Ultraviolet Light Emission Research Articles

Cathodoluminescence of Single Phase ZnAl<sub>2</sub>O<sub>4</sub> and Diffusion Layer Synthesized by Coupling of ZnO and α-Al<sub>2</sub>O<sub>3</sub>

Zinc oxide (ZnO) ceramics was coupled and reacted with alpha aluminum oxide (α-Al2O3) ceramics at 1200oC for 24 h. SEM observation and energy dispersive X-ray spectroscopy (EDS) analysis revealed the existence of diffusion layer of 10 μm thickness on the α-Al2O3 side of interface between ZnO and α-Al2O3. The diffusion layer is considered to consist of a ZnAl2O4, from the result of XRD. The cathodoluminescence of the diffusion layer was compared with that of zinc aluminate (ZnAl2O4) ceramics synthesized by conventional solid state reaction method. The single phase ZnAl2O4 showed weak emission peaked at 4.6 eV. On the other hand, the diffusion layer showed intense emission peaked at 3.75 eV. The ZnAl2O4 on the α-Al2O3 side of the interface synthesized by coupling of ZnO and α-Al2O3 is considered to be a excellent material as an ultraviolet light emitter.

Simple Synthesis of a Variety of Nano-structures Using Silicide Alloys with Ga Droplets

A variety of nano-structures, such as nanofibers, nanotubes, nanocapsules, nanoribbons and nanorods, were synthesized using silicide alloys with Ga droplets. It was found that the growth morphology and the structural property of the nanostructures significantly depended on the silicides seed materials. In addition, the amorphous SiOx nanofibers show strong ultraviolet and/or visible light emissions, and the PL spectra of the nanofibers depended on the seed materials. For some of the nanofiber syntheses, the formation of nanoflakes or nanoribbons of β-Ga2O3 occurs, and a variety of growth morphologies for the β-Ga2O3 nanostructures was obtained. The series of morphological and structural images are shown for a variety of nanostructures. The obtained SiOx nanofibers are expected to be materials for use in cheap, abundant and safe luminescent devices for visible light applications.

Deep Ultraviolet Light-Emitting Hexagonal Boron Nitride Synthesized at Atmospheric Pressure

Materials emitting light in the deep ultraviolet region around 200 nanometers are essential in a wide-range of applications, such as information storage technology, environmental protection, and medical treatment. Hexagonal boron nitride (hBN), which was recently found to be a promising deep ultraviolet light emitter, has traditionally been synthesized under high pressure and at high temperature. We successfully synthesized high-purity hBN crystals at atmospheric pressure by using a nickel-molybdenum solvent. The obtained hBN crystals emitted intense 215-nanometer luminescence at room temperature. This study demonstrates an easier way to grow high-quality hBN crystals, through their liquid-phase deposition on a substrate at atmospheric pressure.

초음파분무법으로 제조한 ZnO:Er막의 UV 발광 특성

The films of Er-doped ZnO (ZnO:Er) were prepared onto MgO wafers by ultrasonic spray pyrolysis at <TEX>$550^{\circ}C$</TEX>. The concentration of Er in the deposition source varied from 0.5 wt% to 3.0 wt%. The crystallographic properties and surface morphologies of the films were investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. The properties of photoluminescence (PL) for the films were investigated by dependence of PL spectra on the Er concentration in the films. The films were grown as polycrystalline with a dominant direction of [002]. The grain size of the films were reduced by Er-doping. Er-doping enhanced the ultraviolet emission of ZnO:Er films. The ZnO:Er films prepared with the deposition source of 2.0 wt% Er showed the strongest ultraviolet light emission peak among the films in this study.

Degradation of a textile azo dye by pulsed arc discharge to the surface of wastewater

The pulsed arc discharge to the surface of wastewater was applied to the degradation of a textile azo dye (Acid Red 27). A high-voltage electrode (discharging electrode) was vertically placed above the surface of the wastewater while the wastewater itself was grounded. The pulsed arc discharge occurred between the tip of the discharging electrode and the surface of the wastewater, producing various oxidative species. Oxygen was used as the working gas instead of air to prevent nitrogen oxides from forming. The effect of several parameters on the chromaticity and chemical oxygen demand (COD) removal was examined. The results obtained showed that the chromaticity of the wastewater was completely removed by this process and the COD also decreased significantly. It has been found that ozone formed in the gas phase mainly affects the removal of the dye. The contribution of other effects such as ultraviolet light emission and OH radical formation during the arc discharge to the degradation of the dye was found to be less than 15%. For the present reactor system, the optimum pH, pulse repetition rate and agitation speed were found to be 3.0, 110 Hz and 300 rpm, respectively.

