Luminescence properties of europium-terbium double activated calcium tungstate phosphor

Double incorporation of Eu³⁺ and Tb³⁺ ions into a CaWO₄ crystalline lattice modifies the luminescence spectrum due to the formation of new emission centers. Depending on the activators concentration and nature, as well as on the interaction between the activators themselves, the luminescence color can be varied within the entire range of the visible spectrum. Variable luminescence was obtained when CaWO₄:Eu,Tb phosphors with 0-5 mol% activator ions were exposed to relatively low excitation energies as UV (365 and 254 nm). Under high energy excitation such as VUV (147 nm) radiation or electron beam, white light has been observed.This material with controlled properties seems to be promising for the applications in fluorescent lamps, colored lightning for advertisement industries, and other optoelectronic devices.     

Luminescence of excitons and antisite defects in Lu<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Ce single crystals and single-crystal films

The luminescence of excitons and antisite defects (ADs) was investigated, as well as the specific features of the excitation energy transfer from excitons and ADs to the activator (Ce³⁺ ion) in phosphors based on Lu₃Al₅O₁₂:Ce (LuAG:Ce) single crystals and single-crystalline films, which are characterized by significantly different concentrations of ADs of the Lu _Al ³⁺ type and vacancy-type defects. The luminescence band with λ_max = 249 nm in LuAG:Ce single-crystal films is due to the luminescence of self-trapped excitons (STEs) at regular sites of the garnet lattice. The excited state of STEs is characterized by the presence of two radiative levels with significantly different transition probabilities, which is responsible for the presence of two excitation bands with λ_max = 160 and 167 nm and two components (fast and slow) in the decay kinetics of the STE luminescence. In LuAG:Ce single crystals, in contrast to single-crystal films, the radiative relaxation of STEs in the band with λ_max = 253.5 nm occurs predominantly near Lu _Al ³⁺ ADs. The intrinsic luminescence of LuAG:Ce single crystals at 300 K in the band with λ_max = 325 nm (τ = 540 ns), which is excited in the band with λ_max = 175 nm, is due to the radiative recombination of electrons with holes localized near Lu _Al ³⁺ ADs. In LuAG:Ce single crystals, the excitation of the luminescence of Ce³⁺ ions occurs to a large extent with the participation of ADs. As a result, slow components are present in the luminescence decay of Ce³⁺ ions in LuAG:Ce single crystals due to both the reabsorption of the UV AD luminescence in the 4f-5d absorption band of Ce³⁺ ions with λ_max = 340 nm and the intermediate localization of charge carriers at ADs and vacancy-type defects. In contrast to single crystals, in phosphors based on LuAG:Ce single-crystal films, the contribution of slow components to the luminescence of Ce³⁺ ions is significantly smaller due to a low concentration of these types of defects.     

Luminescence and EPR studies of Eu doped BaAl12O19 blue light emitting phosphors

Europium doped BaAl₁₂O₁₉ powder phosphors have been synthesized by combustion process within few minutes. The phosphors have been characterized by XRD, SEM, FT-IR, EPR and PL techniques. The EPR spectrum exhibits an intense resonance signal at g=1.96 characteristic of Eu²⁺ ions. In addition to this two weak resonance signals have been observed at g=2.28 and g=4.86. The population of the spin levels (N) for the resonance signal at g=1.96 is calculated as a function of temperature. By post-treating the phosphor at 1350 °C under a reducing atmosphere, it is observed that the population of spin levels has been increased five times. The excitation spectrum shows a peak at 326 nm with a shoulder at 290 nm. Upon excitation at 326 nm, the emission spectrum exhibits a well defined broad band with maximum at 444 nm emitting a blue light corresponding to 4f⁶5d→4f⁷ transition. The luminescence intensity also has been enhanced to 60% by post-treating the phosphor at 1350 °C under a reducing atmosphere.     

