Warm White Light-emitting Diodes Research Articles

Highly thermostable mixed lanthanide organic frameworks with high quantum yield for warm white light-emitting diodes.

A mixed lanthanide organic framework was prepared via hydrothermal methods using m-phthalic acid (m-H2BDC), 1,10-phenanthroline (1,10-Phen), and Ln3+ ions, formulated as [HNMe2][Eu0.095Tb1.905(m-BDC)3(phen)2] (ZTU-6). The structure and stability of ZTU-6 were characterised by X-ray diffraction (XRD) and thermogravimetric analysis (TGA), which revealed a three-dimensional pcu topology with high thermal stability. Fluorescence tests showed that ZTU-6 emitted orange light with a high quantum yield of 79.15%, and it can be effectively encapsulated in a light-emitting diode (LED) device emitting orange light. In addition, ZTU-6 was found to be compatible with BaMgAl10O17:Eu2+ (BAM) blue powder and [(Sr,Ba)2SiO4:Eu2+] silicate yellow and green powder to create a warm white LED with a high colour rendering index (CRI) of 93.4, a correlated colour temperature (CCT) of 3908K, and CIE coordinates of (0.38, 036).

Recycling the Energy of Indoor Light: Highly Efficient Organic Photovoltaics via a Ternary Strategy

A ternary approach in organic photovoltaics (OPVs) is simple and reliable to effectively tune the optical, electrical, and morphological properties of the photoactive layer for high efficiency as well as long-term stability under 1 sun. However, there have been few papers reporting the benefits of ternary systems for recycling the energy of indoor light, which has narrow emission spectra and weak illumination compared to outdoor light. In this study, by using two compatible donor polymers (PM7 and PM7 D1) with slightly different band gaps and similar chemical structures, we demonstrate a simultaneous modulation of light absorption and molecular packing under ambient conditions, which resulted in efficient indoor OPVs exhibiting power conversion efficiency (PCE) over 20% under 1000 lx of warm white light-emitting diode (2900 K). From morphological analysis, we infer that PM7 serves to seed the nucleation of PM7 D1 in the ternary blend, templating its crystallization and alignment along the PM7 backbone. Such templating effect leads to increased domain spacing and relative degree of crystallinity (rDoC) compared to those of each binary system. We further show that higher rDoC helps suppress both bimolecular and trap-assisted recombination of photogenerated charges in the ternary devices. As a result, the complementary absorption and synergistic molecular assembly of the two donor polymers enhance the short-circuit current density as to increase the average PCEs from 9.6 to 10.3% under 1 sun and from 18.7 to 20.0% under 1000 lx. We envision that our strategy of incorporating both a planar and a flexible donor polymer with similar chemical structures can be generally applicable to attain a high-performance ternary OPV under both 1 sun and indoor light.

Designing multicolor luminescence in Tb3+/Eu3+-codoped KGd2F7 nanoparticles through energy transfer engineering for warm white light-emitting diode

By a facile precipitation method at room temperature, series of Tb3+-doped and Tb3+/Eu3+-codoped KGd2F7 nanoparticles (NPs) were synthesized. Upon 377 nm excitation, Tb3+-doped KGd2F7 NPs possess intense green emissions and their strongest intensities are obtained when Tb3+ ion is 30 mol%, where the corresponding concentration quenching mechanism is electric dipole-quadrupole interaction. As for Tb3+/Eu3+-codoped KGd2F7 NPs, dazzling multi-color emissions are achieved, excited by 377 nm, triggered by the energy transfer (ET) from Tb3+ to Eu3+. Specially, the emitting color is changed from green to yellow and finally to red as Eu3+ content increases. Through systematical discussion based on the emission spectra and decay time, the ET mechanism is decided by dipole-dipole interaction. Moreover, Tb3+/Eu3+-codoped KGd2F7 NPs do not only have high thermal stability but also exhibit admirable quantum efficiency of 65.7% excited by 377 nm. Furthermore, through utilizing the resultant NPs as yellow-emitting components, a warm white light-emitting diode with high color rendering index (85.5) and low correlated color temperature (4323 K) is fabricated, which are insensitive to injection current. Our findings may provide promising luminescence materials with multicolor emissions to meet the requirement of phosphor-converted solid-state lighting.

