Luminescence of Eu3+ and Eu2+ in the new BaLnB9O16 borates (Ln = rare earth)

The luminescence of Eu³⁺ in the borates BaLnB₉O₁₆ (Ln = La, Gd and Y) has been investigated. Under UV excitation the Eu³⁺-activated lanthanum and gadolinium compounds show a bright red luminescence at room temperature, whereas the Eu³⁺-activated yttrium borate is characterized by a red-orange luminescence which indicates a change in the symmetry of the rare earth ion sites. As a consequence of the high energy of the charge transfer band the Eu³⁺ emission has a high efficiency under 253.7 nm excitation, a characteristic favorable for lighting applications. The sensitization of the Eu³⁺ emission by Bi³⁺ has been examined. Energy transfer from Bi³⁺ to Eu³⁺ is observed, but in presence of bismuth the efficiency of excitation through the Eu³⁺ charge transfer band is reduced. The luminescence of Eu²⁺ has also been studied. Eu²⁺-activated BaGdB₉O₁₆ shows an intense blue emission band under UV excitation.     

EPR and optical investigation of ultraviolet-emitting Gd<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript> garnet

Gd₃Ga₅O₁₂ garnet was prepared by a solution combustion method and characterized using powder X-ray diffraction, electron paramagnetic resonance (EPR) and optical spectroscopic techniques. EPR spectrum of the samples at ambient and low temperatures exhibited resonance signal at g?≈?2 attributed to Gd³⁺ ions disposed in an octahedral symmetry. The optical absorption spectrum showed a band centered at 274 nm attributed to ⁸S_7/2→⁶I_J transition of the Gd³⁺ ions. The excitation spectrum showed a maximum at 273 nm along with two relatively weaker peaks at 276 and 279 nm. These three excitation bands were assigned to the transitions ⁸S_7/2→⁶I_J. The emission spectrum showed two peaks centered at 311 and 306 nm that were assigned to the transitions ⁶P_7/2→⁸S_7/2 and ⁶P_5/2→⁸S_7/2 of Gd³⁺, respectively. To evaluate the phosphor performance, several magnetic and thermodynamic parameters for the system were calculated adopting standard procedure.     

Characteristics and synthesis mechanism of Gd2O2S:Tb phosphors prepared by vacuum firing method

Characteristics and synthesis mechanism of Gd₂O₂S:Tb phosphors prepared by vacuum firing were investigated by photoluminescence (PL) spectra, X-ray diffraction (XRD), scanning electronic microscopy (SEM) and transmission electron microscopy (TEM). The mixtures of raw materials were tested by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The firing temperature was reduced to about 725 °C by the vacuum firing method. The particle size decreased. Meanwhile the particle size distribution and morphology were improved and the luminescence of Gd₂O₂S:Tb phosphors was also enhanced as the degree of vacuum decreases. With the decrease of the degree of vacuum, the intensity of the excitation spectrum was strengthened and the band was widened being the particle size of the host lattice decreased to nano scales. The peak with high intensity around 272 nm in the excitation spectra (λ_Em = 545 nm) of Gd₂O₂S:Tb nanophosphor may be attributed to the 4d–5f transitions of Gd atoms, which may play a significant role in the energy transfer between Tb³⁺ and Gd³⁺ ions.⁵D₄–⁷F_J transitions of Tb³⁺ ion were mainly concentrated in the narrow green emission spectrum (535–555 nm) with its sharp peak at 545 nm. The synthesis mechanism of Gd₂O₂S:Tb phosphors prepared by vacuum firing was also studied.     

Time-resolved spectroscopy of Bi3+ centers in Y4Al2O9

Steady-state and time-resolved emission and excitation spectra as well as luminescence decay kinetics are studied at 4.2–400 K under excitation in the 3–6 eV energy range for Bi³⁺ ions substituting for Y³⁺ ions in four inequivalent crystal lattice sites of Y₄Al₂O₉:Bi ceramics. Luminescence characteristics of Bi³⁺ centers of all the four types are identified and are shown to arise from the radiative decay of the triplet relaxed excited state (RES) of Bi³⁺ ions. The parameters of the triplet RES, namely, probabilities of the radiative and nonradiative transitions from the metastable and emitting levels as well as the energy distance between these levels, are determined. The influence of the nearest surroundings of Bi³⁺ ions on the luminescence characteristics and the parameters of the triplet RES of Bi³⁺ centers is discussed.     

