Gravity-assisted convective assembly of centimeter-sized uniform two-dimensional colloidal crystals

Super-resolution imaging based on cascaded microsphere compound lenses

In this paper, a cascaded microsphere compound lens (CMCL) is introduced, in which a 20-µm-diameter barium titanate glass (BTG) primary microsphere and a 250-nm-diameter or 200-nm-diameter polystyrene (PS) secondary microsphere array constitute CMCL1 and CMCL2, respectively. The field of view (FOV) depends on the size of the BTG microsphere, while the waist of the photon nanojet (PNJ) can be adjusted by the size of the PS microsphere. The narrower the waist of the PNJ, the higher the imaging resolution. In the experiment, a 200-nm-diameter hexagonally close-packed PS nanoparticle array is successfully observed by the CMCL with a high magnification of  ∼11.6× and a FOV of  ∼14µm, while the single BTG microsphere is incapable of observing the array. The point spread function is used to quantify the resolution of the CMCL. A well-designed CMCL can improve the imaging performances of a microsphere-assisted microscope.

Submicron Nonporous Silica Particles for Enhanced Separation Performance in pCEC

Applications of submicron-scale particles are of rising interest in separation science due to their favorable surface-to-volume ratio and their fabrication of highly ordered structures. The uniformly dense packing beds in columns assembled from nanoparticles combined with an electroosmotic flow-driven system has great potential in a highly efficient separation system. Here, we packed capillary columns using a gravity method with synthesized nanoscale C18-SiO₂ particles having diameters of 300-900 nm. The separation of small molecules and proteins was evaluated in the packed columns on a pressurized capillary electrochromatography platform. The run-to-run reproducibility regarding retention time and peak area for the PAHs using a column packed with 300 nm C18-SiO₂ particles were less than 1.61% and 3.17%, respectively. Our study exhibited a systematic separation analysis of small molecules and proteins based on the columns packed with submicron particles combined with the pressurized capillary electrochromatography (pCEC) platform. This study may provide a promising analytical approach with extraordinary column efficiency, resolution, and speed for the separation of complex samples.

Parallelization of Microfluidic Droplet Junctions for Ultraviscous Fluids

The parallelization of multiple microfluidic droplet junctions has been successfully achieved so that the production throughput of the uniform microemulsions/particles has witnessed considerable progress. However, these advancements have been observed only in the case of a low viscous fluid (viscosity of 10^-2 -10^-3 Pa s). This study designs and fabricates a microfluidic device, enabling a uniform micro-emulsification of an ultraviscous fluid (viscosity of 3.5 Pa s) with a throughput of ≈330 000 droplets per hour. Multiple T-junctions of a dispersed oil phase, split from a single inlet, are connected into the single post-crossflow channel of a continuous water phase. In the proposed device, the continuous water phase undergoes a series circuit, wherein the resistances are continuously accumulated. The independent corrugations of the dispersed oil phase channel, under the theoretical guidance, compromise such increased resistances; the ratio of water to oil flow rates at each junction becomes consistent across T-junctions. Owing to the design being based on a fully 2D interconnection, single-step soft lithography is sufficient for developing the full device. This easy-to-craft architecture contrasts with the previous approach, wherein complicated 3D interconnections of the multiple junctions are involved, thereby facilitating the rapid uptake of high throughput droplet microfluidics for experts and newcomers alike.

Heteroepitaxial fabrication of binary colloidal crystals by a balance of interparticle interaction and lattice spacing

HYPOTHESIS

The colloidal epitaxy utilizing a patterned substrate is used to fabricate colloidal crystals of the same structure and lattice spacing with the substrate, which is an effective technique for creating desired nanoscale architectures. However, this technique has been mainly limited to a single-component system. The colloidal epitaxy is versatile if multicomponent colloidal crystals can be produced, which is inspired by our previous study regarding binary colloidal crystals (b-CCs) fabricated at the edge of single-component crystals.

EXPERIMENTS

We have examined various particle size combinations of binary colloidal mixture and substrates for heteroepitaxial growth of b-CCs. Colloidal crystallization was achieved through depletion attraction induced by added polymers.

