Internal Quantum Yield Research Articles

Rapid synthesis of YAG phosphor by facile mechanical method

AbstractIn this study, synthesis of yttrium aluminum garnet (YAG):Ce3+ phosphor powders for white light emitting diodes was investigated by mechanical method using the attrition‐type mill with no external heating and no flux in dry phase. High mechanical energy input to the starting powder mixture of Y2O3, Al2O3 and CeO2 achieved the synthesis of YAG:Ce3+ without any flux materials. X‐ray diffraction patterns of the processed powders after 5 min processing revealed the peaks of YAG were clearly identified. The maximum temperature of the mill chamber during the processing was 240℃. The YAG phosphor obtained by the mechanical method revealed the internal quantum yield of 65% in the case of the sample mechanically processed under a reducing atmosphere. The synthesized powder showed granule structure consisting of submicron size of YAG particles, which is better handling for the fabrication of light emitting diode devices.

Monolayer‐Thick GaN/AlN Multilayer Heterostructures for Deep‐Ultraviolet Optoelectronics

Recent progress in the development of monolayer (ML)‐thick GaN/AlN multilayer heterostructures for deep‐ultraviolet (UV) optoelectronics is reviewed. Analysis of both plasma‐assisted molecular beam epitaxy and metal–organic vapor phase epitaxy shows that extreme interface sharpness and sub‐ML accuracy in setting the layer thickness are attractive features of the former, whereas the lowest density of threading dislocations and wide possibilities for the implementation of various 2D growth mechanisms are the advantages of the latter. The structural properties of ML GaN/AlN heterostructures are evaluated not only by standard X‐ray diffraction and scanning transmission electron microscopy, but also by Raman spectroscopy. Theoretical and experimental studies of the optical properties of ML‐thick GaN/AlN quantum wells (QWs) reveal that quenching of the quantum‐confined Stark effect, suppression of transverse electric transverse magnetic polarization switching, as well as the excitonic nature of UV‐radiative recombination in ultrathin (1–2 ML) QWs ensure in such structures a high internal quantum yield of 75% for UV radiation at 235 nm at room temperature. The possibilities of using ML‐GaN/AlN heterostructures to fabricate UVC emitters of spontaneous and stimulated emissions in a wide range of output powers with various pumping techniques are considered, and the most important problems are formulated.

Competitive incorporation of Eu(III) and Na(I) ions into citric acid-passivated hydroxyapatite particles

Competitive incorporation of Eu3+ and Na+ ions into citric acid-passivated hydroxyapatite (Cit/HA) particles was investigated for improving the photoluminescence internal quantum yield (ηint). As a result, Eu3+/Na+-co-doped Cit/HA (Cit/HA:Eu/Na) particles were successfully synthesized to control the Eu3+ states inside/outside the HA structure. The optimum Na+ concentration would suppress the surface deactivation of the Cit/HA:Eu/Na particles, leading to the higher ηint value.

Enhanced red emission from Mn4+ activated phosphor induced by fluoride to oxyfluoride phase transformation

Mn4+-activated (oxy)fluoride-based red phosphors have received extensive attention. In this work, a phase transformation from K2NbF7 to K3HF2NbOF5 was successfully achieved by stoichiometric control during coprecipitation process, resulting in a novel red phosphor K3HF2NbOF5:Mn4+ with narrow-band emission. A systematic study was carried out on its crystal structure and luminescent properties. By introducing O ions into the local coordinated structure, the emission intensity of K3HF2NbOF5:Mn4+ phosphors is 2.15 times as high as that of K2NbF7:Mn4+ with a photoluminescence internal quantum yield (IQE) of 73.28% and absorption efficiency (AE) of 49.68%, which could be attributed to the layered structure of the oxyfluoride host. High quenching concentration of 8.39 mol% and short decay lifetime of 4.19 ms are realized. Based on K3HF2NbOF5:Mn4+ and commercial YAG:Ce3+, a white light-emitting diode (WLED) for solid state lighting was fabricated, which has low CCT of 3802 K, high Ra of 86.3 and high luminous efficiency (LE) of 126.23 lm/W. Moreover, by employing commercial β-sialon:Eu2+ and K3HF2NbOF5:8.39%Mn4+, another WLED with high LE of 95.78 lm/W and wide color gamut of 108.6% National Television Standard Committee (NTSC) standard for backlight lighting was packaged, indicating a great potential of K3HF2NbOF5:Mn4+ as a red component in the application of backlight lighting.