Optical properties of aluminophosphate glasses containing silver, tin, and dysprosium

The spectroscopic properties of a Dy³⁺-doped aluminophosphate glass containing silver and tin were reported. Different oxidation and aggregation states of silver were obtained by varying silver concentration and glass thermal history. The addition of silver and tin at the lowest concentration studied results in Dy³⁺ ions emission under nonresonant UV excitation in connection with the appearance of an excitation band around 270 nm, which is associated to isolated Ag⁺ ions and twofold-coordinated Sn centers. The increase in silver and tin concentration leads to a broadening of aforementioned band and to the presence of charged silver dimers as evidenced by the appearance of an excitation band around 330 nm. The data indicated that light absorption might take place at ionic silver species and twofold-coordinated Sn centers, followed by energy transfer to Dy³⁺ ions. After heat treatment, ionic silver species were reduced to atomic Ag by tin with the subsequent formation of Ag nanoparticles (NPs) inside the dielectric host. A quenching effect in Dy³⁺ ions luminescence was shown with the presence of the Ag NPs, most notably for excitation of ⁶H_15/2 → ⁴F_9/2, ⁴I_15/2, ⁴G_11/2 transitions, which were in resonance with the dipole absorption mode of the particles. The silver NPs were believed to provide radiationless pathways for excitation energy loss in Dy³⁺ ions.     

Vacuum ultraviolet optical properties of (La,Gd)PO4:RE (RE=Eu, Tb)

Vacuum ultraviolet excitation spectra of phosphors (La,Gd)PO₄:RE³⁺ (RE=Eu or Tb) and X-ray photoelectron spectra of LaPO₄ and GdPO₄ are investigated. The vacuum ultraviolet excitation intensity of (La,Gd)PO₄:RE³⁺ is enhanced with the increasing of Gd³⁺ content, which implies that Gd³⁺ plays an intermediate role in energy transfer from host absorption band to RE³⁺. When Gd³⁺ is doped into LaPO₄:Eu³⁺, charge transfer band (CT band) begins to shift to higher energy region and the overlap degree of CT band and the host absorption band gets greater with more Gd³⁺ doped into LaPO₄. These results suggest that the dopant (Gd³⁺) gives an important influence on energy transfer efficiency. The top of LaPO₄ valance band is formed by the 2p level of O²⁻, whereas that of GdPO₄ valance band is formed by the 2p level of O²⁻ and the 4f level of Gd³⁺, showing the differences in band structures between LaPO₄ and GdPO₄.     

Luminescent properties of europium-activated yttrium gadolinium phosphates

Nanocrystalline Y_{1 ? x ? y} Gd _x Eu _y PO₄ phosphors have been prepared via precipitation from aqueous solutions. From their luminescence and excitation spectra, the intensity ratio I ₆₁₅/I ₅₉₄ of the Eu³⁺ luminescence bands corresponding to electric dipole and magnetic dipole transitions has been determined as a function of Gd³⁺ content. The critical concentration and effective energy transfer radius in Y_{1 ? x ? y} Gd _x Eu _y PO₄ have been evaluated. Excitation of Gd³⁺⁸ S _J ?⁶ D _J and ⁸ S _J ?⁶ J _J transitions to Eu³⁺ luminescence excitation levels in Y_{0.99 ? x} Gd _x Eu_0.01PO₄ involves efficient energy transfer. Under 250-nm excitation, the Eu³⁺ luminescence yield in Y_{0.99 ? x} Gd _x Eu_0.01PO₄ is a factor of 2.5–3 higher than that in Y_0.99Eu_0.01PO₄.     

Luminescence of SrGdGa3O7:RE(RE = Eu, Tb) phosphors and energy transfer from Gd to RE

Eu³⁺- and Tb³⁺-activated SrGdGa₃O₇ phosphors were synthesized by the solid-state reaction and their luminescence properties were investigated. Sr(Gd_1 − xEu_x)Ga₃O₇ and Sr(Gd_1 − xTb_x)Ga₃O₇ formed continuous solid solution in the range of x = 0-1.0. Unactivated SrGdGa₃O₇ exhibited a typical characteristic excitation and emission of Gd ion. The SrGdGa₃O₇:xEu³⁺ and SrGdGa₃O₇:xTb³⁺ phosphors also showed the well-known Eu³⁺ and Tb³⁺ excitation and emission. The energy transfer from Gd³⁺ to Eu³⁺ and Tb³⁺ were verified by photoluminescence spectra. The dependence of photoluminescence intensity on Eu³⁺ and Tb³⁺ concentration were also studied in detail and the photoluminescence (PL) intensity of SrGdGa₃O₇:Eu and SrGdGa₃O₇:Tb were compared with commercial phosphors, Y₂O₃:Eu and LaPO₄:Ce,Tb. The luminescence decay measurements showed that the lifetimes of Eu³⁺ and Tb³⁺ were in the range of microsecond. The energy transfer from Gd³⁺ to Tb³⁺ was also observed in decay curve.     

