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Chameleon-like photoluminescence of (Sr,Mg)2Si(O,N)4:Eu2+ nitridosilicate phosphors
Abstract
Sr2-xMgxSi(O,N)4:Eu²⁺ nitridosilicate powders were synthesized using a high-temperature nitridation process. X-ray diffraction data and photoluminescence (PL) spectra demonstrated that the Mg²⁺ ions were incorporated into the Sr²⁺ sites rather than the interstitial sites. The Mg²⁺ substitution led to a phase transformation and affected the PL properties. There were two Eu²⁺ sites: Eu(I) and Eu(II). The PL spectra of the Sr2SiO4:Eu²⁺ powders consisted of two emission bands (A- and B′-bands) in the blue and green regions assigned to Eu(I) and Eu(II), respectively. Remarkably, the Sr2-xMgxSi(O,N)4:Eu²⁺ powders exhibited an additional emission band (B-band) in the red region, which originated from the large red-shift of the B′-band due to the coexistence of the Eu‒O and Eu‒N bonds. Furthermore, the PL spectra were hypersensitive to the excitation wavelength, resulting in chameleon-like luminescence properties. The PL properties were explained using a structural analysis, multiple Gaussian fit of the spectra, and thermal quenching.
Advances in solid-state white-light-emitting diodes (WLEDs) necessitate the urgent development of highly efficient single-phase phosphors with tunable photoluminescence properties. Herein, the Tm3+, Dy3+, and Sm3+ ions are incorporated into the orthorhombic NaGdTiO4 (NGT) phosphors, resulting in phosphors that fulfill the aforementioned requirement. The emission spectrum of Tm3+ ions overlaps well with the adsorption spectra of both Dy3+ and Sm3+ ions. Under the excitation at 358 nm, the single-phase NaGdTiO4: Tm3+, Dy3+, Sm3+ phosphor exhibits tunable emission peaks in the blue, yellow, and red regions simultaneously, resulting in an intense white-light emission. The coexisting energy transfer behaviors from Tm3+ to Dy3+ and Sm3+ ions and the energy transfer from Dy3+ to Sm3+ ions are demonstrated to be responsible for this phenomenon. The phosphors with multiple energy transfers enable the development of single-phase white-light-emitting phosphors for phosphor-converted WLEDs.
Phosphors are suffering from decreasing light output due to heat, oxygen and water degradation during the device fabrication process and in the working environment. Here, we report a series of (Ba,Sr)LuAl2Si2O2N5:Ce³⁺,Eu²⁺ phosphors which maintain initial luminescence intensity after annealed in air up to 800 °C or immersed in water for 5 days. Powder X-ray diffraction for fresh and annealed samples and thermogravimetric/differential thermal analysis supported the high stability of the host lattice of (Ba,Sr)LuAl2Si2O2N5. Electron spin resonance demonstrated that the majority of Eu maintained divalence even after heating at 800 °C in air. The robust host lattice and stable valence of activators are responsible for the high stability of (Ba,Sr)LuAl2Si2O2N5:Ce³⁺,Eu²⁺ phosphors. The high chemical stability originates from the featured star-like N[(Al/Si)(O/N)3]4 building block and condensed three dimensional rigid framework. The (Ba,Sr)LuAl2Si2O2N5:Ce³⁺,Eu²⁺ phosphors are potential candidates for lighting and display application, especially for which needs high temperature treatment in the package process.
Experimental data from literature on the thermal quenching of the Eu²⁺ 5d-4f emission in the MxSiyNz (M = alkali, alkaline earth or rare earth) nitridosilicates have been collected and evaluated. No clear correlation was observed between the activation energy for thermal quenching and the Stokes shift, suggesting that non-radiative relaxation via a thermally excited cross-over from the 5d excited state to the 4f ground state is not the main reason for thermal quenching in the nitridosilicates. Based on literature data on rare-earth charge transfer transitions, host-lattice bandgap and Eu²⁺ 5d-4f emission energy, the energy difference between the Eu²⁺ 5d level and the bottom of the host-lattice conduction band has been determined. This energy difference correlates fairly well with the quenching temperature, suggesting that thermal ionization of the 5d electron to the conduction band is responsible for the thermal quenching of the Eu²⁺ 5d-4f emission in the nitridosilicates. The energy difference between the lowest 5d level and the bottom of the conduction band, and consequently the quenching temperature, increases with increasing Si/N ratio of the nitridosilicates. From this, it is concluded that the combined effect of the larger Stokes shift and the raise in energy of the bottom of the host lattice conduction band with increasing Si/N ratio is stronger than the decrease of the centroid shift and crystal field splitting of the Eu²⁺ 5d level.
The effect of Al³⁺ substitution on the enhancement of the luminescence of Lu1–xAlxNbO4:Eu³⁺ and Lu1–xAlxNbO4:Tb³⁺ was investigated. X-ray diffraction patterns confirmed that the Eu³⁺, Tb³⁺, and Al³⁺ ions were fully incorporated into the Lu³⁺ sites. In the case of Lu1–xAlxNbO4:Eu³⁺, the predominant red emission (614 nm) was assigned to the ⁵D0 → ⁷F2 transition of Eu³⁺ and for x = 0–0.05, its intensity increased up to ~125 and 108% under 395 nm (⁷F0 → ⁵L6) and a charge transfer band excitation, respectively. For Lu1–xAlxNbO4:Tb³⁺, the strongest emission band peaking at 551 nm was attained in the green region among multiple emission bands corresponding to the ⁵D4 → ⁷FJ transitions of Tb³⁺. Increasing the x values from 0 to 0.05 increased the green emission significantly by ~137%. These phenomena were explained by the local structural distortions and crystal field asymmetry surrounding Eu³⁺ and Tb³⁺, which were attributed to a large difference in the ionic radii of Al³⁺ and Lu³⁺.
