Search Results (23)

As the atmospheric carbon dioxide (CO₂) concentration rapidly rises, carbon capture, utilization, and storage (CCUS) is an emerging field for climate change mitigation. Various carbon capture technologies are in development with the help of adsorbents, membranes, solvent-based systems, etc. One of the main challenges in this field is the removal of CO₂ from nitrogen (N₂) gas. This paper focuses on mixed matrix membrane technology, for which the CO₂/N₂ separation performance is based on differences in gas permeations. Membrane separation and purification technologies are widely studied for carbon capture. Microporous adsorbents such as zeolites and metal organic frameworks (MOFs) for carbon capture have been attracting researchers’ attention due to their highly porous structures, high selectivity values, and tunable porosities. Utilizing microporous adsorbents dispersed within a novel, blended polymer matrix, fourteen membranes were prepared with the commercial MOF ZIF-8, zeolite 13X, and kaolin, with methyl cellulose (MC) and polyvinyl alcohol (PVA), which were tested using a single gas permeation setup in this study. The addition of polyallylamine (PAH) as a chemisorbent was also investigated. These membranes were synthesized both with and without a polyacrylonitrile (PAN) support to compare their performances. MC was found to be an ideal polymeric matrix component to develop free-standing MMMs. At 24 °C and a relatively low feed pressure of 2.36 atm, a free-standing zeolite-13X-based membrane (MC/PAH/13X/PVA) exhibited the highest N₂/CO₂ selectivity of 2.8, with a very high N₂ permeability of 6.9 × 10⁷ Barrer. Upon the optimization of active layer thickness and filler weight percentages, this easily fabricated free-standing MMM made of readily available materials is a promising candidate for CO₂ purification through nitrogen removal. Full article

We fabricated an air-tunnel structure between a gallium nitride (GaN) layer and trapezoid-patterned sapphire substrate (TPSS) through the in situ carbonization of a photoresist layer to enable rapid chemical lift-off (CLO). A trapezoid-shaped PSS was used, which is advantageous for epitaxial growth on the upper c-plane when forming an air tunnel between the substrate and GaN layer. The upper c-plane of the TPSS was exposed during carbonization. This was followed by selective GaN epitaxial lateral overgrowth using a homemade metal organic chemical vapor deposition system. The air tunnel maintained its structure under the GaN layer, whereas the photoresist layer between the GaN layer and TPSS disappeared. The crystalline structures of GaN (0002) and (0004) were investigated using X-ray diffraction. The photoluminescence spectra of the GaN templates with and without the air tunnel showed an intense peak at 364 nm. The Raman spectroscopy results for the GaN templates with and without the air tunnel were redshifted relative to the results for free-standing GaN. The CLO process using potassium hydroxide solution neatly separated the GaN template with the air tunnel from the TPSS. Full article

Due to the emerging requirements of miniaturization and multifunctionality, monolithic devices with both functions of lighting and detection are essential for next-generation optoelectronic devices. In this work, based on freestanding (In,Ga)N films, we demonstrate a monolithic device with two functions of lighting and self-powered detection successfully. The freestanding (In,Ga)N film is detached from the epitaxial silicon (Si) substrate by a cost-effective and fast method of electrochemical etching. Due to the stress release and the lightening of the quantum-confined Stark effect (QCSE), the wavelength blueshift of electroluminescent (EL) peak is very small (<1 nm) when increasing the injection current, leading to quite stable EL spectra. On the other hand, the proposed monolithic bifunctional device can have a high ultraviolet/visible reject ratio (Q = 821) for self-powered detection, leading to the excellent detection selectivity. The main reason can be attributed to the removal of Si by the lift-off process, which can limit the response to visible light. This work paves an effective way to develop new monolithic multifunctional devices for both detection and display. Full article