Highly luminescent and thermostable R6G@DUT-52 composite for sensing Fe3+ ion and white light emitting diode

Introduction of the yellow rhodamine 6G (R6G) fluorescent dyes into the nanopores of DUT-52 has been successfully achieved the R6G@DUT-52 composite by the one-pot hydrothermal method. Its structure, stability and morphology are well characterized by the XRD, TGA and SEM, whose fluorescent properties are also studied in detail. The obtained R6G@DUT-52 exhibits the highly thermostability with the blue emission of the DUT-52 at 435 nm, the characteristic emission of R6G dyes at 568 nm and the highly quantum yields (QY) of 77.8%, which shows the potential application prospects as the probes for high selectivity and sensitivity of detecting Fe3+ ion from 12 kinds of metals ions. In addition, R6G@DUT-52 is cooperated with the commercial green phosphor powder (GGA) that is further encapsulated on the 455 nm blue LED chip to achieve the warm white light emitting diode (WLED) device with the CIE coordinates of (0.373, 0.364), correlated color temperature (CCT) of 4107 K and luminous efficiency (LE) of 113.08 lm/w.

Green synthesis strategy to construct strong zero-phonon line red emission phosphor Na2SiF6:Mn4+: Photoluminescence properties, thermal stability and application in warm white LEDs

Green synthesis strategy to construct strong zero-phonon line red emission phosphor Na2SiF6:Mn4+: Photoluminescence properties, thermal stability and application in warm white LEDs

Highly efficient and water-resistant K3ZrF7:Mn4+ red-emitting phosphors

Mn4+ activated fluorides are promising red phosphors to be used in warm white light emitting diodes and liquid crystal display (LCD) backlighting due to their unique narrow-band red emission and broad band absorption in blue wavelength region. However, the instability in humid environment of these phosphors limited their practical application. In this work, we developed a facile co-precipitate method to prepare Mn4+ doped K3ZrF7 phosphors with good water-resistant properties. The crystal structure of K3ZrF7 was analyzed by XRD Rietveld refinement. The photoluminescence intensity was maintained at 95% after being immersed in water for 40 min in comparison with only 39.8% for Mn4+ doped K2TiF6 phosphors. The intrinsic seven coordination environment of Mn4+ in K3ZrF7 crystal was proposed as the main reason contributing to the good water resistance of the sample. The internal quantum efficiency and external quantum efficiency of optimized K3ZrF7:Mn4+ was measured to be 70.8% and 19.5% respectively. The thermal stability of the obtained phosphors was studied. A warm white light LED device with color temperature of 4074 K and color rendering index of 76.5 was fabricated by combining a mixture of K3ZrF7:Mn4+ and Y3Al5O12:Ce3+ with a blue InGaN chip.

P3HT vs Spiro-OMeTAD as a hole transport layer for halide perovskite indoor photovoltaics and self-powering of motion sensors

Recent years have witnessed the emergence of indoor photovoltaic (PV) devices with the rapid development of the Internet of things technology field. Among the candidates for indoor PVs, halide perovskites are attracting enormous attention due to their outstanding optoelectronic properties suitable for indoor light harvesting. Here we investigated the indoor PV properties of CH3NH3PbI3-based devices using Spiro-OMeTAD and P3HT as the hole transport layers. The Spiro-OMeTAD-based devices show a consistently higher power conversion efficiency under indoor illumination and 1 sun, with the champion devices showing a power conversion efficiency of 21.0% and 30.1% for the forward and reverse scan under 1000 lux warm white LED illumination. Fewer trap states and higher carrier lifetime were revealed for Spiro-OMeTAD based devices compared to P3HT. The best-performed Spiro-OMeTAD-based devices are used to self-power a wearable motion sensor, which could detect human motion in real-time, to create a primary sensor system with independent power management. By attaching the Spiro-OMeTAD indoor PV device to the strain sensor, the sensor exhibits an accurate and sensitive response with finger bending movements with good repeatability and negligible degradation of mechanical stability, which indicates the success of sensor powering with the indoor PV device.

