TiO2 Nanorods and Pt Nanoparticles under a UV-LED for an NO2 Gas Sensor at Room Temperature.

Because the oxides of nitrogen (NOx) cause detrimental effects on not only the environment but humans, developing a high-performance NO2 gas sensor is a crucial issue for real-time monitoring. To this end, metal oxide semiconductors have been employed for sensor materials. Because in general, semiconductor-type gas sensors require a high working temperature, photoactivation has emerged as an alternative method for realizing the sensor working at room temperature. In this regard, titanium dioxide (TiO2) is a promising material for its photocatalytic ability with ultraviolet (UV) photonic energy. However, TiO2-based sensors inevitably encounter a problem of recombination of photogenerated electron-hole pairs, which occurs in a short time. To address this challenge, in this study, TiO2 nanorods (NRs) and Pt nanoparticles (NPs) under a UV-LED were used as an NO2 gas sensor to utilize the Schottky barrier formed at the TiO2-Pt junction, thereby capturing the photoactivated electrons by Pt NPs. The separation between the electron-hole pairs might be further enhanced by plasmonic effects. In addition, it is reported that annealing TiO2 NRs can achieve noteworthy improvements in sensing performance. Elucidation of the performance enhancement is suggested with the investigation of the X-ray diffraction patterns, which implies that the crystallinity was improved by the annealing process.

Polarized ultraviolet emitters with Al wire-grid polarizers fabricated by solvent-assisted nanotransfer process.

Polarized ultraviolet (UV) emitters are essential for various applications, such as photoalignment devices for liquid crystals, high-resolution imaging devices, highly sensitive sensors, and steppers. To increase the high polarization ratio (PR) of a UV emitter, the grating period should be decreased than that of the visible emitter. However, the fabrication of the short period grating directly on UV emitters is still limited. In this study, we demonstrate that 200, 100, and 50 nm period aluminum (Al)-based wire-grid polarizers (WGPs) can be fabricated directly on UV emitters by a solvent-assisted nanotransfer process. The UV emitter with a grating period of 100 nm shows a PR of 84%, and an electroluminescence efficiency that is 22.5% and 48% higher than those of UV emitters with 50 nm and 200 nm period WGPs, respectively, due to the increased photon extraction efficiency (PEE). The higher PEE is attributed to the optical cavity property of the Al metal reflector with low light loss and the surface plasmon effect of the Al grating layer.

High Efficiency Flip Chip-Based UV Emitter with Indium Tin Oxide Nano Grains/Al Reflector and Periodic Microhole Arrays.

We propose a high efficiency flip chip-based ultraviolet (UV) emitter with aluminum (Al) reflector that includes indium tin oxide (ITO) nano grains for current injection between the Al and p-AlGaN layer. Al has attracted attention as a reflector for high efficiency UV emitters because of its high reflectance in the UV region. To improve the efficiency of UV emitter, we generated periodic microhole arrays on the p-AlGaN layer, which serve as a scattering center in the flip chip structure and enhance the light extraction efficiency. The light output power of the fabricated flip chip-based UV emitter with ITO nano grains/Al reflector and microhole arrays on the p-AlGaN layer is significantly improved by 72% and 45% at an injection current of 20 mA, compared to that of UV emitter with only Al reflector and ITO nano grains/Al reflector.

Transparent Conductive Electrodes of ß-Ga2O3/Ag/ß-Ga2O3 Multilayer for Ultraviolet Emitters.

We investigated the optical and electrical properties of a ß-Ga2O3/Ag/ß-Ga2O3 multilayer transparent conductive electrode deposited on an α-Al2O3 (0001) substrate. For the deposition of a continuous Ag layer, we preliminarily performed anultraviolet-ozone pretreatment of the Ga2O3 bottom layer. To obtain a stable ß-phase of Ga2O3, the ß-Ga2O3/Ag/ß-Ga2O3 multilayer was annealed at 700 °C under N2 atmosphere. The transmittance and sheet resistance of the ß-Ga2O3/Ag/ß-Ga2O3 multilayer were critically affected by the surface morphology and thickness of the Ag interlayer. The multilayer with optimized thicknesses (ß-Ga2O3 top layer: 30 nm; Ag interlayer: 12 nm; ß-Ga2O3 bottom layer: 60 nm) exhibited a resistance of 8.48 Ωsq-1, an average optical transmittance of 87.16% in the ultraviolet wavelength range from 300 to 350 nm, and a figure of merit of 29.81 × 10-3 Ω-1.

