specifically, we have studied the effect of polycrystal size, pulse duration, slope and frequency on the efficiency of the annealing process and found that (i) this strategy is only effective when the polycrystal consists of less than approximately 10 domains; (ii) that the pulse duration should be of the order of magnitude of the time required for the outer particles to travel one diameter during the heating step; (iii) that the quality of larger polycrystals can be slightly improved by applying tilted pulses. The experimental results were corroborated by Brownian dynamics simulations.

Precision nanoengineering for functional self-assemblies across length scales.

As nanotechnology continues to push the boundaries across disciplines, there is an increasing need for engineering nanomaterials with atomic-level precision for self-assembly across length scales, i.e., from the nanoscale to the macroscale. Although molecular self-assembly allows atomic precision, extending it beyond certain length scales presents a challenge. Therefore, the attention has turned to size and shape-controlled metal nanoparticles as building blocks for multifunctional colloidal self-assemblies. However, traditionally, metal nanoparticles suffer from polydispersity, uncontrolled aggregation, and inhomogeneous ligand distribution, resulting in heterogeneous end products. In this feature article, I will discuss how virus capsids provide clues for designing subunit-based, precise, efficient, and error-free self-assembly of colloidal molecules. The atomically precise nanoscale proteinic subunits of capsids display rigidity (conformational and structural) and patchy distribution of interacting sites. Recent experimental evidence suggests that atomically precise noble metal nanoclusters display an anisotropic distribution of ligands and patchy ligand bundles. This enables symmetry breaking, consequently offering a facile route for two-dimensional colloidal crystals, bilayers, and elastic monolayer membranes. Furthermore, inter-nanocluster interactions mediated via the ligand functional groups are versatile, offering routes for discrete supracolloidal capsids, composite cages, toroids, and macroscopic hierarchically porous frameworks. Therefore, engineered nanoparticles with atomically precise structures have the potential to overcome the limitations of molecular self-assembly and large colloidal particles. Self-assembly allows the emergence of new optical properties, mechanical strength, photothermal stability, catalytic efficiency, quantum yield, and biological properties. The self-assembled structures allow reproducible optoelectronic properties, mechanical performance, and accurate sensing. More importantly, the intrinsic properties of individual nanoclusters are retained across length scales. The atomically precise nanoparticles offer enormous potential for next-generation functional materials, optoelectronics, precision sensors, and photonic devices.

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.

Graphene Oxide Liquid Crystals Aligned in a Confined Configuration: Implications for Optical Materials

Orientation control of two-dimensional (2D) colloidal liquid crystals in both microscopic and macroscopic scales holds massive promise for improved multifield applications. In this respect, graphene oxide (GO) is used as a platform substance for 2D carbon-based lyotropic liquid crystal (LC) materials. Here, we design the directed alignment of aqueous GO-LC dispersion confined in a rectangular capillary tube. The isotropic GO dispersion is loaded into a horizontal capillary and subsequently concentrated to the LC phase by slow water evaporation at open ends of the tube. An obtained surprising result, GO layers organize normal to the inner surfaces of the capillary with a highly ordered orientation of the optic axis over tens of millimeters. Such transverse orientation of GO layers originates from the meniscus region and then moves toward the center, caused by the interfacial effect and the sequential isotropic-to-nematic transition. This aligned mesomorphic system is characterized via optical retardation, nematic order parameter, absorption, and modified Cauchy’s transmission equation about the dependency of both birefringence and wavelength with concentration. Finally, the aligned GO-LC layers are preserved in a polymer composite matrix by photopolymerizing the evaporation-aligned GO/acrylamide-LC dispersion confined in a capillary. This composite shows higher thermal stability than polyacrylamide until 500 °C of about 6.1%. Our experimental findings provide efficient advantages for controlling the orientation of GO-LC and 2D materials, beneficial for diverse optical applications due to their directional ordering.

Two-dimensional binary colloidal crystals formed by particles with two different sizes

The formation of mathrm {AB_2} type two-dimensional binary colloidal crystals was studied by performing Monte Carlo simulations with two different size particles. The effect of interactions between particles and between particles and a wall, and the particles size ratios on the formation of mathrm {AB_2} structure were examined. mathrm {AB_2} structures formed efficiently when the interaction between equivalently sized particles was smaller than that between differently sized particles. To create mathrm {AB_2} on a wall, it was necessary to choose a suitable particles size ratios, and the attraction between the particles and the wall was greater than that between particles.

Grain boundary dynamics driven by magnetically induced circulation at the void interface of 2D colloidal crystals.

