Monolayer Colloidal Crystals Research Articles

Investigation of the regulatory mechanism of monolayer colloidal crystals prepared by gas-liquid-solid three-phase interfacial self-assembly method

In recent years, the gas-liquid interface self-assembly method has emerged as a recognized and efficient technique for preparing two-dimensional monolayer colloidal crystals. However, challenges such as limited preparation area, dependence on complex equipment, and high costs persist in the conventional preparation process. This paper proposes a straightforward, rapid, and cost-effective method to implement a gas-liquid-solid three-phase interfacial self-assembly strategy. This approach enhances the gas-liquid interfacial self-assembly method, allowing for the preparation of large-area, high-quality single-layer colloidal crystals (>10 cm2) without using any surfactant. The gas-liquid-solid three-phase interfacial self-assembly process was analyzed in detail, and it was found that the main factors affecting the quality of the monolayer colloidal crystals were the volume ratio of PS colloidal solution to ethanol and the self-assembly temperature. It was shown that the optimal self-assembly conditions were achieved after dispersion by ultrasonic dispersion in a water bath at 32 °C–30 °C at a volume ratio of 1:2.5 of PS colloidal solution to ethanol. In addition, particles of various sizes can be prepared by this method to produce tightly arranged monolayer films that can be transferred to a variety of different substrates with great versatility. The demonstrated efficiency of this self-assembly method underscores its significant potential for application in large-scale manufacturing and practical industrialization.

Band edge induced plasmonic excitation in heterostructures consisting of rod hyperbolic metamaterials and colloidal photonic crystals

Heterostructures consisting of metal coated rod hyperbolic metamaterials (HMMs) and colloidal photonic crystals (PhCs) are proposed and fabricated. There are two different mechanisms for the reflection valleys induced from the excitation of plasmons in HMMs. Besides the common wave vector matching effects from colloidal gratings, band edge effects provide additional excitations in heterostructures. Slow light induced excitation is verified by separately modifying the photonic bandgap and grating parameters on HMMs using multilayer or monolayer colloidal crystals, 1D PhCs, or ellipsoid arrays, and by modifying the interval or metal thickness in heterostructures. Index-dependent sensitivity of the valleys is enhanced by the bandgap effect.

Fabrication of superamphiphobic surface with re-entrant structures via self-assembly colloidal template-assisted electrochemical deposition

Superamphiphobic surfaces with re-entrant structures have attracted widespread attention due to their superior water and oil resistance. However, current methods for fabricating superamphiphobic surfaces mostly rely on expensive equipment and cumbersome processes. This paper represents a facile and controlled preparation method for superamphiphobic surfaces with zinc oxide (ZnO) re-entrant structures using self-assembled polystyrene (PS) monolayer colloidal crystals (MCCs) as templates to assist electrochemical deposition of ZnO film. The prepared surface shows contact angles (CAs) larger than 150° and sliding angles smaller than 10° for water, glycerol, ethylene glycol (EG), and olive oil. The morphology and size of the re-entrant structures were modulated by the deposition potential and time, and the mechanism of the influence of the structures on the wetting properties was investigated. This superamphiphobic surface with re-entrant structures can be used as a surface-enhanced Raman spectroscopy (SERS) substrate for molecular detection with a detection limit of 10−10 M for rhodamine (R6G), benefiting from the enrichment effect of the superamphiphobic surface and the Schottky barrier at the Ag/ZnO contact interface. We hope that this preparation method for superamphiphobic surface has broad application prospects in the fields of self-cleaning, anti-icing, anti-fog, corrosion resistance, microfluidics, photocatalysis and fluid drag reduction.