Highly thermally stable Dy3+/Sm3+ co-doped Na5Y9F32 single crystals for warm white LED

The optical behavior and thermal stability of Na5Y9F32 single crystals doped with fixed 1 mol% Dy3+ and various Sm3+ content grown by Bridgman technique were investigated by means of the fluorescence spectra, decay curves and temperature-dependent emission spectra. Upon 365 nm excitation, the emission color shifted from cool white emission to warm white emission as the Sm3+ concentration rise from 0 mol% to 1.6 mol%, in which the CIE coordinates experienced from (0.3445, 0.3888) to (0.3727, 0.3695) and the values of correlated color temperature (CCT) descended from 5114 K to 4174 K. The energy transfer (ET) from Dy3+ to Sm3+ identified by decay curves with emission spectra played significant effect on the change of emission color and the maximum energy transfer efficiency (ETE) was calculated to be 44.5% from the measured decay curves. Based on the Dexter's energy transfer formula, the mechanism of ET from Dy3+ to Sm3+ was determined as a dipole-dipole interaction. The down-conversion internal quantum yield (QY) of 1 mol% Dy3+/0.8 mol% Sm3+ co-doped Na5Y9F32 single crystal was estimated to be ~15.9%. When excited by 365 nm, Dy3+/Sm3+ co-doped Na5Y9F32 single crystals displayed excellent thermal stability deducing from the temperature-dependent emission spectra, in which the normalized emission intensity can still maintain 83% at 423 K. These results indicated that Dy3+/Sm3+ co-doped Na5Y9F32 single crystal held potential application prospect in warm white light-emitting diodes (w-LEDs.)

Enhancement of luminescence properties of Zn(W–Mo)O4:Eu3+ red phosphors by co-doped charge compensators A+(Li+, Na+, K+)

Using a convenient high-temperature solid-state method, a novel Zn(W–Mo)O4 red phosphor co-doped with alkaline metal ions and Eu3+ was successfully synthesized. The effects of doping with one charge-compensating agent and mixed multiple charge-compensating agents on the phase composition, crystal structure, crystallinity and luminescence properties of phosphors were studied. Using a theoretical study, the crystal environment around the Eu3+ ion was analysed and the internal quantum yield was calculated. The charge-compensating agent decreases the non-radiative transition by reducing the Zn defect in the phosphor and simultaneously changes the crystal environment around the Eu3+ ions. The enhancement of luminescence intensity of phosphors by co-doping with different charge-compensating agent ions is consistent with single doping. The luminescence intensity was increased 82.64% by doping with a charge-compensating agent, and the colour purity increased to 90.36%, indicating that the Zn(W–Mo)O4:Eu3+ red phosphor can be used as a potential candidate material in white-light-emitting diodes.

