Synthesis and luminescence properties of Sr2SiO4: Eu3+, Dy3+ phosphors by the sol-gel method

The Sr2SiO4:Eu3+, Dy3+ phosphors for white light emitting diodes (LEDs) were synthesized by the sol-gel method. The microstructure and luminescent properties of the obtained Sr2SiO4:Eu3+, Dy3+ particles were well characterized. The results demonstrate that the Sr2SiO4:Eu3+, Dy3+ particles, which have spherical morphology, emitted an intensive white light emission under excitation at 386 nm. The phosphors show three emission peaks: the blue emission at 486 nm corresponding to the 4F(9/2)-6H(15/2) transition of Dy3+, the yellow emission at 575 nm corresponding to the 4F(9/2)-6H(13/2) transition of Dy3+, and the red emission at 615 nm corresponding to the 5D0-7F2 transition of Eu3+. At the same time, the effect of Eu3+ concentration on the emission intensities of Sr2SiO4:Eu3+, Dy3+ was investigated in detail. The phosphors used for white LEDs were obtained by combining near ultraviolet (NUV) light (386 nm) with Sr2SiO4:0.04Dy3+, 0.01Eu3+ phosphors with the characteristic of Commission Internationale de l'Eclairage (CIE) chromaticity coordinate (x, y) of (0.33, 0.34), and color temperature Tc of 5,603 K. In addition, the effect of the charge compensators (Li+, Na+, and K+ ions) on the photoluminescence (PL) emission intensities were studied.     

Luminescence properties of nano-crystalline Ba3MgSi2O8:Eu2+, Mn2+ phosphors

Ba3MgSi2O8:Eu2+, Mn2+ phosphors were synthesized by the sol-gel method and high temperature solid-state reaction method, respectively. XRD (X-ray diffraction), FT-IR (Fourier transform infrared spectroscopy), PL (photoluminescence spectra), and PLE (photoluminescence excitation spectra) were measured to characterize the samples. Emission and excitation spectra of our Ba3MgSi2O8:Eu2+, Mn2+ phosphors monitored at 441, 515, and 614 nm are depicted in the paper. The emission intensities of 441 and 515 nm emission bands increase with increasing Eu2+ concentration, while the peak intensity of the 614 nm band increases with increasing Mn2+ concentration. We conclude that the 515 nm emission band is attributed to the 4f(6)5d transition of Eu2+ ions substituted by Ba2+ sites in Ba2SiO4. The 441 nm emission band originates from Eu2+ ions, while the 614 nm emission band originates from Mn2+ ions of Ba3MgSi2O8:Eu2+, Mn2+. Nano-crystalline Ba3MgSi2O8:Eu2+, Mn2+ phosphors prepared by the sol-gel method show higher color rendering and better color temperature in comparison with the samples prepared by high temperature solid-state reaction method.     

溶胶-凝胶法制备CaAl_2SiO_6:Eu~(2+)荧光粉及其光学性质的研究

采用溶胶-凝胶法制备了CaAl2SiO6∶Eu2+荧光粉,利用X射线衍射仪、荧光光谱仪、热分析仪对其结构和光学性质进行了研究.结果表明,样品经1 000℃CaAl2SiO6∶Eu2+荧光粉在430 nm附近的发光峰为Eu2+中心的4f65d1(t2g)→4f7(8S7/2)跃迁;随Eu2+离子浓度的增加,样品的发光强度先增加后降低,在Eu2+浓度为0.7 mol%时达到最大;CaAl2SiO6∶Eu2+系列荧光粉的激发峰波长在280~390 nm之内,可以作为一种新型的UV-LED用三基色荧光粉.     