Microdynamics of active particles in defect-rich colloidal crystals

Hypothesis. Because they are self-propulsive, active colloidal particles can interact with their environment in ways that differ from passive, Brownian particles. Here, we explore how interactions in different microstructural regions may contribute to colloidal crystal annealing.Experiments. We investigate active particles propagating in a quasi-2D colloidal crystal monolayer produced by alternating current electric fields (active-to-passive particle ratio ∼ 1:720). The active particle is a platinum Janus sphere propelled by asymmetric decomposition of hydrogen peroxide. Crystals are characterized for changes in void properties. The mean-squared-displacement of Janus particles are measured to determine how active microdynamics depend on the local microstructure, which is comprised of void regions, void-adjacent regions (defined as within three particle diameters of a void), and interstitial regions.Findings. At active particle energy EA = 2.55 kBT, the average void size increases as much as three times and the average void anisotropy increases about 40% relative to the passive case. The average microdynamical enhancement, <δ(t)>, of Janus particles in the crystal relative to an equivalent passive Janus particle is reduced compared to that of a free, active particle (<δ(t) > is 1.88 ± 0.04 and 2.66 ± 0.08, respectively). The concentration of active particles is enriched in void and void-adjacent regions. Active particles exhibit the greatest change in dynamics relative to the passive control in void-adjacent regions (<δ(t)> = 2.58 ± 0.06). The results support the conjecture that active particle microdynamical enhancement in crystal lattices is affected by local defect structure.

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.

Formation and manipulation of 2D colloidal crystals driven by convective currents and electrostatic forces

Colloidal crystal plays a major role in novel optical devices such as waveguides, photonic crystals, and colloidal crystal lasers, among others. It has been known that colloidal crystals may self-assemble due to optical, electric, or chemical potentials, and light-induced thermal effects, such as thermophoresis and convective currents. In this work, the formation and manipulation of a self-limiting assembly of monolayer colloidal crystals using a 20 nm absorbing thin film of titanium and a CW-laser emitting at 1070 nm is reported. The colloidal particles, dragged by thermally induced convective currents, self-assembling over the titanium thin film around the laser focal spot. The self-assembled monolayer colloidal crystal tends to form a circular shape. Using numerical simulation, we obtained a potential well associated with the convective currents that spatially govern the radial extension of the monolayer colloidal crystal. We also show, that by changing the pH of the solution i.e., by tuning the electrostatic interaction, the monolayer colloidal crystal can be formed or destroyed. In addition, it was possible to create a larger monolayer colloidal structure covering an area of 600 μm2 using an array of six laser spots.

Lead-Free Copper-Based Perovskite Nanonets for Deep Ultraviolet Photodetectors with High Stability and Better Performance.

Considering practical application and commercialization, the research of non-toxic and stable halide perovskite and its application in the field of photoelectric detection have received great attention. However, there are relatively few studies on deep ultraviolet photodetectors, and the perovskite films prepared by traditional spin-coating method have disadvantages such as uneven grain size and irregular agglomeration, which limit their device performance. Herein, uniform and ordered Cs3Cu2I5 nanonet arrays are fabricated based on monolayer colloidal crystal (MCC) templates prepared with 1 μm polystyrene (PS) spheres, which enhance light-harvesting ability. Furthermore, the performance of the lateral photodetector (PD) is significantly enhanced when using Cs3Cu2I5 nanonet compared to the pure Cs3Cu2I5 film. Under deep ultraviolet light, the Cs3Cu2I5 nanonet PD exhibits a high light responsivity of 1.66 AW−1 and a high detection up to 2.48 × 1012 Jones. Meanwhile, the unencapsulated PD has almost no response to light above 330 nm and shows remarkable stability. The above results prove that Cs3Cu2I5 nanonet can be a great potential light-absorbing layer for solar-blind deep ultraviolet PD, which can be used as light absorption layer of UV solar cell.

Recyclable SERS substrate: Optimised by reducing masking effect through colloidal lithography

Surface-enhanced Raman spectroscopy (SERS) has attracted much attention as it can deliver “fingerprint” type selectivity and detect organic compounds and other analytes down to single molecules. Conventional methods to produce reliable SERS sensors require high cost and control in the morphology and topology of the sensors. The lack of recyclability makes these high-cost sensors one-time use only. In this research, we have employed unconventional colloidal lithography techniques to develop a reliable and reproducible SERS sensor with self-cleaning capabilities. The sensor is developed via chemical vapour deposition of titania (TiO2) on 2D colloidal crystal monolayers to form Long-Range Ordered Crystals (LROCs) followed by controlled electroless deposition of silver nanoparticles. The fabricated surfaces are highly ordered and reproducible and the topology of the surface is uniform across the surface due to the seamless formation of TiO2/Ag LROCs. The developed sensors have shown self–cleaning properties from their TiO2 based layers and the formation of Schottky heterojunction between TiO2 sublayers and decorated Ag nanoparticles. Thus, the developed sensors show high sensitivity as well as recyclability enabling these sensors to perform multicycle measurements.

