5D0 State Research Articles

Spectroscopic characterization and pulsed laser operation of Eu3+:KGd(WO4)2 crystal

Europium-doped monoclinic potassium gadolinium tungstate KGd(WO4)2 crystals are grown by the top-seeded solution growth (TSSG) technique. Their absorption spectra are studied in detail for principal light polarizations, E ∥ Np, Nm and Ng. It is accompanied by determination of absorption oscillator strengths by means of the theory of f–f transition intensities for systems with anomalously strong configuration interaction. Spectral and temporal characteristics of luminescence associated with 5D0 → 7FJ transitions are analyzed, and luminescence branching ratios and the radiative lifetime of the 5D0 state are determined. Stimulated-emission cross-section spectra are evaluated for Eu:KGd(WO4)2 crystal. Pulsed Eu:KGd(WO4)2 lasers operating at room temperature at the wavelength of 702.8 nm (5D0 → 7F4 transition) are studied at Eu concentration of 10/25 at.%. Under laser pumping at 533.6 nm, a maximum output energy of 570 μJ is obtained. The main limitation for laser operation at 5D0 → 7F2 transition is strong excited-state absorption via the 5D0 → 5F4 channel.

Novel Red-Emitting Microwave-Assisted-Sintered LiSrPO4: Eu3+ Phosphors for Application in Near-UV White Light-Emitting Diodes

Novel red-emitting LiSr1−xPO4:xEu3+ phosphors with various concentrations (x = 0.03, 0.05, 0.07, 0.1) of Eu3+ ions were synthesized by microwave-assisted sintering at 1200°C for 3 h in air. The microstructural and luminescent characteristics of the LiSrPO4:Eu3+ phosphors were investigated and are discussed here. x-Ray diffraction (XRD) results showed that the prepared LiSr1−xPO4:xEu3+ phosphors presented an impurity phase of Eu2O3 when the Eu3+ ions exceeded x = 0.05. Photoluminescence (PL) results showed a series of emission states 5D0 → 7F0, 5D0 → 7F1, 5D0 → 7F2, 5D0 → 7F3, and 5D0 → 7F4 (corresponding to the typical 4f → 4f intraconfiguration forbidden transitions of Eu3+) with a major emission peak at around 617 nm. The optimum concentration of Eu3+ for LiSr1−xPO4:xEu3+ prepared by microwave-assisted sintering was found to be 0.05. The lifetime values of LiSr1−xPO4:xEu3+ phosphors with doping concentrations of Eu3+ ions of 0.03, 0.05, 0.07, and 0.1 were found to be 3.32 ms, 3.30 ms, 2.84 ms, and 2.60 ms, respectively. Moreover, the chromaticity values (x, y) of all of the LiSr1−xPO4:xEu3+ phosphors were located in the red region (0.65, 0.34).

Site‐dependent excited state absorption in Ce‐doped CaGa2S4 single crystal

AbstractExcited state absorption (ESA) by Ce3+ ions in the 5d1 lowest excited state in Ce‐doped CaGa2S4 single crystal was investigated by nanosecond two‐pulse pump‐CW probe spectroscopy. The first pump pulse of 425 nm excited the 4f ground state electron to the 5d1 first excited state and the second pump pulse of 589 nm was introduced at 13 ns after the first pump in order to selectively excite the 5d1 state. Transient intensity change of the CW probe of a diode laser of 780 nm was observed. Temporal behavior of photoluminescence intensity by the 5d1 to 4f state transition was also observed under excitation of the same two‐pulse pump.The experimental results can be explained by the model of site‐dependent excited state absorption. The photoluminescence is generated from the 5d1 states of Ce3+ at two sites with different decay time, whereas the excited state absorption takes place mainly from Ce3+ at one site as well as metastable Ce2+. The model predicts that optical gain by the 5d1 to 4f transition can be achieved by Ce3+ at the other site if the populations at both the 5d1 state of Ce3+ at the site resulting in ESA and Ce2+ can be bleached by simultaneous secondary optical pumping with an IR (e.g. Nd:YAG) laser. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Controlled growth, intense upconversion emissions and concentration induced luminescence switching of bifunctional Tm3+ doped hexagonal NaYb0.55Gd0.45F4 nanorods