Self-standing ZnO nanotube/SiO2 core-shell arrays for high photon extraction efficiency in III-nitride emitter.

Self-standing ZnO nanotube (ZNT) arrays were fabricated on the surface of a GaN-based emitter with an indium tin oxide (ITO) transparent layer using a hydrothermal method and temperature cooling down process. For the greater enhancement of photon extraction efficiency, ZNT/SiO2 core-shell nanostructure arrays were fabricated on the emitter with a 430 nm wavelength. The optical output power of ZNT/SiO2 core-shell arrays on the emitter with ITO electrode was remarkably enhanced by 18.5%, 28.1%, and 55.9%, compared to those of ZNTs, ZNRs on an ITO film on an emitter and ITO film on an emitter as a conventional emitter, respectively. The large enhancement in optical output is attributable to the synergistic effect of efficient photon injection from the ITO/GaN layer to ZNTs because of the well-matched refractive indices and wave-guiding, in addition to the superior photon extraction by the SiO2 coating layer on the ZNTs.

High Reliability of Ag Reflectors with AgCu Alloy for High Efficiency GaN-Based Light Emitting Diodes.

We propose an Ag reflector layer with an AgCu alloy layer as a thermally reliable reflector for high power flip-chip and vertical light emitting diodes (LEDs). By annealing the deposited Ag and Cu layers, intermixed grains and grain boundaries from the alloyed AgCu layer were formed on the LEDs, and CuO nano dots precipitated at the grain boundaries. A thick AgCu layer was deposited to cover the AgCu alloy layer. The precipitation of the CuO nano dots at the grain boundaries suppressed Ag agglomeration, leading to enhanced light reflectance after the annealing process. Consequently, the alloyed AgCu/Ag reflector produced by annealing at a high temperature of 500 °C demonstrated a higher reflectance of 78% and a lower contact resistance of 7.0 × 10-5 Ω · cm2.

Wide Bandgap Transparent Conducting Electrode of FTO/Ag/FTO Structure for Ultraviolet Light-Emitting Diodes.

We investigated the effect of the Ag interlayer thickness on the structural, electrical and optical properties of FTO/Ag/FTO structures designed for use in wide bandgap transparent conducting electrodes. The top and bottom FTO layers were deposited on α-Al2O3 (0001) substrates via RF magnetron sputtering at 300 °C and Ag interlayers were deposited using an e-beam evaporator system. We optimized the figure of merit by changing the thickness of the inserted Ag interlayer from 10 nm to 14 nm, achieving a maximum value of 2.46 × 10-3 Ω-1 and a resistivity of 6.4 × 10-4 Ω · cm using an FTO (70 nm)/Ag (14 nm)/FTO (40 nm) structure. Furthermore, the average optical transmittance in the deep UV range (300 to 330 nm) was 82.8%.

Nano-patterned dual-layer ITO electrode of high brightness blue light emitting diodes using maskless wet etching.

We propose a dual-layer transparent Indium Tin Oxide (ITO) top electrode scheme and demonstrate the enhancement of the optical output power of GaN-based light emitting diodes (LEDs). The proposed dual-layer structure is composed of a layer with randomly distributed sphere-like nano-patterns obtained solely by a maskless wet etching process and a pre-annealed bottom layer to maintain current spreading of the electrode. It was observed that the surface morphologies and optoelectronic properties are dependent on etching duration. This electrode significantly improves the optical output power of GaN-based LEDs with an enhancement factor of 2.18 at 100 mA without degradation in electrical property when compared to a reference LED.

