Trapping mechanism in the afterglow process of the rare-earth activated Y2O2S phosphors

Phosphorescence properties are investigated in Y₂O₂S phosphors doped with rare-earth (lanthanoid, Ln) ions. Luminescence afterglow with a decay time of several ten milliseconds is observed at room temperature in the phosphors activated by Nd, Sm, Eu, Dy, Ho, Tm, Er, and Yb. The depths (thermal activation energies) of the traps causing the afterglow are measured with the transient luminescence method.It is concluded that the excited electron and the hole in the conduction and valence bands are trapped separately in the states (impurity levels) located in the vicinity of the Ln³⁺ ion. The trapping depths of the level range from 0.3 to 1.1 eV and are dependent on the electron affinity of the Ln³⁺ ion estimated from the energy difference between the 4fⁿ⁺¹ and the 4fⁿ configurations in the 4f shell of the ion.     

Luminescence of Cs2NaScCl6:Pr3+: Effects of Changing the Elpasolite Lattice Parameter

Abstract

The luminescence spectrum of Cs₂NaScCl₆:Pr³⁺ (0.1 at.%) has been recorded at temperatures down to 10 K and assigned between 20,800 and 9900 cm^?1. Twenty‐three energy levels of the 4f² configuration Pr³⁺ ion were located and then fitted by the conventional 4f² crystal field calculation, as well as by a configuration interaction assisted crystal field (CIACF) calculation. The latter gave a much better fit. A comparison of the fit for Cs₂NaScCl₆:Pr³⁺ with fits upon the same set of energy levels in Cs₂NaYCl₆:Pr³⁺ and Cs₂NaPrCl₆, where the fifth nearest neighbor of Pr³⁺ is changed and the lattice parameter increases along this series, shows a decrease in the magnitudes of the crystal field parameters, which were also semiquantitatively simulated. Several facets of the emission spectra are interesting, including the observation of weak progressions in the totally symmetric Pr–Cl stretching vibration and the occurrences of the resonance of electronic and vibronic states. The spectra of Cs₂NaScCl₆:Pr³⁺ (1 at.%) differ considerably from those of the more dilute system and show that other species are formed rather than a complete substitution of the Sc³⁺ sites by Pr³⁺.     

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.     

New perovskite oxides LaBa<Emphasis Type="Italic">M</Emphasis>CoO<Subscript>5 + δ</Subscript> (<Emphasis Type="Italic">M</Emphasis> = Fe,Cu): Synthesis,structure, and properties

The electrical resistivity ρ and the thermopower S of ceramic materials LnBaCuFeO_{5 + δ} (Ln= La, Pr, Nd, Sm, Gd-Lu) are measured in air at temperatures in the range from 300 to 1100 K. All the studied ferrocuprates are p-type semiconductors. The electrical resistivity ρ and the thermopower S of these compounds increase with a decrease in the radius of the Ln ³⁺ cation (with an increase in the number of 4f electrons n in Ln ³⁺). The nonmonotonic behavior of the dependences ρ=f(n) and S=f(n) indicates that the electrical properties of the layered ferrocuprates LnBaCuFeO_{5 + δ} depend on the electronic configuration of the Ln ³⁺ cation. The power factors P calculated for the LnBaCuFeO_{5 + δ} ceramic materials from the experimental values of ρ and S increase with increasing temperature and, at T = 1000 K, reach the maximum values P = 102.0 and 54.1 μW m⁻¹ K⁻² for Ln = Pr(4f ²) and Sm(4f ⁵), respectively, and become close to each other and equal to 30–35 μW m⁻¹ K⁻² for Ln = Gd(4f ⁷), Dy(4f ⁹), and Ho(4f ¹⁰).     

