Luminescent properties and energy transfer of double-emitting NaSrPO4:Eu2+,Tb3+ phosphor for near-UV white LEDs

Novel blue/green NaSrPO₄ phosphors co-doped with Eu²⁺ and Tb³⁺ were synthesized by a conventional solid-state reaction. Their luminescent properties were characterized by using powder X-ray diffraction, photoluminescence excitation and emission spectra, lifetime, and temperature dependent emission spectra, respectively. The NaSrPO₄:Eu²⁺,Tb³⁺,Na⁺ phosphor showed an intense broad excitation band between 250 and 430 nm, which was in agreement with the near-UV chip (350–420 nm), and it exhibited two dominating emission bands at 445 and 545 nm, corresponding to the allowed 4f⁶5d¹→4f⁷(⁸S_7/2) transition of Eu²⁺ ion and the ⁵D₄→⁷F₅ transition of Tb³⁺ ion, respectively. The emission intensity and lifetime of Eu²⁺ ion decreased with the increasing concentration of Tb³⁺ ion, which strongly indicated that an effective energy transfer occurred from Eu²⁺ to Tb³⁺ in NaSrPO₄ host. The principle of the energy transfer should be the combined effect of the non-radiative resonant energy transfer and the phonon-assisted non-radiative process.     

Nonradiative energy transfer from Mn2+ to Eu3+ in K2CaP2O7:Mn2+,Eu3+ phosphor

A series of color tunable phosphors K₂Ca_1?x?yP₂O₇:xMn²⁺, yEu³⁺ are synthesized by solid state reaction method. The energy transfer phenomenon from Mn²⁺ to Eu³⁺ has been observed in the Mn²⁺/Eu³⁺ codoped non-magnetic K₂CaP₂O₇ host, which was confirmed by PL spectra and decay curves. The Mn²⁺→Eu³⁺ energy transfer is controlled by quadrupole–quadrupole interaction between sensitizer and activator. The maximum efficiency of energy transfer is estimated to be 33% with x=0.125 and y=0.03 in K₂Ca_1?x?yP₂O₇:xMn²⁺, yEu³⁺ phosphor. The phosphors can emit light from green to yellow and eventually to orange under 400 nm excitation by changing the Mn²⁺/Eu³⁺ content ratio, indicating that K₂CaP₂O₇: Mn²⁺, Eu³⁺ would be potential candidates for use in lighting and displays applications.     

Fluorescent properties of a green- to red-emitting Eu3+, Tb3+ codoped amorphous calcium silicate phosphor

A Eu³⁺, Tb³⁺ codoped amorphous calcium silicate phosphor was prepared by heating a Eu³⁺, Tb³⁺ codoped calcium silicate hydrate phosphor formed by liquid-phase reaction for 30 min at 900 °C. The excitation peak wavelength of the resulting phosphor was 379 nm and the emission peak wavelengths were at 542 nm, attributed to the ⁵D₄→⁷F₅ transition of Tb³⁺, and at 613 mm, attributed to the ⁵D₀→⁷F₁ transition of Eu³⁺. The intensity ratio of the two peaks could be freely controlled by varying the Eu/Tb atomic ratio of the Eu³⁺, Tb³⁺ codoped amorphous calcium silicate phosphor, allowing light to be emitted over a wide range from green to red. It was clarified that electron transfer from Tb³⁺ to Eu³⁺ is occurring.     

Energy transfer from Tb3+ to Eu3+ ions sorbed on SrTiO3 surface

The energy transfer at room temperature between Tb³⁺ and Eu³⁺ ions sorbed onto SrTiO₃ powders is investigated, using Time-Resolved Laser-induced Fluorescence Spectroscopy (TRLFS). Several published works deal with the energy transfer between two lanthanide ions in co-doped matrices but it is the first time that transfer processes between two lanthanide ions sorbed on a solid surface is reported. The results show that the energy transfer between sorbed Tb³⁺ and Eu³⁺ ions on strontium titanate is a non-radiative process and follows a dipole–dipole type interaction. Moreover, the higher the acceptor ions Eu³⁺ concentration, the more efficient the energy transfer.It is shown that no energy migration between the Tb³⁺ donor ions occurs. A formalism based on the model of Inokuti–Hirayama is used and allows one to fit the non-exponential Tb³⁺ fluorescence decay. It is thus possible to evaluate the critical radius (R₀) of the influence sphere of the sorbed Tb³⁺ ions. According to the previous works, two sorption sites are considered for the sorbed rare-earth. The calculated radii are similar to those obtained for other couples of donor–acceptor lanthanide ions reported in the literature.     

