Luminescence properties of Pr^(3+) and Bi^(3+) co-doped NaCaTiNbO_6 phosphor for red-LEDs

This paper investigates the photoluminescence properties of NaCaTiNbO6：Pr^3＋ and NaCaTiNbO6：Pr^3＋,Bi^3＋ phosphors. NaCaTiNbO6：Pr^3＋ and NaCaTiNbO6：Pr^3＋,Bi^3＋ powder were synthesized successfully by solid state reaction method. Phase purity was checked using X-ray powder diffractometry（XRD）. The excitation and emission spectra were recorded to elucidate the photoluminescence properties of NaCaTiNbO6：Pr^3＋ and NaCaTiNbO6：Pr^3＋,Bi^3＋. Furthermore,fluorescence lifetime measurements were performed. The excitation spectra of NaCaTiNbO6：Pr^3＋ show a main band centered at around 357 nm.The luminescence spectra of NaCaTiNbO6：Pr^3＋ exhibit a red emission peak at 615 nm from the ^1 D2→^3 H4 transition of Pr^3＋ ions. With the introduction of the Bi^3＋ ion into NaCaTiNbO6：Pr^3＋, the luminescence intensity is enhanced nearly two times. Meanwhile,the absorption band edge of NaCaTiNbO6：Pr^3＋ is shifted from 380 to 420 nm. Thus, this study shows that the red phosphor NaCaTiNbO6：Pr^3＋ incorporated with Bi^3＋ is advantageous for light-emitting diode applications.

A novel white emitting phosphor Ca_2PO_4Cl:Dy^(3+): luminescence, concentration quenching and thermal stability

A novel white emitting phosphor Ca2PO4Cl：Dy^3＋ was synthesized by a solid state method. The luminescence, concentration quenching and thermal stability of Ca2PO4Cl：Dy^3＋ were investigated. Ca2PO4Cl：Dy^3＋ showed three emission peaks, which were located at 483, 575 and 660 nm. Though the ratio of yellow to blue emission intensities showed a similar value, the intensities of yellow and blue peaks were influenced by Dy^3＋ concentration, and the concentration quenching effect was observed. The emission intensity of Ca2PO4Cl：Dy^3＋ as a function of temperature was explored and the emission intensity（at 150 °C） of Ca2PO4Cl：Dy^3＋ was 90.0% of the value at 25 °C, and activation energy was 0.18 eV. The results indicated that Ca2PO4Cl：Dy^3＋ might be conducive to development of white LEDs.

Synthesis and luminescence of β-SrGe(PO4)2：RE(RE=Eu^2＋,Eu^3＋,Tb^3＋) phosphors for UV light-emitting diodes

A new series of β-Sr Ge（PO_4）_2：RE（RE=Eu^2＋,Eu^3＋,Tb^3＋） phosphors were synthesized and characterized by using X-ray powder diffraction as well as excitation, and emission spectroscopy. The results exhibited that the singly doping Eu2＋, Tb^3＋ and Eu^3＋ of β-Sr Ge（PO_4）_2 emit strong blue, green and red light under UV irradiation, respectively. Based on the charge transfer transitions of O^2-→RE^3＋, an overlapping excitation band of the as-obtained phosphors could be found in UV region, which made β-Sr Ge（PO_4）_2：RE（RE=Eu^2＋,Eu^3＋,Tb^3＋） serve as a new series of RGB phosphors. Meanwhile, these phosphors could also be excited by 380 nm excitation simultaneously, and hence the three phosphors mixed physically could achieve the tunable hues from blue to white region by adjusting the mixed ratios.

