Structure and luminescence characteristics of self-activated vanadate garnet phosphors.

Vanadate phosphors include [VO4] tetrahedra with Td symmetry. Due to the charge transfer (CT) between V5+-O2-, the phosphor with [VO4] tetrahedra exhibits strong emission in the visible region and can effectively absorb ultraviolet (UV) light. Garnet structured vanadate phosphors have attracted much attention due to their easily controllable composition. In this paper, we successfully prepared phosphors with high quantum efficiency. The lattice structure and luminescent properties of the samples were studied in detail. Vanadate garnet phosphors have the same structure. The phosphors exhibit broadband emission peaks under 266 nm and 355 nm near-ultraviolet (UV) excitation, emitting bright greenish-white and yellowish-white light. The emission wavelength of the phosphor is red-shifted. The phosphors exhibit high internal quantum efficiency (IQE) for the NaSr2Mg2V3O12 and KCa2Mg2V3O12 samples. In this paper, the structure and luminescence characteristics of greenish-white-emitting and yellowish-white-emitting vanadate phosphors with high quantum efficiency are investigated.

Structural characterization of Li2(MgxZn1-x)2W2O9 for bluish-white luminescence in rare earth-free tungstate phosphors.

Self-activated phosphors have attracted considerable attention due to their low synthesis temperature, high excitation threshold, and broad emission spectrum. And self-activated tungstate phosphors are distinguished by their low cost and stable chemical properties. Generally, it is difficult to observe luminescence from tungstate phosphors at room temperature. Furthermore, blue-emitting tungstate phosphors with high quantum efficiency are rarely reported. In this study, we succeeded in discovering high quantum-efficiency bluish-white-emitting Li2(MgxZn1-x)2W2O9 phosphors and investigating their detailed crystal structures. Upon near-ultraviolet excitation at 266 nm, these phosphors exhibit a broadband emission peak. The red shift of emission is slight with increasing Zn content in Li2(MgxZn1-x)2W2O9. A highly compact octahedral [WO6] unit is observed in the Li2(MgxZn1-x)2W2O9 phosphors. The phosphors exhibit high internal quantum efficiencies (IQEs) of 68.70% (M = Mg), 43.90% (M = Mg0.5Zn0.5), and 22.90% (M = Zn), respectively. This study provides a bluish-white-emitting tungstate phosphor with high quantum efficiency.

Investigation of the thermal quenching of two emission centers in Sr9MnLi(PO4)7:Eu2+ using time-resolved technique.

The thermal stability of the phosphors in phosphor-converted light-emitting diodes (LEDs) plays an important role in the practical application of lighting. Herein, the Mn2+-based red-emitting phosphors of pure and Eu2+-doped Sr9MnLi(PO4)7 (SMPO) samples were prepared using the high temperature solid-state reaction method. The crystal field environment around the Mn2+ ions was analyzed by combining the results of photoluminescence excitation spectroscopy and Tanabe-Sugano diagrams. By comparing the results of X-ray photoelectron spectroscopy, two additional bands centered at about 129.8 eV and 130.7 eV were found in the Eu2+-doped sample, which corresponded to the chemical states of P 2p3/2 and P 2p1/2. Two different sets of emission spectra were observed for Sr9MnLi(PO4)7:5%Eu2+ (SMPO:Eu2+) on employing the time-resolved technique. The emission peaks centered at 615 nm and 661 nm were attributed to Mn2+ and Eu2+ ions, respectively. The thermal quenching behaviors of Eu2+ and Mn2+ were investigated in the temperature range of 300-620 K and the thermal quenching mechanisms are given in this work. Systematic research on the luminescent properties of Eu2+ and Mn2+ ions in the SMPO:Eu2+ phosphor contributes to the understanding of the thermal stability and aids in the development of Mn2+-based red-emitting phosphors.

Temperature-dependent persistent luminescence of SrAl2O4:Eu2+, Dy3+, Tb3+: a strategy of optical thermometry avoiding real-time excitation.

A new strategy of optical thermometry is realized by long persistent luminescence phosphor SrAl2O4:Eu2+, Dy3+, Tb3+ (SAEDT). Under different temperatures, SAEDT shows bright afterglow emissions after cessation of the UV excitation. The afterglow color of the SAEDT sample is blue at 60 K and gradually changed into green at 240 K. The normalized afterglow spectra at different temperatures give a dramatic change of fluorescence intensity ratio between the blue band and the green band. Not only has this material exhibited a high absolute sensitivity and relative sensitivity for temperature sensing, but it also has the great advantage of eliminating the heating effect due to avoidance of real-time direct excitation.

