5d 4f Luminescence Research Articles

Solidified Salt Melts of the NaCl-KCl-CeF3-EuF3 System as Promising Luminescent Materials.

This study presents the results of investigating the interaction between the CeF₃-EuF₃ system and the NaCl-KCl salt melt using spectroscopic methods. It was found that CeF₃ ions undergo no significant changes upon dissolution in the NaCl-KCl melt. In contrast, the dissolution of EuF₃, both individually and within the CeF₃-EuF₃ system, is accompanied by redox reactions leading to the formation of Eu2⁺. The diffuse reflectance spectra of both the bottom (insoluble sediment) and upper parts of the solidified salt melt in the UV range indirectly indicate photoluminescence excitation from Ce3⁺ and Eu2⁺ ions. In addition, absorption bands in the near-IR region (1900-2300 cm⁻1) confirm the retention of some Eu3⁺ ions in the salt melt. The study explored the effects of various factors-including sample composition, excitation wavelength, prior and subsequent heat treatment, and medium composition-on the excitation and emission spectra of the samples. Intense 5d-4f luminescence of Ce3⁺ and Eu2⁺ ions (at 330 and 430 nm, respectively) was observed predominantly in the upper part of the salt melts, along with much weaker 4f-4f luminescence from Eu3⁺ ions. Certain parameters were optimized to reduce the luminescence contribution from Ce3⁺ and especially Eu3⁺ ions while enhancing the luminescence of Eu2⁺ ions. Solidified salt solution-melts of the NaCl-KCl-CeF₃-EuF₃ system show promise as materials for developing solar ultraviolet radiation detectors.

Luminescence properties of complex fluoride Na3CaMg3AlF14 phosphors doped with cerium and europium ions

Ceramic samples of two multi-cation fluoride compositions, namely Na3CaMg3AlF14 doped with Ce3+ or Eu2+ and KCaAl2F9 doped with Eu2+, have been synthesized using a conventional solid-state reaction method. The XRD analysis shows that the new polymorphic modification of Na3CaMg3AlF14 with the cubic A2B2X7 pyrochlore crystal structure (space group Fd3‾m, lattice parameter 10.172 Å) has been obtained. This conclusion is confirmed with the simulation of the XRD pattern for the cubic pyrochlore structure by introducing random occupation of Na+ and Ca2+ ions at the A-type sites as well as Mg2+ and Al3+ ions at the B-type sites in the structure of rhombohedral Na3CaMg3AlF14. It has been shown that doping of the Na3CaMg3AlF14 pyrochlore matrix with Ce3+ ions creates several Ce3+ optical centers with different spectral and decay properties of 5d-4f luminescence. In contrast to that, doping with Eu2+ ions creates a single optical center by the substitution for the Na/Ca A-type site in the cubic Na3CaMg3AlF14 pyrochlore structure. In Eu2+-doped Na3CaMg3AlF14, the Eu2+ emission band caused by the interconfigurational 4f65d – 4f7 transition has a rather narrow width (FWHM = 29 nm) which is considerably less than that observed in Eu2+-doped KCaAl2F9 (FWHM = 44 nm) in which the emission band is broadened due to the existence of two types of sites for doping Eu2+ ions. The Eu2+ ions doped into Na3CaMg3AlF14 and KCaAl2F9 hosts emit not only broad-band 4f65d – 4f7 luminescence but also a narrow-line 4f7 – 4f7 (6P7/2 - 8S7/2) emission which is not observed under excitation to the lowest-energy 4f65d state. It has been shown that both Na3CaMg3AlF14:Eu2+ and KCaAl2F9:Eu2+ demonstrate a rather good thermal stability of luminescence with thermal quenching temperature T1/2 equal to 504 and 573 K, respectively.

The Influence of Halide Ion Substitution on Energy Structure and Luminescence Efficiency in CeBr2I and CeBrI2 Crystals.

