James F. Gilchrist

Light Extraction Efficiency and Radiation Patterns of III-Nitride Light-Emitting Diodes With Colloidal Microlens Arrays With Various Aspect Ratios

The fabrication studies of silica/polystyrene (PS) colloidal microlens arrays with various aspect ratios were performed on the III-nitride light-emitting diodes (LEDs). The use of colloidal-based microlens arrays led to significant enhancement in light extraction efficiency for III-nitride LEDs. In varying the aspect ratios of the microlens arrays, the engineering of various PS thicknesses was employed by using high-temperature treatment and redeposition process. The effects of PS thickness on the light extraction efficiency and far-field emission patterns of InGaN quantum-well (QW) LEDs were studied. The total output powers of microlens LEDs with various PS thicknesses exhibited 1.93-2.70 times enhancement over that of planar LEDs, and the use of optimized PS layer thickness is important in leading the enhancement of the light extraction efficiency in large angular direction.

Enhancement of light extraction efficiency of InGaN quantum wells light emitting diodes using SiO2/polystyrene microlens arrays

Yik-Khoon Ee, Ronald A. Arif, Nelson Tansu, Pisist Kumnorkaew, and James F. Gilchrist Citation: Applied Physics Letters 91, 221107 (2007); doi: 10.1063/1.2816891 View online: http://dx.doi.org/10.1063/1.2816891 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/91/22?ver=pdfcov Published by the AIP Publishing

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.

Light extraction efficiency enhancement of InGaN quantum wells light-emitting diodes with polydimethylsiloxane concave microstructures

Improvement of light extraction efficiency of InGaN light emitting diodes (LEDs) using polydimethylsiloxane (PDMS) concave microstructures arrays was demonstrated. The size effect of the concave microstructures on the light extraction efficiency of III-Nitride LEDs was studied. Depending on the size of the concave microstructures, ray tracing simulations show that the use of PDMS concave microstructures arrays can lead to increase in light extraction efficiency of InGaN LEDs by 1.5 to 2.0 times. Experiments utilizing 2.0 micron thick PDMS with 1.0 micron diameter of the PDMS concave microstructures arrays demonstrated 1.70 times improvement in light extraction efficiency, which is consistent with improvement of 1.77 times predicted from simulation. The enhancement in light extraction efficiency is attributed to increase in effective photon escape cone due to PDMS concave microstructures arrays.

Chaotic mixing of granular materials in two-dimensional tumbling mixers

We consider the mixing of similar, cohesionless granular materials in quasi-two-dimensional rotating containers by means of theory and experiment. A mathematical model is presented for the flow in containers of arbitrary shape but which are symmetric with respect to rotation by 180 degrees and half-filled with solids. The flow comprises a thin cascading layer at the flat free surface, and a fixed bed which rotates as a solid body. The layer thickness and length change slowly with mixer rotation, but the layer geometry remains similar at all orientations. Flow visualization experiments using glass beads in an elliptical mixer show good agreement with model predictions. Studies of mixing are presented for circular, elliptical, and square containers. The flow in circular containers is steady, and computations involving advection alone (no particle diffusion generated by interparticle collisions) show poor mixing. In contrast, the flow in elliptical and square mixers is time periodic and results in chaotic advection and rapid mixing. Computational evidence for chaos in noncircular mixers is presented in terms of Poincare sections and blob deformation. Poincare sections show regions of regular and chaotic motion, and blobs deform into homoclinic tendrils with an exponential growth of the perimeter length with time. In contrast, in circular mixers, the motion is regular everywhere and the perimeter length increases linearly with time. Including particle diffusion obliterates the typical chaotic structures formed on mixing; predictions of the mixing model including diffusion are in good qualitative and quantitative (in terms of the intensity of segregation variation with time) agreement with experimental results for mixing of an initially circular blob in elliptical and square mixers. Scaling analysis and computations show that mixing in noncircular mixers is faster than that in circular mixers, and the difference in mixing times increases with mixer size. (c) 1999 American Institute of Physics.

