Unambiguously Enhanced Ultraviolet Luminescence of AlGaN Wavy Quantum Well Structures Grown on Large Misoriented Sapphire Substrate

High‐quality epitaxy consisting of Al_1?xGa_xN/Al_1?yGayN multiple quantum wells (MQWs) with sharp interfaces and emitting at ≈280 nm is successfully grown on sapphire with a misorientation angle as large as 4°. Wavy MQWs are observed due to step bunching formed at the step edges. A thicker QW width accompanied by a greater accumulation of gallium near the macrostep edge than that on the flat‐terrace is observed on 4° misoriented sapphire, leading to the generation of potential minima with respect to their neighboring QWs. Consequently, a significantly enhanced photoluminescence intensity (at least ten times higher), improved internal quantum efficiency (six times higher at low excitation laser power), and a much longer carrier lifetime are achieved. Importantly, the wafer‐level output‐power of the ultraviolet light emitting diodes on 4° misoriented substrate is nearly increased by 2–3 times. This gain is attributed to the introduction of compositional inhomogeneities in AlGaN alloys induced by gallium accumulation at the step‐bunched region thus forming a lateral potential well for carrier localization. The experimental results are further confirmed by a numerical modeling in which a 3D carrier confinement mechanism is proposed. Herein, the compositional modulation in active region arising from the substrate misorientation provides a promising approach in the pursuit of high‐efficient ultraviolet emitters.     

Long Electron–Hole Diffusion Length in High‐Quality Lead‐Free Double Perovskite Films

Developing environmentally friendly perovskites has become important in solving the toxicity issue of lead‐based perovskite solar cells. Here, the first double perovskite (Cs₂AgBiBr₆) solar cells using the planar structure are demonstrated. The prepared Cs₂AgBiBr₆ films are composed of high‐crystal‐quality grains with diameters equal to the film thickness, thus minimizing the grain boundary length and the carrier recombination. These high‐quality double perovskite films show long electron–hole diffusion lengths greater than 100 nm, enabling the fabrication of planar structure double perovskite solar cells. The resulting solar cells based on planar TiO₂ exhibit an average power conversion efficiency over 1%. This work represents an important step forward toward the realization of environmentally friendly solar cells and also has important implications for the applications of double perovskites in other optoelectronic devices.     

Doping and Switchable Photovoltaic Effect in Lead‐Free Perovskites Enabled by Metal Cation Transmutation

Creating defect tolerant lead‐free halide perovskites is the major challenge for development of high‐performance photovoltaics with nontoxic absorbers. Few compounds of Sn, Sb, or Bi possess ns² electronic configuration similar to lead, but their poor photovoltaic performances inspire us to evaluate other factors influencing defect tolerance properties. The effect of heavy metal cation (Bi) transmutation and ionic migration on the defects and carrier properties in a 2D layered perovskite (NH₄)₃(Sb_(1?x)Bi_x)₂I₉ system is investigated. It is shown, for the first time, the possibility of engineering the carriers in halide perovskites via metal cation transmutation to successfully form intrinsic p‐ and n‐type materials. It is also shown that this material possesses a direct–indirect bandgap enabling high absorption coefficient, extended carrier lifetimes >100 ns, and low trap densities similar to lead halide perovskites. This study also demonstrates the possibility of electrical poling to induce switchable photovoltaic effect without additional electron and hole transport layers.     

Passively Q-switched erbium-doped fibre laser using cobalt oxide nanocubes as a saturable absorber

We demonstrate a Q-switched Erbium-doped fibre laser (EDFL) utilizing cobalt oxide (Co₃O₄) nanocubes film based saturable absorber (SA) as a passive Q-switcher. Co₃O₄ nanocubes are embedded into a polyethylene oxide film to produce a high nonlinear optical response, which is useful for SA application. It has saturation intensity and modulation depth of 3 MW/cm² and 0.35%, respectively. The proposed laser cavity successfully generates a stable pulse train where the pulse repetition rate is tunable from 29.8 to 70.92 kHz and the pulse-width reduces from 10.9 to 5.02 μs as the 980 nm pump power increases. This result indicates that the Co₃O₄ is excellent for constructing an SA that can be used in producing a passively Q-switched fibre laser operating at a low pump intensity. To the best of our knowledge, this is the first demonstration of Co₃O₄ film based fibre laser.     

Asymmetric organocatalysis of structurally well-defined chiral quaternary ammonium fluorides

Unique organocatalysis of both naturally occurring cinchona alkaloid-derived and purely synthetic chiral quaternary ammonium fluorides in synthetically useful stereoselective bond-forming reactions is overviewed. The development of this chemistry was initiated by the in situ generation of generally hygroscopic ammonium fluorides from the corresponding easy-to-handle ammonium salts in the presence of excess metal fluorides and their direct use for subsequent enantioselective reactions. On the other hand, chiral ammonium fluorides have been prepared by using ion-exchange resins and successfully applied as catalyst to various asymmetric bond formation reactions under homogeneous conditions. In addition, utilization of chiral quaternary ammonium bifluorides as organocatalysts in asymmetric synthesis is described, featuring their characteristic reactivity and selectivity.     