Submillimeter electron-nuclear excitation spectra in CsCdBr3:Ln3+ (Ln=Tm, Ho) crystals

The ESR spectra of single and pair impurity centers of thulium and holmium ions in CsCdBr₃:Tm³⁺ and CsCdBr₃:Ho³⁺ crystals are measured in the frequency range 160-400 GHz. Analysis of the characteristic features of the hyperfine structure of the ESR lines and analysis of the variations in the spectra as a function of the temperature and external magnetic field shows that the Ln³⁺ ions substitute for Cd²⁺ ions and predominantly form symmetric pair centers of the type Ln³⁺-(vacancy at a neighboring Cd²⁺ site)-Ln³⁺. The ESR spectra of CsCdBr₃: Ln³⁺ crystals are used to make a positive identification of the optical spectra of selective laser excitation. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 7, 535–540 (10 April 1997)     

Luminescence spectroscopy of Ce3+-doped ABaPO4 (A=Li, Na, K) phosphors

Ce³⁺ doped ABaPO₄ (A=Li, Na, K) phosphors were prepared by conventional high temperature solid-state reaction. The phosphors were investigated by XRD, photoluminescence excitation and emission spectra, and luminescence decay curves. The five 5d levels corresponding to the 4f¹→4f⁰5d¹ transition of Ce³⁺ ions were identified. The spectroscopic parameters, e.g., the 5d barycenter, the crystal-field splitting, and the Stokes shift, were discussed. LiBaPO₄:Ce³⁺ phosphor could be efficiently excited by the near-UV lights (330–420 nm) and showed a broad emission band in the range of 430–620 nm with the maximum wavelength at 468 nm. In contrast, Ce³⁺-doped NaBaPO₄ and KBaPO₄ showed only excitation bands in a limited UV region (230–370 nm) and have blue emission at 385 nm and 416 nm, respectively. The temperature quenching of luminescence and the chromaticity coordinates were reported. The luminescence properties were discussed by analyzing the crystal structure and the local surroundings of Ce³⁺ ions on the Ba²⁺ sites.     

5d-4f luminescence of Nd3+, Gd3+, Er3+, Tm3+, and Ho3+ ions in crystals of alkaline earth fluorides

The vacuum ultraviolet emission spectra of alkaline-earth fluoride (CaF₂, SrF₂, BaF₂) crystals with rare earth impurity ions (Nd, Gd, Er, Tm, Ho) have been investigated. The main luminescence bands are described well by the transitions from the lowest excited 5d state to different 4f levels of rare earth ions.     

Self-assembled 3D sphere-like SrMoO4 and SrMoO4:Ln3+ (Ln = Eu,Sm, Tb,Dy) microarchitectures: Facile sonochemical synthesis and optical properties

Three-dimensional (3D) well-defined SrMoO₄ and SrMoO₄:Ln³⁺ (Ln = Eu, Sm, Tb, Dy) hierarchical structures of obvious sphere-like shape have been successfully synthesized using a large-scale and facile sonochemical route without using any catalysts or templates. X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), and photoluminescence (PL) spectra were used to characterize the samples. The intrinsic structural feature of SrMoO₄ and external factor, namely the ultrasonic time and the pH value, are responsible for the ultimate shape evolutions of the product. The possible formation mechanism for the product is presented. Additionally, the PL properties of SrMoO₄ and SrMoO₄:Ln³⁺ (Ln = Eu, Sm, Tb, Dy) hierarchical structures were investigated in detail. The Ln³⁺ ions doped SrMoO₄ samples exhibit respective bright red–orange, yellow, green and white light of Eu³⁺, Sm³⁺, Tb³⁺ and Dy³⁺ under ultraviolet excitation, and have potential application in the field of color display. Simultaneously, this novel and efficient pathway could open new opportunities for further investigating about the properties of molybdate materials.     

Luminescence of europium and ytterbium ions in strontium hexaborate

We have studied the luminescent properties of Eu^2+/3+ and Yb²⁺ ions in strontium hexaborate SrB₆O₁₀ for excitation in the 120–400 nm region. The luminescence spectra of Ln²⁺ ions in SrB₆O₁₀ consist of overlapping bands in the 370–520 nm region, due to 5d → 4f transitions at several nonequivalent centers. In the excitation spectra, besides the bands associated with 4f → 5d transitions in the Ln²⁺ ions, we also observe a band in the 135–160 nm region due to the transitions O(2p) → B(2s,2p) within the borate anions. The luminescence of the Eu³⁺ ions is excited most efficiently in the region of the Eu³⁺ charge transfer band (λ_max = 226 nm). The results obtained are compared with data for Ln in other strontium borates. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 6, pp. 770–774, November–December, 2006.     

Influence of rare-earth ions on the dielectric response in stillwellite glasses

Experimental spectra of the dielectric response function of glasses with the composition LnBGeO₅ (Ln = La, Pr, Tb, Er, Sm) in the terahertz and infrared ranges are compared. Absorption bands are identified, and their contributions to permittivity are established. The contours of the boson peak and the dependence of the dielectric loss on the type of rare-earth ion are determined.     