Origin of the red luminescence in Sr_3Al_2O_6:Eu phosphor ——From the synergetic effects of Eu^(2+) and Eu^(3+)

In order to uncover the real origin of red luminescence from Sr_3Al_2O_6：Eu and the physical mechanisms that were involved in the dynamical process of luminescence, variant amount of Eu and Dy activated Sr_3Al_2O_6 phosphors were synthesized with the solid-state reaction and the combustion-assisted solid-state reaction, respectively, using the fine graphite powder or the mixed H^2＋N_2gases as a reducing agent. The phase was examined with XRD analysis and the photoluminescence properties were characterized by a fluorescence spectrometer. Although the phosphors possessed the same Sr_3Al_2O_6 phase, different emission colors（red or green） were obtained, relying on synthesis conditions. The simultaneous existence of Eu^2＋ and Eu^3＋ was not only observed in the emission and excitation spectra, but also identified with the near edge X-ray absorption fine structure spectroscopy（NEXAFS）.The mixed valence（higher than ＋2 and less than ＋3） of Eu may be related with the six different sites of Sr, whose effective valence ranged from ＋1.5058 to ＋2.2698, in the crystal lattice of Sr_3Al_2O_6 that could accommodate Eu. Moreover, the reduction of Eu^3＋ to forming Eu^2＋ depended on the amount of Eu^3＋ or Dy^3＋ doped, due to the different energy barrier in each site of Sr that Eu had to overcome. The residual Eu^3＋, similar to the doped Dy^3＋, played an important role in supplying the hole for Eu^2＋ to form a bound trap（Eu^2＋）＊ after excitation. During electron returning to the 4f^7 ground state of Eu^2＋, the red luminescence was radiated. Therefore, the synergetic effects of Eu^2＋ and Eu^3＋（Dy^3＋） produce red luminescence.

Luminescence properties of Mn^(2+) ions doped Lu_2CaMg_2Si_3O_(12) garnet phosphors

Yttrium aluminum garnet structure phosphors Lu2CaMg2Si3O12：Mn2＋ were synthesized by conventional high temperature solid-state reaction in reductive atmosphere. The structure and optical properties of samples were characterized by application of powder X-ray diffraction （XRD） and photoluminescence spectroscopy. Results of X-ray diffraction （XRD） analysis showed that the phosphors mainly presented garnet structure with a few weak peaks of impurity phases. Lu2-xCaMg2Si3O12：xMn2＋ （x=0.01-0.8） phosphors showed a broad emission band peaking at around 590 nm under ultraviolet （UV） light of 408 nm when Mn2＋ concentration was less than 0.08 mol. With an increase in the Mn2＋ concentration （above 0.08）, another broad emission band peaking at 720 nm besides 590 nm was observed, which may be due to manganese ion having different valence and occupying different host lattice. The critical quenching concentrations of manganese ion in the wavelength of 590 and 720 nm were about 0.06 and 0.2 mol, respectively. With 408 nm excitation wavelength, emission color of the samples had a red shift trend as the Mn2＋ concentration increased. All the results indicated that the Lu2CaMg2Si3O12：Mn2＋ phosphors could be applicable to n-UV based white LEDs.

Synthesis and photoluminescence properties of ZnTiO_3:Eu^(3+) red phosphors via sol-gel method

ZnTiO3：Eu3＋ phosphors were synthesized with different concentrations of Eu3＋ doping through sol-gel method. The samples were calcined at different temperatures for 2 h in air. The synthesized powders were characterized by X-ray diffraction（XRD）, scanning electron microscopy-energy dispersive spectroscopy（SEM-EDS）, transmission electron microscopy（TEM）, Raman and photoluminescence spectroscopy. The XRD results showed that the Zn Ti O3：Eu3＋ phosphors doped with different concentrations of Eu3＋ ions calcined at 600 oC were of single phase, which indicated that the Eu3＋ ions had been successfully incorporated into the Zn Ti O3 host lattice and did not destroy the lattice structure of Zn Ti O3 host. The Raman spectrum, SEM and TEM also proved that the doping of Eu3＋ did not change the lattice structure of hexagonal Zn Ti O3 host. The photoluminescence（PL） of Eu3＋ ions with the main emission peak at 614 nm was observed to increase with Eu3＋ concentrations from 0.5 mol.% to 2.0 mol.% and decreased when the concentration was increased to 2.5 mol.%. The decrease in the PL intensity at higher Eu3＋ concentrations could be associated with concentration quenching effect. The CIE1931 chromaticity diagram（x, y） of Zn Ti O3：2.0 wt.%Eu3＋ phosphors were located in the red region（x=0.652, y=0.347）. The luminescence properties suggested that Zn Ti O3：Eu3＋ phosphors might be regarded as a potential red phosphor candidate for light emitting diodes（LEDs）.