荧光粉CaWO4：Eu3+中WO2-4与Eu3+间的能量转递*

通过高温固相法分别制备了CaWO4和CaWO4：1%Eu3+样品.测量了样品不同温度(10-300 K)的荧光光谱、荧光衰减曲线和时间分辨荧光光谱.样品的荧光光谱表明：在240 nm紫外光激发下,两个样品在430 nm处都展现出来源于WO2?4的蓝色发射;样品CaWO4：Eu3+的Eu3+(5D0→7F1,2,3,4)的特征发射则归属于WO2?4到Eu3+间的能量传递.由样品室温(300 K)荧光衰减曲线发现：纯CaWO4的荧光寿命为8.85μs, Eu3+掺杂之后WO2?4的荧光寿命缩短至6.27μs,这从另一方面证明了WO2?4与Eu3+间能量传递的存在.由荧光寿命得到T =300 K时, CaWO4：1%Eu3+中WO2?4与Eu3+间的能量传递效率(ηET)为29.2%,能量传递速率(ωET)为4.65×104 s?1.通过时间分辨荧光光谱,获得了从WO2?4到Eu3+之间的能量传递的时间演变过程,当温度由10 K增加到300 K时,能量传递出现的时间单调变小.测试了不同温度(10-300 K)对CaWO4：Eu3+的荧光寿命的影响,发现在10-50 K时, Eu3+的荧光寿命增加,但温度超过50 K时发生猝灭,荧光寿命开始下降;WO2?4的荧光寿命则是随着温度的升高逐渐缩短.     

An efficient blue-emitting phosphor LiCaPO4:Eu2+ for white LEDs

A new blue-emitting phosphor LiCaPO₄: Eu²⁺ was synthesized by solid state reaction at a relatively low temperature of 900 °C. It gives a single intense emission band centering at 470 nm, which corresponds to the 4f⁶5d¹→4f⁷ transition of Eu²⁺. The dependence of luminescence intensities on Eu²⁺ concentration was investigated. The phosphor, with a single excitation band extending from 250 to 400 nm, could be efficiently excited by near-ultraviolet light-emitting diodes and is believed to be a promising blue-emitting phosphor for white light-emitting diodes.     

Luminescence and energy transfer in Eu2+,Mn2+ codoped Na2BaMgP2O8 phosphor

Eu²⁺ single-doped and Eu²⁺/Mn²⁺-codoped Na₂BaMgP₂O₈ phosphors were prepared by a combustion-assisted synthesis method. The phase formation was confirmed by X-ray powder diffraction measurement. Na₂BaMgP₂O₈:Eu²⁺,Mn²⁺ shows a broad blue emission band and a red emission band, which originate from Eu²⁺ occupying the Ba²⁺ sites and Mn²⁺ occupying the Mg²⁺ sites, respectively. The efficient energy transfer from Eu²⁺ to Mn²⁺ is verified by the excitation and emission spectra together with the luminescence decay curves. Based on the principle of energy transfer, the relative intensities of blue and red emissions could be tuned by adjusting the contents of Eu²⁺ and Mn²⁺.     

Spectral tuning and energy transfer in a potential fluorescent lamp phosphor BaMg2Al6Si9O30:Eu2+

BaMg₂Al₆Si₉O₃₀:Eu²⁺ phosphors are synthesized by the solid-state reaction method and their photoluminescence (PL) properties are investigated. The ultraviolet emission originates from Eu²⁺(I) substituting for Ba²⁺ sites, whereas the blue emission is attributed to Eu²⁺(II) substituting for Mg²⁺ sites. With increasing Eu²⁺ doping concentrations, the blue emission band shifts to long wavelength and the PL intensity ratio of blue to ultraviolet emission increases. Energy transfer between the two different Eu²⁺ ions is analyzed by photoluminescence excitation and emission spectra, and lifetimes. Results indicate that the emission spectra can be tuned by changing Eu²⁺ contents. We have also demonstrated that BaMg₂Al₆Si₉O₃₀:Eu²⁺ phosphor is a kind of potential phosphor for fluorescent lamps.     