Spectroscopy of resonant excitation of exciton luminescence of GaSe–GaTe solid solutions

The luminescence excitation spectra of localized excitons in GaSe_0.85Te_0.15 solid solutions have been investigated at the temperature T = 2 K. It has been shown that the excitation spectra of excitons with the localization energy ε > 10 mV exhibit an additional maximum M _E located on the low-energy side of the maximum corresponding to the free exciton absorption band with n = 1. It has been found that the shift in the position of the maximum M _E in the excitation spectrum with respect to the energy of detected photons increases as the energy of detected photons decreases, i.e., with an increase in the localization energy of excitons. Under the resonant excitation of localized excitons by a monochromatic light from the region of the exciton emission band, in the exciton luminescence spectrum on the low-energy side from the excitation line, there is also a maximum of the luminescence (M _L ). The energy distance between the position of the excitation line and the position of the maximum in the luminescence spectrum increases with a decrease in the frequency of the excitation light. The possible mechanisms of the formation of the described structure of the luminescence excitation and exciton luminescence spectra of GaSe_0.85Te_0.15 have been considered. It has been concluded that the maximum M _E in the excitation spectrum and the maximum M _L in the luminescence spectrum are attributed to electronic–vibrational transitions with the creation and annihilation of localized excitons, respectively.     

Specific features of absorption and luminescence in CsBr: EuOBr crystals

The specific features of the absorption, photoluminescence, x-ray luminescence, thermally stimulated luminescence, and photostimulated luminescence spectra of CsBr: Eu²⁺ single crystals grown using the Bridgman method are investigated in the temperature range 80–500 K at the highest possible dopant content (0.1–0.4 mol % EuOBr in the batch) required for preparing perfect crystals. It is shown that an increase in the dopant content leads to a broadening of the absorption and photoluminescence excitation bands with maxima at wavelengths of 250 and 350 nm due to the interconfigurational transitions 4f⁷(⁸S_7/2) → 4f⁶5d(e _g , t_2g) in Eu²⁺ ions. The photoluminescence and photostimulated luminescence spectra of CsBr: EuOBr single crystals (0.1–0.4 mol % EuOBr) contain a band at a wavelength of λ_max=450 nm and bands at wavelengths of λ_max=508–523 and 436 nm. The last two bands are assigned to Eu²⁺-V_Cs isolated dipole centers and Eu²⁺-containing aggregate centers, respectively. It is revealed that the intensity of the luminescence associated with the aggregate centers (λ_max=508–523 nm) is maximum at an EuOBr content of less than or equal to 0.1 mol % and decreases with an increase in the dopant content. The possibility of forming CsEuBr₃-type nanocrystals that are responsible for the green luminescence observed at a wavelength λ_max=508–523 nm in CsBr: Eu crystals is discussed. The intensity of photostimulated luminescence in the CsBr: EuOBr crystals irradiated with x-ray photons is found to increase as the dopant content increases. It is demonstrated that CsBr: EuOBr crystals at a dopant content in the range 0.3–0.4 mol % can be used as x-ray storage phosphors for visualizing x-ray images with high spatial resolution.     

Exciton luminescence of YAlO<Subscript>3</Subscript> single crystals and single-crystal films

The paper reports on a study of exciton luminescence in single crystals (SCs) and single-crystal films (SCFs) of YAlO₃, which have substantially different concentrations of vacancy-type and substitutional defects, under excitation by synchrotron radiation near the fundamental absorption edge. The radiative annihilation of excitons in SCFs was shown to occur primarily at regular perovskite lattice sites and to be accompanied by luminescence in a band peaking at λ_max = 295 nm with τ = 5.2 ns. In contrast to SCFs, the radiative exciton decay in YAlO₃ SCs takes place predominantly near vacancy-type defects (F⁺ and F centers) and is accompanied by luminescence in the bands at λ_max = 350 nm (τ = 2.5 ns) and 440 nm (τ₁ = 1.9 ns, τ₂ = 30 ms). Photoexcitation in the 175-nm band of YAlO₃ SCs revealed photoconversion of the centers F → F⁺.     

