White light emitting from YVO4/Y2O3:Eu3+,Bi3+ composite phosphors for UV light-emitting diodes

A series of (1−x)YVO₄/xY₂O₃:Eu³⁺_0.006,Bi³⁺_0.006 (0≤x≤0.54) composite phosphors was synthesized in one step by high temperature solid state reaction and the photoluminescence properties were investigated. By means of co-doping Eu³⁺ and Bi³⁺ ions into the composite matrices composed of YVO₄ and Y₂O₃ crystals, the YVO₄/Y₂O₃:Eu³⁺,Bi³⁺ phosphor exhibits simultaneously the blue (418 nm), green (540 nm) and orange-red (595, 620 nm) emissions. The broad blue and green emissions are attributed to the ³P₁–¹S₀ transitions of Bi³⁺ ion both in Y₂O₃ and in YVO₄ matrices. Moreover, the sharp orange-red emissions are attributed to the ⁵D₀–⁷F_1,2 transitions of Eu³⁺ ion in YVO₄ matrix. By tuning the mole ratio of YVO₄/Y₂O₃ matrices the white light-emitting could be obtained. The results indicated that when the mole ratio of Y₂O₃ (x) is at 0.11–0.54 mol, the (1−x)YVO₄/xY₂O₃:Eu³⁺_0.006,Bi³⁺_0.006 phosphors emit white light by combining the blue, green and orange-red emissions under the excitation of 360–370 nm wavelength which matches the emission of the commercial UV-LED diode. This implies that the phosphors may be the promising white light materials with broad absorption band for white light-emitting diodes.     

Europium and erbium co-doped yttria-stabilized zirconia as a potential dual-sensor for simultaneous oxygen partial pressure and temperature measurements

In this study, the phosphorescence properties of Eu³⁺ and Er³⁺ co-doped yttria stabilized zirconia (YSZ: Eu³⁺, Er³⁺) as a function of oxygen partial pressure and temperature are investigated to explore the possibility of using this phosphor for pressure and temperature measurements at elevated temperature (up to 600 °C). The YSZ: Eu³⁺, Er³⁺ phosphors have a fixed Eu³⁺ concentration (1 mol%) and a varied Er³⁺ concentration (0.5–5 mol%). The phosphorescence intensity and lifetime of Eu³⁺ bands are sensitive to both oxygen partial pressure and temperature when temperature exceeds 400 °C. This oxygen/pressure sensitivity is related to the oxygen vacancy-induced non-radiative decay through charge transfer state (CTS), and is inversely correlated with Er³⁺ concentration. In contrast, the phosphorescence intensity and lifetime of Er³⁺ bands are temperature-sensitive but oxygen-insensitive, which is related to the dominant oxygen-independent ⁴S_3/2 → ²H_11/2 transition. Accordingly, two measurement strategies are proposed including an intensity ratio method and a lifetime method. The intensity ratio of the oxygen-sensitive Eu³⁺ bands and the oxygen-insensitive Er³⁺ reference bands is capable of 2D pressure measurement using a dual-camera system. But temperature-induced errors exist due to the temperature-sensitivity of intensity ratio. Simultaneous pressure and temperature measurements (point by point) can be achieved utilizing two PMTs that record the lifetimes of Eu³⁺ and Er³⁺, respectively. This method can resolve the issue of temperature sensitivity of Eu³⁺ and accurately correct the temperature-induced errors. The YSZ: Eu, Er phosphors show great potential for applications in high-temperature environment where the existing organic PSPs and TSPs cannot survive.     

