Blue‐light‐emitting poly(arylene ethynylenes) containing alternating sequences of biphenylene or fluorenediyl and p‐phenylene moieties linked through triple bonds

Two new poly(arylene ethynylenes) were synthesized by the reaction of 1,4‐diethynyl‐2.5‐dioctylbenzene either with 4,4′‐diiodo‐3,3′‐dimethyl‐1,1′‐biphenyl or 2,7‐diiodo‐9,9‐dioctylfluorene via the Sonogashira reaction, and their photoluminescence (PL) and electroluminescence (EL) properties were studied. The new poly(arylene ethynylenes) were poly[(3,3′‐dimethyl‐1,1′‐biphenyl‐4,4′‐diyl)‐1,2‐ethynediyl‐(2,5‐dioctyl‐1,4‐phenylene)‐1,2‐ethynediyl] (PPEBE) and poly[(9,9‐dioctylfluorene‐2,7‐diyl)‐1,2‐ethynediyl‐(2,5‐dioctyl‐1,4‐phenylene)‐1,2‐ethynediyl] (PPEFE), both of which were blue‐light emitters. PPEBE not only emitted better blue light than PPEFE, but it also performed better in EL than the latter when the light‐emitting diode devices were constructed with the configuration indium–tin oxide/poly(3,4‐ethylenedioxythiophene) doped with poly(styrenesulfonic acid) (50 nm)/polymer (80 nm)/Ca:Al. The device constructed with PPEBE exhibited an external quantum efficiency of 0.29 cd/A and a maximum brightness of about 560 cd/m², with its EL spectrum showing emitting light maxima at λ = 445 and 472 nm. The device with PPEFE exhibited an efficiency of 0.10 cd/A and a maximum brightness of about 270 cd/m², with its EL spectrum showing an emitting light maximum at λ = 473 nm. Hole mobility (μ_h) and electron mobility (μ_e) of the polymers were determined by the time‐of‐flight method. Both polymers showed faster μ_h values. PPEBE revealed a μ_h of 2.0 × 10^?4 cm²/V·s at an electric field of 1.9 × 10⁵ V/cm and a μ_e of 7.0 × 10^?5 cm²/V·s at an electric field of 1.9 × 10⁵ V/cm. In contrast, the mobilities of the both carriers were slower for PPEFE, and its μ_h (8.0 × 10^?6 cm²/V·s at an electric field of 1.7 × 10⁶ V/cm) was 120 times its μ_e (6.5 × 10^?8 cm²/V·s at an electric field of 8.6 × 10⁵ V/cm). The much better balance in the carriers' mobilities appeared to be the major reason for the better device performance of PPEBE than PPEFE. Their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels were also a little different from each other. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 299–306, 2006     

Electrical properties and the role of inhomogeneities at the polyvinyl alcohol/n‐inp schottky barrier interface

In this work, we have investigated the electrical properties of Au/n‐InP contacts with a thin layer of polyvinyl alcohol (PVA) as an interlayer. The current–voltage (I–V) and capacitance–voltage (C–V) measurements are carried out in the temperature range of 175–425 K. The Au/PVA/n‐InP Schottky structure show nonideal behaviors and indicates the presence of a nonuniform distribution of interface states. The temperature dependent interface states densities (N_SS), ideality factor and barrier height are obtained. An abnormal decrease in zero‐bias barrier height (BH) and increase in the ideality factor ( ) with decreasing temperature have been explained on the basis of the thermionic emission theory with Gaussian distribution (GD) of the BHs due to the BH inhomogeneities. The experimental I–V characteristics of Au/PVA/n‐InP Schottky diode has revealed the existence of a double GD with mean BH values of ( ) of 1.246 and 0.899 eV and standard deviation ( ) of 0.176 and 0.137 V, respectively. Consequently, the modified conventional activation energy versus plot gives and Richardson constants ( ) and the values are 1.17 and 0.71 eV and 9.9 and 6.9 A/cm² K², respectively, without using the temperature coefficient of the BH. The effective Richardson constant value of 9.9 A/cm² K² is very close to the theoretical value of 9.4 A/cm² K² for n‐InP. The discrepancy between Schottky barrier heights estimated from I–V and C–V measurements is also discussed. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39773.     