Luminescence properties of NaGd(PO3)4:Eu3+ and energy transfer from Gd3+ to Eu3+

The luminescence properties of polyphosphates NaEu _x Gd_(1?x)(PO₃)₄ (x = 0–1.00) and the energy transfer from Gd³⁺ to Eu³⁺ were studied. In undoped NaGd(PO₃)₄ sample, the photon cascade emission of Gd³⁺ was observed under ⁸S_7/2 → ⁶G_J excitation (201 nm) in which the emission of a red photon due to ⁶G_J → ⁶P_J transition is followed by an ultraviolet photon emission due to ⁶P_J → ⁸S_7/2 transition. When part of Gd³⁺ ions in the host NaGd(PO₃)₄ were substituted by Eu³⁺ ions, the NaGd(PO₃)₄:Eu³⁺ sample showed intensive red emission under 172-nm vacuum-ultraviolet (VUV) excitation which is suitable for mercury-free fluorescent lamps and plasma display panel applications. Based on the VUV–visible spectroscopic characteristics and the luminescence decay properties of NaGd(PO₃)₄:Eu³⁺, it was found that the quantum cutting by a two-step energy transfer from Gd³⁺ to Eu³⁺ can improve the red emission of Eu³⁺ ions under VUV excitation but only a part of the excitation energy in the excited ⁶P_J states within Gd³⁺ ions can be transferred to Eu³⁺ ions for its red emission, and the nonradiative energy transfer efficiencies from the excited ⁶P_J states within Gd³⁺ to Eu³⁺ were calculated.     

Eu3+ luminescence and Gd3+-Eu3+ energy transfer in K5Li2GdF10:Eu3+

Polycrystalline samples of europium-doped K₅Li₂GdF₁₀ have been obtained by a slow cooling of melted compound and investigated using spectroscopy methods. Luminescence from the ⁵ D ₂ level of Eu³⁺ is found to be weak. Intense visible emission upon excitation into the ⁵ D ₂ or higher energy levels has been attributed to overlapping transitions from long-lived ⁵ D ₁ and ⁵ D ₀ levels. A strong increase of the ⁵ D ₀ emission at the expense of the ⁵ D ₁ emission occurs between 5 K and 25 K without significant change of the ⁵ D ₁ lifetime. To account for this, it is supposed that both the radiative and the nonradiative transition rates are temperature-dependent. Efficient energy transfer from the ⁶ G _J levels of Gd³⁺ to Eu³⁺ ions has been evidenced by excitation spectra in the VUV region and VUV-excited luminescence. It has been concluded that the cross relaxation contributes to the energy-transfer process. Received: 8 May 2001 / Accepted: 11 May 2001 / Published online: 25 July 2001     

Concentration quenching of Eu2+ in a thermal-stable yellow phosphor Ca2BO3Cl:Eu2+ for LED application

A piece-shaped phosphor Ca₂BO₃Cl: Eu²⁺ was synthesized by solid-state reaction method. This phosphor exhibited wide absorption in ultra-violet and visible range, and bright yellow emission band centering at 570 nm. The concentration quenching mechanism was verified to be a dipole–dipole interaction, and its critical transfer distance was about 17 Å by both calculated crystal structural method and experimental spectral method. This phosphor has a good thermal stability with a quenching temperature (T_1/2) of 200 °C. Yellow and white LEDs were fabricated with this phosphor and near UV chips, and the yellow LED has a high color purity of 97.0% and promising current tolerant property, while the white LED shows a luminous efficiency of 11.68 lm/W.     

Photoluminescence characteristics and energy transfer between Bi3+ and Eu3+ in Gd2O3: Eu3+, Bi3+ nanophosphors

Bi³⁺ and Eu³⁺ codoped cubic Gd₂O₃ nanocrystals were prepared by the Pechini sol-gel method. Their photoluminescent properties were investigated under ultraviolet light excitation. The introduction of Bi³⁺ ions broadened the excitation band of Eu³⁺ emission, of which a new strong band occurred ranging from 320 to 380 nm due to the 6s²→6s6p transition of Bi³⁺ ions, implying a very efficient energy transfer from Bi³⁺ ions to Eu³⁺ ions. Upon 325 and 355 nm light excitation, the luminescent intensity of Eu³⁺ ions was remarkably improved by the incorporation of Bi³⁺ ions. But a significant quenching of Eu³⁺ emission was observed under 266 nm light excitation when Bi³⁺ was codoped. The possible energy transfer processes between Bi³⁺ and Eu³⁺ were discussed. The decay curves of Eu³⁺ emission under the excitation of 266 nm pulsed laser were measured and gave further evidence for our discussion.     