High luminescent brightness and thermal stability of red emitting Li3Ba2Y3(WO4)8:Eu3+ phosphor

A series of Li₃Ba₂Y_3−x(WO₄)₈:xEu³⁺ (x=0.1, 1, 1.5, 2 and 2.8) phosphors were synthesized by a high temperature solid-state reaction method. Under the excitation of near ultraviolet (NUV) light, the as-prepared phosphor exhibits intense red luminescence originating from the characteristic transitions of Eu³⁺ ions, which is 1.8 times as strong as the commercial Y₂O₂S:Eu³⁺ phosphor. The optimal doping concentration of Eu³⁺ ions here is confirmed as x=1.5. The electric dipole-quadrupole (D-Q) interaction is deduced to be responsible for concentration quenching of Eu³⁺ ions in the Li₃Ba₂Y₃(WO₄)₈ phosphor. The analysis of optical transition and Huang-Rhys factor reveals a weak electron-phonon coupling interaction. The temperature-dependent emission spectra also indicate that the as-prepared Li₃Ba₂Y₃(WO₄)₈:Eu³⁺ phosphor has better thermal stability than that of the commercial Y₂O₂S:Eu³⁺ phosphor. Therefore, our results show that the as-prepared Li₃Ba₂Y₃(WO₄)₈:Eu³⁺ phosphor is a promising candidate as red emitting component for white light emitting diodes (LEDs).     

A novel single-phase Na3.6Y1.8(PO4)3:Bi3+,Eu3+ phosphor for tunable and white light emission

A new type of Bi³⁺,Eu³⁺ single- and co-doped Na_3.6Y_1.8(PO₄)₃ phosphate phosphors were manufactured using conventional high-temperature solid-state reaction technique to explore their application for solid-state lighting. The crystal structure, luminescent properties, luminescent mechanism and quantum efficiency were thoroughly explored. Results show that there are two crystallization sites for Bi³⁺ and Eu³⁺ ions. Upon the excitation of 342 and 373 nm, Bi³⁺ single-doped phosphors exhibit green and blue emission, derived from the ³P₁ to ¹S₀ transition of Bi³⁺ located in different occupancy sites. Thanks to radiative energy transfer process from Bi³⁺ to Eu³⁺, adjustable emission could be acquired by altering Eu³⁺ content in co-doped phosphors. Pure white-light emission with quantum efficiency value of 22.9% can be realized in Na_3.6Y_1.8(PO₄)₃:0.01Bi³⁺,0.1Eu³⁺ sample and the integrated intensity of white light emission at 417 K remains 85% of that at room temperature. Our results indicate that Na_3.6Y_1.8(PO₄)₃:Bi³⁺,Eu³⁺ phosphors have feasible application in high-power ultraviolet driven solid-state lighting.     

Tunable white light and energy transfer of Dy3+ and Eu3+ doped Y2Mo4O15 phosphors

A series of Dy³⁺ or/and Eu³⁺ doped Y₂Mo₄O₁₅ phosphors were successfully synthesized at a low temperature of 600 °C via solid state reaction. The as-prepared phosphors were characterized by X-ray powder diffraction (XRD), scanning electronic microscope (SEM), photoluminescence (PL) excitation, emission spectra and PL decay curves. XRD results demonstrate that Y₂Mo₄O₁₅: Dy³⁺, Eu³⁺ has the monoclinic structure with the space group of p21/C(14). Under the excitation of ultraviolet (UV) or near-UV light, the Dy³⁺ and Eu³⁺ ions activated Y₂Mo₄O₁₅ phosphors exhibit their characteristic emissions in the blue, yellow and red regions. The emitting light color of the Y₂Mo₄O₁₅: 0.08Dy³⁺, yEu³⁺ phosphors can be adjusted by varying the concentration ratio of Dy³⁺ to Eu³⁺ ions and a white light is achieved when the doping concentration of Eu³⁺ is 5%. In addition, the energy transfer from Dy³⁺ to Eu³⁺ is also confirmed based on the luminescence spectra and decay curves.     