A series of single phase white light emitting CaSrAl2SiO7:Dy³⁺ phosphors were prepared by traditional high temperature solid state reaction method. Structural characterization was done by X-ray diffraction, field emission scanning electron microscopy and energy dispersive spectroscopy analyses. Optical characterization was performed from photoluminescence and thermoluminescence studies. When excited at 350 nm, CaSrAl2SiO7:Dy³⁺ phosphors showed two intense emission bands in the blue region (480 nm and 493 nm) corresponding to ⁴F9/2 → ⁶H15/2 transition and one in yellow region (576 nm) corresponding to ⁴F9/2 → ⁶H13/2 transition. The Commission Internationale de I’Eclairage diagram was drawn for entire series. It confirmed that by manipulating Dy³⁺ content the luminescence color of CaSrAl2SiO7:Dy³⁺ phosphors were tuned from blue to white region. Computation of correlated color temperature suggested that present phosphor was cool in appearance hence CaSrAl2SiO7:Dy³⁺ phosphor can serve as a white light emitting phosphor and may be useful in outdoor illumination. A detailed study on thermoluminescence of ultraviolet exposed samples was done and possible mechanism of thermoluminescence was discussed. TL emission spectrum was also measured.
A single‐phase full‐color emitting phosphor Na3Sc2(PO4)3:Eu²⁺/Tb³⁺/Mn²⁺ has been synthesized by high temperature solid‐state method. The crystal structure is measured by X‐ray diffraction. The emission can be tuned from blue to green/red/white through reasonable adjustment of doping ratio among Eu²⁺/Tb³⁺/Mn²⁺ ions. The photoluminescence, energy transfer efficiency and concentration quenching mechanisms in Eu²⁺‐Tb³⁺/Eu²⁺‐Mn²⁺ co‐doped samples were studied in detail. All as‐obtained samples show high quantum yield and robust resistance to thermal quenching at evaluated temperature from 30 to 200 °C. Notably, the wide‐gamut emission covering the full visible range of Na3Sc2(PO4)3:Eu²⁺/Tb³⁺/Mn²⁺ gives an outstanding thermal quenching behavior‐near zero thermal quenching at 150 °C/less than 20% emission intensity loss at 200 °C, and high quantum yield‐66.0% at 150 °C/56.9% at 200 °C. Moreover, the chromaticity coordinates of Na3Sc2(PO4)3:Eu²⁺/Tb³⁺/Mn²⁺ keep stable through the whole evaluated temperature range. Finally, near‐UV w‐LED devices were fabricated, the white LED device (CCT=4740.4K, Ra=80.9) indicates that Na3Sc2(PO4)3:Eu²⁺/Tb³⁺/Mn²⁺ may be a promising candidate for phosphor‐converted near‐UV w‐LEDs.
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Oxynitride Li2SrSiON2 was synthesized by introducing N³⁻ into the Li2SrSiO4 lattice, which is suitable for Eu²⁺ doping. It crystallizes in an trigonal structure with the space group of P3121 (152) and cell parameter a = b = 5.0131(0) Å, c = 12.4270(1) Å and V = 270.46 ų. Under blue light (450 nm) excitation, the representative sample Li2Sr0.98SiON2:0.02Eu²⁺ exhibits a bright broad band orange emission peaking at 586 nm with an external quantum efficiency of 52.7%. Its emission intensity at 150 °C remained 78% of that measured at 25 °C. Warm white light-emitting diode 1 (W-LED1) was prepared by combining a blue LED chip and LSSON:0.02Eu²⁺. High color rendering index (Ra ∼ 90) warm W-LED2 was fabricated by combing a blue light chip, BaSi2O2N2:Eu²⁺, and Li2Sr0.98SiON2:0.02Eu²⁺. The chromaticity coordinate and correlated color temperature (CCT) of W-LED2 are (0.3625, 0.3539) and ∼4386 K, respectively. Excellent optical properties allow it to be an attractive orange phosphor for warm W-LEDs.
Cyan-emitting (Ba,Ca)Si7N10:Eu²⁺ was synthesized using a high-temperature graphite furnace. Crystallographic data for BaSi7N10:0.1Eu²⁺ were obtained by Rietveld refinement and discussed compared with un-doped BaSi7N10. The excitation spectra peaked at 368 nm and decay times were apparently different from those of earlier papers, and these differences were explained by the structural analyses. The substitution of Ca²⁺ ions for Ba²⁺ ions increased the emission intensity by approximately 130%, as this resulted in local lattice modification. The findings indicate that (Ba,Ca)Si7N10:Eu²⁺ phosphors are highly stable and well suited for use in white LEDs pumped by near ultraviolet sources.