FINDINGS

We demonstrated heteroepitaxial growth of b-CCs on the foreign colloidal crystals as the substrate. Under depletion attraction, deviation from equilibrium interparticle distance because of lattice mismatch between the substrate and epitaxial layers induces strain energy among the particles, yielding the b-CCs to attain minimum strain energy. Various types of b-CCs are created by adjusting the particle size ratio and polymer concentration. The heteroepitaxial growth technique enables the fabrication of complex multicomponent colloidal crystals that greatly facilitate versatile applications of the colloidal crystals.

A versatile technology for colloidal crystal transfer using parylene coatings and hydrosoluble polymers

We propose a novel versatile colloidal crystal transfer technique compatible with a wide range of water-insoluble substrates regardless of their size, material, and wettability. There are no inherent limitations on colloidal particles material and size. The method possibilities are demonstrated via the colloidal transfer on quartz, glass substrates with a flat and curved surface, and via the fabrication of 3D colloidal structure with 5 overlaid colloidal monolayers. The process occurs at a room temperature in water and is independent from the illumination conditions, which makes it ideal for experimental manipulations with sensitive functional substrates. We performed the nanosphere photolithography process on a photosensitive substrate with a transferred colloidal monolayer. The metallized hexagonal arrays of nanopores demonstrated a clear resonant plasmonic behavior. We believe that due to its high integration possibilities the proposed transfer technique will find applications in a large-area surface nanotexturing, plasmonics, and will speed up a device fabrication process.

Recent Advances in Binary Colloidal Crystals for Photonics and Porous Material Fabrication

In the past few years, binary colloidal crystals (BCCs) composed of both large and small particles have attracted considerable attention from the scientific community as an exciting alternative to single colloidal crystals (SCCs). In particular, more complex structures with diverse nanotopographies and desirable optical properties of BCCs can be obtained by various colloidal assembly methods, as compared to SCCs. Furthermore, high accuracy in crystal growth with controllable stoichiometries allows for a great deal of promising applications in the fields of both interfacial and material sciences. The visible-light diffraction property of BCCs is more superior than that of SCCs, which makes them have more promising applications in the fabrication of photonic crystals with full band gaps. On the other hand, their spherical shapes and ease of removal property make them ideal templates for ordered porous material fabrication. Hence, this perspective outlined recent advances in assembly approaches of BCCs, with an emphasis on their promising applications for advanced photonics and multifunctional porous material fabrication. Eventually, some challenging yet important issues and some future perspectives are further discussed.

Improvement of the Centrifugal Force in Gravity Driven Method for the Fabrication of Highly Ordered and Submillimeter-Thick Colloidal Crystal

In this paper, we propose a modified gravity method by introducing centrifugal force to promote the stacking of silica particles and the order of formed colloidal crystals. In this method, a monodispersed silica colloidal solution is filled into empty cells and placed onto rotation arms that are designed to apply an external centrifugal force to the filled silica solution. When sample fabrication is in progress, silica particles are forced toward the edges of the cells. The number of defects in the colloidal crystal decreases and the structural order increases during this process. The highest reflectivity and structural order of a sample was obtained when the external centrifugal force was 18 G. Compared to the samples prepared using the conventional stacking method, samples fabricated with centrifugal force possess higher reflectivity and structural order. The reflectivity increases from 68% to 90%, with an increase in centrifugal force from 0 to 18 G.

Nano-substructured plasmonic pore arrays: a robust, low cost route to reproducible hierarchical structures extended across macroscopic dimensions

Plasmonic nanostructures are important across diverse applications from sensing to renewable energy. Periodic porous array structures are particularly attractive because such topography offers a means to encapsulate or capture solution phase species and combines both propagating and localised plasmonic modes offering versatile addressability. However, in analytical spectroscopic applications, periodic pore arrays have typically reported weaker plasmonic signal enhancement compared to particulate structures. This may be addressed by introducing additional nano-structuring into the array to promote plasmonic coupling that promotes electric field-enhancement, whilst retaining pore structure. Introducing nanoparticle structures into the pores is a useful means to promote such coupling. However, current approaches rely on either expensive top-down methods or on bottom-up methods that yield random particle placement and distribution. This report describes a low cost, top-down technique for preparation of nano-sub-structured plasmonic pore arrays in a highly reproducible manner that can be applied to build arrays extending over macroscopic areas of mm2 to cm2. The method exploits oxygen plasma etching, under controlled conditions, of the cavity encapsulated templating polystyrene (PS) spheres used to create the periodic array. Subsequent metal deposition leads to reproducible nano-structuring within the wells of the pore array, coined in-cavity nanoparticles (icNPs). This approach was demonstrated across periodic arrays with pore/sphere diameters ranging from 500 nm to 3 μm and reliably improved the plasmonic properties of the substrate across all array dimensions compared to analogous periodic arrays without the nano-structuring. The enhancement factors achieved for metal enhanced emission and surface enhanced Raman spectroscopy depended on the substrate dimensions, with the best performance achieved for nanostructured 2 μm diameter pore arrays, where a more than 104 improvement over Surface Enhanced Raman Spectroscopy (SERS) and 200-fold improvement over Metal Enhanced Fluorescence (MEF) were observed for these substrates compared with analogous unmodified pore arrays. The experiments were supported by Finite-Difference Time-Domain (FDTD) calculations used to simulate the electric field distribution as a function of pore nano-structuring.

