Thermostimulated Luminescence Research Articles

Luminescent properties of Pb2MoO5 single crystals

Lanarkite type Pb2MoO5 single crystals were grown using low temperature gradient Czochralski technique and their optical and luminescent properties were studied. The luminescence band at 590–600nm was observed under UV and X-ray excitations and ascribed to emission of self-trapped excitons. The band is thermally quenched at T>15K with the activation energy of process, Eact=11meV. The measurements of thermostimulated luminescence demonstrated the presence of shallow traps, which adversely influence the energy transfer to the emission centers.

Thermoexoelectronic and thermoluminescent properties of transparent YAG:Nd and YAG:Yb nanoceramics

Preliminary investigations of thermoluminescent and thermoexoelectronic properties of transparent microstructured YAG:Nd and YAG:Yb ceramics are performed. Five peaks of thermostimulated luminescence are found for YAG:Nd in the VUV and UV ranges. Four and six peaks of thermostimulated exoemission are found for YAG:Yb and YAG:Nd, respectively. The parameters of carrier traps are calculated.

Some interesting phenomena in the study of rare earth long lasting phosphors

Long lasting phosphors (LLPs) are a kind of very important and usable luminescent materials. Many new LLPs were found and some interesting phenomena were observed. Some distinctive questions on LLPs have been found and reviewed in this paper. For example: why most of LLPs of rare earth cannot be excited by sun light even they have nearly the same excitation spectra as SrAl2O4:Eu2+, Dy3+, Sr2MgSi2O7:Eu2+, Dy3+ which can be excited by sun light ? why there is only the emission of the activator Eu2+ in SrAl2O4:Eu2+, Dy3+, but no emission of the codopant (Dy3+) was observed under usual excitation condition? why different co-dopants have different effect on the LLP? various mechanisms about LLP of rare earth are still discussing widely and unsolved, which is the suitable one? there is similarity between the mechanisms of thermo-stimulated luminescence (TSL), photo-stimulated luminescence (PSL) and force-stimulated luminescence (FSL or mechanoluminescence) with the mechanisms for long lasting luminescence. For synthesis of LLP, which is a kind of electron trapping materials, the aliovalence (unequal valence) substitution method plays the key role. By using of this method, defects, vacancies and traps are produced. The properties of LLP strongly depend on the effects of doping and co-doping ions and some applications of LLP are presented in this paper.

Positronium formation and thermostimulated luminescence: A common nature and combined application to studies of organic systems

Polymeric membrane materials with highly developed intrinsic microporosity for gas separation are studied. The porosity can be essentially improved by filling the polymer with nonorganic components (zeolites). This porosity, herein referred to as artificially induced, turned out, in some cases, to be higher than that of the constituent components. The reasons for this effect are not always clear. To gain additional insights into the nature of this effect, an experimental study is suggested with the use of a combination of two methods related by being based on track processes: positron annihilation lifetime spectroscopy PALS and thermostimulated luminescence (TSL). The present paper summarizes and discusses both recently published results and completely new data.

Thermo-stimulated luminescence of gamma-irradiated polytetrafluoroethylene

The thermo-stimulated luminescence of γ-irradiated commercial polytetrafluoroethylene (PTFE) was explored. There are up to four intense maxima in the luminescence curves of different formulations of PTFE. The position and number of maxima remain constant at doses up to 30 kGy. The differences of the glow curves in the different copolymers may be the result of impurities capable of reacting with ions or radicals formed under irradiation and by differences in the polymer’s topological structures.

Long Lasting Phosphorescence in Mn<sup>2+</sup>-Activeted ZnO-B<sub>2</sub>O<sub>3</sub> Glass

Mn2+ activated ZnO-B2O3 glasses show bright and long-lasting phosphorescence (LLP). After irradiation by 254nm light, a visible red light with dominating peak at 600nm can be observed with the naked eye in the dark for up to 12h after removal of the activating light. We examined the luminescence properties using fluorescence, phosphorescence spectra and thermostimulated luminescence (TL). This phosphorescence arises from d-d transitions of Mn2+ ions. The thermostimulated luminescence (TL) intensity increases with the increasing of Mn2+ concentration from 1×10-3 mol % to 1×10-1 mol %. The decay curve of the LLP is consistent with the TL releasing velocity raised from defects of 420K. Based on the fact, we suggest that LLP is a special kind of thermostimulate luminescence. The storage and releasing of energy is closely associated with properties of defects, while the properties of LLP are related to both the defects and luminescence centers.

