Monolayer Of Microspheres Research Articles

Investigation of the Deposition of Microsphere Monolayers for Fabrication of Microlens Arrays

Convective deposition of a monolayer of microspheres by drawing a meniscus of a suspension across a substrate is used to fabricate microlens arrays to enhance the photon extraction efficiency of light emitting diodes (LEDs). The self-assembly of a colloidal crystal within the blade-drawn thin film is dominated by capillary forces and the thickness of this crystal depends on many parameters, including the deposition rate and particle size. This study investigates these and other parameters such as angle and hydrophobicity of the deposition blade that have not previously been considered. Using a confocal laser scanning microscope, the local and long-range order of the deposited particles are evaluated by the radial distribution function, and the fraction of the number of nearest neighbors and local bond order, demonstrating the dependence of the microstructure on the deposition parameters. Our results suggest previous descriptions of the critical deposition parameters are inadequate for understanding how various processing conditions influence deposition. For instance, increasing the deposition blade angle from 20 degrees up to 90 degrees requires an increase in deposition rate to achieve a monolayer deposition. The microlens arrays were fabricated on LEDs where polystyrene and silica are coated in consecutive depositions. Heat is used to sacrifice the polystyrene layers to result in an ordered array of partially buried silica microspheres that act as lenses to scatter light from the device. Enhancement in light extraction efficiency of 2.66 times was demonstrated for InGaN-based light emitting diodes employing micron scale microlens arrays with 1 um diameter silica microspheres.

Large-area parallel near-field optical nanopatterning of functional materials using microsphere mask

Large-area parallel near-field optical nanopatterning on functional material surfaces was investigated with KrF excimer laser irradiation. A monolayer of silicon dioxide microspheres was self-assembled on the sample surfaces as the processing mask. Nanoholes and nanospots were obtained on silicon surfaces and thin silver films, respectively. The nanopatterning results were affected by the refractive indices of the surrounding media. Near-field optical enhancement beneath the microspheres is the physical origin of nanostructure formation. Theoretical calculation was performed to study the intensity of optical field distributions under the microspheres according to the light scattering model of a sphere on the substrate.

Combinational template-assisted fabrication of hierarchically ordered nanowire arrays on substrates for device applications

By combining two self-organized templates, the authors fabricate hierarchically ordered nanowire arrays on substrates. The nanowire arrays consist of periodic voids at the microscale and hexagonally packed nanowires at the nanoscale. The nanowires are created by selective electrodeposition using nanoporous anodic alumina as a template and a porous gold film as a working electrode that is patterned by microsphere monolayers. In addition to in-plane hierarchical structure, they have sequentially varied the vertical structure of the nanowires. This approach is simple, inexpensive, and high throughput to fabricate nanostructures on substrates.

Seed-mediated growth of gold nanoparticles using self-assembled monolayer of polystyrene microspheres as nanotemplate arrays

Arrays of noble metal nanoparticles show potential applications in (bio-)sensing, optical storage, surface-enhanced spectroscopy, and waveguides. For all such potential devices, controlling the size, morphology, and interparticle spacing of the nanoparticles is very important. Here, we combine seed-mediated growth with nanosphere lithography to study the controllable growth of gold nanoparticles (Au NPs), in which the self-assembly monolayer of polystyrene (PS) on a silicon surface is used to guide the modification of alkanesilanes and the subsequent adsorption of gold seeds; seed-mediated growth is applied to controlling the morphology and size of Au NPs. The size of adsorption region (determining the number of adsorbed gold seeds) is controlled by etching PS microspheres with oxygen plasma or annealing PS microspheres at the glass transition temperature. The size and morphology of the Au NPs are controlled by changing growth conditions. In such a way, we have achieved the dual control of the obtained Au NPs. Preliminary results show that this strategy holds a great promise. This approach can also be extended to a wide range of materials and substrates.

Patterning of silica microsphere monolayers with focused femtosecond laser pulses

We demonstrate the patterning of monolayer silica microsphere lattices with tightly focused femtosecond laser pulses. We selectively removed microspheres from a lattice and characterized the effect on the lattice and the substrate. The proposed physical mechanism for the patterning process is laser-induced breakdown followed by ablation of material. We show that a microsphere focuses radiation in its interior and in the near field. This effect plays an important role in the patterning process by enhancing resolution and accuracy and by reducing the pulse energy threshold for damage. Microsphere patterning could create controlled defects within self-assembled opal photonic crystals.