The complexity of shear-induced grain boundary dynamics has been historically difficult to view at the atomic scale. Meanwhile, two-dimensional (2D) colloidal crystals have gained prominence as model systems to easily explore grain boundary dynamics at single-particle resolution but have fallen short at exploring these dynamics under shear. Here, we demonstrate how an inherent interfacial shear in 2D colloidal crystals drives microstructural evolution. By assembling paramagnetic particles into polycrystalline sheets using a rotating magnetic field, we generate a particle circulation at the interface of particle-free voids. This circulation shears the crystalline bulk, operating as both a source and sink for grain boundaries. Furthermore, we show that the Read-Shockley theory for hard-condensed matter predicts the misorientation angle and energy of shear-induced low-angle grain boundaries based on their regular defect spacing. Model systems containing shear provide an ideal platform to elucidate shear-induced grain boundary dynamics for use in engineering improved/advanced materials.

Surface Plasmon Resonance of Two-Dimensional Gold Colloidal Crystals Formed on Gold Plates.

The free electrons inside precious metals such as Au vibrate when the surface of the metal is irradiated with an electromagnetic wave of an appropriate frequency. This oscillation is referred to as surface plasmon resonance (SPR), and the resonance frequency varies with permittivity of the medium around the metal. SPR sensors are widely applied in the fields of bioscience and pharmaceutical sciences, including biosensing for drug discovery, biomarker screening, virus detection, and testing for food safety. Here, we fabricated a metal-insulator-metal (MIM) SPR sensor by constructing two-dimensional (2D) regular array of Au colloidal particles (2D colloidal crystals) on an insulator layer over a thin Au film coated on a glass substrate surface. The 2D crystals were fabricated by electrostatically adsorbing negatively charged three-dimensional crystals onto a positively charged thin insulator formed on Au film. The plasmon peaks/dips from the MIM structure were measured in aqueous solutions of ethylene glycol (EG) at various concentrations. Multiple plasmon peaks/dips were observed due to the localized SPR (LSPR) of the Au particles and the Fano resonance between the Au particles and thin film. The plasmon peaks/dips shifted to higher wavelengths on increasing EG concentrations due to an increase in the refractive index of the media. The observed peak/dip shift was approximately twice that of LSPR from an isolated Au particle. We expect the present MIM substrate will be useful as a highly sensitive sensor in the pharmaceutical field.

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.

Bio-inspired Janus structural color films as visually flexible electronics

Multifunctional composite flexible sensors are essential components in flexible and stretchable electronics. In this paper, we present novel mussel-inspired Janus structural color films as visually flexible electronics. The Janus composite film with a three-layer structure, which is compozed of a conductive carbon nanotube (CNT) layer, a supporting polydimethylsiloxane (PDMS) layer and a structural color layer formed by two-dimensional colloidal crystals (2D-CCs), is prepared by a hierarchical assembly strategy. The manufactured multi-layer CNT films and the 2D-CCs are integrated on both sides of the supporting PDMS layer, respectively by the self-assembly process and the adhesion of polydopamine (PDA) to provide corresponding excellent conductivity, flexibility and visual optical sensing for the Janus composite film. It is demonstrated that the CNT films could not only increaze the contrast of structural color of the 2D-CC array as their light absorption characteristics in the broadband frequency field, but also impart the composite film with photothermal response characteristics. In addition, because of the outstanding electrical properties, visualized structural color and photothermal response, the resulting Janus structural color films show stable electrical sensing and visualized color-sensing under deformations arose by human motions and near-infrared (NIR) illumination, which could play essential roles in flexible and stretchable electronics.

Active Micromixer of Microfluids via Plasmonic Marangoni Convection

Abstract Chemical reactions can be efficiently carried out in microfluidic devices using a small amount of solution, meaning that fewer resources are required and less liquid waste is generated, making them energy-efficient and eco-friendly devices. However, the microchannel of the device has a narrow channel width, so the reaction solution exhibits laminar flow. Although this laminar flow is good for transporting the solution, efficiently conducting a chemical reaction is difficult in such a flow. Therefore, the aim of the present study is to produce a micromixer in which diffusion can be easily controlled in the microchannel. In this work, efficient stirring of the fluid was achieved only under laser irradiation by utilising the local light-to-heat conversion that occurs on plasmonic metal nanostructures placed in the microchannel. The gold nanostructure that covers a two-dimensional colloidal crystal, which was easily manufactured at low cost, has a high temperature of 200 °C or more only when irradiated with a laser, and the solution is vapourised and forms bubbles with a diameter of approximately 10 µm. The temperature difference on the bubble surface causes Marangoni convection, which can be used to efficiently stir the reaction solution in the assembled microfluidic device.

Symmetry-Dependent Kinetics of Dislocation ReactionSupported by the National Natural Science Foundation of China (Grant No. 11674235, 11635002, and 11974255).

