Comparative study of the luminescence of Al2O3:Ti and Al2O3 crystals under VUV synchrotron radiation excitation

The comparative study of the luminescent properties of Al₂O₃:Ti crystal in comparison with those for undoped Al₂O₃ crystal counterpart is performed under synchrotron radiation excitation with an energy of 3.7–25 eV. Apart from the main emission band peaked at 725 nm related to the ²E → ²T₂ radiative transitions of Ti³⁺ ions, the luminescence of excitons localized around Ti ions in the band peaked at 290 nm and the luminescence of F⁺–Ti and F–Ti centers in the bands peaked at 325 and 434 nm are also found in the emission spectra of Al₂O₃:Ti crystal. We show also that the luminescence of Ti³⁺ ions in Al₂O₃:Ti crystal can be effectively excited by the luminescence of excitons localized around Ti dopant as well as by the luminescence of F–Ti centers.     

Effect of annealing on the spectroscopy performance of YVO4:Ce3+ single crystals

In this study, 1.0 at.% YVO₄:Ce³⁺ single crystals were grown in the protective atmosphere by using the Czochralski method. The crystals were annealed in Ar and H₂ atmospheres at different temperatures. The absorption and fluorescence spectra of the samples before and after annealing were measured. Results showed that the luminescent efficiency of the crystals was significantly enhanced after annealing in H₂ than after annealing in Ar. This phenomenon can be attributed to the existence of some Ce⁴⁺ ions in the crystal lattice. These Ce⁴⁺ ions can be effectively reduced to Ce³⁺ via annealing in H₂. With a fixed annealing time in H₂, the luminescent intensity significantly increased with increasing annealing temperature. The possibility of the crystal as white light material was also discussed according to the luminescence properties.     

Synthesis and efficient blue-emitting of Eu2+-activated borate fluoride BaAlBO3F2

Eu²⁺-doped borate fluoride BaAlBO₃F₂ was synthesized by the conventional high temperature solid state reaction. The crystal phase formations were confirmed by X-ray powder diffraction (XRD) measurements and the structure refinement. The photoluminescence (PL) excitation and emission spectra, and the decay curves were investigated. Eu²⁺-doped BaAlBO₃F₂ phosphor can be efficiently excited by near-UV light and presents narrow blue luminescence band centered at 450 nm. The maximum absolute quantum efficiency (QE) of BaAlBO₃F₂:0.05Eu²⁺ phosphor was measured to be 76% excited at 398 nm light at 300 K. The thermal stability of the blue luminescence was evaluated by the luminescence decays as a function of temperature. The phosphor shows an excellent thermal stability with high thermal activation-energy on temperature quenching effects because of the rigid crystal lattices.     

Structure and optical properties of Li2Ga2GeS6 nonlinear crystal

Structure and optical properties of new nonlinear crystal – Li₂Ga₂GeS₆ single crystal of optical quality, grown by the Bridgman technique were studied. The data on transmission, Raman scattering, luminescence emission, excitation and thermal quenching as well as thermostimulated luminescence are presented. Fundamental absorption edge is determined by the direct allowed electronic transitions: The values of optical band gap are estimated. Absorption band at 8.0 μm is due to S–S vibrations. Features in photoluminescence spectra are associated with excitons: both free (narrow line at 371 nm) and self-trapped ones (broad bands at 596, 730 and 906 nm). Spontaneous emission in the 80–170 K range, both at crystal heating and cooling, is typical of pyroelectrics: This confirms the absence of symmetry center in Li₂Ga₂GeS₆ and an opportunity of laser frequency nonlinear conversion.     