Ultraviolet luminescence of Li6Gd(BO3)3: Ce crystals under selective excitation in the region of 4d → 4f transitions

The subnanosecond time-resolved ultraviolet luminescence of Li₆Gd(BO₃)₃: Ce crystals under selective excitation by ultrasoft X-rays in the region of the 4d??4f core transitions at temperatures of 7 and 293 K has been investigated for the first time. The performed investigation has revealed the following features: an intense fast component of the luminescence decay kinetics in the subnanosecond range due to the high local density of electronic excitations and the processes of Auger relaxation of the core hole; the modulation of the luminescence excitation spectrum by the ??giant resonance?? absorption band of the 4d-4f photoionization in the energy range 135?C160 eV; and a new broad luminescence band at an energy of 4.44 eV due to the direct radiative recombination between the genetically related electron in the states of the conduction band bottom and hole in the 4f ground state of the Ce³⁺ ion.     

N-lines in the luminescence spectra of Cr3+-doped spinels: IV. Excitation spectra

Excitation spectra (T = 75–300 K; λ_exc = 450–630 nm) which were measured for the R-lines of Cr³⁺-doped oxides (α-Al₂O₃, β-Ga₂O₃) and for different luminescence lines (R-lines, N-lines) of Cr³⁺-doped spinels (MgAl₂O₄, ZnAl₂O₄, ZnGa₂O₄) are reported. The excitation maxima observed for different luminescence lines of a given compound exhibit considerable differences: 530 nm ? λ^max_exc ? 565 nm for MgAl₂O₄; 530 nm ? λ^max_exc ? 580 nm for ZnAl₂O₄; 545 nm ? λ^max_exc 555 nm for ZnGa₂O₄. According to the interpretation of N-lines to arise from different classes of Cr³⁺ ion swith different short range orderd, the excitation maximum of one distinct line should entirely correspond to the transition Δ : ⁴T₂ ← ⁴A₂ of that Cr³⁺ class from which the line arises. By this method spectroscopic data about the different kinds of Cr³⁺ ions present in a given sample can, therefore, be obtained which are not available from absorption measurements. The experimetal data were found to be in agreement with the results of model calculations. Restrictions which limit the accuracy and relevance of the data are discussed.     

Study of the protoporphyrin luminescence under selective laser excitation

The structural luminescence spectra of protoporphyrin IX solid solutions in ethanol and hydrochloric acid were obtained under selective laser excitation in the region of the inhomogeneouly broadened pure electronic and vibronic bands at T = 3.8 K. The dependence of the excitation selectivity on the excitation frequency was investigated. The vibrational frequencies of protoporphyrin IX were obtained in the ground and excited electronic states when the excitation frequency has been scanned in the region of pure electronic and vibronic bands, respectively. A universal apparatus is described for the investigation of the absorption and luminescence spectra of polyatomic molecules in the temperature range from 3.5 to 300 K under the dye laser excitation tunable in the spectral region 265–365 nm, 435–730 nm, detection of spectra in the region 300–900 nm with a polychromator and silicon intensified image detector and processing of the spectral information with an optical spectrum analyser controlled with a microcomputer.     

VUV luminescence of Er<Superscript>3+</Superscript> ions in LiYF<Subscript>4</Subscript> and BaY<Subscript>2</Subscript>F<Subscript>8</Subscript> crystals

Vacuum ultraviolet luminescence of Er³⁺ ions in LiYF4 and BaY₂F₈ crystals has been investigated. It is revealed that under excitation by 193 nm radiation from an ArF excimer laser the interconfigurational 5d–4f radiative transitions in Er³⁺ ions are observed. It is shown that from the LiYF₄:Er crystal only the spin-forbidden luminescence (λ = 165 nm) is detected, whereas both the spin-forbidden (λ = 169 nm) and spin-allowed (λ = 160.5 nm) components are observed from the BaY₂F₈:Er crystal.     