Enhancement of light extraction efficiency of GaN-based light-emitting diodes by ZnO nanorods with different sizes.

The improvement of the optical output power of GaN-based light emitting diodes (LEDs) was achieved by employing nano-sized flat-top hexagonal ZnO rods. ZnO nanorods (NRs) with the average diameters of 250, 350, and 580 nm were grown on p-GaN top surfaces by a simple wet-chemical method at relatively low temperature (90 degrees C) to investigate the effect of the diameter of ZnO NRs on the light extraction efficiency. Consequently, the enhancement by the factor of as high as 2.63 in the light output intensity at 20 mA for the LED with 350 nm ZnO NRs was demonstrated without the increase in the operation voltage compared to the reference LED.

Convergence Gas Sensors with One-Dimensional Nanotubes and Pt Nanoparticles Based on Ultraviolet Photonic Energy for Room-Temperature NO2 Gas Sensing.

Zinc oxide (ZnO) is a promising material for nitrogen dioxide (NO2) gas sensors because of its nontoxicity, low cost, and small size. We fabricated one-dimensional (1D) and zero-dimensional (0D) convergence gas sensors activated via ultraviolet (UV) photonic energy to sense NO2 gas at room temperature. One-dimensional ZnO nanorod (ZNR)-based and ZnO nanotube (ZNT)-based gas sensors were synthesized using a simple hydrothermal method. All the sensors were tested under UV irradiation (365 nm) so that they could be operated at room temperature rather than a high temperature. In addition, we decorated 0D Pt nanoparticles (NPs) on the gas sensors to further improve their sensing responsivity. The NO2-sensing response of the ZNT/Pt NP convergence gas sensor was 2.93 times higher than that of the ZNR gas sensor. We demonstrated the complex effects of UV radiation on 1D ZnO nanostructures and 0D metal nanostructures in NO2 gas sensing.

Self-array of one-dimensional GaN nanorods using the electric field on dielectrophoresis for the photonic emitters of display pixel.

Recently, high-efficiency III-nitride photonic emitters (PEs) for next-generation displays have been studied. Although micro-light-emitting diodes (µ-LEDs), one of the III-nitride PEs, have attracted considerable attention because of their high efficiency and size flexibility, they have encountered technical limitations such as high defect rate, high processing cost, and low yield. To overcome these drawbacks of µ-LEDs, a lot of research on PEs using one-dimensional (1D) gallium nitride-related nanorods (GNRs) capable of horizontally self-positioning on the electrodes has been carried out. The degree of array of GNRs on the interdigitated electrodes (IDEs) is an important factor in the efficiency of the PEs using GNRs to obtain excellent single-pixel characteristics. Therefore, in this study, we demonstrate that the improved performance of self-arrayed GNRs was realized using the dielectrophoresis technique by changing the thickness of IDEs. In addition, the shape and size of vertically aligned GNRs were controlled by the wet process, and GNR-integrated PEs (GIPEs) were driven by perfectly horizontally self-arrayed GNRs on IDEs. The electroluminescence (EL) intensity of the GIPEs was measured at 4-20 V and showed a maximum intensity value at 15 V. Over the injection voltage at 20 V, the EL intensity decreased due to the high current density of GIPEs. The external quantum efficiency (EQE) property of the GIPEs showed a similar efficiency droop as that of conventional III-nitride PEs.

High efficiency GaN-based light emitting diode with nano-patterned ZnO surface fabricated by wet process.

The improvement of the optical output power of GaN-based light emitting diodes (LEDs) was achieved by a novel bi-layer transparent top electrode scheme. The proposed bi-layer structure is composed of a Ga-doped ZnO layer with nano-patterns obtained solely by wet etching process and an Indium Tin Oxide p-type transparent conducting electrode layer. We employed various wet-etching conditions to maximize light extraction efficiency and it was observed that the crystal morphologies of nano-patterns and optoelectronic properties are dependent on etching duration. Because of ITO under GZO layer, the current spreading was not affected even after formation of nano-patterned surface on the GZO layer by wet etching. Consequently, an enhancement of as high as 43.1% in optical output power at an injection current of 100 mA for the LED with nano-patterns wet-etched by 0.025% HCl for 30 seconds was realized without significant degradation in electrical property when compared to a reference LED.

