Synthesis and luminescence properties of CaAl2O4:Mn4+ phosphor

CaAl_2−yO₄:yMn⁴⁺ (y = 0–1.6 mol%) phosphors are synthesized by a solid-state reaction method in air, and their crystal structure and luminescence property are investigated. To compare luminescence property, CaAl_3.99O₇:1%Mn⁴⁺ and SrAl_1.99O₄:1%Mn⁴⁺ phosphors are also synthesized at the same condition. Broad band excitation spectra are observed within the range 220–550 nm, and emission spectra cover from 600 to 720 nm with the strongest emission peak at ∼658 nm owing to the ²E → ⁴A₂ transition of Mn⁴⁺ ion. The influence of crystal field to luminous intensity is discussed, and the possible luminous mechanism of Mn⁴⁺ ion is explained by using energy level diagram of Mn⁴⁺ ion. CaAl_1.99O₄:1%Mn⁴⁺, CaAl_3.99O₇:1%Mn⁴⁺, and SrAl_1.99O₄:1%Mn⁴⁺ phosphors under excitation 325 nm light emit red light, and their CIE chromaticity coordinates are (0.7181, 0.2813), (0.7182, 0.2818), and (0.7198, 0.2801), respectively. These contents in the paper are helpful to develop novel and high-efficient Mn⁴⁺-doped phosphor for white LEDs.     

Synthesis and luminescence properties of Ca8NaGd(PO4)6F2: Eu2+, Mn2+ for white LEDs

A series of luminescent emission-tunable phosphors Ca₈NaGd(PO₄)₆F₂: Eu²⁺, Mn²⁺ have been prepared by a combustion-assisted synthesis method. The X-ray diffraction measurement results indicate that the crystal structure of the phosphor is a single phase of Ca₈NaGd(PO₄)₆F₂. The photoluminescence (PL) properties of Eu²⁺ and Mn²⁺-codoped Ca₈NaGd(PO₄)₆F₂ phosphors were also investigated. The phosphors can be efficiently excited by ultraviolet (UV) light and show a blue emission band at about 450 nm and a yellow emission band at about 574 nm, which originated from the Eu²⁺ ions and the Mn²⁺ ions, respectively. The efficient energy transfer from the Eu²⁺ ions to the Mn²⁺ ions was observed and its mechanism should be a resonant type via a nonradiative dipole–quadrupole interaction. A color-tunable emission in Ca₈NaGd(PO₄)₆F₂ phosphors can be realized by Eu²⁺ → Mn²⁺ energy transfer. Our results indicate that the developed phosphor may be used as a potential white emitting phosphor for UV based white LEDs.     

Synthesis and tunable luminescence of RbCaGd(PO4)2:Ce3+, Mn2+ phosphors

RbCaGd(PO₄)₂ doped with Ce³⁺, Mn²⁺ was synthesized by the sol-gel method. The crystal structure and crystallographic location of Ce³⁺ in RbCaGd(PO₄)₂ were identified by Rietveld refinement. Powder X-ray diffraction (XRD) revealed that the structure of RbCaGd(PO₄)₂:Ce³⁺ compounds is hexagonal structure which is similar to that of hexagonal LnPO₄ with the lattice constant of a = b = 7.005(57) Å, c = 6.352(05) Å, and V (cell volume) = 269.980 Å³. The photoluminescence behavior and emission mechanism were studied systematically by doping activators in the RbCaGd(PO₄)₂ host. The Mn²⁺ incorporated RbCaGd(PO₄)₂:Ce³⁺, Mn²⁺ compounds exhibited blue emission from the parity- and spin-allowed f-d transition of Ce³⁺ and orange-to-red emission from the forbidden ⁴T₁ → ⁶A₁ transition of Mn²⁺. The emission chromaticity coordinates of RbCaGd(PO₄)₂:0.10Ce³⁺, xMn²⁺ (x = 0.16, 0.25) are close to the white region due to an energy transfer process and the energy transfer mechanism from Ce³⁺ to Mn²⁺ in the RbCaGd(PO₄)₂ host was dominated by dipole-dipole interactions.     

