Pr3+ luminescence center in Lu2Si2O7 host

Pr³⁺‐doped Lu₂Si₂O₇ (LPS:Pr) microcrystalline phosphor was prepared by the sol–gel method. We study the LPS:Pr luminescence properties under UV and X‐ray excitation within 80–500 K. The emission spectrum is dominated by fast 5d–4f band peaking at 261 nm having 16 ns decay time. By purely optical contactless methods we determine the energy barrier of 300 meV for thermal ionization of the Pr³⁺ 5d₁ relaxed excited state in LPS host. The barrier is high enough to keep the room temperature quantum efficiency of the Pr³⁺ luminescence center close to unity. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermostimulated luminescence of CdF2:Eu crystals

Using the method of fractional thermostimulated luminescence (FTSL), the temperature dependence of the mean activation energy of recombination processes in CdF₂:Eu³⁺ crystal was obtained. After thermal annealing of the crystal, thermostimulated luminescence peaks were identified. Anomalously low frequency factor (s=10⁷ s ^–1) of the recombination processes can be explained by the dependence of the resonance energy transfer probability on intercentre distance.The authors are very grateful to Dr. E. Kotomin for valuable comments and Dr. C. Paracchini for supply of CdF₂:Eu crystals.     

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 (T_q) 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.     

The luminescence of bismuth and europium in Ca4YO(BO3)3

A series of Bi³⁺/Eu³⁺ singly and doubly activated Ca₄YO(BO₃)₃ phosphors were synthesized by solid-state reaction method. The structures and photoluminescent properties of the phosphors were investigated at room temperature. Under UV excitation Bi³⁺ and Eu³⁺ show a high light output. Ca₄YO(BO₃)₃:Eu³⁺ has potential application as a phosphor for fluorescent lamps. The luminescence of Bi³⁺ and Eu³⁺ in Ca₄YO(BO₃)₃ resembles more that in the rare earth oxides than that in borates. The free oxygen ion in the host lattice, which is not bonded to any boron ions seems to be responsible for that. In this host lattice energy migration between linear Eu³⁺ chains occurs. The emission of Bi³⁺ is completely quenched when Eu³⁺ is co-doped. A model was proposed to explain it.     

Intense visible luminescence from Nd-doped yttrium oxysulfide

We report on a novel luminescent phenomenon in Y₂O₂S doped with Nd³⁺. After irradiation by a 261 nm ultraviolet (UV) light into the Y₂O₂S host lattice, the Nd³⁺-doped Y₂O₂S phosphor emits intense blue luminescence in the visible light region. Moreover, this blue luminescence can also be obtained by exciting directly into the Nd³⁺ energy absorption itself. XRD, photoluminescence, and fluorescence decay curve are used to characterize the synthesized phosphor. The spectroscopic data indicate that all the visible emission peaks are originated from the electrical transitions of Nd³⁺, and the strong luminescence of the Nd³⁺ is considered to be due to an efficient energy transfer from the Y₂O₂S host lattice to the Nd³⁺ in Y₂O₂S:Nd³⁺. The optimum concentration for the luminescence Nd³⁺ is determined to be 1 mol% of Y³⁺ in Y₂O₂S host. The critical energy transfer distance has been calculated by the concentration quenching and the possible luminescent process of this blue luminescence-emitting phosphor is also investigated.     

Photon down-shifting by energy transfer from Sm3+ to Eu3+ ions in sol-gel SiO2-LaF3 nano-glass-ceramics for photovoltaics

95SiO₂?C5LaF₃ sol-gel derived nano-glass-ceramics single doped with Eu³⁺ or Sm³⁺ and codoped with both of them were successfully obtained. XRD measurements confirm the precipitation of LaF₃ nanocrystals after the ceramming process, with mean size ranging from 10 to 20?nm which increases with the thermal treatment temperature. The incorporation of rare-earth ions into precipitated LaF₃ nanocrystals was confirmed from luminescence spectra. Intense yellow-red emissions were detected under UV and blue light excitation in single and codoped samples. The effect of codoping with Eu³⁺ and Sm³⁺ ions and the energy transfer mechanism between them have been analyzed in order to increase the yellow-red emissions.     

