Efficient Red Emission Research Articles

Color Control of Pr3+ Luminescence by Electron-Hole Recombination Energy Transfer in CaTiO3 and CaZrO3.

Controlling luminescence in phosphors able to produce several emissions from different stable excited states determines their use in optical devices. We investigate the color control mechanism that quenches the greenish-blue emission in favor of the red one in the archetype phosphor CaTiO3:Pr3+. State-of-the-art ab initio calculations indicate that direct host-to-dopant energy transfer (released by electron-hole recombination following the interband excitation and structural reorganization) selectively populates the 1D2 red luminescent state of Pr3+ and bypasses the 3P0 greenish-blue emitter. Local defects can modulate the electron-hole recombination energy and therefore increase the red emission efficiency, as experimentally observed. The selection of red emission does not happen in CaZrO3:Pr3+ because the electron-hole recombination energy is much higher. The calculations could not support the widely accepted color control mechanism based on metal-to-metal charge transfer states. The conclusion sets new points of view for the color control of lanthanide activated inorganic phosphors.

3d3 system – Comparison of Mn4+ and Cr3+ in different lattices

In the last years the Mn4+ has been successfully used as a dopant in the red-emitting phosphors based on fluoride matrices such as: AMF6 (A=Na, K, Cs, Ba, Rb; M=Si, Ti, Ge), due to its efficient red emission around 630 nm. The peculiarities of the Mn4+ emission are the consequence of the specific energetic structure related to three electrons occupying the 3d orbitals which interact with the crystal field of the lattice. The 3d3 ion system is one of the most extensively investigated systems, specifically concerning the Cr3+, since Al2O3:Cr3+ (ruby) has been used for laser generation. In this contribution the details of energetic structure and kinetics of radiative and nonradiative processes in the 3d3 system are discussed. Special attention is given to consideration of the electron-lattice interaction and spin-orbit coupling. Differences between the spectroscopic properties of Cr3+ and Mn4+ ions in different lattices are analyzed in the framework of the extended crystal field model that includes the vibronic structure of the system. Specifically, we present the proper formula describing the temperature dependence of the Mn4+ luminescence lifetime that includes the interaction with odd-parity phonons.

Facile fabrication of Eu3+ activated YAG:Ce3+ glass ceramics exhibiting high thermal stability and tunable luminescence for warm white LEDs

To relieve the issues of high correlated color temperature and low color rendering index, it is urgent to develop an inorganic color converter materials with both red- and yellow-emission to replace the conventional organic resin/silicon-based color converter and acquire warm white light-emitting diodes (WLEDs). In our work, transparent glass–ceramics (GCs) containing Ce3+:Y3Al5O12 (Ce3+:YAG) yellow phosphors and Eu3+ ions are successfully fabricated by using low temperature co-sintering technique. And then the microstructure and luminescence properties of the GC are investigated in detail. Importantly, the impact of Ce3+:YAG phosphors concentration, Eu3+ ions concentration, co-sintering time and the GC thickness on optical performances are also systematically demonstrated. Impressively, the as-fabricated samples exhibit an efficient red emission at 611 nm owing to the 5D0→7F2 transition of Eu3+. Subsequently, the temperature-dependent luminescence and thermal quenching is also investigated. Finally, the constructed WLEDs show an optimal luminous efficacy of 120.85 lm/W, a color rendering index of 70.2 and a correlated color temperature of 5267 K. All the results indicate that the as-fabricated GC would have potential practical applications in high-power WLEDs.

Highly Stable Red-Light-Emitting Electrochemical Cells.

