Fast d-f emission in Ce, Pr and Nd activated RbCl

In the search for new scintillator materials, Ce³⁺ doped chlorides are a promising class of materials, combining a high efficiency and fast response time. Even shorter response times may be achieved by replacing Ce³⁺ by Pr³⁺ or Nd³⁺ as the lifetime of the d-f emission is substantially shorter for these ions. Here we report on the luminescence properties of Ce³⁺, Pr³⁺ and Nd³⁺ in RbCl and investigate the potential as a scintillator material. Under UV excitation Ce³⁺ shows d-f emission between 325 and 425 nm. The emission originates from multiple (differently charge compensated) Ce³⁺ sites. The luminescence lifetime varies with wavelength and is ∼40 ns for the longer wavelength emission. For RbCl:Pr³⁺ three d-f emission band are observed between 250 and 350 nm which can be assigned to transitions from the lowest energy fd state to different ³H_J (J = 4-6) states within the 4f² configuration of Pr³⁺. The decay time is ∼17 ns. For the Nd³⁺ activated sample a weak emission band around 220 nm is observed only at 8 K which may be due to d-f emission. The very short lifetime (4 ns) is faster than the radiative lifetime, indicating that the d-f emission is quenched by relaxation to lower lying 4f³ states or by the process of photoionization. Under VUV excitation at wavelengths below 175 nm (the bandgap of RbCl) the d-f emission is very weak for Ce³⁺, Pr³⁺ and Nd³⁺ doped RbCl and the emission spectra are dominated by defect related emission. This indicates that energy transfer from the host lattice to the fd states is inefficient which prevents application as a scintillator material.     

Emission properties of (SrTiO3-TiO2):Pr eutectic with self-organized fractal microstructure

An investigation of spectroscopic properties of (SrTiO₃-TiO₂):Pr³⁺ eutectic and, for comparison, of bulk SrTiO₃:Pr³⁺ and TiO₂:Pr³⁺crystals is presented. Luminescence spectra have been measured under both 450 nm and 350 nm excitation wavelength. For UV excitation they are characterized by a dominant red luminescence corresponding to transition from the ¹D₂ level of Pr³⁺ ions. The mechanism responsible for quenching of blue (from ³P₀ state) and intensification of red luminescence is proposed to be thermally-induced radiationless relaxation involving a low-lying Pr³⁺-Ti⁴⁺ intervalence charge transfer state. Measured decay constants of ¹D₂ excited state of Pr³⁺ are compared with values obtained for other praseodymium doped titanate hosts.     

Enhanced G4 emission in NaLaF4: Pr, Yb and charge transfer in NaLaF4: Ce, Yb studied by fourier transform luminescence spectroscopy

A high resolution luminescence study of NaLaF₄: 1%Pr³⁺, 5%Yb³⁺ and NaLaF₄: 1%Ce³⁺, 5%Yb³⁺ in the UV to NIR spectral range using a InGaAs detector and a fourier transform interferometer is reported. Although the Pr³⁺(³P₀ → ¹G₄), Yb³⁺(²F_7/2 → ²F_5/2) energy transfer step takes place, significant Pr³⁺¹G₄ emission around 993, 1330 and 1850 nm is observed. No experimental proof for the second energy transfer step in the down-conversion process between Pr³⁺ and Yb³⁺ can be given. In the case of NaLaF₄: Ce³⁺, Yb³⁺ it is concluded that the observed Yb³⁺ emission upon Ce³⁺ 5d excitation is the result of a charge transfer process instead of down-conversion.     

Photoluminescence characteristics of Li-doped CaTiO3:Pr thin films grown on Si (100) substrate by PLD

The effects of Li-doped CaTiO₃:Pr³⁺ thin films have been investigated by varying the lithium ion concentrations from 0 to 5 wt.%. The films have been deposited on Si (100) substrate using a pulsed laser deposition technique. Structural properties of these films have been studied by the measurement of their XRD, SEM, and AFM. The variation of Li⁺ concentration influences the crystallinity and surface morphology of the CaTiO₃:Pr³⁺ thin films. As Li⁺ content increases from 0 to 1 wt.%, the crystallinity and intensity of emission increases. The dominant emission is from ¹D₂ → ³H₄ transition at 613 nm. The ¹D₂ emission quenching has also been observed in highly doped sample and is related to the cross-relaxation process between Pr³⁺ ions.     