Bifunctional hexagonal Tm3+ doped NaYb0.55Gd0.45F4 nanorods with tunable size are prepared via in situ cation-exchange reaction using hydrothermal method. The measured field dependence of magnetization of the NaYb0.55Gd0.45F4 nanorods shows typical paramagnetic characteristics that can be ascribed to the non-interacting localized nature of the magnetic moment of rare-earth ions. When excited by a 980nm laser, these nanorods exhibit intense multi-color up-conversion (UC) emissions in infrared, red, blue and especially ultraviolet. In addition, luminescent switching between different UC emission wavelengths of 480nm and 450nm is observed by adjusting Tm3+ doping concentration. Based on power-dependent spectral analyses, it is found that with the increase of Tm3+ doping concentration, due to the suppressed saturation effect, the dominative UC process redistribute the populations at 1G4 and 1D2(Tm3+) states of Tm3+ ion resulting in the above luminescent switching. Our results indicate that bifunctional hexagonal NaYb1−xGdxF4 nanocrystals have potential applications in miniaturized solid-state light sources, optical processing sensors and fluorescent biolabels.

Microstructure, luminescence and thermal stability properties of NaSrPO4:Tb3+ phosphors with various doping concentrations prepared using conventional solid-state sintering

This paper reports the microstructure, luminescence and thermal stability properties of the NaSr1−xPO4:xTb3+ powders (x=0.008, 0.01, 0.02, 0.04 and 0.06) via the conventional solid-state sintering at 1200°C for 5h. The X-ray diffraction result verifies all diffraction peaks are pure phase of NaSrPO4. The luminescence results show that the NaSrPO4:xTb3+ powders mainly excited at 370nm have a series of the emission-states, related to the typical 4f→4f intra-configuration forbidden transitions of Tb3+, and a major emission peak of around 546nm. The concentration quenching of the NaSr1−xPO4:xTb3+ phosphors is appeared at x=0.02. The decay time values of the NaSr1−xPO4:xTb3+ phosphors for the 5D4 state of the Tb3+ are around 3.30ms to 3.60ms. It is also found the chromaticity coordinate of NaSrPO4:Tb3+ phosphor varies with the increase of the concentration of Tb3+ ions from blue to green. Moreover, the thermal stability of the NaSrPO4:xTb3+ phosphors is slightly better than that of conventional YAG phosphors.

Optical properties of Tb and Eu doped cubic YAlO3 phosphors synthesized by sol–gel method

In this work, terbium and europium doped YAlO3 phosphors were synthesized by sol–gel method. It has been shown that after annealing at 1000°C these phosphors crystallize in the cubic garnet phase, similar to Y3Al5O12. As evidenced by the time-resolved photoluminescence spectroscopy and Maximum Entropy Method, the photoluminescence decay of both europium 5D0 and terbium 5D4 states has two lifetime components: short and long, both of the order of milliseconds. Moreover, for both phosphors the short photoluminescence lifetime was ascribed to the ions occupying low-symmetry sites in the vicinity of the nanocrystal surface, while the long decay component was related to the ions present in the bulk of YAlO3 nanocrystal. For both dopants, the long lifetime component of Tb3+ and Eu3+ emission is longer than the equivalent lifetimes in bulk Y3Al5O12 crystal.

Luminescence Spectra of β-SiAlON/Pr3+ Under High Hydrostatic Pressure

In this contribution, steady-state and time-resolved photoluminescence spectra of β-Si5.9Al0.1O0.1N7.9/Pr3+0.016 obtained at high hydrostatic pressures up to 260 kbar are presented. At ambient pressure, excitation spectra of the β-SiAlON/Pr3+ luminescence consist of sharp lines in the spectral region of 20000–22200 cm–1 related to the 3H4 → 3P0,1,2 transitions and a broad band with two maxima peaking at 30300 and 35700 cm–1, which are related to the ionization of Pr3+ ions occupying two different crystallographic sites in the β-SiAlON lattice. In both sites, the luminescence spectra consist of the dominant 1D2 red emission and 3P0 blue emission of Pr3+ ions. The intensity of luminescence related to transitions from the 3P0 state and the respective decay time gradually decreases when the pressure increases. The intensity of emission from the 1D2 state is independent of the pressure and remains strong over the entire pressure range. Weak dependence of the Pr3+ emission on applied pressure can be related to the covalent and strong Pr–N bonds and specific location of Pr3+ ions occupying empty lattice points in β-SiAlON.