Formation and luminescent properties of face-centered-cubic Si nanocrystals in silica matrix by magnetron sputtering with substrate bias

Si nanocrystals with face-centered-cubic (fcc) structure were fabricated using common sputtering technique, but applying bias on the substrate, the thermodynamic calculation indicates that the bombardment of Ar+ ions arising from the bias should be responsible for the crystallization and phase transition of Si nanocrystals. The silica films containing fcc-Si nanocrystals give strong blue and ultraviolet light emissions, which suggest that the band structure of Si material could be modified by changing the crystal structure.

Synthesis and photoluminescence properties of well-aligned Ga-doped N-rich SiOxNy nanowire bundles

Well-aligned Ga-doped N-rich SiOxNy bundles were synthesized on Si substrates by chemical vapor deposition. According to the x-ray photoelectron spectroscopy spectra of the Si 2p3∕2 region, the relative shifts of N 1s and chemical stabilities of all the possible tetrahedrons, the most probable four tetrahedral phases in the bundles are Si–N4, Si–N3O, Si–O3N, and Si–O4, corresponding to bonding units of Si3N4, Si2ON2, Si6O9N2, and SiO2. Elemental mappings, by using electron energy loss spectroscopy, confirm that elements Si, O, and N are homogeneously distributed. An intense ultraviolet light emission is observed by using vacuum ultraviolet synchrotron radiation as an excitation light source.

UNDERSTANDING ULTRAVIOLET EMITTER PERFORMANCE USING INTENSITY DEPENDENT TIME-RESOLVED PHOTOLUMINESCENCE

Time-resolved photoluminescence studies of nitride semiconductors and ultraviolet light emitters comprised of these materials are performed as a function of pump intensity as a means of understanding and evaluating device performance. Comparison of time-resolved photoluminescence (TRPL) on UV LED wafers prior to fabrication with subsequent device testing indicate that the best performance is attained from active regions that exhibit both reduced nonradiative recombination due to saturation of traps associated with point and extended defects and concomitant lowering of radiative lifetime with increasing carrier density. Similar behavior is observed in optically pumped UV lasers. Temperature and intensity dependent TRPL measurements on a new material, AlGaN containing nanoscale compositional inhomogeneities (NCI), show that it inherently combines inhibition of nonradiative recombination with reduction of radiative lifetime, providing a potentially higher efficiency UV emitter active region.

Radiation-induced self-structuring of radiative defects complexes in a K-feldspar crystal: A study by thermoluminescence

Ultraviolet light emission at a wavelength of ∼ 290 nm ( ∼ 4.28 eV ) of a natural K-rich feldspar crystal from Cáceres (Spain) has been studied using high sensitivity thermoluminescence (3D-TL) spectrometry. The spectral emission band has a Gaussian profile and could be linked with radiative electron–hole recombination in radiation-induced defect complexes. The glow curve shows a characteristic shape along the temperature axis, with an exponential rise ( E a = 0.98 ± 0.03 eV ) and a power-law decay (scaling exponent τ = 4.49 ± 0.09 at 2.5 °C/s heating rate). This characteristic profile is interpreted as a diffusion-activated luminescence phenomenon in which the trapping-detrapping-transition dynamics is controlled by local cooperative phenomena inside these complexes. The processes involved in the radiative recombinations could represent the reverse sequence of events occurring during defect formation by irradiation. The crystal lattice of this phosphor exhibits the capability of energy storage by luminescent centre formation because of its potential to self-structuring locally into well-organized structural complexes under irradiation. This structural capability must be considered as a systemic property.

