Luminescence characteristics of Eu2+ activated Ca2SiO4 Sr2SiO4 and Ba2SiO4 phosphorsfor white LEDs

<正>This paper investigates the luminescence characteristics of Eu²⁺ activated Ca₂SiO₄,Sr₂SiO₄ and Ba₂SiO₄ phosphors. Two emission bands are assigned to the f-d transitions of Eu²⁺ ions doped into two different cation sites in host lattices,and show different emission colour variation caused by substituting M²⁺ cations for smaller cations.This behaviour is discussed in terms of two competing factors of the crystal field strength and covalence.These phosphors with maximum excitation of around 370 nm can be applied as a colour-tunable phosphor for light-emitting diodes（LEDs） based on ultraviolet chip/phosphor technology.     

Effect of Ca and Sr alkaline earth ions on luminescence properties of BaAl12O19:Eu nanophosphor

Nanosized barium aluminate materials was doped by divalent cations (Ca²⁺, Sr²⁺) and Eu²⁺ having nominal compositions Ba_1−xMxAl₁₂O₁₉:Eu (M=Ca and Sr) (x=0.1-0.5), were synthesized by the combustion method. These phosphors were characterized by XRD, scanning electron microscopy-energy-dispersive spectrometry (SEM-EDS) and photoluminescence measurement. The photoluminescence characterization showed the presence of Eu ion in divalent form which gave emission bands peaking at 444 nm for the 320 nm excitation (solid-state lighting excitation), while for 254 nm it gave the same emission wavelength of low intensity (1.5 times) compared to 320 nm excitation. It was also observed that alkaline earth metal (Ca²⁺ and Sr²⁺) dopants increase the intensity of Eu²⁺ ion in BaAl₁₂O₁₉ lattice, thus this phosphor may be useful for solid-state lighting.     

Photoluminescence and afterglow behavior of Eu, Dy and Eu, Dy in Sr3Al2O6 matrix

This paper reports the photoluminescence and afterglow behavior of Eu²⁺ and Eu³⁺ in Sr₃Al₂O₆ matrix co-doped with Dy³⁺. The samples containing Eu²⁺ and Eu³⁺ were prepared via solid-state reaction. X-ray diffraction (XRD), photo luminescent spectroscope (PLS) and thermal luminescent spectroscope (TLS) were employed to characterize the phosphors. The comparison between the emission spectra revealed that Sr₃Al₂O₆ phosphors doped with Eu²⁺, Dy³⁺ and Eu³⁺, Dy³⁺ showed different photoluminescence. The phosphor doped with Eu³⁺, Dy³⁺ showed an intrinsic f-f transition generated from Eu³⁺, with two significant emissions at 591 and 610 nm. However, the phosphor doped with Eu²⁺, Dy³⁺ revealed a broad d-f emission centering around 512 nm. After the UV source was turned off, Eu²⁺, Dy³⁺ activated Sr₃Al₂O₆ phosphor showed excellent afterglow while Eu³⁺, Dy³⁺ activated phosphor almost showed no afterglow. Thermal simulated luminescence study indicated that the persistent afterglow of Sr₃Al₂O₆: Eu²⁺, Dy³⁺ phosphor was generated by suitable electron traps formed by the co-doped rare-earth ions (Dy³⁺) within the host.     

Enhanced blue emission and EPR study of LaMgAl11O19:Eu phosphors

Europium doped LaMgAl₁₁O₁₉ phosphor was prepared by the combustion method. The as-prepared and post-treated (1350 °C 10 h 5% H₂+95% N₂) phosphors were investigated by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), photoluminescence (PL) and electron paramagnetic resonance (EPR) techniques. XRD patterns show that LaMgAl₁₁O₁₉:Eu phosphors have hexagonal structure. FT-IR spectrum exhibits absorption bands corresponding to the stretching vibration of AlO₄ and AlO₆. Morphological studies reveal that this phosphor has faceted plates of varying sizes and shapes. The as-prepared LaMgAl₁₁O₁₉:Eu phosphor consists of both Eu³⁺ and Eu²⁺ ions. The phosphor exhibits a bright blue emission at 450 nm (4f⁶5d→4f⁷ transition of Eu²⁺). On post-treating the phosphor we are able to enhance the blue emission efficiency by 330%. The process was detected from the evolution of excitation, emission and EPR spectra and the results are discussed.     