Low-bandgap copolymers consisting of 2,1,3-benzoselenadiazole and carbazole derivatives with thiophene or selenophene π-bridges

Two donor–acceptor-type alternating copolymers consisting of 2,1,3-benzoselenadiazole and carbazole derivatives with thiophene or selenophene π-bridges were synthesized by Suzuki cross-coupling polymerization, and their optical, electrochemical, and photovoltaic properties were compared. The selenophene π-bridged copolymer (PCz-DSeBSe) exhibited a smaller band-gap (1.82 eV) than the thiophene-bridged polymer (PCz-DTBSe; 1.89 eV). PCz-DSeBSe also showed a deeper highest occupied molecular orbital energy level (−5.36 eV) than PCz-DTBSe (−5.20 eV). Moreover, the PCz-DSeBSe thin film showed higher crystallinity and hole mobility than the PCz-DTBSe thin film. Organic photovoltaic devices were fabricated using the polymers as the donors and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as the acceptor. The device using PCz-DSeBSe showed a higher open circuit voltage (V_oc), short circuit current density (J_sc), and power conversion efficiency (PCE) than that using PCz-DTBSe. The fabricated indium tin oxide/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/PCz-DSeBSe:PC₇₁BM/LiF/Al device showed the maximum PCE of 2.88% with a J_sc of 7.87 mA/cm², an V_oc of 0.80 V, and a fill factor of 0.50 under AM 1.5G irradiation (100 mW/cm²).     

Polymer particles with a pomegranate-like internal structure via micro-dispersive polymerization in a geometrically confined reaction space I. Experimental study

The synthesis of micron-sized polymer particles with a core-shell pomegranate-like morphology is presented. The proposed polymerization technique takes advantage of a reaction-induced micro-phase separation within a suspended organic liquid droplet containing monomer, a chemical initiator, a steric stabilizer, and a poor solvent for the polymer. With an increase in monomer conversion, the monomer droplet suspended in a continuous aqueous medium is transformed first into a micro-capsule with a thick pericellular membrane, and eventually into a polymer particle packed with 300-500 nm polymer sub-particles. The experimentally observed evolution of particle morphology indicates that the reaction pathway is strongly influenced by micro-phase separation and transport phenomena. In the first stage of polymerization, a pseudo-homogeneous polymerization takes place at the droplet surface, followed by a starved macro-dispersive polymerization in the inner region where polymer precipitates out from the solvent phase as nano-sized sub-particles.     

Effect of field enhancement on inorganic powder electroluminescence using short carbon nanotubes

Inorganic powder electroluminescence (IPEL) devices with the insertion of a carbon nanotube (CNT) layer were investigated to verify the effect of the increased local field produced by CNTs on electroluminescence (EL). To increase the field strength effectively, the CNTs were shortened using the cryogenic crushing method. IPEL devices with the insertion of a short CNT layer exhibited an increase in brightness and efficiency with increasing amount of CNTs. The local field enhancement by CNTs, further enlarged by the triple-junction, could increase the field strength applied to the phosphor, resulting in improved EL performance. In addition, short CNTs in an EL device can lead to field enhancement without an unintentional current flowing into the device.     

High-performance field emission from a carbon nanotube carpet

We have created a field emitter composed of a carbon nanotube (CNT) yarn, which was prepared by direct spinning through chemical vapor deposition and then formed into a carpet structure by tying the yarn to a conductive substrate before cutting it. The structure of the carpet is arranged to induce the tips of the CNT yarn to protrude toward the anode for maximum electron emission. The turn-on field, threshold field, and field enhancement factor of the device are 0.33, 0.48 V/μm, and 19,141, respectively. Extremely low operating electric fields and a high field enhancement factor result from the high density of CNT emitters with high crystallinity, the electrically good contact between the emitters and the substrate, and the effects of the multistage structure. The emission is stable even at a high current density of 2.13 mA/cm², attributed to the strong adhesion between the emitters and the substrate. The emission performance is found to be customizable by adjusting the structure, for example, the CNT pile density. These results are relevant for practical applications, such as large-area flat-panel displays, large-area low-voltage lamps, and X-ray sources.     