Recent advances in self-assemblies and sensing applications of colloidal photonic crystals

Colloidal photonic crystals (PCs), consisting of highly ordered monodisperse nanoparticles, have been carried out a great deal of research in recent decades due to the attributes of readable signal, easy modification and low cost. With these unique features, colloidal PCs have also gradually become a focus of candidates applied in sensing fields. In this review, an overview of recent advances in colloidal PCs including self-assemblies and sensing applications is illustrated. With respect to the development in self-assemblies of colloidal PCs, the review concentrates on the summary of responsive mechanisms, detection methods, responsive materials, unit cells and fabrication methods. In terms of advances in sensing application of colloidal PCs, various types of sensors are summarized based on the kinds and applications of target analytes. Furthermore, the current limitations and potential future directions of colloidal PCs in self-assemblies and sensing areas are also discussed.

Binary colloidal crystals (BCCs): Interactions, fabrication, and applications

The organization of matter into hierarchical structures is a fundamental characteristic of functional materials and living organisms. Binary colloidal crystal (BCC) systems present a diversified range of nanotopographic structures where large and small colloidal particles simultaneously self-assemble into either 2D monolayer or 3D hierarchical crystal lattices. More importantly, understanding how BCCs form opens up the possibility to fabricate more complex systems such as ternary or quaternary colloidal crystals. Monolayer BCCs can also offer the possibility to achieve surface micro- and nano-topographies with heterogeneous chemistries, which can be challenging to achieve with other traditional fabrication tools. A number of fabrication methods have been reported that enable generation of BCC structures offering high accuracy in growth with controllable stoichiometries; however, it is still a challenge to make uniform BCC structures over large surface areas. Therefore, fully understand the mechanism of binary colloidal self-assembly is crucial and new/combinational methods are needed. In this review, we summarize the recent advances in BCC fabrication using particles made of different materials, shapes, and dispersion medium. Depending on the potential application, the degree of order and efficiency of crystal formation has to be determined in order to induce variability in the intended lattice structures. The mechanisms involved in the formation of highly ordered lattice structures from binary colloidal suspensions and applications are discussed. The generation of BCCs can be controlled by manipulation of their extensive phase behavior, which facilitates a wide range potential applications in the fields of both material and biointerfacial sciences including photonics, biosensors, chromatography, antifouling surfaces, biomedical devices, and cell culture tools.

Super-resolution imaging properties of cascaded microsphere lenses

In this paper, the imaging properties of a cascaded microsphere lens are studied. The cascaded microsphere lens consists of two lenses. A hexagonally close-packed 960-nm-diameter array of polystyrene microspheres is used as the first lens. The second lens is a single silica microsphere with a diameter of about 5 μm. A Blu-ray disc is observed by both the cascaded microsphere lens and the single silica microsphere. Studies reveal that the magnification of the cascaded microsphere lens is about 1.4 times greater than that of the single silica microsphere, while the field of view of the cascaded microsphere lens, which is close to the diameter of the polystyrene microsphere, is decreased. The focal position of the cascaded lens microsphere must be close to the sample in order to observe it.