Photoluminescence study of KMgSO4F:X (X=Cu+ or Dy3+ or Eu3+) halosulfate phosphor

Photoluminescence (PL) from Cu+, Dy3+and Eu3+ doped KMgSO4F halosulfate fluoride phosphors have been investigated and synthesized by the wet chemical method. Strong emission of KMgSO4F: Cu+ phosphor is assigned to electronic transition 3d94s1→3d10. Emission bands peaking at 442nm and 462nm in the green region of the spectrum. Dy3+ emission is observed at 484 and 576nm in KMgSO4F host due to 4F9/2→6H15/2 and 4F9/2→6H13/2 transitions respectively. The emission has a yellow-orange color in KMgSO4F host lattice The presence of trivalent Eu in KMgSO4F suggest the presence of Eu3+ in the host compound occupying two different lattice sites which is peaking at 593 and 616nm showing 5D0→7F1 and 5D0→7F2 transitions respectively. The trivalent europium ion is very useful for studying the nature of metal coordination in various systems owing to its non-degenerate emitting 5D0 state. The paper discusses the photoluminescence of Cu+, Dy3+and Eu3+ in KMgSO4F matrix by a simple method.

Effects of OH radicals and the silicon network on the lifetime of Eu^3+-doped sodium silicate glasses

A set of sodium silicate glass matrices were synthesized to study the influence of Na2O concentration on the optical properties of Eu3+. The samples were characterized by optical absorption (OA), time and energy resolved photoluminescence and Fourier Transform Infrared (FTIR). We observed that decreasing sodium oxide concentration affects the absorption of OH radicals in the host matrix. Adjusting the obtained FTIR spectra by Gaussian functions, we observed the existence of two possible non-radiative transfer channels with the transition from the 5D0 to the 7F2 state of the Eu3+ ions. The first was produced by resonance with the 5th harmonic vibration of OH bonded radicals and the second by resonance with the 14th harmonic vibration of the silicon network. A decrease in OH radicals observed by FTIR was followed by an increase in the lifetime of the 5D0 state of the Eu3+ ions. However, resonance with higher harmonic orders did not affect the optical properties of the Eu3+ ions. Increases in the lifetime of this transition (~3.3 ms) were obtained from the synthesized sample with the lowest sodium oxide concentration. This lifetime is comparable with the well-known YAG system. Molecular dynamic results show that decreasing sodium oxide content in the host matrix produces structural changes such as decreases in non-bridge oxygen species, which may explain the decreases in OH radical absorption seen in the experimental results.

Thermally induced ionization of 5d1 state of Ce3+ ion in Gd3Ga3Al2O12 host

We studied the processes occurring in the 5d1 excited state of Ce3+ in Gd3Ga3Al2O12 (GGAG) host. Energy transfer from Gd3+ to Ce3+ was evidenced by photoluminescence excitation spectra of Ce3+ emission. We measured temperature dependences of prompt nanosecond decays and millisecond delayed recombination decays of Ce3+ in GGAG host. The decrease of nanosecond decay times above approximately 200K is due to thermally induced ionization of the 5d1 excited state of Ce3+. Thermal stimulated luminescence glow curve measurements after X-ray or UV (within 4f–5d1 absorption of Ce3+) irradiation at RT provide another support for the ionization effect around this temperature.

