Application Of Ultraviolet Light-emitting Diodes Research Articles

Highly Monochromatic Ultraviolet LED Based on the SnO2 Microwire Heterojunction Beyond Dipole-Forbidden Band-Gap Transition.

SnO2 has been extensively applied in the fields of optoelectronic devices because of its large band gap, high exciton binding energy, and outstanding optical/electrical properties. However, its applications in ultraviolet light-emitting diodes (LEDs) are still hindered by the dipole-forbidden rule. Herein, the dipole-forbidden rule can be conquered by synthesizing Sb-incorporated SnO2 microwires (SnO2:Sb MWs), which are examined by ultraviolet photoluminescence emitting at 363.2 nm and a line width of 11.3 nm. Subsequently, a highly monochromatic ultraviolet light-emitting diode (LED) based on a SnO2:Sb MW heterojunction was constructed with a p-GaN film serving as the hole supplier. In the LED, the presence of a MgO intermediate layer can modulate carrier transport and recombination path, thus achieving band-edge optical transition in the SnO2:Sb MW. As the LED is modified using Ag nanowires, electrical properties, especially for the hole injection efficiency, were dramatically boosted, contributing significantly to the device high brightness. The LED emits at 365.9 nm and a line width of 12.4 nm. Therefore, we have realized a high-brightness and narrow-band ultraviolet LED with the shortest peak wavelength never seen in previously reported SnO2 LEDs. This work will promote the potential applications of low-dimensional SnO2 optoelectronic devices and provide an effective exemplification to overcome the dipole-forbidden rule in metal-oxide materials with "forbidden" energy gaps.

Nonequilibrium quantum transport properties of tetragonal zinc chalcogenide monolayers

Tetragonal zinc chalcogenide monolayers (TZCM) are emerging as interesting electronic materials with a direct band gap and relatively high carrier mobility. In this work, we report a theoretical investigation of electronic transport properties and photoelectric response properties of TZCM with gold contacts by density functional theory (DFT) and non-equilibrium Green's function (NEGF) methods. When there is no gate voltage applied, the current increases nonlinearly as bias voltage increases. Among the four proposed devices, the Au(100)/ZnS/Au(100) device has the best electronic transport performance and is most sensitive to the adjustment of bias voltage and gate voltage. The photocurrent calculation results indicate that the low-frequency oscillatory photocurrent of the Au(100)/ZnSe/Au(100) device in the high photon energy region may have potential applications in ultraviolet light-emitting diodes. The Au(100)/Zn2SeS/Au(100) device has more stable photoelectric response and polarization sensitivity than the Au(100)/Zn2SSe/Au(100) device. The Au(100)/TZCM/Au(100) devices exhibit considerable photocurrent and good extinction ratios. This work could pave the way for the future application of TZCM in the field of optoelectronics and so on.

Photocatalytic oxidation of volatile organic compounds (VOCs) in air using ultraviolet light-emitting diodes (UV-LEDs)

The aim of this study was to experimentally investigate the application of ultraviolet light-emitting diodes (UV-LEDs) for removing volatile organic compounds (VOCs) from air. For this purpose, a 365 nm UV-LED and a photocatalyst were used in a batch reactor to study the photocatalysis of toluene as the representative pollutant. It was demonstrated that combining reduced graphene oxide (rGO) with titanium dioxide (TiO2) and immobilizing the catalyst on the porous substrate could significantly improve the photocatalytic activity compared with the pristine TiO2 catalyst on a solid substrate. Photocatalyst deactivation was also studied by running cyclical tests. The photocatalytic crystal structure remained intact, but catalytic activity declined more than 80% after five consecutive photocatalytic tests for treating relatively high toluene concentrations. The adsorbed intermediates of toluene photocatalysis, such as benzoic acid, benzaldehyde, and benzyl alcohol, were identified, and the deactivation mechanism was proposed. It was shown that the deactivated photocatalyst can be effectively regenerated by UV-LED irradiation of 38.5 mW/cm2 irradiance within two hours. Overall, it was concluded that the UV-LED with a TiO2 on porous support provides an effective method for practical air purification applications.