Luminescence properties of Eu^(2+),Gd^(3+) and Pr^(3+) doped translucent Sialon phosphors

Optical properties of hot pressed Sialon ceramics doped with different rare earth oxides（REOs） i.e. Eu2O3, Gd2O3, and Pr2O3 were investigated. The α-Sialon phase was the main phase obtained after sintering as observed by X-ray diffraction（XRD）. The transparency of different samples of varying thickness measured from UV to IR region revealed that the samples were translucent in the visible region while transparent in IR region. The thin samples of 150 μm thickness had transmittance as high as 30% in the visible region. The luminescence was observed in transmittance mode to investigate the effect of sample thickness on luminescence intensity. We observed blue, yellow and red emissions in Sialon doped with Gd2O3, Eu2O3, and Pr2O3, respectively. The excitation wavelength for Gd2O3 and Pr2O3 doped samples were in UV region i.e. 280 and 270 nm, respectively, whereas, for Eu2O3 doped samples was in the blue region（460 nm）. The Eu2O3 doped Sialon having 300 μm thickness showed better white light extraction as coupled with blue LED. Moreover, the fabricated phosphor samples exhibited high hardness around 20 GPa and fracture toughness above 5 MPa·m1/2.

Optically stimulated luminescence(OSL) of LiMgPO4：Tm,Tb phosphor

LiMgPO4：Tm,Tb phosphors were synthesized using a solid-state diffusion method. Their properties were investigated using X-ray diffraction（XRD）, thermoluminescence（TL）, scanning electron microscopy（SEM）, and OSL dosimetric techniques. The influence of light stimulation and thermal excitation on the TL and OSL, and the reusability of the phosphors for OSL regenaration were also studied. The LiMgPO4：Tm,Tb phosphor exhibited high sensitivity to ionizing radiation, good signal reusability and a broad linear dose response range（0.1–1000 Gy）. Fading of the OSL signal was about 16% in eight days, after that the intensity kept stable. The excellent luminescent and dosimetric properties of these LiMgPO4：Tm,Tb phosphors enable them to be promising candidates in radiation dosimetry.

Effect of Yb^(3+) concentration on upconversion luminescence of AlON:Er^(3+) phosphors

AlON：1.6 mol.%Er3＋, x mol.%Yb3＋（x=0, 2.6, 3.1, 3.6, 4.1, 4.6） phosphors were synthesized successfully by aluminothermic reduction and nitridation（ATRN） method and characterized by X-ray diffraction（XRD）, scanning electron microscopy（FESEM） and upconversion photoluminescence（UCPL） emission spectra. Under the excitation of diode laser 980 nm, the green（556 nm） and red（655 nm） upconverted emissions were observed, attributed to the 4S3/2→4I15/2 and 4F9/2→4I15/2 transition of Er3＋respectively. The emission intensity increased with increasing Yb3＋ concentration due to the energy transfer（ET） between Yb3＋ and Er3＋. The upconverted emission reached the highest as x=3.6, and was pump-power dependent involving a two-photon process.

Mechanisms and absolute quantum yield of upconversion luminescence of fluoride phosphors

Mechanisms of upconversion luminescence（UCL） of SrF2：Er phosphors corresponding to the 4G11∕2→4I15∕2,2H9∕2→4I15∕2,4F5∕2→4I15∕2,4F7∕2→4I15∕2,2H11∕2→4I15∕2,4S3∕2→4I15∕2,4F9∕2→4I15∕2, and 4I9∕2→4I15∕2 transitions upon excitation of the 4I11∕2 level of Er3＋ions were investigated. Energy transfer upconversion processes are responsible for the populating of the 2H9∕2,2H11∕2,4S3∕2, and 4F9∕2 levels. Cooperative process is the dominant mechanism of luminescence from 4S3∕2 and 4F9∕2 levels for SrF2：Er with high concentrations of Er3＋ions. The UCL from 4G11∕2 and 4F5∕2 is explained by excited-state absorption. Cross-relaxation processes take part in the population of 4F9∕2 and 4I9∕2 levels. For quantifying material performance, the Er3＋-concentration dependence of UCL and the absolute quantum yields of SrF2：Er were studied. The most intensive visible luminescence was obtained for SrF2：Er（14.2%） with 0.28% maximum quantum yield.