Novel highly luminescent europium dinitrosalicylates

A series of lanthanide dinitrosalicylates M₃Ln(3,5-NO₂-Sal)₃ · nH₂O (Ln = Eu, Gd; M = Li, Na, K, Cs) was synthesized. It was found that the luminescence efficiency of some M₃Eu(3,5-NO₂-Sal)₃ · nH₂O compounds was near to the high efficiency of europium dibenzoylmethanate with 1,10-phenanthroline, Eu(DBM)₃ · Phen. The luminescence excitation spectra, electron-vibrational luminescence spectra, and vibrational IR spectra were investigated. The energy of the lowest excited triplet state of the ligand was obtained from phosphorescence spectra of M₃Gd(3,5-NO₂-Sal)₃ · nH₂O, M(3,5-NO₂-HSal) · nH₂O, and M₂(3,5-NO₂-Sal) · nH₂O. The details of the structure of compounds were discussed. The influence of different M-cations on the Eu³⁺ luminescence efficiency and on the processes of excitation energy transfer to a Eu³⁺ ion was analyzed. The presence of large alkali metal cations in lanthanide dinitrosalicylates and an increase in the temperature weaken the network of hydrogen bonds and, to some extent, the “ligand–metal” bonds. This is a cause of a long-wavelength shift of the intraligand charge transfer (ILCT) band in Eu³⁺ excitation spectra arising at inclusion of Cs⁺ instead of Li⁺ cations in the crystal lattice and at the heating of compounds. A change of the energies of ligand electronic states at substitution of Li⁺ and Na⁺ for Cs⁺ can give a tenfold enhancement of the Eu³⁺ luminescence efficiency at 300 K.     

UV and VUV characteristics of (YGd)2O3:Eu phosphor particles prepared by spray pyrolysis from polymeric precursors

Red-emitting (YGd)₂O₃:Eu phosphor particles, with high luminescence efficiency under vacuum ultraviolet (VUV) and ultraviolet (UV) excitation, were prepared by a large-scale spray pyrolysis process. To control the morphology of phosphor particles under severe preparation conditions, spray solution with polymeric precursors were introduced in spray pyrolysis. The prepared (YGd)₂O₃:Eu phosphor particles had spherical shape and filled morphology even after post-treatment irrespective of Gd/Y ratio. In the case of solution with polymeric precursors, long polymeric chains formed by esterification reaction in a hot tubular reactor; the droplets turned into viscous gel, which retarded the precipitation of nitrate salts and promoted the volume precipitation of droplets. The brightness of (YGd)₂O₃:Eu phosphor particles increased with increasing gadolinium content, and the Gd₂O₃:Eu phosphor had the highest luminescence intensity under UV and VUV excitation. The maximum peak intensity of Gd₂O₃:Eu phosphor particles under UV and VUV were 118 and 110% of the commercial Y₂O₃:Eu phosphor particles, respectively.     

Down-shifting emission of Yb3+ ions sensitized by broad charge transfer band of V-O and Yb-O in Ca2V2O7 phosphors

Yb³⁺ singly doped pyrovanadate Ca₂V₂O₇ phosphors were prepared by conventional solid state reaction. The crystal structure of the powders was investigated by X-ray diffraction. The diffuse reflection spectrum, photoluminescence excitation, emission spectra were investigated. The intense near infrared down-shifting emission of Yb³⁺ ions were demonstrated upon excitation of ultraviolet (UV) photon in a wide range of 250–450 nm. The broad excitation band in UV is attributed to the overlap of Yb³⁺–O²⁻ and V⁵⁺–O²⁻ charge transfer band. With the combination of wide Yb-O charge transfer band, which is seldom reported to sensitize the down-shifting emission of Yb³⁺ ion, the spectral response of the spectral converting materials is greatly extended. The energy transfer process from V-O and Yb-O charge transfer band to Yb³⁺ ions and involved mechanism have been discussed.     