Application of Ultraviolet Light-Emitting Diodes (UV-LED) to Full-Scale Drinking-Water Disinfection

Ultraviolet light-emitting diodes (UV-LEDs) have recently emerged as a viable technology for water disinfection. However, the performance of the technology in full-scale drinking-water treatment systems remains poorly characterised. Furthermore, current UV disinfection standards and protocols have been developed specifically for conventional mercury UV systems and so do not necessarily provide an accurate indication of UV-LED disinfection performance. Hence, this study aimed to test the hypothesis that a full-scale UV-LED reactor can match the Cryptosporidium inactivation efficiency of conventional mercury UV reactors. Male-specific bacteriophage (MS2) was used as the Cryptosporidium spp. surrogate microorganism. The time-based inactivation efficiency of the full-scale reactor was firstly compared to that of a bench-scale (batch-type) UV-LED reactor. This was then related to mercury UV reactors by comparing the fluence-based efficiency of the bench-scale reactor to the USEPA 90% prediction interval range of expected MS2 inactivation using mercury UV lamps. The results showed that the full-scale UV-LED reactor was at least as effective as conventional mercury UV reactors at the water-quality and drive-current conditions considered. Nevertheless, comparisons between the bench- and full-scale UV-LED reactors indicated that improvements in the hydraulic flow profile and power output of the full-scale reactor could help to further improve the efficiency of UV-LED reactors for municipal drinking water disinfection. This represents the world’s first full-scale UV-LED reactor that can be applied at municipal water treatment works for disinfection of pathogenic microorganisms from drinking water.

NaBaY(BO3)2:Ce3+,Tb3+: A novel sharp green‐emitting phosphor used for WLED and FEDs

AbstractA new borate phosphor NaBaY(BO3)2: Ce3+, Tb3+ (NBY:Ce3+, Tb3+) was successfully synthesized under low temperature designed to put into application in the fields of ultraviolet (UV)‐excited light emitting diodes (LEDs) and field emission displays (FEDs). The structure distortion between Ce3+, Tb3+ single‐ and co‐doping NBY was discussed by X‐ray powder diffraction Rietveld refinement, high‐resolution transmission electron microscopy (HRTEM) and spectra. NBY: Ce3+, Tb3+ presents a wide absorption band ranging from 310 to 400 nm and efficient green emission (λmax = 542 nm) with a full‐width at half‐maximum of 3.3 nm. The remarkable thermal stability has also been tested, indicating that the intensity at 200°C is still beyond 70% of the original intensity. In addition, a white LED device was manufactured by connecting a 370 nm UV chip with a blend of BaMaAl10O17: Eu2+ (BAM: Eu2+), NBY: Ce3+, Tb3+ and CaAlSiN3: Eu2+. The color coordinate, correlated color temperature and color rendering index of the manufactured LED device were (0.335, 0.347), 5511 K and 80.16, respectively. Meanwhile, the cathodoluminescence (CL) spectra under the various conditions of probe currents and accelerating voltages were also analyzed. Through successive excitation of low‐voltage electron‐beam, the wonderful performances of degradation property and color stability were obtained. These results suggest that the green‐emitting NBY: Ce3+, Tb3+ phosphor has the prospect of becoming applications in white UV LEDs and FEDs.

ITO/Ag/AlN/Al2O3 multilayer electrodes with conductive channels: Application in ultraviolet light-emitting diodes

There have been considerable efforts to use oxide/metal/oxide (OMO) multilayers as transparent conductive electrodes (TCEs) for various optoelectronic devices. However, it was difficult to achieve high transmittances in the UV region because relatively narrow-bandgap oxide films such as indium tin oxide (ITO) and ZnO have been used to achieve low series resistances. In this study, ITO/Ag/AlN/Al2O3-based TCEs were fabricated by incorporating conductive channels in AlN/Al2O3 wide-bandgap bilayers under electric fields. These multilayered films exhibited much higher transmittances in the UV region and lower sheet resistances compared to the reference ITO and ITO/Ag/ITO electrodes. These films were then applied in GaN-based near-UV LEDs, following the investigation of ohmic conduction mechanisms at the interface between p-GaN and ITO/Ag/AlN/Al2O3 TCE layers. The LEDs with ITO/Ag/AlN/Al2O3 multilayers exhibited much lower forward voltages and higher output powers than those of the other two LEDs. This method offers a new and effective route to enhance the transmittance of OMO-based TCEs at a shorter wavelength without losses in electrical conductivity.