Tunable color emission in LaScO3:Bi3+,Tb3+,Eu3+ phosphor

LaScO₃:xBi³⁺,yTb³⁺,zEu³⁺ (x = 0 − 0.04, y = 0 − 0.05, z = 0 − 0.05) phosphors were prepared via high-temperature solid-state reaction. Phase identification and crystal structures of the LaScO₃:xBi³⁺,yTb³⁺,zEu³⁺ phosphors were investigated by X-ray diffraction (XRD). Crystal structure of phosphors was analyzed by Rietveld refinement and transmission electron microscopy (TEM). The luminescent performance of these trichromatic phosphors is investigated by diffuse reflection spectra and photoluminescence. The phenomenon of energy transfer from Bi³⁺ and Tb³⁺ to Eu³⁺ in LaScO₃:xBi³⁺,yTb³⁺,zEu³⁺ phosphors was investigated. By changing the ratio of x, y, and z, trichromatic can be obtained in the LaScO₃ host, including red, green, and blue emission with peak centered at 613, 544, and 428 nm, respectively. Therefore, two kinds of white light-emitting phosphors were obtained, LaScO₃:0.02Bi³⁺,0.05Tb³⁺,zEu³⁺ and LaScO₃:0.02Bi³⁺,0.03Eu³⁺,yTb³⁺. The energy transfer was characterized by decay times of the LaScO₃:xBi³⁺, yTb³⁺, zEu³⁺ phosphors. Moreover absolute internal QY and CIE chromatic coordinates are shown. The potential optical thermometry application of LaScO₃:Bi³⁺,Eu³⁺ was based on the temperature sensitivity of the fluorescence intensity ratio (FIR). The maximum S_a and S_r are 0.118 K⁻¹ (at 473.15 K) and 0.795% K⁻¹ (at 448.15 K), respectively. Hence, the LaScO₃:Bi³⁺,Eu³⁺ phosphor is a good material for optical temperature sensing.     

Gel-combustion assisted synthesis of eulytite-type Sr3Y(PO4)3 as a single host for narrow-band Eu3+ and broad-band Eu2+ emissions

A series of eulytite-type Sr₃Y_1-x(PO₄)₃:xEu³⁺ (x = 0–0.13) and Sr_3-yY(PO₄)₃:yEu²⁺ (y = 0–0.10) phosphors were successfully synthesized via gel-combustion and subsequent calcination in O₂ and Ar/H₂ atmospheres at 1250 °C, respectively. Detailed crystal structure analysis via Rietveld refinement showed that the phosphors were crystallized in the cubic system (space group I-43d, No. 220), in which the Eu³⁺ and Eu²⁺ activators reside at the Y³⁺ and Sr²⁺ sites, respectively. The trivalent Eu³⁺ ions (CN = 6) exhibited typical narrow-band luminescence via intra-4f⁶ transitions, with the red emission at ~ 615 nm being dominant (⁵D₀ → ⁷F₂ transition, FWHM = 15.9 ± 0.2 nm). The divalent Eu²⁺ ions (CN = 6 and 9) showed broad-band luminescence ranging from light-blue to blue via 4f⁶5d¹ → 4f⁷ transitions (FWHM = 115 ± 2 nm). The optimal Eu³⁺ and Eu²⁺ concentrations were determined to be 10 at% (x = 0.10) and 7 at% (y = 0.07), respectively, and the mechanisms of concentration quenching were discussed. The excitation/emission properties, fluorescence decay kinetics, CIE chromaticity, and particularly the rarely addressed thermal stability of the phosphors were investigated in detail.     

