Synthesis, characterization, thermo- and photoluminescence properties of Bi3+ co-doped Gd2O3:Eu3+ nanophosphors

Gd₂O₃:Eu³⁺ (4 mol%) co-doped with Bi³⁺ (Bi = 0, 1, 3, 5, 7, 9 and 11 mol%) ions were synthesized by a low-temperature solution combustion method. The powders were calcined at 800°C and were characterized by powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), Fourier transform infrared and UV–Vis spectroscopy. The PXRD profiles confirm that the calcined products were in monoclinic with little cubic phases. The particle sizes were estimated using Scherrer’s method and Williamson–Hall plots and are found to be in the ranges 40–60 nm and 30–80 nm, respectively. The results are in good agreement with TEM results. The photoluminescence spectra of the synthesized phosphors excited with 230 nm show emission peaks at ～590, 612 and 625 nm, which are due to the transitions ⁵D₀→⁷F₀, ⁵D₀→⁷F₂ and ⁵D₀→⁷F₃ of Eu³⁺, respectively. It is observed that a significant quenching of Eu³⁺ emission was observed under 230 nm excitation when Bi³⁺ was co-doped. On the other hand, upon 350 nm excitation, the luminescent intensity of Eu³⁺ ions was enhanced by incorporation of Bi³⁺ (5 mol%) ions. The introduction of Bi³⁺ ions broadened the excitation band of Eu³⁺ of which a new strong band occurred ranging from 320 to 380 nm. This has been attributed to the 6s²→6s6p transition of Bi³⁺ ions, implying a very efficient energy transfer from Bi³⁺ ions to Eu³⁺ ions. The gamma radiation response of Gd₂O₃:Eu³⁺ exhibited a dosimetrically useful glow peak at 380°C. Using thermoluminescence glow peaks, the trap parameters have been evaluated and discussed. The observed emission characteristics and energy transfer indicate that Gd₂O₃:Eu³⁺, Bi³⁺ phosphors have promising applications in solid-state lighting.     

Photoluminescence characteristics of energy transfer between Er and Bi in Gd2O3: Er, Bi

The phosphors, Bi³⁺- activated Gd₂O₃:Er³⁺, were prepared by sol-gel combustion method, and their photoluminescent properties were investigated under ultraviolet light excitation. The emission spectrum exhibited sharp peaks at about 520, 535, 545, 550 and 559 nm due to (²H_11/2, ⁴S_3/2)→⁴I_15/2 transitions of Er³⁺ ions. The luminescent intensity was remarkably improved by the incorporation of Bi³⁺ ions under 340 nm light excitation, which suggested very efficient energy transfer from Bi³⁺ ions to Er³⁺ions. The introducing of Bi³⁺ ions broadened the excitation band of the phosphor, of which a new strong peak occurred ranging from 320 to 360 nm due to the 6s²→6s6p transition of Bi³⁺ ions. There is significant energy overlap between the emission band of Bi³⁺ ions and the excitation band of Er³⁺ ions. Under 340 nm light excitation, Bi³⁺ absorbed most of the energy and transferred it to Er³⁺. The energy transfer probability from Bi³⁺ to Er³⁺ is strongly dependent on the Bi³⁺ ion concentration. Also, the sensitization effectiveness was studied and discussed in this paper.     

Photon down-shifting by energy transfer from Sm3+ to Eu3+ ions in sol-gel SiO2-LaF3 nano-glass-ceramics for photovoltaics

95SiO₂?C5LaF₃ sol-gel derived nano-glass-ceramics single doped with Eu³⁺ or Sm³⁺ and codoped with both of them were successfully obtained. XRD measurements confirm the precipitation of LaF₃ nanocrystals after the ceramming process, with mean size ranging from 10 to 20?nm which increases with the thermal treatment temperature. The incorporation of rare-earth ions into precipitated LaF₃ nanocrystals was confirmed from luminescence spectra. Intense yellow-red emissions were detected under UV and blue light excitation in single and codoped samples. The effect of codoping with Eu³⁺ and Sm³⁺ ions and the energy transfer mechanism between them have been analyzed in order to increase the yellow-red emissions.     