GaN and its alloys with InN and AlN are of technological importance for a variety of optical, electronic, and optoelectronic devices due to its high thermal conductivity, wide band gap, high breakdown voltage and high saturation velocity. GaN-based devices now provide superior performance for a variety of high power, high frequency, high temperature, and optical applications. The major roadblock for the full realization of Nitride semiconductor potential is still the availability of affordable large-area and high-quality native substrates with controlled electrical properties. Despite the impressive accomplishments recently achieved by techniques such as hydride vapor phase epitaxy and ammonothermal for GaN growth, much more must be attained before establishing a fully satisfactory bulk growth method for this material. Recent results suggest that ammonothermal GaN wafers can be successfully used as seeds to grow thick freestanding GaN wafers by hydride vapor phase epitaxy. A brief review of defect-sensitive optical and paramagnetic spectroscopy techniques employed to evaluate structural, optical, and electronic properties of the state-of-the-art bulk and thick-film (quasi-bulk) Nitride substrates and homoepitaxial films is presented. Defects control the performance of devices and feeding back knowledge of defects to growth efforts is key to advancing technology. Full article

The effect of layer spacing and halogenation on the gas separation performances of free-standing smectic LC polymer membranes is being investigated by molecular engineering. LC membranes with various layer spacings and halogenated LCs were fabricated while having a planar aligned smectic morphology. Single permeation and sorption data show a correlation between gas diffusion and layer spacing, which results in increasing gas permeabilities with increasing layer spacing while the ideal gas selectivity of He over CO₂ or He over N₂ decreases. The calculated diffusion coefficients show a 6-fold increase when going from membranes with a layer spacing of 31.9 Å to membranes with a layer spacing of 45.2 Å, demonstrating that the layer spacing in smectic LC membranes mainly affects the diffusion of gasses rather than their solubility. A comparison of gas sorption and permeation performances of smectic LC membranes with and without halogenated LCs shows only a limited effect of LC halogenation by a slight increase in both solubility and diffusion coefficients for the membranes with halogenated LCs, resulting in a slightly higher gas permeation and increased ideal gas selectivities towards CO₂. These results show that layer spacing plays an important role in the gas separation performances of smectic LC polymer membranes. Full article

With the advances in nanofabrication technology, horizontally aligned and well-defined nitrogen-doped ultrananocrystalline diamond nanostripes can be fabricated with widths in the order of tens of nanometers. The study of the size-dependent electron transport properties of these nanostructures is crucial to novel electronic and electrochemical applications. In this paper, 100 nm thick n-type ultrananocrystalline diamond thin films were synthesized by microwave plasma-enhanced chemical vapor deposition method with 5% N₂ gas in the plasma during the growth process. Then the nanostripes were fabricated using standard electron beam lithography and reactive ion etching techniques. The electrical transport properties of the free-standing single nanostripes of different widths from 75 to 150 nm and lengths from 1 to 128 μm were investigated. The study showed that the electrical resistivity of the n-type ultrananocrystalline diamond nanostripes increased dramatically with the decrease in the nanostripe width. The nanostripe resistivity was nearly doubted when the width was reduced from 150 nm to 75 nm. The size-dependent variability in conductivity could originate from the imposed diffusive scattering of the nanostripe surfaces which had a further compounding effect to reinforce the grain boundary scattering. Full article

We propose and demonstrate both flexible and stretchable blue light-emitting diodes based on core/shell InGaN/GaN quantum well microwires embedded in polydimethylsiloxane membranes with strain-insensitive transparent electrodes involving single-walled carbon nanotubes. InGaN/GaN core-shell microwires were grown by metal-organic vapor phase epitaxy, encapsulated into a polydimethylsiloxane film, and then released from the growth substrate. The fabricated free-standing membrane of light-emitting diodes with contacts of single-walled carbon nanotube films can stand up to 20% stretching while maintaining efficient operation. Membrane-based LEDs show less than 15% degradation of electroluminescence intensity after 20 cycles of stretching thus opening an avenue for highly deformable inorganic devices. Full article