Spectroscopy and frequency upconversion of Er ions in fluorotellurite glasses

In this work we report the optical properties and upconversion luminescence of Er³⁺ ions in TeO₂–WO₃–PbO–BaF₂ and TeO₂–TiO₂–Nb₂O₅–BaF₂ fluorotellurite glasses and their comparison with those of TeO₂–TiO₂–Nb₂O₅ glass. The optical properties of Er³⁺ ions have been established in terms of absorption and emission spectra and lifetime measurements. The 1.5 μm emission cross-section has been determined from the line shape of the emission spectrum and the calculated emission probability for the ⁴I_13/2 level. The highest emission cross section (6.9 × 10⁻²¹ cm²) corresponds to the TeO₂–TiO₂–Nb₂O₅–BaF₂ glass with a figure of merit for the bandwidth of 524.4 cm² nm. Upconversion emissions at 530, 548 nm, and 660 nm have been obtained under infrared excitation at 800 nm in the ⁴I_9/2 level and compared with those obtained under one photon excitation. The green emission corresponding to the ⁴S_3/2 → ⁴I_15/2 transition is dominant in all glasses. The excitation wavelength dependence of the upconverted luminescence together with its time evolution after infrared pulsed excitation suggest that energy transfer upconversion processes are responsible for the green upconversion luminescence.     

Two-photon excitation of anti-stokes photoluminescence in Y1.80Er0.10Yb0.10O2S

Phosphors with the composition Y_1.80Er_0.10Yb_0.10O₂S have been prepared by solid-state reaction. We have analyzed the diffuse reflectance, luminescence excitation, and photoluminescence spectra of this material under Stokes and anti-Stokes optical excitation by two light sources. Visible luminescence was observed when two resonance infrared photons of different energies were absorbed successively. This effect can be understood in terms of the metastable states of the ⁴ I _11/2 excited-state level of the erbium ion, from which transitions to higher energy levels occur when infrared photons are absorbed.     

Synthesis and luminescence of LaPO4:Eu nanoparticles from different phosphate sources

LaPO₄:Eu nanoparticles were synthesized from different phosphate sources by the method of precipitation. The influences of phosphate sources on properties of LaPO₄:Eu nanoparticles were studied. The LaPO₄:Eu sample synthesized from (NH₄)₂HPO₄ has the uniform morphology and good dispersion. The excitation spectra show the Eu–O charge-transfer band and f–f orbital transitions of Eu³⁺. The strong absorption of charge transfer band (CTB) can be measured when H₃PO₄ is used as the phosphate source. However, there is a red shift of CTB when (NH₄)₂HPO₄ and Na₅P₃O₁₀ are used as phosphate sources, which indicates the increase in O^2?–Eu³⁺ distance and the decrease in charge transfer energy. The emission spectra show different emission transitions originating from ⁵D₀ state (⁵D₀ → ⁷F_j) of Eu³⁺. The life times of the Eu³⁺ ions are determined to be 2.176, 4.576, and 5.608 ms for the samples synthesized from H₃PO₄, Na₅P₃O₁₀ and (NH₄)₂HPO₄, respectively.     

Photoluminescence and energy transfer of terbium doped titania film

Strong visible luminescence of Tb³⁺ions due to intra-4f shell transitions are obtained from Tb³⁺-doped titania (TiO₂) films fabricated by sol-gel method. Based on the overlap of excitation band of Tb³⁺-doped TiO₂ and absorption band of undoped TiO₂, we propose an energy transfer mechanism from TiO₂ host to Tb³⁺ions. Photoluminescence (PL) intensity is found to have a well matching relation with the doping concentration of Tb³⁺ ions. Concentration quenching of PL occurs when Tb³⁺ concentration exceeds a certain value (9.5 mol%). Luminescence intensity is improved obviously after co-doped with Ce³⁺ ions because of the sensitization effect of Ce³⁺ ions and the dispersion of Tb³⁺ ions in TiO₂ system.     