Sensitized red luminescence from Bi co-doped Eu: ZnO-B2O3 glasses

Photoluminescence properties of Bi³⁺ co-doped Eu³⁺ containing zinc borate glasses have been investigated and the results are reported here. Bright red emission due to a dominant electric dipole transition ⁵D₀→⁷F₂ of the Eu³⁺ ions has been observed from these glasses. The nature of Stark components from the measured fluorescence transitions of Eu³⁺ ions reveal that the rare earth ions could take the lattice sites of C_s or lower point symmetry in the zinc borate glass hosts. The significant enhancement of Eu³⁺ emission intensity by 346 nm excitation (¹S₀→³P₁ of Bi³⁺ ions) elucidates the sensitization effect of co-dopant. The energy transfer mechanism between sensitizer (Bi³⁺) and activator (Eu³⁺) ions has been explained.     

Synthesis and luminescence properties of YVO4:Eu,Bi phosphor with enhanced photoluminescence by Bi doping

YVO₄:Eu³⁺,Bi³⁺ phosphors have been prepared by the high-temperature solid-state (HT) method and the Pechini-type sol-gel (SG) method. Spherical SiO₂ particles have been further coated with YVO₄:Eu³⁺,Bi³⁺ phosphor layers by the Pechini-type SG process, and it leads to the formation of core-shell structured SiO₂/YVO₄:Eu³⁺,Bi³⁺ phosphors. Therefore, the phase formations, structures, morphologies, and photoluminescence properties of the three types of as-prepared YVO₄:Eu³⁺,Bi³⁺ phosphors were studied in detail. The average diameters for the phosphor particles are 2-4 μm for HT method, 0.1-0.4 μm for SG method, and 0.5 μm for core-shell structured SiO₂/YVO₄:Eu³⁺,Bi³⁺ particles, respectively. Photoluminescence spectra show that effective energy transfer takes place between Bi³⁺ and Eu³⁺ ions in each type of as-prepared YVO₄:Eu³⁺,Bi³⁺ phosphors. Introduction of Bi³⁺ into YVO₄:Eu³⁺ leads to the shift of excitation band to the long-wavelength region, thus the emission intensities of ⁵D₀-⁷F₂ electric dipole transition of Eu³⁺ at 615 nm upon 365 nm excitation increases sharply, which makes this phosphor a suitable red-emitting materials that can be pumped with near-UV light emitting diodes (LEDs).     

Luminescent properties of (Y,Gd)BO3:Bi,RE (RE=Eu, Tb) phosphor under VUV/UV excitation

Bi³⁺- and RE³⁺-co-doped (Y,Gd)BO₃ phosphors were prepared and their luminescent properties under vacuum ultraviolet (VUV)/UV excitation were investigated. Strong red emission for (Y,Gd)BO₃:Bi³⁺,Eu³⁺ and strong green emission for (Y,Gd)BO₃:Bi³⁺,Tb³⁺ are observed under VUV excitation from 147 to 200 nm with a much broader excitation region than that of single Eu³⁺-doped or Tb³⁺-doped (Y,Gd)BO₃ phosphor. Strong emissions are also observed under UV excitation around 265 nm where as nearly no luminescence is observed for single Eu³⁺-doped or Tb³⁺-doped (Y,Gd)BO₃. The luminescence enhancement of Bi³⁺- and RE³⁺-co-doped (Y,Gd)BO₃ phosphors is due to energy transfer from Bi³⁺ ion to Eu³⁺ or Tb³⁺ ion not only in the VUV region but also in the UV region. Besides, host sensitization competition between Bi³⁺ and Eu³⁺ or Tb³⁺ is also observed. The investigated phosphors may be preferable for devices with a VUV light 147-200 nm as an excitation source such as PDP or mercury-free fluorescent lamp.     