Photonic properties of silicon-coated monolayers of colloidal silica microspheres

The spectral dependence of the optical transmission through close-packed monolayers of silica (a-SiO2) microspheres covered with different amounts of amorphous silicon (a-Si) is studied. The strong dips in the transmission can be understood by the coupling of the incident electromagnetic field into the modes of the combined photonic slab structure consisting of the close-packed lattice of a-SiO2 microspheres and a-Si. The dependence of the transmission minima positions on the a-Si thickness and the angle of incidence agree quite well with model calculations.

2P314 アガロース薄層に固定したマイクロビーズ上での細胞培養と選択的細胞回収技術の開発(バイオエンジニアリング)

2P314 アガロース薄層に固定したマイクロビーズ上での細胞培養と選択的細胞回収技術の開発(バイオエンジニアリング)

Influence of Surfactants on an Evaporating Drop: Fluorescence Images and Particle Deposition Patterns

The insoluble surfactant pentadecanoic acid is spread at the interface of aqueous droplets, which are then deposited on a well-defined substrate and allowed to evaporate. The surface state of the surfactant is imaged as the drop evaporates using fluorescence microscopy, and the mean rate of evaporation is calculated from successive digitized silhouettes of the sessile drops. The drops contain suspended microspheres that act as tracers in the flow within the drop and deposit on the substrate in patterns that vary with the surfactant surface state. Patterns observed include circular mounds of particles at the original drop periphery termed “coffee rings”, created by an outward flow to the three-phase contact line; connected polygons formed by Marangoni−Bénard flow; and uniform monolayers of microspheres deposited on the substrate. These results establish that surfactants can be used to alter flow fields and deposition patterns from evaporating aqueous drops. This is potentially useful in the organization of nanoparticles and in the deposition of materials functionalized with specific binding sites whose folded structures might be destroyed by other solvents. The deposition of biologically functionalized materials is demonstrated by depositing streptavidin-labeled microspheres, which retain their ability to bind biotin after deposition.

Nonlinear compressional pulses in a 2D crystallized dusty plasma.

Compressional pulses were launched in a two-dimensional Yukawa lattice, a hexagonal monolayer of polymer microspheres suspended in a plasma. The pulsed wave was excited by a laser beam, and nonlinear effects were observed for Mach numbers M>0.07 and for variation of particle number density delta(n)/n>0.1, but no steepening of the pulse was detected. The pulse propagation speed was found to be comparable to the sound speed of compressional waves launched with sinusoidal excitation.

Changes in the Morphology of Self-Assembled Polystyrene Microsphere Monolayers Produced by Annealing

Ordered monolayers of polystyrene (PS) microspheres were prepared on both hydrophilic and hydrophobic substrates by convective self-assembly. Changes in the degree of ordering in the microsphere monolayers during annealing at temperatures below and above the glass transition temperature Tg of PS was quantitatively assessed using a customized image analysis method. This method overcomes the shortcomings of traditional Fourier transform methods for the analysis of images containing a small number of objects. In general, large ordered domains in the monolayers break up into smaller regions separated by cracks, due to thermal expansion of the polymer and the attractive dispersion force between microspheres. The substrate influences this phenomenon: the evolution was much slower for instance for monolayers on glass than for monolayers on PMMA substrates. In annealing experiments above Tg of PS, measurements of the widths of cracks in the monolayers showed differences between substrates: At later stages the width of the cracks in monolayers on PS substrates were observed to decrease more quickly than for monolayers on PMMA substrates. After very long times at relatively high temperatures, the monolayers on both substrates became essentially smooth films.

A Novel Preparation of Nonsymmetrical Microspheres Using the Langmuir−Blodgett Technique

We have utilized the Langmuir−Blodgett (L−B) technique to develop the generic technology that can prepare nonsymmetrical microspheres. The dispersion of the microspheres in butanol was spread on a water surface and compressed by a L−B apparatus to obtain a microsphere monolayer. Then, an amphiphilic and reactive polymer was prepared by terpolymerization of N-octadecyl acrylamide, p-nitrophenyl acrylate, and 2,2,2,-trifluoroethyl methacrylate. A stable polymer monolayer was transferred onto the microsphere monolayer by the L−B technique. It was found from contact angle measurement that the surface became hydrophobic with deposit of the terpolymer. X-ray photoelectron spectroscopy data demonstrated that only the top side of modified microspheres was covered with the terpolymer.