Luminescence properties of SrSi2O2N2 doped with divalent rare earth ions

The optical properties of SrSi₂O₂N₂ doped with divalent Eu²⁺ and Yb²⁺ are investigated. The Eu²⁺ doped material shows efficient green emission peaking at around 540 nm that is consistent with 4f⁷→4f⁶5d transitions of Eu²⁺. Due to the high quantum yield (90%) and high quenching temperature (>500 K) 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.     

Color tunability of nanophosphors by changing cations for solid-state lighting

X₃MgSi₂O₈: Eu²⁺, Mn²⁺ (X=Ba, Sr, Ca) phosphors with the mean particle size of 200 nm and the spherical shape are synthesized through combustion method. They show three emission colors under near-ultraviolet light: the blue and green colors from Eu²⁺ ions and the red color from Mn²⁺ ions. Three emission bands show the different emission colors with changing X²⁺ cations. These color shifts are discussed in terms of two competing factors of the crystal field strength and the covalency. These phosphors with maximum excitation of around 375 nm can be applied as color-tunable phosphors for white-light-emitting diode based on ultraviolet/phosphor technology.     

Unusual luminescence of octahedrally coordinated divalent europium ion in Cs2MP2O7 (M=Ca, Sr)

The excitation and emission spectra of octahedrally coordinated europium ion (Eu²⁺) ions in Cs₂M²⁺P₂O₇ (M²⁺=Ca, Sr) are reported and discussed. The remarkable features of the Eu²⁺ luminescence in these phosphate materials include (a) very large Stokes shift of emission (∼1 eV), (b) high luminescence quenching temperature, and (c) unusually low energy of the emitted photons for Eu²⁺ luminescence in phosphate-based materials. The broad emission bands of Eu²⁺ in Cs₂CaP₂O₇ and Cs₂SrP₂O₇ peak at 607 and 563 nm, respectively. The Stokes shift, crystal field splitting, centroid shift and the red shift of the Eu²⁺ 4f⁶5d¹ electronic configuration have been estimated from the relevant optical data. The radiative lifetime of the Eu²⁺ emission in Cs₂M²⁺P₂O₇ is ∼1.2 μs. The nature of the Eu²⁺ emission in Cs₂M²⁺P₂O₇ is discussed and arguments are presented to associate the luminescence with an extreme case of normal 4f⁶5d¹→4f⁷[⁸S_7/2] emission.     

White light-emitting Mg0.1Sr1.9SiO4:Eu phosphors

Divalent europium-activated strontium orthosilicate Sr₂SiO₄:Eu²⁺ and Mg_0.1Sr_1.9SiO₄:Eu²⁺ phosphors were synthesized through the solid-state reaction technique. Their luminescent properties under ultraviolet excitation were investigated. The X-ray diffraction (XRD) results show that these phosphors are of α′-Sr₂SiO₄ phase with a trace of β-Sr₂SiO₄. Doping of Eu²⁺ ion into the crystal lattice results in the lattice constant being expended, while Mg²⁺ makes the lattice constant shrinking. A solid solution with the same crystal structure is formed when Eu²⁺ or Mg²⁺ substitutes part of Sr²⁺ ions and occupies the same lattice sites. The Sr₂SiO₄:Eu²⁺ phosphors show two emission spectra peaked at 535 and 473 nm originated from the 5d-4f transition of Eu²⁺ ion doped in two different Sr²⁺ sites in the host lattice. By substitution of 0.1 mol of Sr²⁺ with Mg²⁺, these two emission bands are tuned to be in the blue and yellow region (459 and 564 nm for Mg_0.1Sr_1.88SiO₄:Eu_0.02), respectively. The tuning effect is discussed. With a combination of the blue and yellow emission bands the phosphors show white color, indicating that these phosphors may become promising phosphor candidates for white light-emitting diodes (LEDs).     