Strategic Tuning of Photophysical Response in the Polyhedral Framework of the Garnet Structure toward White Light-Emitting Devices with Enhanced Color Rendering.

Designing a single-phase phosphor with high quantum efficiency and full spectrum emission is inevitable for today's scientific world. Herein, an optimal strategy for realizing white emission in a single component matrix is envisaged based on the structure-property-design-device policy. Cationic substitution corresponding to polyhedral expansion and contraction in A2A'B2V3O12 confirms the existence of strong and intricate linkage in the garnet structure. The dodecahedral expansion causes compression of VO4 tetrahedra and a blue shift. The direct correlation of V-O bond distance with red shift validates the distortion of the VO4 tetrahedra. The interdependence of photophysical properties via cationic substitution and subsequent correlation of the V-O bond distance with emission bands enabled the tailoring of phosphor-CaSrNaMg2V3O12 with a high quantum efficiency of 52% and excellent thermal stability of 0.39 eV. Bright warm white light-emitting diode (WLED) devices are fabricated based on Eu3+ and Sm3+-activators. A high quantum efficiency-74% is obtained for the designed Eu3+ phosphor. CIE coordinates near the achromatic point (0.329, 0.366), low CCT-5623 K, and high color rendering index (CRI)-87 are obtained for the single-phase WLED device. This work puts forth a new direction for designing and engineering promising WLEDs with enhanced color rendering based on single-phase phosphors with full spectrum emission.

Remarkable solution and solid-state color-tunable luminescence of diethylamino-coumarin derivatives and application in warm-white LEDs

Organic materials with solution and solid dual efficient, color-tunable luminescence are potentially useful for sensors and optical devices. Herein, a series of donor-π-acceptor luminophores, named BMs, with different phenyl substituents at the 3-positions of 7-diethylamino-coumarin was developed. All four compounds with slightly twisted structure rendering BMs have bright emission in solution and solid state. BMs not only possess solvatochromic properties in solution but also realize color tuning from cyan (480 nm) to deep red (661 nm) in solid states. Among them, the fluorescence quantum yield of compound BM1 in solid state reached 98.2%, and the fluorescence quantum yield in solution was 77.1%. Time dependent density functional theory calculation and single crystal analysis show that with the increase in the electron-withdrawing ability of 4-phenyl moiety at the 3-position of coumarin, the charge transfer effect was strengthened gradually, and the photoluminescence mechanism changed from intramolecular charge transfer (ICT) to twisted ICT on account of the electron acceptor changed accordingly. Moreover, the cyan phosphor BM1 and the deep-red phosphor BM4 were laminated to fabricate an impressive warm-white light-emitting diode device with a maximum brightness of 52,090 cd/m2 at an operating voltage of 4.0 V.

Ultrahighly Efficient Narrowband Red Luminescence of Uniquely Distorted Mn4+ Octahedron in the Feldspar‐Type LED Phosphor