Luminescence and excitation spectra of YAG:Nd excited by synchrotron radiation

The low-temperature 4f²5d→4f³ fast emission of Nd³⁺ from YAG:Nd³⁺ has been studied under excitation by synchrotron radiation. Additionally, 4f³→4f³ luminescence of Nd³⁺ has been observed and assigned to transitions from the ²F(2)_5/2 and ⁴F_3/2 multiplet terms. The observed experimental spectra of Nd³⁺ d-f emission and f-d excitation are well simulated by crystal-field calculations.     

Hybrid density functional study of optically active Er3+ centers in GaN

Understanding the luminescence of GaN doped with erbium (Er) requires a detailed knowledge of the interaction between the rare‐earth dopant and the nitride host, including intrinsic defects and other impurities that may be present in the host material. We address this problem through a first‐principles hybrid density functional study of the structure, energetics, and transition levels of the Er impurity and its complexes with N and Ga vacancies, substitutional C and O impurities, and H interstitials in wurtzite GaN. We find that, in the interior of the material, Er_Ga is the dominant Er³⁺ center with a formation energy of 1.55 eV, Er_Ga–V_N possesses a deep donor level at 0.61 eV which can assist in the transfer of energy to the 4f ‐electron core. Multiple optically active Er³⁺ centers are possible in Er‐doped GaN. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Spectroscopic characterisation of Er-doped LuVO4 single crystals

The LuVO₄:Er single crystals were grown by the Czochralski technique. The crystal-field split energy levels of Er³⁺ ion were derived experimentally employing absorption and emission spectra measured at T=10 K. The Judd–Ofelt phenomenological method was used to estimate intensity parameters, radiative lifetimes and branching ratios of luminescence. The excited state dynamics of the LuVO₄:Er systems was investigated and experimental lifetimes of emitting levels were measured. The emission cross section of the ⁴I_13/2→⁴I_15/2 transition in the infrared was calculated by the Füchtbauer–Ladenburg method. The gain cross section, estimated for several inverse-population parameters, allowed us to evaluate a potential laser activity of the LuVO₄:Er system at 1.6 μm. Also, the potential range of the optical pumping was assessed based on absorption spectra achieved at the room temperature. The optical losses related to the green up-converted emission, encountered under the 978 nm excitation between 300 and 670 K were indicated and discussed. Spectroscopic peculiarities of the Er³⁺-doped LuVO₄ crystal were discussed in relation to optical properties of the YVO₄:Er and GdVO₄:Er crystals. Taking into account the high quantum efficiency of the ⁴I_13/2 level, and satisfactory absorption and emission features, the LuVO₄:Er crystal can be considered as a promising active material for laser operation near 1.6 μm.     

Unusual luminescence of octahedrally coordinated divalent europium ion in Cs2MP2O7 (M=Ca, Sr)

The excitation and emission spectra of octahedrally coordinated europium ion (Eu²⁺) ions in Cs₂M²⁺P₂O₇ (M²⁺=Ca, Sr) are reported and discussed. The remarkable features of the Eu²⁺ luminescence in these phosphate materials include (a) very large Stokes shift of emission (∼1 eV), (b) high luminescence quenching temperature, and (c) unusually low energy of the emitted photons for Eu²⁺ luminescence in phosphate-based materials. The broad emission bands of Eu²⁺ in Cs₂CaP₂O₇ and Cs₂SrP₂O₇ peak at 607 and 563 nm, respectively. The Stokes shift, crystal field splitting, centroid shift and the red shift of the Eu²⁺ 4f⁶5d¹ electronic configuration have been estimated from the relevant optical data. The radiative lifetime of the Eu²⁺ emission in Cs₂M²⁺P₂O₇ is ∼1.2 μs. The nature of the Eu²⁺ emission in Cs₂M²⁺P₂O₇ is discussed and arguments are presented to associate the luminescence with an extreme case of normal 4f⁶5d¹→4f⁷[⁸S_7/2] emission.     