Design and fabrication of vertical-injection GaN-based light-emitting diodes.

The fabrication process and design issues for the fabrication of vertical-injection GaN-based light-emitting diodes were investigated. The process yield was reduced according to the adhesion of reflective p-electrodes, the exposure of electroplated metal in plasma, and wet-etching induced surface textures. The chip design utilizing current blocking layer and branched n-electrode was found to significantly affect the power efficiency of LEDs.

Ultraviolet light-emitting diode-assisted highly sensitive room temperature NO2 gas sensors based on low-temperature solution-processed ZnO/TiO2 nanorods decorated with plasmonic Au nanoparticles.

Nanostructured semiconducting metal oxides such as SnO2, ZnO, TiO2, and CuO have been widely used to fabricate high performance gas sensors. To improve the sensitivity and stability of gas sensors, we developed NO2 gas sensors composed of ZnO/TiO2 core-shell nanorods (NRs) decorated with Au nanoparticles (NPs) synthesized via a simple low-temperature aqueous solution process, operated under ultraviolet irradiation to realize room temperature operation. The fabricated gas sensor with a 10 nm-thick TiO2 shell layer shows 9 times higher gas sensitivity and faster response and recovery times than ZnO NR-based gas sensors. This high performance of the fabricated gas sensor can be ascribed to band bending between the ZnO and TiO2 core-shell layers and the localized surface plasmon resonance effect of Au NPs with a sufficient Debye length of the TiO2 shell layer.

Improved Performance of GaN-Based Light-Emitting Diodes Grown on Si (111) Substrates with NH3 Growth Interruption.

We demonstrate the highly efficient, GaN-based, multiple-quantum-well light-emitting diodes (LEDs) grown on Si (111) substrates embedded with the AlN buffer layer using NH3 growth interruption. Analysis of the materials by the X-ray diffraction omega scan and transmission electron microscopy revealed a remarkable improvement in the crystalline quality of the GaN layer with the AlN buffer layer using NH3 growth interruption. This improvement originated from the decreased dislocation densities and coalescence-related defects of the GaN layer that arose from the increased Al migration time. The photoluminescence peak positions and Raman spectra indicate that the internal tensile strain of the GaN layer is effectively relaxed without generating cracks. The LEDs embedded with an AlN buffer layer using NH3 growth interruption at 300 mA exhibited 40.9% higher light output power than that of the reference LED embedded with the AlN buffer layer without NH3 growth interruption. These high performances are attributed to an increased radiative recombination rate owing to the low defect density and strain relaxation in the GaN epilayer.

Self-Aligned Hierarchical ZnO Nanorod/NiO Nanosheet Arrays for High Photon Extraction Efficiency of GaN-Based Photonic Emitter.

Advancements in nanotechnology have facilitated the increased use of ZnO nanostructures. In particular, hierarchical and core-shell nanostructures, providing a graded refractive index change, have recently been applied to enhance the photon extraction efficiency of photonic emitters. In this study, we demonstrate self-aligned hierarchical ZnO nanorod (ZNR)/NiO nanosheet arrays on a conventional photonic emitter (C-emitter) with a wavelength of 430 nm. These hierarchical nanostructures were synthesized through a two-step hydrothermal process at low temperature, and their optical output power was approximately 17% higher than that of ZNR arrays on a C-emitter and two times higher than that of a C-emitter. These results are due to the graded index change in refractive index from the GaN layer inside the device toward the outside as well as decreases in the total internal reflection and Fresnel reflection of the photonic emitter.

Quantitative analysis of adsorption and desorption of volatile organic compounds on reusable zeolite filters using gas chromatography.

