Phosphorescent and thermoluminescent properties of SrAl2O4:Eu,Dy phosphors prepared by solid state reaction method

Long persistent SrAl₂O₄:Eu²⁺ phosphors co-doped with Dy³⁺ were prepared by the solid state reaction method. The main diffraction peaks of the monoclinic structure of SrAl₂O₄ were observed in all the samples. The broad band emission spectra at 497 nm for SrAl₂O₄:Eu²⁺, Dy³⁺ were observed and the emission is attributed to the 4f⁶5d¹ to 4f⁷ transition of Eu²⁺ ions. The samples annealed at 1100–1200 °C showed similar broad TL glow curves centered at 120 °C. The similar TL glow curves suggest that the traps responsible for them are similar. The long afterglow displayed by the phosphors annealed at different temperatures, may be attributed to the Dy³⁺ ions acting as the hole trap levels, which play an important role in prolonging the duration of luminescence.     

Low-voltage cathodoluminescence property of Li-doped Gd2−xYxO3:Eu

Cathodoluminescent (CL) spectra of Li-doped Gd_2−xY_xO₃:Eu³⁺ solid-solution (0.0?x?0.8) were investigated at low voltages (300 V-1 kV). The CL intensity is maximum for the composition of x=0.2 and gradually reduces with increasing the amount of substituted Y content. In particular, small (∼100 nm) particles of Li-doped Gd_1.8Y_0.2O₃:Eu³⁺ are obtained by firing the citrate precursors at only 650°C for 18 h. Relative red-emission intensity at 300 V of this phosphor is close to 180% in comparison with that of commercial red phosphor Y₂O₃:Eu³⁺. An increase of firing temperature to 900°C results in 400-600 nm sized spherical particles. At low voltages (300-800 V), the CL emission of 100 nm sized particles is much stronger than that of 400-600 nm sized ones. In contrast, the larger particles exhibit the higher CL emission intensity at high voltages (1-10 kV). Taking into consideration small spherical morphology and effective CL emission, Li-doped Gd_1.8Y_0.2O₃:Eu³⁺ appears to be an efficient phosphor material for low voltage field emission display.     

Luminescent properties of HTP AgGd1−xW2O8:Eux and AgGd1−x(W1−yMoy)2O8:Eux phosphor for white LED

Intense red phosphors, AgGd_1−xEu_x(W_1−yMo_y)₂O₈ (x=0.0-1.0, y=0.0-1.0), have been synthesized through traditional solid-state reaction and characterized by X-ray diffraction (XRD) and photoluminescence (PL). XRD results reveal that AgGd_1−xEu_xW₂O₈ synthesized at 1000 °C has a tetragonal crystal structure, which is named as high temperature phase (HTP) AgGdW₂O₈. All phosphors compositions with Eu³⁺ show red and green emission on excitation either in the charge-transfer or Eu³⁺ levels. Analysis of the emission spectra with different Eu³⁺ concentrations reveal that the optimum dopant concentration for Eu³⁺ is x=0.6 in the HTP AgGd_1−xEu_xW₂O₈ (x=0.0-1.0). Studies on the AgGd_0.4Eu_0.6(W_1−yMo_y)₂O₈ (y=0.0-1.0) and AgGd_1−xEu_x(W_0.7Mo_0.3)₂O₈ (x=0.0-1.0) show that the emission intensity is maximum for compositions with y=0.3 and x=0.5, respectively, and a decrease in emission intensity is observed for higher y or x values. The Mo⁶⁺ and Eu³⁺ co-doped AgGd(WO₄)₂ phosphors show higher emission intensity in comparison with the singly Eu³⁺-doped AgGd(WO₄)₂ in UV region. The intense emission of the tungstate/molybdate phosphors under 394 and 465 nm excitations, respectively, suggests that these materials are promising candidates as red-emitting phosphors for near-UV/blue GaN-based white LED for white light generation.     

节能灯用BaMgAl10O17:Eu2+,Mn2+荧光粉的劣化机理研究

对节能灯用BaMgAl₁₀O₁₇: Eu²⁺,Mn²⁺荧光粉的热劣化和紫外辐照劣化机理进行了对比研究. 发现热处理和紫外辐照处理均对BaMgAl₁₀O₁₇: Eu²⁺,Mn²⁺产生明显的发光劣化作用. 研究结果表明：热劣化主要涉及到Eu²⁺ 的氧化及其格位偏移, 而紫外辐照劣化与上述过程无关. 紫外辐照劣化主要源自高能紫外辐照使Eu²⁺ 处于更加不稳定的状态, 从而降低Eu²⁺ 的直接吸收和发射强度.     