Photon down-shifting by energy transfer from Sm3+ to Eu3+ ions in sol-gel SiO2-LaF3 nano-glass-ceramics for photovoltaics

95SiO₂?C5LaF₃ sol-gel derived nano-glass-ceramics single doped with Eu³⁺ or Sm³⁺ and codoped with both of them were successfully obtained. XRD measurements confirm the precipitation of LaF₃ nanocrystals after the ceramming process, with mean size ranging from 10 to 20?nm which increases with the thermal treatment temperature. The incorporation of rare-earth ions into precipitated LaF₃ nanocrystals was confirmed from luminescence spectra. Intense yellow-red emissions were detected under UV and blue light excitation in single and codoped samples. The effect of codoping with Eu³⁺ and Sm³⁺ ions and the energy transfer mechanism between them have been analyzed in order to increase the yellow-red emissions.     

Time resolved investigations of energy transfer in Eu3+∶Y2O3

The concentration dependence of the energy transfer from the Eu³⁺ ions at lattice sites of point symmetryC ₃ i to the Eu³⁺ ions at lattice sites of point symmetryC ₂ in Y₂O₃ has been investigated by analyzing the nonexponential fluorescence decay curves of the donors measured after excitation by a short optical light pulse. The simple model of a fixed interaction rangeR ₀ proved to be adequate to explain the experimental observations. An interaction range ofR ₀=8.7 Å was found in agreement with previous steady state experiments.     

Preparation and luminescence of blue-emitting phosphor Ca2PO4Cl:Eu2+ for n-UV white LEDs

A series of blue-emitting Ca_2 ? xEu_xPO₄Cl phosphors were synthesized by a solid state method in a reducing atmosphere. The factors those affect the structure and the photoluminescence (PL) intensities of phosphors, including the dosage of chlorine source CaCl₂, reaction time and annealing temperature, have been investigated in detail. X-ray diffraction (XRD) and photoluminescence measurements were performed to testify the crystal structure and luminescent properties. The optimal Eu²⁺ concentration was determined, and the mechanism of the concentration quenching was predominated by dipole–dipole interaction. The present phosphor exhibits a strong absorption in the near-UV region, emits an intense blue emission centered at 451 nm and presents excellent thermal stability, suggesting that the phosphor is competitive as a promising blue-emitting phosphor for near ultraviolet (n-UV) light-emitting diodes (LEDs).     

Photoluminescence and efficient energy transfer from Ce3+ to Tb3+ or Mn2+ in Ca9ZnLi(PO4)7 host

Structural and spectroscopic characterizations of the Ce³⁺/Tb³⁺(Mn²⁺) solely and Ce³⁺–Tb³⁺(Mn²⁺) doubly doped phosphate compound Ca₉ZnLi(PO₄)₇ with β-Ca₃(PO₄)₂ structure have been performed by powder X-ray diffraction and photoluminescence spectra measurements. The weak green emission from Tb³⁺ and red emission from Mn²⁺ are significantly enhanced by introduction of sensitizer Ce³⁺ ions due to an efficient resonant-type energy transfer from Ce³⁺ to activators Tb³⁺ or Mn²⁺. The energy transfer efficiency and the mechanism have been estimated based on spectroscopic data. Meanwhile, the critical distances for energy transfer between the Ce³⁺ and Tb³⁺ or Mn²⁺ ions are also calculated by the method of spectral overlapping.     

Effect of Tb3+ concentration on luminescence properties of Ce/Tb/Eu co-doped borosilicate glasses for white LEDs

Ce, Tb and Eu single and ternary doped borosilicate glasses were prepared and effect of Tb³⁺ concentration on luminescence properties of ternary co-doped glasses were analyzed by utilizing emission spectra, excitation spectra, the Commission International de I’Eclairage (CIE) colorimetric system and fluorescence decay curves. The results show that Tb³⁺ concentration significantly affects spectral intensities of ternary co-doped glasses when excited by near ultraviolet (NUV). With the increasing of Tb³⁺ concentration, the blue emission of Ce³⁺ is weakened and the red emission of Eu³⁺ is slightly enhanced. Both the color coordinates and correlated color temperatures (CCTs) can be adjusted by Tb³⁺ concentration. Besides, the energy transfers from Ce³⁺ to Tb³⁺ and from Tb³⁺ to Eu³⁺ were observed. Measured characteristic lifetimes of Tb³⁺ indicate that the energy transfer from Ce³⁺ to Tb³⁺ tends to predominate in whole process. The studies show that Ce/Tb/Eu ternary doped borosilicate glasses might be promising luminescence materials for NUV pumped white LEDs.