A set of multifunctional equipment for deposition of colloidal photonic crystal films and monolayers

It is impossible to obtain colloidal photonic crystal film of given properties without special equipment. In order to minimize the influence of external disturbing influences such devices should be automated. To obtain large-area or hetero structures it is rational to combine in one universal device equipment for the implementation of several depositional methods. In this paper the authors present two automated devices for both special and universal purposes. They can be used to obtain films and monolayers by vertical lifting of the substrate from the colloidal suspension, vertical deposition by pumping out the suspension, electrophoresis and Langmuir-Blodgett technique and its combinations.

Tension gradient-driven rapid self-assembly method of large-area colloidal crystal film and its application in multifunctional structural color displays

Liquid-air interface self-assembly (such as the classic L-B method) is the most popular way to form high-quality colloidal crystal monolayers, yet still limited by sophisticated equipment, time-consuming processes and small preparation area. Here, a tension gradient-driven nanoparticle self-assembly method is proposed to realize the high-efficiency and low-cost fabrication of large-area (25 × 18 cm2) colloidal crystal film. The periodic nanoparticle structures composed of various particles with different materials and diameters can be easily obtained in a short time and transferred to various substrates under this self-assembly method. And the preparation area is one to two orders of magnitude higher than the conventional self-assembly method. The simulation results align well with experiments, indicating that the Marangoni effect induced by tension gradient is the main driving force for self-assembly. The influence of various self-assembly parameters is investigated. Inspired by amber, a periodic particle/PDMS/nano-silica particles composite film is further designed to form a large-area durable structural color display. The structural color of the composite film is surprisingly different from that of the periodic particle which was mainly due to the unique semi-coated structure. The composite film has outstanding superhydrophobic and self-cleaning performance with a broad impact on durable structural color displays/sensors, anti-counterfeiting and other aspects.

Accelerated annealing of colloidal crystal monolayers by means of cyclically applied electric fields

External fields are commonly applied to accelerate colloidal crystallization; however, accelerated self-assembly kinetics can negatively impact the quality of crystal structures. We show that cyclically applied electric fields can produce high quality colloidal crystals by annealing local disorder. We find that the optimal off-duration for maximum annealing is approximately one-half of the characteristic melting half lifetime of the crystalline phase. Local six-fold bond orientational order grows more rapidly than global scattering peaks, indicating that local restructuring leads global annealing. Molecular dynamics simulations of cyclically activated systems show that the ratio of optimal off-duration for maximum annealing and crystal melting time is insensitive to particle interaction details. This research provides a quantitative relationship describing how the cyclic application of fields produces high quality colloidal crystals by cycling at the fundamental time scale for local defect rearrangements; such understanding of dynamics and kinetics can be applied for reconfigurable colloidal assembly.

Controllable preparation of silver nano-bowl coatings for suppressing secondary electron emission

Multipactor leads to the unrepairable damage to high-power microwave devices in the satellite. Restraining the secondary electron yield (SEY) is an effective method to suppress the multipactor of microwave devices. In this work, silver nano-bowl coating was fabricated via electrodeposition in AgNO3 electrolyte at room temperature on the surface of silver-plated aluminum alloy using the monolayer colloidal crystal of polystyrene (PS) spheres as templates. The scanning electron microscope images show that the surface structure of the silver-plated aluminum alloy sample could be modulated by altering the deposition time and adjusting the size of the PS spheres. And the pore diameter and the aspect ratio of the nano-bowl have a great influence on the SEY of the silver nano-bowl coating. When the size of the PS spheres is 2000 nm and the electrochemical deposition time is 90 min, the maximum value of SEY value of silver nano-bowl coating drastically decreases by 39.3% from 2.44 to 1.48, compared with that of the initial silver aluminum alloy samples. The silver nano-bowl coating shows great multipactor suppression for potential application in a satellite.

Polyhedral metal–organic framework monolayer colloidal crystals with sharpened and crystal facet-dependent selectivity for organic vapor sensing

Polyhedral metal–organic framework monolayer colloidal crystals demonstrate sharpened and crystal facet-dependent selectivity for organic vapors by eliminating grain boundaries and randomly exposed crystal surfaces.

