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.     

Time-resolved spectroscopy of intrinsic luminescence of Y3Ga5O12 and (LaLu)3Lu2Ga3O12 single crystals

The nature of intrinsic luminescence of Y₃Ga₅O₁₂ (YGG) and (LaLu)₃Lu₂Ga₃O₁₂ (LLGG) single crystals grown from a melt was determined. In the case of a YGG single crystal containing Y_Ga antisite defects with a concentration of 0.25–0.275 at.% the intrinsic luminescence was considered as a superposition of luminescence of self-trapped excitons (STE), luminescence of excitons localized near antisite defects (LE(AD) centers) and luminescence caused by a recombination of an electron with a hole captured at Y_Ga antisite defects. Due to a large (2–3%) concentration of Lu_La antisite defects in LLGG single crystals the intrinsic luminescence was a superposition mainly of the LE(AD) center emission and the recombination luminescence of Lu_La antisite defects. The energy structure of the mentioned centers in YGG and LGGG hosts was determined from the excitation spectra of their luminescence under excitation by synchrotron radiation in the range of the fundamental absorption edge of these garnets.     

Visible luminescence in photo-electrochemically etched p-type porous silicon: Effect of illumination wavelength

The effect of low power density of ~ 5 μW/cm² monochromatic light of different wavelengths on the visible photoluminescence (PL) properties of photo-electrochemically formed p-type porous silicon (PS) has been investigated. Two-peak PL “red” and “green” is resolved in PS samples etched under blue-green wavelength illumination; 480, 533 and 580 nm. It is found that the weight of “green” PL has maxima for the sample illuminated with 533 nm wavelength. Whereas, PL spectra of PS prepared under the influence of red illumination or in dark does not exhibit “green” PL band, but shows considerable enhancement in the “red” PL peak intensity. Fourier transform infrared (FTIR) spectroscopic analysis reveals the relationship between the structures of chemical bonding in PS and the observed PL behavior. In particular, the PL efficiency is highly affected by the alteration of the relative content of hydride, oxide and hydroxyl species. Moreover, relative content of hydroxyl group with respect to oxide bonding is seen to have strong relationship to the blue PL. Although, the estimated energy gap value of PS samples shows a considerable enlargement with respect to that of bulk c-Si, the FTIR, low temperature PL and Raman measurements and analysis have inconsistency with quantum confinement of PS.     

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.     

Photoluminescence properties of MgAl2O4:Dy powder phosphor

Trivalent dysprosium (Dy³⁺) activated magnesium alluminate phosphors were synthesized by high temperature solid state reaction method. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FT-IR), photoluminescence (PL) spectra as well as lifetimes were used to characterize the resulting phosphors. The results show that the obtained MgAl₂O₄:Dy³⁺ phosphors have good crystallinity, spherical morphology with sizes ranged from 120 to 140 nm and strong blue emission under an excitation of 258 nm. The emission spectrum of this phosphor consists of two emission bands: blue band and yellow band, and the emission intensity of the former is stronger than that of the later. Luminescence quenching is explained and the corresponding luminescence mechanisms have been proposed.     

Luminescence and energy transfer of Mn co-doped SrSi2O2N2:Eu green-emitting phosphors

Eu²⁺ and Mn²⁺ co-doped SrSi₂O₂N₂ green-phosphors, with promising luminescent properties (examined by their powder diffuse reflection, photoluminescence excitation and emission spectra) suitable for UV converted white LEDs, were produced by high temperature solid-state reaction method. The produced materials exhibited intense broad absorption bands at 220–500 nm and a broad emission band centered at ca. 530 nm, attributed to 4f–5d transitions of Eu²⁺. The emission intensity of Eu²⁺ ions was greatly enhanced by introducing Mn²⁺ ions into SrSi₂O₂N₂:Eu²⁺ due to the energy transfer from Mn²⁺ to Eu²⁺. The energy transfer probability from Mn²⁺ to Eu²⁺ depends strongly on the Mn²⁺ concentration, which is maximized at a Mn²⁺ concentration of 3 mol%. It drastically decreases for higher concentrations. The results indicated that SrSi₂O₂N₂:Eu²⁺, Mn²⁺ is a promising green-emitting phosphor for white-light emitting diodes with near-UV LED chips.     