Synthesis and luminescence properties of nanocrystalline Gd2O3:Eu by combustion process

The nanocrystalline Gd₂O₃:Eu³⁺ powders with cubic phase were prepared by a combustion method in the presence of urea and glycol. The effects of the annealing temperature on the crystallization and luminescence properties were studied. The results of XRD show pure phase can be obtained, the average crystallite size could be calculated as 7, 8, 15, and 23 nm for the precursor and samples annealed at 600, 700 and 800 °C, respectively, which coincided with the results from TEM images. The emission intensity, host absorption and charge transfer band intensity increased with increasing the temperature. The slightly broad emission peak at 610 nm for smaller particles can be observed. The ratio of host absorption to O²⁻-Eu³⁺ charge transfer band of smaller nanoparticles is much stronger compared with that for larger nanoparticles, furthermore, the luminescence lifetimes of nanoparticles increased with increasing particles size. The effects of doping concentration of Eu³⁺ on luminescence lifetimes and intensities were also discussed. The samples exhibited a higher quenching concentration of Eu³⁺, and luminescence lifetimes of nanoparticles are related to annealing temperature of samples and the doping concentration of Eu³⁺ ions.     

Combustion synthesis and luminescence properties of NaY1−xEux(WO4)2 phosphors

The red phosphors NaY_1−xEu_x(WO₄)₂ with different concentrations of Eu³⁺ were synthesized via the combustion synthesis method. As a comparison, NaEu(WO₄)₂ was prepared by the solid-state reaction method. The phase composition and optical properties of as-synthesized samples were studied by X-ray powder diffraction and photoluminescence spectra. The results show that the red light emission intensity of the combustion synthesized samples under 394 nm excitation increases with increase in Eu³⁺ concentrations and calcination temperatures. Without Y ions doping, the emission spectra intensity of the NaEu(WO₄)₂ phosphor prepared by the combustion method fired at 900 °C is higher than that prepared by the solid-state reaction at 1100 °C. NaEu(WO₄)₂ phosphor synthesized by the combustion method at 1100 °C exhibits the strongest red emission under 394 nm excitation and appropriate CIE chromaticity coordinates (x=0.64, y=0.33) close to the NTSC standard value. Thus, its excellent luminescence properties make it a promising phosphor for near UV InGaN chip-based red-emitting LED application.     

The effect of different gas atmospheres on luminescent properties of pulsed laser ablated SrAl2O4:Eu,Dy thinfilms

SrAl₂O₄:Eu²⁺,Dy³⁺ thin films were grown on Si (1 0 0) substrates using the pulsed laser deposition (PLD) technique to investigate the effect of vacuum, oxygen (O₂) and argon (Ar) deposition atmospheres on the structural, morphological, photoluminescence (PL) and cathodoluminescence (CL) properties of the films. The films were ablated using a 248 nm KrF excimer laser. Atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS) and fluorescence spectrophotometry were used to characterize the thin films. Auger electron spectroscopy (AES) combined with CL spectroscopy were employed for the surface characterization and electron-beam induced degradation of the films. Better PL intensities were obtained from the unannealed films prepared in Ar and O₂ atmospheres with respect to those prepared in vacuum. A stable green emission peak at 515 nm, attributed to 4f⁶5d¹→4f⁷ Eu²⁺ transitions were obtained with less intense peaks at 619 nm, which were attributed to transitions in Eu³⁺. After annealing the films prepared in vacuum at 800 °C for 2 h, the intensity of the green emission (520 nm) of the thin film increased considerably. The amorphous thin film was crystalline after the annealing process. The CL intensity increased under prolonged electron bombardment during the removal of C due to electron stimulated surface chemical reactions (ESSCRs) on the surface of the SrAl₂O₄:Eu²⁺, Dy³⁺ thin films. The CL stabilized and stayed constant thereafter.     