LGSO:Ce scintillation crystal optimization by thermal treatment

Impact of post-growth thermal treatment parameters (temperature, duration, atmosphere composition) on optical, luminescent, and scintillation characteristics of Lu2xGd2−2xSiO5:Ce LGSO:Ce has been analyzed. Absorption, X-ray luminescence spectra, thermostimulated luminescence curves, as well as light yield and energy resolution, have been measured before and after thermal treatment in different regimes. Scintillator optimization by thermal treatment allows to improve the energy resolution up to 6.7% at 662keV.

Characterization of defects in colloidal CdSe nanocrystals by the modified thermostimulated luminescence technique

The temperature dependencies of the luminescence spectra of 5-nm-diameter CdSe semiconductor nanocrystals synthesized by colloidal-chemistry methods are investigated. The two bands observed in these spectra around 2.01 and 1.37 eV correspond to band-to-band transitions and luminescence of defect states, respectively. A model explaining the temperature behavior of the luminescence band intensities both upon cooling and heating is put forward. A new modification of spectrally resolved thermostimulated luminescence technique making it possible to determine the activation energies and the character of traps responsible for the temperature dependence of the luminescence intensities is suggested. This technique is used to obtain the activation energies of the emission and capture of electrons at traps (190 and 205 meV, respectively) and to determine the depth of the electron level (57 meV) responsible for luminescence in the 1.37-eV region.

Thermo-stimulated luminescence analysis of powder of the gamma-irradiated tetrafluoroethylene/ethylene copolymer

The thermo-stimulated luminescence of Γ-irradiated commercial copolymers of tetrafluoroethylene with ethylene were explored. There are five intense maxima in the luminescence curves. Position, halfwidth and number of maxima remain constant at doses of irradiation from 10 kGy up to 30 kGy. The first peak at 112 K to 119 K has the greatest intensity and is found on the luminescence curves of all of the investigated copolymers except for Tefzel®. For Tefzel®, the maximum occurs at 141 K. The differences of the glow curves in the different copolymers may be the result of impurities capable of reacting with ions or radicals formed under irradiation and by differences in the copolymer’s topological structures.

Comparative study of thermo-stimulated luminescence and electron emission of nitrogen nanoclusters and films

We have studied thermo-stimulated luminenscence and electron emission of nitrogen films and nanoclusters containing free radicals of atomic nitrogen. Thermo-stimulated electron emission from N2 nanoclusters was observed for the first time. Thermo-stimulated luminescence spectra obtained during the destruction of a N2–He sample are similar to those detected from N2 films pre-irradiated by an electron beam. This similarity reveals common mechanisms of energy transfer and relaxation. The correlation of luminescence intensity and electron current in both systems points to the important role of ionic species in relaxation cascades. Sublimation of solid helium shells isolating nitrogen nanoclusters is a trigger for the initiation of thermo-stimulated luminescence and electron emission in these nitrogen–helium condensates.

Photo- and thermostimulated luminescence of ZnO nanowires

This is a study of the low-temperature photo- and thermostimulated luminescence of zinc oxide nanowires produced by gas-transport reactions. The parameters of the electron trapping centers are determined: trap ionization energy 0.53 ± 0.03 eV, electron capture cross section 4.7∙10–17 cm2, and frequency factor 1.1∙104 s–1. It is found that defects of the crystalline structure, such as interstitial zinc atoms, control the formation of the “green” emission band.

Cathodoluminescence of oxyfluoride glass-ceramics

Abstract Tb, Ce, Eu activated oxyfluoride glass-ceramics with the composition SiO2 · Al2O3 · Li2O · LaF3 have been studied by cathodoluminescence (CL). We compared CL intensities and decay times of the Tb, Ce, Eu activated glass-ceramic samples and observed that the Tb activated sample has the most intense luminescence, but the Ce activated sample has the shortest decay times. Induced optical absorption and thermostimulated luminescence have been observed after X-ray irradiation of samples.