Synthesis and luminescence investigation of RE (Eu, Tb and Ce)-doped lithium silicate (Li2SiO3)

Synthesis and photoluminescence (PL) investigations of lithium metasilicate doped with Eu³⁺, Tb³⁺ and Ce³⁺ were carried out. PL spectra of Eu-doped sample showed peaks corresponding to the ⁵D₀→⁷F_j (j=1, 2, 3 and 4) transitions under ultraviolet excitation. Strong red emission coming from the hypersensitive ⁵D₀→⁷F₂ transition of Eu³⁺ ion suggested the presence of the dopant ion in structurally disordered environment. Tb³⁺-doped silicate sample showed blue-green emission corresponding to the ⁵D₄→⁷F_j (j=6, 5 and 4) transitions. Ce-doped sample under excitation from UV, showed a broad emission band in the region 350-370 nm with shoulders around 410 nm. The fluorescence lifetimes of Eu³⁺ and Tb³⁺ ions were found out to be 790 and 600 μs, respectively. For Ce³⁺, the lifetime was of the order of 45 ns. PL spectra of the europium- and terbium-doped samples were compared with commercial red (Y₂O₃:Eu³⁺) and green (LaPO₄:Tb³⁺) phosphors, respectively. It was found that the emission from the doped silicate sample was 37% of the commercial phosphor in case of the Tb-doped sample and 8% of the commercial phosphor in case of the Eu-doped sample.     

Photoluminescence of Eu single doped and Eu/Dy codoped Sr2Al2SiO7 phosphors with long persistence

This paper reports the preparation of long persistent Sr₂Al₂SiO₇:Eu²⁺ and Sr₂Al₂SiO₇:Eu²⁺, Dy³⁺ phosphors and the comparison of their photoluminescent properties. The silicate phosphors prepared by solid-state reaction routine showed a broad blue emission peaking at 484 nm when activated by UV illumination. Such a bluish-green emission can be attributed to the intrinsic 4f-5d transitions of Eu²⁺. After the UV source was switched off, long persistent phosphorescence could be observed by naked eyes for both samples in darkness. Afterglow measurements revealed that Eu/Dy codoped phosphor possesses better afterglow properties than the Eu single doped one, since the maximum lifetime (τ_max=99 s) of the photons calculated from the decay profile is much larger than that of the Eu single doped phosphor (τ_max=82 s). TSL results suggested that the difference in afterglow properties was caused by the difference in the electron traps within the crystal lattice. For Eu/Dy codoped phosphor, the doping of Dy ions produced electron traps with trap depth of 0.52 eV, which is suitable and therefore leads to good persistence. However, in the case of Eu single doped phosphor, the trap depth is 0.88 eV, which is really too deep an energy barrier to overcome, and therefore a poor persistence was observed in the experiment.     

Photoluminescence of Y2O3:Eu thin film phosphors by sol-gel deposition and rapid thermal annealing

Y₂O₃:Eu³⁺ phosphor films have been developed by using the sol-gel process. Comprehensive characterization methods such as Photoluminescent (PL) spectroscopy, X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy were used to characterize the Y₂O₃:Eu³⁺ phosphor films. In this experiment, the XRD profiles show that the Y₂O₃:Eu³⁺ phosphor films crystallization temperature and optimum annealing temperature occur at about 650 and 750 °C, respectively. The optimum dopant concentration is 12 mol% Eu³⁺ and the critical transfer distance (R_c) among Eu³⁺ ions is calculated to be about 0.84 nm. Vacuum environment is more efficient than oxygen and nitrogen to eliminate the OH content and hence yields higher luminescent phosphor films. The PL emission intensity of Y₂O₃:Eu³⁺ phosphor films is also dependent on the annealing time. It was found that the H₂O impurities were effectively eliminated after annealing time of 25 s at 750 °C in vacuum environment. From the experiment results, the schematic energy band diagram of Y₂O₃:Eu³⁺ phosphor films is constructed.     