Fabrication of silver nanowires and metal oxide composite transparent electrodes and their application in UV light-emitting diodes

In this paper, we prepared the silver nanowires (AgNWs)/aluminum-doped zinc oxide (AZO) composite transparent conducting electrodes for n-ZnO/p-GaN heterojunction light emitting-diodes (LEDs) by drop casting AgNW networks and subsequent atomic layer deposition (ALD) of AZO at 150 °C. The contact resistances between AgNWs were dramatically reduced by pre-annealing in the vacuum chamber before the ALD of AZO. In this case, AZO works not only as the conformal passivation layer that protects AgNWs from oxidation, but also as the binding material that improves AgNWs adhesion to substrates. Due to the localized surface plasmons (LSPs) of the AgNWs resonant coupling with the ultraviolet (UV) light emission from the LEDs, a higher UV light extracting efficiency is achieved from LEDs with the AgNWs/AZO composite electrodes in comparison with the conventional AZO electrodes. Additionally, the antireflective nature of random AgNW networks in the composite electrodes caused a broad output light angular distribution, which could be of benefit to certain optoelectronic devices like LEDs and solar cells.

Photoluminescence characterization and energy transfer of color-tunable Li6Y(BO3)3:Ce3+,Tb3+ phosphors

Ce3+ and Tb3+ singly doped and co-doped Li6Y(BO3)3 (LYB) phosphors were synthesized through a solid-state reaction. The phosphors were effectively excited by 350nm, which matched the near-UV emitting InGaN chip. Luminescence spectra and decay lifetime curves of LYB:Ce3+,Tb3+ were measured to prove energy transfer from Ce3+ to Tb3+. Through energy transfer, the intensity of the typical emission peak of Tb3+ at 546nm in LYB:0.05Ce3+,0.03Tb3+ was approximately 1.8 times stronger than that in LYB:0.03Tb3+. The mechanism of Ce3+→Tb3+ energy transfer was a dipole–dipole interaction, and the energy transfer efficiency gradually increased to 29.27% with increasing Tb3+ doping concentration. Furthermore, the emission colors of LYB:Ce3+,Tb3+ varied from blue to green by adjusting the Ce3+/Tb3+ ratio, indicating that the phosphors could be used as blue-to-green emitting phosphors for application in ultraviolet light-emitting diodes.

Red LaPO4:Eu Nanophosphor for Near UV LED and Field Emission Display Applications