Luminescent properties of Eu3+ activated MLa2(MoO4)4 based (M = Ba,Sr and Ca) novel red-emitting phosphors

Eu³⁺-activated novel red phosphors, MLa₂(MoO₄)₄ (M = Ba, Sr and Ca) were synthesized by the conventional solid state method. The excitation and emission spectra indicate that these phosphors can be effectively excited by UV (395 nm) and blue (466 nm) light, and exhibit a satisfactory red performance at 614 nm. Upon excitation with a 466 nm light, our synthesized phosphors have stronger emission intensity than the sulfide red phosphors used in white LEDs. Due to high emission intensity and a good excitation profile, the Eu³⁺-doped CaLa₂(MoO₄)₄ phosphor may be a promising candidate in solid-state lighting applications.     

VUV–UV luminescence of Ce3+, Tb3+, Eu3+, and Dy3+ doped GdOCl

The vacuum ultraviolet spectroscopic properties of GdOCl:Re³⁺ (Re³⁺ = Ce³⁺, Tb³⁺, Eu³⁺, and Dy³⁺) are investigated in detail for the first time. The host absorption band is determined to be around 179 nm, and the f–d transition bands as well as the charge transfer bands are assigned. Upon 179 nm excitation, Re³⁺ (Re³⁺ = Ce³⁺, Tb³⁺, Eu³⁺, Dy³⁺) ions shown their characteristic emissions. Energy transfers from Gd³⁺ to Re³⁺ ion were observed. A broad band ranging from 350 to 400 nm corresponding to the d–f transition of Ce³⁺ is observed. Eu³⁺ has typical red emission with the strongest peak at 620 nm; Tb³⁺ shows characteristic transition of ⁵D_3,4 → ⁷F_j, and its spin-forbidden and spin-allowed f–d transitions in VUV region are calculated with Dorenbos’ equations, these calculated values agree well with the experimental results. Dy³⁺ presents yellow emission (⁴F_9/2 → ⁶H_13/2) with the strongest peak at 573 nm.     

Synthesis and two-color emission properties of BaGd2(MoO4)4:Eu3+,Er3+,Yb3+ phosphors

Eu³⁺, Er³⁺ and Yb³⁺ co-doped BaGd₂(MoO₄)₄ two-color emission phosphor was synthesized by the high temperature solid-state method. The structure of the sample was characterized by XRD, and its luminescence properties were investigated in detail. Under the excitation of 395 nm ultraviolet light, the BaGd₂(MoO₄)₄:Eu³⁺,Er³⁺,Yb³⁺ phosphor emitted an intense red light at 595 and 614 nm, which can be attributed to ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions of Eu³⁺, respectively. The phosphor will also show bright green light under 980 nm infrared light excitation. The green emission peaks centred at 529 and 552 nm, were attributed to ⁴H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺, respectively. It indicated that the two-color emission can be achieved from the same BaGd₂(MoO₄)₄:Eu³⁺,Er³⁺,Yb³⁺ host system based on the different pumping source, 395 nm UV light and 980 nm infrared light, respectively. The obtained results showed that this kind of phosphor may be potential in the field of multi-color fluorescence imaging and anti-counterfeiting.     

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.     

Effects of aging time on phase,morphology, and luminescence by two-photon processes of BiPO4:Er3+, Yb3+ in the solvothermal synthesis

BiPO₄:Er³⁺, Yb³⁺ phosphors were synthesized by the solvothermal process. The phase transformation, morphology, and UC luminescent property were characterized by different analytical techniques. The aging time has obvious influence on the phase, morphology, and luminescence of the samples. With the extension of aging time, the phase of BiPO₄:Er³⁺, Yb³⁺ phosphors changes from hexagon to monocline. The morphology changes from nanorods through nanorugbies to microoctahedra. Under the excitation at 980 nm, BiPO₄:Er³⁺, Yb³⁺ phosphors show green and red UC emissions, which originate from the (²H_11/2, ⁴S_3/2) → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ions. The green and red UC emission intensities increase gradually with the increase of pumping power. On the basis of the luminescent properties, one can conclude that the two-photon process is involved in green and red UC emissions.     

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.     