This study aims to determine the optimum composition of the CeBr1-xIx compound to achieve the maximum light output. It is based on calculations of the band energy structure of crystals, specifically taking into account the characteristics of the mutual location of local and band 5d states of the Ce3+ ions. The band energy structures for CeBr2I and CeBrI2 crystals were calculated using the projector augmented wave method. The valence band was found to be formed by the hybridized states of 4p Br and 5p I. The 4f states of Ce3+ are located in the energy forbidden band gap. The conduction band is formed by the localized 5d1 states, which are created by the interaction between the 5d states of Ce3+ and the 4f0 hole of the cerium ion. The higher-lying delocalized 5d2 states of Ce3+ correspond to the energy levels of the 5d states of Ce3+ in the field of the halide Cl0 (Br0) hole. The relative location of 5d1 and 5d2 bands determines the intensity of 5d-4f luminescence. The bottom of the conduction band is formed by localized 5d1 states in the CeBr2I crystal. The local character of the bottom of the conduction band in the CeBr2I crystal favors the formation of self-trapped Frenkel excitons. Transitions between the 5d1 and 4f states are responsible for 5d-4f exciton luminescence. In the CeBrI2 crystal, the conduction band is formed by mixing the localized 5d1 and delocalized 5d2 states, which leads to quenching the 5d-4f luminescence and a decrease in the light output despite the decrease in the forbidden band gap. CsBr2I is the optimum composition of the system to achieve the maximum light output.

Novel optical-guiding crystal scintillator composed of an Eu-doped SrI2 core and glass cladding

Scintillators are used not only as single crystals in radiation detection systems but also as arrays, columns, and fibers. In this study, a novel optical-guiding crystal scintillator (OCS) was fabricated using an Eu:SrI2 crystal as the core and borosilicate glass as the cladding. The OCS has an elongated shape and can be flexibly processed like an optical fiber. Furthermore, because inorganic scintillator crystals are used as the core, the OCS has high sensitivity to high-energy radiation, such as γ- and X-ray radiation, unlike plastic scintillators. In this study, by observing the cross section of the fabricated OCS, identifying the crystal structure, and investigating the radioluminescence spectrum under X-ray irradiation, OCS fabrication in accordance with the OCS design specifications was confirmed, and the SrI2 crystal structure and Eu2+ 5d–4f luminescence were maintained. The γ-ray responses, such as light yield and decay time, were also investigated.

Luminescence properties and energy transfer processes in LiSrPO4 doped with Pr3+ and co-doped with Na+ and Mg2+

The paper studies the influence of co-doping with alkali metal ions (Na+, Mg2+) on fast interconfigurational 5d-4f luminescence in LiSrPO4 doped with Pr3+. Na+ and Mg2+ ions were introduced in order to provide charge compensation and modify the electronic configuration of the materials. Luminescent properties of the powder samples were investigated using time-resolved pulsed cathodoluminescence and luminescence spectroscopy in the UV-VUV spectral region. The obtained results were compared with those obtained earlier on LiSrPO4:Pr3+ without co-dopants. With introduction of the co-dopants, slight changes in relative luminescence intensity and decay time of Pr3+ ion 4f15d1-4f2 radiative transitions were found. The study revealed a complex interplay between defects of crystalline structure and impurity ions leading to the peculiarities in energy relaxation processes. Energy transfer leading to the delayed recombination processes is responsible for significant afterglow in Pr3+4f15d1-4f2 emission.

X- and γ-ray response of Sm-doped SrBr2 crystalline scintillators emitting red-NIR photons

Sm:SrBr2 crystals with various Sm concentrations were prepared by the vertical Bridgman method, and the detector properties including emission spectra, decay time, and pulse height were investigated. The samples exhibit a broad emission band peaking at 690 nm and decay time of about 10 μs, and the emission is attributable to the allowed 5d-4f luminescence of Sm2+. The pulse height analyses under 137Cs γ-rays (662 keV) demonstrate that the light yields of the 0.03, 0.1, 0.3, and 1% Sm:SrBr2 are 31000, 32000, 23000 and 13000 photons/MeV, respectively.