The synthesis and characterization of a series of new cyclometalated iridium(III) complexes [Ir(ppy)2(N∧N)][PF6] in which Hppy = 2-phenylpyridine and N∧N is (pyridin-2-yl)benzo[d]thiazole (L1), 2-(4-(tert-butyl)pyridin-2-yl)benzo[d]thiazole (L2), 2-(6-phenylpyridin-2-yl)benzo[d]thiazole (L3), 2-(4-(tert-butyl)-6-phenylpyridin-2-yl)benzo[d]thiazole (L4), 2,6-bis(benzo[d]thiazol-2-yl)pyridine (L5), 2-(pyridin-2-yl)benzo[d]oxazole (L6), or 2,2'-dibenzo[d]thiazole (L7) are reported. The single crystal structures of [Ir(ppy)2(L1)][PF6]·1.5CH2Cl2, [Ir(ppy)2(L6)][PF6]·CH2Cl2, and [Ir(ppy)2(L7)][PF6] have been determined. The new complexes are efficient red emitters and have been used in the active layers in light-emitting electrochemical cells (LECs). The effects of modifications of the 2-(pyridin-2-yl)benzo[d]thiazole ligand on the photoluminescence and LEC performance have been examined. Extremely stable red-emitting LECs are obtained, and when [Ir(ppy)2(L1)][PF6], [Ir(ppy)2(L2)][PF6], or [Ir(ppy)2(L3)][PF6] are used in the active layer, device lifetimes greater than 1000, 6000, and 4000 h, respectively, are observed.

Multi-chromatic silicon nanocrystals

Silicon nanocrystals (SiNCs) have great potential to become environmental friendly alternatives to heavy-metal containing nanocrystals for applications including medical imaging, lighting and displays. SiNCs exhibit excellent photostability, non-toxicity and abundant resources, but their often reported inefficient and spectrally limited light emission seriously impair their applications. Here we demonstrate a new method that converts SiNCs into an efficient and robust multi-chromatic phosphor. Using ~15 keV electron-beam irradiation of oxide-capped SiNCs, we introduce several types of color centers into the nanocrystal’s oxide shell with efficient blue, green and red emission bands, together yielding warm-white photoluminescence, even for a single SiNC. Introduced centers are not native to the original system and we relate them to known defects in silica. Unlike in the silica host, however, here the centers are efficiently optically excitable. Provided further optimization and up-scaling of this method, e-beam irradiated SiNCs can be of great interest as white phosphors for applications such as LEDs.

Europium doped low cost Zn2SiO4 based glass ceramics: A study on fabrication, structural, energy band gap and luminescence properties

Nowadays, the demand for better light emitting diodes (LEDs) has led to a growing interest in producing Zn2SiO4 based glass ceramic phosphors using waste materials. In this research, Eu3+ doped Zn2SiO4 based glass ceramics were prepared based on a solid state method using recyclable glass wastes as silica source. The effect of sintering temperatures, ranging from 600 to 1000°C and the influence of Eu3+ ions (x=0, 1, 2, 3, 4 and 5 wt%) on the structural, energy band gap and luminescence properties of the phosphors were investigated using XRD, FTIR spectroscopy, UV-Vis-NIR spectroscopy, Photoluminescence spectroscopy, and FESEM. XRD results showed the formation of α-Zn2SiO4 at higher sintering and poor crystallization when the dopant's concentration was increased. FTIR spectra showed that the progression of sintering temperature had sharpened the absorption bands. The band gap energies were identified by the measurement of the absorption edge. Results indicated that the absorption edges were blue shifted, thus increasing the band gap values when the powders were sintered at 1000°C. The luminescent properties were studied by the measurement of excitation and emission spectra under 600nm and 400nm, respectively. It was observed that higher sintering temperatures of 900 and 1000°C had decreased the red emission intensity, while the doping effect of Eu3+ concentration had gradually enhanced the red emission efficiency of 5D0 →7F2 transition. The morphologies from FESEM analysis showed the formation of densely packed grains with the increment of sintering temperatures.