Preparation and luminescence properties of yellow long-lasting phosphor Ca2ZnSi2O7:Eu2+, Dy3+

Long-lasting phosphors Ca₂ZnSi₂O₇:Eu²⁺, Dy³⁺ are prepared by solid-state reaction method assisted with different fluxes. Broadband emission, peaked at 580 nm and originating from 4f to 5d transition of Eu²⁺, is observed. The emission intensities of the phosphors can be enhanced by 4.84 and 7.73 times with the introduction of H₃BO₃ and CaF₂, respectively. Moreover, their afterglow times are also respectively prolonged to 11 h and 12 h. The yellow afterglow can be excited by both ultraviolet and visible light, thus permitting its application in both room and outdoor environment. In terms of the crystal structure and trap feature of the phosphors added with different fluxes, the mechanism for the improved luminescence and afterglow properties is discussed.     

Synthesis and luminescent properties of spindle-like CaWO4:Sm3+ phosphors

Spindle-like CaWO₄:Sm³⁺ phosphors were prepared via a Polyvinylpyrrolidone (PVP)-assisted sonochemical process, and characterized by using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and photoluminescence spectroscopy (PL). The XRD results suggested that the prepared samples are single-phase. The FE-SEM images indicated that the prepared CaWO₄:Sm³⁺ phosphors are composed of many spindles with maximum average diameter of 150 nm and maximum average length of 500 nm. Under 404 nm excitation, the characteristic emissions corresponding to ⁴G_5/2 → ⁶H_J (J = 5/2, 7/2, 9/2 and 11/2) transitions of Sm³⁺ in CaWO₄ phosphors were observed. The color coordinates for 1 mol% Sm³⁺ doped CaWO₄ phosphor were calculated to be (0.595, 0.404). The fluorescent concentration quenching of Sm³⁺ doped spindle-like phosphors was studied based on the Van Uitert's model, and it was found that the electric dipole–dipole (D–D) interaction is the dominant energy transfer mechanism between Sm³⁺ ions in the CaWO₄:Sm³⁺ phosphors. The critical energy transfer distance was estimated.     

Synthesis and photoluminescence spectroscopy of BaGeF6:Mn4+ red phosphor

We synthesized Mn⁴⁺-activated BaGeF₆ red phosphor by the chemical reaction method from HF, H₂SiF₆, BaF₂, KMnO₄, and GeO₂ powder. The structural and optical properties of BaGeF₆:Mn⁴⁺ were investigated using X-ray diffraction analysis, secondary electron microscopy observation, electron spin resonance measurement, photoluminescence (PL), PL excitation (PLE) and Raman scattering spectroscopies, and luminescence decay time measurement. Temperature dependence of the PL intensity was measured from T = 20 to 500 K and analyzed by taking into consideration the Bose–Einstein phonon occupation number. The PLE spectra measured at T = 20 and 300 K and luminescence decay time at T = 20–460 K were also analyzed based on the Franck–Condon and conventional thermal quenching models, respectively. Comprehensive discussion was given on the Mn⁴⁺-related PL properties and Raman scattering behaviors in a family of the barium hexafluorometallate phosphors.     

Photoluminescence,photo-stimulated luminescence and thermoluminescence properties of CaB2O4 crystals activated with Ce3+

Photoluminescence (PL), photo-stimulated luminescence (PSL), and thermoluminescence (TL) properties of a Ce-doped CaB₂O₄ crystal were studied. The Ce-doped crystal was prepared by the simple solidification method using a Pt crucible under nitrogen atmosphere. A PL emission band in the 350–370 nm wavelength range was obtained under excitation at 325 nm owing to the 5d (t_2g)–4f (²F_5/2, ²F_7/2)-allowed transition of the Ce³⁺ emission center. The fluorescence quantum efficiency and the decay time of Ce³⁺ were estimated to be about 70% and 29 ns, respectively. The 5d–4f emission band of Ce³⁺ also appeared in the 350–370 nm wavelength range in the TL and PSL spectra. Good linear TL and PSL responses were observed in the 1–1000 mGy and 1–10,000 mGy X-ray dose range, respectively.     

