Improvement of Er:I11/2→Ce:F5/2 energy transfer rate in Er/Ce co-doped TeO2-ZnO-Na2O-Nb2O5 glasses

The B₂O₃ component was introduced into Er³⁺/Ce³⁺ co-doped TeO₂-ZnO-Na₂O-Nb₂O₅ glass to improve energy transfer rate of Er³⁺:⁴I_11/2→Ce³⁺:²F_5/2 phonon-assisted cross-relaxation process. With the 6 mol% substitution of B₂O₃ for TeO₂, the energy transfer rate increased from 1300 to 1831 s⁻¹ and the fluorescence intensity increased by about 13.4%. However, the more B₂O₃ substitution in the same glass system reduced the quantum efficiency of Er³⁺:⁴I_13/2→⁴I_15/2 transition due to the higher OH⁻ group concentration. The results show that an appropriate amount of B₂O₃ component can be used to improve the phonon-assisted energy transfer rate and enhance 1.53 μm fluorescence emission by increasing the phonon energy of host glass. The effect of B₂O₃ on the energy transfer process, the lifetimes of the ⁴I_11/2 and ⁴I_13/2 levels, and the upconversion emission have also been investigated.     

Effects of Ce on the spectroscopic properties of transparent phosphate glass ceramics co-doped with Er/Yb

We have prepared Er³⁺/Yb³⁺ co-doped transparent phosphate glass ceramics by the high-temperature melting technique, and demonstrated the influence of energy acceptors Ce³⁺ ions on the up-conversion and 1.54 μm emission properties of Er³⁺. The energy transfer mechanism is discussed based on the energy matching and the energy level structure. The phonon-assisted energy transfer between Er³⁺ and Ce³⁺ favors population feeding from the ⁴I_11/2 to the ⁴I_13/2 level, and therefore drastically decreases the up-conversion emission intensity of Er³⁺. Meanwhile, 1.54 μm fluorescence enhances greatly with the introduction of Ce³⁺ ions at the proper concentration.     

Ion beam mixing induced by electronic energy deposition in Fe-Si multilayers

Abstract

Spectroscopic properties of Ce³⁺ ions in GdAlO₃ crystal are presented. At least three Ce³⁺ nonequivalent centres (multisites) are present in this crystal. Energy transfer from the Ce³⁺ main in the UV emitting centres to the Ce³⁺ green emitting centres is observed. Ce³⁺ fluorescence decays are either fast (1.5–20 ns) or slower due to complicated processes of energy transfer and migration (Ce³⁺)_i → (Gd³⁺)_n-steps → (Ce³⁺)_j (energy transfer through Gd³⁺ sublattice).     

Heat-treatment-induced luminescence degradation in Tb-doped CePO4 nanorods

CePO₄:Tb nanorods were synthesized via a simple wet-chemical route. The as-synthesized CePO₄:Tb nanorods present high photoluminescence efficiency due to an efficient energy transfer form Ce³⁺ to Tb³⁺. However, heat treatment at 150 °C in air leads to a significant decrease of photoluminescence. X-ray photoelectron spectroscopy and excitation spectra revealed the oxidation of Ce³⁺ to Ce⁴⁺ in the heat-treatment process, which should be responsible for significant photoluminescence degradation due to the breakage of Ce³⁺→Tb³⁺ energy transfer. This conclusion is further supported by atmosphere and size effects of photoluminescence of CePO₄:Tb under the heat treatment.     

Photoluminescence characterization of Ce and Dy doped Li2CaGeO4 phosphors

Ce³⁺ and Dy³⁺ activated Li₂CaGeO₄ phosphors were prepared by a solid-state reaction method, and characterized by XRD (X-ray diffraction) and photoluminescence techniques. The characteristic emission bands of Dy³⁺ due to ⁴F_9/2→⁶H_15/2 (blue) and ⁴F_9/2→⁶H_13/2 (yellow) transitions were detected in the emission spectra of Li₂CaGeO₄:Dy³⁺. Ce³⁺ broad band emission was observed in Li₂CaGeO₄:Ce³⁺ phosphors at 372 and 400 nm due to 5d→4f transition when excited at 353 nm. Co-doping of Ce³⁺ enhanced the luminescence of Dy³⁺ significantly and concentration quenching occurs when Dy³⁺ is beyond 0.04 mol%. White-light with different hues can be realized by tuning Dy³⁺ concentration in the phosphors.     

