Multiplication of electronic excitations in nanophosphors Lu2O3:Eu and Lu2O3:Tb

Luminescence properties of Lu₂O₃:Eu³⁺ and Lu₂O₃:Tb³⁺ nanocrystalline powders with the particle size varying from 46 to 6 nm were studied under excitation by synchrotron radiation in the photon energy range (up to ∼22.5 eV) covering the region where the processes of multiplication of electronic excitation occur. It was found that the excitation spectra of Tb³⁺ emission from all Lu₂O₃:Tb³⁺ nanopowders have similar behavior, whereas the shape of the excitation spectra of Eu³⁺ emission from Lu₂O₃:Eu³⁺ nanopowders strongly depends on the particle size. The difference in the behavior of Lu₂O₃:Eu³⁺ and Lu₂O₃:Tb³⁺ nanophosphor systems was explained by different mechanisms of the energy transfer from the host to Eu³⁺ or Tb³⁺ ions (either the hole or electron recombination mechanism, respectively), which are differently influenced by losses of electronic excitations near the particle surface.     

Low temperature quenching and high efficiency Tm, La or Tb co-doped CaSc2O4:Ce phosphors for light-emitting diodes

Low temperature quenching and high efficiency CaSc₂O₄:Ce³⁺ (CSO:Ce³⁺) phosphors co-doped with Tm³⁺, La³⁺ and Tb³⁺ ions were prepared by a solid state method and the phase-forming, morphology, luminescence and application properties of these phosphors were investigated. The results showed that co-doping of Tm³⁺, La³⁺ and Tb³⁺ ions can improve the luminescence properties and decrease temperature quenching of CSO:Ce³⁺ phosphor remarkably. High efficiency green-light-emitting diodes were fabricated with the prepared phosphors and InGaN blue-emitting (∼460 nm) chips. The good performances of the green-light-emitting LEDs made from co-doped CSO:Ce³⁺ phosphors confirm the luminescence enhancement and indicate that Tm³⁺, La³⁺ and Tb³⁺ co-doped CSO:Ce³⁺ phosphors are suitable candidates for the fabrication of high efficiency white LEDs.     

Luminescence and energy transfer mechanism in Eu<Superscript>3+</Superscript>/Tb<Superscript>3+</Superscript>-co-doped ZrO<Subscript>2</Subscript> nanocrystal rods

Nanocrystal rods of Eu³⁺/Tb³⁺-co-doped ZrO₂ were synthesized using a simple chemical precipitation technique. Both ions were successfully doped into the Zr⁴⁺ ion site in a mixed structure containing both monoclinic and tetragonal phases. The Eu³⁺ or Tb³⁺ singly doped zirconia produced red and green luminescence which are characteristics of Eu³⁺ and Tb³⁺ ions, respectively. The co-doped zirconia samples produced blue emission from defect states transitions in the host ZrO₂, red and green luminescence from dopant ions giving cool to warm white light emissions. The phosphors were efficiently excited by ultraviolet and near-ultraviolet/blue radiations giving white and red light, respectively. The decay lifetime was found to increase with increasing donor ion concentration contrary to conventional observations reported by previous researchers. Weak quadrupole–quatdrupole multipolar process was responsible for energy transfer from Tb³⁺ (donor) ion to Eu³⁺ ion. No energy back-transfer from Eu³⁺ to Tb³⁺ ion was observed from the excitation spectra. Temperature-dependent photoluminescence shows the presence of defects at low temperature, but these defects vanished at room temperature and beyond. The Eu³⁺/Tb³⁺-co-doped ZrO₂ nanocrystal rod is a potential phosphor for white light application using UV as an excitation source. Thermoluminescence measurements show that the inclusion of Tb³⁺ ion increases trap depths in the host zirconia.     

Thermally stable luminescence of blue LiSrPO4:Eu2+ phosphor for near-UV light-emitting diodes

Blue phosphor, LiSrPO₄:Eu²⁺, was prepared by solid-state reaction method under a weak reductive atmosphere and investigated by means of photoluminescence, concentration quenching process, and temperature dependence of luminescence. These results show that LiSrPO₄:Eu²⁺ can be efficiently excited by the UV-visible light of 250–440 nm and exhibits bright blue emission. Furthermore, Eu²⁺-doped LiSrPO₄ phosphor shows high thermally stable luminescence comparable to commercial phosphor BaMgAl₁₀O₁₇:Eu²⁺ (BAM). Two bright blue LEDs were fabricated by incorporating an InGaN-based near-UV chip with the obtained phosphor LiSrPO₄:Eu²⁺ and BAM, respectively. Their luminescence properties were compared based on different forward-bias currents. All the characteristics suggest that LiSrPO₄:Eu²⁺ is a good blue phosphor candidate for creating white light in phosphor-conversion white LEDs.     