Luminescence properties of NaGd(PO3)4:Eu3+ and energy transfer from Gd3+ to Eu3+

The luminescence properties of polyphosphates NaEu _x Gd_(1?x)(PO₃)₄ (x = 0–1.00) and the energy transfer from Gd³⁺ to Eu³⁺ were studied. In undoped NaGd(PO₃)₄ sample, the photon cascade emission of Gd³⁺ was observed under ⁸S_7/2 → ⁶G_J excitation (201 nm) in which the emission of a red photon due to ⁶G_J → ⁶P_J transition is followed by an ultraviolet photon emission due to ⁶P_J → ⁸S_7/2 transition. When part of Gd³⁺ ions in the host NaGd(PO₃)₄ were substituted by Eu³⁺ ions, the NaGd(PO₃)₄:Eu³⁺ sample showed intensive red emission under 172-nm vacuum-ultraviolet (VUV) excitation which is suitable for mercury-free fluorescent lamps and plasma display panel applications. Based on the VUV–visible spectroscopic characteristics and the luminescence decay properties of NaGd(PO₃)₄:Eu³⁺, it was found that the quantum cutting by a two-step energy transfer from Gd³⁺ to Eu³⁺ can improve the red emission of Eu³⁺ ions under VUV excitation but only a part of the excitation energy in the excited ⁶P_J states within Gd³⁺ ions can be transferred to Eu³⁺ ions for its red emission, and the nonradiative energy transfer efficiencies from the excited ⁶P_J states within Gd³⁺ to Eu³⁺ were calculated.     

Luminescence spectroscopy of Eu3+ and Mn2+ ions in MgGa2O4 spinel

Photoluminescence and excitation spectra of the spinel-type MgGa₂O₄ with 0.5 mol. % Mn²⁺ ions and Eu³⁺ content from 0 to 8 mol. % have been investigated in this work at room temperature. Polycrystalline samples were synthesized via high-temperature solid-state reaction method. Photoluminescence spectra of all samples exhibit host emission presented by a broad “blue” band peaking ∼430 nm, which consists of at least three elementary bands that correspond to different host defects. Excitation of the host luminescence showed the broad band with a maximum at 360 nm. Characteristic bands of d–d transitions of Mn²⁺ ions and f–f transitions of Eu³⁺ ions together with charge-transfer bands (CTB) of these ions were also found on the excitation spectra. Mn²⁺ and Eu³⁺ co-doped samples emit in green and red spectral regions. Mn²⁺ ions are responsible for the green emission band at 505 nm (⁴Т₁→⁶А₁ transition). The studies of photoluminescence spectra of activated samples with different Eu³⁺ ions content show characteristic f–f luminesecence of Eu³⁺ ions. The maximum of Eu³⁺ emission was found at 618 nm (⁵D₀→⁷F₂) and optimal concentration of activator ions was around 4 mol. %.     

Photoluminescence properties of color-tunable SrMgAl10O17:Eu2+,Mn2+ phosphors for UV LEDs

Aluminate phosphors SrMgAl₁₀O₁₇ codoped with Eu²⁺ and Mn²⁺ ions were prepared by solid-state reaction. The phase structure and photoluminescence properties of the as-prepared phosphors were characterized by powder X-ray diffraction, photoluminescence excitation and emission spectra. Upon excitation of UV light, two broad emission bands centered at 470 and 515 nm were observed, and they were assigned to Eu²⁺ and Mn²⁺ emissions, respectively. The emission color of the phosphors can be tuned from blue to cyan and finally to green by adjusting the concentration ratios of Eu²⁺ and Mn²⁺. Effective energy transfer occurs from Eu²⁺ to Mn²⁺ in the host due to the spectral overlap between the emission band of Eu²⁺ and the excitation bands of Mn²⁺. The energy transfer mechanism was demonstrated to be electric dipole–quadrupole interaction. The energy transfer efficiency and critical distance were also calculated. The phosphors exhibit strong absorption in near UV spectral region and therefore they are potentially useful as UV-convertible phosphors for white LEDs.     

