To enhance the light extraction efficiency of traditional light-emitting diodes (LEDs) by reducing the total internal reflection, an omni-directional reflector (ODR) and photonic crystal are adopted in the paper. The structures of photonic crystal and the ODR are designed by diffraction theory and finite difference time domain (FDTD) method. The photonic crystal is employed in the p-GaN layer and the ODR composed of TiO 2 /SiO 2 is designed between the active region and substrate. The simulation results indicate that the light extraction of LEDs can be enhanced by 11.6 times, and the external quantum efficiency of LEDs will be effectively improved.The development of LEDs is limited because of their low output power. Hence, it is a crucial issue to improve the quantum efficiency of LEDs. This can be achieved by improving internal quantum efficiency or light extraction efficiency. The internal quantum efficiency has been greatly enhanced owing to high quality materials [1] and crystal growth [2] technology. Therefore, how to improve light extraction has been one of the hotspots at home and abroad recently.There are two reasons to explain the low light extraction efficiency of traditional LEDs. First, because of the total internal reflection at the interface between the outer medium and semiconductor, most light forms guided modes in the active region rather than radiation modes [3] . Second, the substrate of LEDs is generally thick, so a large part of energy radiated into the substrate will be absorbed. Generally the light extraction efficiency is only 4% [4] .Aiming at the former, there are many methods to increase the efficiency [5][6][7][8] . Among them, photonic crystal is considered as a potential structure because of its large enhancement of light efficiency [4] . Currently, the feasible fabrication of photonic crystal incorporated in GaN-based LEDs has been investigated and implemented.For the latter, an ODR employed between the substrate and active region reflects the downward light to reduce energy loss in the substrate. The efficiency could be effectively increased by the ODR [3] . Lin et al. [9] have proved that the ODR composed of TiO 2 /SiO 2 does have better performance than a metallic ODR.The simulation methods adopted in the paper are diffraction theory and FDTD. Photonic crystal is designed on the top surface to extract the guided mode, while a composite distributed Bragg reflector (DBR) as an ODR is employed between quantum wells and the substrate to reflect the downward light. Simulation results indicate that the designed structure can enhance the light extraction by 11.6 times, which effectively increases the external quantum efficiency of LEDs. Fig.1 shows the LED structure with photonic crystal and a composite DBR. The structure consists of a 5 m-thick substrate, two DBRs with different parameters as an ODR, a 3.5 m-thick n-GaN layer, a multiple quantum-well (MQW) which is composed of five periods of 3 nm/7 nm-thick InGaNGaN well, and a 0.2 m p-GaN layer. The photonic crystal is designed in...
In this work, a novel corrosion‐inhibiting polycarboxylate superplasticizer for concrete was synthesized through two‐step esterification reaction and one‐step polymerization. Firstly, arginine‐polyethylene glycol monoester was synthesized by p‐toluenesulphonic acid‐catalyzed esterification reaction of one hydroxyl of polyethylene glycol and carboxylic of arginine containing the main functional group of corrosion‐inhibiting polycarboxylate superplasticizer. Secondly, under the condition of hydroquinone as a polymerization inhibitor, polycarboxylate macromonomer was also synthesized by p‐toluenesulphonic acid‐catalyzed esterification reaction of another hydroxyl of as‐prepared monoester and carboxylic of methyl acrylic acid (96 % esterification rate). Finally, a corrosion‐inhibiting polycarboxylate superplasticizer for concrete was synthesized by ammonium peroxydisulphat‐initiated polymerization, polycarboxylate macromonomer, methyl acrylic acid, and sodium methyl acrylic acid. The structures of monoester, macromonomer, and superplasticizer were characterized by FTIR. Moreover, the fluidity of cement paste, the water reduction, and the compressive strength ratio of as‐prepared polycarboxylate macromonomer (0.5 %) as a corrosion‐inhibiting superplasticizer were more than 290 mm, 25 %, and 130 % respectively.
An
anthraquinone-2-sulfonate and reduced graphene oxide nanocomposite
(AQS@rGO) was prepared and the improvement on the biotic reduction
of a pollutant,
i.e.
, azo dye, was demonstrated.
Electron paramagnetic resonance signal of the semi-quinone radical
in the well-dispersed solid AQS@rGO solution was detected. Although
the as-prepared AQS@rGO has a negligible adsorption capacity toward
methyl orange (MO) dye, the decolorization efficiencies in both flask
experiments and sequencing operation reactors in the presence of AQS@rGO
were increased by more than 1.5 times as compared to that with graphene
oxide, and an efficient and continuable catalytic effect on the decolorization
of azo dyes in seven operation periods was maintained. The catalytic
effect on reduction was caused by the formation of a space-charge
layer, which facilitates the efficient e
–
transfer
from the conductive rGO sheets to the C=O of the AQS molecule.
The results suggested that the AQS@rGO may act as an efficient insoluble
redox mediator, which is important for the pollution control by accelerating
the extracellular electron transfer.
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