Synchrotron and laser excitation of luminescence in PbWO<Subscript>4</Subscript>:Tb crystals at different temperatures

The effect of temperature on the spectral luminescence characteristics of PbWO₄:Tb³⁺ crystals with synchrotron and laser excitation is studied. If PbWO₄:Tb³⁺ is excited by synchrotron radiation with λ = 88 nm at 300 K, a faint recombination luminescence of the impurity terbium is observed against the matrix luminescence. When the temperature is reduced to 8 K, the luminescence intensity of PbWO₄:Tb³⁺ increases by roughly an order of magnitude and the characteristic luminescence of the unactivated crystal is observed. Excitation of PbWO₄:Tb³⁺ by a nitrogen laser at 300 K leads to the appearance of emission from Tb³⁺ ions. At 90 K, a faint matrix luminescence is observed in addition to the activator emission. The formation of the luminescence excitation spectra for wavelengths of 60–320 nm is analyzed and the nature of the emission bands is discussed.     

Thermoelectric properties of layered ferrocuprates <Emphasis Type="Italic">Ln</Emphasis>BaCuFeO<Subscript>5 + δ</Subscript> (<Emphasis Type="Italic">Ln</Emphasis>= La,Pr, Nd,Sm, Gd-Lu)

The electrical resistivity ρ and the thermopower S of ceramic materials LnBaCuFeO_{5 + δ} (Ln= La, Pr, Nd, Sm, Gd-Lu) are measured in air at temperatures in the range from 300 to 1100 K. All the studied ferrocuprates are p-type semiconductors. The electrical resistivity ρ and the thermopower S of these compounds increase with a decrease in the radius of the Ln ³⁺ cation (with an increase in the number of 4f electrons n in Ln ³⁺). The nonmonotonic behavior of the dependences ρ=f(n) and S=f(n) indicates that the electrical properties of the layered ferrocuprates LnBaCuFeO_{5 + δ} depend on the electronic configuration of the Ln ³⁺ cation. The power factors P calculated for the LnBaCuFeO_{5 + δ} ceramic materials from the experimental values of ρ and S increase with increasing temperature and, at T = 1000 K, reach the maximum values P = 102.0 and 54.1 µW m^?1 K^?2 for Ln = Pr(4f ²) and Sm(4f ⁵), respectively, and become close to each other and equal to 30–35 µW m^?1 K^?2 for Ln = Gd(4f ⁷), Dy(4f ⁹), and Ho(4f ¹⁰).     

Synthesis and luminescence properties of Na2Ba2Si2O7:Eu2+ phosphor

Green-emitting phosphor Na₂Ba₂Si₂O₇:Eu²⁺ has been synthesized by a conventional high-temperature solid-state reaction. The phase structure and luminescence properties are characterized by the X-ray powder diffraction, diffuse reflectance spectra, photoluminescence excitation and emission spectra, temperature-dependent emission spectra, respectively. It can be efficiently excited in the wavelength range of 325–400 nm and consists of a strong broad green band centered at about 501 nm, which is ascribed to 4f⁶6s⁰5d¹ → 4f⁷6s²5d⁰ transition of Eu²⁺. The critical quenching concentration of Eu²⁺ in the Na₂Ba₂Si₂O₇ host is about 0.8 mol % and corresponding quenching behavior is ascribed to be electric dipole–dipole interaction. Furthermore, the phosphor has good thermal stability property, and the activation energy for thermal quenching is calculated as 0.34 eV.     

Spectral studies on erbium doped potassium lithium tantalate niobate single crystal grown by the step-cooling technique

An erbium doped K_0.603Li_0.397Ta_0.428Nb_0.572O₃ single crystal was grown by the step-cooling technique. The crystal has a tetragonal tungsten bronze-type structure at room temperature with a Curie temperature of 303°C. There are Er ions characteristic absorption bands around 449, 485, 521, 550, and 652 nm in the visible absorption spectrum. Upconversion fluorescence spectra and power dependence centered at 527 nm, 548 nm, and 660 nm under 975 nm excitation were measured at room temperature. Decay lifetimes of the 548 nm and 660 nm emission bands are 281 μs and 420 μs, respectively. The lifetime of the 548 nm emission corresponding to the transition of ?⁴ S _3/2→⁴ I _15/2 is ten times the lifetime of the same transition of Er³⁺ in LiNbO₃ crystal and twice in KYb(WO₄)₂ crystal. The crystal might become a promising upconversion laser material. The upconversion mechanism of Er³⁺ in the sample was discussed based on decay curves and pump power dependence analyses in this work.