Energy transfer and luminescence studies of Pr, Yb co-doped lead borate glass

Lead borate glass samples doped with the tripositive lanthanide ions Pr³⁺ and Yb³⁺ were synthesized by the conventional melting-quenching method. The luminescence properties and energy transfer process from Pr³⁺ to Yb³⁺ were investigated. Upon ultraviolet excitation, the room temperature luminescence decay curve of a sample containing only a low concentration of Pr³⁺ exhibited monoexponential decay from ¹D₂ with the lifetime 37 μs, without emission from ³P₀. The room temperature Pr³⁺ emission intensity decreased with the increase of Yb³⁺ mole ratio in the glass. Under the excitation of 454.5 nm at 10 K, a broad red emission band centered at 605 nm, and an NIR emission band at 995 nm were observed in the co-doped lead borate glass, originating from Pr³⁺ and Yb³⁺ ions, respectively. The decay curves of the ¹D₂ emission from Pr³⁺ with addition of Yb³⁺ in lead borate glass show non-monoexponential character, and are best described by a stretched exponential function. The average ¹D₂ decay time decreases considerably with the addition of Yb³⁺ in the glass. Decay curve fitting using a modified Inokuti-Hirayama expression indicates dipole-dipole energy transfer from Pr³⁺ to Yb³⁺, which is consistent with the expected cross-relaxation scheme. There is a good agreement of the estimated overall energy transfer efficiency obtained from the integrals under the normalized decay curves, or from the lifetimes fitted by the stretched exponential function, or from the average decay times.     

The influence of the Pr 4f5d configuration on the S0 emission efficiency and lifetime in LaPO4

The photoluminescence and excitation spectra of Pr³⁺ activated LaPO₄ has been investigated in the 1.6-300 K temperature region. At room temperature, the luminescence of LaPO₄:Pr³⁺ is composed of the interconfigurational 4f¹5d¹ → 4f² emission transitions. However, in the 1.6-60 K temperature range, the emission spectrum also consists of the intraconfigurational emission transitions that emanate from the ¹S₀ state. A radiative lifetime of 145 ns is measured for the Pr³⁺¹S₀ → ¹I₆ emission transition in LaPO₄. This is one of the shortest radiative lifetime observed for this transition in a solid. The energy position of the Pr³⁺¹S₀ state in LaPO₄ is established by high-resolution emission spectrum at 46 375 ± 5 cm⁻¹. A detailed analysis of the thermal quenching of the ¹S₀ lifetime and emission intensity is presented. It is proposed that the lowest energy state of the relaxed 4f¹5d¹ configuration is situated energetically below that of the ¹S₀ state.     

High pressure and time resolved luminescence spectra of Gd3Ga5O12:Pr crystal

Luminescence spectra and time resolved luminescence spectra of GGG crystal doped with Pr³⁺ were measured at high hydrostatic pressure from ambient to 220 kbar. Effect of pressure results in the red shift of all luminescence lines related to Pr³⁺ ion emission equals from −0.32 to −1.02 cm⁻¹/kbar and in the diminishing of the luminescence lifetimes. The luminescence decay related to emission from ³P₀ state was single-exponential and diminished with pressure from 23 μs at ambient pressure to 6.5 μs at 165 kbar. Luminescence decay related to transition form ¹D₂ state was two-exponential with longer decay equal to 162 μs at ambient pressure and 120 μs at 165 kbar. We discussed effect of pressure on the ¹D₂ → ³H₄ luminescence and emission from ³P₀ state in the context of non-radiative processes that depopulate the ³P₀ and populate the ¹D₂ state, considering mainly multiphonon relaxation processes and depopulation via the praseodymium trapped exciton state.     