A novel Dy3+-doped GdPO4 white-light phosphors under vacuum ultraviolet excitation for Hg-free lamps application

Novel Dy3+-doped GdPO4 white light phosphors with monoclinic system were successfully synthesised by hydrothermal method at 240 ℃. This paper investigates the luminescence properties of white-light Gd1-xPO4 : xDy3+ under vacuum ultraviolet (VUV) excitation. The strong absorption at around 147 nm in excitation spectrum energy can be transferred to the energy levels of Dy3+ ion from the host absorption. Additionally, this white light phosphors are activated by a single Dy3+ ion and with a lower preparation temperature, which tend to decrease the consumption of rare earth resource and energy. Therefore, the luminescence of Gd1-xPO4 : xDy3+ under VUV excitation is effective, and proves to be promising in application to mercury-free lamp.     

Scintillation kinetics and thermoluminescence of SrI2:Eu2+ single crystals

Single crystal of strontium iodide doped with 1% europium (SrI₂:1% Eu²⁺) was grown by Vertical Gradient Freeze technique. UV excited emission spectra were studied as a function of temperature. Results indicate the thermal quenching of Eu²⁺ emission starts from ～400 K with a thermal activation energy of 0.39 eV. Gamma and UV excited decay measurements indicate that the scintillation decay time of SrI₂:Eu²⁺ is longer than the lifetime of Eu²⁺ luminescence center in the SrI₂ host. The thermoluminescence glow curve revealed a highly concentrated charge carrier trap at 50 K. Elimination of this trap is expected to enhance the energy migration of charge carriers and result in faster scintillation decay.     

Energy transfer from the host to Er<Superscript>3+</Superscript> dopants in semiconductor SnO<Subscript>2</Subscript> nanocrystals segregated in sol–gel silica glasses

Undoped and Er³⁺-doped glass–ceramics of composition (100−x)SiO₂–xSnO₂, with x = 5 or 10 and with 0.4 or 0.8 mol% of Er³⁺ ions, were synthesised by thermal treatment of precursor sol–gel glasses. Structural studies were developed by X-Ray Diffraction. Wide band gap SnO₂ semiconductor quantum-dots embedded in the insulator SiO₂ glass are obtained. The mean radius of the SnO₂ nanocrystals, ranging from 2 to 3.2 nm, is comparable to the exciton Bohr radius. The luminescence properties have been analysed as a function of sample composition and thermal treatment. The results show that Er³⁺ ions are partially partitioned into the nanocrystalline phase. An efficient UV excitation of the Er³⁺ ions by energy transfer from the SnO₂ nanocrystal host is observed. The Er³⁺ ions located in the SnO₂ nanocrystals are selectively excited by this energy transfer mechanism. On the other hand, emission from the Er³⁺ ions remaining in the silica glassy phase is obtained by direct excitation of these ions.     

Photoluminescence properties of red-emitting phosphors Ln3BWO9:Eu3+ (Ln=Y,La, Gd)

Undoped and Eu³⁺ activated Ln₃BWO₉ (Ln=Y, La, Gd) were prepared by the Pechini method and characterized with X-ray diffraction (XRD) and ultraviolet (UV) spectroscopy. All the samples have the hexagonal phase after heat treatment in the range of 850–1000 °C. The Eu³⁺ doped samples emit high-purity red light with peak maximum at about 617 nm under excitation of UV light (～285 nm) at room temperature. When the doping concentration of Eu³⁺ is about 20–30%, luminescence intensity reaches the maximum. Luminescence decay curves indicate that Ln₃BWO₉:Eu³⁺ exhibits a fast decay time of about 0.5 ms. A possible luminescence mechanism has also been proposed. It is worth noting that both the absorption of host lattice and the charge transfer (CT) transition of Eu³⁺ are of great importance to the promising luminescent performance of Ln₃BWO₉:Eu³⁺.     

Interplay of spin-allowed and spin-forbidden 5d–4f luminescence from rare earth ions

The conditions for co-existence of emissions from spin-allowed and spin-forbidden 5d–4f transitions in rare earth ions and for thermal quenching of these emissions have been analyzed by taking Tm³⁺ 5d–4f luminescence in LiYF₄:Tm³⁺ and Lu³⁺ 5d–4f luminescence in LuF₃ as examples. It is shown that temperature behavior of 5d–4f luminescence agrees with the common trends in decreasing energy splitting between the lowest high-spin and low-spin 5d levels as well as decreasing energy gap between the lowest 5d level and the bottom of the conduction band of the host crystal towards heavier rare earth ions (from Er³⁺ to Lu³⁺).     