Low-temperature wet chemical synthesis and photoluminescence properties of YVO4: Bi, Eu nanophosphors

YVO₄: Bi³⁺, Eu³⁺nanophosphors are prepared by the citrate-assisted low-temperature wet chemical synthesis. When the colloidal solution is aged at 60 °C, the crystalline YVO₄: Bi³⁺, Eu³⁺ nanorods are formed from the amorphous gel precursors, as confirmed by transmission electron microscopy and X-ray diffractometry (XRD). YVO₄: Bi³⁺, Eu³⁺ nanophosphors emit red through energy transfer from Bi³⁺ to Eu³⁺ under near-UV-light excitation. The emission intensity increases with increasing the fraction of the crystalline phase during aging. The excitation peak corresponding to Bi³⁺-V⁵⁺ charge transfer relative to those of O²⁻-V⁵⁺ and O²⁻-Eu³⁺ charge transfers gradually becomes strong until the completion of the crystallization, although the contents of individual Bi³⁺ and Eu³⁺ ions incorporated into YVO₄ keep constant. When the aging is continued after the completion of the crystallization, the content of incorporated Bi³⁺ gradually increases, and hence the emission intensity decreases as a result of the energy migration among Bi³⁺ ions. These results suggest that in addition to the fraction of the crystalline phase and the contents of incorporated Bi³⁺ and Eu³⁺ ions, the local chemical states around Bi³⁺ play significant roles in photoluminescence properties.     

Effects of the homogeneous Bi doping process on photoluminescence properties of YVO4:Bi,Eu nanophosphor

YVO₄:Bi³⁺,Eu³⁺ nanophosphors at a high Bi³⁺ concentration of 15 at% are synthesized from a Bi³⁺ source, nitrates of yttrium and europium(III), and sodium orthovanadate(V) by a low-temperature aqueous precipitation in the presence of citrate ions. When an ethylene glycol solution of bismuth(III) nitrate is used as a Bi³⁺ source, YVO₄:Bi³⁺,Eu³⁺ nanophosphors of ∼20 nm in size crystallize during aging at 85 °C without any by-products where the contents of Bi³⁺ and Eu³⁺ incorporated into crystalline YVO₄ are close to the respective nominal contents, as confirmed by transmission electron microscopy, X-ray diffractometry and X-ray fluorescent analysis. These nanophosphors show red emission corresponding to the f-f transition of Eu³⁺ under the excitation of Bi³⁺-V⁵⁺ charge transfer. When aging is continued after the completion of the crystallization, the photoluminescence intensity of nanophosphors reaches the constant value. This is the improved behavior in comparison to our previous work, where the photoluminescence intensity decreases after the prolonged aging because of the inhomogeneous doping of Bi³⁺ ions, and hence the concentration quenching.     

Luminescence properties of Y2O3:Bi3+, Ln3+ (Ln=Sm,Eu, Dy,Er, Ho) and the sensitization of Ln3+ by Bi3+

The spectroscopic characterization of yttria, singly and doubly doped with Ln³⁺ (Ln=Sm, Eu, Dy, Er, Ho) and Bi³⁺ ions, is performed through excitation spectra, emission spectra and decay time measurements. The obtained spectroscopic data clearly indicate that energy transfer takes place from Bi³⁺ to Ln³⁺ ions. The energy transfer efficiency of Bi³⁺→Ln³⁺ and quantum efficiency of Ln³⁺ were calculated. Upon excitation of 370 nm (Bi³⁺ excitation band), the quantum efficiency of Ln³⁺ varies from ～4% to ～44%. The energy transfer efficiency increases continuously with increasing Ln³⁺ concentrations, whereas the variation of the quantum efficiency of Ln³⁺ is complicated. The quantum efficiency of Ln³⁺ is discussed in terms of electron transfer and cross relaxation.     