Synthesis and luminescent properties of Y6W2O15:Eu phosphors

In this paper, a novel phosphor, Y₆W₂O₁₅:Eu³⁺ was synthesized by thermal decomposition and phase transition of its decatungstate gel precursor. With stepwise increase of temperature to 750 °C, a crystalline phase of Y₆W₂O₁₅:Eu³⁺forms that gives intense red emission when excited at 466 nm, the emission is attributed to the Eu³⁺ ions transitions from ⁵D₀ excited states to ⁷F_J (J=0-4) ground states. The long excitation wavelength proves the Eu³⁺ transition follows the photoexcitation of the oxygen-metal (O→W lmct) charge transfer bands in yttrium tungstate. Some structural information regarding Y₆W₂O₁₅ provided by luminescence is in accord with that characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The long-wavelength excitation properties of this material may find application in the production of red phosphors for white light-emitting diodes (LEDs).     

Synthesis of nano-size BaMgAl10O17:Eu blue phosphor by a rapid exothermic reaction

Ultrafine particles of BaMgAl₁₀O₁₇:Eu²⁺ (BAM) phosphor were synthesized by a solid-state combustion reaction in a powder bed of 0.9BaCO₃+MgO+5Al₂O₃+0.05Eu₂O₃+k(KClO₃+1.5C) composition. A large exothermic reaction of the mixture (KClO₃+1.5C) leads to a self-sustaining combustion mode. Under optimized combustion conditions, the product consisted of BAM powder and KCl was obtained. BAM ultrafine particles resulting from the combustion process were easily obtained by simply washing the salt by-product with water. Combustion-processed BAM phosphor shows a homogeneous grain size of 100-500 nm, good dispersity, regular morphology, and improved luminescence properties.     

Surface characterization and luminescent properties of SrAl2O4:Eu,Dy nano thin films

SrAl₂O₄:Eu²⁺, Dy³⁺ thin films were grown on Si (1 0 0) substrates in different atmospheres using the pulsed laser deposition (PLD) technique. The effects of vacuum, oxygen (O₂) and argon (Ar) deposition atmospheres on the structural, morphological and photoluminescence (PL) properties of the films were investigated. The films were ablated using a 248 nm KrF excimer laser. Improved PL intensities were obtained from the unannealed films prepared in Ar and O₂ atmospheres compared to those prepared in vacuum. A stable green emission peak at 520 nm, attributed to 4f⁶5d¹→4f⁷ Eu²⁺ transitions was obtained. After annealing the films prepared in vacuum at 800 °C for 2 h, the intensity of the green emission (520 nm) of the thin film increased considerably. The amorphous thin film was crystalline after the annealing process. The diffusion of adventitious C into the nanostructured layers deposited in the Ar and O₂ atmospheres was most probably responsible for the quenching of the PL intensity after annealing.     

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.     

Luminescent properties of green- or red-emitting Eu-doped Sr3Al2O6 for LED

Eu²⁺-doped Sr₃Al₂O₆ (Sr_3−xEu_xAl₂O₆) was synthesized by a solid-state reaction under either H₂ and N₂ atmosphere or CO atmosphere. When H₂ was used as the reducing agent, the phosphor exhibited green emission under near UV excitation, while CO was used as the reducing agent, the phosphor mainly showed red emission under blue light excitation. Both emissions belong to the d-f transition of Eu²⁺ ion. The relationship between the emission wavelengths and the occupation of Eu²⁺ at different crystallographic sites was studied. The preferential substitution of Eu²⁺ into different Sr²⁺ cites at different reaction periods and the substitution rates under different atmospheres were discussed. Finally, green-emitting and red-emitting LEDs were fabricated by coating the phosphor onto near UV- or blue-emitting InGaN chips.     

Luminescence properties of Y2WO6:Eu incorporated with Mo or Bi ions as red phosphors for light-emitting diode applications

In this study, the red phosphors, Y₂W_1−xMo_xO₆:Eu³⁺ and Y₂WO₆:Eu³⁺,Bi³⁺, have been investigated for light-emitting diode (LED) applications. In Y₂WO₆:Eu³⁺, the excitation band edge shifts to longer wavelength with the incorporation of Mo⁶⁺ or Bi³⁺ ions. The emission spectra exhibit ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transition of Eu³⁺ ion at 588, 593, and 610 nm, respectively. Moreover, the bluish-green luminescence of the WO₆⁶⁻ at about 460 nm is observed to decrease with the incorporation of Mo⁶⁺, which results in pure red color. Thus, this study shows that the red phosphor, Y₂WO₆:Eu³⁺, incorporated with Mo⁶⁺ or Bi³⁺ ions is advantageous for LEDs applications.     