Large-Area Self-Assembly of Silica Microspheres/Nanospheres by Temperature-Assisted Dip-Coating

This work reports a temperature-assisted dip-coating method for self-assembly of silica (SiO₂) microspheres/nanospheres (SPs) as monolayers over large areas (∼cm²). The area over which self-assembled monolayers (SAMs) are formed can be controlled by tuning the suspension temperature (T_s), which allows precise control over the meniscus shape. Furthermore, the formation of periodic stripes of SAMs, with excellent dimensional control (stripe width and stripe-to-stripe spacing), is demonstrated using a suitable set of dip-coating parameters. These findings establish the role of T_s, and other parameters such as withdrawal speed (V_w), withdrawal angle (θ_w), and withdrawal step length (L_w). For T_s ranged between 25 and 80 °C, the morphological analysis of dip-coatings shows layered structures comprising of defective layers (25-60 °C), single layers (70 °C), and multilayers (>70 °C) owing to the variation of SP flux at the meniscus/substrate assembling interface. At T_s = 70 °C, there is an optimum V_w, approximately equal to the downshift speed of the meniscus (V_m = 1.3 μm/s), which allows the SAM formation over areas (2.25 cm²) roughly 10 times larger than reported in the literature using nanospheres. Finally, the large-area SAM is used to demonstrate the enhanced performance of antireflective coatings for photovoltaic cells and to create metal nanomesh for Si nanowire synthesis.

Driving Chemical Reactions in Plasmonic Nanogaps with Electrohydrodynamic Flow

Nanoparticles from colloidal solution-with controlled composition, size, and shape-serve as excellent building blocks for plasmonic devices and metasurfaces. However, understanding hierarchical driving forces affecting the geometry of oligomers and interparticle gap spacings is still needed to fabricate high-density architectures over large areas. Here, electrohydrodynamic (EHD) flow is used as a long-range driving force to enable carbodiimide cross-linking between nanospheres and produces oligomers exhibiting sub-nanometer gap spacing over mm² areas. Anhydride linkers between nanospheres are observed via surface-enhanced Raman scattering (SERS) spectroscopy. The anhydride linkers are cleavable via nucleophilic substitution and enable placement of nucleophilic molecules in electromagnetic hotspots. Atomistic simulations elucidate that the transient attractive force provided by EHD flow is needed to provide a sufficient residence time for anhydride cross-linking to overcome slow reaction kinetics. This synergistic analysis shows assembly involves an interplay between long-range driving forces increasing nanoparticle-nanoparticle interactions and probability that ligands are in proximity to overcome activation energy barriers associated with short-range chemical reactions. Absorption spectroscopy and electromagnetic full-wave simulations show that variations in nanogap spacing have a greater influence on optical response than variations in close-packed oligomer geometry. The EHD flow-anhydride cross-linking assembly method enables close-packed oligomers with uniform gap spacings that produce uniform SERS enhancement factors. These results demonstrate the efficacy of colloidal driving forces to selectively enable chemical reactions leading to future assembly platforms for large-area nanodevices.

Influence of the photonic nanojet of microspheres on microsphere imaging

The imaging properties of BaTiO₃ glass (BTG) microspheres in the diameter range of 5-50 µm which are fully immersed in a polydimethylsiloxane layer are experimentally studied. Our experimental results show that for both Blu-ray disc samples and the single-layer hexagonally close-packed microsphere array samples, with the increase of the diameter of BTG microspheres, the range of focal image positions (RFIP) increases linearly. When the diameter of BTG microspheres increases from 5 to 50 μm, the RFIP changes from 4 to 25 μm. For the microsphere array samples, Talbot effect is observed, and both the position of Talbot images and the Talbot distance depend on the diameter of BTG microspheres. Numerical simulations indicate that the length of the photonic nanojet changes from 2.9 to 7.1 μm when the BTG microsphere size increases from 5 to 50 μm, and the calculated RFIP is between 6 and 24 μm. The calculated RFIPs match well with the experimental ones. Our researches reveal that the RFIP depends on the length of the photonic nanojet of the BTG microsphere.

Uniformly spaced nanoscale cracks in nanoparticle films deposited by convective assembly

Rapid convective deposition is used to assemble nanoparticle coatings from suspension, with controllable thickness. Varying film thickness generates stress-induced linear cracks with highly monodisperse spacing. Film thickness is controlled through mechanical means, suspension volume fraction, and the use of applied thermal gradients. These cracks extend in the deposition direction, and a uniform crack spacing from 2 to 160μm is observed. The nanoparticle film thickness is the relevant length scale for hydrodynamic flow, and films will crack with this spacing, in a characteristic manner to minimize the system energy and capillary stresses. As expected from this energy minimization problem and relevant theory, the correlation between coating thickness and crack spacing is highly linear. Because this process is continuous, continuous cracks have potential as a high-throughput method of fabricating nanoscale channels for microfluidics and MEMS.

