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Micro-structure patterned substrates attract our attention due to the special and programmable wettabilities. The interaction between the liquid and micro/nano structures gives rise to controllable spreading and thus evaporation. For exploration of the application versatility, the introduction of nanoparticles in liquid droplet results in interaction among particles, liquid and microstructures. In addition, temperature of the substrates strongly affects the spreading of the contact line and the evaporative property. The evaporation of sessile droplets of nanofluids on a micro-grooved solid surface is investigated in terms of liquid and surface properties. The patterned nickel surface used in the experiments is designed and fabricated with circular and rectangular shaped pillars whose size ratios between interval and pillars is fixed at 5. The behavior is firstly compared between nanofluid and pure liquid on substrates at room temperature. For pure water droplet, the drying time is relatively longer due to the receding of contact line which slows down the liquid evaporation. Higher concentrations of nanoparticles tend to increase the total evaporation time. With varying concentrations of graphite at nano scale from 0.02% to 0.18% with an interval at 0.04% in water droplets and the heating temperature from 22 to 85°C, the wetting and evaporation of the sessile droplets are systematically studied with discussion on the impact parameters and the resulted liquid dynamics as well as the stain. The interaction among the phases together with the heating strongly affects the internal circulation inside the droplet, the evaporative rate and the pattern of particles deposition.

Textured surfaces have been extensively employed to investigate the dynamics, wetting phenomena, and shape of liquid droplets. Droplet shape can be controlled via the manipulation of topographic or chemical heterogeneity of a solid surface by anchoring the three-phase line at specific sites. In this study, we demonstrate that droplet shape on a topographically patterned surface can be modified by varying the concentration of salt potassium chloride (KCl) in the droplet solution. It is found that at the beginning of evaporation the octagonal shape of the solid-liquid interface is changed to a rectangle with corners cut upon increasing the salt concentration. Such a variation in the solid-liquid interface versus the salt concentration is explained by the analysis of free energy difference. It indicates that the increases in solid-liquid and liquid-vapor surface tensions by raising the salt concentration result in a favored extension of the three-phase line intersecting the micropyramid bottom sides than the counterpart intersecting the micropyramid diagonal edges. The saline droplets experience a pinning stage at first and a depinning one afterward. The onset of depinning is delayed, and at which the instantaneous contact angle is larger upon raising the salt concentration. The three-phase line which intersects the micropyramid diagonal edges recedes ahead of the one along the micropyramid bottom sides, making the octagonal wetting interface evolve toward a circle. A close view at the droplet edge indicates that the three-phase line repeats "slow slip-rapid slip" across row by row of micropyramids during the depinning stage.

Subwavelength gratings have been of great interest recently due to their ability to eliminate multiple orders. However, high index contrast ( &Delta, n &sim, 3 ) is typically achieved using metals or high-index dielectrics surrounded by vacuum in order to maintain good optical selectivity. Here, we theoretically propose and experimentally realize a selective subwavelength grating using an index contrast of &Delta, 1.2 without vacuum. Despite its low index contrast, our simulation and experiments show that good optical selectivity is achieved using the same physics as subwavelength gratings made of high-index contrast. Such polymer-based encapsulated gratings are easier to scale up for use in large-area applications such as photovoltaics and lighting.

Wetting transitions induced by varying the components in a solution of a drying droplet can lead to its evolving shape on a textured surface. It can provide new insights on liquid pattern control through manipulating droplet solutions. We show the pronounced transitions of wetting for surfactant solution droplets drying on a micropyramid-patterned surface. At low initial surfactant concentrations, the droplet maintains an octagonal shape until the end of drying. At intermediate initial surfactant concentrations, the early octagon spreads to a square, which later evolves to a stretched rectangle. At high initial surfactant concentrations, the droplet mainly exhibits the "octagon-to-square" transition, and the square shape is maintained until the end. The octagon-to-square transition occurs at similar temporal volume-averaged surfactant concentrations for the various initial surfactant concentrations. It results from the dependence of the surface energy change of spread over the micropyramid structure on the temporal volume-averaged surfactant concentration. At high initial surfactant concentrations, the accumulation of the surfactant near the contact line driven by outward flows could raise the local viscosity and enhance the pinning effect, leading to the great suppression of the "square-to-rectangle" transition. Agency for Science, Technology and Research (A*STAR) X.Z., J.R., and F.D. thank the financial support from A*Star SERC A1783c0006. K.S.-L.C. and K.-S.O. thank the AStar Nanoimprint Foundry under grants (1325307091 and 1525300037).