Morphology control and photoluminescence of BaMgAl10O17:Eu2+, Mn2+ phosphors prepared by flux method

The blue phosphor Ba_0.9 Mg_0.98Al₁₀O₁₇:0.1Eu²⁺, 0.02Mn²⁺ (BAM:Eu²⁺, Mn²⁺) was prepared by flux assisted solid-state reaction method. The effects of (NH₄)₂CO₃ and LiF on the morphology and luminescent properties were studied. The usually obtained BAM:Eu²⁺, Mn²⁺ particles had hexagonal shape. We found that the thickness of the particle was affected by the amount of LiF and spherical-like particles can be obtained. The significant change on morphology from plate-like to spherical-like shape led to a highly enhanced luminescence of BAM:Eu²⁺, Mn²⁺. The maximum luminance intensity of prepared samples was 108% in comparison with the commercial BAM:Eu²⁺, Mn²⁺ under UV excitation.     

Sr3Bi(PO4)3:Eu2+, Mn2+: Single-phase and color-tunable phosphors for white-light LEDs

Sr₃Bi(PO₄)₃:Eu²⁺, Sr₃Bi(PO₄)₃:Mn²⁺, and Sr₃Bi(PO₄)₃:Eu²⁺, Mn²⁺ phosphors were synthesized by solid state reaction. The structure and luminescent characteristics were investigated by X-ray powder diffraction and fluorescent spectrophotometer. All samples have the structural type of eulytine. The excitation and emission spectra of Sr₃Bi(PO₄)₃:0.01Eu²⁺ sample show characteristic bands of Eu²⁺ ions. Also, the excitation and emission spectra of Sr₃Bi(PO₄)₃:0.06Mn²⁺ sample show characteristic bands of Mn²⁺ ions. The emission color of Sr₃Bi(PO₄)₃:Eu²⁺, Mn²⁺ sample could be tuned through tuning the co-dopant concentration of Mn²⁺ ions. The decay times for the Eu²⁺ ions decrease with the increase of Mn²⁺ dopant concentration, but the energy transfer efficiency increases with the increase of Mn²⁺ dopant concentration. On the basis of the luminescent spectra and fluorescence decay curves, we confirm that the energy transfer process from the Eu²⁺ to Mn²⁺ ions takes place in the co-doped Sr₃Bi(PO₄)₃ phosphor. Sr₃Bi(PO₄)₃:Eu²⁺, Mn²⁺ sample shows the good thermostability. The emission intensity of the sample at 400 K is about 60% of the value at 300 K. These results show Sr₃Bi(PO₄)₃:Eu²⁺, Mn²⁺ phosphors could be anticipated for UV-pumped white-light-emitting diodes.     

Luminescence properties and energy transfer of Ba2Mg(PO4)2:Eu2+, Mn2+ phosphor synthesized by co-precipitation method

The Ba₂Mg(PO₄)₂:Eu²⁺, Mn²⁺ phosphor is synthesized by a co-precipitation method. Crystal phase, morphology, excitation and emission spectra of sample phosphors are analyzed by XRD, SEM and FL, respectively. The results indicate particles synthesized by a co-precipitation method have a smaller size in diameter than that synthesized by conventional solid-state reaction method. Emission spectra of BMP:Eu²⁺, Mn²⁺ phosphor show a broad blue and a broad yellow emission bands with two peaks at about 456 nm and 575 nm under 380 nm excitation. An overlap between Eu²⁺ emission band and Mn²⁺ excitation band proves the existence of energy transfer from Eu²⁺ to Mn²⁺. Emitting color of the BMP:Eu²⁺, Mn²⁺ phosphor could be tuned by adjusting relative contents of Eu²⁺ and Mn²⁺ owing to energy transfer formula. Therefore, BMP:Eu²⁺, Mn²⁺ may be considered as a potential candidate for phosphor for near-UV white LED.     