Near-infrared engineering for broad-band wavelength-tunable in biological window of NIR-Ⅱ and -Ⅲ: A solid solution phosphor of Sr1-xCaxTiO3:Ni2+

In this study, near-infrared (NIR)-emitting Sr1-xCaxTiO3:0.01Ni2+ (x = 0, 0.25, 0.5, 0.75, 1) solid-solution phosphors were prepared by a high-temperature solid-state reaction method. The effect of Ni2+-doping phase transformation on the crystallographic, luminescence properties, and electron density performance of Sr1-xCaxTiO3:1%Ni2+ phosphors are investigated. Upon 350 nm excitation, the phosphors could produce broadband NIR light within the biological windows of NIR-Ⅱ and -Ⅲ, originating from the 3T2g(F) → 3A2g(F) spin-allowed electron transition of Ni2+ in the octahedron, in full-width half-maximum spectra of over 200 nm. Through the Rietveld refinement analysis and density functional theory calculations, the results show that the NiO6 octahedron is twisted and distorted, reducing the crystal field strength and effective coordination around Ni2+ ions are the main reason for the emission peaks of SrTiO3:Ni2+ shift from 1311 to 1377 nm by substituting Ca2+ under excitation by UV light at 350 nm. In addition, SrTiO3:Ni2+ has an internal quantum yield of up to 6.5%. All these results indicate that Sr1-xCaxTiO3:Ni2+ phosphors have potential applications as a source for NIR lighting.

Investigating photoluminescence properties of Ca-doped ZnS nanoparticles prepared via hydrothermal method

The structural, morphological and optical properties of pure and Ca-doped zinc sulfide (ZnS:Ca) nanoparticles (NPs) prepared by hydrothermal method at different percentages of Ca are reported herein. The resulting NPs were characterized by X-ray diffraction analysis (XRD), Raman scattering, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. The XRD patterns confirmed the crystalline nature of all pure and doped NPs showing a single phase cubic blende structure, whereas the SEM micrographs showed sphere-shaped NPs. The sizes of the as-prepared NPs estimated by TEM were found to be in the range 44–147 nm. The PL emission spectra of ZnS:Ca nanocrystals excited at 400 nm consist of a broad band (420 nm–640 nm) characterized by two main contributions peaking at 484 and 533 nm. The overall maximum emission intensity is found for ZnS:3% Ca with internal quantum yield of 8.4%.

A New Copper(I) Iodide Based Organic-Inorganic Hybrid Structure with Red Emission

A new organic-inorganic hybrid structure based on copper (I) iodide staircase chain 1D-Cu2I2(5-chloropyrimidine)2 (1) has been synthesized by a slow-diffusion method. It emits red emission peaking at 620 nm. The internal quantum yield (IQY) measured for this compound is 6.5% under 360 nm excitation. This compound exhibits potential as a non-rare-earth light-emitting phosphor alternative.

NaBr-Assisted Photoelectrochemical and Photochemical Integrated Process for Isomerization of Maleate Esters to Fumarate Esters

We developed a NaBr-assisted photoelectrochemical and photochemical integrated process for the complete isomerization of dimethyl maleate to dimethyl fumarate under solar-simulated light irradiation. This process also produced bulky fumarate ester derivatives at high yields (85–100%). A plausible mechanism of the bromine-initiated radical chain reaction was suggested by the results of the experimental data and density function theory calculations. The photoelectrochemical oxidation of NaBr generated the oxidized bromine species, which were then photolyzed to produce the bromine radical initiators induced by the photochemical process. This resulted in the high faradaic efficiency (98%) in the photoelectrochemical step and internal quantum yield (>100%) in the photoisomerization step. This newly developed process is an innovative and environmentally friendly strategy that goes beyond the conventional thermochemical process to achieve the precise synthesis of fumarate derivatives under mild conditions.