Photoluminescence properties of Sr3(PO4)2: Eu2+, Dy3+ double-emitting blue phosphor for white LEDs

Double-emitting blue phosphor Sr₃(PO₄)₂: Eu²⁺, Dy³⁺ was synthesized by solid state reaction under H₂ atmosphere. XRD exhibited the pure hexagonal phase of the prepared phosphor. The photoluminescence results showed that all samples had intense broad absorption band between 250 and 450 nm, which matched well with the near-UV (350–420 nm) emission band of InGaN-based chips. The emission spectrum of Sr₃(PO₄)₂: Eu²⁺, Dy³⁺ consisted of two broad bands, peaked at 485 nm and 410 nm, which originated from two luminescent centers, related to 4f⁶5d¹ → 4f⁷ transition of Eu²⁺ in six-coordinated Sr(I) and ten-coordinated Sr(II) sites respectively. The intensity ratio of two emission bands could be easily tuned by adjusting Dy³⁺ co-doping content, which resulted in color-tunable luminescence in bluish green region to purplish blue region.     

Novel red phosphors LaBSiO5 co-doped with Eu3+, Al3+ for near-UV light-emitting diodes

Red-emitting phosphors LaBSiO₅:Eu³⁺ and LaBSiO₅:Eu³⁺, Al³⁺ were synthesized by the conventional solid state method at 1100 °C. The structure and luminescent properties of these phosphors are investigated. LaBSiO₅:Eu³⁺ and LaBSiO₅:Eu³⁺, Al³⁺ could be efficiently excited by near ultraviolet light with the strongest excitation peak at 395 nm. The main emission peak is located at around 616 nm, which corresponds to the transition of ⁵D₀ → ⁷F₂ of Eu³⁺ ions. The emission intensity of LaBSiO₅:Eu³⁺ was enhanced by introducing Al³⁺ ions. Compared with Y₂O₂S:0.05Eu³⁺, the sample La_0.70B_0.75SiO₅:0.30Eu³⁺, 0.25Al³⁺ shares the intense red emission, and its emission intensity is about 3.8 times as strong as that of Y₂O₂S:0.05Eu³⁺ under 395 nm light excitation. Bright red light can be observed from the red LED based on La_0.70B_0.75SiO₅:0.30Eu³⁺, 0.25Al³⁺, hence La_0.70B_0.75SiO₅:0.30Eu³⁺, 0.25Al³⁺ maybe find application on near-UV InGaN-based white LEDs.     

Improved optical photoluminescence by charge compensation in the phosphor system CaMoO4:Eu3+

It has been found that charge compensated CaMoO₄:Eu³⁺ phosphors show greatly enhanced red emission under 393 and 467 nm-excitation, compared with CaMoO₄:Eu³⁺ without charge compensation. Two approaches to charge compensation, (a) 2Ca²⁺ → Eu³⁺ + M⁺, where M⁺ is a monovalent cation like Li⁺, Na⁺ and K⁺ acting as a charge compensator; (b) 3Ca²⁺ → 2Eu³⁺ + vacancy, are investigated. The influence of sintering temperature and Eu³⁺ concentration on the luminescent property of phosphor samples is also discussed.     

Concentration quenching,surface and spectral analyses of SrF2:Pr3+ prepared by different synthesis techniques

Pr³⁺ doped strontium fluoride (SrF₂) was prepared by hydrothermal and combustion methods. The phosphors were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and Photoluminescence (PL) spectroscopy. XRD patterns indicated that the samples were completely crystallized with a pure face-centred cubic (space group: Fm3m) structure. SEM images showed different morphologies which is an indication that the morphology of the SrF₂:Pr³⁺ phosphor strongly depends on the synthesis procedure. Both the SrF₂:Pr³⁺ samples exhibit blue–red emission centred at 488 nm under a 439 nm excitation wavelength (λ_exc) at room temperature. The emission intensity of Pr³⁺ was also found to be dependent on the synthesis procedure. The blue–red emission has decreased with an increase in the Pr³⁺ concentration. The optimum Pr³⁺ doping level for maximum emission intensity was 0.4 and 0.2 mol% for the hydrothermal and combustion samples, respectively. The reduction in the intensity for higher concentrations was found to be due to dipole–dipole interaction induced concentration quenching effects.     

