Floating body effect in indium-gallium-zinc-oxide (IGZO) thin-film transistor (TFT).

In this paper, the floating body effect (FBE) in indium-gallium-zinc-oxide (IGZO) thin-film transistor (TFT) and the mechanism of device failure caused by that are reported for the first time. If the toggle AC pulses are applied to the gate and drain simultaneously for the switching operation, the drain current of IGZO TFT increases dramatically and cannot show the on/off switching characteristics. This phenomenon was not reported before, and our study reveals that the main cause is the formation of a conductive path between the source and drain: short failure. It is attributed in part to the donor creation at the drain region during the high voltage (Vhigh) condition and in part to the donor creation at the source region during the falling edge and low voltage (Vlow) conditions. Donor creation is attributed to the peroxide formation in the IGZO layer induced by the electrons under the high lateral field. Because the donor creation features positive charges, it lowers the threshold voltage of IGZO TFT. In detail, during the Vhigh condition, the donor creation is generated by accumulated electrons with a high lateral field at the drain region. On the other hand, the floating electrons remaining at the short falling edge (i.e., FBE of the IGZO TFT) are affected by the high lateral field at the source region during the Vlow condition. As a result, the donor creation is generated at the source region. Therefore, the short failure occurs because the donor creations are generated and expanded to channel from the drain and source region as the AC stress accumulates. In summary, the FBE in IGZO TFT is reported, and its effect on the electrical characteristics of IGZO TFT (i.e., the short failure) is rigorously analyzed for the first time.

Lifetime estimation of thin-film transistors in organic emitting diode display panels with compensation.

Oxide semiconductor thin-film transistors (TFTs) are used in the pixel array and gate driver circuits of organic light emitting diode (OLED) display panels. Long-term reliability characteristics of the TFTs are a barometer of the lifetime of OLED display panels. The long-term reliability of the driver TFTs is evaluated in a short time under high voltages and high temperature for an accelerated degradation test. If reliability parameters from the power law or stretched-exponential functions are the same for individual devices and devices in an operating panel, the lifetime of the panel can be accurately estimated. However, since compensation circuits are designed into operating panels, an environmental discrepancy exists between the accelerated test of single devices and the operation of devices in the panel. Herein, we propose a novel compensation stretched-exponential function (CSEF) model which captures the effect of the threshold voltage compensation circuit in the panel. The CSEF model not only bridges the discrepancy between individual devices and panel devices, but also provides a method to accurately and efficiently estimate the long-term lifetime of all display panels that utilize compensation circuits.

Effects of multiple scattering on angle-independent structural color in disordered colloidal materials.

Disordered packings of colloidal spheres show angle-independent structural color when the particles are on the scale of the wavelength of visible light. Previous work has shown that the positions of the peaks in the reflectance spectra can be predicted accurately from a single-scattering model that accounts for the effective refractive index of the material. This agreement shows that the main color peak arises from short-range correlations between particles. However, the single-scattering model does not quantitatively reproduce the observed color: the main peak in the reflectance spectrum is much broader and the reflectance at low wavelengths is much larger than predicted by the model. We use a combination of experiment and theory to understand these features. We find that one significant contribution to the breadth of the main peak is light that is scattered, totally internally reflected from the boundary of the sample, and then scattered again. The high reflectance at low wavelengths also results from multiple scattering but can be traced to the increase in the scattering cross section of individual particles with decreasing wavelength. Both of these effects tend to reduce the saturation of the structural color, which limits the use of these materials in applications. We show that while the single-scattering model cannot reproduce the observed saturations, it can be used as a design tool to reduce the amount of multiple scattering and increase the color saturation of materials, even in the absence of absorbing components.

Photonic Microcapsules Containing Single-Crystal Colloidal Arrays with Optical Anisotropy.

Colloidal particles with a repulsive interparticle potential spontaneously form crystalline lattices, which are used as a motif for photonic materials. It is difficult to predict the crystal arrangement in spherical volume as lattices are incompatible with a spherical surface. Here, the optimum arrangement of charged colloids is experimentally investigated by encapsulating them in double-emulsion drops. Under conditions of strong interparticle repulsion, the colloidal crystal rapidly grows from the surface toward the center of the microcapsule, forming an onion-like arrangement. By contrast, for weak repulsion, crystallites slowly grow and fuse through rearrangement to form a single-crystal phase. Single-crystal structure is energetically favorable even for strong repulsion. Nevertheless, a high energy barrier to colloidal rearrangement kinetically arrests the onion-like structure formed by heterogeneous nucleation. Unlike the isotropic onion-shaped product, the anisotropic single-crystal-containing microcapsules selectively display-at certain orientations but not others-one of the distinct colors from the various crystal planes.

Photonic Capsule Sensors with Built-In Colloidal Crystallites.