In this study, we propose a method to quantitatively analyze the concentration of VOCs adsorbed on zeolite filters via gas chromatography (GC). The sampled VOCs from the filters with ethanol as a solution were characterized using GC to determine the concentration of the adsorbed VOCs by comparing the areas of GC peaks of the detected VOCs and ethanol. The proposed method also enabled determination of the desorption (regeneration) conditions of the zeolite filters according to heating temperature and time for various VOCs. Repeated adsorption and desorption of VOCs on zeolite filters and GC analyses allow us to evaluate the durability and reusability of the filter and could help predict the lifetime of zeolite filters in practice.

Ultraviolet Photoactivated Room Temperature NO2 Gas Sensor of ZnO Hemitubes and Nanotubes Covered with TiO2 Nanoparticles.

Prolonged exposure to NO2 can cause lung tissue inflammation, bronchiolitis fibrosa obliterans, and silo filler's disease. In recent years, nanostructured semiconducting metal oxides have been widely used to fabricate gas sensors because of their unique structure and surface-to-volume ratio compared to layered materials. In particular, the different morphologies of ZnO-based nanostructures significantly affect the detection property of NO2 gas sensors. However, because of the large interaction energy of chemisorption (1-10 eV), metal oxide-based gas sensors are typically operated above 100 °C, overcoming the energy limits to attain high sensitivity and fast reaction. High operating temperature negatively affects the reliability and durability of semiconductor-based sensors; at high temperature, the diffusion and sintering effects at the metal oxide grain boundaries are major factors causing undesirable long-term drift problems and preventing stability improvements. Therefore, we demonstrate NO2 gas sensors consisting of ZnO hemitubes (HTs) and nanotubes (NTs) covered with TiO2 nanoparticles (NPs). To operate the gas sensor at room temperature (RT), we measured the gas-sensing properties with ultraviolet illumination onto the active region of the gas sensor for photoactivation instead of conventional thermal activation by heating. The performance of these gas sensors was enhanced by the change of barrier potential at the ZnO/TiO2 interfaces, and their depletion layer was expanded by the NPs formation. The gas sensor based on ZnO HTs showed 1.2 times higher detection property than those consisting of ZnO NTs at the 25 ppm NO2 gas.

Ni-Cu Nanoparticles-Embedded Ag-Based Reflector for High-Efficiency Light-Emitting Diodes.

We investigated the use of a silver reflector embedded with Ni-Cu nanoparticles to achieve low resistance and high reflectivity in GaN-based flip-chip light-emitting diodes. Compared to a single layer of Ag, the NC-NPs/Ag reflector exhibits a higher light reflectance of ~90% at a wavelength of 450 nm, a lower contact resistance of 4.75 × 10-5 II cm², and improved thermal stability after annealing at 400°C. The NC-NPs formed after the annealing process prevents agglomeration of the Ag layer, while also reducing the Schottky barrier height between the p-GaN layer and metal reflector. The LED fabricated with a NC-NPs/Ag reflector exhibited a forward-bias voltage of 3.13 V and an improvement in light output power of 36.6% (at 20 mA), when compared with the LED composed of a Ag SL. This result indicates that the NC-NPs/Ag reflector is a promising p-type reflector for high-intensity light-emitting diodes.

Enhanced Photon Emission Efficiency Using Surface Plasmon Effect of Pt Nanoparticles in Ultra-Violet Emitter.

We demonstrate the surface plasmon (SP)-enhanced ultraviolet (UV) emitter using Pt nanoparticles (NPs). The UV emitter is hole-patterned on the p-AlGaN layer to consider the penetration depth of Pt NPs. The Pt NPs with sizes under 50 nm are required to realize the plasmonic absorption in UV wavelength. In this study, we confirm the average Pt NP sizes of 10 nm, 20 nm, and 25 nm, respectively, at an annealing temperature of 600 °C. The absorption of annealed Pt NPs is covered with the 365-nm wavelength. The electroluminescence intensity of SP-UV is 70% higher than that of reference UV emitter without hole-patterns and Pt NPs. This improvement can be attributed to the increase of spontaneous emission rate through resonance coupling between the excitons in multiple quantum wells and Pt NPs deposited on the p-AlGaN layer.