Synthesis and luminescence properties of red long-lasting phosphor Y2O2S:Eu3+, Mg2+, Ti4+ nanoparticles

We report an effective method to synthesize Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ nanoparticles. Tube-like Y(OH)₃ were firstly synthesized by hydrothermal method to serve as the precursor. Nanocrystalline long-lasting phosphor Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ was obtained by calcinating the precursor with co-activators and S powder. XRD investigation shows a pure phase of Y₂O₂S, indicating no other impurity phase appeared. SEM and TEM observation reveals that the precursor synthesized via a hydrothermal routine has tube-like structure and the final phosphor reveals a hexagonal shape. The fine nanoparticles which have the particle size ranging from 30 to 50 nm show uniform size and well-dispersed distribution. From the spectrum, the main emission peaks are ascribed to Eu³⁺ ions transition from ⁵D_J (J = 0, 1, 2) to ⁷F_J (J = 0, 1, 2, 3, 4). After irradiation by 325 nm for 10 min, the Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ long-lasting phosphor shows very bright red afterglow and the longest could last for more than 1 h even after the irradiation source had been removed. It is considered that the long-lasting phosphorescence is due to the contribution from the electron traps with suitable trap depth.     

Luminescence properties and energy transfer of a novel bluish-green tunable NaBa4(BO3)3:Ce3+, Tb3+ phosphor

A series of single-phased emission tunable NaBa₄(BO₃)₃:Ce³⁺, Tb³⁺ phosphors were synthesized by solid-state reaction. The crystal structure, photoluminescence properties, concentration quenching and energy transfer of NaBa₄(BO₃)₃:Ce³⁺, Tb³⁺ were systematically investigated. The wavelength-tunable bluish-green light can be realized by coupling the emission bands centered at 425 and 543 nm ascribed to the contribution from Ce³⁺ and Tb³⁺, respectively. The energy transfer from Ce³⁺ to Tb³⁺ in NaBa₄(BO₃)₃ host was studied and demonstrated to be a resonant type via a dipole–dipole interaction mechanism. The energy transfer efficiency (Ce³⁺ → Tb³⁺) obtained by decay curves was consistent with the result calculated by the emission intensity, which gradually increased from 0% to 84.5% by increasing the Tb³⁺ doping content from 0 to 0.45. The results indicate that the NaBa₄(BO₃)₃:Ce³⁺, Tb³⁺ phosphors have potential applications as an ultraviolet-convertible phosphor due to its effective excitation in the ultraviolet rang.     

Photoluminescence of CaAlSiN3:Eu2+-based fine red-emitting phosphors synthesized by carbothermal reduction and nitridation method

In this research, we have presented the synthesis and characterization of the various Ca_1−xEu_xAl_0.76Si_1.18N₃ (x = 0.01 ∼ 0.1) red-emitting phosphors, which were successfully prepared by carbothermal reduction and nitridation (CTRN) method without the strict needs of high pressure. Here, raw materials were CaCO₃, AlN, Si₃N₄, Eu₂O₃, and C. In particular, C was considered as efficient and robust reducing agent. The influences of reaction temperature, holding time, C content, and Eu²⁺ concentration were investigated in the crystal phase compositions and photoluminescence properties of the as-prepared phosphors. Importantly, CaAlSiN₃:Eu²⁺-based red phosphors with interesting properties were obtained with reaction temperature at 1600 °C for 4 h by atmospheric N₂–10%H₂ pressure, and the C/O ratio of 1.5:1, respectively. The emission peak positions of as-prepared phosphors were red-shifted from 607 nm to 654 nm with Eu²⁺ concentration from 1 mol% to 10 mol%. Meanwhile the highest luminescence intensity was achieved with 2 mol% of Eu²⁺ concentration, which showed high external quantum efficiency up to 71%. Combining the phosphor blend of green-emitting β-sialon:Eu²⁺, yellow-emitting Ca-α-sialon:Eu²⁺, and red-emitting Ca_0.98Eu_0.02Al_0.76Si_1.18N₃ with a blue LED (light emitting diodes), warm white LED can be generated, yielding the color rendering index (Ra) of 93 at correlated color temperature (CCT) of 3295 K. These results indicate that CaAlSiN₃:Eu²⁺-based red-emitting phosphors prepared by facile CTRN are highly promising candidates for warm white LEDs.