Synthesis,defect characterization and photocatalytic degradation efficiency of Tb doped CuO nanoparticles

Monoclinic undoped and Tb doped CuO are prepared by solution combustion method and annealed at different temperatures. The effect of annealing and doping on their structural and optical properties of CuO are examined using XRD, FTIR and DRS. The surface and lattice defects in CuO and Tb doped CuO is analyzed qualitatively and quantitatively using positron lifetime and Doppler broadening spectroscopy. The average positron lifetime and electron momentum (energy) S parameter increases owing to the number of vacancies in the CuO lattice upon doping and decreases with increasing temperature. The migration of vacancies from grain to grain boundary region is observed at 600 °C annealed samples. At 800 °C, the overall behavior of lifetime value denotes that the vacancy type defect is recovered, cluster vacancy and microvoids exists with reducing size. The photocatalytic performance of undoped and Tb doped CuO on degradation of methylene blue (MB) and methyl orange (MO) is investigated under visible light for two different lamp power and dye concentration. The influence of annealing temperature and dopant ion on the efficiency is also elaborated. Enhanced photocatalytic efficiency in Tb doped CuO is observed upon annealing. X-ray photoelectron spectroscopy (XPS) result indicates that the valence states of Cu, O and Tb ions exist at the surface of the particles. Brunauer–Emmett–Teller N₂ adsorption–desorption analyses were employed to characterize specific surface area and porosity of Tb doped CuO. The doped CuO with pore size of about ～34 nm have a surface area of 16–28 m²/g. The surface area effect plays an important role in the enhanced catalytic performance on Tb doped catalysts.     

Nature of intrinsic and impure luminescence of MAlP2O7 crystals

The results of the photoluminescence (PL) investigation of pure and chromium-doped MAlP₂O₇ (M = Na, K, Cs) compounds are presented. The spectra of the intrinsic luminescence of MAlP₂O₇ crystals consist of a separated UV band at a peak position near 330 nm and a complex wide band which covers the region of visible light up to 750 nm at excitation by VUV synchrotron radiation. The “red” band in 600–1000 nm diapason appears in the PL spectra of crystals doped with chromium ions. The effect of the temperature on the structure, the peak positions and intensities of the luminescence bands was studied. An assumption about the nature of the intrinsic PL was made. The “red” luminescence was considered as a result of the ⁴Т₂ → ⁴А₂ radiation transitions in the impurity Cr³⁺ ions located in the intermediate crystal field.     

Photoluminescence and thermoluminescence studies of Tb3+ doped ZnO nanorods

Here in, the synthesis of the terbium doped zinc oxide (ZnO:Tb³⁺) nanorods via room temperature chemical co-precipitation was explored and their structural, photoluminescence (PL) and thermoluminescence (TL) studies were investigated in detail. The present samples were found to have pure hexagonal wurtzite crystal structure. The as obtained samples were broadly composed of nanoflakes while the highly crystalline nanorods have been formed due to low temperature annealing of the as synthesized samples. The diameters of the nanoflakes are found to be in the range 50–60 nm whereas the nanorods have diameter 60–90 nm and length 700–900 nm. FTIR study shows ZnO stretching band at 475 cm^?1 showing improved crystal quality with annealing. The bands at 1545 and 1431 cm^?1 are attributed to asymmetric and symmetric CO stretching vibration modes. The diffuse reflectance spectra show band edge emission near 390 nm and a blue shift of the absorption edge with higher concentration of Tb doping. The PL spectra of the Tb³⁺-doped sample exhibited bright bluish green and green emissions at 490 nm (⁵D₄ → ⁷F₆) and 544 nm (⁵D₄ → ⁷F₅) respectively which is much more intense then the blue (450 nm), bluish green (472 nm) and broad green emission (532 nm) for the undoped sample. An efficient energy transfer process from ZnO host to Tb³⁺ is observed in PL emission and excitation spectra of Tb³⁺-doped ZnO ions. The doped sample exhibits a strong TL glow peak at 255 °C compared to the prominent glow peak at 190 °C for the undoped sample. The higher temperature peaks are found to obey first order kinetics whereas the lower temperature peaks obey 2nd order kinetics. The glow peak at 255 °C for the Tb³⁺ doped sample has an activation energy 0.98 eV and frequency factor 2.77 × 10⁸ s^?1.     