Synthesis and luminescence properties of Eu3+,Bi3+-doped BaWO4 phosphors

BaWO₄:Eu³⁺,Bi³⁺ phosphors have been prepared by the conventional high-temperature solid-state reaction and chemical precipitation. The materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL) technologies. When the phosphors are prepared by the high-temperature solidstate reaction, Bi³⁺ doping into BaWO₄:Eu³⁺ can increase the emission intensity of 613 nm. However, maximum emission at about 595 nm was observed in Eu³⁺,Bi³⁺-doped BaWO₄ phosphors prepared by the chemical precipitation. The decay constants (monitored at 595 and/or 613 nm) are within 45–100 s. The color purity of the Ba_0:865WO₄: Eu_0:11,Bi_0:025 phosphor (prepared by chemical precipitation) was 100%. The emission mechanism of Eu³⁺,Bi³⁺ in the BaWO₄ phosphors is briefly discussed.     

Synthesis and photoluminescence properties of the high-brightness Eu3+-doped Sr3Bi(PO4)3 phosphors

A series of orange reddish emitting phosphors Eu³⁺-doped Sr₃Bi(PO₄)₃ have been successfully synthesized by conventional solid-state reaction, and its photoluminescence (PL) properties have been investigated. The excitation spectra reveal strong excitation bands at 392 nm, which match well with the popular emissions from near-UV light-emitting diode chips. The emission spectra of Sr₃Bi(PO₄)₃:Eu³⁺ phosphors invariably exhibit five peaks assigned to the ⁵D₀→⁷F_J (J=0, 1, 2, 3, 4) transitions of Eu³⁺ and have dominating emission peak at 612 nm under 392 nm excitation. The luminescence intensity was enhanced with increasing Eu³⁺ content and the emission reached the maximum intensity at x=0.05 in Sr₃Bi(PO₄)₃:xEu³⁺. The energy transfer behavior in the phosphors was discussed. The Commission Internationale de lEclairage (CIE) chromaticity coordinates, the quantum efficiencies, and the decay curves of the entitled phosphors excited under 392 nm are also investigated. The experimental results indicate that the Eu³⁺-doped Sr₃Bi(PO₄)₃ phosphors are promising orange reddish-emitting phosphors pumped by near-UV light.     

Efficient VUV sensitization of Eu3+ emission by Tb3+ in potassium rare‐earth double phosphate

The luminescence properties of K₃Tb(PO₄)₂ activated by Eu³⁺ were studied at excitation over the 120–300 nm wavelength range. It is demonstrated that Tb³⁺ ions, exhibiting a strong absorption band in the vacuum‐ultraviolet (VUV), can provide efficient sensitisation of Eu³⁺ emission in this wave length range, giving rise to intense red luminescence at 150 nm excitation. A proof is given for the concept of VUV sensitisation enabling the engineering of luminescence materials with enhanced conversion efficiency of VUV radiation into visible light. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fluorescent properties of a green- to red-emitting Eu3+, Tb3+ codoped amorphous calcium silicate phosphor

A Eu³⁺, Tb³⁺ codoped amorphous calcium silicate phosphor was prepared by heating a Eu³⁺, Tb³⁺ codoped calcium silicate hydrate phosphor formed by liquid-phase reaction for 30 min at 900 °C. The excitation peak wavelength of the resulting phosphor was 379 nm and the emission peak wavelengths were at 542 nm, attributed to the ⁵D₄→⁷F₅ transition of Tb³⁺, and at 613 mm, attributed to the ⁵D₀→⁷F₁ transition of Eu³⁺. The intensity ratio of the two peaks could be freely controlled by varying the Eu/Tb atomic ratio of the Eu³⁺, Tb³⁺ codoped amorphous calcium silicate phosphor, allowing light to be emitted over a wide range from green to red. It was clarified that electron transfer from Tb³⁺ to Eu³⁺ is occurring.     