Fabrication of floating colloidal crystal monolayers by convective deposition

HypothesisWell-defined two-dimensional colloidal crystal monolayers (CCM) have numerous applications, such as photonic crystal, sensors, and masks for colloidal lithography. Therefore, significant effort was devoted to the preparation of preparing CCM. However, the fabrication of CCM that can float in the continuous phase and readily transfer to other substrate remains an elusive challenge. ExperimentsIn this article a facile approach to prepare floating CCM from polymeric colloids as building blocks is reported. The key to obtain floating CCM is the selection of an appropriate solvent to release the formed CCM from the substrate. There are two steps involved in the preparation of floating CCM: formation and peeling off. FindingsFirst, colloids are dispersed in a solvent. Evaporation of this solvent results in the formation of a meniscus structure of the air–liquid interface between the colloids that are on the substrate. The deformation of the meniscus gives rise to capillary attraction, driving the colloids together in a dense monolayer. Once a crystallization nucleus is formed, a convective flow containing additional colloids sets in, resulting in the formation of CCM on the substrate. Second, the remaining bulk dispersion is replaced by an extracting solvent that wets the substrate and peels the formed CCM off. The influence of the several solvents, the substrate materials, and the types of colloids on the CCM formation are investigated systematically. The robustness of the approach facilitates the preparation of CCM. Furthermore, the floating feature of the CCM in principle makes transfer of the CCM to other substrates possible, which broadens its applications.

Interfacial Capillary‐Force‐Driven Self‐Assembly of Monolayer Colloidal Crystals for Supersensitive Plasmonic Sensors

Colloidal lithography technology based on monolayer colloidal crystals (MCCs) is considered as an outstanding candidate for fabricating large-area patterned functional nanostructures and devices. Although many efforts have been devoted to achieve various novel applicatons, the quality of MCCs, a key factor for the controllability and reproducibility of the patterned nanostructures, is often overlooked. In this work, an interfacial capillary-force-driven self-assembly strategy (ICFDS) is designed to realize a high-quality and highly-ordered hexagonal monolayer MCCs array by resorting the capillary effect of the interfacial water film at substrate surface as well as controlling the zeta potential of the polystyrene particles. Compared with the conventional self-assembly method, this approach can realize the reself-assembly process on the substrate surface with few colloidal aggregates, vacancy, and crystal boundary defects. Furthermore, various typical large-scale nanostructure arrays are achieved by combining reactive ion etching, metal-assisted chemical etching, and so forth. Specifically, benefiting from the as-fabricated high-quality 2D hexagonal colloidal crystals, the surface plasmon resonance (SPR) sensors achieve an excellent refractive index sensitivity value of 3497 nm RIU-1 , which is competent for detecting bovine serum albumin with an ultralow concentration of 10-8 m. This work opens a window to prepare high-quality MCCs for more potential applications.

Archimedean lattices emerge in template-directed eutectic solidification.

Template-directed assembly has been shown to yield a broad diversity of highly ordered mesostructures1,2, which in a few cases exhibit symmetries not present in the native material3-5. However, this technique has not yet been applied to eutectic materials, which underpin many modern technologies ranging from high-performance turbine blades to solder alloys. Here we use directional solidification of a simple AgCl-KCl lamellar eutectic material within a pillar template to show that interactions of the material with the template lead to the emergence of a set of microstructures that are distinct from the eutectic's native lamellar structure and the template's hexagonal lattice structure. By modifying the solidification rate of this material-template system, trefoil, quatrefoil, cinquefoil and hexafoil mesostructures with submicrometre-size features are realized. Phase-field simulations suggest that these mesostructures appear owing to constraints imposed on diffusion by the hexagonally arrayed pillar template. We note that the trefoil and hexafoil patterns resemble Archimedean honeycomb and square-hexagonal-dodecagonal lattices6, respectively. We also find that by using monolayer colloidal crystals as templates, a variety of eutectic mesostructures including trefoil and hexafoil are observed, the former resembling the Archimedean kagome lattice. Potential emerging applications for the structures provided by templated eutectics include non-reciprocal metasurfaces7, magnetic spin-ice systems8,9, and micro- and nano-lattices with enhanced mechanical properties10,11.