Photoluminescence of hexagonal-shaped SiC nanowires prepared by sol–gel process

New hexagonal-shaped SiC nanowires were synthesized through a sol–gel and direct heating process. Electron microscopy, X-ray diffraction and Raman spectrum indicate the nanowires possess a β-SiC structure and have typical diameters of 50–100 nm and length up to several tens of micrometers. Two strong broad ultraviolet light emission peaks at about 290 and 396 nm are observed at room temperature from the hexagonal-shaped nanowires, which are excitated by 250 nm ultraviolet fluorescent light. Compared with the previous reports, the synthesized nanowires exhibit two larger buleshifts in the photoluminescence (PL) spectrum. The buleshift of former PL peak is attributed to the oxygen defects in SiO x layer and the second one is due to morphology and size confinement effects of the SiC nanowires. Memory effect may be responsible for the hexagonal shape formation of SiC nanowires during vapor–solid growth process.

Synthesis and photoluminescence of needle-shaped 3C–SiC nanowires on the substrate of PAN carbon fiber

Needle-shaped 3C–SiC nanowires were obtained on the substrate of polyacrylonitrile (PAN) carbon fibers using a simple thermal evaporation of silicon on the carbon fibers. The growth of SiC nanowires may be dominated by the vapor–solid reaction, and the skin/core heterogeneity of PAN carbon fibers may play a crucial effect to synthesize SiC nanowires. A strong photoluminescence peak located around 468 nm was observed at room temperature and the luminescence mechanism was discussed. The advantages of this process are that SiC nanoneedles were grown radially from the substrate of PAN carbon fibers and PAN carbon fiber is electrically conductive. This sort of radial SiC nanowire array may find practical applications in blue and ultraviolet light emitters, field electron emission, reinforced composites, sensors, etc.

Synthesis of high-purity boron nitride single crystals under high pressure by using Ba–BN solvent

High-purity cubic boron nitride (cBN) and hexagonal boron nitride (hBN) single crystals were synthesised at 4.5 GPa and 1500 °C using barium boron nitride as a solvent. Secondary ion mass spectrometry was used to analyse impurities in the crystals. Fine cBN and hBN crystals, whose oxygen and carbon concentrations were less than 10 18 atoms/cm 3, were obtained, and their band-edge optical properties were measured by cathodoluminescence spectroscopy. High-purity hBN single crystals exhibited intense ultraviolet emission, demonstrating their promise for use as deep ultraviolet-light emitters.

Principal issues in producing new ultraviolet light emitters based on transparent semiconductor zinc oxide

In order to develop electroluminescent and laser devices based on the ultraviolet exciton emission of ZnO, it will be important to fabricate p–n junctions with adequate characteristics. Attempts to observe p-type conductivity have been to some extent successful, resulting in hole concentrations and mobilities, respectively up to 1019cm−3 and 100cm2/Vs in thin layers of ZnO grown by conventional techniques. A new technique is also introduced by Tsukazaki et al. [Tsukazaki et al., Nat. Mater. 4 (2005) 42] to fabricate p-type ZnO reproducibility. Moreover, in order to produce quantum well lasers from an optically active ZnO layer similar to the previously introduced AlxGa1−xAs/GaAs hetero-structures, ZnO based hetero-structures using generally magnesium as a third component have been built-in and two-dimensional electronic confinement has been achieved. Nevertheless, stable and reproducible light emitting devices with sufficient light output and expected UV wavelength around 370nm have yet been fabricated, neither from the p–n homo- and hetero-junctions nor from the double hetero-structures. Therefore, in order to produce ultraviolet light emitters based on ZnO, we outline this paper as a comprehensive study involving our and some other important recent results to underline the crucial key points concerned with the production of p-type materials, observation of electroluminescent devices, improvement of minority carrier lifetime and modification of the bandgap. It is aimed that the predicted interpretations and proposed models will draw an appropriate path towards fabrication of optoelectronic devices working at short wavelength ultraviolet range of the electromagnetic spectrum.

The ultraviolet emission mechanism of ZnO thin film fabricated by sol–gel technology

ZnO thin films were successfully deposited on SiO 2/Si substrate by sol–gel technology. The as-grown ZnO thin films were annealed under an ambient atmosphere from 600 to 900 °C by rapid thermal annealing (RTA) process. X-ray diffraction and scanning electron microscopy analyses reveal the physical structures of ZnO thin films. From PL measurement, two ultraviolet (UV) luminescence bands were obtained at 375 and 380 nm, and the intensity became stronger when the annealing temperature was increased. The strongest UV light emission appeared at annealing temperature of 900 °C. The chemical bonding state in ZnO films was investigated by using X-ray photoelectron spectrum. The mechanism of UV emission was also discussed.