Divalent europium-doped nitride phosphors, Ca1-xEuxAlSiN3 (x = 0−0.2), were successfully prepared by the self-propagating high-temperature synthesis (SHS) by using Ca1-xEuxAlSi alloy powder as a precursor. The Rietveld refinement analysis was carried out on the CaAlSiN3 host lattice to elucidate the luminescence properties of dopant Eu2+ on the tetrahedrally coordinated site. For the Eu2+ doped samples, strong absorption peaking at about 460 nm was observed on the excitation spectra, which matched perfectly with the current blue light of InGaN/GaN light-emitting diodes (LEDs). The optimized sample, Ca0.98Eu0.02AlSiN3, gave the red emission peaking at 649 nm of which the intensity was competitive with the sample prepared from the metal nitride raw materials (Ca3N2, AlN, Si3N4, and EuN). The CIE chromaticity index (0.647, 0.347) with high color saturation indicated that it was a promising candidate as a red-emitting phosphor for the InGaN/GaN-based down-conversion white LEDs for general illumination or displays.
This letter reports a β-SiAlON:Eu2+ green phosphor with the composition of Eu0.00296Si0.41395Al0.01334O0.0044N0.56528. The phosphor powder exhibits a rod-like morphology with the length of ∼ 4 μm and the diameter of ∼ 0.5 μm. It can be excited efficiently over a broad spectral range between 280 and 480 nm, and has an emission peak at 535 nm with a full width at half maximum of 55 nm. It has a superior color chromaticity of x = 0.32 and y = 0.64. The internal and external quantum efficiencies of this phosphor is 70% and 61% at λex = 303 nm, respectively. This newly developed green phosphor has potential applications in phosphor-converted white LEDs.
In this letter, a yellow oxynitride phosphor α- SiAlON with compositions of Ca 0.625 Eu x Si 0.75-3x Al 1.25+3x O x N 16-x ( Ca -α- SiAlON : Eu , x=0–25 ) was prepared by gas pressure sintering. The diffuse reflection spectrum, photoluminescence spectrum, and chromaticity of the powder phosphors were presented. It absorbs light efficiently in the UV–visible spectral region, and shows a single intense broadband emission at 583–603 nm . This phosphor may become a good candidate for creating white light, typically warm white light, when coupled to a blue light-emitting diode (λ em =450 nm ) .
Fluorescence emission was investigated in a series of Ba2−xEuxSi5N8 compounds. Two maxima corresponding to the two crystallographic Eu2+ sites are observed. Excitation with intense laser light at 1.047 μm reveals strong fluorescence emission at ∼600 nm due to two-photon-excitation. In Ba1.89Eu0.11Si5N8 the long lasting luminescence was studied. The emission peaking at about 590 nm has been observed in the dark with the naked eye even 15 min after removal of the activating lamp. This effect is due to the recombination of holes and traps consisting of nitrogen vacancies formed by reducing synthesis conditions. The maximum emission was observed at −7°C.
We examined non-stoichiometric CaAlxSi(7-3x)/4N3:Eu²⁺ phosphors that were intentionally prepared with x = 0.7 ~1.3 to identify the origin of the deconvoluted Gaussian components that constitute the emission spectra of stoichiometric CaAlSiN3:Eu²⁺ phosphors. The Al/Si molar ratio around the Eu²⁺ activator caused the deconvoluted Gaussian peaks. The Eu²⁺ activator sites in Al-rich environments gave rise to the lower-energy emission peak, while those in Si-rich environments were related to the higher-energy emission peaks. Active energy transfer from the Eu²⁺ activator site in the Si-rich environment to the Eu²⁺ activator site in the Al-rich environment was confirmed. Particle swarm optimization was employed to estimate the nine unknown decision parameters that control the energy transfer process. All of the decision parameters were estimated within the range of reasonable values.
Red-emitting phosphors (usually Eu 2+-activated) play indispensable roles in manipulating the color rendition and color temperature of InGaN-based solid state lighting, but they usually suffer from luminance saturation under high-power density irradiation and thus are hard to use to fabricate high-brightness lighting sources. Ce 3+-doped red phosphors with short decay times therefore need to be developed to solve the bottleneck problem. In this work, we proposed an empirical structure criterion for screening Ce 3+-doped red phosphors and discovered HP-CaSiN 2 :Ce 3+ , which has an emission peak at 610 nm, a full-width at half maximum (FWHM) of 160 nm and a quantum efficiency of 40.3% under 450 nm excitation. Under blue laser irradiation (5.47 W), HP-CaSiN 2 :Ce 3+ shows a luminance saturation threshold of 10.89 W mm À2 , luminous flux of 1087 lm and luminous efficacy of 198.7 lm W À1 , which are 5.4 and 7.3 times higher than the reported CaAlSiN 3 :Eu 2+ counterparts. In addition, it can also serve as a red component for creating warm white LEDs with an excellent color-rendering index (Ra) of 91.5 and a small chromaticity shift (DE: 1.1 Â 10 À2). The discovery of the red-emitting HP-CaSiN 2 :Ce 3+ makes it possible to search for and design long-wavelength Ce 3+-doped phosphors by the proposed criterion, and paves an avenue for it to be used in high-power solid state lighting.