Tuning upconversion fluorescence emission of -NaLuF4:Yb3+/Ho3+ nanocrystals through codoping Ce3+ ions

Rare-earth-doped up-conversion (UC) fluoride materials have been widely used in phosphors, color displays, optical storages, solid-state lasers, solar cells and biomedical imaging, due to the fact that their low phonon energy can effectively suppress the nonradiative multiphonon relaxation process. In this work, the NaLuF4:Yb3+/Ho3+ nanocrystals are successfully synthesized by a facile solvothermal method. The crystal structure and morphology of the NaLuF4 nanocrystals are characterized by the X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM) respectively. The diffraction peaks are well consistent with those of high-purity hexagonal NaLuF4 (JCPDS No. 77-2042, P63/m space group). The TEM image reveals that the product is composed of monodisperse hexagonal rods with an average length of about 170 nm and an average diameter of 30 nm. The crystal structure and morphology do not present obvious change with the increasing Ce3+ ion concentration, which is due to the similarity in ion radius between Ce3+ and Lu3+. Under 980 nm excitation, the UC emissions of -NaLuF4:Yb3+/Ho3+ nanocrystals with different Ce3+ codoping concentrations are carefully studied. The strong green and red UC emissions of Ho3+ ions are observed in -NaLuF4 nanocrystals. It can be found that the UC emission of Ho3+ ions is tuned from green to red in -NaLuF4 nanocrystals through increasing Ce3+ ion concentrations from 0 to 12%, and the red-to-green (R/G) ratio is enhanced from 0.34 to 8.44. According to the level structure of Ho3+ ions, the red UC emission originates from the excited state 5F5. However, the population of the 5F5 excited state mainly depends on the two nonradiative relaxation processes of 5S2/5F45F5 and 5I65I7 transitions. In fact, the two nonradiative relaxation processes are very difficult to occur according to multiphonon nonradiative relaxation rate. When Ce3+ ion is introduced into the system, the red UC emission intensity and R/G ratio of Ho3+ are increased, because the energy gap from the excited state 5F7/2 to the ground state 2F5/2 is about 3000 cm-1 for Ce3+ ions, which is similar to the gaps of 5S2/5F45F5 and 5I65I7 transitions of Ho3+ ions. According to the energy conservation law, the two inefficient nonradiative processes from the 5S2/5F4 and 5I6 states of Ho3+ ions are substituted in order by resonant cross relaxation (CR) processes 5S2 (5F4) (Ho3+) + 2F5/2 (Ce3+5F5 (Ho3+) + 2F7/2 (Ce3+) and 5I6 (Ho3+) + 2F5/2 (Ce3+)5I7 (Ho3+) +2F7/2 (Ce3+) between Ho3+ and Ce3+ ions. These two resonant CR processes can transfer populations from the 5S2/5F4 state and 5I6 state to the 5F5 state and its intermediate 5I7 state, respectively. The resonant modality and the strong interaction between Ho3+ and Ce3+ ions are employed to enhance the red emission and suppress the green emission. The occurrence of CR process between Ho3+ and Ce3+ ions is further proved by the down-conversion emission spectra of Ho3+ ions under 532 and 980 nm laser excitation, respectively. We demonstrate that the highly efficient red UC emission of -NaLuF4:Yb3+/Ho3+/Ce3+ nanocrystals offers opportunities as desired optical materials for color displays, anticounterfeiting techniques and multiplexed labeling applications.

Rigidly linking cyclometallated Ir(iii) and Pt(ii) centres: an efficient approach to strongly absorbing and highly phosphorescent red emitters.

The synthesis and photophysical properties of an unprecedented tetranuclear complex are described, in which a fac-tris-cyclometallated Ir(iii) centre is rigidly connected to three cyclometallated Pt(ii) centres. The complex absorbs strongly up to ∼600 nm and emits red light with unusually high efficiency.

A novel mechanism for red emission carbon dots: hydrogen bond dominated molecular states emission.

Carbon dots (CDs) have emerged as novel fluorescent probes due to their remarkable optical properties; however, red emission is still rare, has a relatively low efficiency, and its mechanism remains ambiguous. Herein, relatively efficient red-emission CDs based on p-phenylenediamine were prepared through various solvothermal means, where the highest quantum yield approached 41.1% in n-amyl alcohol, which was the most efficient quantum yield reported to date. Various structural characterizations were performed and confirmed that the red emission originated from the molecular states consisting of a nitrogen-containing organic fluorophore. The CDs were dispersed in different organic solvents and showed tunable emission, evolving from green to orange-red in aprotic solvents and a red emission in protic solvents. Further solvent correlation studies indicated that the hydrogen bond effect between the CDs and solvents was the main mechanism leading to the spectral shift. Accordingly, solid-state luminescent CDs-polymers were fabricated, which also demonstrated continuously tunable emission properties. This work opens a new window for recognizing the generation of tunable and red-emission CDs.