Effects of Gd<Superscript>3+</Superscript> modifications on the photoelectrochemical properties of TiO<Subscript>2</Subscript>-based dye-sensitized solar cells

In this paper, TiO₂ particles (~30 nm) modified with Gd₂O₃-coating layer (~2 nm) for dye-sensitized solar cells (DSSCs) were fabricated via the hydrothermal method. Among the solar cells based on the Gd³⁺-doped TiO₂ photoanodes, the optimal conversion efficiency was obtained from the 0.025Gd³⁺-modified TiO₂-based cell, with a 17.7% improvement in the efficiency as compared to the unmodified one (7.18%). This enhancement was probably due to the improved UV radiation harvesting via a down-conversion luminescence process by Gd³⁺ ions, enhancement of visible light absorption and improved dye loading capacity. In addition, after Gd modification, a thin coating could be formed on the TiO₂ nanoparticles, which worked as an energy barrier and resulted in a lower charge recombination.     

High brightness and precise adjustment of multicolor-tunable luminescence of Lu2GeO5:Tb3+, Eu3+ phosphors for white LEDs

High brightness and precise adjustment of luminescence colour of phosphors are two main targets in the research of phosphor-converted white LEDs. However, few feasible strategy can be employed to achieve the multicolor-tunable luminescence under the premise of maintaining high quantum efficiency. Here, we demonstrate a high-efficiency energy-transfer process from Tb³⁺ to Eu³⁺ ions with a higher luminescent quantum efficiency (64.5% and 53.4%, respectively), and green-red multicolor emission in Lu₂GeO₅ host via varying the doping content of Tb³⁺ and Eu³⁺ ions. Besides, Lu₂GeO₅:Tb³⁺, Lu₂GeO₅: Eu³⁺ and Lu₂GeO₅: Tb³⁺, Eu³⁺ all exhibit weak thermal quenching which ensures the stable use of white LED device in the high temperature environment. This paper provides a novel multicolor-tunable phosphor with high brightness, efficient energy transfer and weak thermal quenching, which presents a potential application for UV-converted 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.     

VUV-UV excited luminescent properties of LnCa4O(BO3)3:RE (Ln=Y, La, Gd; Re=Eu, Tb, Dy, Ce)

Calcium lanthanide oxyborate doped with rare-earth ions LnCa₄O(BO₃)₃:RE³⁺ (LnCOB:RE, Ln=Y, La, Gd, RE=Eu, Tb, Dy, Ce) was synthesized by the method of solid-state reaction at high temperature. Their fluorescent spectra were measured from vacuum ultraviolet (VUV) to visible region at room temperature. Their excitation spectra all have a broadband center at about 188 nm, which is ascribed to host absorption. Using Dorenbos’ and Jφrgensen's work [P. Dorenbos, J. Lumin. 91 (2000) 91, R. Resfeld, C.K. Jφrgensen, Lasers and Excite States of Rare Earth [M], Springer, Berlin, 1977, p. 45], the position of the lowest 5d levels E(Ln,A) and charge transfer band E_ct were calculated and compared with their excitation spectra.Eu³⁺ and Tb³⁺ ions doped into LnCOB show efficient luminescence under VUV and UV irradiation. In this system, Ce³⁺ ions do not show efficient luminescence and quench the luminescence of Tb³⁺ ions when Tb³⁺ and Ce³⁺ ions are co-doped into LnCOB. GdCOB doped with Dy³⁺ shows yellowish white light under irradiation of 254 nm light for the reason that Gd³⁺ ions transfer the energy from itself to Dy³⁺. Because of the existence of Gd³⁺, the samples of GdCOB:RE³⁺ show higher excitation efficiency than LaCOB:RE³⁺ and YCOB:RE³⁺, around 188 nm, which indicates that the Gd³⁺ ions have an effect on the host absorption and can transfer the excitation energy to the luminescent center such as Tb³⁺, Dy³⁺ and Eu³⁺.     

The effect of Mg ions on the photoluminescence of Ce-doped silica

In this paper, we report enhanced luminescence of Mg-co-doped silica gels, which were prepared by a sol-gel method. The total amount of Ce³⁺ ions was kept constant in this experiment at 0.5 mol% total doping. Structural, morphological, thermal, optical absorption and photoluminescence studies were employed. The XRD spectra show that all the samples are non-crystalline. Scanning electron microscopy (SEM) images show that the particles were in nano-range and spherical in shape. Differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) of samples depict that the presence of dopant and co-dopant decreases the endothermic peak temperature; while Ce³⁺ increase the yield, Mg²⁺ reduces it. UV analyses revealed that the presence of Ce³⁺ ions increases transmittance but lowers absorbance of annealed silica xerogels, while that of Mg²⁺ ions reduces transmittance but increases absorbance. Luminescence intensities were compared for different gels with and without Mg particles by varying the different concentrations of Mg. Silica containing Mg²⁺ ions had broad blue emission due to energy transfer from Mg²⁺ to Ce³⁺, which is due to radiative recombination. An increase in luminescence intensity was observed as the Mg²⁺ to Ce³⁺ ratio increased for the range investigated.     

