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1.
Time-resolved emission and excitation spectra and luminescence decay kinetics were studied at 150-300 K for the green emission of PbWO4:Mo crystals. It was found that the slow (μs-ms) decay component observed under excitation in the defect-related absorption region (around 3.8-3.9 eV) arises from the G(II) emission which appears at the tunneling recombination of optically created electron and hole centers. The study of the emission decay kinetics at different temperatures and excitation intensities allowed concluding that both the monomolecular and the bimolecular tunneling recombination process can be stimulated in the mentioned energy range. The monomolecular process takes place in the isolated spatially correlated pairs of electron and hole centers produced without release of electrons into the conduction band. The bimolecular process takes place in the pairs of randomly distributed centers created at the trapping of free electrons from the conduction band. The formation of electron centers under irradiation in the defect-related absorption region was investigated by the electron spin resonance (ESR) and thermally stimulated luminescence (TSL) methods. The possibility of various photo-thermally stimulated defects creation processes, which take place with and without release of free electrons into the conduction band, was confirmed. 相似文献
2.
The photoluminescence, photoconductivity and absorption in GaSe0.9Te0.1 alloy crystals have been investigated as a function of temperature and external electric field. It has been observed that the exciton peaks shift to lower energy in GaSe0.9Te0.1 alloy crystals compared to GaSe crystals. The long wavelength tails of interband photoluminescence, photoconductivity and absorption spectra are determined by the free exciton states and show an Urbach-Martienssen-type dependence to the photon energy. The maxima of the extrinsic photoluminescence and photoconductivity spectra were found to be determined by the acceptor centers with an energy of EA=EV+0.19 eV formed by the polytypism and defects complexes that include Se and Te anions. 相似文献
3.
The nonmodulated and wavelength-modulated reflection spectra of CuGaS2 crystals for the polarization EIIc of 10 K are studied. The states n = 1, 2 and 3 of the excitons Γ4 (A-excitons) and n = 1, n = 2 of B- and C-excitons are found. The nonmodulated absorption spectra for the polarization E⊥c at 10 K have been studied. The states n = 1, 2 and 3 of Γ5 excitons are found. The main parameters of the A (Γ4, Γ5) and B, C exciton series at the energies of the longitudinal and transverse excitons Γ4 for the states n = 1 and n = 2, the effective masses of electrons and holes are determined. The photoluminescence peaks were observed at n = 3 and n = 4 of the excitons Γ5 in the luminescence spectra excited by the line 4880 Å of Ar+ laser. In the luminescence spectra the interference is found. 相似文献
4.
Emissions from the high-lying excited states, energy transfer and upconversion processes are investigated in Ho3+-activated KGd(WO4)2 crystal. The spectral assignment based on time-resolved emission spectra allowed to identify various near ultra-violet (UV), blue and green emissions starting from the excited 3H5, 5G4, 5G5, 5F3 and 5S2 levels. The temporal behavior of these transitions after pulsed excitation was analyzed as a function of temperature and holmium ions concentration. The shortening and nonexponentiality of the decays, observed with increasing activator concentrations, indicated cross-relaxation (CR) among the Ho3+ ions. Cross-relaxation rates were experimentally determined as a function of activator concentration and used to evaluate the values of the nearest-neighbor trapping rates X01 and to model the decays. It was observed that KGW, despite higher than in YAG maximum phonon energy of about 900 cm−1, is more efficient short-wavelength emitter than YAG. Examples of the excited-state absorption (ESA) and energy transfer (ET) mechanisms responsible for the upconverted, short-wavelength emissions were identified by analyzing fluorescence dynamics and possible energy resonances. 相似文献
5.
Tb3+:NaGd(WO4)2 (Tb:NGW) phosphors with different Tb3+ concentrations have been synthesized by a mild hydrothermal process directly without further sintering treatment. X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence excitation and emission spectra and decay curve were used to characterize the Tb:NGW phosphors. XRD analysis confirmed the formation of NGW with scheelite structure. SEM study showed that the obtained Tb:NGW phosphors appeared to be nearly spherical and their sizes ranged from 1 to 1.5 μm. The excitation spectra of these systems showed an intense broad band with maximum at 270 nm related to the O→W ligand-to-metal charge-transfer state. Photoluminescence spectra indicated the phosphors emitted strong green light centered at 545 nm under UV light excitation. Analysis of the photoluminescence spectra with different Tb3+ concentrations revealed that the optimum dopant concentration for Tb3+ is about 15 at% of Tb3+ ions in Tb:NGW phosphors. 相似文献
6.