Light is as a fuel for photosynthesis and plays an irreplaceable role in seed germination, stem elongation, leaf expansion, flowering induction, and accumulation of nutrients. However, the current phosphors combined with LEDs for photosynthesis have some unsolved problems, such as low efficiency, poor thermal stability, changeless emission wavelength or nonmatched spectrum, which restrict their application. In this work, a series of novel efficient Lu3Al5O12:10%Ca2+,xBi3+,yMn4+ phosphors with blue-violet plus deep red dual-emission peaking at 419 and 669 nm are first reported, which can meet the different light requirements of plants through controlling the concentration ratio of Bi3+ and Mn4+, and possess at most 90.64% spectrum resemblance to the fitting spectrum of chlorophyll A and B. Furthermore, these phosphors exhibit a high internal efficiency and at most reserve 95% of initial intensity at 425 K. Finally, as a light source for monitoring fluorescence spectra of evergreen, the pc-LEDs are fabricated by UV chip and our synthesized phosphors. The results reveal that these phosphors can achieve as high as 83% photosynthetic efficiency for evergreen and obviously improve it by 10% than single emission phosphor, which show a good application prospect for improving plant photosynthesis.
Eu ²⁺ activated silicate phosphor, Sr 2 SiO 4 :Eu ²⁺ were synthesized by polymerized complex method to investigate the effect of Eu doping on room temperature phases. Firstly, synthesis condition for β phase was optimized and found that β-Sr 2 SiO 4 :Eu ²⁺ phosphors can be obtained at reduction temperature of 1150 °C using HCl, which provide SrCl 2 flux during synthesis process. The characterization for various Eu concentration of Sr 2 SiO 4 :Eu ²⁺ phosphors show that pure β-Sr 2 SiO 4 :Eu ²⁺ phosphors were obtained at 1%, 3% and the samples which are almost in β phase can be achieved until 5%. Temperature dependence of crystal structure and PL properties of Sr 2 SiO 4 :Eu ²⁺ (3%)phosphors were also characterized to investigate the phase transition and thermal hysteresis between β and α′ phase. Phase transition between β ↔ α′ phase was observed from XRD around 60 °C–100 °C and change in PL spectra owing to phase transition was observed around same temperature range. The further analysis show that the thermal hysteresis between β → α′ and α’→ β directions of Sr 2 SiO 4 :Eu ²⁺ (3%)phosphors were about 40 K, which was much larger than the reported thermal hysteresis of undoped Sr 2 SiO 4 .
Designing the two-step tuning based on multiple cation lattices substitution and energy transfer successfully accomplish versatile colour output for single-phase phosphor-converted white LEDs in this work. Herein, we choose whitlockite-type Ca9LiY0.667(PO4)7 compound as a host, which provides multiple cation sites for Eu2+ occupation. Spectral result indicates that Ca9LiY0.667(PO4)7:Eu2+ phosphor emits a cold white emission due to the multiple bands emissions at about 420, 480, and 530 nm, which are corresponding to the emission from M(3), M(1)/M(2) and M(5) cation sites ascertained by Retiveld refinement method. In order to enhance the red emission of CLYPO:Eu2+ phosphors, Mn2+ was codoped into CLYPO:Eu2+ phosphors. A warm white emission in single-phase host based on energy transfer is successfully obtained in CLYPO:Eu2+, Mn2+ phosphors. The excitation band of CLYPO:Eu2+, Mn2+ phosphors in 360−450 nm range perfectly coincides with commercial LED chip, implying they are desirable for white LED applications. White LEDs with higher color rendering index (Ra = 90.2) and lower correlated color temperature (CCT = 3614 K) are achieved by using a single-phase CLYPO: 0.03Eu2+, 0.10Mn2+ phosphor and a 395 nm InGaN-LED chip.
A series of novel Eu2+/Ce3+/Tb3+ ion doped La8Ca2(Si4P2O22N2)O2-based apatite structure oxynitride phosphors have been synthesized by the solid-state method. The phase purity has been checked by X-ray diffraction (XRD) and the Rietveld method. The photoluminescence excitation (PLE) spectra indicate that the obtained phosphors have broad excitation bands in the n-UV range, which is a favourable property for application as n-UV-WLED phosphors. The concentration quenching has been investigated in detail. The temperature-dependent luminescence and efficient study shows that as-prepared phosphors have a good thermal stability and high quantum efficiency. Moreover, a warm white LED device with low color temperature of 4288 K and high color rendering index (Ra) of 80.8 has been obtained by coating La8Ca1.99(Si4P2O22N2)O2:0.01Eu2+ phosphor with (Sr,Ca)AlSiN3:Eu2+ red phosphor on a n-UV chip. Our results reveal that La8Ca2(Si4P2O22N2)O2:Eu2+ phosphor is a promising bluish-green phosphor for WLEDs.