Eu/Tb ions co-doped white light luminescence Y2O3 phosphors

Y₂O₃:Eu³⁺, Tb³⁺ phosphors with white emission are prepared with different doping concentration of Eu³⁺ and Tb³⁺ ions and synthesizing temperatures from 750 to 950 °C by the co-precipitation method. The resulted phosphors were characterized by X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy. The results of XRD indicate that the crystallinity of the synthesized samples increases with enhancing the firing temperature. The photoluminescence spectra indicate the Eu³⁺ and Tb³⁺ co-doped Y₂O₃ phosphors show five main emission peaks: three at 590, 611 and 629 nm originate from Eu³⁺ and two at 481 and 541 nm originate from Tb³⁺, under excitation of 250-320 nm irradition. The white light luminescence color could be changed by varying the excitation wavelength. Different concentrations of Eu³⁺ and Tb³⁺ ions were induced into the Y₂O₃ lattice and the energy transfer from Tb³⁺→Eu³⁺ ions in these phosphors was found. The Commission International de l’Eclairage (CIE) chromaticity shows that the Y₂O₃:Eu³⁺, Tb³⁺ phosphors can obtain an intense white emission.     

Intramolecular Energy Transfer and Co-luminescence Effect in Rare Earth Ions (La, Y, Gd and Tb) Doped with Eu3+ β-diketone Complexes

In this paper, Eu³⁺ β-diketone Complexes with the two ligands 1-(2-naphthoyl)-3, 3, 3-trifluoroacetonate (TFNB) and 2’2-bipyridine (bpy) have been synthesized. Furthermore, we reported a systematical study of the co-fluorescence effect of Eu(TFNB)₃bpy doped with inert rare earth ions (La³⁺, Gd³⁺ and Y³⁺) and luminescence ion Tb³⁺. The co-luminescence effect can be found by studying the luminescence spectra of the doped complexes, which means that the existence of the other rare earth ions (La³⁺, Y³⁺, Gd³⁺ and Tb³⁺) can enhance the luminescence intensity of the central Eu³⁺, which may be due to the intramolecular energy transfer between rare earth ions and Eu³⁺. The efficient intramolecular energy transfer in all the complexes mainly occurs between the ligand TFNB and the central Eu³⁺. Full characterization and detail studies of luminescence properties of all these synthesized materials were investigated in relation to co-fluorescence effect between the central Eu³⁺ and other inert ions. Further investigation into the luminescence properties of all the complexes show that the characteristic luminescence of the corresponding Eu³⁺ through the intramolecular energy transfers from the ligand to the central Eu³⁺. Meantime, the differences in luminescence intensity of the ⁵D₀→⁷F₂ transition, in the ⁵D₀ lifetimes and in the ⁵D₀ luminescence quantum efficiency among all the synthesized materials confirm that the doped complex Eu_0.5Tb_0.5(TFNB)₃bpy exhibits higher ⁵D₀ luminescence quantum efficiency and longer lifetime than the pure Eu(TFNB)₃bpy complex and other materials.     

Influence of phase and morphology on thermal conductivity of alumina particle/silicone rubber composites

Eu²⁺- and Eu³⁺-Zn₂GeO₄ were prepared by the high temperature solid-state reaction method. The phase purity and crystallinity of Zn₂GeO₄:Eu samples were characterized by X-ray diffraction (XRD). The excitation spectra, the emission spectra and the luminescence decay curves of the Eu²⁺- and Eu³⁺-Zn₂GeO₄ were investigated. Zn₂GeO₄:Eu²⁺ gives a bluish-green luminescence with one emission band located at 467 nm, and Zn₂GeO₄:Eu³⁺ presents an reddish-orange color due to the transition (⁵D₀→⁷F_J, J = 1 and 2) of the Eu³⁺ ions. The luminescence decay curves of Eu²⁺ and Eu³⁺ provide complementary evidence to the mixed valence of europium (Eu²⁺, Eu³⁺) in Zn₂GeO₄ host. These indicate that the mixed valence of europium (Eu²⁺, Eu³⁺) coexists in Zn₂GeO₄ host prepared in an oxidizing atmosphere. The abnormal reduction phenomenon of Eu³⁺→Eu²⁺ in Zn₂GeO₄ host prepared in an oxidizing atmosphere was reported and discussed on the basis of the charge compensation model.     