Spectroscopic studies of impurity-helium condensates containing stabilized N and O atoms

We present optical spectra of impurity-helium condensates during the process of their formation by injection of gas mixtures N2-Rg-He (Rg=Ne, Kr) into bulk superfluid helium after passing through a RF discharge. Atomic lines of He, Ne, Kr, N, O atoms as well as bands (1+ and 2+ systems of N2) are present in the spectral range 320–1100 nm studied in these experiments. We also detected spectra emitted by the samples during their destruction, stimulated by warming through the temperature range 1.5–15 K The luminescence spectra of the N-N2-He and N-N2-Ne-He samples contain only intense N(2D-4S) (α-group) and O(1D-1S) (β-group) lines. For the N-N2-Ne-He sample we observed the transformation of spectra of the α-group. In thermo-stimulated luminescence spectra of the N-N2-Kr-He samples, the intense β-group of O atoms and M-bands of NO molecules were found. Differences in the spectra obtained during destruction of N-N2-Ne-He and N-N2-Kr-He samples may be explained by a different shell structure of the nanoclusters formed during sample preparation.

Energy Transfer Mechanism in Pr-Doped SrF$_{2}$ Crystals

Results of investigation of energy transfer mechanism in SrF2 crystals doped with Pr3+ ions are presented. Temperature dependences of 5d-4f and exciton emission in this material under X-ray excitation, and thermostimulated luminescence (TSL) in temperature range 80-300 K are studied. It has been established that in SrF2-Pr3+ crystals the consecutive electron-hole capture mechanism of energy transfer takes place. Also we found that hole centers made a contribution to energy transfer from primary electrons and holes to Pr3+ ions.

Low temperature luminescence of ZnMoO4 single crystals grown by low temperature gradient Czochralski technique

ZnMoO4 single crystals of high optical quality were grown using low temperature gradient Czochralski technique and their luminescence properties were studied. At low temperatures two overlapping luminescence bands with the maxima 1.93eV and 2.45eV were detected at Eex>3.9eV. The bands are attributed to the radiative recombination of self-trapped excitons of MoO42- parentage. High-intensity non-elementary peak of thermostimulated luminescence (TSL) observed at 65K is due to the thermal release of self-trapped holes. The threshold of TSL excitation spectrum at 6.2eV exceeds the bandgap value by 1.9eV and corresponds to electronic transitions from the valence band to the upper subband of the conduction band.

Temperature dependence features of the photoluminescence spectral distribution of Li2B4O7 and Li2B4O7:Cu

The temperature dependence of the photoluminescence (PhL) spectral distributions has been reported for the lithium tetraborate Li2B4O7 (LTB) and Li2B4O7:Cu (LTB:Cu) single crystals and for glassy LTB:Cu. It was found that the emission peaks of the LTB and LTB:Cu single crystals are non-elementary and splittable by temperature increase into several elementary peaks. By the sample heating the temperature quenching of PhL as well as the redistribution of the PhL intensity among elementary peaks was observed. Heating of the LTB:Cu single crystal samples caused no shift of the spectral maxima of the individual PhL peaks. The curve describing the temperature dependency of individual PhL peaks for the LTB:Cu single crystal is characterized by maxima resulting from combination of PhL and thermostimulated luminescence (TSL). The PhL intensity for glassy LTB:Cu is significantly lower than that for LTB:Cu single crystal with the equivalent copper dopant content. As compared to the LTB:Cu single crystal, the PhL spectral maximum for glassy LTB:Cu is wider and shifted to the lower energy range. Heating of the glassy LTB:Cu sample results in the PhL temperature quenching without any shift of the spectral maximum.

Photo- and thermally stimulated luminescence of polyminerals extracted from herbs and spices

Ionizing radiation processing is a widely employed method for preservative treatment of foodstuffs. Usually it is possible to detect irradiated herbs and spices by resorting to luminescence techniques, in particular photo- and thermostimulated luminescence. For these techniques to be useful, it is necessary to characterize the response to radiation of each particular herb or spice. In this work, the thermoluminescence (TL) and photostimulated luminescence (PSL) properties of inorganic polymineral fractions extracted from commercial herbs and spices previously irradiated for disinfestation purposes have been analyzed. Samples of mint, cinnamon, chamomile, paprika, black pepper, coriander and Jamaica flower were irradiated from 50 to 400Gy by using a beta source. The X-ray diffraction (XRD) analysis has shown that the mineral fractions consist mainly of quartz and feldspars. The PSL and TL response as a function of the absorbed dose, and their fading at room temperature have been determined. The TL glow curves have been deconvolved in order to obtain characteristic kinetics parameters in each case. The results of this work show that PSL and TL are reliable techniques for detection and analysis of irradiated foodstuffs.