Synthesis and blue to near-infrared quantum cutting of Pr/Yb co-doped Li2TeO4 phosphors

Near-infrared (NIR) quantum cutting luminescent materials Li₂TeO₄ doped with Pr³⁺ and Yb³⁺ were synthesized by solid-state reaction method. The dependence of Yb³⁺ doping concentration on the visible- and NIR-emissions, decay lifetime, and quantum efficiencies of the phosphors are investigated. Quantum cutting down-conversion involving 647 nm red emission and 960-1050 nm broadband near-infrared emission for each 487 nm blue photon absorbed is realized successfully in the resulting phosphors, of which the process of near-infrared quantum cutting could be expressed as ³P₀(Pr³⁺) → ²F_5/2(Yb³⁺) + ²F_5/2(Yb³⁺). The maximum quantum cutting efficiency approaches up to 166.4% in Li₂TeO₄: 0.3 mol%Pr³⁺, 1.8 mol%Yb³⁺ sample corresponding to the 66.4% value of energy transfer efficiency.     

Preparation and upconversion property of TeO2:Er/Yb nanoparticles

Different crystal structure of TeO₂ nanoparticles were used as the host materials to prepare the Er³⁺/Yb³⁺ ions co-doped upconversion luminescent materials. The TeO₂ nanoparticles mainly kept the original morphology and phase after having been co-doped the Er³⁺/Yb³⁺ ions. All the as-prepared TeO₂:Er³⁺/Yb³⁺ nanoparticles showed the green emissions (525 nm, 545 nm) and red emission (667 nm) under 980 nm excitation. The green emissions at 525 nm, 545 nm and red emission at 667 nm were attributed to the ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of the Er³⁺ ions, respectively. For the α-TeO₂:Er³⁺/Yb³⁺ (3/10 mol%) nanoparticles, three-photon process involved in the green (²H_11/2 → ⁴I_15/2) emission, while two-photon process involved in the green (⁴S_3/2→⁴I_15/2) and red (⁴F_9/2 → ⁴I_15/2) emissions. For the β-TeO₂:Er³⁺/Yb³⁺ (3/10 mol%) nanoparticles, two-photon process involved in the green (²H_11/2 → ⁴I_15/2), green (⁴S_3/2 → ⁴I_15/2) and red (⁴F_9/2 → ⁴I_15/2) emissions. It suggested that the crystal structure of TeO₂ nanoparticles had an effect on transition processes of the Er³⁺/Yb³⁺ ions. The emission intensities of the α-TeO₂:Er³⁺/Yb³⁺ (3/10 mol%) nanoparticles and β-TeO₂:Er³⁺/Yb³⁺ (3/10 mol%) nanoparticles were much stronger than those of the (α + β)-TeO₂:Er³⁺/Yb³⁺ (3/10 mol%) nanoparticles.     

High pressure luminescence and time resolved spectra of La2Be2O5:Pr

We present luminescence, luminescence excitation and luminescence time resolved spectra of La₂Be₂O₅:Pr³⁺ system. We used high pressure spectroscopy approaches, with high pressure applied in diamond anvil cell (DAC) and sapphire anvil cell (SAC), for detailed analysis of luminescence related to the 4f5d → 4f² and 4f² → 4f² transitions. We present effect of up-converted luminescence related to 4f5d → 4f² transition excited with 488 nm. We also discussed possibility of existence of praseodymium trapped exciton (PTE) states in La₂Be₂O₅:Pr³⁺ system. Lack of the PTE is attributed to high quantity of bulk modulus of this material.     

White-light long-lasting phosphor Sr2SiO4:Pr

A white-emitting phosphor Sr₂SiO₄: Pr³⁺ was synthesized through a solid-state reaction, and characterized by XRD, scanning electron microscopy (SEM), fluorescence spectrophotometer and thermo luminescence (TL) meter. Its emission spectra is composed of bluish purple (peaking at 390 nm), green (peaking at 535 nm) and red (peaking at 604 nm) light emission. They originate from the transitions of 4f → 5d, ³P₀ → ³H₅ and ¹D₂ → ³H₄ of Pr³⁺. The afterglow emission spectrum is similar to the emission spectra. And the afterglow can last over 40 min in darkness. The TL curve shows that there is only one thermo luminescence band peak at about 376.480 K, which is responsible for the long-lasting emission.     