Ga-modified YAG:Pr3+ dual-mode tunable luminescence thermometers

• Dual-mode thermometry is executed in YAG:Pr 3+ -YGG:Pr 3+ systems. • Bandgap engineering allows enhancing the performance of the thermometers. • YAG:Pr-YGG:Pr present manageable high relative sensitivity and high accuracy. • Garnet-based luminescence thermometers work effectively in the range 17–700 K. • Relative temperature errors are calculated and compared for all the garnets. The temperature determination using luminescent materials is nowadays considered the perspective remote technique for temperature gauging, despite the few examples reported so far combining wide operating temperature range with satisfying relative thermal sensitivity and temperature uncertainty values. In this paper, we study the Y 3 (Al,Ga) 5 O 12 :0.1%Pr phosphors with controlled Ga:Al composition to deliberately affect their luminescent properties. We demonstrate that the energy barrier for thermal quenching of the 5d-4f luminescence can be effectively tailored, yielding the fine tune of the thermometric parameters of these phosphors. By exploiting time-resolved and time-integrated approaches we show that the thermometers can cover the 17–700 K temperature range with a maximum relative sensitivity up to 3.6%·K −1 and a temperature uncertainty as lower as 0.02 K. For each sample, the temperature readout of the distinct thermometric parameters is compared illustrating that the performance of the thermometers should also consider the relative temperature error between the calculated and the measured temperatures, besides relative thermal sensitivity and temperature uncertainty.

Effect of coordination environment of Eu2+ ion on the 5d-4f luminescence of molecular compounds EuL2(THF)x (L = Cl, Br, I, NO3, Ac, fod, tmhd, and acac; x = 0, 2)

The photoluminescence spectra of crystalline europium dihalides EuL2 (L = Cl, Br, and I) and solutions of complexes of divalent europium EuL2(THF)2 (L = Cl, NO3, fod, tmhd, acac, Ac) in tetrahydrofuran were registered. The bathochromic shift of the Eu2+ 5d-4f luminescence maxima is typical for all studied compounds. Its value decreases in the series Cl– > Br– > NO3– > Ac– > acac– > fod– > tmhd– and correlates well with the mean polarizabilities of the anionic ligands.

Luminescence and scintillation characteristics of Cs3ScCl6:Ce crystals

In this study, we fabricated Cs3ScCl6 crystals doped with different concentrations of Ce and investigated their luminescence properties. In both the photoluminescence (PL) and scintillation spectra, emission bands attributable to Ce3+ 5d–4f transition were observed. The PL temporal profiles revealed decay time constants of ~35 ns, which are typical values for Ce3+ 5d–4f transition. In the scintillation temporal profiles, two decay components were observed. The fast and slow components could be attributed to Ce3+ 5d–4f luminescence and self-trapped excitons (STEs) or a slow energy transfer from STEs to Ce3+, respectively. The maximum light yield was 9200 photons/MeV for a Ce concentration of 0.5 mol%.

Luminescence of 5d–4f transitions of Pr3+ in natural fluorite CaF2, anhydrite CaSO4 and apatite Ca5(PO4)3F

Natural fluorite, anhydrite and apatite under deep UV excitation exhibits several narrow emission bands in the spectral range of 225–300 nm with the strongest peaks at 230–240 nm and 260–290 nm and with half-widths of 20–30 nm. They are characterized by narrow excitation band peaking at 220 nm and very short decay times of 20–50 ns. Those spectral-kinetic parameters enable their interpretation as 4f5d–4f transitions in Pr3+ luminescence center. Such kind of luminescence has been studied extensively over the past several decades in synthetic phosphors, including those which are analogous to natural compounds, but it is the first time 5d–4f luminescence transitions of Pr3+ is being proved in minerals.

Down-conversion luminescence of Ce-Yb ions in YF3

The single-phase Y1-x-yCexYbyF3 solid solution powders were prepared by melt crystallization technique. The 5d-4f interconfigurational luminescence of Ce3+ ions and 2F5/2-2F7/2 luminescence of Yb3+ ions in YF3 crystalline powder doped with Ce3+/Yb3+ ions pair upon 266 nm excitation were studied. The energy transfer from Ce3+ to Yb3+ after 266 nm excitation has complex character and deals with the UV excitation induced dynamic processes and color centers formation in YF3:Ce3+ powders. It was shown that UV light irradiation leads to in absorbing in 525–650 nm spectral range which is not observed for Ce3+-Yb3+ containing samples. This speaks for additional channels of Ce3+ and Yb3+ excitation as a result of recombination of free charge carriers originated from excited state absorption by Ce3+ ions and trapped by Yb3+ ions. The efficiency of energy transfer from Ce3+ to Yb3+ was 22,6% for YF3:Ce(0.05 mol. %):Yb(10.0 mol.%) solid solution with 0,91% down-conversion luminescence external quantum yield.