Photoluminescence and gamma-ray irradiation of SrAl2O4:Euand Y2O3:Eu phosphors

Y₂O₃:Eu³⁺ phosphor is a very attractive material for use as a red phosphor in many fields. SrAl₂O₄:Eu²⁺ belongs to long lasting phosphor (LLP) and it is a useful bluish-green luminescence material, which can also be a promising candidate as a simple and easy-to-use radiation detection element for visual display of two dimensional radiation distributions. In the present study, both these two kinds of phosphors were synthesized using high temperature solid state reactions. In our work, the influence of gamma-ray irradiation on the properties of these two kinds of phosphors was studied by comparing photoluminescence, brightness and the decay curve of unirradiated and gamma-ray-irradiated samples. Conclusions from the present work can be briefly summarized as follows. In irradiated samples, the brightness is decreased without sensible change in the wavelength distribution of the luminescence spectrum and in the decay kinetic upon gamma exposure. Moreover, the emission due to Eu³⁺→Eu²⁺ conversion in Y₂O₃:Eu³⁺ phosphors was not observed in our sample after irradiation to high exposure. Also the brightness of SrAl₂O₄:Eu²⁺ phosphor turned out to decrease after the exposition to ionizing radiation while the luminescence wavelength distribution remained unchanged. The reason for the effect of gamma-ray irradiation on the properties of phosphors is also discussed in the paper.     

The influence of the number of pulses on the morphological and photoluminescence properties of SrAl2O4:Eu,Dy thin films prepared by pulsed laser deposition

The current work reports on the influence of the number of laser pulses on the morphological and photoluminescence properties of SrAl₂O₄:Eu²⁺,Dy³⁺ thin films prepared by the pulsed laser deposition (PLD) technique. Atomic force microscopy (AFM) was used to study the surface topography and morphology of the films. The AFM data showed that the film deposited using a higher number of laser pulses was packed with a uniform layer of coarse grains. In addition, the surface of this film was shown to be relatively rougher than the films deposited at a lower number of pulses. Photoluminescence (PL) data were collected using the Cary Eclipse fluorescence spectrophotometer equipped with a monochromatic xenon lamp. An intense green photoluminescence was observed at 517 nm from the films prepared using a higher number of laser pulses. Consistent with the PL data, the decay time of the film deposited using a higher number of pulses was characteristically longer than those of the other films. The effects of laser pulses on morphology, topography and photoluminescence intensity of the SrAl₂O₄:Eu²⁺,Dy³⁺ thin films are discussed.     

Luminescence and EPR studies of Mn-activated SrAl12O19 phosphor prepared by facile combustion approach

Manganese-activated strontium hexa-aluminate (SrAl₁₂O₁₉) phosphor has been prepared at low temperature (500 °C) and in a very short time (<5 min) by urea combustion route. Powder X-ray diffraction pattern showed the presence of hexagonal SrAl₁₂O₁₉ phase. Scanning electron microscopy (SEM) indicated the presence of several particles with sizes of 200 nm. The luminescence of Mn²⁺ activated SrAl₁₂O₁₉ exhibits a strong green emission peak around 515 nm from the synthesized phosphor particles under excitation (451 nm). The luminescence is assigned to a transition from the upper ⁴T₁→⁶A₁ ground state of Mn²⁺ ions. EPR investigations also indicated the presence of Mn²⁺ ions in the prepared material. From the observed EPR spectrum, the spin-Hamiltonian parameters have been evaluated. The magnitude of the hyperfine splitting (A) constant indicates that there exists a moderately covalent bonding between Mn²⁺ ions and the surrounding ligands. The variation of zero-field splitting parameter (D) with temperature is measured and discussed. The mechanism involved in the generation of a green emission has been explained in detail.     