Luminescence of CaWO4:Pr3+ and CaWO4:Tb3+ at ambient and high hydrostatic pressures

Photoluminescence spectra of CaWO4 doped with Pr3+ and Tb3+ obtained at high hydrostatic pressures up to 315 kbar applied in a diamond anvil cell (DAC) are presented. The intensities of the luminescence from the 3P0 state of Pr3+ and from the 5D3 state of Tb3+ decreased with increasing pressure. At pressures greater than 50 kbar, the 1D2 → 3HJ transitions in Pr3+ and the 5D4 → 7FJ transitions in Tb3+ dominated the spectra. At pressures greater than 100 kbar, only emissions from the lower excited states were observed. At pressures greater than 150 kbar, luminescence from the 1D2 and 5D4 states also decreased with increasing pressure, and at a pressure of 315 kbar for CaWO4:Pr3+ and 190 kbar for CaWO4:Tb3+, the emissions related to the Pr3+ and Tb3+ were quenched. These effects were related to the influence of impurity trapped excitons (ITEs) on the efficiency of the f–f emission in the Pr3+ and Tb3+ ions. Analysis of the emission spectra collected at different pressures allowed the energies of the ground states of the Pr3+ and Tb3+ ions with respect to the band edges of the CaWO4 host to be estimated.

Preparation of aligned Eu(DBM)3phen/PS fibers by electrospinning and their luminescence properties

Electrospinning is a technique employed to prepare nanofibers of polymer, ceramics, and composite that are used in the high electrostatic field. Alignment is an important step toward the exploitation and applications of these fibers. In the present report, super-long aligned luminescent composite Eu(DBM)3phen/PS fibers (DBM=dibenzoylmethane, phen=1,10-phenanthroline, PS=polystyrene) were prepared via an electrospinning method. The key to the success of this method was the usage of high molecular weight PS (Mw=4.4×105) in the electrospinning solution and the low speed (>0.5m/s) collecting drum. Luminescent properties of the composite fibers were systemically studied in comparison with that of the corresponding pure europium complex Eu(DBM)3phen. The results showed that the fluorescence lifetime of 5D0 state and the luminescent intensity of Eu3+ in Eu(DBM)3phen/PS composite fibers increased gradually with the fiber diameter. The composite fibers with smaller diameter exhibited better photoluminescence stability than that of the pure complex Eu(DBM)3phen.

Structural and luminescence properties of Eu3+, Dy3+ and Tb3+ ions in lead germanate glasses obtained by conventional high-temperature melt-quenching technique

The subject of this paper is the structural and luminescence properties of selected rare earth ions in lead germanate glasses. Glasses were obtained by conventional high-temperature melt-quenching technique. Europium, dysprosium and terbium ions were chosen as active dopants. The spectroscopic parameters for Eu3+, Dy3+ and Tb3+ ions were determined based on excitation and emission measurements as well as luminescence decay analysis. Especially, the luminescence intensity ratios R/O (Eu3+), Y/B (Dy3+) and G/B (Tb3+) were calculated. Luminescence lifetimes for 5D0 state of Eu3+ ions, 4F9/2 state of Dy3+ ions and 5D4 state of Tb3+ ions were also determined. The amorphous nature and local structure of the studied lead germanate glass systems was confirmed by X-ray diffraction (XRD) and infrared (FT-IR) spectroscopy.

Formation and down/up conversion luminescence of Ln3+doped NaY(MoO4)2microcrystals

NaY(MoO(4))(2) microcrystals with different morphologies including spherical, rhombic, sheet-like, and rectangular plate-like morphologies have been successfully synthesized via a simple hydrothermal method by varying the molar ratios of Y(NO(3))(3)/Na(2)MoO(4) and pH values of the resultant solutions. NaY(MoO(4))(2) was demonstrated to be a good host material for different lanthanide ions. In the up-conversion luminescence spectrum of NaY(MoO(4))(2):Yb(3+)/Er(3+) under 980 nm excitation, the green emissions ((2)H(11/2)/(4)S(3/2)→(4)I(15/2)) were observed and the red emission ((4)F(9/2)→(4)I(15/2)) almost disappeared. The CIE chromaticity coordinates of NaY(MoO(4))(2):Yb(3+)/Er(3+) were calculated to be (0.27, 0.65). In the up-conversion luminescence spectrum of NaY(MoO(4))(2):Yb(3+)/Tm(3+), the (1)G(4)→(3)H(6) as well as (3)F(3)→(3)H(6) emission was observed and the CIE chromaticity coordinates of NaY(MoO(4))(2):Yb(3+)/Tm(3+) were calculated to be (0.25, 0.24). The down-conversion luminescence properties of NaY(MoO(4))(2):Eu(3+) were studied in detail. In addition, the decay lifetimes at the (4)S(3/2) (Er(3+)), (1)G(4) (Tm(3+)), and (5)D(0) (Eu(3+)) states were determined to be 5.63 μs, 19.76 μs, and 487.39 μs, respectively.