Producing and maintaining specific liquid patterns during evaporation holds great potential for techniques of printing and coating. Here we report the control over the evolution of surfactant solution droplets on the micropyramid substrates during evaporation. The polygonal droplet shape is achieved during the drying rather than solely at the beginning. As the initial surfactant concentration is 0.04 mM, the droplet maintains its initial octagonal shape throughout the lifetime. Interestingly, the initial octagonal shape transforms into a square during the evaporation as the initial surfactant concentration reaches 0.8 mM. These findings can shed light on wetting pattern control for complex solutions required in various applications.

The wettability and evaporation of water-ethanol binary droplets on the substrate with micropyramid cavities are studied by controlling the initial ethanol concentrations. The droplets form octagonal initial wetting areas on the substrate. As the ethanol concentration increases, the side ratio of the initial wetting octagon increases from 1.5 at 0% ethanol concentration to 3.5 at 30% ethanol concentration. The increasing side ratio indicates that the wetting area transforms from an octagon to a square if we consider the octagon to be a square with its four corners cut. The droplets experience a pinning-depinning transition during evaporation. The pure water sessile droplet evaporation demonstrates three stages from the constant contact line (CCL) stage, and then the constant contact angle (CCA) stage, to the mixed stage. An additional mixed stage is found between the CCL and CCA stages in the evaporation of water-ethanol binary droplets due to the anisotropic depinning along the two different axes of symmetry of the octagonal wetting area. Droplet depinning occurs earlier on the patterned surface as the ethanol concentration increases.

A planar micro-optic solar concentrator is reported as part of a natural daylight collection system for illumination of indoor spaces. A ray-tracing model is used to evaluate the full system efficiency, from lens array to planar waveguide and to optical fiber array. The lens array and waveguide modules are feasible to be fabricated using cost-effective nanoimprinting methods. Suitability of such daylighting systems for implementation in tropical regions is systematically presented using meteorological data.

High performance organic light emitting diodes (OLEDs) devices were fabricated using transparent and conductive single wall carbon nanotube (SWCNT) anode. The turn-on voltage of 4–4.5 V and a maximum brightness of 2695 cd/m2 and luminance efficiency of > 10 d/A are achieved by adding a thin nano-thick-layer of CF x coating between the SWCNT anode and hole transport layer (HTL). The interaction of this CF x-layer and the CNT anode is studied by atomic force microscopy (AFM). A simple and effective way of patterning the SWCNT anode is demonstrated.

A composite silver–indium zinc oxide (Ag–IZO) layer is deposited on a polyethylene naphthalate (PEN) film by DC and RF sputter respectively. Tof–SIMS and XPS studies showed that an interface was formed when Ag has successfully plugged into the softer layer of IZO forming a composite Ag-IZO layer to achieve a water vapor transmission rate (WVTR) of ∼1.6 × 10 −4 g/day/m 2 . Poly (3-hexylthiophene) (P3HT) based organic thin-film transistors (OTFT) encapsulated with the Ag–IZO–PEN film kept in glove box and ambient condition are tested and shown that this film is capable to provide the OTFT with adequate protection against moisture and oxygen for more than 3600 h. Mechanical bending tests on the Ag–IZO film also revealed that it has only 0.9% increase in sheet resistance after 10,000 cycles of bending which make this barrier film very suitable for encapsulation of flexible OTFT devices.

Hyperbranched organic–inorganic hybrid conjugated polymers P1 and P2 were prepared via FeCl3-oxiditive polymerization of 4,7-bis(3-ethylhexyl-2-thienyl)-2,1,3-benzothiadiazole (A) and octa(3-ethylhexyl-2-thienyl-phenyl)polyhedral oligomeric silsesquioxane (POSS) (B) at different POSS concentrations. Compared to linear polymer PM derived from A, P1, and P2 exhibit much higher PL quantum efficiency (ϕPL-f) in condensed state with improved thermal stability. ϕPL-f of P1 and P2 increased by 80% and 400%, and the thermal degradation temperatures of P1 and P2 are increased by 35 °C and 46 °C, respectively. Light-emitting diodes were fabricated using P1, P2, and PM. While the electroluminescent spectra of both P1 and PM show λmax at 660 nm, P1 exhibits a much narrower EL spectrum and higher electroluminescence (∼500%) compared with PM at a same voltage and film thickness. The maximum current efficiency of P1 is more than seven times of that of PM. The turn-on voltages of the LEDs are in the order of P2 > PM > P1. LED prepared by blending P1 with MEH-PPV shows a maximum luminescence of 2.6 × 103 cd/m2 and a current efficiency of 1.40 cd/A, which are more than twice (1.1 × 103 cd/m2) and five times (0.27 cd/A) of LED of PM/MEH-PPV blend, respectively. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5661–5670, 2009