Giant topological luminophor with high-intensity luminescent performance

Improving the luminescent intensity and reducing the critical voltage of rare-earth-doped phosphor materials has long been suffered from a significant technical requirement and is a scientific problem. Although luminescent intensity can be improved to some extent by nanocomposite method like coating precious metals on the surface of the material or providing a porous structure, the improvement in luminescent performance is still unsatisfactory. Inspired by the structure and performance of metamaterials and laser resonator, a fundamental model of giant topological luminophor Y2O3:Eu3++Octadecylamine(ODA)+Dodecylamine(DDA)+Ag with a gradient porous structure was designed and synthesized following the Y2O3:Eu3++Ag topological luminophor to realize a sharply enhance luminescent performance of this nanocomposite. Experiment results and theoretical calculation indicated that the luminescence performance of giant topological luminophor Y2O3:Eu3++ODA+DDA+Ag increased by approximately 1400% compared with that of Y2O3:Eu3+ phosphors under the same conditions and internal quantum yield is 116% while the critical voltage was reduced by 100% as well as the stability is also improved greatly, under the synergy mechanism of Ag topological structures and the gradient porous which composed of differently sized pores. This giant topological luminophor which improves the luminescent performance via the synergistic mechanism of surface micro/nano structures provides a new thought and approach for designing and synthesizing of high-performance rare-earth-doped nanocomposites phosphors.

Doped lanthanum cerate pyrochlore for multicolor luminescent phosphor: Decisive role of energy transfer and defects

Designing of efficient luminescent materials requires proper understanding of energy transfer, defect evolution and dopant local structure. The present work is a perfect amalgam wherein we have thoroughly investigated the concentration dependent host sensitized energy transfer, defect evolution and local structure of europium ion in La2Ce2O7 (LCO) pyrochlore material. Raman spectroscopy suggested stabilization of defect fluorite structure for LCO as well as LCO:Eu3+ (LCOE) but degree of structural distortion as well as oxygen vacancies (OVs) increases with increase in europium ion concentration. Diffuse reflectance spectroscopy suggested band gap narrowing at higher doping level owing to enhanced density of OVs. Positron annihilation lifetime spectroscopy suggested an increase in the formation of oxygen vacancies and vacancy clusters near the surfaces at higher doping. As a result, LCO, on irradiation with 250 nm ultraviolet photon showed visible emission due to OVs as well charge transfer transition. On Eu3+ doping emission spectra was rich in host as well as europium emission and host to europium energy transfer increases with increase in europium ion concentration. This get's reflected as violet blue emission at lower doping and yellowish white at higher doping level ≥7.5% under host excitation and orange-red emission under 471 nm dopant excitation. Europium ion emission spectral profiles in LCOE suggested stabilization of Eu3+ in CeO6 octahedra as is also confirmed using lifetime spectroscopy. Judd-Ofelt measurement further reflected higher branching ratio for 5D0→7F2, internal quantum yield ~48% and high Ω2 compared to Ω4. This work highlights the importance of local dopant site, HSET, excitation photon and defects in designing color tunable luminescent materials and is expected to play an important role in designing the phosphor converted light emitting diodes (pc-LEDs).

Isostructural coordination chains assembled with aminopyrazine: Decreasing photoluminescence yield by Cd(II) replacement for Mn(II)

A cadmium coordination polymer assembled with aminopyrazine (ampyz) and acetate (AcO) was recently found to be a high-efficiency material able to convert ultraviolet radiation into blue light, with internal and external photoluminescence quantum yields (iPLQY and ePLQY) approaching 80%. Here we were inspired to prepare a blue-light-emitter based on its structure by replacing the toxic cadmium element for Mn2+. The isostructural Mn2+ coordination polymer was obtained and its structure was elucidated by single-crystal X-ray diffraction. Its optical properties were assessed and understood based on TD-DFT calculations. Low-gap d-shaped HOMO-LUMO orbitals were found intercalating those responsible for UV absorption. Therefore, low-frequency radiative decay occurs before the target luminescence, depleting much its blue-light-emission efficiency which doesn't achieve 3%. This novel material allowed us to conclude that the cadmium d10 closed-shell is decisive to the high-efficiency brightness of the literature compound.