Enhanced luminescence properties of Eu3+ in La2MoO6 by nonsensitization of Bi3+

It was unusual for Bi³⁺ ions to enhance the emission intensity of phosphors via nonsensitization. Here, La₂MoO₆:Eu³⁺, Bi³⁺ phosphors were successfully synthesized by a high temperature solid-state reaction method in air atmosphere. As the increase of doping concentration of Bi³⁺, the emission spectra of La₂MoO₆:Eu³⁺, Bi³⁺ phosphors had obvious shifts, splits and the enhancement of intensities, which indicated that the characteristics of the phosphors were modified. To analyze these phenomena, the crystal structure refinements, spectral characteristic analyze and Judd-Ofelt theoretical calculation were mainly performed. Bi³⁺ ions played the role of the nonsensitizer and affected the distortion of the crystal, the sites of Eu³⁺ ions, the field splitting energy and the internal quantum yield. Moreover the nephelauxetic effects of Bi³⁺ ions and the ET process caused synergistically the life times of La₂MoO₆:Eu³⁺, Bi³⁺ phosphors to increase and then gradually decrease. The CIE coordinates of phosphors changed within a small range. This study might be instrumental in promoting the further application of Bi³⁺ ions in rare earth luminescent materials.     

Energy transfer and color-tunable emission in Ba2Y2Si4O13:Bi3+,Eu3+ phosphors

Single-composition Ba₂Y₂Si₄O₁₃:Bi³⁺,Eu³⁺ (BYSO:Bi³⁺,Eu³⁺) phosphors with color-tunable and white emission were prepared by conventional high temperature solid-state reaction method. The structural and luminescent properties of these phosphors were thoroughly investigated through X-ray diffraction, photoluminescence, and decay curves. BYSO:Bi³⁺ phosphors show two excitation peaks at 342 and 373 nm, and give two emission peaks at 414 and 503 nm, respectively, indicating that there are two sites of Bi³⁺ in BYSO. The energy transfer from Bi³⁺ to Eu³⁺ was investigated in detail. Varied hues from blue (chromaticity coordinate [0.219, 0.350]) to white (0.288, 0.350) and orange-red light (0.644, 0.341) can be generated by adjusting the content of Eu³⁺. Pure white light emission (0.311, 0.338) can be obtained under the excitation of 355 nm in BYSO:3%Bi³⁺,20%Eu³⁺ phosphor. Besides, BYSO:Bi³⁺,Eu³⁺ phosphors exhibit distinct thermal quenching properties, whose emission intensity at 473 K is 82.6% of that at 298 K. Our results indicate that BYSO:Bi³⁺,Eu³⁺ may be applied as conversion phosphors for n-UV-based W-LEDs.     

Red,green and blue-violet emission coexistence in white-light-emitting Ca3SiO4Cl2:Bi3+, Li+, Eu2+, Eu3+ phosphor

A series of emission-tunable Ca₃SiO₄Cl₂:Bi³⁺, Li⁺, Euⁿ⁺(n =2, 3) (CSC:Bi³⁺, Li⁺, Euⁿ⁺) phosphors have been synthesized via sol-gel method. The X-ray diffraction results indicate that the as-synthesized phosphors crystallize in a low temperature phase with the space group of P21/c. Energy transfer from Bi³⁺ to Eu³⁺/Eu²⁺ exists in CSC:Bi³⁺, Li⁺, Euⁿ⁺ phosphors. Under the excitation of 327 or 365 nm, the Ca_2.98−ySiO₄Cl₂:0.01Bi³⁺, 0.01Li⁺, yEuⁿ⁺(y=0.0001–0.002) phosphors show an intense green emission band around 505 nm, while under the excitation of 264 nm, three emission bands centered around 396 nm (Bi³⁺), 505 nm (Eu²⁺) and 614 nm (Eu³⁺) are observed and tunable colors from blue-violet to green or white are achieved in these phosphors by varying the content of Eu. White-light emission with the color coordinate (0.312, 0.328) is obtained in Ca_2.978SiO₄Cl₂:0.01Bi³⁺, 0.01Li⁺, 0.002Euⁿ⁺(n =2, 3). Based on these results, the as-prepared CSC:Bi³⁺, Li⁺, Eu²⁺, Eu³⁺ phosphors can act as color-tunable and single-phase white emission phosphors for potential applications in UV-excited white LEDs.     