Excitation dependent quenching of luminescence in LED phosphors

Droop, the decrease of efficiency with increased power density, became a major topic with InGaN LEDs, after its introduction in 2007. This paper provides insight into droop in localized center luminescence phosphors, exemplified here by Eu²⁺ doped materials. This topic is of increasing importance, as high brightness blue LEDs have reached outputs >1 W/mm². The nonlinearities in phosphor quantum efficiency result in drive‐dependent color point shift and reduction of overall efficiency of phosphor converted white LEDs which utilize Eu²⁺ activated phosphors. The efficiency quenching can be traced back to two processes, well‐known in laser physics, excited state absorption or/and cross relaxation by Foerster/Dexter transfer. Both processes lead to reduction in phosphor efficiency, but they can be differentiated. Understanding the root cause of efficiency quenching opens ways to minimize the practical consequences. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Luminescence properties of SrSi2O2N2 doped with divalent rare earth ions

The optical properties of SrSi₂O₂N₂ doped with divalent Eu²⁺ and Yb²⁺ are investigated. The Eu²⁺ doped material shows efficient green emission peaking at around 540 nm that is consistent with 4f⁷→4f⁶5d transitions of Eu²⁺. Due to the high quantum yield (90%) and high quenching temperature (>500 K) of luminescence, SrSi₂O₂N₂:Eu²⁺ is a promising material for application in phosphor conversion LEDs. The Yb²⁺ luminescence is markedly different from Eu²⁺ and is characterized by a larger Stokes shift and a lower quenching temperature. The anomalous luminescence properties are ascribed to impurity trapped exciton emission. Based on temperature and time dependent luminescence measurements, a schematic energy level diagram is derived for both Eu²⁺ and Yb²⁺ relative to the valence and conduction bands of the oxonitridosilicate host material.     

Study on the color tunability and energy transfer mechanism in Tb3+, Sm3+ co-doped LiLaSiO4 phosphors

A several of LiLaSiO₄: xTb³⁺, ySm³⁺ (LLSO) phosphors were synthesized by high-temperature solid-phase reaction. Through SEM, XRD and fluorescence spectrometer, the phase, morphology, luminescence properties and energy transfer of the samples were systematically analyzed and discussed. Under an excitation of 378 nm wavelength, LLSO: xTb³⁺ phosphors emit green light, and the concentration quenching point of Tb³⁺ ions was x = 0.08. In LLSO: xTb³⁺, ySm³⁺ phosphors, When Sm³⁺ ions doping molar mass fraction increases, the fluorescence intensity of Tb³⁺ ion decreases while the fluorescence intensity of Sm³⁺ ions first strengthen and then weaken. The concentration quenching point of Sm³⁺ ions was y = 0.04. By changing the proportion of Sm³⁺ and Tb³⁺ ions, the luminous color can be adjusted from green to red. There is effective energy transfer between Tb³⁺→Sm³⁺. The molar mass fraction of doping Sm³⁺ ions is y = 0.10, the energy transfer efficiency reaches 96.67%. The energy transfer mechanism is the quadrupole-quadrupole interaction. The quantum yield is 22.34%. Therefore, LLSO: xTb³⁺, ySm³⁺ phosphors have certain potential application value in the field of ultraviolet-near ultraviolet white LEDs.     

Effect of non-stoichiometry on the luminescence of Eu2+-doped aluminates with the β-alumina-type crystal structure

In Al₂O₃ rich Ba-aluminate: Eu phosphors a green luminescence band involving Eu is found next to the blue Eu²⁺-luminescence. The green band is ascribed to associates of Eu and oxygen ions at Ba sites. These O_Ba ions also influence the concentration quenching behaviour of the Ba aluminate with high Eu content. The drop in the quantum efficiency of Ba-aluminate: Eu²⁺ on decreasing the Eu content of phosphors is not related to the presence of O_Ba. It is explained by the presence of Ba-vacancies. No green luminescence was observed in Mg-rich β-alumina type Eu²⁺ phosphors. Also, the quenching of the luminescence at higher Eu concentrations is less. Most probably, Mg²⁺ ions, positioned in between O_Ba and Eu²⁺, block the interaction between last-named ions.     