Reactions between dislocations are investigated in two-dimensional colloidal crystals. It is found that, because of the conservation of total Burgers vectors, the kinetics of the reaction is dependent on the the symmetry of the crystal lattice. Merging is possible only when the total Burgers vector of the reacting dislocations is in line with existing crystal lines. In non-merging reactions, the number of dislocations cannot be reduced but the interacting dislocations can exchange their Burgers vectors and migrate to different gliding lines. The changing of gliding lines promises additional annihilation in multi-dislocation reactions. The bonding of non-merging dislocations determines the configuration and the orientation of the grain boundaries. The findings in this study may shed new light on understanding of dislocations and have potential applications in fabrication of crystalline materials.

Fabrications of Three- and Two-dimensional Colloidal Crystals

Fabrications of Three- and Two-dimensional Colloidal Crystals

Mechanical properties of colloidal crystals at fluid interfaces

We characterise the local mechanical properties of two-dimensional colloidal crystals with hexagonal symmetry assembled at the flat interface between oil and water. Our experiments elucidate the conditions under which the material behaves isotropically, as opposed to those where the microstructure plays a major role. Brownian fluctuations are used to extract the stiffness of the lattice under the continuum approximation, whereas at larger displacements, obtained by optically driving one particle through the structure, the mechanical resistance of the lattice depends on both the area fraction and the direction of the applied force. Remarkably, the minimum resistance does not necessarily correspond to a probe being driven between neighbours, i.e. at 30° with respect to the crystal axes.

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.

Fabrication of two-dimensional silica colloidal crystals via a gravity-assisted confined self-assembly method

• Silica particles are transported to the crystallizing region by gravity instead of solvent flux. • The particles settled on the substrate can be spread out by gas-liquid interface to form 2D colloidal crystals. • The sedimentation of silica particles has limited effect on the self-assembly process. • This method is applicable to silica particles with a size of hundreds of nanometers to several micrometers.

Dynamical processes of interstitial diffusion in a two-dimensional colloidal crystal

In two-dimensional (2D) solids, point defects, i.e., vacancies and interstitials, are bound states of topological defects of edge dislocations and disclinations. They are expected to play an important role in the thermodynamics of the system. Yet very little is known about the detailed dynamical processes of these defects. Two-dimensional colloidal crystals of submicrometer microspheres provide a convenient model solid system in which the microscopic dynamics of these defects can be studied in real time using video microscopy. Here we report a study of the dynamical processes of interstitials in a 2D colloidal crystal. The diffusion constants of both mono- and diinterstitials are measured and found to be significantly larger than those of vacancies. Diinterstitials are clearly slower than monointerstitials. We found that, by plotting the accumulative positions of five- and sevenfold disclinations relative to the center-of-mass position of the defect, a sixfold symmetric pattern emerges for monointerstitials. This is indicative of an equilibrium behavior that satisfies local detailed balance that the lattice remains elastic and can be thermally excited between lattice configurations reversibly. However, for diinterstitials the sixfold symmetry is not observed in the same time window, and the local lattice distortions are too severe to recover quickly. This observation suggests a possible route to creating local melting of a lattice (similarly one can create local melting by creating divacancies). This work opens up an avenue for microscopic studies of the dynamics of melting in colloidal model systems.

Multiband Hypersound Filtering in Two-Dimensional Colloidal Crystals: Adhesion, Resonances, and Periodicity.

The hypersonic phonon propagation in large-area two-dimensional colloidal crystals is probed by spontaneous micro Brillouin light scattering. The dispersion relation of thermally populated Lamb waves reveals multiband filtering due to three distinct types of acoustic band gaps. We find Bragg gaps accompanied by two types of hybridization gaps in both sub- and superwavelength regimes resulting from contact-based resonances and nanoparticle eigenmodes, respectively. The operating GHz frequencies can be tuned by particle size and depend on the adhesion at the contact interfaces. The experimental dispersion relations are well represented by a finite element method model enabling identification of observed modes. The presented approach also allows for contactless study of the contact stiffness of submicrometer particles, which reveals size effect deviating from macroscopic predictions.

Anti-Counterfeiting Layer of 2D Colloidal Crystal Based Photonic Material

A facile approach of robust polymer-based two-dimensional (2D) colloidal crystal (CC) layers was presented. This technology enables the convenient fabrication of an anti-counterfeiting coating with a polymeric 2D CC, allowing the fast preparation of functional polymer photonic materials. Briefly, a 2D CC was prepared by self-assembly of polystyrene (PS) submicrospheres, which was then transferred to substrates by adhesive polymeric solution and cured to form a photonic film. Such photonic films strongly and angle dependently diffract visible light, and the high transparency of the photonic layers ensured the readout from the substrate. The PC layers can also prevent the re-write or re-print on the substrate, indicating the potential applications in colorful and anti-counterfeiting coating materials.