Study of acceptor states in CdTe by donor-acceptor pair excitation luminescence

Excitation of donor-acceptor pair luminescence has been studied in CdTe doped with lithium or chlorine. The excitation spectrum of the lithium acceptor is determined and fitted with the effective mass theory of Baldereschi and Lipari. Revised values of the valence band parameters are deduced: μ = 0.8, δ = 0.054, Ry = 24 meV. The analysis of the 1.45 eV luminescence band in compensated Cl-doped crystals shows the existence of donor-acceptor pair transitions. Three acceptor centers are identified: E_A = 89, 111 and 119 meV, and the contribution of a deep donor (E_D > 40 meV) is demonstrated. Besides intracentre type excitation transitions of the 1.45 eV band have been observed in non-compensated chlorine-doped crystals. Thus several recombination channels and distinct acceptor states contribute to the composite 1.45 eV luminescence band.     

Time resolved luminescence of CdS at high excitation

We report a study at low temperature of the time resolved luminescence of CdS, excited by two photon absorption. Concerning the so called EHP-LO and P bands, we confirm our results previously obtained on CdSe[1, 2]. (a) At high excitation a broad band (peak position at λ > 495 nm) occurs due to radiative recombination in an electron hole plasma, assisted by the emission of one LO phonon. (b) The simultaneous kinetics of the P line (λ ? 490.5 nm) and A-LO line (λ ? 492.5 nm) are conflicting with the interpretation of the P line as resulting of radiative exciton-exciton collisions. We interpret the P line as due to biexciton recombination.We have studied the luminescence in the (I₂-I₁) region (486nm < λ < 490nm) at low excitation. We observe clearly the following, (a) A broad band (488 nm < λ < 490.5 nm) which corresponds to the gain observed in previous experiments of pulse and probe spectroscopy and interpreted as direct recombination in an electron hole liquid (EHL). (b) After the disappearance of the EHL band, one single line (M_D), which shifts continuously towards the I₂ position during the time resolved kinetics. We suggest it to be connected with the high excitation effect on donor impurities (bound polyexcitons).     

MOVPE 生长GaN薄膜的 光致黄带发光与激发光源的相关性

用四种不同光源作为激发光源,研究了蓝宝石衬底金属有机物汽相外延方法生长的氮化镓薄膜的光致发光特性。结果发现用连续光作为激发光源时,光致发光谱中除出现365 nm的带边发射峰外,同时观察到中心波长位于约550 nm 的较宽黄带发光;而用脉冲光作为激发光源时其发光光谱主要是365 nm附近的带边发光峰,未观察到黄带发光。氮化镓薄膜的光致发光特性依赖于所用的激发光源性质。     

Study of polarized luminescence in erbium-doped laser glasses

A partial polarization of luminescence in laser phosphate and silicate erbium-doped glasses was found to take place for the fundamental laser transition ⁴ I _13/2→⁴ I _15/2 (λ=1.55 μm) under excitation by linearly polarized laser radiation (532 and 790–990 nm). The shape of the luminescence spectrum depends on the wavelength of the exciting light and on the composition of the glass matrix. The degree of polarization of the luminescence depends on the spectral range of both the excitation and the detection, attaining a maximum of ～1%. The concentration dependence of the degree of polarization is studied.     

An emission spectrum of InCl+

A spectrum, found in the 360- to 420-nm region following hollow-cathode excitation of InCl₃/In/He mixtures has been attributed to the InCl⁺ ion. Sixteen bands have been classified for the system which has been identified through rotational analysis to be B²Σ-X²Σ, consistent with photoelectron observations by Berkowitz and Dehmer. A second, very much weaker set of bands has been observed between 320 and 335 nm under the same experimental conditions. These latter bands constitute either a new system of InCl or a second InCl⁺ system.     

Photoinduced Degradation of the Optical Properties of Colloidal Ag<Subscript>2</Subscript>S and CdS Quantum Dots Passivated by Thioglycolic Acid

The results of studying degradation of the optical properties of colloidal Ag₂S and CdS quantum dots (QDs) 2.6–3.2 nm in size passivated by thioglycolic acid (TGA) are presented. The photoluminescence intensity of colloidal Ag₂S QDs has been found to decrease under laser irradiation at a wavelength of 445 nm, beginning with the effective power of 10 mW. The observed effect is interpreted as a photochemical reaction of formation of new nonradiative-recombination channels in Ag₂S QDs upon excitation. It is established for colloidal CdS QDs passivated by TGA that a decrease in the optical density in the entire absorption spectrum and the luminescence intensity is accompanied by precipitation of the colloidal particles in a cell and related to photodegradation of the passivating shell.     