Optical properties of rare-earth doped epitaxial Sr0.5Ba0.5Nb2O6 thin films grown by pulsed laser deposition

Optical quality rare-earth (RE) (Nd³⁺, Eu³⁺, Gd³⁺ and Yb³⁺) doped Sr_0.5Ba_0.5Nb₂O₆ (SBN) epitaxial films of ~ 170 nm thick have been successfully grown on MgO (100) single crystal substrates using pulsed laser deposition technique. Strong residual stress in these films has been revealed by Raman spectroscopic studies. Two kinds of in-plane orientations with respect to the MgO substrate co-exist. All the film samples show high transparency in the visible wavelength. Their band-gap energies appear to be independent of the types of dopant. Photoluminescence (PL) spectra of RE-doped SBN ceramics show a strong and broad emission band at around 600 nm (2.07 eV). The peak position of this emission band changes slightly with different RE-dopants. Thin film samples, however, yield a broad PL band at around 385 nm (3.22 eV). This UV emission shows no observable shift in the peak position for different dopants. Apart from these broad emission bands, conspicuous emission lines from Eu³⁺ and Nd³⁺ ions are also noted. The origins of these PL spectra are discussed.     

Effect of Mg and Ti doping on the luminescence of Y2O2S:Eu

The Y₂O₂S:Eu³⁺,Mg²⁺,Ti^IV (x_Eu = 0.028, x_Mg = 0.086, x_Ti = 0.03) materials were prepared with the flux fusion method. According to X-ray powder diffraction, the materials had the hexagonal crystal structure. The emission of Y₂O₂S:Eu³⁺,Mg²⁺,Ti^IV was centered at 627 nm (λ_exc : 250 nm) due to the ⁵D₀ → ⁷F₂ transition of Eu³⁺. The excitation spectra (λ_em : 627 nm) showed broad bands at 240 and 320 nm due to the O²⁻ → Eu³⁺ and S²⁻ → Eu³⁺ charge transfer transitions, respectively. The latter band can also overlap with the Ti → Eu³⁺ energy transfer. In the excitation spectra with synchrotron radiation, in addition to the O²⁻ → Eu³⁺ and S²⁻ → Eu³⁺ charge transfer transitions, excitation over the band gap was observed at 4.8 eV (258 nm). The red persistent luminescence due to the ⁵D₀ → ⁷F₂ emission from Eu³⁺ residing in the regular Y³⁺ site of the host was ca. 10 min with 1 min fluorescent lamp irradiation. In addition, a very broad band was observed at 600 nm probably due to the Ti³⁺ emission.     

Spectroscopic characteristics of Olgite (Ba,Sr)(Na,Sr)2Na[PO4]2 crystals doped with Sm ions

Sm³⁺ ions doped Olgite crystals (Ba,Sr)(Na,Sr)₂Na[PO₄]₂ were prepared by high temperature solid-state reaction. Sm²⁺ ions were obtained by X-ray irradiation reduction. The samples were investigated by X-ray diffraction, SEM, photoluminescence and decay curves measurements. In (Ba,Sr)(Na,Sr)₂Na[PO₄]₂, the influence of different mole ratios of Sr to Ba atoms on the crystal structure, reducing efficiencies of Sm³⁺ to Sm²⁺ and luminescence properties of Sm²⁺ ions were discussed. It is found that the conversion of Sm³⁺ → Sm²⁺ after X-ray irradiation is efficient in this phosphate. The emission of Sm²⁺ in this host after excitation into the 4f⁵ 5d¹ levels shows ⁵D₀ → ⁷F_J (J = 0, 1, 2) emission together with a broad emission band. The characteristic of Sm²⁺ ions luminescence was discussed.     

Synthesis and luminescent properties of ZnNb2O6 nanocrystals for solar cell

The phase formation, morphology and luminescent properties of ZnNb₂O₆ nanocrystals by the sol-gel method were investigated at a lower temperature than that of the traditional solid-state reaction method. The products were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), photoluminescence spectroscopy (PL) and absorption spectra. The activation energy of ZnNb₂O₆ grain growth is obtained about 18.4 kJ/mol. The diameters of the nanocrystals are in the range of 20-40 nm. The PL spectra excited at 276 nm have a broad and strong blue emission band maximum at 450 nm, corresponding to the self-activated luminescence of the niobate octahedra group [NbO₆]⁷⁻. The optical absorption spectrum of the sample at a calcination temperature of 800 °C has a band gap energy of 3.68 eV.     