We have improved the material quality of the high indium composition InGaN/GaN multiple quantum wells (MQWs) grown on free-standing GaN substrates using the graded-indium-content superlattice. We found that by adopting a graded-indium-content superlattice structure, the spectral FWHM of the yellow emitting InGaN/GaN MQW was reduced from 181 meV to 160 meV, and the non-radiative recombination lifetime increased from 13 ns to 44 ns. Besides, the graded-indium-content superlattice can mitigate strain relaxation in high indium composition MQWs as shown by the TEM diffraction patterns. Full article

An experimental and theoretical investigation on microwave plasma-based synthesis of free-standing N-graphene, i.e., nitrogen-doped graphene, was further extended using ethanol and nitrogen gas as precursors. The in situ assembly of N-graphene is a single-step method, based on the introduction of N-containing precursor together with carbon precursor in the reactive microwave plasma environment at atmospheric pressure conditions. A previously developed theoretical model was updated to account for the new reactor geometry and the nitrogen precursor employed. The theoretical predictions of the model are in good agreement with all experimental data and assist in deeper understanding of the complicated physical and chemical process in microwave plasma. Optical Emission Spectroscopy was used to detect the emission of plasma-generated ‘‘building units’’ and to determine the gas temperature. The outlet gas was analyzed by Fourier-Transform Infrared Spectroscopy to detect the generated gaseous by-products. The synthesized N-graphene was characterized by Scanning Electron Microscopy, Raman, and X-ray photoelectron spectroscopies. Full article

In this paper, we fabricated Gallium Nitride (GaN) vertical p-i-n diodes grown on free-standing GaN (FS-GaN) substrates. This homogeneous epitaxy led to thicker GaN epi-layers grown on the FS-GaN substrate, but a high crystalline quality was maintained. The vertical GaN p-i-n diode showed a low specific on-resistance of 0.85 mΩ-cm² and high breakdown voltage (BV) of 2.98 kV. The high breakdown voltage can be attributed to the thick GaN epi-layer and corresponds to the mesa structure. Improvement of the device characteristics by the mesa structure was investigated using device simulations. We proved that a deeper mesa depth is able to decrease the electric field at the bottom of the mesa structure. Furthermore, a smaller mesa bevel angle will assist the BV up to 2.98 kV at a 60° bevel angle. Our approach demonstrates structural optimization of GaN vertical p-i-n diodes is useful to improve the device performance. Full article

Among the different semiconductors, GaN provides advantages over Si, SiC and GaAs in radiation hardness, resulting in researchers exploring the development of GaN-based radiation sensors to be used in particle physics, astronomic and nuclear science applications. Several reports have demonstrated the usefulness of GaN as an α-particle detector. Work in developing GaN-based radiation sensors are still evolving and GaN sensors have successfully detected α-particles, neutrons, ultraviolet rays, x-rays, electrons and γ-rays. This review elaborates on the design of a good radiation detector along with the state-of-the-art α-particle detectors using GaN. Successful improvement in the growth of GaN drift layers (DL) with 2 order of magnitude lower in charge carrier density (CCD) (7.6 × 10¹⁴/cm³) on low threading dislocation density (3.1 × 10⁶/cm²) hydride vapor phase epitaxy (HVPE) grown free-standing GaN substrate, which helped ~3 orders of magnitude lower reverse leakage current (I_R) with 3-times increase of reverse breakdown voltages. The highest reverse breakdown voltage of −2400 V was also realized from Schottky barrier diodes (SBDs) on a free-standing GaN substrate with 30 μm DL. The formation of thick depletion width (DW) with low CCD resulted in improving high-energy (5.48 MeV) α-particle detection with the charge collection efficiency (CCE) of 62% even at lower bias voltages (−20 V). The detectors also detected 5.48 MeV α-particle with CCE of 100% from SBDs with 30-μm DL at −750 V. Full article