Growth and luminescent properties of Ce and Eu doped Cesium Hafnium Iodide single crystalline scintillators

In order to obtain new scintillators with high light output and high effective atomic number (Zeff), we performed anion-substitution for Cs2HfCl6 (CHC) scintillator, and then, we succeeded in growing Cs2HfI6 (CHI) single crystalline scintillator. It had Zeff of 58, which is the same as that of CHC, and had high light output of ∼70,000 photons/MeV with 700 nm emission. However, its scintillation decay time of ∼2.5 µs was slow for practical use as gamma-ray monitor. In this study, we performed Ce3+/Eu2+ doping to Hf4+ site to improve decay time of CHI, introducing the fast 5d-4f luminescence. Ce:CHI and Eu:CHI single crystals were finally obtained by the vertical Bridgman-Stockbarger method. The luminescence spectra of the Ce:CHI and Eu:CHI were very similar to that of the non-doped CHI, which would mean that no 5d-4f luminescence of Ce3+/Eu2+ was observed. The measured light output and decay time of Ce:CHI were ∼48,000 photon/MeV and 2.3 ± 0.1 µs, respectively. As for Eu:CHI, light output and decay time were ∼69,000 photon/MeV and 2.8 ± 0.1 µs, respectively.

Visualizing hidden electron trap levels in Gd3Al2Ga3O12:Ce crystals using a mid-infrared free-electron laser

The energy levels of electron traps (namely, defect complexes associated with oxygen vacancies) in Gd3Al2Ga3O12:Ce (GAGG:Ce) were studied at 12 K using mid-infrared (MIR) light pulses from a free-electron laser (FEL) as the probe light. Ce3+ 5d–4f luminescence was stimulated by the MIR light pulse following an ultraviolet light pulse. Stimulation of Ce3+ 5d–4f luminescence by MIR light pulses was pronounced above 0.31 eV. This result is consistent with that of previous work based on a trap-mediated luminescence model. It is concluded that the electron trap levels are located 0.31 eV below the bottom of the conduction band. This study demonstrates that MIR-FEL is applicable for the determination of hidden electron trap levels.

Modeling and decoding of fine structure of electron-vibrational 5d – 4f luminescence spectra in LiRF4:Ce3+ (R=Y, Lu) crystals

The 5d-4f luminescence spectra in the LiYF4:Ce3+, LiLuF4:Ce3+ crystals at zero temperature are simulated using a microscopic model of electron-phonon interaction and realistic phonon spectrum of host crystal lattices. Calculations show that three vibrational peaks with the energies 65cm−1, 223cm−1 and 420cm−1 with respect to zero-phonon transition energy dominate in vibrational structure in these spectra. Comparison of simulation results with available experimental data allows to identify zero-phonon lines and vibrational peaks in the measured spectra and thus determine energies of several 4f1 electronic configuration levels: 514, 2221 and 2316cm−1 for LiYF4:Ce3+ and 514, 2188, 2283 and, possibly, 3134cm−1 for LiLuF4:Ce3+ with respect to the ground 4f level energy.

(Invited) Investigation of Thermal Quenching Process for 5d-4f and 3d-3d Luminescence