Preparation and luminescence properties of Mn-doped ZnGa2O4 nanofibers via electrospinning process

One-dimensional Mn²⁺-doped ZnGa₂O₄ nanofibers were prepared by a simple and cost-effective electrospinning process. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric and differential thermal analysis (TG-DTA), scanning electron microscopy (SEM), energy-dispersive X-ray spectrum (EDS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL) and cathodoluminescence (CL) spectra as well as kinetic decays were used to characterize the samples. SEM results indicated that the as-formed precursor fibers and those annealed at 700 °C are uniform with length of several tens to hundred micrometers, and the diameters of the fibers decrease greatly after being heated at 700 °C. Under ultraviolet excitation (246 nm) and low-voltage electron beams (1–3 kV) excitation, the ZnGa₂O₄:Mn²⁺ nanofibers presents the blue emission band of the ZnGa₂O₄ host lattice and the strong green emission with a peak at 505 nm corresponding to the ⁴T₁–⁶A₁ transition of Mn²⁺ ion.     

Luminescence characteristics of LuAG:Pr and YAG:Pr single crystalline films

Emission and excitation spectra and luminescence decay kinetics were studied for Pr³⁺-doped Lu₃Al₅O₁₂ and Y₃Al₅O₁₂ single crystalline films (SCF) grown by the liquid phase epitaxy method from a PbO-based flux. The influence of lead-induced centers on their scintillation characteristics was clarified. It was found that the influence of single Pb²⁺-based centers on the characteristics of Pr³⁺ centers due to the Pb²⁺ → Pr³⁺ energy transfer was weak. However, an overlap of the emission spectra of single and dimer lead-induced centers with the emission spectrum of Pr³⁺ ions, and especially a strong overlap of the 4f–5d₁ absorption band of Pr³⁺ ions with the slow emission of localized excitons in the 290 nm band had a considerable influence on the scintillation characteristics of the Pr³⁺-doped SCF.     

Novel blue-emitting Sr3Ga2O5Cl2:Eu phosphor for UV-pumped white LEDs

A novel blue-emitting Sr₃Ga₂O₅Cl₂:Eu²⁺ phosphor has been synthesized by a two-step solid-state reaction. The luminescence properties have been investigated by photoluminescence (PL) spectra, and temperature-dependent PL spectra. It shows an efficient broad absorption band around 400 nm, which matches well with the commercial near-ultraviolet light-emitting chips, and an efficient blue emission. It shows a higher thermal quenching temperature than that of Sr₃Al₂O₅Cl₂:Eu²⁺ phosphor. Sr₃Ga₂O₅Cl₂:Eu²⁺ phosphor is a promising blue-emitting component for UV chip excited white light-emitting-diodes.     

Synthesis of CaAl2O4:Eu,Nd long persistent phosphor by combustion processes and its optical properties

Eu²⁺,Nd³⁺ co-doped calcium aluminate with high brightness and long persistent luminescence was prepared by the combustion method. The luminescent properties of CaAl₂O₄-based luminescent materials have been studied systematically. The phosphor powders were further investigated by X-ray diffractometer (XRD), photoluminescence excitation and emission spectra (PL) and brightness meter. The analytical results indicated that the phase of CaAl₂O₄ was formed when the initiating combustion temperature was 400 °C. The broad band UV excited luminescence of the CaAl₂O₄:Eu²⁺,Nd³⁺ was observed at the blue region (λ_max = 440 nm) due to transitions from the 4f⁶5d¹ to the 4f⁷ configuration of the Eu²⁺ ion. The decay time of the persistence indicated that the persistent luminescence phosphor has bright phosphorescence and maintains a long duration.     