Transverse waves in a two-dimensional screened-coulomb crystal (Dusty plasma)

Transverse shear waves were observed experimentally in a two-dimensional screened Coulomb crystal. They were excited by applying a chopped laser beam to a 2D dusty plasma, i.e., a monolayer of charged microspheres levitated in a plasma. Measurements of the dispersion relation reveal an acoustic, i.e., nondispersive, character over the entire range of wave numbers measured, 0.2<k(r)a/pi<0.7, where a is the interparticle spacing. Comparison to theory provides a measurement of the particles' charge.

Organic light-emitting device with an ordered monolayer of silica microspheres as a scattering medium

Periodic dielectric structures, consisting of hexagonally closed-packed arrays of silica microspheres with the diameter of 550 nm, were incorporated into organic light-emitting devices with a conventional two-layer structure made with vacuum-sublimation. The arrays acted as a two-dimensional diffraction lattice which behaved as a light scattering medium for the light propagated in waveguiding modes within the device. Strongly scattered light emission through the front surface of the devices was observed. An increase in the device coupling-out factor for electroluminescent efficiency by using the scattering structure is demonstrated.

Cell response to micropatterned surfaces produced with polymeric microspheres

Surface topography plays an important role in cell function. We fabricated microstructured patterns from monolayers of polymeric microspheres, which were arranged on a flat polystyrene dish. Such patterns could be quickly formed by spin-coating microspheres and were easily controlled by changing the size of the settled microspheres. Polystyrene (PS) microspheres having different sizes were prepared by soap-free emulsion polymerization in the presence of sodium chloride with different concentrations. The zeta potential of these microspheres was approx. −25 mV regardless of their diameter. Electron microscopic observations revealed that the microspheres were deposited on a PS dish in hexagonally-packed monolayers with a pitch equal to the diameter of each microsphere. The water contact angles gradually decreased with an increase of the pitch. To investigate the topographical effect of these surfaces on the neutrophil-like cell function, microscopic observations and measurements of active oxygen released from cells were carried out. Microscopic observation revealed that spreading cells adhered on PS microsphere-settling surfaces as well as on a PS film. As for active oxygen release, cell response to a PS microsphere-settling surface depended on the diameter of settling microspheres and showed a maximum for diameters of approx. 1 μm. This tendency was observed in phosphate-buffered solutions both with and without serum. Continuous activation which followed by the initial attachment seems to relate to the change of cytoskeletal systems. These results indicate that cells recognize the roughness of the micropatterned surface covered with PS microspheres, and a pitch between the tops of settled microspheres is an important factor for cell recognition.

Preparation of Unsymmetrical Microspheres at the Interfaces

Many synthetic microspheres are spherical and symmetrical because of the thermodynamical limitations of the reaction systems. We developed modification techniques to prepare spherical microspheres with intrinsic unsymmetry, for example, a microsphere with both an anionic and a cationic part. The modification was performed at the liquid−solid interface. Reactive microspheres with p-nitrophenyl moieties were settled onto the IgG-preadsorbed substrate, and the reaction between the activated ester moieties and IgG molecules proceeded only at the interface upon attachment. After the reaction, the modified microspheres were detached from the substrate with ultrasonication. In another modification at the air−water interface, the dispersion of reactive microspheres in ethanol was first spread on a water surface to produce the monolayer of microspheres, and then hydrolysis at the water side of the monolayer was performed by adding a NaOH aqueous solution to the subphase. After this unsymmetrical hydrolysis, we carried out IgG immobilization with the remaining reactive moieties. In both interface methods, the immobilized IgG molecules were labeled with colloidal gold particles conjugated with anti-IgG antibodies. Transmission electron microscopy indicated that gold particles could be observed on only the immobilized side. Small microspheres were coupled to the reactive microsphere, and its unsymmetry was confirmed by scanning electron microscopy. Such unsymmetrical microspheres will be applicable to the electrical rheology, diagnosis, display technology, and creation of functional devices through the assembling of microspheres.