PL and CL degradation and characteristics of SrAl2O4: Eu,Dy phosphors

Strontium aluminate phosphors are ideal for luminescent infrastructure materials. Their brightness and persistent glow time are much higher than previously used sulphide phosphors. Strontium aluminates prepared by the sol–gel and combustion methods are compared with commercially available strontium aluminate. High luminescent efficient SrAl₂O₄:Eu²⁺,Dy³⁺ pulsed laser deposited (PLD) thin films were also produced using the commercially available powder. Photoluminescence (PL) degradation studies showed that the phosphor intensity decreased about 20% over a period of 2 weeks under ultraviolet (UV) irradiation. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) showed that cathodoluminescence (CL) degradation is due to the formation of SrO due to electron stimulated surface reactions. The light output mechanism of the phosphor is also discussed in more detail.     

Emission properties of Eu, Mn in MAl2Si2O8 (M=Sr, Ba)

The emission properties of Eu²⁺ and Mn²⁺ in monoclinic SrAl₂Si₂O₈ (M-SAS) and hexagonal BaAl₂Si₂O₈ (H-BAS), both of which have only one alkaline-earth site, were studied. The emission peaks of both Eu²⁺ (405 nm) and Mn²⁺ (564 nm) in SrAl₂Si₂O₈, are located at longer wavelengths, compared with those in H-BAS (373 nm for Eu²⁺ and 518 nm for Mn²⁺), because of the stronger crystal field strength at the Sr site. EPR spectra showed that the g values of Mn²⁺ are 4.5065 in M-SAS:Mn and 2.0247 in H-BAS:Mn. Magnetic measurements proved that Mn²⁺ was at high-spin state in both hosts. The large g value of Mn²⁺ in M-SAS was ascribed to the mixing of the first excitation state to the ground state, both of which have lower d orbital degeneracy due to the lower symmetry of Mn²⁺ site. The transfer efficiency from Eu²⁺ to Mn²⁺was about 10% in M-SAS, higher than that in H-BAS (5%). This was probably because Eu²⁺ emission overlaps the relatively low excitation level of Mn²⁺ in M-SAS. In order to obtain high transfer efficiency, it was necessary for the Eu²⁺ emission to overlap the lowest excitation level of Mn²⁺. The results obtained in this work may be helpful to design the new white or red phosphors for white-light emitting diode (w-LED) applications.     

Synthesis and luminescent properties of nanoparticles GdCaAl3O7:RE (RE=Eu, Tb) via the sol-gel method

By using metal nitrates as starting materials and citric acid as complexing agent, GdCaAl₃O₇:Eu³⁺ and GdCaAl₃O₇:Tb³⁺ powder phosphors were prepared by a citrate-gel method. Thermal analysis (TG-DTG), X-ray diffraction (XRD), transmission electron microscope (TEM) and scanning electron microscope (SEM), photoluminescence excitation and emission, as well as kinetic decays were employed to characterize the resulting samples. The results of the XRD indicated the precursor samples began to crystallize at 800 °C and the crystallinity increased with elevation the annealing temperature. TEM images showed that the phosphor particles were basically of spherical shape, with good dispersion about a particle size of around 40-70 nm. Upon excitation with UV irradiation, it is shown that there is a strong emission at around 617 nm corresponding to the forced electric dipole ⁵D₀-⁷F₂ transition of Eu³⁺, and at around 543 nm corresponding to the ⁵D₄-⁷F₅ transition of Tb³⁺. The dependence of photoluminescence intensity on Eu³⁺ (or Tb³⁺) concentration and annealing temperature were also studied in detail.     

Luminescence studies on ZnO-P2O5 glasses doped with Gd2O3:Eu nanoparticles and Eu2O3

Zinc phosphate glasses doped with Gd₂O₃:Eu nanoparticles and Eu₂O₃ were prepared by conventional melt-quench method and characterized for their luminescence properties. Binary ZnO-P₂O₅ glass is characterized by an intrinsic defect centre emission around 324 nm. Strong energy transfer from these defect centres to Eu³⁺ ions has been observed when Eu₂O₃ is incorporated in ZnO-P₂O₅ glasses. Lack of energy transfer from these defect centres to Eu³⁺ in Gd₂O₃:Eu nanoparticles doped ZnO-P₂O₅ glass has been attributed to effective shielding of Eu³⁺ ions from the luminescence centre by Gd-O-P type of linkages, leading to an increased distance between the luminescent centre and Eu³⁺ ions. Both doped and undoped glasses have the same glass transition temperature, suggesting that the phosphate network is not significantly affected by the Gd₂O₃:Eu nanoparticles or Eu₂O₃ incorporation.