Interaction of Rare Earth Ions in Sr2MgSi2O7：Eu^2＋,Dy^3＋ Material

To discuss the function of Eu and Dy and their interaction in Sr2 Mg Si2O7：Eu^2＋,Dy^3＋ long afterglow material,the Eu and Dy single doped and their co-doped Sr2 Mg Si2O7：Eu^2＋,Dy^3＋ were prepared.The samples were characterized by X-ray diffraction（XRD）,decay curves,photoluminescence（PL）,and thermoluminescence（TL）.The results indicate that Sr2 Mg Si2O7：Eu has afterglow properties,and the doping of Eu ion in Sr2 Mg Si2O7：Eu^2＋,Dy^3＋ can lower the depth of traps.Eu ion can not only serve as luminescence center,but also produce traps in the matrix,meanwhile,it also exerts certain influences on the traps produced by Dy in Sr2 Mg Si2O7：Eu^2＋,Dy^3＋.The Dy ion can not act as luminescence center but relates to the change of the traps in the Sr2 Mg Si2O7 matrix.

Effect of RE^3+electronegativities on luminescence property of Ba5Si8O21:Eu^2+persistent phosphor

A series of Ba5Si8O（21）：0.02Eu^2＋,0.09RE^3＋ persistent phosphors were synthesized by the solid-state reaction method.The measurement results of photoluminescence（PL）,phosphorescence and thermoluminescence（TL）were analysed and discussed.The XRD results showed that samples codoped with different RE^3＋ were Ba5Si8O（21） single pure phase.Under the excitation,all samples exhibited a broad Eu^2＋ characteristic emission,and the La^3＋ co-doped sample emitted the brightest photoluminescence even though its persistent luminescence property was the worst because of the weakest electronegativity.However,Nd^3＋ electronegativity was suitable,thus after activation,the Ba5Si8O（21）：Eu^2＋,Nd^3＋ sample had the best persistent luminescence performance with the highest phosphorescence intensity and the persistent luminescence decay time beyond 8 h.The Nd^3＋ co-doped sample also had the largest thermoluminescence integral area which proved effectively it had longer persistent luminescence time.The luminescence mechanism was also proposed to study the photoluminescence and persistent luminescence process.These results showed that RE^3＋ electronegativities were distinctly important for persistent phosphors and choosing suitable electronegativity codopant was conducive to enhancing the phosphorescent performance.

Luminescence properties of tunable red-green emitting phosphor Ba2Ca(BO3)2：Eu3＋,Tb3＋

A series of Eu^(3+) or Tb^(3+) doped Ba_2Ca(BO_3)_2 phosphors were synthesized by a high temperature solid state method, and the luminescence properties are investigated. Ba_2Ca(BO_3)_2:Tb^(3+) can show an obvious green emission, and the peak locates at 551 nm, which corresponds to the 5D34→7F5 transition of Tb^(3+). Ba_2Ca(BO_3)_2:Eu+ can present the characteristic emission of Eu^(3+), and the peak locates at 600 nm, which is ascribed to the 5D70→F2 transition of Eu^(3+). In order to achieve the emission-tunable phosphors, the Eu^(3+)/Tb^(3+) co-doped Ba_2Ca(BO_3)_2 are synthesized. When tuning the Eu^(3+) or Tb^(3+) concentration, Ba_2Ca(BO_3)_2:Eu^(3+), Tb^(3+) can both show the tunable emission, which may be induced by the energy transfer from Tb^(3+) to Eu^(3+).