Vacuum-UV excitation and visible luminescence of nano-scale and micro-scale NaLnF4:Pr3+ (Ln = Y,Lu)

The UV–Vis luminescence of NaLnF₄:Pr³⁺ (Ln = Y, Lu) materials can be efficiently excited by vacuum UV radiation (VUV) such as the 172 nm emission of mercury-free Xe-discharge lamps. In this work, the optical properties of the cubic α-phase and the hexagonal β-phase of NaLnF₄:Pr³⁺ (Ln = Y, Lu) powders are compared regarding particle sizes in the nano- and micrometer regime. Upon VUV excitation, the emission spectra of both crystal phases are found to be dominated by intraconfigurational [Xe]4f²–[Xe]4f² transitions, which is explained by the chemical properties of the ternary fluorides. Furthermore it is observed that the emission and excitation spectra of nano- and micro-scale powders are very similar, but that the luminescence intensity is affected by the average particle size.     

The effect of Ce4 + incorporation on structural,morphological and photocatalytic characters of ZnO nanoparticles

We report a simple chemical precipitation method for the preparation of undoped and cerium doped ZnO nanocrystals. The concentration of cerium in the products can be controlled in the range of 0.025–0.125 mol. The structure and chemical compositions of the products were characterized by X-ray diffraction, X-ray photoelectron spectroscopy; energy dispersive spectrum and Fourier transform infrared spectroscopy. The results demonstrate that Ce^4 + ions were successfully incorporated into the lattice position of Zn^2 + ions in ZnO. The morphology of the products was analyzed by field emission scanning electron microscopy and confirmed by high resolution transmission electron microscope analysis. The optical properties of the products were studied by ultraviolet–visible and room temperature photoluminescence measurements. The photoluminescence emission spectra of Ce-doped ZnO showed enhanced visible emissions as a result of 5d → 4f transition of cerium. In particular, a novel photocatalytic activity of the products was assessed using methylene blue. The obtained result reveals that Ce-doped products show higher reduction efficiency for methylene blue than the undoped ZnO.     

Radiation resistance diagnostics of wide-gap optical materials

Novel approach in the detection of radiation damage created by ion beams in optical materials was demonstrated. Protons of the energy of 100 keV and fluence of 10¹⁷ cm⁻² create sufficient amount of crystal lattice defects in the thin surface layer for testing of optical materials needed for future fusion reactors. These structural defects can be detected and analysed using the spectra of cathodoluminescence excited in the irradiated layer by an electron beam with adjustable energy. The method was verified by the enhanced intensity of F-type luminescence that reflects the creation of radiation-induced oxygen vacancies in MgO and Al₂O₃ crystals. Low radiation resistance of nominally pure (Lu_1-xGd_x)₂SiO₅ crystals was demonstrated by almost total suppression of intrinsic luminescence after the same irradiation.     

Effect of oxygen vacancies on ferroelectric behavior of Na1/2Bi1/2TiO3–BaTiO3 single crystals

One of the important lead-free ferroelectric solid solutions, sodium bismuth titanate–barium titanate Na_1/2Bi_1/2TiO₃–BaTiO₃ (NBT–BT) was grown by the conventional flux technique. In order to study the role of oxygen vacancies on the dielectric/ferroelectric properties, some of the crystal samples oriented along (0 0 1) and (1 0 0) planes were subjected to oxygen and nitrogen annealing processes to create different concentrations of oxygen vacancies in the samples. Dielectric and its loss measurements were carried out to analyze the role of oxygen vacancies and their corresponding dielectric behavior on NBT–BT crystals. Electron energy loss spectrum (EELS) has revealed that increasing oxygen vacancies has reduced oxidation states of Ti. X-ray rocking curve analysis has confirmed the degradation in the structural quality also on increasing the oxygen vacancies.     