Luminescent properties of Sr2Al2SiO7:Ce3+,Eu2+ phosphors for near UV-excited white light-emitting diodes

The Sr₂Al₂SiO₇:Eu²⁺, Ce³⁺ phosphors were synthesized by a high temperature solid-state reaction. Effective energy transfer occurs in Ce³⁺ and Eu²⁺ co-doped Sr₂Al₂SiO₇ due to large spectral overlap between the emission of Ce³⁺ and excitation of Eu²⁺ ions. Co-doping of Ce³⁺ enhances the emission intensity of Eu²⁺ greatly by transferring its excitation energy to Eu²⁺ ions. The critical distance has been estimated to be about 1.83 nm by spectral overlap method. Furthermore, the developed phosphors can generate lights from blue to green region under the excitation of UV radiation by appropriately tuning the activator content. The Sr₂Al₂SiO₇:Eu²⁺,Ce³⁺ phosphors are promising phosphors for warm-white-light-emitting diode because of its effective excitation in the near ultraviolet range.     

Luminescence enhancement of Tb3+ in Sr3SiO5:Eu2+ phosphor

Sr₃SiO₅ phosphors co-doped with Eu²⁺ and Tb³⁺ were prepared by a conventional solid-state reaction method. The prepared Sr₃SiO₅:Eu²⁺,Tb³⁺,Li⁺ phosphors had characteristic luminescent spectra excited under near-UV excitation in which both the broadband spectrum assigned to Eu²⁺ and the line spectrum assigned to Tb³⁺ are observed, although Tb³⁺ is inactive with this photon energy in general. For Eu²⁺–Tb³⁺ codoped Sr₃SiO₅, energy transfer process takes place and the mechanism is ascribed to the overlap between the shorter Eu²⁺ luminescence band from the Sr₃SiO₅ crystal structure with two Sr sites and ₅D⁴ energy level of Tb³⁺ ion. Due to the energy transfer, PL intensity of Eu²⁺ emission increased about 26 %. We suggest that this enhancement mechanism could shed light on the potential applications in white light-emitting diodes excited by near-UV light. In addition, the emission peak position near the orange region indicates that our system is a step towards a new class of wavelength sources for artificial lighting with improved PL intensity and lower energy consumption.     

Luminescence of nanostructured SnO2-SiO2 glass-ceramics prepared by sol-gel method

Nanostructured silica based glass-ceramics samples of composition (100 - x)SiO2-xSnO2, with x from 1 to 10, have been synthesized by thermal treatment of precursor sol-gel glasses. The average size of the obtained SnO2 nanocrystals, calculated by using the X-ray diffraction, can be predetermined by using well-controlled concentration of tin precursor. The mean radius ranging from 1.6 to 5.5 nm, is comparable to the exciton Bohr radius, corresponding to wide band-gap semiconductor quantum-dots in an insulator SiO2 glass. A spectroscopy study in terms of optical absorption and photoluminescence spectra has been carried out as a function of SnO2 concentration. Size-dependent red-shifts of excitation and emission bands, with increasing of tin precursor concentration, point to the quantum confinement effect. The nanocrystal sizes have been obtained and compared by using the Brus and Scherrer equations. The band gap increase is in agreement with results, based on the effective mass model. The recombination of conduction band electron with oxygen vacancies is proposed to explain the luminescence red-shift.     

Eu2+,Mn2+在CaZn2(PO4)2中的发光及Eu2+→Mn2+能量传递

Mn2+作为激活剂加入一些基质中,发光比较弱。因此常常选择使用合适的敏化剂来提高Mn2+的发光效率,本文的研究目就是验证Eu2+是Mn2+的良好的敏化剂。采用高温固相法合成了Eu2+,Mn2+掺杂激活的CaZn2(PO4)2荧光粉,并对其发光性质进行了研究。单掺杂Eu2+时呈现发射峰位于504nm的带谱,属于Eu2+离子的5d-4f能级跃迁辐射,激发峰位于380nm,属于Eu2+的f-d跃迁特征激发谱带。单掺Mn2+时CaZn2(PO4)2不发光。当Eu2+和Mn2+共掺时,出现Mn2+的673nm发射峰,样品发红光,表明Eu2+对Mn2+的发光有很强的敏化作用。研究了Eu2+和Mn2+掺杂浓度对激发光谱和发射光谱的影响,证明在CaZn2(PO4)2:Eu2+,Mn2+中Eu2+对Mn2+的能量传递属于共振能量传递。     