Photoluminescence Properties and Energy Transfer in Color-Tunable Garnet Phosphor for High Quality LED Lighting

Recently, the performance of phosphor-converted white light-emitting diodes (WLED) has been improved mainly by diversifying the components, encapsulation of the device, phosphor synthesis, as well as designing new strategies for the development of phosphors. Nowadays, single-composition white-emitting phosphors which are excited by near UV or blue LED have been developed to prevent the reabsorption, instability of color temperature, and cost issues. We report a garnet phosphor with partially displaced Tb3+ and Pr3+ as green and red components respectively. A green-emitting and white-emitting phosphor was synthesized by a solid-state reaction, exhibiting an efficient green emission and red emission at 543 nm and 609 nm under blue excitation. The structural and optical properties of the phosphor were studied using powder X-ray diffraction and photoluminescence analysis. The compound crystalizes in cubic phase with space group Ia d. The partial substitution of Tb3+ and Pr3+ in garnet phosphor led to a considerably broad emission spectrum. The WLED was fabricated by using phosphor-in-glass with a blue LED chip and exhibited a warm white light with an excellent color rendering index of 92. The garnet phosphor with Ce3+, Tb3+, and Pr3+ is thus a potential green and white-emitting phosphor for LED lighting. Figure 1. PL excitation and emission spectrum of garnet phosphor with (a) Ce3+, (b) Ce3+, Tb3+, and (c) Ce3+, Pr3+. Figure 1

Synthesis and characterization of red phosphorescent iridium(Ⅲ) complexes based on electron-acceptor modulation of main ligand for high efficiency organic light-emitting diodes

ABSTRACTA new series of red phosphorescent iridium(Ⅲ) complexes, (PT-TFP)2Ir(tmd), (PT-P)2Ir(tmd), and (PT-MP)2Ir(tmd) were synthesized for highly efficient red emitter of the organic light-emitting diodes (OLEDs). The main ligands consisted of electron-donor (phenanthren) and electron-acceptor (trifluoromethyl pyridine, pyridine, and methyl pyridine) were synthesized by Suzuki coupling reaction. The iridium(Ⅲ) complexes based on main ligands and 2,2,6,6-tetramethyl-3,5-heptanedione(tmd) ancillary ligand were synthesized by Nonoyama reaction. Their luminescence properties were investigated by UV-visible spectroscopy and photoluminescence (PL) spectroscopy. The OLEDs devices were manufactured by vacuum deposition and These OLEDs devices were characterized by investigation of current density-voltage-luminance (J-V-L), current efficiency, power efficiency, external quantum efficiency (EQE), EL spectrum. The Ir(Ⅲ) complexes will be used as a solution process dopant for the hybrid OLED devices.

Synthesis, crystal structure and temperature dependent luminescence of Eu3+ doped SrGd2O4 host: An approach towards tunable red emissions for display applications

In the present paper, an investigation on the structural and photoluminescence (PL) properties of SrGd2O4:Eu3+ ceramic phosphors synthesized by homogeneous precipitation method followed by combustion process has been reported. The samples, annealed at 1200°C, were crystallized into orthorhombic phase without any impurities. Microscopic studies revealed the irregular morphology of the obtained ceramic phosphor particles having sizes in the range of 0.3–3µm. The characteristic photoluminescence properties and decay curves were studied in detail as a function of Eu3+ concentration and temperature. The Eu3+ doped ceramic samples illuminated with UV light revealed the characteristic red luminescence corresponding to 5D0→7FJ transitions of Eu3+. The concentration quenching phenomenon of Eu3+ ions in the present host, analyzed in the light of ion-ion interaction, indicated multipolar interaction between Eu3+ ions. Finally, the intensity parameters (Ω2, Ω4) and various radiative properties such as stimulated emission cross-section (σe), gain band-width (σe×Δλeff) and optical gain (σe×τexp) of Eu3+ in the SrGd2O4 ceramic phosphors have been calculated by using Judd-Ofelt theory. The present phosphor system exhibited efficient red emission with high red color purity (95%) and adequate thermal stability even at 200°C. Present research broadly indicated the suitability of SrGd2O4:Eu3+ ceramic phosphor for display applications.