The wetting, spreading and drying of pure liquid and nanofluid sessile droplets on a patterned solid surface were investigated systematically in terms of liquid and surface property. The patterned nickel surface was characterized with diamond, circular, hexagon and rectangular pillars. The size ratio between interval and pillars varies from 1.0 to 5.0. The study was firstly carried out for the effect of pure water droplet size on liquid spreading and droplet evaporation process on diamond-shape micro structured substrate with LInterval/LPillar=1.0. Larger amount of liquid leads to a larger wetting area. With fixed substrate (diamond, LInterval/LPillar=1.0) and droplet size (1 μm), mixture of DI water and Ethanol (volume ratio varies from 0.5 to 2.0) was used for generating droplets with different surface tension and evaporation coefficient. Fingering shape would generate on the contact line. With higher concentration of ethanol, the fingering effect is stronger and appeared in a shorter time. The contact area shrinks when increase the size ratio of interval and pillar. This would reduce the length of the contact line, and thus slow down the liquid evaporation. The role of pillar shape was examined based on time for complete evaporation. The effect of surface material on evaporation process was conducted on nickel and PMMA substrate fabricated with the same design. Additionally, investigations were conducted with solutions consisted with nanoparticles and DI water. The mixture were made at different weight ration to achieve concentration of nanoparticles varies from 0.02% to 0.18% with an interval at 0.04%.Copyright © 2016 by ASME

There has been an increased interest in developing top emission organic light-emitting diodes (OLEDs) that are able to emit light from both sides of the OLED display. One important application of the top emission device structure is to achieve monolithic integration of a top-emitting OLED on a polycrystalline or amorphous silicon thin film transistors used in active matrix displays. A high performance dual-sided top-emitting polymer OLED developed in this work exhibited a total luminous efficiency of ∼5.0 cd/A at 4.0 V, which is comparable to that observed for a control device having bottom emission structure. A laser ablation technology was developed to define the pixels. The cathode separation was achieved without using the conventional reverse trapezoid type separators that are normally used for pixellated OLED displays. A prototype of semitransparent polymer light-emitting passive matrix display has a matrix of 100 × 32 with a display area of 32.25 mm by 11.15 mm.

Flexible poly(p-phenylene vinylene) organic light-emitting diodes (OLEDs) with top-emitting architecture were developed on metal-sandwiched polyethylene terephthalate (PET) substrates. Commercial aluminum-laminated PET (Al-PET) was adopted as initial substrates. Different kinds of metal layers (silver and nickel) were then deposited on Al-PET foil to form metal-sandwiched plastic substrates and also to serve as anodes for the top emitting OLEDs. Interface modification was employed to enhance carrier injection from the metal anodes to the polymeric organic stack. Semitransparent top cathodes were used for the flexible top-emitting diodes. The performance of these OLEDs was characterized and compared among different metal anodes. The flexible top-emitting OLEDs on metal-sandwiched PET foil can be bent to a substantial degree without breaking. Moreover, increased contrast ratio of OLEDs can be achieved with specific anodes due to lower reflectance of nickel material.

Transparent conducting aluminum-doped zinc oxide (AZO) films have been prepared on glass substrates by radio frequency magnetron sputtering using a hydrogen-argon gas mixture at room temperature. The effect of hydrogen partial pressure on the optical and electrical properties of AZO films is investigated. The hydrogen partial pressure was varied from 0 to 1.25 x 10 -3 Pa during film growth. Polycrystalline Al-doped ZnO films having a preferred orientation with the c-axis perpendicular to the substrate were obtained with a root mean square roughness of ∼2 nm. At an optimal condition, the AZO films with a resistivity of 5.1 x 10 -4 Ωcm and an optical transparency of over 83% in the visible wavelength region are achieved. The AZO films with desired properties were used as an anode contact in polymeric electroluminescent devices and the electroluminescence performance of these OLEDs was studied.