Synthesis and optical characterization of Eu^(2+),Tb^(3+)-codoped Sr_3Y(PO_4)_3 green phosphors

A series of Eu^2＋,Tb^3＋-codoped Sr3 Y（PO4）3（SYP） green phosphors were synthesized by hightemperature solid-state reaction. Several techniques, such as X-ray diffraction, UV-vis spectrum,and photoluminescence spectrum, were used to investigate the obtained phosphors. The present study investigates in detail photoluminescence excitation and emission properties, energy transfer between the two dopants, and effects of doping ions on optical band gap. SYP：0.05 Eu2＋ phosphor shows an intense and broad excitation band ranging from 220 to 400 nm and exhibits a bright green emission band with CIE chromaticity coordinates（0.189, 0.359） under 350 nm excitation. Green emission of SYP：0.03 Tb3＋ is intensified by codoping with Eu^2＋, and energy transfer mechanism between them is demonstrated to be a dipole-dipole interaction. Upon 350 nm excitation, SYP：Eu^2＋,Tb^3＋ phosphors exhibits two dominating bands peaking at 466 and 545 nm, which are assigned to 4 f^65 d^1→4 f^7 transition of Eu^2＋ ions and ~5 D4→~7 F5 transition of Tb^3＋ ions, respectively. Optimal doping concentrations of Eu^2＋ and Tb^3＋ in the SYP host are 5 mol% and 15 mol%, respectively. Results indicate that SYP：Eu^2＋,Tb^3＋ phosphors are potentially used as green-emitting phosphors for white light-emitting diodes.

Synthesis and thermometric properties of Yb^3＋-Er^3＋ co-doped K2GdF5 up-conversion phosphors

Yb^3＋-Er^3＋ co-doped K2GdF5 up-conversion phosphor was successfully synthesized by a solid-state reaction method. The phase purity and structure of the sample were characterized by powder X-ray diffraction. The sample emitted orange light at room temperature and its up-conversion spectra at different temperatures were recorded under the excitation of a 980 nm diode laser. The energy transfer from Yb^3＋ to Er^3＋ notably enhanced the up-conversion luminescence intensity. The possible up-conversion mechanisms and processes were proposed based on the power dependence of the luminescence intensities. The temperature-dependent up-conversion luminescence and temperature sensing performances of the sample were discussed according to the fluorescence intensity ratio of green emissions originating from ~2H（11/2）/~4S（3/2）→~4I（15/2） transitions of Er^3＋ in the range from 307 K to 570 K under the excitation of 980 nm laser with power of 260 mW. The dependence of the fluorescence intensity ratio on temperature was fitted with an exponential function and the effective energy difference obtained was 690 cm^（–1）, which further gave a relative temperature sensitivity of 1.1%/K at 307 K. The results suggested that the Yb^3＋-Er^3＋ co-doped K2GdF5 sample is a promising candidate for optical temperature sensor.

White light emission in Pr^(3+),Tb^(3+):CaYAlO_4 phosphor

In this work, the synthesis and photoluminescence characteristics of two new phosphors Pr3+: CaYAlO4(CYA) and Pr3+/Tb3+: CYA for light emitting diodes(LEDs) are investigated. 0.5%(atom percentage) Pr3+: CYA exhibits the largest bright yellow emission by varying the Pr3+ concentration, owing to the cross-relaxation process of 3P0 +3H4→ 1G4+1G4. The energy level diagram in Pr3+: CYA, especially the positions of 4f5 d level and 1S0 level, is discussed. By co-doping Tb3+ ions, the color coordinates of Pr3+/Tb3+: CYA phosphor can be tuned from yellow to white region. Finally, the strongest luminescence emission with color coordinates of(0.339, 0.364) located in the white region can be obtained in 0.3%Tb3+/0.5%Pr3+: CYA phosphor.

Visible to ultraviolet up-conversion in a novel rare earth-free up-converting phosphor

A novel up-converting phosphor,Na_xAl_yF_（x＋3y）/CaF2（0.3 〈 x 〈 0.5,0 〈 y 〈 0.1） composite,is prepared by a hydrothermal process at 180°C and an annealing treatment at 500℃.X-ray diffraction（XRD） pattern indicates that Na_xAl_yF_（x＋3y）/CaF_2 is a composite consisting of NaF,Na_3 AlF_6,and CaF_2.The up-conversion luminescence properties of this phosphor are investigated by fluorescence spectrometer.Results show that Na_xAl_yF_（x＋3y）/CaF_2 exhibits visible-to-ultraviolet up-conversion luminescence properties.The emission peaks at 316 and 336 nm are observed when excited by 610 nm,the luminescence lifetime of the emissions is great than 18 ms,and the internal fluorescence quantum yield is 4.2%.Additionally,the luminescent mechanism of this phosphor is also mentioned.