The site-selective excitation and emission spectroscopy, and luminescence decay have been investigated under a pulsed, tunable, narrowband dye laser of the 5D0→7F0 region in the europium ions-doped lead tungstate PbWO4 (PWO) in single crystal. In as-grown sample, the experimental results show that there is only one 7F0→5D0 excitation transition indicating the only one Eu3+ site in PbWO4 lattices. The sequential annealing treatments were conducted to investigate the effects of oxygen components on the microstructure environments of Eu3+ in the lattices. The site distribution of Eu3+ was changed by the annealing in air atmosphere, which could create new sites in PWO lattices. Confirmation of interstitial oxygen and interpretations of charge compensation mechanism for the observed new sites were discussed in the context of site-selective excitation and emission spectra. The main Eu3+ site is related to the charge compensation by the [(EuPb3+)-V″Pb-(EuPb3+)] complex; the other minor new sites after annealing are originated from [(EuPb3+)-O″i-(EuPb3+)] defects. Emission spectra excited by 355-laser and RT-Raman spectra were also measured. 相似文献
7.
High-quality CsPbCl3 films composed of crystallites with narrow size distributions are achieved for various size levels, from microcrystalline to polycrystalline, by a novel heat-treatment method applied to the same amorphous films. Their photoluminescence is dominated by free-exciton emission at every size level without showing trapped-exciton emission in great contrast to the case for single crystals. The microcrystalline state shows more than an order of magnitude stronger free-exciton emission than the polycrystalline state, and exhibits intense stimulated emission under high-power excitation. 相似文献
8.
We have determined the photophysical properties of [Eu(C12H8N2)2](NO3)3, (EuPhen), a complex which is very promising for photonic and optoelectronic applications, because of its easy synthetic procedure and high thermal stability (up to 300 °C) combined with large sensitization efficiency and good emission quantum yield. Available experimental absorption and emission data have been analyzed by using Judd-Ofelt analysis. Moreover, semi-empirical calculations have been used to determine the structure of the complex and to interpret the convoluted shape of the absorption spectrum. 相似文献
9.
It is well known that Auger-free luminescence (AFL) is observable when the condition Eg>EVC is satisfied, where Eg is the band-gap energy between the lowest unoccupied band and the highest occupied band and EVC the energy difference between the top of the highest occupied band and the top of the next lower occupied band. From measurements of reflection and X-ray photoelectron spectra, CeF3 is demonstrated to really satisfy this condition. No evidence for AFL is found, nevertheless. The absence of AFL in CeF3 is related to a characteristic nature of its highest and next lower occupied bands, which are quite different from those of previously studied AFL-materials. 相似文献
10.
Feng Song Lin Han Hao Tan Jing Su Jia Yang Jian-guo Tian Guang-yin Zhang Zhen-xiang Cheng 《Optics Communications》2006,259(1):179-186
Using Czochralski (CZ) pulling method, an Er3+/Yb3+-codoped NaY(WO4)2 crystal was prepared. Absorption spectra, emission spectra and excitation spectra of this crystal were measured at room temperature. Some optical parameters, such as intensity parameters, spontaneous emission probabilities and lifetimes, were calculated from absorption spectra with Judd-Ofelt (J-O) theory. Upconversion luminescence excited by a 970 nm diode laser was studied. In this crystal, green upconversion luminescence is particularly intensive. Energy transfer mechanisms that play an important role in upconversion processes were analyzed. Two cross-relaxation processes: 4G11/2 + 4I9/2 → 2H11/2 (or 4S3/2) + 2H11/2 (or 4S3/2), and 4G11/2 + 4I15/2 → 2H11/2 (or 4S3/2) + 2I13/2, which contribute to the intensive green luminescence under 378 nm excitation, were put forward. Background energy transfer 4G11/2(Er3+) + 2F7/2(Yb3+) → 4F9/2(Er3+) + 2F5/2(Yb3+) was also demonstrated. 相似文献
11.