Scintillation and optical properties of Pb-doped YCa4O(BO3)3 crystals

This communication reports optical properties and radiation responses of Pb²⁺ 0.5 and 1.0 mol%-doped YCa₄O(BO₃)₃ (YCOB) single crystals grown by the micro-pulling-down (μ-PD) method for neutron scintillator applications. The crystals had no impurity phases according to the results of X-ray powder diffraction. These Pb²⁺-doped crystals demonstrated blue-light luminescence at 330 nm because of Pb²⁺¹S₀-³P_0,1 transition in the photoluminescence spectra. The main emission decay component was determined to be about 250-260 ns under 260 nm excitation wavelength. When irradiated by a ²⁵²Cf source, the relative light yield of 0.5% Pb²⁺-doped crystal was about 300 ph/n that was determined using the light yield of a reference Li-glass scintillator.     

Investigations on luminescence characteristics of Eu and Cr codoped BaAl2O4

Green phosphor BaAl₂O₄:Eu²⁺, Cr³⁺ with varying concentrations of Cr codoping were prepared by solid-state synthesis method. The synthesized compositions were characterized for their phase, morphology and crystallinity by powder X-ray diffraction, SEM and TEM techniques. Photoluminescence properties were investigated measuring PL and decay time. Broad band UV excited luminescence was observed for BaAl₂O₄:Eu²⁺, Cr³⁺ in the green region (λ_max = 500 nm) due to transitions from 4f⁶ 5d¹ to the 4f⁷ configuration of the Eu²⁺ ion. The effect of Cr doping on crystalline quality, morphology and photoluminescence characteristics of prepared compositions was investigated.     

Dependence of photoluminescence (PL) emission intensity on Eu and ZnO concentrations in Y2O3:Eu and ZnO·Y2O3:Eu nanophosphors

Y₂O₃:Eu³⁺ and ZnO·Y₂O₃:Eu³⁺ nanophosphor powders with different concentrations of Eu³⁺ ions were synthesized by a sol-gel method and their luminescence properties were investigated. The red photoluminescence (PL) from Eu³⁺ ions with the main emission peak at 612 nm was observed to increase with Eu³⁺ concentration from 0.25 to 0.75 mol% and decreased notably when the concentration was increased to 1 mol%. The decrease in the PL intensity at higher Eu³⁺ concentrations can be associated with concentration quenching effects. The red emission at 612 nm was shown to increase considerable when ZnO nanoparticles were incorporated in Y₂O₃:Eu³⁺ while green emission from ZnO was suppressed. The increase is attributed to energy transfer from ZnO to Eu³⁺.     

Effects of gamma-irradiation on Al2O3 crystals grown under reducing atmosphere

Gamma-ray irradiation-induced color centers in Al₂O₃ crystals grown by temperature gradient techniques (TGT) under a strongly reducing atmosphere were studied. The transition F⁺ → F takes place during the irradiation process. Glow discharge mass spectroscopy (GDMS) and annealing treatments show that Fe³⁺ impurity ions are present in the crystals. A composite (F⁺-Fe³⁺) defect was presented to explain the origin of the 255 nm band absorption in the TGT-Al₂O₃ crystals.     

Spectroscopy of Yb3+ in Cubic ZrO2 Crystals

The absorption and luminescence spectra of Yb³⁺ ions in yttria-stabilized zirconia crystals are studied. The site-selective luminescence spectra of a ZrO₂–12 mol % Y₂O₃–0.3 mol % Yb₂O₃ crystal are obtained under excitation at different wavelengths within the absorption band. For this crystal, Yb³⁺ luminescence spectra are also measured with 3-ms and 5-s delays relative to the excitation pulse. The results are interpreted under the assumption that the crystals contain three distinct types of optical centers.     

Catalytic-free growth of ZnGa2O4 nanowires on amorphous carbon layers

Zinc gallate (ZnGa₂O₄) nanowires were directly grown on the amorphous carbon-coated silicon substrates using a facile chemical vapor deposition method without any metal catalysts. The growth mechanism can be attributed to a self-organization vapor-liquid-solid (VLS) process. The amorphous carbon layer plays an important role in the nucleation and growth process of the ZnGa₂O₄ nanowires. The photoluminescence (PL) of the nanowires shows a broad, strong green emission band centered at 532 nm and a weak UV emission band at 381 nm, which can be attributed to a large amount of ionized oxygen vacancies and the combination of Ga³⁺ ions with free electrons in coordinated oxygen vacancies, respectively.