Capacitance–voltage property of composite microsphere monolayer with polymer/Fe 2O 3/ polymer structure

Inorganic nanoparticles Fe 2O 3 3–5 nm in diameter were assembled on the surface of polymer microspheres and then encapsulated in the polymer to form a trilayer composite structure. The capacitance–voltage ( C– V) curve of the composite microsphere monolayer shows the presence of a plateau. The electron transportation process in the special structure was investigated combining with the energy band model. The results show that Fe 2O 3 layer acts as a charge trap between polymer layers which are regarded as potential barriers.

Effect of Interparticulate Interaction on Release Kinetics of Microsphere Ensembles

□ The release kinetics of microsphere ensembles is complicated by the mutual influence of the microspheres which are entrapped in small compartments such as body cavities. This work focused on the effect of interparticulate interaction on the release kinetics of microsphere ensembles with limited spreading. Experiments and finite element modeling were conducted to investigate diffusional drug release from a single sphere, a monolayer, and multiple layers of microspheres. Poly(methyl methacrylate-co-methacrylic acid) (P(MMA/MAA)) microspheres and azidothymidine (AZT) were used in the experiments. The order of the release rate of AZT from various microsphere populations was observed to be single sphere>monolayer>multiple layers. This evidenced the importance of interparticulate interaction. The finite element simulations elucidated the influence of various factors on the release kinetics of microsphere ensembles including the separation distance, location of the spheres, and the drug accumulation in the medium. Calibration of overall release kinetics for the neighboring effect was proposed on the basis of the spreading factor. overall release profiles of microsphere ensembles were predicted using the release profiles of individual microspheres at various locations.

Immobilized polymeric microspheres: Polyacrolein microspheres covalently bound in a monolayer structure onto glass surfaces

Novel materials composed of an unpacked monolayer of polyacrolein microspheres covalently bonded onto glass surfaces have been prepared and characterized. These surfaces were synthesized by the covalent binding of long- and short-chain length ω-cyano-alkylsilane compounds (SiCl 3(CH 2) n CN and Si(OMe) 3(CH 2) n CN, where n = 3 and 16) onto glass substrates. The nitrile groups at the ω position were then reduced to primary amino groups. Polyacrolein microspheres of 0.7 μm average diameter were then bonded to the modified surfaces. This binding of the microspheres to the derivatized surfaces is achieved via polyvalent Schiff-base bonds formed by the interaction between aldehyde groups of the microspheres and ω-primary amino groups of the derivatized glass surfaces. The percent coverage of the glass surfaces by the microspheres was higher for surfaces coated with the trichloroalkylsilane compounds compared to trimethoxyalkylsilane compounds, and higher also for surfaces coated with the short-chain length alkylsilane compounds compared to surfaces coated with the long ones. The residual aldehyde groups of the immobilized microspheres can then be used for the covalent binding of amino ligands, e.g. proteins, in a single step and at physiological pH (or any other desired pH). The binding kinetics of bungarotoxin to the immobilized microspheres has been investigated. Fourier transform-infrared/attenuated total reflection, electron spectroscopy for chemical analysis, contact angle measurements and scanning electron microscopy have been used for the characterization of the alkylsilane coatings and the binding of the microspheres to these coatings.

Fabrication of high-T/sub c/ Josephson effect devices by natural lithography

Thin films of YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) have been deposited on LaAlO/sub 3/ substrates previously textured with submicron features by the use of natural lithography (the use of monolayers of polystyrene microspheres as lithographic masks). This weakens the superconducting properties, due to reduced connectivity in the film. Devices fabricated using localized textured regions have shown Josephson coupling. Weak links have shown Shapiro steps at the expected voltage intervals. Magnetic-field-induced modulation in the detected RF voltage with the geometrically correct periodicity has been observed in RF SQUIDs over a limited temperature range.

Fracture in microsphere monolayers studied by experiment and computer simulation

The formation of irregular (often fractal1) patterns under non-equilibrium conditions has become a subject of considerable scientific and practical interest. One of the most important processes of this type is the formation of cracks and other extended defects in materials under stress2,3. Here we present an experimental study of crack growth in a two-dimensional system, using a monolayer of uniformly sized microspheres confined between two parallel sheets of glass. The cracking patterns observed in this system closely resemble those found in more complex systems of practical importance, such as paint films or ceramic-coated metals. A simple two-dimensional computer model for elastic fracture leads to structures that closely resemble those observed in the experiments. In both the experimental and computer models an early stage in which isolated defects are formed is followed by a period in which rapid growth of almost linear cracks occurs. At later times the crack growth process slows down and the shapes of the cracks become increasingly irregular.