Luminescence and electronic excitations in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript>: Ce<Superscript>3+</Superscript> crystals

This paper reports on a study of the luminescence emitted by Li₆Gd(BO₃)₃: Ce³⁺ crystals under selective photoexcitation to lower excited states of the host ion Gd³⁺ and impurity ion Ce³⁺ within the 100–500-K temperature interval, where the mechanisms of migration and relaxation of electronic excitation energy have been shown to undergo noticeable changes. The monotonic 10–15-fold increase in intensity of the luminescence band at 3.97 eV has been explained within a model describing two competing processes, namely, migration of electronic excitation energy over chains of Gd³⁺ ions and vibrational energy relaxation between the ⁶ I _j and ⁶ P _j levels. It has been shown that radiative transitions in Ce³⁺ ions from the lower excited state 5d ¹ to ² F _5/2 and ² F _7/2 levels of the ground state produce two photoluminescence bands, at 2.08 and 2.38 eV (Ce1 center) and 2.88 and 3.13 eV (Ce2 center). Possible models of the Ce1 and Ce2 luminescence centers have been discussed.     

Luminescence in LaBaB9O16 prepared by combustion synthesis

LaBaB₉O₁₆ phosphors activated by various ions belonging to ns², 3dⁿ and 4fⁿ configurations were prepared by combustion synthesis. Phosphors’ synthesis and luminescence spectra are reported. Most of the activators displayed intense characteristic emission. Pr³⁺→Gd³⁺, Ce³⁺→Tb³⁺, Ce³⁺→Dy³⁺, Ce³⁺→Mn²⁺ and Bi³⁺→Mn²⁺ energy transfers were also observed. In particular, Ce³⁺→Tb³⁺ energy transfer leads to an efficient green emitting phosphor.     

Ultraviolet upconversion luminescence of Gd from Ho and Gd codoped oxide ceramic induced by 532-nm CW laser excitation

Ultraviolet upconversion emissions around 314 nm from ⁶P_J states of Gd³⁺ ions have been observed in Y_1.98 − xGd_xHo_0.02O₃ (x = 0.02, 0.10, 0.20, and 0.30) oxide ceramics under the excitation of a continuous-wave 532-nm laser. We found that the energy transfer process from Ho³⁺ to Gd³⁺ plays an important role in populating the ⁶P_J states of Gd³⁺. The doping of Gd³⁺ ions does not affect ⁵G₄ and ⁵G₅ states but only the ³D₃ state of Ho³⁺. The emissions from ³D₃ state decrease with the increase of Gd³⁺ concentration. Power dependence curves and time-resolved spectra have been measured to identify the proposed upconversion mechanism.     

Enhanced luminescent properties of Y3Al5O12:Tb,Ce phosphor prepared by spray pyrolysis

Yttrium aluminum garnet (YAG) particles doped with Tb³⁺ or double doped with Tb³⁺ and Ce³⁺ were prepared by spray pyrolysis and characterized by photo- and cathode-luminescence. It was tried to incorporate a broad band of Ce³⁺ activator into the line peaks of Tb³⁺ in YAG host without the reduction of emission intensity. Ce-codoped YAG:Tb particles showed a broad band emission due to the d-f transition of Ce³⁺ and a reduction in the intensity of emission peaks due to ⁵D₃-⁷F_j (j=3, 4, 5, 6) transition of Tb³⁺ when they were excited by the ultraviolet light of 270 nm. These results supported that an effective energy transfer occurs from Tb³⁺ to Ce³⁺ in YAG host. Codoping Ce³⁺ ions greatly intensified the excitation peak at 270 nm for the emission at 540 nm of Tb³⁺, which means that more lattice defects, involving in the energy absorption and transfer to Tb³⁺, are formed by the Ce³⁺ codoping. The finding gives a promising approach for enhancing the luminescence efficiency.     