Luminescence and structural properties of Y(Ta,Nb)O4:Eu3+,Tb3+ phosphors

Yttrium tantalate (YTaO₄), yttrium niobium-tantalate (YTaNbO₄) and yttrium niobate (YNbO₄) doubly doped by Eu³⁺ and Tb³⁺, were investigated using X-ray diffraction and X-ray excitation luminescence in order to study their structural and luminescent properties. By means of X-ray diffraction, the crystallographic data for YTaO₄ and YNbO₄ with double activation by Eu³⁺ and Tb³⁺ were first calculated. Under X-ray excitation luminescence, the rare earth emission centers contribute to the overall luminescence. Due to their various luminescence chromaticities, the proposed rare earth activated phosphors are promising materials for optoelectronics as well as for X-ray intensifying screens for medical diagnosis providing the broad variation of visible photoluminescence from blue to red.     

Luminescence properties of long-lasting phosphor SrMg2(PO4)2:Eu2+, Ho3+, Zr4+

Novel long lasting phosphors SrMg₂(PO₄)₂:Eu²⁺, SrMg₂(PO₄)₂:Eu²⁺, Zr⁴⁺, SrMg₂(PO₄)₂:Eu²⁺, Ho³⁺ and SrMg₂(PO₄)₂:Eu²⁺, Ho³⁺, Zr⁴⁺ were synthesized by conventional solid-state reaction method. The luminescent properties were systematically characterized by X-ray diffraction, photoluminescent excitation and emission spectra, as well as thermoluminescence spectrum and decay curves. The XRD patterns indicated that the samples belonged to monoclinic phase and co-doping Eu²⁺, Ho³⁺ and Zr⁴⁺ ions had no effect on the basic crystal structure. These phosphors emitting purplish blue light is related to the characteristic emission of Eu²⁺. The afterglow time of Eu²⁺ activated SrMg₂(PO₄)₂ can be greatly enhanced by the co-doping of Ho³⁺, Zr⁴⁺. After the 365 nm UV light excitation source switching off, the Sr_0.92Mg_1.95(PO₄)₂:Eu²⁺_0.01, Zr⁴⁺_0.05, Ho³⁺_0.07 phosphorescence can be observed for more than 1013 s in the limit of light perception of dark-adapted human eyes (0.32 mcd/m²). Different kinds of TL peaks at 423, 448 and 473 K have appeared, and traps densities have increased compared with the Eu²⁺ single doped SrMg₂(PO₄)₂ phosphor. By analyzing the TL curve the depths of traps were calculated to be 0.846, 0.896 and 0.946 eV, respectively, which suggested that the co-doping of Ho³⁺, Zr⁴⁺ improved the electron storage ability of material. Besides, the mechanism was discussed in this report.     

Synthesis of the uniform hollow spherical Sr2SiO4:Eu2+ phosphors via an h-BN protective method

The uniform hollow spherical Sr₂SiO₄:Eu²⁺ green emitting phosphors have been successfully synthesized using hollow silica spheres as templates by an h-BN protective method. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results directly confirmed the existence of the hollow spherical structure with a narrow size distribution and a shell thickness of 15–25 nm. The h-BN protective film, observed by high resolution TEM, plays an important role in the formation of the hollow spherical morphology and the improvement of photoluminescence properties. Comparing with the Sr₂SiO₄:Eu²⁺ micron-phosphor prepared by the traditional solid state reaction method, the hollow spherical phosphor with nano-sized grains exhibits stronger green emission under ultraviolet–blue light excitation. This could be attributed to the elimination of surface defects by the h-BN coating. This research gives an economic and convenient way to synthesize uniform spherical phosphors with high quantum efficiency.     