Synthesis and luminous characteristics of Ba2MgSi2−xAlxO7: 0.1Eu, 0.1Mn phosphor for WLED

The shift of the emission band to longer wavelength (yellow-orange) of the Ba₂MgSi_2−xAl_xO₇: 0.1Eu²⁺ phosphor under the 350-450 nm excitation range has been achieved by adding the codoping element (Mn²⁺) in the host. The single-host silicate phosphor for WLED, Ba₂MgSi_2−xAl_xO₇: 0.1Eu²⁺, 0.1Mn²⁺ was prepared by high-temperature solid-state reaction. It was found experimentally that, its three-color emission peaks are situated at 623, 501 and 438 nm, respectively, under excitation of 350-450 nm irradiation. The emission peaks at 438 and 501 nm originate from the transition 5d to 4f of Eu²⁺ ions that occupy the two Ba²⁺ sites in the crystal of Ba₂MgSi_2−x Al_xO₇, while the 623 nm emission is attributed to the energy transfer from Eu²⁺ ions to Mn²⁺ ions. The white light can be obtained by mixing the three emission colors of blue (438 nm), green (501 nm) and red (623 nm) in the single host. When the concentrations of the Al³⁺, Eu²⁺ and Mn²⁺ ions were 0.4, 0.1 and 0.1 mol, respectively, the sample presented intense white emission. The addition of Al ion to the host leads to a substantial change of intensity ratio between blue and green emissions. White light could be obtained by combining this phosphor with 405 nm light-emitting diodes. The near-ultraviolet GaN-based Ba₂MgSi_1.7 Al_0.3O₇: 0.1Eu²⁺, 0.1Mn²⁺ LED achieves good color rendering of over 85.     

KBaPO4:Ln (Ln=Eu, Tb, Sm) phosphors for UV excitable white light-emitting diodes

This letter reports the novel three emission bands based on phosphate host matrix, KBaPO₄ doped with Eu²⁺, Tb³⁺, and Sm³⁺ for white light-emitting diodes (LEDs). The phosphors were synthesized by solid-state reaction and thermal stability was elucidated by measuring photoluminescence at higher temperatures. Eu²⁺-doped KBaPO₄ phosphor emits blue luminescence with a peak wavelength at 420 nm under maximum near-ultraviolet excitation of 360 nm. Tb³⁺-doped KBaPO₄ phosphor emits green luminescence with a peak wavelength at 540 nm under maximum near-ultraviolet excitation of 370 nm. Sm³⁺-doped KBaPO₄ phosphor emits orange-red luminescence with a peak wavelength at 594 nm under maximum near-ultraviolet excitation of 400 nm. The thermal stabilities of KBaPO₄:Ln (Ln=Eu²⁺, Tb³⁺, Sm³⁺), in comparison to commercially available YAG:Ce³⁺ phosphor were found to be higher in a wide temperature range of 25-300 °C.     

Temperature-dependent emission spectra of M2SiO4:Eu (M=Ca, Sr, Ba) phosphors for green and greenish white LEDs

The temperature dependence of emission spectra of alkaline earth ortho-silicates M₂SiO₄ (M=Ca, Sr, Ba) doped with Eu²⁺ ions is investigated. Two emission bands of Sr₂SiO₄:Eu²⁺ show the normal redshift with broadening bandwidth and decreasing emission intensity as an increase in temperature. On the other hand, emission bands of Ca₂SiO₄:Eu²⁺ and Ba₂SiO₄:Eu²⁺ show the anomalous blueshift with increasing temperature. For Ca₂SiO₄:Eu²⁺ and Ba₂SiO₄:Eu²⁺, the temperature dependence of the emission color can be described in terms of back tunneling from the excited state of low-energy emission band to the excited state of high-energy emission band in the configuration coordinate diagram. Our phosphors have a promising potential as phosphors for green or greenish white-light-emitting diode pumped by ultraviolet chip.     