White light LEDs (light-emitting diodes) are rapidly replacing incandescent lamps and (compact) fluorescent tubes in all lighting markets (consumer, automotive, and professional). The most widely used type of white LEDs (w-LEDs) is a phosphor-converted w-LED (PC-LED), which is composed of an InGaN-based blue LED and visible light emitting inorganic phosphors, such as oxides and nitrides doped with Ce3+, Eu2+ , Mn4+ as an active center. These phosphors are required to have excellent thermal quenching behavior because the LED chip reaches temperatures up to ∼200 °C in recent high-power w-LED. In these phosphors, two processes of thermal quenching are considered generally. One is the thermally activated cross over from the excited state to the next lower state. The other is the thermal ionization from the excited state to the bottom of the conduction band (CB). In this study, we investigated the thermal quenching mechanism for the 5d-4f and 3d-3d luminescence in w-LED phosphors doped with Ce3+ or Mn4+. Ce3+-doped Y3Al5O12 garnet (yttrium aluminum garnet,YAG) is the most prominent phosphor in w-LEDs. The excellent thermal quenching behavior of the Ce3+ luminescence in YAG:Ce3+ is well established, but the mechanism for thermal quenching remains unclear. To elucidate the mechanism of thermal quenching of YAG:Ce3+, thermoluminescence excitation (TLE) spectra were recorded at room temperature and 300 °C. At room temperature, the lowest 5d1 band at 450 nm does not contribute to the charging process for TL while at 300 °C, a temperature corresponding to the onset of thermal quenching of the Ce3+ luminescence, the excitation in 5d1 band gives rise to a TL signal. This result indicates that thermal ionization is responsible for thermal quenching of the Ce3+luminescence. We also investigated the thermal quenching process for calcium aluminum magnetoplumbite, CaAl12O19, doped with Mn4+ (CAO:Mn4+), which is narrow red luminescence phosphor. We tried to understand the quenching mechanism of Mn4+ compared with the luminescence quenching properties of Mn2+. From the temperature dependence of Mn4+: 2E-4A2 red luminescence and Mn2+:4T1- 6A1 green luminescence intensities in CaAl12O19, the quenching temperatures of Mn4+ and Mn2+ are 330K and 800K, respectively. There is a big difference between the quenching temperatures of Mn4+ and Mn2+ in spite of the same host material and the similar luminescence transtion energy. The Mn4+:2E-4A2 red luminescence is quenched by the thermally activated crossover through the 4T2 excited state whose potential curve possesses a large configurational offset in a configurational coordinate diagram. Mn2+ green luminescence is also quenched by the thermally activated crossover, but the activation energy for non-radiative process is much larger than that for Mn4+ because there are no other excited states related with the quenching process.

Lanthanide-Activated Phosphors Based on 4f-5d Optical Transitions: Theoretical and Experimental Aspects.

The synthesis of lanthanide-activated phosphors is pertinent to many emerging applications, ranging from high-resolution luminescence imaging to next-generation volumetric full-color display. In particular, the optical processes governed by the 4f-5d transitions of divalent and trivalent lanthanides have been the key to enabling precisely tuned color emission. The fundamental importance of lanthanide-activated phosphors for the physical and biomedical sciences has led to rapid development of novel synthetic methodologies and relevant tools that allow for probing the dynamics of energy transfer processes. Here, we review recent progress in developing methods for preparing lanthanide-activated phosphors, especially those featuring 4f-5d optical transitions. Particular attention will be devoted to two widely studied dopants, Ce3+ and Eu2+. The nature of the 4f-5d transition is examined by combining phenomenological theories with quantum mechanical calculations. An emphasis is placed on the correlation of host crystal structures with the 5d-4f luminescence characteristics of lanthanides, including quantum yield, emission color, decay rate, and thermal quenching behavior. Several parameters, namely Debye temperature and dielectric constant of the host crystal, geometrical structure of coordination polyhedron around the luminescent center, and the accurate energies of 4f and 5d levels, as well as the position of 4f and 5d levels relative to the valence and conduction bands of the hosts, are addressed as basic criteria for high-throughput computational design of lanthanide-activated phosphors.

Thermal quenching of luminescence of BaY2F8 crystals activated with Er3+ and Tm3+ ions

Thermal quenching of interconfigurational 5d-4f luminescence of Er3+ and Tm3+ ions in BaY2F8 crystals is studied in the temperature range of 330–790 K. The quenching temperatures are ~575 and ~550 K for Er3+ and Tm3+, respectively. It is shown that quenching of 5d-4f luminescence of Tm3+ ions is caused by thermally stimulated ionization of 5d electrons to the conduction band.

Optical, Scintillation and Dosimeter Properties of Ce-Doped and Eu-Doped LiCaAlF6 Crystals