Molten salt synthesis and luminescent properties of YVO_4:Eu nanocrystalline phosphors

YVO4：Eu nanocrystalline phosphors were successfully prepared at 400 oC in equal moles of NaNO3 and KNO3 molten salts. NaOH concentration and annealing temperature played important roles in phase purity and crystallinity of the nanocrystallines, and the optimum NaOH concentration and annealing temperature were 6：40 and 400 oC, respectively. The nanocrystallines were well crystallized with a cubic morphology in an average grain size of 18 nm. Upon excitation of the vanadate groups at 314 nm, YVO4：Eu nanocrystallines exhibited the characteristic emission of Eu3＋, which indicated that there was an energy transfer from vanadate groups to Eu3＋. Moreover, the influence of superficial effect, especially the dangling bonds on the structure and luminescent properties of the nanocrystallines was discussed in detail.

White light-emitting Ba0.05Sr0.95WO4:Tm3+Dy3+phosphors

Tm3＋ and Dy3＋） co-doped Ba（0.05）Sr0.95WO4 phosphors were synthesized by a low temperature combustion method. The structures of the samples were SrWO_4 phase and were identified by X-ray diffraction. The surface topographies of Ba_（0.05）Sr_（0.91）WO_4：0.01 Tm^（3＋） 0.03 Dy^（3＋） were tested by scanning electron microscopy. The particles are ellipsoid, and their average diameter is approximately 0.5 μm. The emission spectra of Ba_（0.05）Sr_（0.95）WO_4：Tm^（3＋） show a peak at 454 nm which belongs to the ~3 H_6→~1 D_2 transition of Tm^（3＋）, and the optimum doping concentration of Tm^（3＋） ions was 0.01. The emission spectra of Ba_（0.05）Sr_（0.95）WO_4：Dy^（3＋） consist of the ~4 F_（9/2）→~6 H_（13/2） dominant transition located at 573 nm, the weaker ~4 F_（9/_2→~6 H_（15/2） transition located at 478 and 485 nm. And the weakest ~4 F_（9/2）→~6 H_（11/2） transition located at660 nm, and the optimum doping concentration of Dy^（3＋） ions was 0.05. A white light is achieved from Tm^（3＋） and Dy^（3＋） co-doped Ba_（0.05）Sr_（0.95）MoO_4 crystals excited at 352-366 nm. With the doping concentration of Tm^（3＋） fixed at 0.01, the luminescence of Ba_（0.05）Sr_（0.95）MoO_4：Tm^（3＋）Dy^（3＋） is closest to standard white-light emissions when the concentration of Dy^（3＋） is 0.03; the chromaticity coordinates are（0.321,0.347）, and the color temperature is 6000 K.

Facile room-temperature coprecipitation of uniform barium chlorapatite nanoassemblies as a host photoluminescent material

As an emerging host phosphor material, barium chlorapatite （Bas（PO4）3CI）, is attracting growing attention. However, rare earth-doped Bas（PO4）3CI phosphors have mainly been obtained via high temperature-based, energy-consuming techniques. In this contribution, we developed a straight- forward, facile room-temperature coprecipitation method in the presence of a specific amount of ethylenediaminetetraacetic acid disodium salt that provided Bas（PO4）3CI nanoparticles self-assembled to construct uniform Bas（PO4）3CI nanoassemblies （diameter： 80-120 nm） as well as rare earth Tb3＋- doped Bas（PO4）3CI：xTb3＋ nanophosphors. The nanoassemblies were transparent within the ultraviolet and visible spectral range. The Bas（PO4）3CI：xTb3＋ nanophosphors exhibited four emission peaks under 228-nm excitation, and the optimal doping amount of Tb3＋ was 4.0%. In contrast to traditional energy-consuming, high-temperature techniques, the facile room-temperature coprecipitation method developed here represents an attractive alternative route to obtain uniform Ba5（PO4）3CI nanoassemblies and Bas（PO4）3CI：xTb3＋ nanopbosphors that are candidate luminescent hosts.