GaSe single crystals were N-implanted along c-axis with ion beams of 1014 and 1016 ions/cm2 doses having energy values of 60 and 100 keV. The photoluminescence (PL) spectra of undoped and N-implanted GaSe crystals were measured at different temperatures. The PL intensity was observed to decrease with increasing implantation dose while the FWHM of the exciton peaks increased. In heavily doped crystals, due to the interaction with the radiation induced disorders, the wave vector selection rules are satisfied and an indirect exciton PL band is observed 36 meV below the direct exciton states. 相似文献
12.
The photoluminescent (PL) emission and excitation behaviour of green-emitting CaAl2S4:Eu2+ powder phosphor is reported in detail. CaAl2S4:Eu2+ emission provides good CIE colour coordinates (x=0.141; y=0.721) for the green component in display applications. Powder with a dopant concentration of 8.5 mol% shows the highest luminescence efficiency. Temperature dependence of the radiative properties, such as luminescence intensity and decay time, was investigated. In particular, the Stokes shift, the mean phonon energy, the redshift, the energy of the f→d and d→f transition and the crystal field splitting of the CaAl2S4:Eu2+ emission were determined. The thermal quenching of the emission was examined. 相似文献
13.
Ba2+-doped Sr2SiO4:Eu2+ phosphors were synthesized with the high-temperature solid-state reaction technique. The experimental results, summarized in the successful production of a single-phase powder with fine microstructure of spherical particles with smooth surface, suggest that Ba2+-doping favors the stabilization of α′-Sr2SiO4. Rietveld refinement of X-ray diffractograms suggests that Ba2+ and Eu2+ ions occupy the sites of Sr2+ in the lattice of α′-Sr2SiO4. The produced phosphors show two intense emission bands at green and yellow regions of spectrum, originated from Eu2+ ions accommodated at two different sites in the host crystal, whose peaks depend on the concentrations of Ba2+ and Eu2+. Intense and broad excitation spectra extend from ultraviolet to the blue region. 相似文献
14.
When certain trivalent rare-earth ions (Ln's) are co-doped in CaGa2S4:Mn2+ as sensitizers, the Mn red emission of the compound is strikingly enhanced. In this work, efficiency of each lanthanide is studied. The best efficiencies are achieved with La3+-, Pr3+-, and Tb3+- co-doped compounds, for each of which the effects of concentrations of the co-doped ions on the Mn2+ emission are investigated. The energy-transfer mechanisms and the location of electronic energy levels of both the trivalent and the divalent lanthanides in the energy band gap of the host material are discussed. Depending on Ln's, charge transfer or cross-relaxation should be taken into account. 相似文献
15.
Characteristics of the defects created at 4.2 K by the UV-irradiation of CsI : Tl crystals in the Tl+-related absorption bands (by photons of 5.8-4.8 eV energy) have been studied. The dependences of the intensities of the thermally stimulated luminescence peaks appearing near 60, 90 and 125 K and of the recombination luminescence photostimulation bands peaking at 2.35, 1.92, 1.33 and 0.89 eV on the irradiation energy and duration, uniaxial stress and thallium concentration have been examined. The mechanisms of the processes, responsible for the appearance of the intense visible (2.55 and 2.25 eV) luminescence of excitons localized near Tl+ ions and creation of defects pairs of the type of Tl0-VK and Tl+-VK with various distances between the components, have been discussed. 相似文献
16.
The thermal annealing behavior of the Y3Al5O12, CaF2 and LiF single crystals bombarded at Algiers with reactor neutrons has been monitored by optical absorption spectroscopy. The irradiation was performed at about 315 K. On heating samples after irradiation, the optical absorption bands decrease and disappear completely at 873 and 523 K in the case of Y3Al5O12 and CaF2, respectively. Activation energies of 1.2±0.02 and 0.9±0.2 eV are estimated for Y3Al5O12 and CaF2, respectively. On the other hand, the LiF crystal shows a complex annealing behavior. Here, the optical absorption spectrum presents different shapes after each annealing temperature. Four steps are distinguished and discussed on heating samples from 300 to 673 K. Above 673 K, the absorption drops by about 50%; it completely disappears at 773 K. 相似文献
17.