Shaped single crystal growth and scintillating application of Yb:(Gd,Lu)3(Ga,Al)5O12 solid solutions

Shaped single crystals of (Yb_0.05Lu_xGd_0.95−x)Ga₅O₁₂ (0.0x0.9) and Yb_0.15Gd_0.15Lu_2.7(Al_xGa_1−x)O₁₂ (0.0x1.0) were grown by the modified micro-pulling-down method. Continuous solid solutions with garnet structure and a linear compositional dependency of crystal lattice parameter in the system Yb:(Gd,Lu)₃(Ga,Al)₅O₁₂ are formed. Measured optical absorption spectra of the samples show 4f–4f transitions related to Gd³⁺ ion at 275 and 310 nm, and also an onset of charge transfer transitions from oxygen ligands to Gd³⁺ or Yb³⁺ cations below 240 nm. A complete absence of Yb³⁺ charge transfer luminescence under X-ray excitation in any of the investigated samples was explained by the overlapping of charge transfer absorption of Yb³⁺ by that of Gd³⁺ ions. For specific composition of Lu_1.5Gd_1.5Ga₅O₁₂ an intense defect––host lattice-related emission, which achieve of about 40% integrated intensity compared with Bi₄Ge₃O₁₂, was found.     

A novel red phosphor NaLa4(SiO4)3F: Eu

A novel red phosphor NaLa₄(SiO₄)₃F: Eu³⁺ was synthesized by the conventional solid-state reaction at 950 °C for the first time. The luminescence properties of NaLa₄(SiO₄)₃F: Eu³⁺ were investigated, and the critical concentration of the activator concentration (Eu³⁺) was found to be 0.1 mol per formula unit. The phosphor presented red luminescence under the ultraviolet excitation of 254 or 395 nm, attributed to the transitions from ⁵D₀ excited states to ⁷F_J ( J = 0-4) ground states of Eu³⁺ ions. The results indicated that this newly-developed phosphor could find applications in tricolor fluorescent lamp, phosphor-liquid crystal displays and white lighting devices utilizing GaN-based excitation in the near UV.     

Infrared to visible upconversion luminescence in Er/Yb co-doped CeO2 inverse opal

Infrared to visible upconversion luminescence has been investigated in Er³⁺/Yb³⁺ co-doped CeO₂ inverse opal. Under the excitation of 980 nm diode lasers, visible emissions centered at 525, 547, 561, 660 and 680 nm are observed, which are assigned to the Er³⁺ transitions of ²H_11/2 → ⁴I_15/2 (525 nm), ⁴S_3/2 → ⁴I_15/2 (547, 561 nm), ⁴F_9/2 → ⁴I_15/2 (660 and 680 nm), respectively. The effect of photonic band gap on the upconversion luminescence intensity was also obtained. Additionally, the upconversion luminescence mechanism was studied. The dependence of Er³⁺ upconversion emission intensity on pump power reveals that it is a two-photon excitation process.     

Combustion synthesis and photoluminescence of SrAl2O4:Eu,Dy phosphor nanoparticles

Eu²⁺,Dy³⁺ co-doped strontium aluminate (SrAl₂O₄) phosphor nanoparticles with high brightness and long afterglow were prepared by glycine–nitrate solution combustion synthesis at 500 °C, followed by heating the resultant combustion ash at 1100 °C in a weak reductive atmosphere of active carbon. The average particle size of the SrAl₂O₄:Eu,Dy phosphor nanoparticles ranges from 15 to 45 nm as indicated by transmission electron microscopy (TEM). The broad-band UV-excited luminescence of the SrAl₂O₄:Eu,Dy phosphor nanoparticles was observed at λ_max=513 nm due to transitions from the 4f⁶5d¹ to the 4f⁷ configuration of the Eu²⁺ ion. The results indicated that the main peaks in the emission and excitation spectrum of phosphor nanoparticles shifted to the short wavelength compared with the phosphor obtained by the solid-state reaction synthesis method. The decay speed of the afterglow for phosphor nanoparticles was faster than that obtained by the solid-state reaction method.     

Synthesis of CaAl2O4:Eu,Nd long persistent phosphor by combustion processes and its optical properties

Eu²⁺,Nd³⁺ co-doped calcium aluminate with high brightness and long persistent luminescence was prepared by the combustion method. The luminescent properties of CaAl₂O₄-based luminescent materials have been studied systematically. The phosphor powders were further investigated by X-ray diffractometer (XRD), photoluminescence excitation and emission spectra (PL) and brightness meter. The analytical results indicated that the phase of CaAl₂O₄ was formed when the initiating combustion temperature was 400 °C. The broad band UV excited luminescence of the CaAl₂O₄:Eu²⁺,Nd³⁺ was observed at the blue region (λ_max = 440 nm) due to transitions from the 4f⁶5d¹ to the 4f⁷ configuration of the Eu²⁺ ion. The decay time of the persistence indicated that the persistent luminescence phosphor has bright phosphorescence and maintains a long duration.