Bioinspired polydopamine coating as a versatile platform for synthesizing asymmetric Janus particles at an air-water interface

Janus particles with controlled asymmetries and functionalities represent promising building blocks and functional materials due to their unique anisotropic features. Herein, we show a facile and general approach towards preparing colloidal Janus particles with adjustable surface asymmetries and functionalities based on monolayer colloidal crystal (MCC) templates combined with mussel-inspired polydopamine (PDA) chemistry at an air-water interface. First, monodisperse colloidal polystyrene/polydopamine (PS/PDA) Janus particles with tunable asymmetrical geometries are synthesized by polymerizing dopamine onto two-dimensional (2D) polystyrene (PS) monolayer colloidal crystals formed at the air-water interface. Moreover, the good chemical reactivity of the PDA coating makes it a versatile platform to introduce a variety of functional materials, e.g., metal nanoparticles and organic molecules, producing colloidal Janus particles with adjustable functionalities. Finally, we demonstrate this synthetic strategy not only provides a controllable method for the fabrication of monodisperse colloidal Janus particles but also opens up a new Janus platform for artificially designed Janus particles with tunable asymmetries and functionalities.

Ordered porous BiVO4 based gas sensors with high selectivity and fast-response towards H2S

Seeking for novel materials that show pronounced gas sensing performance has been of great interests from both academic and technological perspectives. Herein, we report for the first time the gas sensing properties of monoclinic phase BiVO4 porous films with high selectivity and low detection limit towards H2S. The porous film with honeycomb-like morphology was in situ synthesized based on the solution dipping strategy using monolayer colloidal crystal as template. The BiVO4 porous film based sensor exhibits excellent sensing performance with high selectivity of 15.9 and the response time as fast as 5 s toward 25 ppm H2S gas at low working temperature (75 °C). The sensing detection limit can reach as low as 62.5 ppb. Moreover, the general feature of the colloidal crystal template approach allows for the accessing of porous BiVO4 films based gas sensors with tunable sensing performance. Based on a five-axe spider-web diagram which could evaluate the optimal practical work condition including sensitivity, selectivity, operating temperature and response/recovery rate, the porous film with 200 nm pore size demonstrates the best potential to keep the good balance of sensing performance. These results suggest that BiVO4, which is a well-known photocatalyst, can be potentially used in high performance gas sensing.

Importance of solvent singularity on the formation of highly uniform hexagonal close packed (HCP) colloidal monolayers during spin coating

Highly uniform hexagonal close packed (HCP) colloidal monolayers are extremely essential for the development of photonic, optical, and biosensing devices. Even though high quality HCP colloidal structures can be obtained with vertical deposition (VD), this traditional method is time-consuming and not scalable. Spin coating (SC) has recently been used as an alternative method due to its high production rate, scalability and versatility. However, a few attentions have been paid on the key process parameters that affect the packing quality of the colloidal crystal monolayers. This work therefore investigated how the choice of solvent system, rotational speed, and weight fraction of colloidal particles affect the arrangement of the spin-coated HCP monolayer films. Radial distribution function (RDF) analysis was employed to determine the uniformity of particle deposition. The two sample Kolmogorov-Smirnov statistical testing was then used to compare the spin-coated HCP structures to those obtained from the VD method.

Ultrathin Photonic Polymer Gel Films Templated by Non-Close-Packed Monolayer Colloidal Crystals to Enhance Colorimetric Sensing.

Responsive polymer-based sensors have attracted considerable attention due to their ability to detect the presence of analytes and convert the detected signal into a physical and/or chemical change. High responsiveness, fast response speed, good linearity, strong stability, and small hysteresis are ideal, but to gain these properties at the same time remains challenging. This paper presents a facile and efficient method to improve the photonic sensing properties of polymeric gels by using non-close-packed monolayer colloidal crystals (ncp MCCs) as the template. Poly-(2-vinyl pyridine) (P2VP), a weak electrolyte, was selected to form the pH-responsive gel material, which was deposited onto ncp MCCs obtained by controlled O2 plasma etching of close-packed (cp) MCCs. The resultant ultrathin photonic polymer gel film (UPPGF) exhibited significant improvement in responsiveness and linearity towards pH sensing compared to those prepared using cp MCCs template, achieving fast visualized monitoring of pH changes with excellent cyclic stability and small hysteresis loop. The responsiveness and linearity were found to depend on the volume and filling fraction of the polymer gel. Based on a simple geometric model, we established that the volume increased first and then decreased with the decrease of template size, but the filling fraction increased all the time, which was verified by microscopy observations. Therefore, the responsiveness and linearity of UPPGF to pH can be improved by simply adjusting the etching time of oxygen plasma. The well-designed UPPGF is reliable for visualized monitoring of analytes and their concentrations, and can easily be combined in sensor arrays for more accurate detection.