Preparation and luminescent modulation of KGaSiO4:Eu3+ phosphors using for multiple anti-counterfeiting

Anti-counterfeiting technology is of great significance to information security. To obtain high-quality anti-counterfeiting materials, the developments of inorganic materials are crucial. In this paper, a series KGaSiO₄:xEu³⁺ phosphors with persistent luminescence, photoluminescence, and thermochromic have been successfully prepared and the application of quadruple anti-counterfeiting is realized. The X-ray diffraction and Rietveld refinement indicate that the phosphors are pure phase. With Eu³⁺ ions doping, the structure change, site occupancies, and color-tunable phenomenon are carefully investigated. Different from another Eu³⁺ doping phosphor, the emission of KGaSiO₄:0.2% Eu³⁺ phosphor changes with the excitation light in the region of 240 nm–306 nm. The emission color can be modulated with the surrounding temperature. Surprisingly, this phosphor can emit green afterglow light, which is attributed to the different luminescent properties of the matrix and doping of Eu³⁺ ions. The series of phosphors exhibit abundant luminescent properties. Based on their wavelength dependence, concentration quenching, long afterglow, and thermochromic properties, the KGaSiO₄:xEu³⁺ phosphors can be effective materials for quadruple-modal anti-counterfeiting devices.     

A novel scheelite-type LiCaGd(WO4)3:Eu3+ red phosphors with prominent thermal stability and high quantum efficiency

A series of LiCaGd(WO4)3 : xEu³⁺ (0 ≤ x ≤ 1.0) red phosphors with tetragonal scheelite structure were synthesized via the conventional solid-state reaction. Their crystal structure, photoluminescence excitation (PLE), and photoluminescence (PL) spectra, thermal stability and quantum efficiency were investigated. The phosphors exhibit a typical red light upon 395 nm near ultraviolet excitation, and the strongest emission peak at 617 nm is dominated by the ⁵D₀→⁷F₂ transition of Eu³⁺ ions. The PL intensity of the phosphors gradually increases with the increase of Eu³⁺ doping concentration, and the concentration quenching phenomenon is hardly observed. The quantum efficiency and the color purity of the phosphor reach maximum values of about 94.2 and 96.6% at x = 1.0, respectively. More importantly, LiCaGd(WO₄)₃:xEu³⁺ phosphors have prominent thermal stability. The temperature-dependent PL intensity of the phosphors at 423 K is only reduced to 89.1% of the PL intensity at 303 K, which is superior to that of commercial red phosphors Y₂O₃:Eu³⁺. Finally, LiCaGd(WO₄)₃:Eu³⁺ phosphor is packaged with near ultraviolet InGaN chips to fabricate white light emitting diodes, which has a low color temperature (CCT = 4622 K) and a high color rendering index (CRI= 89.6).     

Novel color tunable LaCaGaO4:Bi3+,Eu3+ phosphors for high color rendering warm white LEDs

A series of LaCaGaO₄:xBi³⁺,yEu³⁺ (x = 0.002–0.04, y = 0.02–0.45) phosphors with adjustable emission colors were synthesized by high-temperature solid-state reaction. The samples were identified as pure phases by X-ray diffraction and Rietveld refinement, and the crystal structures were analyzed in detail. The LaCaGaO₄:xBi³⁺ phosphor shows an intense blue emission under near-ultraviolet excitation, originating from the ³P₁ → ¹S₀ transition. The spectrum analysis reveals that the Bi³⁺ ions occupy two luminescence centers in the LaCaGaO₄ host and that energy transfer can occur. A model of the energy transfer between the Bi³⁺ and Eu³⁺ ions was also created and studied in detail. As the Eu³⁺-concentration increased, the emission color of the LaCaGaO₄:0.005Bi³⁺,yEu³⁺ phosphor changed from blue to pink to red. In addition, the fluorescence lifetime, quantum yield, thermal stability, and other properties of the phosphors were characterized and analyzed. Finally, two white light-emitting diode devices with R_a values of 96.6 and 95 and correlated color temperatures of 4578 and 3324 K were fabricated, indicating the potential of phosphors for warm white lighting applications.     