Blue-emitting long-lasting phosphor, Sr3Al10SiO20:Eu,Ho

A novel blue-emitting long-lasting phosphor Sr₃Al₁₀SiO₂₀:Eu²⁺,Ho³⁺ is prepared by the conventional high-temperature solid-state technique and their luminescent properties are investigated. XRD, photoluminescence (PL) and thermoluminescence (TL) are used to characterize the synthesized phosphors. These phosphors are well crystallized by calcinations at 1500-1600 °C for 3 h. The phosphor emits blue light and shows long-lasting phosphorescence after it is excited with 254/365 nm ultraviolet light. TL curves reveal the introduction of Ho³⁺ ions into the Sr₃Al₁₀SiO₂₀:Eu²⁺ host produces a highly dense trap level at appropriate depth, which is the origin of the long-lasting phosphorescence in this kind of material. The long-lasting phosphorescence lasts for nearly 6 h in the light perception of the dark-adapted human eye (0.32 mcd/m²). All the results indicate that this phosphor has promising potential practical applications.     

Synthesis and crystal structures and luminescent properties of divalent europium-doped Ba2ZnSi2O7 and BaZn2Si2O7

The monoclinic Ba₂ZnSi₂O₇:Eu²⁺ blue-green-emitting phosphor and the orthorhombic BaZn₂Si₂O₇:Eu²⁺ green-emitting phosphor were prepared by combustion-assisted synthesis method as the fluorescent materials for ultraviolet-light-emitting diodes (UV-LEDs) performed as a light source. The crystallinity and luminescence were investigated using X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy. Pure monoclinic Ba₂ZnSi₂O₇ and orthorhombic BaZn₂Si₂O₇ crystallize completely at 1100 °C. The doped Eu²⁺ ions did not cause any significant change in the host structure. The emission spectra presented an emission position red shift of up to 16 nm from Ba₂ZnSi₂O₇:Eu²⁺ to BaZn₂Si₂O₇:Eu²⁺. The excitation spectra of Ba₂ZnSi₂O₇:Eu²⁺ and BaZn₂Si₂O₇:Eu²⁺ were broad-banding, extending from 260 to 465 nm, which match the emission of UV-LEDs.     

A white light emitting phosphor Sr1.5Ca0.5SiO4:Eu, Tb, Eu for LED-based near-UV chip: Preparation, characterization and luminescent mechanism

In this work, we report preparation, characterization and luminescent mechanism of a phosphor Sr_1.5Ca_0.5SiO₄:Eu³⁺,Tb³⁺,Eu²⁺ (SCS:ETE) for white-light emitting diode (W-LED)-based near-UV chip. Co-doped rare earth cations Eu³⁺, Tb³⁺ and Eu²⁺ as aggregated luminescent centers within the orthosilicate host in a controlled manner resulted in the white-light phosphors with tunable emission properties. Under the excitation of near-UV light (394 nm), the emission spectra of these phosphors exhibited three emission bands: one broad band in the blue area, a second band with sharp lines peaked in green (about 548 nm) and the third band in the orange-red region (588-720 nm). These bands originated from Eu²⁺ 5d→4f, Tb³⁺⁵D₄→⁷F_J and Eu³⁺⁵D₀→⁷F_J transitions, respectively, with comparable intensities, which in return resulted in white light emission. With anincrease of Tb³⁺ content, both broad Eu²⁺ emission and sharp Eu³⁺ emission increase. The former may be understood by the reduction mechanism due to the charge transfer process from Eu³⁺ to Tb³⁺, whereas the latter is attributed to the energy transfer process from Eu²⁺ to Tb³⁺. Tunable white-light emission resulted from the system of SCS:ETE as a result of the competition between these two processes when the Tb³⁺ concentration varies. It was found that the nominal composition Sr_1.5Ca_0.5SiO₄:1.0%Eu³⁺, 0.07%Tb³⁺ is the optimal composition for single-phased white-light phosphor. The CIE chromaticity calculation demonstrated its potential as white LED-based near-UV chip.     