Highly uniform and monodisperse ß-NaYF4 : Sm3+ nanoparticles for a nanoscale optical thermometer.

Monodisperse ß-NaYF4:1%Sm3+ nanoparticles were fabricated successfully via the thermal decomposition technique. Strong temperature dependence of the Sm3+ emission was observed when its thermally populated state H7/26 was directly excited to the G5/24 level. This strategy not only can eliminate laser heating and background Stokes-type scattering noise but also has a high quantum yield as a result of one-photon excitation process. Under 594.0 nm laser excitation, the emission intensity of G5/24-H5/26 enhances monotonously with rising temperature from 300 K to 430 K, including a physiological temperature range (27°C-60°C). The relative temperature sensitivity can reach 1.1% K-1 and 0.91% K-1 at 300 K and 330 K, respectively. In addition, the repeatability of temperature sensing was evaluated under several heating-cooling cycles, and the decay curves of the emission at 560.0 nm (G5/24-H5/26) at different temperatures were also investigated. These results raise the prospects of monodisperse ß-NaYF4:1%Sm3+ nanoparticles for optical temperature sensing in biomedicine fields.

Temperature Sensing Using Thermal Population of Low-Lying Energy Levels with (Sm0.01Gd0.99)VO4.

A temperature sensing scheme is proposed that is based on the dramatic temperature dependence of the photoluminescence when Sm3+ dopants are excited from thermally populated 6H7/2,9/2 levels, rather than the ground level 6H5/2, to the 4G5/2 luminescent level. The scheme has the advantage of eliminating laser heating and background Stokes-type scattering noise. Experimental realization was carried out on a (Sm0.01Gd0.99)VO4 sample by detecting the intensities at 550-580 nm using excitation wavelengths of 601.6 nm (process A) and 644.0 nm (process B) to excite Sm3+ to the 4G5/2 level from the 6H7/2 and 6H9/2 levels, which are ca. 1160 and ca. 2270 cm-1 above the ground 6H5/2 level, respectively. The sensitivities achieved are 1267 K/T2 in the temperature range of 183-413 K for process A and 2600 K/T2 in 393-603 K for process B. At even higher temperatures (600-800 K), a complementary process C based on the temperature-dependent luminescence decay lifetime resulted in a relative temperature sensitivity increase from 0.52% K-1 at 640 K to a top value of 3.23% K-1 at around 750 K. Furthermore, factors affecting the temperature dependence of the luminescence intensities have been successfully explored by taking into account the broadening of the thermally activated energy levels and the quantum efficiency of the luminescent level.

Investigation on the site occupation of rare-earth ions in CaIn2O4 with the fluorescence probe of Eu3.

Rare-earth doped CaIn2O4 phosphors have been widely investigated due to their excellent luminescent property, but the site occupation of rare-earth ions in CaIn2O4 is not very clear and needs to be clarified. Using Eu3+ as a fluorescence probe, such a clarification has been made in this work. 1% and 2% Eu3+ doped CaIn2O4 powder samples have been prepared by the sol-gel method. The X-ray diffraction results indicate that the lanthanide doping does not influence the structure of CaIn2O4. Site selective excitation at low temperature disclosed five different luminescent centers marked as A, B, C1, C2 and C3. The spectral analysis revealed that the A and B sites belong to Eu3+ embedded in In3+ sites; the other three are attributed to Eu3+ substitution on Ca2+ sites, which show slight distortion. Energy transfers from the B site to the A and C1 sites were observed in the 2% Eu3+ doped CaIn2O4 sample. The transitions of Eu3+ ions in the Ca2+ sites make the main contribution to the emission spectra excited at room temperature. These results may provide a guide for optimizing rare-earth doped CaIn2O4 phosphors for their application in the solid state lighting field.

Nonlinear Composition-Dependent Optical Spectroscopy of Ba2xSr2-2xV2O7.

In general, adjusting the composition of a fluorescent material is an effective way to tune its luminescent properties such as peak energy and bandwidth. In most solid-solutions, the emission peak shifts linearly with the materials' composition, which is referred to as Vegard's Law. However, we found extraordinary variations in our samples Ba2xSr2-2xV2O7, that is, both the excitation and emission peaks show nonlinear dependence on the composition x, and the same is true for the spectral bandwidths. The nonlinearities are not due to structural anomaly, as all the samples are confirmed to be solid-solutions by X-ray diffraction measurements. To explain these phenomena, we proposed a model by considering the disorder of Ba(2+) and Sr(2+) distributions in solid-solutions and the changes of configurations between the ground and excited electronic states. This novel phenomenon could be applied to further exploit new fluorescent materials.