Synthesis, characterization and photoluminescence properties of (Gd0.99,Pr0.01)2O2S sub-microphosphor by homogeneous precipitation method

Homogeneous precipitation method for synthesizing (Gd_0.99,Pr_0.01)₂O₂S sub-microphosphor was developed, using the commercially available Gd₂O₃, Pr₆O₁₁, H₂SO₄ and (NH₂)₂CO (urea) as the starting materials. It was found that the as-synthesized precursor is mainly composed of (Gd_0.99,Pr_0.01)₂(OH)₂(CO₃)(SO₄)·nH₂O. Pure quasi-spherical shaped (Gd_0.99,Pr_0.01)₂O₂S particles can be synthesized by calcining the precursor at a temperature higher than 700 °C for 1 h in flowing hydrogen. The (Gd_0.99,Pr_0.01)₂O₂S particles have a narrow size distribution with a mean grain size of about 300-400 nm. Photoluminescence spectra of (Gd_0.99,Pr_0.01)₂O₂S under 303 nm UV excitation show a green emission at 515 nm as the most prominent peak, which corresponds to the ³P₀ → ³H₄ transition of Pr³⁺ ions. Decay study reveals that the ³P₀ → ³H₄ transition of Pr³⁺ ions in Gd₂O₂S host lattice has a single exponential decay behavior.     

Photoluminescence of Tb/Ce co-doped aluminum oxynitride powders

Aluminum oxynitride(AlON) phosphors co-doped by Tb³⁺ and Ce³⁺ were synthesized by nitridation of the precursor which was co-precipitated from Al(NO₃)₃ solution and nanosized carbon black at 1750 °C for 2 "hrs" in flowing nitrogen atmosphere. The obtained AlON based powders were composed of polycrystalline spinel typed particles with sizes in the range of 1-3 μm. Under an excitation of 275 nm, it was found that co-doping of Ce³⁺ could drastically enhance the luminescence of AlON:Tb³⁺ powder by energy transfer. The product with 0.5 mol% Ce³⁺ and 0.67 mol% Tb³⁺ exhibited a strong broad green emission at 540 nm. The critical quenching concentration of Tb³⁺ in AlON:0.5 mol% Ce³⁺/xmol% Tb³⁺ phosphor was determined to be 0.67 mol%. It was supposed that the mechanism of concentration quenching of Tb³⁺ in AlON:0.5 mol% Ce³⁺ xmol% Tb³⁺ phosphor was dipole-dipole interaction.     

Rapid synthesis of pure and narrowly distributed Eu doped ZnO nanoparticles by solution combustion method

Pure ZnO:Eu³⁺ nanoparticles (~ 50 nm) were prepared by a solution combustion method. ZnO and Eu₂O₃ were used as starting materials and dissolved in nitric acid. Citric acid was used as a fuel. The reaction mixture was heated at 350 °C resulting into a rapid exothermic reaction yielding pure nanopowders. The atomic weight concentration of Eu³⁺ doped in ZnO was 20%. Transmission electron microscopy (TEM) was used to study the particle size and morphology. The nanopowders were characterized for phase composition using X-ray diffractrometry (XRD). Particle size distribution (PSD) analysis of ZnO: Eu³⁺ showed particle sizes ranging from 30 to 80 nm.The photoluminescence emission spectra of ZnO:Eu³⁺ nanostructures showed a strong band emission around 618 nm when excited with 515 nm wavelength.     

Crystal growth and spectral properties of Pr:La2(WO4)3

Pr³⁺-doped La₂(WO₄)₃ single crystal with dimensions up to Ø 20 mm × 35 mm has been grown by the Czochralski method. The structure of the Pr³⁺:La₂(WO₄)₃ crystal was determined by the X-ray powder diffraction and the Pr³⁺ concentration in this crystal was determined. The absorption and fluorescence spectra of Pr³⁺:La₂(WO₄)₃ crystal were measured at room temperature, and the fluorescence lifetime of main emission multiplets were estimated from the recorded decay curves. The spectral properties related to laser performance of the crystal were evaluated.     