Scintillators are one of the luminescent materials which have a function to convert the ionizing radiation to UV-NIR photons. Among such applications, scintillators for thermal neutron detectors have attracted much attentions in recent years since the lack of 3He gas causes some serious problems in this field. To develop the alternative to 3He gas, many groups are developing new scintillators containing 6Li or 10B since these elements have a high cross section against thermal neutrons. For this purpose, we have developed Ce- [1] and Eu-doped [2] LiCaAlF6 crystals. In addition to scintillators, dosimeter materials are also important, and these materials are used in the individual and environmental dose monitors. In dosimeters, optically stimulated luminescence (OSL), thermally stimulated luminescence (TSL), and radiophotoluminescence (RPL) have played an important role. In dosimeter materials, human tissue equivalency is one of the most important properties, and LiCaAlF6 crystals are interesting because the effective atomic number of this material is close to the human tissue. If the dosimeter materials have the same effective atomic number with the tissue, ideally no mathematical corrections are required to calibrate the dose. Furthermore, the complementary relation of scintillators and dosimeter materials was found by us [3, 4] so the investigation of these properties on the same material is important to develop high performance materials for ionizing radiation detectors and to understand the energy migration process from the host matrix to emission centers. In this work, we have grown Ce and Eu differently doped LiCaAlF6 crystals to investigate optical, scintillation, OSL and TSL properties. In transmittance, absorption was proportional to dopant concentrations and typical optical quantum yield of Ce and Eu-doped LiCaAlF6 were 40 and 100%, respectively. Scintillation wavelength and decay time profiles were investigated under X-ray irradiation. Ce3+ and Eu2+ 5d-4f luminescence appeared around 300 nm and 370 nm with typical decay time of 40 ns and 1.5 ms, respectively. OSL of Ce-doped ones appeared under 405 nm stimulation and that of Eu-doped ones was observed under 470 nm stimulation. TSL of Ce- and Eu-doped LiCaAlF6 were enough strong and they exhibited good response function from 1 to 1000 mGy exposure. In Eu-doped LiCaAlF6, the clear relation between the scintillation and dosimeter properties were observe while it was not observed in Ce-doped ones. [1] T. Yanagida, et al., Opt. Mater. 32 311 (2009). [2] T. Yanagida, et al., Opt. Mater., 33 1243 (2011). [3] T. Yanagida, et al., Rad. Meas., 71 162 (2014). [4] T. Yanagida, J. Lumin., 169 544 (2016).

VUV spectroscopy of complex fluoride systems Na0.4(Y1−xREx)0.6F2.2 (RE3+ = Nd3+, Tm3+)

Emission and excitation spectra as well as luminescence decay kinetics of complex non-stoichiometric fluoride crystals Na0.4(Y1−xNdx)0.6F2.2 (x=0.005, 0.05, 0.2, 1) and Na0.4(Y1−xTmx)0.6F2.2 (x=0.0005, 0.01, 0.05, 0.1) have been studied in the VUV spectral range at liquid-helium (T∼10K) temperatures. It has been shown that these crystals show intense broad-band VUV luminescence due to the interconfiguration 5d-4f transitions in Nd3+ and Tm3+ ions. Remarkable concentration quenching is observed for Nd3+ 5d-4f luminescence whereas fast (spin-allowed) 5d-4f luminescence of Tm3+ shows no concentration quenching for the studied doping level up to 10%. The spin-allowed 5d-4f luminescence of Tm3+ in these crystals was found to be rather weak compared to spin-forbidden 5d-4f luminescence because of efficient nonradiative relaxation from higher-energy 5d states of Tm3+ to the lowest-energy 5d level responsible for spin-forbidden 5d-4f luminescence. The studied fluoride systems can be considered as promising active media for the development of VUV solid state lasers with optical pumping.

High-temperature VUV spectroscopy of KYF4 crystals doped with Nd3+, Er3+ and Tm3+ ions

Thermal quenching of 5d-4f luminescence from Nd3+, Er3+ and Tm3+ ions doped into KYF4 crystals has been investigated in the temperature range up to ∼750 K where this luminescence is completely quenched. The obtained temperatures of thermal quenching (Tq) are ∼270, 495, 450 K for Nd3+, Er3+, Tm3+, respectively. At high temperatures, thermal quenching of 5d-4f luminescence from Nd3+ and Er3+ is accompanied by the appearance of 4f-4f luminescence from the lower-energy 4f levels. It has been shown that the dominating mechanism of thermal quenching for Nd3+ and Er3+ ions is thermally stimulated non-radiative transitions (intersystem crossing) from the 5d states to lower-energy 4f levels, namely 2G(2)9/2 and 2F(2)7/2, respectively, whereas for the Tm3+ ion, thermally stimulated ionization of 5d electrons to the conduction band states is responsible for thermal quenching of 5d-4f luminescence. The energy gap between the lowest Tm3+ 5d level and the bottom of the KYF4 conduction band has been estimated to be 0.66 eV.