Near-infrared Downconversion in LuPO4: Tm3+, Yb3+ Phosphors.

Tm3+ and Yb3+ codoped LuPO4 phosphors were synthesized by the reverse-strike co-precipitation method. The obtained LuPO4:Tm3+,Yb3+ phosphors were characterized by X-ray diffraction (XRD), diffuse reflectance spectra, photoluminescence (PL) spectra, and decay lifetime to understand the observed near-infrared downconversion (DC) phenomena. The XRD results show that all the prepared phosphors can be readily indexed to the pure tetragonal phase of LuPO4 and exhibit good crystallinity. The experimental results showed that the strong visible emission around 649 nm from Tm3+(1G4 --> 3F4) and near-infrared (NIR) emission around 1003 nm from Yb3+(2F5/2 --> 2F7/2) of LuPO4:Tm3+,Yb3+ phosphors were observed under 468 nm excitation, respectively. The Yb3+ concentration dependence of luminescent properties and lifetimes of both the visible and NIR emissions have also been investigated. The quenching concentration of Yb3+ ions approaches 30 mol%. The DC mechanism is also discussed in detail.

Temperature sensor based on ladder-level assisted thermal coupling and thermal-enhanced luminescence in NaYF4: Nd³âº.

NaYF4: Nd³âº microprisms were synthesized by a hydrothermal method. The bands of near-infrared (NIR) luminescence originating from the 4F3/2, 4F5/2 and 4F7/2 levels of Nd³âº ions in NaYF4: Nd³âº microcrystals were measured under 574.8 nm excitation at various temperatures from 323 to 673 K. The fluorescence intensity ratios (FIRs) between any two of the three bands change monotonically with temperature and agree with the prediction assuming thermal couplings. A large relative temperature sensitivity of 1.12% K⁻¹ at 500K is reached with the FIR of 4F7/2 to 4F3/2 levels. In addition, anti-Stokes fluorescence from 4F5/2 level (740 nm) and 4F5/2,7/2 levels (740 nm and 803 nm) of Nd³âº ions was studied meticulously under 793.8 nm and 864.2 nm excitations, respectively. The intensities were shown to be greatly enhanced as temperature increases, and the 740 nm band from 4F7/2 level at 458 K increases in intensity by 170 fold relative to that at 298 K under the 793.8 nm excitation.

Strategy for thermometry via Tm³âº-doped NaYF4 core-shell nanoparticles.

Optical thermometers usually make use of the fluorescence intensity ratio of two thermally coupled energy levels, with the relative sensitivity constrained by the limited energy gap. Here we develop a strategy by using the upconversion (UC) emissions originating from two multiplets with opposite temperature dependences to achieve higher relative temperature sensitivity. We show that the intensity ratio of the two UC emissions, ³F(2,3) and ¹G4, of Tm³âº in ß-NaYF4:20%Yb³âº, 0.5%Tm³âº/NaYF4:1%Pr³âº core-shell nanoparticles under 980 nm laser excitation exhibits high relative temperature sensitivity between 350 and 510 K, with a maximum of 1.53% K⁻¹ at 417 K. This demonstrates the validity of the strategy, and that the studied material has the potential for high-performance optical thermometry.

Spectroscopic properties of Yb3+ doped CaNb2O6 phosphor.

The spectroscopic measurements of orthorhombic CaNb2O6 phosphor doped with Yb3+ ions have been performed. Low temperature spectra indicate that Yb3+ ions may occupy three types of nonequivalent sites inside the CaNb2O6 lattice: a main Yb3+ site dominating the near-infrared emission, and two other minor sites of similar behaviors to each other. An attempt to determine the Stark sublevels energies of the 2F7/2 and 2F5/2 manifolds of three Yb3+ nonequivalent ions was carried out. The fluorescence lifetimes associated to the emission lines of Yb3+ ions located at the main site and minor sites are also quite different. Furthermore, there exists energy transfer from the main site to the minor sites.

Synthesis and luminescence properties of NaSrPO4:Eu2+, Tb3+, Mn2+ for WLED.