VUV spectroscopic properties of Ce and Pr-doped AREP2O7-type alkali rare earth diphosphates (A=Na, K, Rb, Cs; RE=Y, Lu)

Spectroscopic properties of Ce³⁺ and Pr³⁺-doped AREP₂O₇-type alkali rare earth diphosphates (A=Na, K, Rb, Cs; RE=Y, Lu) have been investigated using VUV spectroscopy technique. Ce³⁺-doped samples show typical Ce³⁺ emission in the range of 325-450 nm. The strong host absorption band starting at around 160 nm indicates that the optical band gap of AREP₂O₇ hosts is at least 7.7 eV, and the host→Ce³⁺ energy transfer process is rather efficient. However, AREP₂O₇:Pr³⁺ samples show less efficient host→Pr³⁺ energy transfer. The direct Pr³⁺ 4f²→4f¹5d¹ excitation, which are 12160±640 cm⁻¹ higher respect to that of Ce³⁺, leads to strong 4f¹5d¹→4f² emission bands in the range of 230-325 nm but no obvious 4f²→4f² emission lines.     

Ultraviolet emissions from Gd ions excited by energy transfer from Ho ions

Ultraviolet (UV) upconversion (UC) emissions of Gd³⁺ ion were investigated in Y_1.838−xGd_xYb_0.16Ho_0.002O₃ (x=0, 0.16, 0.4, 1, 1.4) bulk ceramics under 976 nm laser diode (LD) excitation. The UC emissions centered at 309 and 315 nm are assigned to the transition of ⁶P_5/2→⁸S_7/2 (Gd) and ⁶P_7/2→⁸S_7/2 (Gd). The ⁶P_J levels of Gd³⁺ ions are populated by an energy transfer (ET) process from ⁸S_7/2 (Gd)+(³P₁, ³L₈, ³M₁₀) (Ho)→⁶P_J (Gd)+⁵I₈ (Ho). A four-photon ET UC process was confirmed by the dependence of the ⁶P_7/2 level emission intensity on the pumping power. We found that the intensity of the UC emissions increased with Gd³⁺ ion concentration and peaked at 8 mol%, then starts to decrease until the Gd³⁺ ion concentration reached 70 mol%. The variation in the UV emission intensity is the result of the competition between the ET process and concentration quenching effect. Theoretical calculations based on steady-state equations validated the proposed UC mechanisms.     

Photoluminescence and energy transfer of Tm doped LiIn (WO4)2 blue phosphors

Room temperature steady and time resolved emission spectra of LiIn_1−xTm_x(WO4)2 (where thulium concentration is 0, 0.5, 1, 5 and 10 at%) blue phosphors, under UV excitation energy have been investigated. The concentration quenching effect on the blue emission, due to the (WO₄)⁻² groups and ¹G₄→³H₆ emission transition of Tm³⁺ were studied. Two energy transfer mechanisms are shown. The first takes place between excited (WO₄)⁻² groups and the ¹G₄ energy level of Tm³⁺, and is mainly analyzed by phonon-assisted energy transfer. The second mechanism is due to an energy transfer from the excited Tm³⁺ ions to the surrounding ground state Tm³⁺ ions. The non-exponential decay curves of the ¹G₄ level observed for higher concentrations are analyzed by the Inokuti–Hirayama model. We think that the quenching effect between Tm³⁺ ions is mainly linked to the dipole–dipole interactions.     

Luminescence of Ce and Pr doped Sr2Mg(BO3)2 under VUV-UV and X-ray excitation

This report presents the luminescence properties of Ce³⁺ and Pr³⁺ activated Sr₂Mg(BO₃)₂ under VUV-UV and X-ray excitation. The five excitation bands of crystal field split 5d states are observed at about 46 729, 44 643, 41 667, 38 314 and 29 762 cm⁻¹ (i.e. 214, 224, 240, 261 and 336 nm) for Ce³⁺ in the host lattice. The doublet Ce³⁺ 5d→4f emission bands were found at about 25 840 and 24 096 cm⁻¹ (387 and 415 nm). The influence of doping concentration and temperature on the emission characteristics and the decay time of Ce³⁺ in Sr₂Mg(BO₃)₂ were investigated. For Pr³⁺ doped samples, the lowest 5d excitation band was observed at about 42017 cm⁻¹ (238 nm), a dominant band at around 35714 cm⁻¹ (280 nm) and two shoulder bands were seen in the emission spectra. The excitation and emission spectra of Ce³⁺ and Pr³⁺ were compared and discussed. The X-ray excited luminescence studies show that the light yields are ∼3200±230 and ∼1400±100 photons/MeV of absorbed X-ray energy for the samples Sr_1.86Ce_0.07Na_0.07Mg(BO₃)₂ and Sr_1.82Pr_0.09Na_0.09Mg(BO₃)₂ at RT, respectively.     