New single‐component multicolor emission Na1−xAl1+2xSi1−2xO4:xBi3+/Eu3+ phosphors via energy transfer

A series of emission‐tunable Na_1?xAl_1+2xSi_1?2xO₄:xBi³⁺/Eu³⁺ phosphors were synthesized via high temperature solid‐state reaction method. The luminescence properties, energy transfer from Bi³⁺ to Eu³⁺ ions, color tuning, thermal stability and quantum efficiency were systematically investigated. Especially, in the host, a certain amount of Si⁴⁺ were replaced by Al³⁺ in order to remedy the charge compensating defect, so that, the emission intensity had been improved. The results of Rietveld refinements, the analysis of SEM mapping and the fourier transform infrared (FT‐IR) indicated that this charge balance strategy was an effective method. Meanwhile, the energy transfer from Bi³⁺ to Eu³⁺ can be inferred and confirmed and the mechanisms were demonstrated to quadrupole–quadrupole interaction. The emission hue can be tuned from blue to pink, and finally to orange red light by properly varying the ratio of Bi³⁺ and Eu³⁺. Importantly, when the temperature was raised to 150°C, the integrated emission intensity was 71.20% of the initial value for NAS:1%Bi³⁺,2%Eu³⁺ samples indicating that these phosphors had excellent thermal stability and stable color (no emission shift). All these properties indicate that the developed phosphors may be potentially used as single‐component color‐tunable‐emitting phosphors for UV light‐emitting diodes.     

Effects of Eu3+ doping on the luminescence properties of novel Sr2CaLa(VO4)3 from partial substitution of Sr2+ by Ca2+ in Sr3La(VO4)3

Eu³⁺-activated Sr_3−xCa_xLa(VO₄)₃ phosphors were fabricated via citric-acid-assisted sol combustion. Characterization of the Sr_3−xCa_xLa(VO₄)₃:Eu³⁺ samples with different concentrations of Ca²⁺ revealed a hexagonal crystal structure belonging to the R-3m space group. The amount of Ca²⁺ added (x) was controlled within 0 ≤ x ≤ 2 to yield high-purity phosphors. Scanning electron microscopy results showed that an increase in Ca²⁺ concentration resulted in a decrease in the particle size of Sr_3−xCa_xLa(VO₄)₃:Eu³⁺, with the shape gradually changing from nearly equiaxed to lath-shaped. The Sr₂CaLa(VO₄)₃:Eu³⁺ phosphor (denoted as SCLVO:Eu³⁺) exhibited the strongest photoluminescence (PL) intensity at 618 nm among the samples under excitation of 394-nm near-UV (NUV) light. The study of Eu³⁺ doping concentration confirmed that Eu³⁺ could enter the lattice of the SCLVO matrix without altering its crystal structure. SCLVO:Eu³⁺ was found to strongly absorb 394 nm NUV light and 464 nm blue light. The optimal concentration of the Eu³⁺ dopant in the SCLVO host was 0.11, which resulted in the phosphor achieving an excellent PL intensity and a color purity of 98.68%. Tunable luminescence from the orange area (0.5280, 0.4522) of Commission Internationale de l'éclairage (CIE) to the red area (0.6313, 0.3650) was achieved by adjusting the concentration of Eu³⁺. Under 394 nm excitation, SCLVO:0.11Eu³⁺ phosphor has a quantum yield (QY) of 28.2% and excellent thermal stability with 0.383 eV activation energy. Consequently, White-light-emitting diode (WLED) based on SCLVO:0.11Eu³⁺ phosphor yielded a high color rendering index (CRI), low correlated color temperature (CCT), and CIE coordinates of 91.8, 5196 K, and (0.3407, 0.3612), respectively, under the 20 mA driven current. These results indicated the tremendous potential of SCLVO:0.11Eu³⁺ phosphors for application in WLEDs excited by NUV or blue light.     