In order to obtain a single-host-white-light phosphor used for near ultraviolet (NUV) light emitting diodes (LEDs), the NaSrPO4:Eu2+, Tb3+, Mn2+ powder samples were synthesized via a high temperature solid-state reaction. XRD investigation shows a single phase. Energy transfer processes is discussed by analyzing the photoluminescence (PL) and photoluminescence excitation (PLE) spectra. White light emitting was observed upon the excitation of a wide range of ultraviolet (UV) wavelengths. The emission spectra are made up of blue, green and red emissions from Eu2+, Tb3+ and Mn2+ ions, respectively. The color shift is insignificant when altering the excitation wavelength from 260 nm to 400 nm. This indicates that the phosphor could exhibit good color stability when used in combination with a NUV LED.

Near-infrared to visible upconversion in Tm3+ and Yb3+ codoped Lu2O3 nanocrystals synthesized by hydrothermal method.

Lutetium oxide nanocrystals codoped with Tm3+ and Yb3+ have been successfully synthesized via adjusting the pH values of the precursor solution in a hydrothermal method followed by a subsequent calcination process. The samples were systematically characterized by X-ray diffraction, field-emission scanning microscopy, Fourier transform infrared transmittance spectroscopy, and upconversion luminescent spectra. The experimental results show that the pH values of the precursor solution have great effects on the structural, morphological, and upconversion luminescent properties of Lu2O3:2%Yb3+, 0.2%Tm3+ nanocrystals. The as-formed lutetium oxide precursors could transform to cubic Lu2O3 with the same morphology and a slight shrinkage in size after a calcination process. The upconversion emission intensity of Lu2O3:2%Yb3+, 0.2%Tm3+ nanocrystals obtained from the precursor solution with pH = 9 is the strongest. The enhancement of the upconversion luminescence is suggested to be the consequence of reducing the number of OH- groups and the enlarged nanocrystals size. Strong blue and weak red emissions from the prepared nanocrystals were observed under 980 nm laser excitation, which were attributed to the 1G4 --> 3H6 and 1G4 --> 3F4 transitions of Tm3+ ion, respectively.

Luminescent properties of LuVO4:Eu3+,Bi3+ red phosphor for light-emitting diodes.

White light-emitting diodes have recently attracted great attention as promising candidates for next-generation lighting. The LuVO4:Eu3+,Bi3+ as new near-ultraviolet excited phosphors were synthesized via high-temperature solid-state reactions. The X-ray diffraction, excitation spectra, emission spectra and decay lifetimes of the phosphors were measured to characterize the structure and luminescent properties. With Bi3+ doping, the edge of excitation band corresponding to the Eu3+ emission shifts from 350 nm to 400 nm with the help of Bi(3+)-V5+ metal-metal charge transfer. Consequently, the phosphor exhibits efficient absorption of near-ultraviolet excitation, and it also exhibits excellent performance in emission intensity compared with the Y2O2S:Eu3+ phosphor in current use. This red-emitting material may be applied as a promising red phosphor for near-ultraviolet excited white light-emitting diodes.

Pr(3+)-doped beta-NaYF4 for temperature sensing with fluorescence intensity ratio technique.

Pure beta-NaYF4:0.8%Pr3+ powder sample was synthesized by the hydrothermal method. The temperature dependence of the fluorescence intensity ratio (FIR) of emission bands corresponding to the 3P1 --> 3H5 and 3P0 --> 3H5 transitions was measured in the temperature range of 120 K to 300 K excited by a 473 nm continuous wave (CW) laser. The dependence of the FIR on temperature is well fitted with an exponential function and the effective energy difference obtained is 457 cm(-1), which gives further an absolute temperature sensitivity of 0.01352 K(-1) at 300 K. The monotonous increase of FIR with temperature and high absolute temperature sensitivity demonstrate that this material can be used as temperature sensor. In addition, mono-dispersed NaYF4:1%Pr3+ nanoparticles were also synthesized.

Local field effect on the photoluminescent spectra of Eu3+ ions in glass.

To investigate the effect of the dielectric medium on the spontaneous emission rate of an isolated emitter, two series of glass samples of various compositions lightly doped with the Eu3+ ion were prepared by melt-quenching method. According to the enhancement factors for emission rates due to the refractive index of the dielectric medium, we qualitatively analyzed the intensities of the electric dipole and magnetic dipole transitions by comparing the emission spectra of the samples with different compositions, viz. various refractive indices. This preliminary result indicates that the local-field effect on the spontaneous emission rates follows the virtual-cavity model, which is derived by assuming that single-ion emitters enter the medium without disturbing the medium, i.e., as interstitial ions or by replacing host ions of low polarizability.