Luminescence and scintillation of Eu2+‐doped high silica glass

High silica glass doped with Eu²⁺ ions was prepared as a scintillating material by impregnation of Eu ions into a porous silica glass followed by reduction sintering in CO atmosphere. A dominant emission band of the Eu²⁺ 5d–4f transition peaking around 430 nm was observed in the luminescence spectrum with the excitation peak around 280 nm and no emission from Eu³⁺ was present. Photoluminescence decay kinetics was governed by decay times of a few microseconds. The Eu²⁺‐doped high silica glass exhibited comparable energy resolution and slightly higher photoelectron yield with respect to the Bi₄Ge₃O₁₂ crystal in the pulse height spectra for X‐ray photon energies within 22–60 keV. Furthermore, a factor of 1.2 higher radioluminescence intensity was observed as well. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Quantum cutting downconversion by cooperative energy transfer from Bi3+ to Yb3+ in Y2O3 phosphor

Bi~(3+) and Yb~(3+) codoped cubic Y2O3 phosphors are prepared by pechini sol-gel method.Strong near-infrared (NIR) emission around 980 nm from Yb~(3+)(2F5/2 → 2F7/2) is observed under ultraviolet light excitation.A broad excitation band ranging from 320 to 360 nm,owing to the 6s 2 →6s6p transition of Bi~(3+) ions,is recorded when the Yb~(3+) emission is monitored,which suggests a very efficient energy transfer from Bi~(3+) ions to Yb~(3+) ions.The Yb~(3+) concentration dependences of both the Bi~(3+) and the Yb~(3+) emissions are investigated.The decay curve of Bi ~(3+) emission under the excitation of 355 nm pulse laser is used to explore the Bi~(3+) →Yb~(3+) energy transfer process.Cooperative energy transfer (CET) is discussed as a possible mechanism for the near-infrared emission.     

Photoluminescence in GdVO4:Bi3+, Eu3+ red phosphor

The doubly doped (Bi³⁺ and Eu³⁺) GdVO₄ powder is synthesized by hydrolyzed colloid reaction (HCR) technique and formation of material is confirmed by XRD measurement. Surface morphology has been studied by SEM measurement and the result shows uniform surface morphology. The average particle size observed by SEM is about 1 7m. The Fritsch particle sizer is used to study the particle size distribution. It distributes from O.15 to 3.57 7m. The small particle size (less than 5 7m) and the narrow particle size distribution, are the necessary requirements of good phosphor material. Photoluminescence result shows a narrow emission line of Eu³⁺ ion (4 nm FWHM) at 618 nm. The Eu³⁺ emission intensity is enhanced by a factor of five with the addition of small amount of Bi³⁺. The emission bands of VO₄^3- and Bi³⁺ partially overlap with the excitation band of Eu³⁺. The process of energy transfer from Bi³⁺ to Eu³⁺ is discussed here. The energy transfer probability is strongly dependent upon the Bi³⁺ and Eu³⁺ concentrations, with a maximum for 0.2 mol % of Bi³⁺ and 3 mol % of Eu³⁺. It drastically decreases for higher concentrations. For photoluminescent applications, the quantum efficiency (QE) of a phosphor material is an important parameter. The QE of GdVO₄:Bi,Eu(0.2,3) is 76%. The GdVO₄:Bi,Eu(0.2,3) material is proposed as an efficient photoluminescent phosphor.     

Optical absorption and emission from irradiated RbMgF3:Eu2+ and KMgF3:Eu2+

The optical properties of irradiated RbMgF₃:Eu²⁺ and KMgF₃:Eu²⁺ have been investigated. Previous research has shown that Eu²⁺ ions in unirradiated RbMgF₃ give rise to broad band absorption around 250 nm and sharp intense line emission at 360 nm. When this material is irradiated little or no change occurs in the 250 nm absorption, but the lifetime of the Eu²⁺ 360 nm transition is reduced. In addition, new emission is observed at 680 nm. In the case of irradiated KMgF₃:Eu²⁺ two new emission bands are observed at 600 and 800 nm. All of these transitions have short lifetimes and are not due to Eu³⁺ ions.