The microscopic interaction parameters for Er/Ho energy transfer in tellurite glasses

We investigate the energy transfer between Er³⁺/Ho³⁺ in tellurite glasses. The main channels of energy transfer between Er³⁺/Ho³⁺ are analyzed in detail. The microscopic interaction parameters of resonant and non-resonant (phonon-assisted) energy transfer parameters via Er³⁺→Ho³⁺ are calculated. The result shows that the resonant energy transfers Er³⁺(²H_11/2(⁴S_3/2))→Ho³⁺(⁵F₄(⁵S₂)) and Er³⁺(⁴F_9/2)→Ho³⁺(⁵F₅) are very efficient and non-resonant energy transfers Er³⁺(⁴I_13/2)→Ho³⁺(⁵I₇) and Er³⁺(⁴I_11/2)→Ho³⁺(⁵I₆), which are a phonon-assisted energy transfer process because of energy mismatch are also existed and cannot be neglected.     

Energy transfer in Y3Al5O12:Ce, Pr and CaMoO4:Sm, Eu phosphors

Non-radiative energy transfers (ET) from Ce³⁺ to Pr³⁺ in Y₃Al₅O₁₂:Ce³⁺, Pr³⁺ and from Sm³⁺ to Eu³⁺ in CaMoO₄:Sm³⁺, Eu³⁺ are studied based on photoluminescence spectroscopy and fluorescence decay patterns. The result indicates an electric dipole-dipole interaction that governs ET in the LED phosphors. For Ce³⁺ concentration of 0.01 in YAG:Ce³⁺, Pr³⁺, the rate constant and critical distance are evaluated to be 4.5×10⁻³⁶ cm⁶ s⁻¹ and 0.81 nm, respectively. An increase in the red emission line of Pr³⁺ relative to the yellow emission band of Ce³⁺, on increasing Ce³⁺ concentration is observed. This behavior is attributed to the increase of spectral overlap integrals between Ce³⁺ emission and Pr³⁺ excitation due to the fact that the yellow band shifts to the red spectral side with increasing Ce³⁺ concentration. In CaMoO₄:Sm³⁺, Eu³⁺, Sm³⁺-Eu³⁺ transfer occurs from ⁴G_5/2 of Sm³⁺ to ⁵D₀ of Eu³⁺. The rate constant of 8.5×10⁻⁴⁰ cm⁶ s⁻¹ and the critical transfer distance of 0.89 nm are evaluated.     

Synthesis and photoluminescence properties of Sm-doped LaMgB5O10 and GdMgB5O10

Luminescence and reflection spectra as well as luminescence kinetics of the 1 mol% Sm³⁺-doped crystalline lanthanum magnesium meta borate (LaMgB₅O₁₀) and gadolinium magnesium meta borate (GdMgB₅O₁₀) were analyzed. Materials were synthesized by conventional solid state route and showed bright orange-red emission under UV excitation. Emission spectra contain sharp and well resolved Sm³⁺⁴G_5/2→⁶H_J transitions indicating a strong crystal-field effect. In case of gadolinium compound energy transfer between Gd³⁺ and Sm³⁺ was detected. The luminescent kinetics of the Sm³⁺ in analyzed powders is characterized by single exponential decay and experimental values vary in the range 2.2-2.4 ms.     

Influence of high hydrostatic pressure on Eu-luminescence in KMgF3:Eu crystal

Spectroscopic investigations are presented of KMgF₃:Eu²⁺ crystal under high hydrostatic pressure from ambient to 310 kbar. The sample was excited by 30 ps pulses generated by optical parametric generator (OPG) system with wavelength controlled between 210 and 325 nm. The Grüneisen parameters of individual phonons are obtained from the pressure shift of the Eu²⁺ emission related to the ⁶P_7/2→⁸S_7/2 transition accompanied by phonon sideband. The luminescence decays exponentially for the pressure below 135 kbar with lifetime of 3.30 ms and slightly nonexponential above 135 kbar, while the average decay time is nearly independent of the pressure. The results obtained for KMgF₃:Eu²⁺ are compared with those for LiBaF₃:Eu²⁺ in which the ⁶P_7/2→⁸S_7/2 emission is replaced by the broadband emission of the 4f⁶5d¹→4f⁷ transition at high hydrostatic pressure.     