Synthesis, crystal structure and luminescence characteristics of a novel red phosphor Ca19Mg2(PO4)14:Eu3+ for light emitting diodes and field emission displays

In order to explore a new family of phosphate phosphors, the synthesis and crystal structure of a novel phosphate family, Ca19M2(PO4)14 (M = Mg, Zn, Mn), were investigated. Eu3+ doped Ca19Mg2(PO4)14 red phosphor was also successfully synthesized with the objective of application in ultraviolet-based light-emitting diodes (LEDs) and field emission displays (FEDs). The characteristic photoluminescence properties were studied in detail by photoluminescence excitation (PLE), emission (PL) spectra and decay times. The Ca19Mg2(PO4)14:0.06Eu3+ phosphor offers higher brightness and thermal stability than the commercial Y2O3:Eu3+. The unexpected temperature-dependent luminescence from higher 5D1 states was observed and was explained via the configuration coordinate diagram. The cathodoluminescence (CL) spectra as a function of accelerating voltage and probe current were also measured. Excellent degradation properties with good color stability were obtained by continuous low-voltage electron-beam excitation of the phosphor. The results indicate that the phosphor Ca19Mg2(PO4)14:0.06Eu3+ can be a suitable red-emitting phosphor candidate for LEDs and FEDs.

Lanthanide coordination compounds with 1H-benzimidazole-2-carboxylic acid: syntheses, structures and spectroscopic properties

A flexible multidentate ligand, 1H-benzimidazole-2-carboxylic acid, was synthesized to construct a series of lanthanide coordination polymers [Ln(HBIC)3]n (Ln = Eu (1), Tb (2), Gd (3), Pr (4), Nd (5); H2BIC = 1H-benzimidazole-2-carboxylic acid) under hydrothermal conditions. All the compounds were fully characterized by elemental analysis, IR spectroscopy, single-crystal X-ray diffraction, thermal analysis and various spectroscopic techniques. Structural analyses reveal that they are isostructural and feature a 2D wave-like layer structure with distorted grids, in which the adjacent Ln3+ centers are bridged by the HBIC− ligands with two kinds of new coordination modes and the adjacent HBIC− ligands are tightly bound by two types of distinct intra-layer hydrogen bonds. The adjacent 2D layers are further interconnected by strong inter-layer hydrogen bond ring motifs R22(10) to generate a 3D supramolecular architecture. Optical studies indicate that the compounds 1, 2, 4 and 5 exhibit characteristic luminescence emission bands of the corresponding lanthanide ions in the visible or near-infrared regions at room temperature. In particular, compound 2 displays bright green luminescence in the solid state with a satisfactory 5D4 lifetime of 1.2 ms and a high overall quantum yield of 31%, due to an ideal energy gap between the lowest triplet state energy level of H2BIC ligand and the 5D4 state energy level of Tb3+. The energy transfer mechanisms in compounds 1 and 2 were also described and discussed.

Luminescence and scintillation mechanism in Ce3+ and Pr3+ doped (Lu,Y,Gd)3(Ga,Al)5O12 single crystal scintillators

AbstractLuminescence processes occurring in the 5d1 excited state of Ce3+ and Pr3+ centers in the (Gd,RE)3(Ga,Al)5O12, RE=Lu,Y, multicomponent garnet host were studied by purely optical contactless methods. Combination of measurements of the prompt nanosecond decay and slow delayed radiative recombination decay allows to resolve the presence of thermal quenching and thermally induced ionization processes acting upon the 5d1 excited state. Additional energy transfer process depleting the 5d1 state of Pr3+ center was found and is due to the resonance of 5d1‐3H4 emission transition of Pr3+ and 8S‐6Px absorption transition of Gd3+ which might be responsible for noticeably lower light yield in Pr‐doped multicomponent garnets. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Hydrothermal synthesis and upconversion luminescence properties of β-NaGdF4:Yb3+/Tm3+ and β-NaGdF4:Yb3+/Ho3+ submicron crystals with regular morphologies