Luminescence properties of phosphors based on Tb<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> (TbAG) terbium-aluminum garnet

The processes of excitation energy transfer in phosphors based on single-crystal Tb₃Al₅O₁₂:Ce (TbAG:Ce) and Tb₃Al₅O₁₂:Ce,Eu (TbAG:Ce,Eu) garnet films have been investigated. These films are considered to be promising materials for screens for X-ray images and luminescence converters of blue LED radiation. The conditions for excitation energy transfer from the matrix (Tb³⁺ cations) to Ce³⁺ and Eu³⁺ ions in TbAG:Ce and TbAG:Ce,Eu phosphors have been analyzed in detail. It is established that a cascade process of excitation energy transfer from Tb³⁺ ions to Ce³⁺ and Eu³⁺ ions and from Ce³⁺ ions to Eu³⁺ ions is implemented in TbAG:Ce,Eu via dipole-dipole interaction and through the Tb³⁺ cation sublattice.     

The effect of Gd3+ on the Eu3+ luminescence in RbGd3F10: Eu3+ compounds

Eu³⁺ luminescence is studied in the two polymorphic forms of RbGd₃F₁₀: Eu³⁺. In the phases where Gd³⁺ is partly replacedby Y³⁺ in order to obtain an independent variation of Eu³⁺ concentration, the quenching of Gd³⁺ luminescence by Eu³⁺ ions takes place very quickly with increasing Eu³⁺ concentration. So the transfer between Gd³⁺ and Eu³⁺ is very efficient. Besides, an important self quenching occurs as well between Gd³⁺ ions as between Eu³⁺ ions, showing strong interactions.     

Photoluminescence investigations of Zn2SiO4 co-doped with Eu and Tb ions

Zinc silicate phosphors co-doped with Eu³⁺ ions and also with both Eu³⁺ and Tb³⁺ ions were prepared by high temperature solid state reaction in air or reducing atmosphere. The luminescence characteristics of the prepared phosphors were investigated. While in the samples prepared in air, Eu³⁺ emission was found to be dominant over Tb³⁺ emission, in the samples prepared in reducing atmosphere, intense Eu²⁺ emission at 448 nm was found to be predominant over narrow Tb³⁺ emission. Luminescence studies showed that Eu³⁺ ions occupy asymmetric sites in Zn₂SiO₄ lattice. The intense f-f absorption peak of Eu³⁺ at 395 nm observed in these phosphors suggests their potential as red emitting phosphors for near ultra-violet light emitting diodes.     

UV-induced photoluminescence and thermally stimulated luminescence of CaSO4:Eu and CaF2:Tb3+ phosphors

Ultraviolet radiation induced changes in photoluminescence (PL) and thermally stimulated luminescence (TSL) of europium activated calcium sulphate (CaSO₄:Eu³⁺, Eu²⁺) and terbium doped calcium fluoride (CaF₂:Tb³⁺) phosphors have been studied. PL measurements suggest conversion of Eu³⁺ to Eu²⁺ on 254 nm irradiation corresponding to charge transfer band of Eu³⁺ ions and reduction of Eu²⁺ ions with 365 nm illumination representing a f–d transition of Eu²⁺ ions. Similar studies carried out on CaF₂:Tb³⁺ phosphor, however, do not show any significant wavelength specific changes. The integrated TSL output appears to be rate-dependent for both phosphors. The wavelength dependent changes in TSL output observed for CaSO₄:Eu phosphor have been correlated with those obtained in PL studies. The changes in TSL and PL characteristics of CaF₂:Tb³⁺ phosphor have been explained on the basis of stabilisation of traps based on matrix specific charge similarities.     

KBaPO4:Ln (Ln=Eu, Tb, Sm) phosphors for UV excitable white light-emitting diodes

This letter reports the novel three emission bands based on phosphate host matrix, KBaPO₄ doped with Eu²⁺, Tb³⁺, and Sm³⁺ for white light-emitting diodes (LEDs). The phosphors were synthesized by solid-state reaction and thermal stability was elucidated by measuring photoluminescence at higher temperatures. Eu²⁺-doped KBaPO₄ phosphor emits blue luminescence with a peak wavelength at 420 nm under maximum near-ultraviolet excitation of 360 nm. Tb³⁺-doped KBaPO₄ phosphor emits green luminescence with a peak wavelength at 540 nm under maximum near-ultraviolet excitation of 370 nm. Sm³⁺-doped KBaPO₄ phosphor emits orange-red luminescence with a peak wavelength at 594 nm under maximum near-ultraviolet excitation of 400 nm. The thermal stabilities of KBaPO₄:Ln (Ln=Eu²⁺, Tb³⁺, Sm³⁺), in comparison to commercially available YAG:Ce³⁺ phosphor were found to be higher in a wide temperature range of 25-300 °C.     