Facile Assembly of Large-Area 2D Microgel Colloidal Crystals Using Charge-Reversible Substrates

2D colloidal crystals (CCs) have important applications; however, the fabrication of large-area, high-quality 2D CCs is still far from being trivial, and the fabrication of 2D microgel CCs is even harder. Here, we have demonstrated that they can be facilely fabricated using charge-reversible substrates. The charge-reversible substrates were prepared by modification with amino groups. The amino groups were then protected by amidation with 2,2-dimethylsuccinic anhydride. At acidic pH, the surface charge of the modified substrate will change from negative to positive as a result of the hydrolysis of the amide bonds and the regeneration of the amino groups. 2D microgel CCs can be simply fabricated by applying a concentrated microgel dispersion on the modified substrate. The negatively charged surface of the substrate allows the negatively charged microgel spheres, especially those close to the substrate, to self-assemble into 3D CCs. With the gradual hydrolysis of the amide bonds and the charge reversal of the substrate, the first 111 plane of the 3D assembly is fixed in situ on the substrate. The resulting 2D CC has a high degree of ordering because of the high quality of the parent 3D microgel CC. Because large-area 3D microgel CCs can be facilely fabricated, this method allows for the fabrication of 2D CCs of any size. Nonplanar substrates can also be used. In addition, the interparticle distance of the 2D array can be tuned by the concentration of the microgel dispersion. Besides rigid substrates (such as glass slides, quartz slides, and silicon wafers), flexible polymer films, including polyethylene terephthalate and poly(vinyl chloride) films, were also successfully used as substrates for the fabrication of 2D microgel CCs.

Large-area 2D microgel colloidal crystals fabricated via benzophenone-based photochemical reaction

Large area, high quality 2D microgel CCs were fabricated by first assembling the microgel spheres into highly ordered 3D CCs, followed by fixing the first 111 plane of the 3D CC onto the substrate via a benzophenone-based photochemical reaction.

Controlled convective self-assembly of silver nanoparticles in volatile organic solvent and its application in electronics

A controlled convective self-assembly approach is reported for producing conductive coatings and patterns using ultra-small Ag nanoparticles in volatile solvents.

Highly ordered, hierarchically porous TiO₂ films via combination of two self-assembling templates

Hierarchically mesoporous TiO2 films with a high degree of order and connectivity on the sub-100-nm scale are successfully fabricated by the dual-templating methods sequentially applied. In this approach, the self-assembly of block copolymers combined with sol-gel reaction is first used to generate highly ordered mesoporous films by optimizing their self-assembling behavior. At the next step, 50-nm PS nanoparticles are deposited on the mesoporous films generated in the previous step to produce the colloidal crystals, and their inverse structure with a high degree of order, otherwise, would not form the colloidal crystals but generate a colloidal glassy phase with poor order. In addition to the exceptionally high degree of order, mesoporous films exhibit a high porosity that spans from the top surface to the bottom surface throughout the entire film without clogging. Especially wide pores at the top layer enable functional materials of large size to access the inside the films with small pores, maximizing their performance. As a proof of concept, photocatalytic effects are examined for the mesoporous films with different structures. In terms of the template pattern, the line pattern and bowl structure are also shown to guide the self-assembly of colloidal particles when their characteristic size matches with the particle diameter, in addition to the hexagonal packing.

Gas-flow-induced reorientation to centimeter-sized two-dimensional colloidal single crystal of polystyrene particle

Centimeter-sized two-dimensional (2D) colloidal single crystals of polystyrene (PS) particles were fabricated at the water/air interface by capillary-modulated self-assembly. Different from previous reports, in this work, emulsifier was used to facilitate the stress release during 2D colloidal crystal formation by adjusting the interparticle lateral interactions. With the assistance of compressed nitrogen flow, 2D hexagonal colloidal single crystals of centimeter size were obtained under appropriate emulsifier concentrations. A new method was also developed to transfer the 2D colloidal crystals from the air/water interface to the desired substrate without obvious disturbance. This new transferring method was proven not to be sensitive to surface wettability nor curvature, thus 2D colloidal single crystals with large areas could be obtained on different kinds of substrate.