Effect of Li+ doping on the luminescence performance of a novel KAlSiO4:Tb3+ green-emitting phosphor

A new green-emitting phosphor, KAlSiO4:1.5 mol% Tb3+, x mol% Li+, was prepared via a high-temperature solid-phase method, and its crystal structure, diffuse reflectance spectrum, and luminescence were studied. The results show that the Li+ doping shifts the strongest diffraction peak to a high angle direction, reducing grain size by 11.4%. The entry of Li2CO3 improves the luminescence performance of KAlSiO4:1.5 mol% Tb3+. At a Li+ concentration of 1.5 mol%, the sample has strong absorption in the ultraviolet light range from 250 to 400 nm. The luminous intensity of the sample at 550 nm approximately quadruples after Li+ doping. Additionally, the colour purity of the sample and the internal quantum yield increase to 83.3% and 42%, respectively. The sample changes colour with time when exposed to air without an obvious fading phenomenon. The emission intensity at 200 °C is 95.1% of its value at room temperature, indicating that the phosphor has excellent thermal stability when x = 1.5. These results show the feasibility of using the silicate phosphor for generating the green light component of white light-emitting diodes for solid-state lighting.

Photoelectrochemical investigation of charge injection efficiency for quantum dot light-emitting diode

When we applied colloidal quantum dots (QDs) for quantum dot light emitting diodes, it was well known that shell thickness played an important role in core protection, confinement of electrons and holes, and charge injection efficiency. However, although the shell thickness dependence of electroluminescence properties was reported, carrier injection efficiency has not been discussed in detail. In this paper, we investigated the effect of shell thickness on the carrier injection efficiency that was evaluated by photoelectrochemical measurements. By comparing the product of internal quantum yield of photoluminescence and the evaluated carrier injection efficiency with external quantum efficiency (EQE) for QDs with various shell thicknesses, we found that the optimal shell thickness for increasing EQE is determined by the balance between protection of QD's surface and carrier injection efficiency.

A novel Mn4+-activated layered oxide-fluoride perovskite-type KNaMoO2F4 red phosphor for wide gamut warm white light-emitting diode backlights.

Mn4+-activated oxide-fluoride phosphors are attractive for application in a wide range of solid-state lighting devices because of their distinct red emission at about 630 nm and the abundant storage of Mn ions. However, the zero-phonon line (ZPL) of Mn4+ ions is too weak to be detected in most host materials due to the magnetic dipole nature. In this article, we introduce a co-precipitation method for synthesizing a Mn4+-doped oxyfluoride perovskite KNaMoO2F4 phosphor containing [MoO2F4]2- building units. The electron paramagnetic resonance (EPR) spectra are consistent with the presence of a MnF62- species at g = 1.991. The KNaMoO2F4:0.01Mn4+ phosphor exhibits strong absorption under blue light and an internal quantum yield (IQE) of 65.8%. Attributed to the distorted octahedral environment of the Mn4+ ions, visible ZPL emission was detected at 625 nm. Based on theoretical calculations, the Mn4+ ions in the KNaMoO2F4 host exist in a strong crystal field with a high Dq/B value of ∼3.86. A series of photoluminescence-dependent low-temperature spectra indicates that the Mn4+ emissive state experiences weak electron-phonon interactions upon calculating the Huang-Rhys factor. Benefiting from the ZPL, a warm white-light-emitting diode is achieved using YAG:Ce3+ and KNaMoO2F4:Mn4+ as color converters, in which the color rendering index was Ra = 83.5 and CCT = 4490 K.

Multiphoton light emission in barium stannate perovskites driven by oxygen vacancies, Eu3+ and La3+: accessing the role of defects and local structures.