Iongel-based CO₂ separation membranes were prepared by fast (< 1 min) UV-initiated polymerization of poly(ethylene glycol) diacrylate (PEGDA) in the presence of different ionic liquids (ILs) with the [C₂mim]⁺ cation and anions such as [TFSI]⁻, [FSI]⁻, [C(CN)₃]⁻ and [B(CN)₄]⁻. The four ILs were completely miscible with the non-ionic PEGDA network. Transparent and free-standing iongels containing between 60 and 90 %wt of IL were obtained and characterized by diverse techniques (FTIR, TGA, DSC, DMTA, SEM, CO₂ solubility and pure gas permeability). The thermal and mechanical stability of the iongels, as well as CO₂ solubility, were found to be strictly dependent on the IL content and the anion’s nature. The TGA results indicated that the iongels mostly follow the thermal profile of the respective neat ILs. The DMTA analysis revealed that the iongels based on fluorinated anions have higher storage modulus than those of cyano-functionalized anions. Conversely, the PEGDA–C(CN)₃ iongels presented the highest CO₂ solubility values ranging from 72 to 80 mmol/g. Single CO₂ permeabilities of 583 ± 29 Barrer and ideal CO₂/N₂ selectivities of 66 ± 3 were obtained with the PEGDA–70 C(CN)₃ iongel membrane. This work demonstrates that the combination of PEGDA with high contents of the best performing ILs is a promising and simple strategy, opening up new possibilities in the design of high-performance iongel membranes for CO₂ separation. Full article

The progress of nitride technology is widely limited and hindered by the lack of high-quality gallium nitride (GaN) wafers. Therefore, a large number of GaN epitaxial devices are grown on heterogeneous substrates. Although various additional treatments of substrate have been used to promote crystal quality, there is still plenty of room for its improvement, in terms of direct and continuous growth based on the hydride vapor phase epitaxy (HVPE) technique. Here, we report a three-step process that can be used to enhance the quality of GaN crystal by tuning V/III rate during successive HVPE process. In the growth, a metal-organic chemical vapor deposition (MOCVD) grown GaN on sapphire (MOCVD-GaN/Al₂O₃) was employed as substrate, and a high-quality GaN polyporous interlayer, with successful acquisition, without any additional substrate treatment, caused the growth stress to decrease to 0.06 GPa. Meanwhile the quality of GaN improved, and the freestanding GaN was directly obtained during the growth process. Full article

In this study, the charge transport mechanism of Pd/Si-based FS-GaN Schottky diodes was investigated. A temperature-dependent current–voltage analysis revealed that the I-V characteristics of the diodes show a good rectifying behavior with a large ratio of 10³–10⁵ at the forward to reverse current at ±1 V. The interface states and non-interacting point defect complex between the Pd metal and FS-GaN crystals induced the inhomogeneity of the barrier height and large ideality factors. Furthermore, we revealed that the electronic conduction of the devices prefers the thermionic field emission (TFE) transport, not the thermionic emission (TE) model, over the entire measurement conditions. The investigation on deep level transient spectroscopy (DLTS) suggests that non-interacting point-defect-driven tunneling influences the charge transport. This investigation about charge transport paves the way to achieving next-generation optoelectronic applications using Si-based FS-GaN Schottky diodes. Full article

A low voltage (−20 V) operating high-energy (5.48 MeV) α-particle detector with a high charge collection efficiency (CCE) of approximately 65% was observed from the compensated (7.7 × 10¹⁴ /cm³) metalorganic vapor phase epitaxy (MOVPE) grown 15 µm thick drift layer gallium nitride (GaN) Schottky diodes on free-standing n+-GaN substrate. The observed CCE was 30% higher than the bulk GaN (400 µm)-based Schottky barrier diodes (SBD) at −20 V. This is the first report of α–particle detection at 5.48 MeV with a high CCE at −20 V operation. In addition, the detectors also exhibited a three-times smaller variation in CCE (0.12 %/V) with a change in bias conditions from −120 V to −20 V. The dramatic reduction in CCE variation with voltage and improved CCE was a result of the reduced charge carrier density (CCD) due to the compensation by Mg in the grown drift layer (DL), which resulted in the increased depletion width (DW) of the fabricated GaN SBDs. The SBDs also reached a CCE of approximately 96.7% at −300 V. Full article