Morphology and crystal quality investigation of hydrothermally synthesized ZnO micro-rods

Micro-structural and room and low temperature photoluminescence response of undoped one-dimensional ZnO were investigated. ZnO rods of different morphology and size were synthesized by controlling growth parameters through hydrothermal technique. The phase and microstructure analysis were carried out by X-ray diffraction and scanning electron microscopy. The room and low temperature photoluminescence spectra of the samples were studied. Near band edge sharp exciton emission peaks and broad defect-related peaks were observed. The ratio of band edge emission to deep level emission was controlled by tuning the initial concentration, pH and reaction time period. Optimal growth condition for growth of micro-rods with improved crystal quality was obtained with initial Zn²⁺ concentration of 0.5 M, at reaction temperature of 120 °C, pH of 9.9 and in a reaction time period of 6 h.     

Annealing effect on the photoluminescence properties of ZnO nanorod array prepared by a PLD-assistant wet chemical method

Well-aligned ZnO nanorod arrays were synthesized by a wet chemical method on the glass substrate with ZnO thin film as seed layer prepared by pulsed laser deposition. The effect of annealing temperature on the luminescence characteristics was investigated. As the annealing temperature increased, the photoluminescence properties show a general enhancing tendency. The nanorod array with high ultraviolet emission and negligible visible light emission (designated by the photoluminescence intensity ratio of ultraviolet to visible emission of 66.4) is obtained by annealing the sample at 700 °C for 1 h. Based on the results of X-ray photoelectron spectroscopy and photoluminescence spectra, the mechanisms of visible emission were discussed.     

Photoluminescent properties of LiInO2 nanocrystals

Synthesis and luminescence properties of LiInO₂ nanocrystals by the sol–gel process were investigated. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence spectroscopy and absorption spectra. The well-crystallized tetragonal LiInO₂ can be obtained by heat treatment above 600 °C from XRD. The excitation wavelengths at about 246 nm were associated with charge transfer between In and O with In³⁺ ions in octahedral coordination. The PL spectra excited at 246 nm have a broad and strong emission band maximum at 391 nm, corresponding to the self-activated luminescence. The optical absorption spectra of the 600 °C sample exhibited the band gap energies of 3.7 eV.     

Effect of thermal annealing on the spectroscopic parameters of Er3+-doped sodium silicate glass

This paper presents the optical characteristics of Er³⁺ ions in sodium silicate glass (SiO₂–Na₂O), synthesized by the fusion method and later annealed for 0, 30, 60 and 90 min. Thermograms, X-ray diffraction, optical absorption, luminescence spectroscopy measurements were performed in order to determine the thermal and structural of the samples and the radiative characteristics of Er³⁺ ions under influence of thermal annealing of the samples. Differential thermal analysis provided evidence of a phase change in the system. This phase change was confirmed by X-ray diffraction, which showed the formation and growth of Na₂SiO₃ crystals for the annealed samples. These crystals affect the neighborhood (from second vicinity) of Er³⁺ ions. These effects were noted by the J–O parameters (Ω₂ and Ω₄), which were calculated from the optical absorption spectra. Judd–Ofelt calculations also confirmed that heat treatment induced structural rearrangement of the samples that was dependent on Er₂O₃ concentration. This resulted in changes in the optical and physical properties of the samples, including stimulated emission cross section and rigidity. Analysis of the spectroscopy parameters after of thermal annealing indicate samples are potential materials for in optical device applications.     

Structural and photophysical properties of lanthanide complexes with N-(diphenylphosphoryl)-4-methylbenzenesulfonamide

Lanthanide complexes with N-(diphenylphosphoryl)-4-methylbenzenesulfonamide (HPMSP) as new sensitizers of visible luminescence were obtained. The series of stable lanthanide complexes Na[Ln(PMSP)₄], where Ln = Eu³⁺, Gd³⁺, Tb³⁺ were characterized by X-ray diffraction, IR, absorption, emission, and excitation spectra at 295 and 77 K as well as luminescence decay times and intrinsic emission quantum yields. The Tb complex, exhibiting relatively efficient ligand-to-metal energy transfer and strong metal-centred emission, is a promising candidate for effective UV-to-visible energy converters. Temperature dependent quenching of sensitized ⁵D₀ europium emission and presence of ⁵D₁ emission are discussed.     