Luminescence of Cr ions associated with surpassing the green-emissive defect centers in β-Ga2O3

Photoluminescence of undoped and Cr³⁺-doped β-Ga₂O₃ was investigated. The transparent, undoped β-Ga₂O₃ film was successfully prepared by thermal conversion from GaOOH. The film exhibited predominant green luminescence in response to ultraviolet light excitation at 250 nm. This luminescence behavior, which was proposed to result from the oxygen defect centers, was used in examining excitation and emission mechanisms for Cr³⁺ ions doped in β-Ga₂O₃. It was found that red luminescence of Cr³⁺ surpasses green luminescence of the host lattice, as evidenced by the dependence of the spectral structure on the Cr³⁺ concentration. The excitation of Cr³⁺ was then suggested to be caused by the energy transfer from Ga³⁺O₆ octahedra present in the monoclinic β-Ga₂O₃ lattice.     

Luminescence and thermally stimulated recombination processes in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript>:Ce<Superscript>3+</Superscript> crystals

The luminescence and thermally stimulated recombination processes in lithium borate crystals Li₆Gd(BO₃)₃ and Li₆Gd(BO₃)₃:Ce have been studied. The steady-state luminescence spectra under X-ray excitation (X-ray luminescence), temperature dependences of the intensity of steady-state X-ray luminescence (XL), and thermally stimulated luminescence (TSL) spectra of these compounds have been investigated in the temperature range of 90–500 K. The intrinsic-luminescence 312-nm band, which is due to the ⁶ P _J → ⁸ S _7/2 transitions in Gd³⁺ matrix ions, dominates in the X-ray luminescence spectra of these crystals; in addition, there is a wide complex band at 400–420 nm, which is due to the d → f transitions in Ce³⁺ impurity ions. It is found that the steady-state XL intensity in these bands increases several times upon heating from 100 to 400 K. The possible mechanisms of the observed temperature dependence of the steady-state XL intensity and their correlation with the features of electronic-excitation energy transfer in these crystals are discussed. The main complex TSL peak at 110–160 K and a number of minor peaks, whose composition and structure depend on the crystal type, have been found in all crystals studied. The nature of the shallow traps that are responsible for TSL at temperatures below room temperature and their relation with defects in the lithium cation sublattice are discussed.     

Recombination kinetics in nonlinear defective LiB3O5 crystals

A study of recombination kinetics in LiB₃O₅ (LBO) crystals by time-resolved luminescence and absorption spectroscopy is reported. An investigation of the kinetics of transient optical absorption (TOA) and luminescence under ns-scale electron-beam excitation performed within a broad temperature range of 77–500 K and a 1.2–5-eV spectral interval has established that the specific features in the recombination kinetics observed in LBO involve electronic, B²⁺, and hole, O⁻, trapping centers. The TOA and luminescence kinetics, as well as their temperature dependence, are interpreted by a model of competing hole centers. Relations connecting the kinetics parameters and the temperature dependence to the parameters of the main LBO point defects are presented. Fiz. Tverd. Tela (St. Petersburg) 40, 2008–2014 (November 1998)     

Photo- and thermo-stimulated luminescence of CsI—Tl crystal after UV light irradiation at 80 K

Abstract

Thermo- and photo-stimulated luminescence are studied for CsI—Tl crystal after the irradiation with the UV light at 80 K. Creation spectrum of the photostimulated luminescence coincides with the D absorption band of Tl⁺ ions. Nature of the defects created by UV light at low temperatures is discussed basing on the correspondence between the thermostimulated glow curve peaks and thermal evolution of the photostimulation spectra observed after irradiation in the D absorption band. Three bands at 1400, 950 and 580 nm have been observed in the stimulation spectrum at 80 K. The 1400 and 950 nm stimulation bands are presumably explained as the optical transitions in the Tl⁰ centre forming the spatially correlated defect pair with V_k centre while the 580 nm stimulation band is connected with the unperturbed Tl⁰ centres. It is concluded that the Tl⁺ luminescence at low temperature is connected with the electron recombination with the Tl²⁺ centre.