Polyethersulfone composite hollow-fiber membrane prepared by in-situ growth of silica with highly improved oily wastewater separation performance

In order to reduce surface aggregation and enhance the performance of PES membranes, a hydrophilic PES/TEOS HF membrane was developed for the treatment of wastewater containing oil. PES/TEOS was prepared via a sol-gel self assembly and dry–wet spinning method. Silicon dioxide sol was prepared from a mixture of tetraethoxysilane, ethanol, water, and acetic acid (acting as the catalyst). HF hybrid membranes were produced from dope solutions containing polyethersulfone, polyethylene glycol, silicon sol, and NMP. The membranes were characterized by field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDX), porosity, fourier transform infrared spectroscopy (FTIR), and contact angle measurements. The composite membranes were successfully used to treat wastewater containing oil and their separation performance were evaluated. The PES/TEOS-2 membrane displayed the best performance, with a permeate flux of 90.937 L/m² h and an oil retention of 99.98%. In addition, this membrane showed a higher pure water flux of 102.43 L/m² h as compared to PES-0 and PES/SiO₂–1 membranes (87.347 L/m² h and 91.949 L/m² h, respectively). The PES/TEOS-2 membrane also presented enhanced antifouling behavior with a FRR and a RFR of 93.33% and 11.22%, respectively. In addition, this membrane displayed excellent long-term recycling properties, making it a desirable candidate for oily wastewater separation applications.     

Low‐cost multi‐standard simultaneous forward and inverse video transform core

This paper presents a cost‐effective, two‐dimensional (2‐D) discrete cosine transform (DCT) and inverse discrete cosine transform (IDCT) capable of MPEG1/2/4, H.264 4 × 4/8 × 8, and VC‐1 4 × 4/8 × 8/4 × 8/8 × 4 standards. We developed multilevel factor sharing in conjunction with distributed arithmetic in a scheme referred to as common sharing distributed arithmetic to enable sharing of the coefficient matrix circuit and replace multipliers with adders and shifters. By taking advantage of the similarities between DCT and IDCT transforms, we were able to implement an interlaced sorting method in a single circuit of the DCT and IDCT transform core in order to reduce area overhead while enabling the simultaneous operation of DCT and IDCT. The proposed design arranges the data of the first dimension and second dimension in order to reuse the same 1‐D core to compute 2‐D data. In this manner, first dimension and second dimension data of DCT and IDCT can be processed simultaneously in a single transform core. The efficacy of the proposed approach has been verified by fabricating a test chip using the Taiwan Semiconductor Manufacturing Company (TSMC) 0.18 µm complementary metal‐oxide semiconductor process. The inverse transform core was shown to have an operating frequency of 227 MHz and throughput of 454 Mpel/s with a gate count of 32.5 k. Copyright © 2015 John Wiley & Sons, Ltd.     

Charge-trapping effects caused by ammonia in carbon nanotubes

Hysteretic behaviors caused by low-concentration ammonia gas are found in single-walled carbon nanotube based field-effect transistors. The transfer curves are found to shift towards negative gate voltage when the gate voltage is swept forwardly upon introducing ammonia. In contrast, no significant change in the transfer curves is observed for the backward sweeping of the gate voltage. This phenomenon is repeatable even after the devices are annealed in dry air at 200 degrees C for 2 hrs. Our findings can be interpreted in terms of additional charge traps induced by the adsorbed ammonia molecules. The observed hysteretic behavior enables the devices to work as memory cells, in which the carbon nanotube field-effect transistors act as readout and ammonia molecules play roles of charge storage.     

The indispensability of dispensable indexes

The design of new indexes has been driven by many factors, such as data types, operations, and application environment. The increasing demand for database systems to support new applications such as online analytical processing (OLAP), spatial databases, and temporal databases has continued to fuel the development of new indexes. In this paper, we summarize the major considerations in developing new indexes, paying particular attention to progress made in the design of indexes for spatial, temporal databases, and object-oriented databases (OODB). Our discussion focuses on the general concepts or features of these indexes, thus presenting the building blocks for meeting the challenges of designing new indexes for novel applications to be encountered in the future     

Current equalization in SPWM FACTS controllers at lowest switchingrates

This paper shows that sinusoidal pulsewidth modulation (SPWM) flexible AC transmission system (FACTS) controllers can be synthesized by employing the phase-shifted triangle carrier technique to multiconverters at a switching rate of only three pulses. The options of parallel, series and series/parallel module connections are examined for current equalization. This paper presents simulation and experimental results