Optical data of the Eu²⁺ doped nitridosilicates (MxSiyNz) have been collected from literature and have been analysed with regard to their dependence on structure and composition. Nitridosilicates with a higher degree of condensation, i.e. a higher Si/N ratio, generally have a higher Eu²⁺ 4f-5d absorption energy, a higher 5d-4f emission energy and a larger Stokes shift. The higher absorption and emission energy are due to the increase of the N by Si coordination number with increasing Si/N ratio. This results in more electrons on N that participate in the bonding with Si, and thus less electrons available for Eu-N bonding, reducing the covalency of the Eu-N bonds. The lower covalency gives a weaker nephelauxetic effect, reducing the centroid shift of the 5d level. The lowest 4f-5d absorption energy further increases by the reduction of the crystal field splitting of the 5d levels, as the Eu-N bonds become longer with increasing Si/N ratio. The Stokes shift increases with increasing degree of condensation despite an increase of lattice rigidity, ascribed to a decrease of local rigidity around the Eu²⁺ ion caused by the larger Eu-N bond lengths. Some nitridosilicates show deviations to the general trends attributed to peculiarities in their crystal structure and the way Eu²⁺ is substituted in the lattice. The relationships established in the present work will be helpful for the design and exploration of new Eu²⁺ doped nitride-based luminescent materials for practical applications.
Sr2-xBaxSi(O,N)4:Eu2+ (SBxSON:Eu2+) oxynitridosilicate phosphors were prepared via incorporation of N3−, Eu2+, and Ba2+ ions into Sr2SiO4 (SSO) lattices. X-ray diffraction patterns of the prepared powders revealed that SBxSON:Eu2+ was a solid-solution form of SSO. An increase in x values caused a phase transition and an expansion of the unit cell. The photoluminescence excitation (PLE) spectra of SBxSON:Eu2+ were broad, covering the ultraviolet range to the visible range. Corresponding PL emission spectra strongly depended on the excitation wavelengths and consisted of two emission bands, one in the green-blue region (A-band) and the other in the red region (B-band), which were assigned to Eu(I) and Eu(II), respectively. The B-band resulted from a dramatic red-shift of the green emission band assigned to Eu(II) of SSO:Eu2+, revealing that the nitridation process preferentially affected the Eu(II) sites. This behavior was explained by crystal field splitting, the fluorescence decay time, and thermal quenching. The Ba2+ substitution caused evolution of the PL spectra, and its effects on the spectra were discussed under consideration of ionic size and covalence.
Phosphor-converted white light-emitting diodes (pc-WLEDs) are efficient light sources used in lighting, high-tech displays, and electronic devices. One of the most significant challenges of pc-WLEDs is the thermal quenching, in which the phosphor suffers from emission loss with increasing temperature during high-power LED operation. Here, we report a blue-emitting Na3-2xSc2(PO4)3:xEu(2+) phosphor (λem = 453 nm) that does not exhibit thermal quenching even up to 200 °C. This phenomenon of zero thermal quenching originates from the ability of the phosphor to compensate the emission losses and therefore sustain the luminescence with increasing temperature. The findings are explained by polymorphic modification and possible energy transfer from electron-hole pairs at the thermally activated defect levels to the Eu(2+) 5d-band with increasing temperature. Our results could initiate the exploration of phosphors with zero thermal quenching for high-power LED applications.
The effects of substitution of Al³⁺ and Ga³⁺ ions on the upconversion luminescence of (Lu,M)NbO4:Yb³⁺,Er³⁺ (M: Al³⁺, Ga³⁺) were studied for the first time. Under infrared (980 nm) radiation, the upconversion spectra consisted of strong green and weak red emission bands, which were assigned to the transition of the ²H11/2/⁴S3/2 and ⁴F9/2 levels, respectively, to the ground state (⁴I15/2) of the Er³⁺ ions through an energy transfer process from Yb³⁺ to Er³⁺. A two-photon upconversion process was involved in the upconversion emission of (Lu,M)NbO4:Yb³⁺,Er³⁺. Substitution of Al³⁺ and Ga³⁺ ions increased the green (red) emission intensity to approximately 120% (120%) and 135% (130%), respectively. These behaviors were explained in terms of the crystal field asymmetry and energy transfer probability.
A series of β-Sr1.98-yMgySiO4-1.5xNx: 0.02Eu²⁺ (x = 0, 0.3, 0.6, 0.9, 1.2 and 4/3, 0 ≤ y ≤ 0.5, β-S(M)SON: Eu²⁺) phosphors were synthesized by a conventional solid state reaction method. The coordination environments of Eu²⁺ are modified effectively via the introduction of N³⁻ and Mg²⁺ using different concentration, which is confirmed by XRD refinement results and PL spectra. Both fourier transform infrared (FT-IR) spectra and energy dispersive spectroscopy (EDS) analysis qualitatively confirm that the existence of N³⁻. The crystal structures of β-Sr2SiO4-1.5xNx (β-SSON): Eu²⁺ and β-Sr2-yMgySi(O,N)4: Eu²⁺ (β-SMSON, x = 4/3, y = 0.1) have been determined effectively. Rietveld refinement indicates that N³⁻ dopants most likely partly substitute O2²⁻ site. With the increase of N content, the unit cell volume and partial SiO and SrN bond lengths gradually decrease, which are coincided each other. With the introduction of N³⁻, the emission of Eu(I) (∼465 nm) hardly change but that of Eu(II) (∼540 nm) splits into two emission bands as Eu(II) (∼540 nm) and Eu(II)′ (∼616 nm). According to the analysis of XRD data, unit cell volume, FT-IR spectra, photoluminescence (PL) and reflectance spectra of β-SMSON: Eu²⁺, it can be concluded that the incorporation of Mg²⁺ into the host leads to interstitial sites rather than substitutional ones. Mg²⁺ doping couldn't change the dominant peak wavelengths (DPWs) of Eu(I), Eu(II) and Eu(II)′ sites but only affects the PL intensity of title phosphors and could offset the nitridation effects. The concentration quenching mechanism and thermal activation energy of β-SSON: Eu²⁺ phosphors are determined effectively. The β-S(M)SON: 0.02Eu²⁺ phosphor have the feature that it can achieve color-tunable white light-emitting combined with blue light emitting diodes (LEDs) with single activator doped and single-phase host, which enables these materials to be enormous possibilities used in white light emitting diodes.