Technologies to monitor microenvironmental conditions and its spatial distribution are in high demand, yet remain unmet need. Herein, photonic microsensors are designed in a capsule format that can be injected, suspended, and implanted in any target volume. Colorimetric sensors are loaded in the core of microcapsules by assembling core-shell colloids into crystallites through the depletion attraction. The shells of the colloids are made of a temperature-responsive hydrogel, which enables the crystallites to rapidly and widely tune the structural color in response to a change in temperature while maintaining close-packed arrays. The spherical symmetry of the microcapsules renders them optically isotropic, i.e., displaying orientation-independent color. In addition, as a solid membrane is used to protect the delicate crystallites from external stresses, their high stability is assured. More importantly, each microcapsule reports the temperature of its microenvironment so that a suspension of capsules can provide information on the spatial distribution of the temperature.

Full-spectrum photonic pigments with non-iridescent structural colors through colloidal assembly.

Structurally colored materials could potentially replace dyes and pigments in many applications, but it is challenging to fabricate structural colors that mimic the appearance of absorbing pigments. We demonstrate the microfluidic fabrication of "photonic pigments" consisting of microcapsules containing dense amorphous packings of core-shell colloidal particles. These microcapsules show non-iridescent structural colors that are independent of viewing angle, a critical requirement for applications such as displays or coatings. We show that the design of the microcapsules facilitates the suppression of incoherent and multiple scattering, enabling the fabrication of photonic pigments with colors spanning the visible spectrum. Our findings should provide new insights into the design and synthesis of materials with structural colors.

Osmotic-pressure-controlled concentration of colloidal particles in thin-shelled capsules.

Colloidal crystals are promising structures for photonic applications requiring dynamic control over optical properties. However, for ease of processing and reconfigurability, the crystals should be encapsulated to form 'ink' capsules rather than confined in a thin film. Here we demonstrate a class of encapsulated colloidal photonic structures whose optical properties can be controlled through osmotic pressure. The ordering and separation of the particles within the microfluidically created capsules can be tuned by changing the colloidal concentration through osmotic pressure-induced control of the size of the individual capsules, modulating photonic stop band. The rubber capsules exhibit a reversible change in the diffracted colour, depending on osmotic pressure, a property we call osmochromaticity. The high encapsulation efficiency and capsule uniformity of this microfluidic approach, combined with the highly reconfigurable shapes and the broad control over photonic properties, make this class of structures particularly suitable for photonic applications such as electronic inks and reflective displays.

Absence of red structural color in photonic glasses, bird feathers, and certain beetles.

Colloidal glasses, bird feathers, and beetle scales can all show structural colors arising from short-ranged spatial correlations between scattering centers. Unlike the structural colors arising from Bragg diffraction in ordered materials like opals, the colors of these photonic glasses are independent of orientation, owing to their disordered, isotropic microstructures. However, there are few examples of photonic glasses with angle-independent red colors in nature, and colloidal glasses with particle sizes chosen to yield structural colors in the red show weak color saturation. Using scattering theory, we show that the absence of angle-independent red color can be explained by the tendency of individual particles to backscatter light more strongly in the blue. We discuss how the backscattering resonances of individual particles arise from cavity-like modes and how they interact with the structural resonances to prevent red. Finally, we use the model to develop design rules for colloidal glasses with red, angle-independent structural colors.

Investigation of the flatband voltage (V(FB)) shift of Al2O3 on N2 plasma treated Si substrate.

The relationships between the physical and electrical characteristics of films treated with N2 plasma followed by forming gas annealing (FGA) were investigated. The Si substrates were treated with various radio frequency (RF) power levels under a N2 ambient. Al2O3 films were then deposited on Si substrates via remote plasma atomic-layer deposition. The plasma characteristics, such as the radical and ion density, were investigated using optical emission spectroscopy. Through X-ray photoelectron spectroscopy, the chemical-bonding configurations of the samples treated with N2 plasma and FGA were examined. The quantity of Si-N bonds increased as the RF power was increased, and Si--O--N bonds were generated after FGA. The flatband voltage (VFB) was shifted in the negative direction with increasing RF power, but the VFB values of the samples after FGA shifted in the positive direction due to the formation of Si--O--N bonds. N2 plasma treatment with various RF power levels slightly increased the leakage current due to the generation of defect sites.

Two-dimensional directed assembly of dicolloids.

The assembly of ordered dicolloid monolayers is directed by an electric field. The dicolloid particles are polystyrene latex with a maximum equatorial diameter 3.45 µm and length 4.63 µm. The monolayer structure is characterized using small-angle light scattering and bright-field microscopy. With increasing field strength from 26.7 to 200 V(RMS)/cm, a transition from a disordered monolayer, to first orientationally ordered, and then translationally ordered two-dimensional (2D) arrays occurs. A c2mm plane group symmetry dominates the ordered structure but is present alongside structures with p2 symmetry, leading to a spread in the angular distribution of the light scattering peaks. The order-disorder transition dependence on field strength and frequency is similar to that observed for colloidal spheres; at higher frequencies, stronger fields are required to assemble particles. Optimal ordered structures reflect a balance between inducing sufficiently strong interparticle interactions while limiting the rate of formation to ensure the growth of large crystalline domains.

The effects of radio frequency plasma power on Ai2O3 films deposited at room-temperature by remote plasma atomic layer deposition.