Synthesis and photoluminescent properties of NaYF4:Eu3+ core and NaYF4:Eu3+/NaYF4 core/shell nanocrystals.

NaYF4:Eu3+ core and NaYF4:Eu3+/NaYF4 core/shell nanocrystals (NCs) were synthesized via a wet chemical method. The transmission electron microscope photographs show that the core and core/shell nanoparticles are monodisperse and uniform NCs with average diameters of 22 and 26 nm respectively. The photoluminescence (PL) properties of the samples, including the PL excitation and emission spectra, and luminescent decay curves, are investigated in detail. The results show that the intensity of 5D2 emission relative to that of 5D0 is stronger in NaYF4:Eu3+/NaYF4 core/shell NCs than that in NaYF4:Eu3+ core NCs, and a longer decay lifetime of 5D2 is observed in core/shell samples. In addition, from the corrected emission spectra of 5D0, the 5D0 radiative lifetimes were calculated. These together with the measured decay lifetime of 5D0 emission give the intrinsic quantum yields of 5D0. The results were well interpreted by considering the surface effects.

Investigation of SrB4O7:Tm2+ luminescence for temperature imaging with high sensitivity based on time-resolved luminescence.

In recent years, with the growing demand for non-contact and real-time optical temperature measurements, it has become imperative to develop new luminescent thermometry materials as well as novel temperature detection schemes with higher sensitivity. In this work, a series of Sr1-xB4O7:xTm2+ (x = 0.001-0.01) polycrystalline powder samples were prepared using a high temperature solid state reaction under ambient atmospheric conditions. The emission spectra and the luminescence decay curves of the red emission corresponding to the 4f125d → 4f13(2F7/2) transition of Tm2+ were recorded at intervals of 10 K from 280 K to 380 K. Both the emission intensity and the fluorescence lifetime exhibited remarkable temperature correlation in the studied temperature range, which reached the best relative temperature sensitivities of 3.55% K-1 and 3.86% K-1 at 363 K and 346 K, respectively. Furthermore, taking into account the highly sensitive variation of the fluorescence lifetime of Tm2+ in the SrB4O7 matrix as the temperature increased near room temperature, a time-resolved temperature measurement scheme was performed to realize real-time temperature field imaging. By utilizing the intensified charge coupled device (ICCD) camera with a time gate to acquire the integral intensity of distinct time intervals and calibrate the relationship between the ratio and the temperature, the experimental results demonstrated that this temperature measurement scheme can achieve a maximum relative sensitivity of 9.39% K-1 at 313 K, which significantly surpasses the relative sensitivity of the other two conventional temperature measurement schemes.

Investigation of the luminescence properties and energy transfer mechanisms in Gd3TaO7:Bi3+,Eu3+ phosphors for their potential application in full-spectrum WLEDs.

In recent years, there has been increasing effort devoted to the development of single-phase white phosphors due to drawbacks such as severe reabsorption and color deviation in traditional white light-emitting diodes (WLEDs). A new feasible strategy has emerged for achieving white light emission through the Bi3+-Eu3+ energy transfer in suitable single-phase phosphors. Therefore, a series of Gd3TaO7:xBi3+ and Gd3TaO7:0.01Bi3+,yEu3+ phosphors were synthesized via a high-temperature solid-state method, and their properties were systematically characterized. In Gd3TaO7, Bi3+ occupies two kinds of Gd3+ site, resulting in two broad emission bands peaking at 427 nm and 500 nm respectively under ultraviolet (UV) excitation, which arise from 3P1 → 1S0 transitions. By adjusting the concentration of Eu3+ in Gd3TaO7:0.01Bi3+,yEu3+, effective energy transfer can occur between Bi3+ and Eu3+, thus enabling the regulation of green-white-red luminescence under 332 nm excitation and blue-white-red luminescence under 365 nm UV light irradiation. Upon stimulation with a 365 nm UV chip, Gd3TaO7:0.01Bi3+,0.02Eu3+ emits white light with CIE coordinates of (0.3509, 0.3202), a color temperature of 4629 K, and an impressive color rendering index of 87.96. The above results indicate the potential of Gd3TaO7:0.01Bi3+,yEu3+ phosphor as a viable candidate for WLED applications.