A series of Sr1.98-xBaxSi(O,N)4: 0.02Eu2+ (0 ≤ x ≤ 0.5) phosphors were synthesized by a conventional solid state reaction method. The Sr-N and Si-N bonds could be observed in FT-IR spectra. The XRD refinement results indicated that N3- would substitute for O22- and form Si-NO3 tetrahedrons during the process of forming the Sr1.98Si(O,N)4: Eu2+ (SSON: Eu2+) substitutional solid solution. The lattice constants of Sr1.98-xBaxSi(O,N)4: 0.02Eu2+ (SBSON) were increased due to the longer bond length of Ba-O. Compared with Sr2SiO4: Eu2+, the SSON: Eu2+ showed a remarkable red-shift, which is due to the nephelauxetic effect and the strong crystal field splitting originating from the stronger covalent bonding effect of the Eu-N bond. The SSON: Eu2+ presented a β phase structure before the introduction of Ba2+, whereas the α′-SBSON phase was obtained by the substitution of Ba2+ for Sr2+. Ba2+ doping led to an obvious blue-shift under 375 nm and 460 nm excitation. The 5d orbital of Ba2+ is coupled with the 5d orbital of Eu2+ on the higher energy level position in the host crystal. Under 375 nm excitation, the PL intensity gradually increased with the increase of the Ba2+ content. Under 460 nm excitation, the PL intensity gradually declined with the increase of Ba2+ content. The thermostability of α′-SBSON: Eu2+ was significantly improved compared with β-SSO: Eu2+ and β-SSON: Eu2+. On the basis of their adjustable emission wavelength, enhanced PL intensity and excellent thermostability in SBSON: Eu2+, we anticipate that these materials can be used as green or red phosphors in white light emitting diodes.
Sr2-xEuxSiO4 powders were prepared by a flux method, and then a correlation between a phase transition and luminescent properties were investigated. The β↔α' transition depended on the synthesizing conditions such as firing temperature, duration time, and europium concentration, resulting in a change of luminescence. Eu2+ ions contributed to the stabilization of the α'-form even at room temperature, which led to a decrease of the β/α' phase ratio with an increase of the Eu2+ concentration. The β-form exhibited a higher PL intensity and a blue-shift compared with the α'-form.
Sr2Si(O1-xNx)(4):Eu2+ (SSON) powders were prepared by a solid-state reaction method and their structural and luminescent properties were investigated. The X-ray diffraction patterns of SSON exactly coincided with those of alpha'- and/or beta-Sr2SiO4 phases, whereas the luminescent spectral properties of SSON were completely different from those of Sr2SiO4:Eu2+ powders, showing broad excitation spectra and strong red-shift of the emission spectra (green to red) due to Eu-N bonds that caused the nephelauxetic effect and strong crystal field. When M2+ ions (Mg2+, Ca2+, Ba2+) were partly substituted for Sr2+ ions, the emission spectra apparently showed red-or blue-shift of the emission, depending on the relative ionic sizes of the M2+ ions and the nature of the Eu2++ sites, i.e., Eu(I) or (II). (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.062205jes]
This book describes the bond valence model, a description of acid-base bonding which is becoming increasingly popular particularly in fields such as materials science and mineralogy where solid state inorganic chemistry is important. Recent improvements in crystal structure determination have allowed the model to become more quantitative. Unlike other models of inorganic chemical bonding, the bond valence model is simple, intuitive, and predictive, and can be used for analysing crystal structures and the conceptual modelling of local as well as extended structures. This is the first book to explore in depth the theoretical basis of the model and to show how it can be applied to synthetic and solution chemistry. It emphasizes the separate roles of the constraints of chemistry and of three-dimensional space by analysing the chemistry of solids. Many applications of the model in physics, materials science, chemistry, mineralogy, soil science, surface science, and molecular biology are reviewed. The final chapter describes how the bond valence model relates to and represents a simplification of other models of inorganic chemical bonding.
Melt-grown beta -Sr2SiO4 (a 5.663(1), b 7.084(2), c 9.767(2) A, beta 92.67(2)o, space group P21/n, Z = 4, Dcalc. 4.536 g/cm3) has a similar structure to that of beta -Ca2SiO4 (M.A. 79-153). The structure of alpha '- Sr1.9Ba0.1SiO4 (a 5.674(1), b 7.086(2), c 9.745(2) A, space group Pmnb, Z = 4, Dcalc. 4.616 g/cm3) is similar to those of alpha '-Ca2SiO4 and Ba2SiO4. -J.E.C.