Optical properties of the Tm and energy transfer between Tm→Pr ions in P2O5-CaO-SrO-BaO phosphate glass

A P₂O₅-CaO-SrO-BaO phosphate glass doped with Tm³⁺ and glasses doped with (Tm³⁺, Pr³⁺) were used for this study. The photo-luminescence behaviors of Tm³⁺ and Pr³⁺ in phosphate glass were investigated by absorption, excitation and emission spectroscopy. The energy transfer between Tm³⁺ and Pr³⁺ in phosphate glasses (which exhibit a variety of transfer efficiencies) was studied. The experimental quantum efficiencies of the luminescence of Tm³⁺ η₀ and (Tm³⁺, Pr³⁺) doped phosphate glasses were measured to give η/η₀ = 0.447, 0.305, and 0.179 for (0.4 mol% Pr³⁺, 1.0 mol% Tm³⁺), (0.8%Pr³⁺, 1.0%Tm³⁺) and (1.6 mol% Pr³⁺, 1.0 mol% Tm³⁺), respectively. In order to verify the nature of the ion coupling in our phosphate glass system, we applied the Inokuti-Hirayama model. The non-radiative energy transfer rate from Tm³⁺ to Pr³⁺, transfer efficiencies, and the donor-acceptor distance have been calculated and compared with obtained experimental data. As usual, the efficiency and the probability of energy transfer increase with the concentration of the acceptor.     

Luminescence properties of Ba2LiB5O10:Dy phosphor

A novel warm white phosphor Ba₂LiB₅O₁₀:Dy³⁺ with various Dy³⁺ concentrations was synthesized by the conventional solid-state reaction at 800 °C. The crystal structure of the phosphor was characterized by X-ray diffraction (XRD). The photoluminescence properties of Ba₂LiB₅O₁₀:Dy³⁺ were investigated, and the critical concentration of the activator ion (Dy³⁺) was found to be 0.04 mol per formula unit. Under the ultraviolet excitation of 348 nm, the phosphor presented warm white luminescence with dominating emissions at 484.6 and 577 nm, corresponding to ⁴F_9/2-⁶H_15/2, ⁴F_9/2-⁶H_13/2 transitions, respectively. The chromatic properties of the typical sample Ba₂LiB₅O₁₀:0.04Dy³⁺ phosphor have been found to have chromaticity coordinates of x = 0.31 and y = 0.35.     

Luminescence quenching processes in Gd2O2S:Pr,Ce scintillating ceramics

Temperature dependent radioluminescence under X-ray excitation (XRL) and luminescence decay time measurements following 430 nm laser excitation have been performed in the 10-775 K range on Gd₂O₂S:Pr³⁺,Ce³⁺ scintillating ceramics. From 200 K to both low and high temperature, XRL light yield decreases by 60%. High temperature luminescence quenching has been revisited. Temperature dependent lifetime measurements imply non-radiative de-excitation mechanism at electronic defects spatially correlated to Pr³⁺ emitting ions. At low temperatures, decreasing XRL light yield with irradiation time is linked to very intense thermoluminescence (TL) peak around 120 K ascribed to sulfur vacancies. These traps cause efficient electron trapping which competes with the prompt recombination mechanism.     

(Eu-Nb)-codoped TiO2 nanopowders synthesized via Ar/O2 radio frequency thermal plasma oxidation processing: Phase composition and photoluminescence properties through energy transfer

(Eu³⁺-Nb⁵⁺)-codoped TiO₂ nanopowders have been prepared by Ar/O₂ radio frequency (RF) thermal plasma oxidizing liquid precursor mists, with various addition contents of dopants (molar ratio of Eu³⁺:Nb⁵⁺ = 1:1). Characterizations have been performed by the combined studies of XRD, TEM, Raman spectra, UV-vis spectroscopy, and excitation and PL spectra. The plasma-generated nanopowders mainly consist of anatase and rutile polymorphs. Doping Nb⁵⁺ cannot have appreciable influence on Eu³⁺ solubility (0.5 at.%) in the TiO₂ host lattice, but can significantly inhibit the increase of rutile weight fraction for TiO₂. 617 nm PL intensity at 350 nm indirect excitation through energy transfer is considerably weaker than that at 467 nm direct excitation, indicating that a defect state level in the TiO₂ host lattice might be lowered below the excited state of Eu³⁺ by doping Nb⁵⁺, which is conceivable from a relatively large amount of oxygen deficiencies yielded in the TiO₂ host lattice.