AbstractThe mineral structure‐inspired discovery of red phosphors has become a research hotspot that can help compensate for the missing red components in cold white light‐emitting diodes (WLEDs). Herein, first the feldspar‐type structure is utilized to develop narrowband red‐emitting CaAl2Si2O8:Mn4+/Mn4+, Mg2+ phosphors. Rietveld refinement results confirm the Mn4+ occupation of the distorted [Ca1O6] octahedron, resulting in a strong nephelauxetic effect, and thus exhibits bright red luminescence with a narrow full width at half maximum (43 nm). Compared with CaAl2Si2O8:Mn4+, the fluorescence intensity of CaAl2Si2O8:Mn4+, Mg2+ is further enhanced by up to 210% owing to the decreasing nonradiative decay rate from 2Eg excited state, which weakens the concentration quenching effect resulting from energy migration along the adjacently aggregated Mn4+. CaAl2Si2O8:Mn4+, Mg2+ shows preferable thermal stability with an ultrahigh quantum efficiency (90.3%), surpassing other current Mn4+‐doped oxide phosphors and UCr4C4‐type nitride phosphors. A fabricated prototype WLED obtains ideal warm white light with a low correlated color temperature (3081 K) and a wide color gamut covering 112% of the National Television System Committee standard. Guided by natural mineral‐type structural prototypes, this study demonstrates the possibility of designing a uniquely distorted local structure to achieve a superior luminescence performance of Mn4+ for warm WLEDs.

Excitation‐dependent energy transfer and color tunability in Dy3+/Eu3+ co‐doped multi‐component borophosphate glasses

AbstractUsing the melt‐quench technique, potassium zinc borophosphate (KZnBP) glasses incorporated with Dy3+, Eu3+, and Dy3+/Eu3+ ions individually and combinedly were prepared, and their photoluminescence (PL)‐related features were investigated. The KZnBP glass containing an optimized content of Dy3+ (0.5 mol%) is co‐doped with Eu3+ in various contents, and the energy transfer (ET) process between them was studied at λexci = 349, 364, 387 (Dy3+), and 394 nm (Eu3+). The Dy3+/Eu3+ co‐doped system, when excited with Dy3+ excitations has resulted in a significant decrease in the intensity of Dy3+ peaks observed at 480 nm (4F9/2→6H15/2, blue) and 574 nm (4F9/2→6H13/2, yellow), with simultaneous enhancement of the intensity of Eu3+ peaks at 591 nm (5D0→7F1, orange) and 617 nm (5D0→7F2, red). This trend is due to the efficient energy transfer from Dy3+ to Eu3+, indicating that Eu3+ ions were sensitized by Dy3+ ions. Dexter's theory and the Inokuti–Hirayama (I–H) model revealed that the dipole–dipole interaction is accountable for the energy transfer from Dy3+ to Eu3+ through energy‐transfer channels [4F9/2(Dy3+)+7F1,2(Eu3+)→6H15/2(Dy3+)+5D2(Eu3+)] and [4F9/2(Dy3+)+7F0(Eu3+)→6H13/2(Dy3+)+5D0(Eu3+)]. The color coordinates of the Dy3+/Eu3+ co‐doped glasses under various excitations fall within the white light emission spectrum, indicating their potential application in warm white LEDs.

Ce,Tb,Gd:YAG-Al2O3 eutectic crystal phosphor plates for high-power warm white LED applications

Ce,Tb,Gd:YAG-Al2O3 eutectic crystal phosphor plates (ECPPs) were grown by the micro-pulling-down (μ-PD) method to adjust correlated color temperature (CCT) and color rendering index (Ra) for high-power warm white LED applications. Tb3+ and Gd3+ ions in Ce:YAG-Al2O3 play a positive role in spectra adjustment based on the structural and luminescent properties investigations. Tb3+ ions enhance the fluorescence lifetime of Ce3+ ions from 61.1 to 63.8 ns and improve the luminescence efficiency, and the highest optical-optical conversion efficiency (OCE) is 270.56 lm/W. Corresponding luminescence shift from cold white to greenish yellow. Gd3+ ions increase the peak wavelength from 554 to 564 nm and improve the Ra value, and the maximum Ra is 75.5. Corresponding luminescence is shifted from cold white to reddish violet. In addition, the luminescence of Ce,Tb,Gd:YAG-Al2O3 ECPPs is adjusted to warm white with CIE coordinate (0.374, 0.364) and CCT of 4102 K, and Ra of 73.0, which is of great significance for realizing the high-power warm white LED lighting.