Sr2–y–zCazSi(O1–xNx)4:yEu2+ (SCSON:Eu2+) solid solutions were prepared by substituting N3-, Eu2+, and Ca2+ ions into Sr2SiO4 (SSO). These ions contributed differently to the evolution of luminescence of SCSON:Eu2+. SSON:Eu2+ (z = 0) has two activation centers: Eu(I) and Eu(II). The nitridation effects led to a dramatic change in the crystal field surrounding the Eu(II) site but rarely affected the Eu(I) site. Accordingly, SSON:Eu2+ exhibited broad excitation spectra from ultraviolet to visible wavelengths. In comparison with the Eu(II) green emissions of SSO:Eu2+, the dominant peak wavelengths (DPWs) of the Eu(II) emissions were at red emission regions (605–630 nm), depending on the amount of Eu2+ ions. The Ca2+ ions of SCSON:Eu2+ preferentially changed the emission wavelength assigned to Eu(I) and affected the Eu(II) emission intensity. In addition to the spectral properties, the chromaticity coordinates and a low thermal quenching behavior of SCSON:Eu2+ powders demonstrated that they can be a novel red phosphors for use in white light emitting diodes.
White light-emitting diodes (WLEDs) as new solid-state light sources have a greatly promising application in the field of lighting and display. So far much effort has been devoted to exploring novel luminescent materials for WLEDs. Currently the major challenges in WLEDs are to achieve high luminous efficacy, high chromatic stability, brilliant color-rending properties, and price competitiveness against fluorescent lamps, which rely critically on the phosphor properties. In recent years, numerous efforts have been made to develop single-phase white-light-emitting phosphors for near-ultraviolet or ultraviolet excitation to solve the above challenges with certain achievements. This review article highlights the current methods to realize the white light emission in a single-phase host, including: (1) doping a single rare earth ion (Eu(3+), Eu(2+) or Dy(3+)) into appropriate single-phase hosts; (2) co-doping various luminescent ions with different emissions into a single matrix simultaneously, such as Tm(3+)/Tb(3+)/Eu(3+), Tm(3+)/Dy(3+), Yb(3+)/Er(3+)/Tm(3+)etc.; (3) codoping different ions in one host to control emission color via energy transfer processes; and (4) controlling the concentration of the defect and reaction conditions of defect-related luminescent materials.
A novel red-emitting oxonitridosilicate phosphors, Sr2SiNzO4−1.5z:Eu2+ (0.7 < z < 1.2), was prepared by solid state reaction in NH3–N2 atmosphere. The crystal structure was determined by Rietveld analysis on powder X-ray data. Sr2SiNzO4−1.5z (0.7 < z < 1.2) crystallizes in an orthorhombic structure with the space group of Pmnb: ba−c (no. 62), and cell parameter a = 5.67366(5) Å, b = 7.09777(4) Å, c = 9.75112(1) Å. Sr2SiNzO4−1.5z:Eu2+ (0.7 < z < 1.2) exhibited broad-band red emission centred at 620 nm (FWHM ≈ 95 nm) under blue light irradiation with a high QE value of 78.0% and good thermal stability, its emission intensity remains 87% at 150 °C of that measured at room temperature. The outstanding luminescent properties allow it to be an attractive red luminescent material for white LEDs.
Sr2SiO4:Eu2+ phosphors were prepared by a flux method. Two emission bands at 495nm and 560nm were observed, which originated from Eu(I) and Eu(II) that were substituted for Sr(I) and Sr(II), respectively. The preference of Eu2+ ions for Sr(I) and Sr(II) strongly depended on the amounts of flux and firing temperatures. The increase of Eu2+ concentration led to the energy transfer from Eu(I) to Eu(II) emitting center, resulting in the red-shift, and the phase transformation from β- to α’-Sr2SiO4 were observed.
A single-host white-light-emitting phosphor, Eu2+ doped BaSrMg(PO4)2 (BSMP: Eu2+), was prepared by solid-state reaction. BSMP: Eu2+ shows two main emission bands peaking at 447 and 536nm, respectively. The emission band peaking at 447nm is attributed to the 4f65d1-4f7 transition of Eu2+ substituting Sr2+, while the emission band peaking at 536nm originates from the 4f65d1-4f7 transition of Eu2+ replacing Ba2+ in host lattice. A white light-emitting diode (WLED) was fabricated by combination of a 380nm emitting InGaN chip and the prepared white phosphor BSMP: Eu2+. In comparison with commercial GaN-pumped YAG: Ce3+ phosphor, BSMP: Eu2+ shows higher color stability against input forward-bias current and excellent color rendering index.
White light-emitting diodes (LEDs) were fabricated by associating an InGaN-based blue LED chip with highly luminescent phosphors, Y3A l5O12:Ce3+ (YAG:Ce) and CaSiAlN3: Eu2+. The thermal stability of these phosphors, depending weakly on the composition and the activator concentration, remains high over a wide range of temperatures (25-300°C). When a mixture of YAG:Ce and CaSiAlN 3: Eu2+ was coated on a blue LED, the resultant white LED had a high luminous efficiency of νL = 68 lm/W, a high color rendering index of Ra = 93, and a color temperature of TC = 3007 K (at 50 mA). Additionally, the color coordinates, Ra and TC, of the white LED tend to remain constant against an appreciable variation in applied current.