Color-tunable emissions in Bi3+/Eu3+ activated phosphors for multi-mode optical thermometers

Accurate, self-referential temperature measurements of inaccessible workpieces, bodies in hazardous work environments, or fluids are of tremendous significance. Here, a series of Ca₃Y₂Ge₃O₁₂(CaYGO):1.0%Bi³⁺/yEu³⁺ phosphors were manufactured, which simultaneously presents the characteristic radiation of dopant ions, and the luminescence mechanism was investigated via emission spectroscopy and decay lifetime characterization. Upon excitation at 285 nm, energy transfer (Bi³⁺ → Eu³⁺) of the as-prepared materials exhibited adjustable polychromatic emissions (blue → red). The temperature dependence was evaluated via thermal quenching. The CaYGO phosphors demonstrated a high relative sensitivity of 0.019 K^?1 at 297.8 K according to FIR (the fluorescence intensity ratio) operation rules. Furthermore, temperature-controlled emission spectroscopy displayed a significant chromaticity shift (Δs = 0.05679 for 297.8–480 K). The temperature-dependent lifetime of Eu³⁺ which regarded as a basis of the self-reference measurement provided the maximum relative sensitivity was 0.038%. The excellent signal resolution, high sensitivity values, and results produced by three temperature measurement systems revealed that Ca₃Y₂Ge₃O₁₂: Bi³⁺/Eu³⁺ luminescent materials have excellent development prospects in multi-mode and self-referenced luminous thermometry fields.     

White emitting Dy3+ activated perovskite titanates and energy transfer by Eu3+ codoping

Perovskite type titanate phosphors Sr_0.97−xDy_0.03Li_xTi_1−xNb_xO₃, Sr_0.9−xDy_xLi_0.1Ti_0.9Nb_0.1O₃ and Sr_0.87−yDy_0.03Eu_yLi_0.1Ti_0.9Nb_0.1O₃ were prepared by conventional solid state method. Herein, white light emission from Sr_0.9−xDy_xLi_0.1Ti_0.9Nb_0.1O₃ phosphors and the lowering of its color temperature through codoping with Eu³⁺ ions are reported. Raman measurements have shown that the incorporation of dopants alters the vibrational properties of these phosphors significantly, indicating the reduction of the local symmetry in the crystal lattice. The addition of LiNbO₃ in SrTiO₃:Dy³⁺ phosphor enhances the luminescence intensity and the yellow to blue ratio resulting in emission of high quality white light with color coordinates corresponding to that of standard white. Life time measurements and data fits of Sr_0.9−xDy_xLi_0.1Ti_0.9Nb_0.1O₃ phosphors revealed the biexponential behaviour of luminescence decay profiles. From Judd-Ofelt analysis it is found that the intensity parameter Ω₂ increases with Dy³⁺ concentration and a quantum efficiency of 90.4% was obtained for optimum concentration. In the case of Dy³⁺ and Eu³⁺ codoped phosphors, the color coordinates are found to be sensitive to the Eu³⁺ concentration and the highest energy transfer efficiency of 92% was obtained for the phosphor doped with 10 mol% Eu³⁺. The emission color changes from cold white to reddish orange when the wavelength of excitation alters from 452 to 388 nm, since the energy transfer mechanism alone take place under 452 nm excitation and both direct absorption and the energy transfer mechanism occurs under 388 nm excitation.     