Luminescence properties of Eu2+-activated Sr5(PO4)2(SiO4) for green-emitting phosphor

A green-emitting phosphor of Eu²⁺-activated Sr₅(PO₄)₂(SiO₄) was synthesized by the conventional solid-state reaction. It was characterized by photoluminescence excitation and emission spectra, and lifetimes. In Sr₅(PO₄)₂(SiO₄):Eu²⁺, there are at least two distinguishable Eu²⁺ sites, which result in one broad emission situating at about 495 nm and 560 nm. The phosphor can be efficiently excited in the wavelength range of 250–440 nm where the near UV (～ 395 nm) Ga(In)N LED is well matched. The dependence of luminescence intensities on temperature was investigated. With the increasing of temperature, the luminescence of the phosphor shows good thermal stability and stable color chromaticity. The luminescence characteristics indicate that this phosphor has a potential application as a white light emitting diode phosphor.     

Photoluminescence mechanisms of color-tunable Sr2CeO4: Eu3+, Dy3+ phosphors based on experimental and first-principles investigation

We report single-phased color-tunable phosphors (Sr₂CeO₄: Eu³⁺, Dy³⁺) synthesized by a polymer-network gel method for UV–LED. The photoluminescence properties and possible energy transfer mechanisms of Eu³⁺ and Dy³⁺ in Sr₂CeO₄ were investigated by experiments and first principles calculations. The results show that the ⁵D₀ → ⁷F₂ emission of Eu³⁺ is enhanced by the increase in the amount of Eu³⁺ ions and Eu³⁺ substitution makes more stable defect than Dy³⁺ substitution does. The photoluminescence mechanism of Sr_1.994−xEu_xDy_0.006CeO₄ can be explained by the energy transfer model with the consideration of the defect conditions in the crystals.     

Color tunable Sr2SiO4:Eu2+ phosphors through the modification of crystal structure

Eu²⁺ doped Sr₂SiO₄ phosphors were prepared through a solid-state reaction method. The phase-composition and photoluminescence of the obtained phosphors were systematically studied in terms of calcination temperature, Eu and Ba doping. High calcination temperature promoted the phase transformation from α′–Sr₂SiO₄ (orthorhombic) to β–Sr₂SiO₄ (monoclinic), while the doping of Eu or Ba ions could stabilize α′–Sr₂SiO₄ phase due to their long bond length with oxygen. Small amount of Eu/Ba doping prefers to occupy Sr(I) sites in the crystal lattice of Sr₂SiO₄, acting as nucleation sites for both α′– and β–Sr₂SiO₄ phases. After nucleation, Eu²⁺ ions distribute equally in the two sites. Through structural modification, the Sr₂SiO₄:Eu²⁺ phosphors could be controlled to emit different colors in a wide range, from blue to yellow, making them good candidates for tuning the chromaticity in application.     

Si3N4掺杂氮化对Sr3SiO5:Eu2+荧光粉发光性能的影响

采用高温固相法制备Si₃N₄掺杂氮化Sr_2.99SiO_5-6xN_4x:0.01Eu²⁺荧光粉。采用XRD、EDS和SEM测试结果表明: N^3-进入Sr₃SiO₅基质晶格中取代部分O^2-离子, 形成了单一相Sr_2.99SiO_5-6xN_4x:0.01Eu²⁺固溶体。PL&PLE荧光光谱测试结果显示, Sr_2.99SiO_5-6xN_4x:0.01Eu²⁺荧光粉在344nm紫外光的激发下发射出红橙光, 属于Eu²⁺离子典型的 4f⁶5d¹→4f⁷电子跃迁。随着N浓度的增加, Sr_2.99SiO_5-6xN_4x:0.01Eu²⁺荧光粉发射光谱和激发光谱的强度明显增强。热稳定性测试结果表明, Si₃N₄掺杂氮化能够显著提高Sr₃SiO₅:Eu²⁺荧光粉的热稳定性。通过Arrhennius模型拟合结果表明横向穿越过程(crossover)引起的Sr₃SiO₅:Eu²⁺荧光粉氮化前后的温度猝灭。