Excitation and luminescence of rare earth-doped lead phosphate glasses

Excitation and luminescence properties of Eu³⁺, Tb³⁺ and Er³⁺ ions in lead phosphate glasses have been studied. From excitation spectra of Eu³⁺ ions, the electron–phonon coupling strength and phonon energy of the glass host were calculated and compared to that obtained by Raman spectroscopy. Main intense and long-lived luminescence bands are related to the ⁵D₀–⁷F₂ (red) transition of Eu³⁺, the ⁵D₄–⁷F₅ (green) transition of Tb³⁺ and the ⁴I_13/2–⁴I_15/2 (near-infrared) transition of Er³⁺. The critical transfer distances, the donor–acceptor interaction parameters and the energy transfer probabilities were calculated using the fitting of the luminescence decay curves from ⁵D₀ (Eu³⁺), ⁵D₄ (Tb³⁺) and ⁴I_13/2 (Er³⁺) excited states. The energy transfer probabilities for Eu³⁺ (⁵D₀), Tb³⁺ (⁵D₄) and Er³⁺ (⁴I_13/2) are relatively small, which indicates low self-quenching luminescence of rare earth ions in lead phosphate glasses.     

Luminescence properties of SrSi2AlO2N3 doped with divalent rare-earth ions

The optical properties of SrSi₂AlO₂N₃ doped with Eu²⁺ and Yb²⁺ are investigated towards their applicability in LEDs. The Eu²⁺-doped material shows emission in the green, peaking around 500 nm. The emission is ascribed to the 4f⁶5d¹–4f⁷ transition on Eu²⁺. In view of the too low quantum efficiency and the considerable thermal quenching of the emission at the operation temperature of high power LED (>1W/mm²) this phosphor is only suitable for application in low power LEDs. The Yb²⁺ emission shows an anomalously red-shifted emission compared to Eu²⁺, which is characterized by a larger FWHM, a larger Stokes shift and lower thermal quenching temperature. The emission is ascribed to self-trapped exciton emission. The Yb²⁺ activated phosphor is found to be unsuitable for the use in any phosphor-converted LEDs.     

Feasibility Study of VUV Sensitization Effect of Tb3+

The possibility to use Tb³⁺ as luminescence sensitizer for enhancement of the conversion efficiency of vacuum-ultraviolet (VUV) radiation into visible light was examined. We studied the luminescence properties of K₃Tb(PO₄)₂ and Ba₃Tb(PO₄)₃ activated by Eu³⁺, and of SrAl₁₂O₁₉ co-doped with Mn²⁺ and Tb³⁺ at excitation over the 120 to 300 nm wavelength range. It is shown that Tb³⁺ ions, exhibiting a strong absorption band in the VUV, can provide efficient sensitization of Eu³⁺ and Mn²⁺ emissions for excitation in this spectral range, giving rise to intense red and green luminescence, respectively. This study provides a proof for the concept of VUV sensitization, which enables the engineering of luminescence materials with improved efficiency for excitation from a noble gas discharge.     

Sensitized luminescence of Eu and Tb macrocyclic complexes bearing benzophenone antennae

Sensitized luminescence behavior of lanthanide (Ln=Eu³⁺, Tb³⁺) macrocyclic cyclen (1,4,7,10-tetraazacyclododecane) complexes bearing one or four benzophenone (BP) moieties as antenna (LnL1 and LnL4) has been studied in water. Despite higher molar extinction coefficient of EuL4 owing to four antennae, it shows only one-thirtieth the luminescence intensity of EuL1. Energy level of triplet excited-state of BP antenna (E_T) is only a few kJ mol⁻¹ higher than that of ⁵D₂ excited-state of Eu³⁺, thus promoting a back energy transfer (BET) from ⁵D₂ of Eu³⁺ to ground-state BP antennae. On EuL4 bearing four antennae, BET occurs more rapidly than that on EuL1, thus exhibiting much weaker luminescence. For Tb complexes, the energy gap between E_T of BP antenna and ⁵D₄ excited state of Tb³⁺ is large enough (>13 kJ mol⁻¹), such that practically no BET occurs. The luminescence intensity of TbL4 is, however, lower (two-third) than that of TbL1. Time-resolved luminescence measurement reveals that hydration number of Tb³⁺ within TbL4 is twice that within TbL1. This is because the structural distortion of ligands on TbL4, caused by an intramolecular dipole-dipole interaction among the BP antennae, allows coordination of higher number of H₂O molecules to Tb³⁺, thus leading to a strong Tb luminescence quenching via O-H oscillators.