High-purity red up-conversion emission of Ba5Zn4Y8O21:Er3+,Yb3+ phosphor excited by 1550 nm laser diode

A novel Ba5Zn4Y8O21:Er3+,Yb3+ up-conversion phosphor was synthesized via solid-state reaction method. The up-conversion luminescence properties of the phosphor excited by 1550nm laser diode were investigated. Results showed that the characteristic emission peaks of Er3+ were located at 530, 550, 675, and 810nm, which correspond to the transitions of 2H11/2→4I15/2, 4S3/2→4I15/2, 4F9/2→4I15/2, and 4I9/2→4I15/2, respectively. The up-conversion mechanisms were systematically studied through concentration and laser power dependence of the up-converted emissions. An evident tuning effect of Yb3+ on the luminescence of the phosphor was observed. An efficient red emission with high purity was obtained through the Er3+→Yb3+→Er3+ energy transfer process, and the intensity ratio of the red and green emissions could reach 62.28. Therefore, Ba5Zn4Y8O21:Er3+,Yb3+ can be an excellent up-conversion red emitter.

Luminescent properties of Eu3+-doped α-NaYF4 single crystal under NUV-excitation

This paper reports on successful synthesis of α-NaYF4 single crystal doped with Eu3+ at various concentrations by a modified Bridgman method. The crystal structure is characterized by means of X-ray diffraction. The absorption spectra, excitation spectra and emission spectra were measured to investigate the optical properties of the single crystals. An intense red emission located at 611 nm with long lifetime of 9.03 ms was observed in single crystal under the excitation of 394 nm light. It benefits from the low maximum phonon energy of α-NaYF4 single crystal matrix (390 cm−1). The CIE chromaticity coordinate of the α-NaYF4 single crystal doped Eu3+ in 4 mol% concentration was calculated (x = 0.6055, y = 0.388), which was close to the National Television Standard Committee standard values for red phosphor (x = 0.67, y = 0.33). All these spectral properties suggest that that this kind of fluoride crystal with high thermal stability and high efficiency of red emission may be used as potential red phosphors for optical devices.

Enhanced performances for top-emitting white organic light-emitting diodes by utilizing green phosphor as energy transfer medium

In top-emitting white organic light-emitting diodes (TWOLEDs), the device performances attribute to the several important factors, such as exciton profile, energy transfer, and microcavity effect. In this paper, a TWOLED containing a heterojunction blue emission layer (EML) and a red EML is reported. A host material with high triplet energy level is employed for the adjacent blue and red EML, while the inefficient red emission reduces the emission efficiency of the TWOLED. In order to enhance the red emission efficiency, mixed-host and co-doping technologies are used in the red EML. By mixing the hole transporting and electron transporting host materials, the exciton recombination zone extends to the red EML to increase the red emission intensity and reduce the efficiency roll-off. And by co-doping a green phosphor into the red EML as the energy transfer medium, the energy transfer rate is enhanced, and then the current efficiency increases. Besides, both the mixed-host and co-doping change the carrier transport and the exciton recombination zone, which further affects the microcavity resonance in the devices. Due to the enhancement on the red emission intensity and the shift of resonant wavelength, the chromaticity of the TWOLED is improved.

P-155: Synthesis and Characterization of Red Phosphorescent Iridium(III) Complexes based on Electron-Acceptor Modulation of Main Ligand for High Efficiency Organic Light-Emitting Diodes

A new series of red phosphorescent iridium(lll) complexes, (PT-MQ)2Ir(tmd), (PT-P)2Ir(tmd), and (PT-TFP)2Ir(tmd) were synthesized for highly efficient red emitter of the organic light-emitting diodes (OLEDs). The devices give very deep red EL spectra with much narrower full width at half maximum (FWHM). Maximum current efficiency of devices is 8.56 Cd/A, 5.52 Cd/A, and 2.62 Cd/A respectively.