For the phosphor materials, lanthanum phosphate (LaPO4) phosphor is one of potential materials for use in energy-efficient lightings, especially for ultraviolet (UV) light emitting diode (LED) and field emission display (FED) applications. In recent years, with the development of nanotechnologies, the synthesis of LaPO4 phosphor with nanosize particles is of increasing scientific and technical interest. In this study, the LaPO4:Eu nanophosphor was prepared with the coprecipitation (CP) method using different phosphate sources and anionic surfactants. The phosphor particle size analyzed by the SEM was about 0.5–1.2 μm. The phosphor synthesized by the CP method had improved crystallinity, smooth morphology, and decreased nonradiative transition, consequently, a twofold increase in luminescence intensity was obtained when compared with the phosphor prepared by the solid state reaction method. Under the excitation at wavelength of 396 nm (the emission wavelength of a UV LED), the LaPO4:Eu phosphor showed a red emission with peak at 594 nm (5 D 0 → 7 F 1). The maximum luminescence intensity of phosphor was obtained when it was doped with 7 mol% Eu3+ and sintered at 1200°C for 2 h. Emission degradation measurement shows that the emission intensity at wavelength of 594 nm was decayed with single-exponential mechanism. The measured decay time was 2.95 ms. The absorption and emission spectra of the phosphor varied with the kinds of phosphate sources. Using H3PO4 as phosphate source, a strong charge transfer band (CTB) absorption and high 5 D 0 → 7 F 2 emission were measured. When (NH4)2HPO4 and Na5P3O10 were used, a red-shifted and decreased CTB absorption was observed. This shift was caused by the increase in O2- - Eu3+ distance and the crystallinity variation of the phosphor, which led to the decrease in charge transfer energy. On the other hand, for the f-f orbital absorption transitions of Eu3+ inner electrons, the energy absorptions of phosphor prepared with phosphate sources of (NH4)2HPO4 and Na5P3O10 were stronger than that of phosphor prepared with H3PO4. Hence, an increased 5 D 0 → 7 F 1 emission was observed. The highest crystallographic symmetry of Eu3+ in the lattice and a high color-purity red emission were obtained when the (NH4)2HPO4 was used as phosphate source material. To further enhance the photoluminescence (PL) of the phosphor for LED application, anionic dispersants were used to modify the surface of phosphor particles. The dominant growth plane was changed from (200) to (120) plane when the phosphor was synthesized with dispersants. The decay time of phosphor was 3.02, and 3.15 ms when SDS and (NaPO3)6 dispersants were used, respectively. A certain improvement in the decay time was observed. Apparently, the surface states of phosphor particles were modified, hence, the PL intensity was enhanced. The optimal crystallinity and the maximum PL intensity of phosphor were obtained when 1 and 3 mol% of (NaPO3)6 and SDS were used, respectively, and the corresponding increment in the PL intensity was 10 and 7%. Moreover, conductive material of carbon nanotubes (CNTs) was doped to improve the cathodoluminescence (CL) of phosphor for FED application. The addition of CNTs reduced the accumulation of electrons on the phosphor surface. Consequently, the CL intensity was increased. The maximum CL intensity was obtained when 1 wt% of CNT was doped. The CL intensity was enhanced by 1.35 times.

Point-of-use water disinfection using UV light-emitting diodes to reduce bacterial contamination

The treatment process described in this research explores the impact of exposing water samples containing fecal coliforms to the radiation produced by single ultraviolet (UV) light-emitting diodes (LEDs) operating at 265 nm. UV LEDs are long lasting, compact in size and produce more efficient light output than traditional mercury-vapour bulbs, making them ideal for application in point-of-use disinfection systems, such as in remote areas. In this study, contaminated water samples containing either a pure culture of Escherichia coli or tertiary effluent from the City of Regina Wastewater Treatment Plant were used to study the application and efficiency of using UV LEDs for water disinfection. The results indicate that bacterial inactivation was achieved in a time-dependent manner, with 1- and 2.5-log E. coli reductions in water following 20 and 50 min of UV LED exposure, respectively. Ultraviolet radiation was less effective in reducing coliform bacteria in wastewater samples due to the elevated turbidity levels. Further work remains to be completed to optimize the application of UV LEDs for point-of-use disinfection systems; however, the results from this study support that bacterial inactivation using UV LEDs is possible, meriting further future technological development of the LEDs.

Long Wavelength Extension of the Excitation Band of LiEuMo2O8 Phosphor with Bi3 + Doping

Bi-doped red phosphor series were prepared by solid-state reactions which present bright red luminescence dominated by a transition of . With doping, the excitation bands of the phosphors extend to longer wavelength (from ), and the excitation efficiency for optimized content is dramatically increased, which favors the phosphors for the white-light ultraviolet light emitting diode applications. The luminescent model for the phosphors studied in this work is proposed. Because the 6p level of is immerged in the conduction band (CB), the excitation of is overlapped by the excitation , which results in extension of the excitation band. The absorbed energy by the excitation can be effectively transferred to , but the emission of disappears.