Mass independently fractionated sulfur components in chondrites

We report high precision sulfur isotopic data obtained by sequential extraction from various physically separated phases (bulk, matrix, and chondrules) from chondrites. A significant excess of 33S (up to Δ 33S of 0.112‰ for Dhajala Chondrule) has been observed and is most likely carried by chondrule rims, though chondrule interiors cannot be ruled out as a carrier. Stellar nucleosynthesis and spallation are ruled out as a cause for this anomaly. Photochemical irradiation of sulfur gaseous species in the early solar nebula has, most likely, produced this anomaly. Observations of mass independent sulfur of photochemical origin suggest that chondrules and their rims must have formed in an optically thin nebular region. This also suggests that the chondrules were formed near the protoSun when it was active in ultraviolet light emission.

Ultraviolet free-exciton light emission in Er-passivated SnO2 nanocrystals in silica

Sn O 2 nanocrystals are grown in silica starting from a sol-gel method and using Er doping to passivate the cluster boundaries. As a result, emission at 3.8eV from the decay of SnO2 free excitons is observed in nanostructured SnO2:SiO2, besides the extrinsic 2eV luminescence of defects in SnO2 and ascribable to substoichiometric nanocluster boundaries. The analysis of the extrinsic emission competitive with the ultraviolet (UV) luminescence evidences the involvement of a phonon mode at 210cm−1 from a SnO-like phase. The feasibility of passivated wide-band-gap nanocrystals in silica gives interesting perspectives for UV-emitting optical devices.

Intrinsic ultraviolet light-emission from Si nanocrystals prepared by reactive sputtering

Thin films with ultrafine Si nanocrystals and ultraviolet (UV) light emission were prepared by using reactive sputtering of SiO2 and Al targets. The reaction of Al and SiO2 leads to the formation of ultrafine Si nanocrystals with compact Al2O3 shell. Al2O3 shell retards the growth of Si nanocrystals and provides ideal passivation on Si nanocrystals. Intrinsic UV photoluminescence is ascribed to the quantum-confinement effect of Si nanocrystals.

High-Density, Aligned SiO2 Nanowire Arrays: Microscopic Imaging of the Unique Growth Style and Their Ultraviolet Light Emission Properties

High-density, free-standing SiO2 nanowire arrays were successfully fabricated by a simple chemical vapor deposition method through a controlled pattern of the micrometer-sized alloyed balls on the Si substrate combined with a local balanced and steady-state reaction vapor environment. The direct observation of temporal evolution of the SiO2 nanowire growth process via the microscopic imaging approach offers us amazing pictures related to the unique vapor-liquid-solid (VLS) growth styles. These novel results are beneficial to understanding the formation mechanism of silica nanowire arrays, and at the same time, they extend our knowledge of VLS growth phenomena. The stable and strong ultraviolet emission properties of the as-grown products are of significant interest for their potential applications related to nanoscale optoelectronic device including ultraviolet-light-emitting devices, etc.

Sn O 2 target with controllable microstructure and thickness for generating extreme ultraviolet light

The target of weaved tin-oxide nanofibers were fabricated by electrospinning method. The microstructure, density, and thickness of the tin-oxide fibers could be well controlled by different fabrication condition. Extreme ultraviolet emission from Sn and SnO2 nanofibers was experimentally investigated. It was found that the microscopic structure of the target could have great affection on the extreme ultraviolet light emission.

Low-density tin targets for efficient extreme ultraviolet light emission from laser-produced plasmas

Influence of initial density of tin (Sn) targets has been quantitatively investigated for efficient extreme ultraviolet light emission from laser-produced plasmas. With a decrease in the initial density, conversion efficiency (CE) from incident laser energy to output 13.5nm light energy in a 2% bandwidth increases; 2.2% of the peak CE was attained with use of 7% low-density SnO2 targets (0.49g∕cm3) irradiated with a Nd:YAG laser, of which wavelength, pulse duration, and intensity are, respectively, 1.064μm, 10ns, and 5×1010W∕cm2. The peak CE is 1.7 times higher than that obtained with the use of solid density Sn targets. Experimental results may be attributed to the influence of the initial density and/or microstructure of the targets on expansion dynamics of the plasmas.