Performance Comparison of Tungsten-Halogen Light and Phosphor-Converted NIR LED in Soluble Solid Content Estimation of Apple.

A Tungsten-Halogen (TH) lamp is the most popular light source in NIR spectroscopy and hyperspectral imaging, which requires a warm-up to reach very high temperatures of up to 250 °C and take a long time for radiation stabilization. Consequently, it has a large enough volume to enable heat dissipation to prevent the thermal runaway of the electric circuit and turn out its power efficiency very low. These are major barriers for miniaturizing spectral systems and hyperspectral imaging devices. However, TH lamps can be replaced by pc-NIR LEDs in order to avoid high temperature and large volume. We compared the spectral emission of the available commercial pc-NIR LEDs under the same condition. As a replacement for the TH lamp, the VIS + NIR LED module was developed to combine a warm-white LED and pc-NIR LEDs. In order to feature out the availability of the VIS + NIR LED module against the TH lamp, they were used as the light source for evaluating the Soluble Solid Content (SSC) of an apple through VIS-NIR spectroscopy. The results show a remarkable feasibility in the performance of the partial least square (PLS) model using the VIS + NIR LED module; during PLS calibration, the correlation coefficient (R) values are 0.664 and 0.701, and the Mean Square Error (MSE) values are 0.681 and 0.602 for the TH lamp and VIS + NIR LED module, respectively. In VIS-NIR spectroscopy, this study indicates that the TH lamp could be replaceable with a warm-white LED and pc-NIR LEDs.

Quantitative Calculation of the Most Efficient LED Light Combinations at Specific Growth Stages for Basil Indoor Horticulture: Modeling through Design of Experiments

Indoor farms are a promising way to obtain vegetables in standard quantity and quality. As opposed to previous studies, this study attempts to calculate optimized LED light conditions for different growth stages (five-days time step) of basil (Ocimum basilicum) to enhance its indoor growth through a statistical approach. Design of Experiments (DoE) was used to plan a limited number of experiments (20) and to calculate quantitatively the effect of different light recipes on four responses: the number of plants, their height, the Leaf Area Index, and the amount of water used. Different proportions (from 25% to 77%) of Hyper Red (660 nm) and Deep Blue (451 nm), intensities in terms of LEDs–plant distance (60, 70 and 80 cm), and the addition of Warm White (3000 K) LEDs were considered as independent variables. The obtained models suggest that a light recipe tailored for every growth step in the plant’s life is beneficial. Appropriate LEDs must be carefully chosen at the beginning of growth, whereas distance becomes relevant at the end. This is confirmed by the results analysis carried out at the end of an additional growth test where the optimal light recipe extracted from the DoE’s results were used.

Hydrothermal synthesis of orange fluorescent carbon dots and their application in fabrication of warm WLEDs and fluorescent ink

Herein, we are reporting the synthesis of orange fluorescent carbon quantum dots (OFCDs) using o-phenylenediamine and semicarbazide by hydrothermal method and the synthesized OFCDs were applied for warm white light emitting diodes (WLEDs) and fluorescent ink applications. The fluorescence emission spectrum of OFCDs shows the emission maximum at 625 nm with an excitation wavelength of 540 nm. The quantum yield of OFCDs was calculated to be 23%. We have prepared warm WLED device by coating PVA-OFCDs polymer matrix on near UV LED light (λem: 420 nm). The OFCDs based warm WLED device exhibits chromaticity coordinate (0.42, 0.48), Colour Correlated Temperature (CCT) 2500 K and Color Rendering Index (CRI) value 82. We also prepared the highly stable fluorescent ink using the same PVA-OFCDs for security information encryption and anti-counterfeiting applications. This OFCDs and their WLEDs device would be an efficient one in optoelectronics and forensic fields.