Highly conducting transparent thin films of aluminium(Al)-doped zinc oxide (ZnO:Al) were deposited by a radio frequency magnetron-sputtering technique using an argon and hydrogen gas mixture at room temperature. Hydrogen serves as a shallow donor and plays a critical role in improving the Al doping efficiency to enhance the conductivity of thin films. The effect of hydrogen partial pressure on the properties of ZnO:Al films was investigated in detail. Polycrystalline ZnO:Al films with a surface roughness of about 2 nm, conductivity of 1.97 ? 103 S cm?1, transmittance of over 83% in the visible wavelength region and an optical band gap of 3.93 eV were achieved at a hydrogen partial pressure of 7.5 ? 10?4 Pa. A ZnO:Al film with the desired properties was used as an anode contact in a bi-layer polymeric light-emitting diode. A polyethylene dioxythiophene?polystyrene sulfonate doped with poly(styrenesulfonic acid) (PEDOT: PSS) and phenyl-substituted poly(p-phenylene vinylene) (Ph-PPV) were employed as a hole transport layer and a light-emitting layer, respectively. The electro-luminescence performance of the aforementioned diode was studied and compared to a control device with an indium tin oxide anode.

Polymeric light-emitting diodes (PLEDs) have been fabricated with top emissive architecture and a shift of the light emission wavelength is observed for a single emitting material—phenyl-substituted poly(p-phenylenevinylene) (Ph-PPV). The device is built on a glass substrate and its structure consists of a bi-layer anode (including a high reflective metal thin film and indium tin oxide anode—ITO thin film), polymer stack and semitransparent multi-layer cathodes. Poly(styrene sulfonate)-doped poly(3,4-ethylene dioxythiophene) (PEDOT: PSS) and Ph-PPV are employed as the hole transport layer and light-emitting layer, respectively. Changes in the electroluminescence (EL) characteristics of the PLEDs are examined by adjusting the ITO thickness. The EL peaks of the devices with a single emissive layer have a wide variation in the wavelength range, from 547 to 655 nm, due to the different ITO thicknesses.

A series of hyperbranched alternating copolymers were synthesized by either Suzuki or Grignard coupling polycondensation reactions between two AB4-type tetrahedral monomers [tetra(4-bromobiphenyl)silane (1) and tetra(2-bromo-9,9-dihexylfluorenyl)silane (2)] with AB2-type dibromo or diboronic acid monomers of 9,9-dihexylfluorene or oligothiophenes. Polymers (3, 5−8) (derived from 1 and 9,9-dihexylfluorene-2,7-diboronic acid, 2,2‘-bithiophene-5,5‘-diboronic acid, 2,2‘:5‘,2‘ ‘-terthiophene-5,5‘ ‘-diboronic acid, 5,5‘-dibromo-3,3‘-dihexyl-2,2‘-bithiophene, and 5,5‘ ‘-dibromo-3,3‘-dihexyl-2,2‘:5‘,2‘ ‘-terthiophene, respectively) emitted blue to red light in the solid state and their absorption and emission maxima were red shifted with the increase in conjugation length. Polymer 4 (derived from 2 and 9,9-dihexylfluorene-2,7-diboronic acid) emitted blue light highly efficiently in both solution and the condensed state, but its absorption and emission maxima were slightly blue-shifted from those of 3, revealing t...

High-quality low-temperature indium tin oxide (ITO) thin films were deposited on polymer-reinforced ultrathin glass for flexible polymer light emitting devices (PLEDs). For a 130-nm thick ITO film, a sheet resistance of 25 Ω/sq was obtained when it was prepared at a hydrogen partial pressure of 2.6×10−3 Pa. The transmittance of ITO film was above 85% over the visible spectrum. A maximum electroluminescent (EL) efficiency of 5.1 cd/A for the flexible PLEDs at an operating voltage of 5 V was obtained. The bending properties of ultrathin glass with a reinforcement polymer layer were investigated. The minimum-bending radius of reinforced glass was calculated. The improvement in the flexibility of ultrathin glass sheet due to the polymer reinforcement was clearly demonstrated.

The first soluble hyperbranched tetrahedral polymers were prepared by Suzuki coupling polycondensation reaction between tetrabromoarylmethane/silane and 9,9-dihexylfluorene-2,7-diboronic acid at low concentrations. The polymers exhibited high thermal stability with their decomposition temperatures (Tds) in the range 352−449 °C. The polymers emitted blue light highly efficiently in both solution and the solid state. The photoluminescence quantum efficiencies of the polymers in THF solution were in the range 73−99%, and those in thin films were 38−82%. The film PL spectra of the polymers exhibited similar spectral patterns to those determined in solutions, with 1−8 nm red shift in their emission maxima and 0−8 nm increase in their full width at half-maximum (fwhm) values being observed. No long wavelength excimer-like emissions at 500−600 nm, which were typical for polyfluorenes due to their self-aggregation in the solid state, were observed. Thus, the polymers were less prone to self-aggregation in the sol...