Effect of annealing on natural calcitic crystals—A thermostimulated luminescence (TSL) study

The quality crystals (Calcitic limestone) were selected using the UV–visible methylene blue adsorption method. The thermostimulated luminescence (TSL) glow curve characteristics of six well crystallized limestone samples were analyzed. The glow curves of unannealed sample show only one peak in the range 320–330 °C. The sample irradiated with a gamma dose of 100 Gy shows two additional peaks in the range of 113–125 °C and 242–260 °C when recorded with linear heating rate of 10 °C/s. The annealed sample also shows the same trend as that of irradiated sample. Annealing treatment above 250 °C increases the sensitivity of all TSL peaks except 320 °C. On the other hand, annealing at 750 °C caused a collapse in the TSL sensitivity. The enhancement in TSL sensitivity was found to depend on the annealing temperature and time. Annealing treatment at 650 °C for 4 h followed by quenching in air is the optimum condition for TSL sensitization. The response to gamma irradiation is linear in the range from 0.5 Gy to 10 4 Gy. The emission spectra of all the samples show an emission at around 610 nm but with different intensities for each TSL peak. With reference to earlier work, it may be assumed that the recombination site always involves Mn 2+ ions. The observation made through infra-red (IR) and X-ray diffraction (XRD) studies with thermal treatment shows the structural changes of calcite from D 3h to C s symmetry at 750 °C. The Thermogravimetric-Differential Thermal Analysis (TG-DTA) analysis shows the calcite gets disordered at 760 °C. Hence, the collapse in the TSL sensitivity at 750 °C is due to structural change or structural disorderedness.

Thermo-stimulated luminescence of x-ray- and alpha-irradiated yttria-stabilized zirconia

Yttria-stabilized zirconia (ZrO2 : Y3+) single crystals (with 9.5 mol% Y2O3) were irradiated with x-rays and α particles. Thermally stimulated luminescence (TSL) data show a main broad peak centred at ∼500–550 K in the glow curves of all irradiated samples. The TSL peak maximum temperature is consistent with the characteristic recovery temperature (∼450 K) of colour centres (T centres) deduced from isochronal annealing curves measured by electron paramagnetic resonance (EPR) spectroscopy. However, the trap-depth energies (ranging between 0.8 and 1.2 eV) deduced from the initial rise of partially cleaned TSL peaks (and from a rough approximation using Urbach’s formula) are much larger than the activation energies for defect recovery of 0.3 eV deduced from the EPR data. A second TSL peak centred at ∼350–450 K found in freshly irradiated samples is seen to decay substantially in aged samples. The processes involved in TSL are discussed in relation to the defect annealing processes, and available defect-level energy and TSL data.

Thermo-stimulated luminescence of ion-irradiated yttria-stabilized zirconia

Yttria-stabilized zirconia (ZrO2:Y3 + )single crystals (with 9.5 mol% Y2O3) were irradiated with ions (from 1 MeV He to 2.6 GeV U). Electronparamagnetic resonance (EPR) data show that two kinds of colour centres (F + -typeand T centres) are produced. Thermo-stimulated luminescence (TSL) data exhibit a quite strong peakat ∼ 500–550 K in the glow curves of all irradiated samples regardless of the ion species and energy. Moreover,the 3D-TSL measurements reveal that this peak is correlated with a light emission at a wavelength of ∼ 620 nm(i.e. photon energy ∼ 2 eV). The TSL peak maximum temperatures are consistent with characteristic temperaturesof about 500 K of annealing stages of colour centres. However, the trap-depthenergies (ranging between 0.7 and 1.4 eV) deduced from the initial rise of partiallycleaned TSL peaks, or from a rough approximation using Urbach’s formula, arerather larger than the activation energies for defect recovery, ranging between 0.3and 0.7 eV, as deduced from the EPR data. The processes involved in TSL arediscussed in relation to available photoluminescence and defect energy-level data.