Enhanced luminescent properties and optical absorption of γ-irradiated KI: Ce, Tb crystals

This paper reports that KI doped with Ce³⁺ or double doped with Tb³⁺ and Ce³⁺ were prepared by the Bridgman-Stockbarger method and characterized by optical absorption photoluminescence (PL), thermoluminescence (TL), photostimulated emission (PSL) and TL emission. The optical absorption measurement indicates that F and V₁, V₂ centers are formed in the crystals during the γ irradiation process. It was attempted to incorporate a broad band of Ce³⁺ activator into the narrow band emission of Tb³⁺ in the KI host without the reduction of emission intensity. Ce³⁺-co-doped KI and Tb crystals showed a broad band emission due to the d-f transition of Ce³⁺ and a reduction in the intensity of emission peaks due to the ⁵D₃-⁷F_j (j=3,4,5,6) transition of Tb³⁺, when they were excited at 240 nm.These results supported that an effective energy transfer occurs from Tb³⁺ to Ce³⁺ in the KI host. Co-doping Ce³⁺ ions greatly intensified the excitation peak at 260 nm for the emission at 393 nm of Tb³⁺, which means that more lattice defects, involved in the energy absorption and transfer to Tb³⁺, are formed by the Ce³⁺ co-doping. The integrated light intensity is an order of magnitude higher as compared to the undoped samples for similar doses of irradiation and heating rates. The defects generated by irradiation were monitored by optical absorption and TSL Trap parameters for the TL process are calculated and presented.     

Optical properties of (ZnO)0.5(P2O5)0.5 glasses doped with Gd2O3:Eu nanoparticles and Eu2O3

Binary (ZnO)_0.5(P₂O₅)_0.5 glasses doped with Eu₂O₃ and nanoparticles of Gd₂O₃:Eu were prepared by conventional melt-quench method and their luminescence properties were compared. Undoped (ZnO)_0.5(P₂O₅)_0.5 glass is characterized by a luminescent defect centre (similar to L-centre present in Na₂O-SiO₂ glasses) with emission around 324 nm and having an excited state lifetime of 18 ns. Such defect centres can transfer the energy to Eu³⁺ ions leading to improved Eu³⁺ luminescence from such glasses. Based on the decay curves corresponding to the ⁵D₀ level of Eu³⁺ ions in both Gd₂O₃:Eu nanoparticles incorporated as well as Eu₂O₃ incorporated glasses, a significant clustering of Eu³⁺ ions taking place with the latter sample is confirmed. From the lifetime studies of the excited state of L-centre emission from (ZnO)_0.5(P₂O₅)_0.5 glass doped with Gd₂O₃:Eu nanoparticles, it is established that there exists weak energy transfer from L-centres to Eu³⁺ ions. Poor energy transfer from the defect centres to Eu³⁺ ions in Gd₂O₃:Eu nanoparticles doped (ZnO)_0.5(P₂O₅)_0.5 glass has been attributed to effective shielding of Eu³⁺ ions from the luminescence centre by Gd-O-P type of linkages, leading to an increased distance between luminescent centre and Eu³⁺ ions.     

Electronic excitation dynamics and energy transfer in lithium-gadolinium borates doped by rare earths

This paper reports on luminescence studies of lithium borate Li₆Gd(BO₃)₃ doped with Eu³⁺ and Ce³⁺ and Li₆Eu(BO₃)₃ crystals upon selective excitation by synchronous radiation in the pump energy region 3.7–27 eV at temperatures of 10 and 290 K. The effective energy transfer between the rare-earth ions Gd³⁺ → Ce³⁺ and Gd³⁺ → Eu³⁺ is found to operate by the resonant mechanism, as well as through electron-hole recombination. A study is made of the fast decay kinetics of the Ce³⁺-center activator luminescence under intracenter photoexcitation and excitation in the interband transition region. The mechanisms underlying luminescence excitation and radiative relaxation of electron states of rare-earth ions are analyzed and energy transfer processes active in these crystals are discussed.