Effect of Nanoparticle Concentration on the Convective Deposition of Binary Suspensions

We investigate the coupling between the suspension properties and the deposition process during convective deposition of aqueous binary suspensions of 1 microm silica microspheres and 100 nm polystyrene (PS) nanoparticles. The structures formed from this rapid and scalable process have use in a variety of optical, chemical, and biochemical sensing applications. At conditions that produce a well-ordered microsphere monolayer at a silica volume fraction of 20% in the absence of nanoparticles, we examine the effect of varying the concentration of nanoparticles from 0% to 16% on the quality of the microsphere deposition and the exposure of the microspheres within the PS layer. At low concentrations of nanoparticles, the deposition results in an instability that forms stripes parallel to the receding contact line. Optimum deposition occurs between 6% and 8% PS and forms a monolayer having the same high degree of uniformity as the monodisperse suspension is fabricated. For higher concentrations, the deposition is increasingly less uniform as a result of nanoparticle depletion destabilizing the microspheres. The degree to which each microsphere is buried by the nanoparticles in the deposited thin film increases with nanoparticle concentration. This variation in coverage also suggests interplay between deposition and nanoparticle engineered properties of the suspension that influence the deposited morphology.

Effect of Surface Nanotopography on Immunoaffinity Cell Capture in Microfluidic Devices

Immunoaffinity microfluidic devices have recently become a popular choice to isolate specific cells for many applications. To increase cell capture efficiency, several groups have employed capture beds with nanotopography. However, no systematic study has been performed to quantitatively correlate surface nanopatterns with immunoaffinity cell immobilization. In this work, we controlled substrate topography by depositing close-packed arrays of silica nanobeads with uniform diameters ranging from 100 to 1150 nm onto flat glass. These surfaces were functionalized with a specific antibody and assembled as the base in microfluidic channels, which were then used to capture CD4+ T cells under continuous flow. It is observed that capture efficiency generally increases with nanoparticle size under low flow rate. At higher flow rates, cell capture efficiency becomes increasingly complex; it initially increases with the bead size then gradually decreases. Surprisingly, capture yield plummets atop depositions of some particle diameters. These dips likely stem from dynamic interactions between nanostructures on the substrate and cell membrane as indicated by roughness-insensitive cell capture after glutaraldehyde fixing. This systematic study of surface nanotopography and cell capture efficiency will help optimize the physical properties of microfluidic capture beds for cell isolation from biological fluids.

Mixing of granular materials: A test-bed dynamical system for pattern formation

Mixing of granular materials provides fascinating examples of pattern formation and self-organization. More mixing action — for example, increasing the forcing with more vigorous shaking or faster tumbling — does not guarantee a better-mixed final system. This is because granular mixtures of just barely different materials segregate according to density and size; in fact, the very same forcing used to mix may unmix. Self-organization results from two competing effects: chaotic advection or chaotic mixing, as in the case of fluids, and flow-induced segregation, a phenomenon without parallel in fluids. The rich array of behaviors is ideally suited for nonlinear-dynamics-based inspection. Moreover, the interplay with experiments is immediate. In fact, these systems may constitute the simplest example of coexistence between chaos and self-organization that can be studied in the laboratory. We present a concise summary of the necessary theoretical background and central physical ideas accompanied by illustrative experimental results to aid the reader in exploring this fascinating new area.

Enhanced colloidal monolayer assembly via vibration-assisted convective deposition

We demonstrate a purely mechanical technique for enhancing evaporation-driven convective deposition of particle monolayers from suspension. Lateral vibration in the deposition direction results in monolayer deposition at faster speeds, over a wider range of withdraw rates, and with higher order versus traditional convective deposition. These enhancements and phenomena are a result of variation in the thin film, where capillary interactions result in self-assembly by dynamically changing the air-liquid interface. This enhancement in fabricating ordered particle thin films may enable development of optical and biological applications and efforts to scale-up this process for commercial application.

Matching Constituent Fluxes for Convective Deposition of Binary Suspensions

Rapid convective deposition is an effective method for depositing well-ordered monolayers from monodisperse suspensions; however, much less is known about polydisperse suspension deposition. The addition of a much smaller species can enhance deposition by extending the range of ordered deposition and can induce instability, producing stripes and other complex morphologies. By considering relative species flux, we predict the volume fraction ratio of smaller to larger constituents necessary for steady well-ordered deposition. Experiments varying the 1 microm microsphere and 100 nm nanoparticle concentrations exhibit an optimum nanoparticle to microsphere volume fraction ratio at moderate volume fractions that agrees well with theory. Average local bond order and surface density characterize crystallinity and coverage, respectively. At lower microsphere volume fraction, monolayer crystallinity is optimized at a constant nanoparticle volume fraction of 0.04. At lower-than-optimum nanoparticle concentrations for each microsphere concentration, instability occurs and alternating stripes of monolayer and submonolayer morphologies form. At higher-than-optimum nanoparticle concentration, the microspheres become disordered and/or form multilayer regions. Additionally, the degree of microsphere burial in deposited nanoparticles depends solely on nanoparticle concentration.