Enriching the Deep-Red Emission in (Mg, Ba)3M2GeO8: Mn4+ (M = Al, Ga) Compositions for Light-Emitting Diodes

Red emission from Mn4+-containing oxides inspired the development of high color rendering and cost-effective white-light-emitting diodes (WLEDs). Aiming at this fact, a series of new crystallographic site modified (Mg, Ba)3M2GeO8: Mn4+ (M = Al, Ga) compositions were developed with strong deep-red emission in the reaction to UV and blue lights. The Mg3Al2GeO8 host is composed of three phases: orthorhombic-Mg3Ga2GeO8, orthorhombic-Mg2GeO4, and cubic-MgAl2O4. However, Mg3Ga2GeO8 secured an orthorhombic crystal structure. Interestingly, Mg3Al2GeO8: Mn4+ showed a 13-fold more intense emission than Mg3Ga2GeO8: Mn4+ since Mn4+ occupancy was preferable to [AlO6] sites compared to [GaO6]. The coexisting phases of MgAl2O4 and Mg2GeO4 in Mg3Al2GeO8: Mn4+ contributed to Mn4+ luminescence by providing additional [AlO6] and [MgO6] octahedrons for Mn4+ occupancy. Further, these sites reduced the natural reduction probability of Mn4+ to Mn2+ in [AlO4] tetrahedrons, which was confirmed using cathodoluminescence analysis for the first time. A cationic substitution strategy was employed on Mg3M2GeO8: Mn4+ to improve the luminescence, and Mg3-xBaxM2GeO8: Mn4+ (M = Al, Ga) phosphors were synthesized. Partial substitution of larger Ba2+ ions in Mg2+ sites caused structural distortions and generated a new Ba impurity phase, which improved the photoluminescence. Compositionally tuned Mg2.73Ba0.27Al1.993GeO8: 0.005Mn4+ exhibited a 35-fold higher emission than that of Mg3Ga1.993GeO8: 0.005Mn4+. Additionally, this could retain 70% of its ambient emission intensity at 453 K. A warm WLED with a correlated color temperature (CCT) of 3730 K and a CRI of 89 was fabricated by combining the optimized red component with Y3Al5O12: Ce3+ and 410 nm blue LED. By tuning the ratio of blue (BaMgAl10O17: Eu2+), green (Ce0.63Tb0.37MgAl11O19), and red (Mg2.73Ba0.27Al2GeO8: 0.005Mn4+) phosphors, another WLED was developed using a 280 nm UV-LED chip. This showed natural white emission with a CRI of 79 and a CCT of 5306 K. Meanwhile, three red LEDs were also fabricated using the Mg2.73Ba0.27Al1.993GeO8: 0.005Mn4+ phosphor with commercial sources. These could be potential pc-LEDs for plant growth applications.

Phantom array effect can be observed above 15 kHz in high speed eye movement group for high luminance warm white LED

A phantom array can be observed when there are saccadic eye movements across a time-modulated light source. In this study, the highest frequency limit of the phantom array effect that can be observed was investigated using an LED light source by estimating the participant’s threshold frequency for the visibility of the phantom array effect using the weighted up-down method. A high luminance, narrow slit, warm white LED, and long saccadic eye movement range were used as the experimental stimuli and conditions. The mean threshold frequency for young adults was approximately 10 kHz under the proposed experimental conditions. The phantom array effect was observed even above 15 kHz in the high threshold frequency group when we divided participants into three groups according to the threshold frequency. In addition, the high threshold frequency group showed a higher eye movement speed than the other groups.