Al2O3 films were deposited by a remote plasma atomic layer deposition (RPALD) method at room temperature (25 degrees C) in a reactor using alternating exposures of Al(CH3)3 and O2 plasma. Oxygen plasma was used as a reactant gas to decompose the trimethylaluminum [TMA, Al(CH3)3] precursor at room temperature. The RF plasma power was increased to produce enough radicals for the deposition of the Al2O3 films at room temperature. Then, changes in the interfacial and bulk properties of the deposited Al2O3 films were investigated according to increasing RF power. Al2O3 films deposited by RPALD with RF powers over 100 W showed similar bulk properties, indicating that radicals over a certain threshold did not have a decisive effect on the additional decomposition of precursors for a low impurity content in the films. An increase in RF plasma power could improve the interfacial stability due to an increase in radicals and ions in the plasma and the minimization of plasma-induced substrate damage by adopting remote plasma.

Assembly of optical-scale dumbbells into dense photonic crystals.

We describe the self-assembly of nonspherical particles into crystals with novel structure and optical properties combining a partial photonic band gap with birefringence that can be modulated by an external field or quenched by solvent evaporation. Specifically, we study symmetric optical-scale polymer dumbbells with an aspect ratio of 1.58. Hard particles with this geometry have been predicted to crystallize in equilibrium at high concentrations. However, unlike spherical particles, which readily crystallize in the bulk, previous experiments have shown that these dumbbells crystallize only under strong confinement. Here, we demonstrate the use of an external electric field to align and assemble the dumbbells to make a birefringent suspension with structural color. When the electric field is turned off, the dumbbells rapidly lose their orientational order and the color and birefringence quickly go away. In this way, dumbbells combine the structural color of photonic crystals with the field addressability of liquid crystals. In addition, we find that if the solvent is removed in the presence of an electric field, the particles self-assemble into a novel, dense crystalline packing hundreds of particles thick. Analysis of the crystal structure indicates that the dumbbells have a packing fraction of 0.7862, higher than the densest known packings of spheres and ellipsoids. We perform numerical experiments to more generally demonstrate the importance of controlling the orientation of anisotropic particles during a concentration quench to achieve long-range order.

High-yield synthesis of monodisperse dumbbell-shaped polymer nanoparticles.

We describe a method for producing highly monodisperse dumbbell-shaped polymer nanoparticles with dimensions on the order of a few hundred nanometers in extremely high yields. Our technique is based on seeded polymerization, where suspended core-shell particles (linear polystyrene core with polystyrene-co-trimethoxysilylpropylacrylate shell) are used as seeds. When an aqueous suspension of seed particles is mixed with monomer solution, the core-shell particles display dramatic changes in their morphology. Subsequent heating drives the polymerization of monomer, resulting in the formation of dumbbell-shaped particles. The relative sizes of the two lobes can be controlled by varying the relative volume of the monomer with respect to the seed particle. These particles are well-suited for future studies of the assembly of photonic crystals of anisotropic particles.

Cell stimulation with optically manipulated microsources.

Molecular gradients are important for various biological processes including the polarization of tissues and cells during embryogenesis and chemotaxis. Investigations of these phenomena require control over the chemical microenvironment of cells. We present a technique to set up molecular concentration patterns that are chemically, spatially and temporally flexible. Our strategy uses optically manipulated microsources, which steadily release molecules. Our technique enables the control of molecular concentrations over length scales down to about 1 microm and timescales from fractions of a second to an hour. We demonstrate this technique by manipulating the motility of single human neutrophils. We induced directed cell polarization and migration with microsources loaded with the chemoattractant formyl-methionine-leucine-phenylalanine. Furthermore, we triggered highly localized retraction of lamellipodia and redirection of polarization and migration with microsources releasing cytochalasin D, an inhibitor of actin polymerization.

Synthesis of colloidal particles with the symmetry of water molecules.

We describe a method for synthesizing "colloidal water": monodisperse micrometer-sized polymer particles with the symmetry of water molecules. Our approach is based on multistep seeded polymerization with variation of cross-linker concentration in each step. This enables precise control of swelling selectivity and phase-separation, which control the particle morphology.

Anti-complementary activity of triterpenoides from fruits of Zizyphus jujuba.

In order to determine on the anti-complement activity of triterpenes, following eleven triterpenoides were isolated from the fruits of the Zizyphus jujuba MILL: ceanothane-type triterpenes: colubrinic acid (1), zizyberenalic acid (11); lupane-type triterpenes: alphitolic acid (2), 3-O-cis-p-coumaroyl alphitolic acid (3), 3-O-trans-p-coumaroyl alphitolic acid (4), betulinic acid (7), betulonic acid (9); and oleanane-type triterpenes: 3-O-cis-p-coumaroyl maslinic acid (5), 3-O-trans-p-coumaroyl maslinic acid (6), oleanolic acid (8), oleanonic acid (10). These compounds were examined for their anti-complement activity against the classical pathway of the complement system. Among them, compounds 5, 6, and 8 exhibited significant anti-complement activity with IC(50) values of 101.4, 143.9, and 163.4 microM, respectively, whereas the ceanothane-type and the lupane-type triterpenes were inactive. This suggests that the oleanane-structure plays an important role in inhibiting the hemolytic activity of human serum against erythrocytes.