Hydrogenation of carbon monoxide to methane over mesoporous nickel-M-alumina (M = Fe,Ni, Co,Ce, and La) xerogel catalysts

Mesoporous nickel(30 wt%)-M(10 wt%)-alumina xerogel (30Ni10MAX) catalysts with different second metal (M = Fe, Ni, Co, Ce, and La) were prepared by a single-step sol–gel method for use in the methane production from carbon monoxide and hydrogen. In the methanation reaction, yield for CH₄ decreased in the order of 30Ni10FeAX > 30Ni10NiAX > 30Ni10CoAX > 30Ni10CeAX > 30Ni10LaAX. Experimental results revealed that CO dissociation energy of the catalyst and H₂ adsorption ability of the catalyst played a key role in determining the catalytic performance of 30Ni10MAX catalyst in the methanation reaction. Optimal CO dissociation energy of the catalyst and large H₂ adsorption ability of the catalyst were favorable for methane production. Among the catalysts tested, 30Ni10FeAX catalyst with the most optimal CO dissociation energy and the largest H₂ adsorption ability exhibited the best catalytic performance in terms of conversion of CO and yield for CH₄ in the methanation reaction. The enhanced catalytic performance of 30Ni10FeAX was also due to a formation of nickel–iron alloy and a facile reduction.     

Synthesis and optical characterization of novel Sr3Ga2O6:Eu phosphor

Alkaline earth metal gallets have been identified as an important ceramic material. The crystal chemistry of many of these gallets is well explored; however, very rare studies regarding optical properties of rare earth (RE) ions doped in such gallets, particularly in Sr₃Ga₂O₆ host, have been carried out. The present study reports on synthesis and characterization of novel Sr₃Ga₂O₆:Eu³⁺ phosphors. The phosphors have been synthesized using a conventional solid state reaction method. Crystal structure, morphology and luminescence properties (excitation, emission and CIE coordinate) of these phosphors have been studied as a function of sintering temperature and Eu³⁺ concentration. X-ray diffraction study reveals that the phosphor sintered at low temperature (900 °C) contains an impurity phase which is removed at higher sintering temperatures and results into cubic crystalline phase of Sr₃Ga₂O₆. Particle size of the phosphor increases with an increase in sintering temperature which results to a red shift in the peak position of excitation band lying in a broad range from 250 to 370 nm. Optimum emission intensity is attained for 0.12 mol% concentration of Eu³⁺ ions; above this concentration, a quenching in emission intensity is observed.     

Preparation and characterization of antifouling poly(vinylidene fluoride) blended membranes

Poly(vinylidene fluoride) (PVDF) membranes have been widely used in microfiltration and ultrafiltration because of their excellent chemical resistance and thermal properties. However, PVDF membranes have exhibited severe membrane fouling because of their hydrophobic properties. In this study, we investigated the antifouling properties of PVDF blended membranes. Antifouling PVDF blended membranes were prepared with a PVDF‐g‐poly(ethylene glycol) methyl ether methacrylate (POEM) graft copolymer. The PVDF‐g‐POEM graft copolymer was synthesized by the atom transfer radical polymerization (ATRP) method. The chemical structure and properties of the synthesized PVDF‐g‐POEM graft copolymer were determined by NMR, Fourier transform infrared spectroscopy, and gel permeation chromatography. To investigate the antifouling properties of the membranes, we prepared microfiltration membranes by using the phase‐inversion method, which uses various PVDF/PVDF‐g‐POEM concentrations in dope solutions. The pure water permeabilities were obtained at various pressures. The PVDF/PVDF‐g‐POEM blended membranes exhibited no irreversible fouling in the dead‐end filtration of foulants, including bovine serum albumin, sodium alginate, and Escherichia coli broth. However, the hydrophobic PVDF membrane exhibited severe fouling in comparison with the PVDF/PVDF‐g‐POEM blended membranes. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Onset of Soret convection in a nanoparticles-suspension heated from above

The onset of Soret-driven convection in a nanoparticles suspension heated from above is analyzed theoretically based on linear theory and relative instability concept. A new set of stability equations are derived and solved by using the dominant mode method. The dimensionless critical time τ _c to mark the onset of instability is presented here as a function of the Rayleigh number, the Lewis number and the separation ratio. Available experimental data indicate that for large Rayleigh number convective motion is detected starting from a certain time τ≈3 τ _c . This means that the growth period of initiated instabilities is needed for convective motion to be detected experimentally. It seems evident that during τ _c ≤ τ ≤ 3 τ _c convective motion is relatively very weak and the primary diffusive transfer is dominant. This article is dedicated to Professor Chang Kyun Choi for celebrating his retirement from the School of Chemical and Biological Engineering, Seoul National University.