β-NaYF4 hexagonal microprisms and microrods with different aspect ratios have been prepared via a simple hydrothermal route. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence (PL) spectra as well as kinetic decays were used to characterize the samples. The influences of reaction temperature and the molar ratio of NaF to Y3+ on the crystal phases and shapes of final products have been studied in detail. The aspect ratios of products increase gradually with the increase of reaction temperature and NaF/Y3+ molar ratio. The growth mechanisms of crystals prepared under the different conditions are presented systematically. More importantly, the systematical investigation on the luminescence properties of β-NaYF4:xEu3+ (x = 0.5, 1, 2, 3, 5, and 10 mol %) with hexagonally microprismatic morphology shows the characteristic emissions of Eu3+ (5DJ−7FJ′, J, J′ = 0, 1, 2, 3). Under the excitation of single wavelength light of 397 nm, the luminescence colors of the corresponding products can be tuned feasibly from bluish white to yellow to red by changing the doping concentration of Eu3+. This merit of multicolor emissions in the visible region endows this kind of materials with potential application in the field of light display systems, lasers, and optoelectronic devices.
Unit cell data and crystal structure refinements of synthetic Ca0.5Sr1.5SiO4, CaSrSiO4, Ca1.5Sr0.5SiO4 and Ca1.8Sr0.2SiO4 are reported. The cations are positionally disordered over mirror-related sites with an accompanying tilt and distortion of the SiO4 tetrahedron; this positional disorder increases with Ca content. Preferences of Ca and Sr for smaller and larger sites were detected and the Ca/Sr interchange energy estimated.-J.E.C.
The optical properties of SrSi2O2N2 doped with divalent Eu2+ and Yb2+ are investigated. The Eu2+ doped material shows efficient green emission peaking at around 540nm that is consistent with 4f7→4f65d transitions of Eu2+. Due to the high quantum yield (90%) and high quenching temperature (>500K) of luminescence, SrSi2O2N2:Eu2+ is a promising material for application in phosphor conversion LEDs. The Yb2+ luminescence is markedly different from Eu2+ and is characterized by a larger Stokes shift and a lower quenching temperature. The anomalous luminescence properties are ascribed to impurity trapped exciton emission. Based on temperature and time dependent luminescence measurements, a schematic energy level diagram is derived for both Eu2+ and Yb2+ relative to the valence and conduction bands of the oxonitridosilicate host material.
The reciprocal salt pair Sr2SiO4-Sr2GeO4-Ba2GeO4-Ba2SiO4 was investigated using X-ray powder diffraction and DTA. Unlimited solubility in the low-K2SO4 structure type (′) occurs throughout the system above 85°C. The nonlinear changes of some lattice constants of the solid solutions are discussed. A stable monoclinic low-temperature (β) form of Sr2SiO4 was found which converts reversibly to the high-temperature ′-modification at 85°C. The enthalpy of the β-’ transition is 51 cal/mol. In the reciprocal salt pair the β-form solid solutions occur in a very narrow region below 85°C.
White-light-emitting diodes are fabricated by using 375 nm emitting InGaN chip with Sr 3 MgSi 2 O 8: Eu 2+ (blue and yellow) or Sr 3 MgSi 2 O 8: Eu 2+ , Mn 2+ (blue, yellow, and red). At a color temperature of 5892 K , the color coordinates are x=0.32 , y=0.33 , and the color rendering index is 84%; at a color temperature of 4494 K , the color coordinates are x=0.35 , y=0.33 , and the color rendering index is 92%. The blue (470 nm ) and yellow (570 nm ) emission bands are originated from Eu 2+ ions, while the red (680) emission band is originated from Mn 2+ ions in Sr 3 MgSi 2 O 8 host. The energy transfer among three bands occurs due to the spectral overlap between emission and absorption bands. It is confirmed by the faster decay time of the energy donor. Our white-light-emitting diodes show higher color reproducibility, higher color stability on forward-bias current, and excellent color rendering index in comparison with a commercial YAG : Ce 3+ -based white-light-emitting diode.
A relation for the variation of the energy gap (Eg) with temperature (T) in semiconductors is proposed. Eg ≐ E0 - αT2/(T+β) where α and β are constants. The equation satisfactorily represents the experimental data for diamond, Si, Ge, 6H-SiC, GaAs, InP and InAs.
Uniform LaOF and LaOF : Eu(3+) nanocrystals of the γ-form have been successfully synthesized under mild conditions via a facile hydrothermal method followed a heat treatment of their bastnaesite-type precursor (LaCO(3)F). The synthetic details, investigations into the phase purity and the presence of the oxocarbonate anion CO(3)(2-) proven by IR measurements and EDX, as well as X-ray powder diffraction data, are given. Photoluminescence (PL) and cathodoluminescence (CL) spectra were utilized to characterize the luminescence properties of the LaCO(3)F : Eu(3+) and LaOF : Eu(3+) samples. Under ultraviolet light excitation, the LaCO(3)F : Eu(3+) precursor shows an orange emission of Eu(3+) (dominated by (5)D(0)→(7)F(1)), while the product of heat treatment, LaOF : Eu(3+), shows the characteristic emissions of Eu(3+) ((5)D(J)→(7)F(J')J, J' = 0, 1, 2, 3 transitions). Under the excitation of UV and low-voltage electron beams, the emission color (including white) of LaOF : Eu(3+) can be tuned by adjusting the doping concentration of Eu(3+). The corresponding luminescence mechanisms have been discussed in detail.
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