Single phase β-NaGdF4:Yb3+/Tm3+ and β-NaGdF4:Yb3+/Ho3+ submicron crystals with various morphologies including hexagonal prisms, spindles, and spheres were synthesized via the one-step hydrothermal method by controlling the pH values and sort of chelators (EDTA and citric acid). The prepared products showed intense up-converted luminescence (UCL) pumped by infrared laser at 980nm. The hexagonal prisms that meaning high degree crystallinity demonstrated strong UCL in comparison with other morphologies such as spindles and spheres. In β-NaGdF4:Yb3+/Tm3+, UCL not only appeared transitions from 1G4, 1D2, and 1I6 states to the lower lying states of Tm3+, but also 6PJ→8S7/2 transition (310nm) of Gd3+. These UCL were responsible for three, five, and six photons processes determined by pump power dependence of UCL intensities. The observation of UCL of Gd3+ implied occurrence of energy transfer from Tm3+:1I6 to Gd3+:6PJ.

A Theoretical Study on the Structural and Energy Spectral Properties of Ce3+ Ions Doped in Various Fluoride Compounds

Geometry optimization and wave function-based complete-active-space self-consistent field-embedded cluster calculations have been performed for a series of Ce3+-doped fluoride compounds (CaF2, YF3, LaF3, KMgF3, LiYF4, K2YF5, and KY3F10) to investigate local coordination structures, crystal field parameters, and 5d1 energy-level structures of doping Ce3+ ions. The crystal-field parameters of Ce3+ are extracted from the calculated energies and wave functions. The calculated crystal-field parameters and 5d1 energy-level structures show excellent consistency with the experimental results. Our calculations show that the onset of 4f → 5d absorption, which is important in phosphors and scintillators, can be well-predicted. Apart from that, the distortion of local structure due to doping, the wave functions, and the crystal-field parameters of 4f1 and 5d1 states of Ce3+ in the hosts can be obtained. Those can seldom be obtained by fitting empirical crystal-field Hamiltonian to experimental data but are required by some detailed theoretical analysis, such as the calculation of transition intensities and hyperfine splittings. The obtained crystal-field parameters of Ce3+ may also be useful for other lanthanide ions in the same hosts.

Luminescence and Energy Transfer in Lu3Al5O12 Scintillators Co-Doped with Ce3+ and Tb3+

Lu(3)Al(5)O(12) (LuAG) doped with Ce(3+) is a promising scintillator material with a high density and a fast response time. The light output under X-ray or γ-ray excitation is, however, well below the theoretical limit. In this paper the influence of codoping with Tb(3+) is investigated with the aim to increase the light output. High resolution spectra of singly doped LuAG (with Ce(3+) or Tb(3+)) are reported and provide insight into the energy level structure of the two ions in LuAG. For Ce(3+) zero-phonon lines and vibronic structure are observed for the two lowest energy 5d bands and the Stokes' shift (2 350 cm(-1)) and Huang-Rhys coupling parameter (S = 9) have been determined. Tb(3+) 4f-5d transitions to the high spin (HS) and low spin (LS) states are observed (including a zero-phonon line and vibrational structure for the high spin state). The HS-LS splitting of 5400 cm(-1) is smaller than usually observed and is explained by a reduction of the 5d-4f exchange coupling parameter J by covalency. Upon replacing the smaller Lu(3+) ion with the larger Tb(3+) ion, the crystal field splitting for the lowest 5d states increases, causing the lowest 5d state to shift below the (5)D(4) state of Tb(3+) and allowing for efficient energy transfer from Tb(3+) to Ce(3+) down to the lowest temperatures. Luminescence decay measurements confirm efficient energy transfer from Tb(3+) to Ce(3+) and provide a qualitative understanding of the energy transfer process. Co-doping with Tb(3+) does not result in the desired increase in light output, and an explanation based on electron trapping in defects is discussed.