Influence of K/Na ions on structure and luminescence properties of red phosphors K0.5Na0.5Gd(WO4)2: Eu3+

In this paper, we demonstrated the influence of K/Na alkali ions on the structural and photo-cathodoluminescence properties of novel K0.5Na0.5Gd(WO4)2:Eu3+ red phosphor synthesized by the solid-state method. The purity of phosphors is determined from the X-ray diffraction. Rietveld refinement studies confirms that the compounds both K0.5Na0.5Gd(WO4)2:Eu3+ and NaGd(WO4)2:Eu3+ were crystallized in the tetragonal structure with I41/a space group and KGd(WO4)2:Eu3+ was crystallized in monoclinic structure with C2/c space group. The FTIR results indicate that the presence of characteristic functional groups in synthesized samples. The optical bandgap determined from the UV-Visible reflectance spectra in the order K0.5Na0.5Gd(WO4)2:Eu3+ <KGd(WO4)2:Eu3+<NaGd(WO4)2:Eu3+. The photoluminescence excitation spectrum reveal that all the synthesized phosphors can be efficiently excited near UV and Ultraviolet (UV) indicating suitable for solid state display applications. The PL emission spectra indicate the two characteristic emissions located at 614 nm (5D0→7F1) and 590 nm (5D0→7F1) under the excitation of 395 nm. The Judd-Offelt (JO) intensity parameters are calculated to investigate the luminescence behavior and to determine the local structure around the activator related to the Eu3+ ion. The Ω2 > Ω4 of JO parameters indicates the asymmetric environment around the activator suggesting the co-valency of Eu-O bonds with respect to host matrix. Radiative lifetime, branching ratio, electronic dipole transition probabilities were calculated based on the Judd-Ofelt formalism. Cathodoluminescence studies shows the characteristic emission of the Eu3+ ions for low current and voltage which further confirms the pure red color emission of phosphors. All the synthesized phosphors can emit the red emissions with correlated color temperature range from 1500 to 2000 K. For K0.5Na0.5Gd(WO4)2:Eu3+ sample the chromaticity coordinate (x = 0.667, y = 0.329) are closer to the NTSC trichromatic coordinated (x = 0.67,y = 0.33) than that of Y2O2S:Eu3+ suggesting that K0.5Na0.5Gd(WO4)2:Eu3+ can be a potential red phosphor material for warm white LED's applications.

Highly Stable White Light Emission from III-Nitride Nanowire LEDs Utilizing Nanostructured Alumina-Doped Mn4+ and Mg2.

In this report, red-emitting alumina nanophosphors doped with Mn4+ and Mg2+ (Al2O3:Mn4+, Mg2+) are synthesized by a hydrothermal method using a Pluronic surfactant. The prepared samples are ceramic-sintered at various temperatures. X-ray diffraction shows that Al2O3:Mn4+, Mg2+ annealed at 500 °C exhibits a cubic γ-Al2O3 phase with the space group Fd3m-227. The tetragonal δ-Al2O3 and rhombohedral α-Al2O3 phase is obtained at 1000 and 1300 °C, respectively. Cube-like nanoparticles in a size of ∼40 nm are observed for the alumina heated at 500-1000 °C. The size and red-emitting intensity of the phosphors remarkably increased with annealed temperature ∼1300 °C. Emission spectra of the phosphors show strong peaks at 678 and 692 nm due to 2 E g → 4 A 2 transitions of the Mn4+ ion, under a light excitation of 460 nm. A strong zero-phonon line (ZPL) emission is observed in the luminescence spectra of δ-Al2O3:Mn4+, Mg2+ at 298 K, whereas a weak one is observed in those of α- and γ-Al2O3:Mn4+, Mg2+. The alumina phosphors exhibited an excellent waterproof ability during 60 days in water and good thermal stability in the range of 77-573 K. A warm-white light-emitting diode (WLED) fabricated using In x Ga1-x N nanowire chips with Al2O3:Mn4+, Mg2+ red-emitting nanophosphors presents a high color rendering index of ∼95.1 and a low correlated color temperature of ∼4998 K. Moreover, the current-voltage characteristic of the nanowire LEDs could be improved using Al2O3:Mn4+, Mg2+ nanophosphors which is attributed to the increased heat dissipation in the nanowire LEDs.

Temperature Spectrum Optimization of Mixed White LED Cluster Comprising Red/Green/Blue/Warm White LED

Temperature Spectrum Optimization of Mixed White LED Cluster Comprising Red/Green/Blue/Warm White LED