A potential red phosphor LiGd(MoO4)2:Eu for light-emitting diode application

Eu³⁺-doped LiGd(MoO₄)₂ red phosphor was synthesized by solid-state reaction, and its photoluminescent properties were measured. The effect of Eu³⁺ doping concentration on PL intensity was investigated, and the optimum concentration of Eu³⁺ doped in LiGd(MoO₄)₂ was found to be 30 mol%. Compared with Y₂O₂S:0.05Eu³⁺, Na_0.5Gd_0.5MoO₄:Eu³⁺ and KGd(MoO₄)₂:Eu³⁺, the LiGd(MoO₄)₂:Eu³⁺ phosphor showed a stronger excitation band around 395 nm and a higher intensity red emission of Eu³⁺ under 395 nm light excitation. For the first time, intensive red light-emitting diodes (LEDs) were fabricated by combining phosphor and a 395 nm InGaN chip, confirming that the LiGd(MoO₄)₂:Eu³⁺ phosphor is a good candidate for LED applications.     

The luminescence and structural characteristics of Eu- doped NaSrB5O9 phosphor

A red-emitting phosphor NaSrB₅O₉:Eu³⁺ was synthesized by employing a solid-state reaction (SSR) method. The structures of the phosphors were analyzed by X-ray diffraction (XRD), Fourier-transform infrared (FTIR) and Raman studies. The band at ~282 nm in the excitation spectra indicated the charge transfer band (CTB) of B-O in the host, whereas the CTB of Eu-O was observed at ~275 nm for the NaSrB₅O₉:Eu³⁺ (Eu³⁺=1 at.%) phosphor, which was supported by diffuse reflectance spectroscopy (DRS) measurements. The photoluminescence (PL) measurements exhibited a strong red emission band centered at about 616 nm (⁵D₀→⁷F₂) under an excitation wavelength of 394 nm (⁷F₀→⁵L₆). Upon host excitation at 282 nm, the pristine NaSrB₅O₉ exhibited a broad UV emission centered at ~362 nm. The energy transfer from host to Eu³⁺ ions was confirmed from luminescence spectra, excited with a 355 nm Nd:YAG laser. In addition, the asymmetric ratios indicate a higher local symmetry around the Eu³⁺ ion in the host. The calculated CIE (Commission International de l′Eclairage) coordinates displayed excellent color purity efficiencies (around 99.7%) compared to other luminescent materials.     

Synthesis of Gd2Ti2O7:Eu, V phosphors by sol-gel process and its luminescent properties

A red-emitting phosphor material, Gd₂Ti₂O₇:Eu³⁺, V⁴⁺, by added vanadium ions is synthesized using the sol-gel method. Phosphor characterization by high-resolution transmission electron microscopy shows that the phosphor possesses a good crystalline structure, while scanning electron microscopy reveals a uniform phosphor particle size in the range of 230-270 nm. X-ray photon electron spectrum analysis demonstrates that the V⁴⁺ ion promotes an electron dipole transition of Gd₂Ti₂O₇:Eu³⁺ phosphors, causing a new red-emitting phenomenon, and CIE value shifts to x=0.63, y=0.34 (a purer red region) from x=0.57, y=0.33 (CIE of Gd₂Ti₂O₇:Eu³⁺). The optimal composition of the novel red-emitting phosphor is about 26% of V⁴⁺ ions while the material is calcinated at 800  °C. The results of electroluminescent property of the material by field emission experiment by CNT-contained cathode agreed well with that of photoluminescent analysis.