Ag and Dy doped zeolite as a broadband phosphor

Photoluminescence (PL) properties of silver (Ag) and dysprosium (Dy) codoped zeolites were investigated. It was found that PL from the ⁴F_9/2–⁶H_13/2 transition of Dy³⁺ ions at 575 nm is more than 50 times enhanced by the presence of Ag⁺ ions under ultraviolet excitation. The excitation wavelength dependence of the PL intensity coincided well with the absorption spectra of Ag⁺ ions, indicating that Dy³⁺ ions are excited by the energy transfer from Ag⁺ ions. In addition, by carefully optimizing annealing condition and Dy and Ag concentration, white light was realized due to the combination of blue emission of Ag⁺ ions, yellow emission of Dy³⁺ ions and red emission of Ag clusters.     

Thermal stability and annealing behavior of photoluminescence from Eu doped YAG

Eu₂O₃ doped Y₃Al₅O₁₂ (YAG) crystals have been grown using a floating zone technique and evaluated thermal stability and annealing behavior of PL for a fluorescence thermo-sensor application. Color of the crystals grown varies from deep purple to colorless with O₂ concentration of the growth atmosphere and annealing in air. Photoluminescence (PL) peaking at λ = 590, 607, 624, 647 and 709 nm due to Eu³⁺ ions are observed from the crystals under UV excitation. Anomalous temperature dependence of PL intensity, which is observed in as-grown crystals, is improved greatly by annealing through the heat cycle. From annealing behavior of optical absorption spectra, residual Eu²⁺ ions are suggested to be responsible for the de-coloration and the improvement of anomalous temperature dependence of Eu doped YAG crystals.     

Li doping effects on the upconversion luminescence of Yb3+/Er3+-doped ABO4 (A = Ca,Sr; B = W,Mo) phosphors

ABO₄ (A = Ca, Sr; B = W, Mo):Er³⁺/Yb³⁺/Li⁺ phosphors tri-doped with different concentrations of Li⁺ ion ranging from 0 to 22.5 mol% were prepared by using a solid-state reaction method. And their upconversion (UC) luminescence properties were in estimated under a 975 nm laser-diode excitation. The four kinds of phosphors (CaWO₄, CaMoO₄, SrWO₄, and SrMoO₄) tri-doped with Er³⁺, Yb³⁺ and Li⁺ ions showed strong green UC emission peaks at 530 nm and 550 nm and weak red UC emission. The intensity of green UC emission of Li⁺ doped samples was several higher than that of Li⁺ un-doped samples due to the reduction of lattice constant and the local crystal field distortion around rare-earth ions. The optimum doping concentration of Li⁺ ions was investigated and the effects of Li⁺ concentration for UC emission intensity were studied in detail.     

Synthesis and characterization of Yb3+,Tm3+:Ba2YF7 nanocrystalline with efficient upconversion fluorescence

A novel upconversion luminescence nanocrystals Yb³⁺,Tm³⁺:Ba₂YF₇ were synthesized via the hydrothermal method. They have uniform morphology with a mean size of 30 nm even if annealed at 600 °C. Pumped by 980 nm laser diode the as-synthesized powers emit ultraviolet/blue light, which is in the range of the specific upconversion luminescent spectra of Tm³⁺ ions. After post-annealing at 600 °C in an argon atmosphere for 2 h, their upconversion luminescence intensity is 5 multiple improved and the ultraviolet/blue light can even be seen by the naked eyes under a low excitation power of 20 mW. This indicates that Ba₂YF₇ is a very effective luminescent host material. Excitation power dependences of individual upconversion emission intensity are plotted, which partly uncover the upconversion luminescence mechanism of Tm³⁺ ions.