Luminescence of neodymium-doped yttria

Pulsed cathodoluminescence of Nd³⁺: Y₂O₃ nanopowders of the cubic and monoclinic phases and the ceramics synthesized from these nanopowders has been investigated in the spectral range 350–850 nm. It is found that the IR emission band of neodymium ions in the Nd³⁺: Y₂O₃ cubic phase is located at λ₁ ≈ 825 nm. When there is a monoclinic phase admixture, two additional luminescence bands of Nd³⁺ arise in the spectrum at λ₂ ≈ 750 nm and λ₃ ≈ 720 nm. The emission spectrum of all Nd³⁺: Y₂O₃ materials also contains a wide intrinsic band of yttrium oxide at λ ≈ 485 nm; however, the presence of neodymium decreases the intensity of this band and increases the its structurization. It is suggested that the structure of this band in Nd³⁺: Y₂O₃ materials is mainly determined by local absorption (self-absorption) of neodymium ions.     

Upconverted 5d-4f luminescence from Er and Nd ions doped into fluoride hosts excited by ArF and KrF excimer lasers

We report on observation of upconverted VUV luminescence due to 5d-4f radiative transitions in Er³⁺ and Nd³⁺ ions doped into some fluoride crystals, under excitation by ArF and KrF excimer lasers, respectively. Only spin-forbidden 5d-4f luminescence of Er³⁺ (at 165 nm) was detected from the LiYF₄:Er³⁺ crystal whereas both spin-forbidden (at 169 nm) and spin-allowed (at 160.5 nm) components are observed from the BaY₂F₈:Er³⁺ crystal, the latter being much weaker than in the case of one-photon excitation. Nd³⁺ 5d-4f luminescence at 180 and 173 nm has been detected from the LiYF₄:Nd³⁺ and LaF₃:Nd³⁺ crystals, respectively. The shift of short-wavelength edge of 5d-4f emission spectra towards longer wavelengths is observed under temperature increase from 15 to 293 K. The observed effects in the spectra of Er³⁺ and Nd³⁺ doped crystals were interpreted as a result of reabsorption of 5d-4f luminescence escaping from the bulk of the crystals.     

Luminescence of Sc3+ and La3+ isoelectronic impurities in Lu3Al5O12 single-crystal films

The time-resolved luminescence and luminescence excitation spectra, and luminescence decay kinetics at 8 and 300 K of Lu₃A₁₅O₁₂ (LuAG) single-crystal films doped with Sc³⁺ and La³⁺ isoelectronic impurities and excited by synchrotron radiation are investigated. It is established that the La³⁺ isoelectronic impurity in the ?ub;c?ub; positions of the garnet lattice forms La _Lu ³⁺ luminescence centers emitting in the band with λ_max = 280 nm and the decay time of the main component τ = 300 ns at 300 K. The Sc³⁺ isoelectronic impurity located in the ?ub;c?ub; and (a) positions of the LuAG lattice forms two luminescence centers, Sc _Lu ³⁺ and Sc _Al ³⁺ , emitting in the bands with λ_max = 290 nm and τ = 240 ns and λ_max = 335 nm and τ = 375 ns, respectively, at 300 K. It is shown that the luminescence excitation of the La³⁺ and Sc³⁺ isoelectronic impurities in LuAG single-crystal films occurs through radiative decay of excitons localized near La _Lu ³⁺ , Sc _Lu ³⁺ , and Sc _Al ³⁺ centers. The energies of formation of these excitons are determined to be 6.8, 6.88, and 7.3 eV, respectively. It was found that the excited state of the excitons genetically related to the La _Lu ³⁺ , Sc _Lu ³⁺ , and Sc _Al ³⁺ enters has two radiative levels with different transition probabilities. This configuration leads to the presence of fast (2.3–8.4 ns) and slow (150–375 ns) main components in the luminescence of the centers formed by isoelectronic impurities in garnets.