Influence of Bi3+ as a sensitizer and SiO2 shell coating as a protecting layer towards the enhancement of red emission in LnVO4: Bi3+, Eu3+ @ SiO2 (Ln=Gd,Y and Gd/Y) powder phosphors for optical display devices

Enhanced red luminescence in LnVO₄: Bi³⁺, Eu³⁺ @ SiO₂ phosphors has been improved mainly in three stages by investigating the effects of: (i) host composition (Gd, Y and Gd/Y), (ii) co-doping Bi³⁺ as a sensitizer and finally (iii) SiO₂ shell coating. XRD data revealed that the produced phosphors possess crystalline, pure phase with tetragonal structure. Silica coating on phosphor particles have been characterized by SEM/EDAX, TEM, PL and with the presence Si–O–Si, Si–O vibrational modes from the FT-IR spectra. Absorption band edges due to VO₄^3?, shifted to higher wavelength with Bi-concentration, owing to the presence of Bi–O bond in addition to V–O. The emission intensities of ⁵D₀→⁷F₂ transition are stronger than ⁵D₀→⁷F₁; indicating the lower inversion symmetry near Eu³⁺, ions. Red emission intensity due to the efficient energy transfer from VO₄^3? to Eu³⁺ via Bi³⁺ ions in Y_0.949VO₄: Bi³⁺_0.001, Eu³⁺_0.05 phosphor was improved significantly, i.e. 1.6 times compared to Y_0.95VO₄: Eu³⁺_0.05. This was further enhanced 2.25 times by SiO₂ shell coating. Thus, Y_0.949VO₄: Bi³⁺_0.001, Eu³⁺_0.05 @ SiO₂ are suggested to be a promising red phosphor for application in display devices or lighting.     

Effect of crystal structures on energy transfer behavior from Bi3+ to Eu3+ in alkaline earth metalstannates

A single phased color-tunable phosphor via energy transfer has drawn much attention, whereas it is still a question that how to choose a suitable host to realize an efficient energy transfer. In this work, novel single Bi³⁺ or Eu³⁺ doped and Bi³⁺, Eu³⁺ co-activated Ca₂SnO₄ (CSO) and Sr₂SnO₄ (SSO) phosphors have been prepared through a high temperature solid state reaction. The energy transfer happening from hosts to Eu³⁺ has been confirmed. By co-doping of Bi³⁺, a brighter red light is realized which is attributed to the much more efficient energy transfer from Bi³⁺ to Eu³⁺. After comparison, the energy transfer efficiency from Bi³⁺ to Eu³⁺ is much higher in CSO:Bi³⁺, Eu³⁺ than that in SSO:Bi³⁺, Eu³⁺. Furthermore, the experiment reveals that the single-phased color-tunable phosphors exhibit excellent resistance against thermal quenching, suggesting that they can be potential candidates in UV-pumped white light emitting diodes (LEDs).     

Dual-mode optical thermometry based on Bi3+/Eu3+ co-activated BaGd2O4 phosphor with high sensitivity and signal discriminability

A series of BaGd₂O₄:Bi³⁺,Eu³⁺ phosphors with dual-emitting centers were prepared by high-temperature solid-state method. X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), fluorescence spectroscopy, lifetime decay curve and variable temperature emission spectroscopy were used to systematically study the structure, luminescence performance and temperature characteristics. Under ultraviolet (UV) excitation, the BaGd₂O₄:Bi³⁺,Eu³⁺ phosphor showed a broad-band emission in the blue region corresponding to transitions of Bi³⁺ ions and the sharp red light emission corresponding to Eu³⁺ ions. The Bi³⁺ and Eu³⁺ ion emission peaks were well-separated, which meets a prerequisite for efficient temperature signal resolution measurement. The fluorescence intensity ratio (FIR) technique was used to measure the different temperature response characteristics between Bi³⁺ blue emission and Eu³⁺ red emission. When the temperature varies from 293 K to 473 K, the relative temperature sensitivity (S_r) of BaGd₂O₄:Bi³⁺,Eu³⁺ phosphors is obtained, was determined as 1.0182%K⁻¹. In addition to calculating the relative sensitivity by FIR technology, we can also obtain the value of S_r through experiments and formulas related to the decay life, and found to be 1.0651%K⁻¹. Therefore, BaGd₂O₄: Bi³⁺,Eu³⁺ phosphor is an excellent non-contact optical temperature measurement material.