A new family of Mn(II) complexes stabilized by carboxylate-rich hybrid ligands has been synthesized, characterized and applied as catalytic precursors for MnO2 nanoparticles in water oxidation by (NH4)2Ce(NO3)6.

The pre-defined selective positioning of a controlled number of vesicles on a rigid substrate is crucial in many potential applications such as diagnostics, biosensors, lab-on-a chip, microanalyses and reaction chambers. In this paper, the vesicles made up of block copolymer using Poly [-(2-methyloxazoline) -poly- (dimethylsiloxane)-poly- (2-methyloxazoline)] (ABA) with dimensions of 100-200 nm are trapped by physisorption on hydrophilic surfaces. We discuss the protocols established for vesicle trapping. The optimum conditions obtained for physisorption is 15 minutes incubation followed by one cycle of DI water rinse. Trapping of 1-10 vesicles in lobe shape micro-wells fabricated by photo lithography using photoresist on UltraStick(™) slides was demonstrated. To overcome the issue of amalgamation of emitted light from optically sensitive photoresist and fluorescently tagged vesicles, an alternative approach of Si/SiO(2) microwell array coupled with APTES (3-AminoPropylTriEthoxySilane) treated bottom surfaces was developed.

Thin form-factor planar sunlight concentrators fabricated using low-cost materials in conjunction with high-efficiency solar cells may prove economically competitive with silicon photovoltaics. A periodic structure with planar interfaces employing an array of microlenses and coupling structures is amenable to high-volume imprint manufacturing and hence a drastic cost reduction. A numerical study of periodic prismatic void structures aligned with microlenses to efficiently couple incoming light into a constant cross-section channel waveguide is reported. Three different prism-like cut-out structures: a bare prism, a planar connected prism, and a pointed prism are proposed. Numerical results show that the proposed pointed prism design offers a high-efficiency coupling with one-sided output (90.8% and 85.4% peak efficiency at concentration ratios of 320 × and 400 ×, respectively).

We report the results of an effort to understand the effect of surface electronic structure of indium tin oxide (ITO) on luminance efficiency of organic light-emitting devices (OLED)s. Nitric oxide (NO) plasma was used to modify the ITO. NO plasma induced an increase in the sheet resistance of ITO. The surface electronic structure of ITO was studied using X-ray photoelectron spectroscopy. An approximately 4-nm thick low conductivity layer with a production of N-O type species was formed near the ITO surface region. It is demonstrated that the barrier for hole-injection from an ITO anode to a hole transporting layer can be engineered by NO plasma treatment. The increase in luminance efficiency of the OLEDs reflects an improved current balance in the device.

Displays based on organic electroluminescent (EL) materials have entered the marketplace already and demonstrated remarkable contrast, high brightness and crisp colors. However, one of the key advantages of this new technology has not been commercially exploited yet: Fabrication of a display that is still fully functional even when it is bent or flexed. This is possible since organic EL devices comprising only thin, amorphous solid state films and optical properties have no critical dependence on the film thickness. In this paper we address the important elements that are required to produce a flexible organic EL display. Most crucial is the selection of a flexible substrate. Here we present results obtained with ultra-thin inorganic glass materials as well as polymeric foils. For the glass substrates we determined the ultimate mechanical properties for different device configurations. In the case of polymeric substrates permeation of water and oxygen molecules through the substrate is the governing factor. We compare the performance of different barrier systems. In summary we demonstrate that OLED devices with certain flexibility can be reliably built on ultra-thin glass substrates. For polymeric substrates a lot of progress has been achieved in terms of the required barrier properties and other necessary ingredients and this might result later into a commercial organic EL product.

An aluminum-laminated polyethylene terephthalate (Al-PET) is used as the substrate for flexible organic light-emitting devices (OLEDs). The efficient flexible electroluminescent devices have a top-emitting OLED architecture. An acrylic layer is formed on the Al-PET surface to improve the surface morphology and also the adhesion between the substrate and the anode. Poly(styrene sulfonate)-doped poly(3,4-ethylene dioxythiophene) was used as hole transporting layer. The light-emitting polymer used is a phenyl-substituted poly(p-phenylenevinylene). Bilayer anodes of Ag∕CFX and Ag/indium-tin oxide and a semitransparent top cathode were used for the flexible polymer OLEDs. For a flexible polymer OLED with a 110-nm-thick light-emitting polymer, it exhibited superior electrical and optical characteristics with a luminous efficiency of 4.56cd∕A at an operating voltage of 7.5V.