Luminescence of Cr3+ ions in ZnAl2O4 and MgAl2O4 spinels: correlation between experimental spectroscopic studies and crystal field calculations

Details of preparation, spectroscopic and structural studies along with crystal field calculations for two Cr3+ doped spinels MgAl2O4 and ZnAl2O4 are given in the present paper. Both compounds show efficient red emission at about 685nm, which is due to the 2Eg → 4A2g spin-forbidden transition of Cr3+ ions located at the sites with D3d local symmetry. Analysis of structure of the CrO6 clusters was performed; comparison of the crystal field effects in both compounds revealed that the low-symmetry splitting of the orbital triplet states is more pronounced in ZnAl2O4. Both compounds show potential for applications as red-emitting phosphors.

Efficient green and red up-conversion emissions in Er/Yb co-doped TiO2 nanopowders prepared by hydrothermal-assisted sol–gel process

In this work, erbium and ytterbium co-doped titanium dioxide (Er–Yb:TiO2) nanopowders have been successfully prepared by hydrothermal-assisted sol–gel method using supercritical drying of ethyl alcohol and annealing at 500°C for 1h. Nanopowders were prepared with fixed 5mol% Erbium concentration and various Ytterbium concentrations of 5 and 10mol%. The powders were characterized by studying their structural, morphology and photo-luminescent properties. The annealing treatment at 500°C was found to enhance the crystallinity of the TiO2 anatase structure and the upconversion (UC) emission of the nanopowders. UC emissions were investigated under 980nm excitation, and the Er–Yb:TiO2 nanopowders exhibited the intense green (520–570nm) and red (640–690nm) upconverted emissions of Er ions originating from an efficient Yb–Er energy transfer process. The absolute upconversion quantum yield (UC-QY) of each nanopowders was measured for the UC emissions centered at 525, 550 and 655nm at varying excitation power densities. UC-QY analysis has revealed that 5mol% Er–5mol% Yb:TiO2 nanopowders possess the highest total quantum yield of 2.8±0.1% with a power density of 16.7W/cm2. These results make these nanopowders promising materials for efficient upconversion in photonic applications.

Growth and characterization of Sm3+ doped cerium oxalate single crystals

Single crystals of Sm3+ doped cerium oxalate decahydrate were synthesized using single diffusion gel technique and the conditions influencing the size, morphology, nucleation density and quality of the crystals were optimized. Highly transparent single crystals of average size 3mm×2mm×1mm with well-defined hexagonal morphology were grown during a time period of two weeks. X-ray powder diffraction analysis revealed that the grown crystals crystallize in the monoclinic system with space group P21/c as identical with the pure cerium oxalate. The various functional groups of the oxalate ligand and the water of crystallization were identified by Fourier transform infrared spectroscopy. The photoluminescence spectrum of the Sm3+ doped cerium oxalate indicated that the Sm3+ ions are optically active in the cerium oxalate matrix. The crystal has a strong and efficient orange red emission with a wavelength peak at 595nm and hence can be effectively used for optical amplification. Microhardness measurements of the crystal revealed that they belong to the soft material category.

Efficient red-emission InGaN/GaN multilayered structure on Si with surface-nitrified HfO2 film as buffer layer

A four-period InGaN/GaN (8 nm/48 nm) layered structure was deposited on a Si substrate with a surface-nitrified HfO2 film as a buffer layer (5 nm). A high In concentration of In0.36Ga0.64N was obtained in the InGaN layers. Red photoluminescence of 648 nm was observed from the layered structure. The internal quantum efficiency of the red emission from the InGaN layers on the surface-nitrified HfO2/Si was 52 %, which was more than 18 times larger than that on the Si substrate without HfO2. The surface-nitrified HfO2 provides another effective buffer layer to grow the InGaN/GaN layered structure on the Si substrate.