We have synthesized blue-emitting CaMgSi2O6:Eu2+ (CMS) and evaluated its thermal stability after baking process. To evaluate its thermal stability, CMS was baked in air at 500 and 600 °C for 20 min, respectively, and compared with BaMgAl10O17:Eu2+ (BAM) treated in the same condition. After baking process, CMS showed somewhat increased photoluminescence (PL) intensity with baking temperature. To investigate the reasons behind the increase of PL intensity after baking process, vacuum ultraviolet (VUV)/PL, electron spin resonance (ESR), X-ray photoelectron spectroscopy (XPS) techniques were applied. From the ESR and the XPS analyses, it is noted that spectral intensity of Eu2+ ion somewhat increased. It was believed that due to charge balance Eu3+ ions reduced to Eu2+ ions during the baking process in air. It is clear that the concentration of Eu2+ increased after the baking process in air and it leads to slight increase of the VUV/PL intensity of CMS phosphor. 相似文献
18.
Marcin Sobczyk 《Journal of luminescence》2009,129(5):430-418
In an attempt to find a neodymium-vanadate system with long lifetime of 4F3/2 level and relatively strong 4F3/2→4I11/2 emission for laser applications, the optical properties of Nd3+ in a new KZnLa(VO4)2 host is reported. The crystalline samples were obtained at 900 °C in air. The samples were crystallized in monoclinic system and were isostructural with KZnLa(PO4)2. KZnLa0.99Nd0.01(VO4)2 strongly emits in the near infrared range with the maxima at 871.6 and 1057 nm upon excitation through the 4F5/2 level (808 nm) or by the charge transfer bands of VO43−. The lifetime of 4F3/2 level of Nd3+ ion is larger than that observed in other neodymium-vanadates systems. 相似文献
19.
The paper is devoted to investigation of the processes of excitation energy transfer between the host cations (Tb3+ ions) and the activators (Ce3+ and Eu3+ ions) in single-crystalline films of Tb3Al5O12:Ce,Eu (TbAG:Ce,Eu) garnet which is considered as a promising luminescent material for the conversion of LED's radiation. The cascade process of excitation energy transfer is shown to be realized in TbAG:Ce,Eu: (i) from Tb3+ ions to Ce3+ and Eu3+ ions; (ii) from Ce3+ ions to Eu3+ ions by means of dipole-dipole interaction and through Tb3+ ion sublattice. 相似文献
20.
Er3NbO7 phosphor was synthesized by sintering a mixture of Er2O3 and Nb2O5 powder in a molar ratio of 3:1 at 1600 °C over 55 h. Optical absorption and emission characteristics of Er3+ ions in the calcined Er3NbO7 powder were investigated and discussed compared with ErNbO4 phosphor and a Z-cut congruent Er (2 mol%):LiNbO3 single crystal. The absorption and emission studies show that, due to different crystal structures, the spectroscopic properties of these niobates have some differences in spectral shape, peak position, and relative intensity, especially at 1.5 μm. The most obvious spectral feature of the Er3NbO7 is that the spectral structure of band instead of peak is observed in its absorption or emission spectrum due to the existence of local structural disorder and multiple Er3+ sites. The Er3NbO4 shows stronger upconversion emission than the single crystal but weaker than the ErNbO4. Experimental results show that energy transfer upconversion and/or excited state absorption play a dominant role in the upconversion emissions, and, at higher pump level (>200 mW), the thermal effect becomes significant and results in drop of the upconversion intensity. The 1.5 μm lifetimes of Er3+ ion in the Er3NbO7, ErNbO4 phosphor, and in the Er:LiNbO3 crystal are measured to be ∼5.3, 2.0, and 2.4 ms, respectively. In combination with the measured Raman spectra, the quantum efficiency, multiphonon nonradiative decay rate, and theoretical radiative lifetime of the 1.5 μm emission of the two powder materials are expected. The differences in upconversion intensity and measured 1.5 μm lifetime between the three materials are explained qualitatively. 相似文献