Defect engineering in perovskites has been found to be the most efficient approach to manipulate their performance in ultraviolet-to-visible photon conversion. Under UV irradiation, BaSnO3 exhibited multicolor photoluminescence (MCPL) in the bluish white region. Its origin has not been well studied in the literature and has been probed in this work using synchrotron radiation, positron annihilation and density functional theory. To achieve desirable performance of doped BaSnO3 in optoelectronics, it is imperative to have correct information on the dopant local site, doping induced defect evolution and efficacy of host to dopant energy transfer (HDET). Extended X-ray absorption fine structure (EXAFS) showed that Eu3+ ions stabilize at both Ba2+ and Sn4+ sites consistent with the highly negative formation energy of around -6.26 eV. Eu3+ doping leads to an intense 5D0→7F1 orange emission and a feeble 5D0→7F2 red emission and an internal quantum yield (IQY) of ∼21% mediated by ET from the defect level of EuBa and EuSn sites to the valence band maximum (VBM). X-ray absorption near edge structure (XANES) ruled out any role of Sn2+ in the PL of BaSnO3 or Eu2+ in the PL of BaSnO3:Eu3+. Interestingly, when co-doped, Eu3+ stabilizes at Sn4+ sites whereas La3+ stabilizes at Ba2+ sites with a formation energy value of -6.44 eV. Based on the asymmetry ratio in emission spectra, it was found that La3+ ions lead to lowering of symmetry around Eu3+ due to increased vacancies and structural distortions, and also suppress the luminescence IQY. We have performed experimental positron annihilation lifetime spectroscopy (PALS) to probe the defects in BaSnO3 in pristine samples and on doping/co-doping. The positron lifetimes for saturation trapping of positrons in various kinds of defects envisaged in BaSnO3 and in the defect free system were calculated using the MIKA Doppler program. Such deep insight into the effect of local structures, dopant sites, defect evolution, ET, etc. on the optical properties of BaSnO3 is expected to provide very deep insight for material scientists into the fabrication of perovskite-based optoelectronic and light-emitting devices.

An antimony based organic-inorganic hybrid coating material with high quantum efficiency and thermal quenching effect.

An antimony based luminescent organic-inorganic hybrid compound H3SbCl6(L)6 (1, L = 2-(3-methyl-1H-imidazol-3-ium-1-yl)acetate) has been prepared by the solvothermal method. It emits bright green light peaking at 525 nm, with an internal quantum yield (IQY) of 73% under 360 nm excitation. The negative thermal quenching (NTQ) effect has been observed in the temperature range of 77 K to 297 K. Due to its ionic structure, compound 1 is soluble in numerous organic solvents, including methanol, dimethyl sulfoxide (DMSO), etc. The solution processability combined with high quantum efficiency makes 1 a promising candidate as a luminescent coating material for optoelectronic devices.

Luminescence properties and energy transfer of La3Ga5SiO14:Eu3+, Tb3+ phosphors

The La3Ga5SiO14 based phosphors were generated by substitutions of Tb3+/Eu3+ to La3+. The Tb3+ → Eu3+ energy transfer occurs in Tb3+/Eu3+ codoped phosphors. The La2.70Eu0.12Tb0.18Ga5Si14 phosphor shows an internal quantum yield about 88.82%.

Enhanced luminescence properties of Eu3+ in La2MoO6 by nonsensitization of Bi3+

AbstractIt was unusual for Bi3+ ions to enhance the emission intensity of phosphors via nonsensitization. Here, La2MoO6:Eu3+, Bi3+ phosphors were successfully synthesized by a high temperature solid‐state reaction method in air atmosphere. As the increase of doping concentration of Bi3+, the emission spectra of La2MoO6:Eu3+, Bi3+ phosphors had obvious shifts, splits and the enhancement of intensities, which indicated that the characteristics of the phosphors were modified. To analyze these phenomena, the crystal structure refinements, spectral characteristic analyze and Judd‐Ofelt theoretical calculation were mainly performed. Bi3+ ions played the role of the nonsensitizer and affected the distortion of the crystal, the sites of Eu3+ ions, the field splitting energy and the internal quantum yield. Moreover the nephelauxetic effects of Bi3+ ions and the ET process caused synergistically the life times of La2MoO6:Eu3+, Bi3+ phosphors to increase and then gradually decrease. The CIE coordinates of phosphors changed within a small range. This study might be instrumental in promoting the further application of Bi3+ ions in rare earth luminescent materials.