Thermally Stimulated Luminescence Curves Research Articles

Size effects and radiation resistance of BaF2 nanophosphors

Abstract The stationary X-ray excited luminescence (XEL) spectra at 80 and 294 K as well as the thermally stimulated luminescence (TSL) in the 80–300 К range of a single crystal and nanoparticles of BaF2 with various grain sizes of 20–113 nm were studied. Under the influence of size effects in BaF2 nanophosphors, a short-wave displacement of self-trapped excitons emission band was observed compared to the corresponding single crystal, and its splitting into two components peaked at 324 and 305 nm for 294 K, and at 321 and 305 nm – for 80 K. A slight decrease of radiation resistance of BaF2 nanophosphors was observed in comparison to single crystal, which led to additional band of X-ray excited luminescence at ∼410 nm that may be associated with emission of exciton localized in the vicinity of anionic vacancies of the matrix. At the same time, it was established that radiation resistance of BaF2 nanophosphors is much higher compared to the corresponding CaF2 nanophosphors. With the use of integral TSL curves and TSL spectra, it has been shown that VH-centers with delocalization temperature of ∼140 K are the main hole centers that occur in BaF2 nanoparticles under X-ray irradiation influence, while in ВаF2 single crystals – these are Vk - and H-centers with delocalization temperatures at 113 and 198 К, respectively. Significant long-wave displacement of emission bands associated with (VH-F)- and (H-F)- recombinations, compared to the spectra of corresponding single crystal, was observed in BaF2 nanophosphors.

Spectrally resolved thermoluminescence in UV excited hexagonal boron nitride nanopowder

Thermally stimulated luminescence (TSL) excitation and emission processes in h-BN nanopowders after UV irradiation were studied. TSL showed the highest response within the 5.4 eV band. 3.0 and 3.9 eV bands, resulting from the radiative donor-acceptor recombination between the electron (1B-H ON-centers) and the hole traps (CN-center), were found. TSL curves quantitative analysis in the context of general order kinetics was made. It was shown that 1B-centers had 0.85±0.05 eV activation energy and were responsible for the TSL peak at 350±10 K. Band structure for TSL excitation and emission mechanisms in h-BN nanopowder was suggested.

A new method for unambiguous determination of trap parameters from afterglow and TSL curves connection: Example on garnets

Due to presence of charge carrier traps, many scintillating materials exhibit afterglow. The de-trapping mechanisms are usually studied separately via either thermally stimulated luminescence (TSL) or isothermal decay (afterglow) measurements.In this paper, we develop procedures to determine trap parameters such as thermal trap depth and frequency factor in an unambiguous manner by connecting TSL and afterglow measurements. In order to accomplish that, we have devised a special method of extracting the lifetime of trapped carriers from afterglow measurements, independent of kinetic order. The procedures are first shown on simulated TSL and afterglow curves and then illustrated using (Y,Gd)3Al5O12:Ce garnets as example.

Effect of thermal annealing on scintillation properties of Ce:Gd2Y1Ga2.7Al2.3O12 under different atmosphere

Cerium-doped 1% Ce:Gd2Y1Ga2.7Al2.3O12(GYGAG) single crystal samples grown via Czochralski method were annealed under air, O2 and N2 atmospheres from 350 to 1400 °C. The X-ray excited luminescence spectra, energy spectra and UV as well as thermally stimulated luminescence (TSL) spectra were performed comparatively on “as-grown” and thermally annealed samples. It was found that the luminescence efficiency after annealing in air and O2 was significantly enhanced compared to the non-annealed samples and this phenomenon was suggested to be caused by the existence of some oxygen vacancies in the Ce:GYGAG crystals. And the oxygen vacancies in the as-grown GYGAG crystals can be effectively eliminated by means of annealing in O2 containing atmosphere without changing the luminescence mechanism. From the TSL curves before and after annealing, three traps within 77–650 K were found to be related to oxygen vacancies.

Effect of Plastic Deformation on Charge-Carrier Energy States in Poly-N-Epoxypropylcarbazole Films

Thermally stimulated luminescence (TSL) curves of poly-N-epoxypropylcarbazole (PEPC) and polystyrene (PS) fi lms were studied. The high-temperature tails of the TSL curves were approximated by Gaussian functions, the halfwidths of which characterized disordering of trap energy states. It was established that the disordering parameter in the initial PEPC varied from sample to sample and increased after application to the fi lm of uniaxial pressure of 1·108 Pa or upon grinding it into a powder. Unlike for PEPC, the disordering parameter for PS decreased after application of pressure to the PS fi lm. It was concluded that the side carbazolyl groups became spatially less ordered after plastic deformation of PEPC due to the high fl exibility of its macromolecule.

Influence of post-radiation deformation on TSL and absorption spectra of irradiated MgO crystals

Thermally stimulated luminescence (TSL) and optical and paramagnetic absorption spectra of MgO and MgO:V2+ have been investigated after quasi-elastic and plastic deformations. Splitting of the low-temperature peak (∼450 K) in TSL curves caused by the deformation in both nominally pure and impure crystals is explained by the formation of an excess of vacancies. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Analysis of thermally stimulated luminescence and conductivity without quasi-equilibrium approximation

Thermally stimulated luminescence (TSL) and conductivity (TSC) are considered using the classical insulator model that assumes one kind of active trap, one kind of inactive deep trap and one kind of recombination centre. Kinetic equations describing the model are solved numerically without and with the use of quasi-equilibrium (QE) approximation. The QE state is characterized by the parameter qI = (dnc/dt)/Ie, where dnc/dt is the rate of change of free electron density, and Ie is the TSL intensity. The QE state parameter qI, the relative recombination probability γ = Ie/(Ie + It) (It is the trapping intensity) and a new parameter called a quasi-stationary (QS) state parameter q* = qIγ = (dnc/dt)/(Ie + It) are used for the analysis of the TSL and TSC. The QE and QS states are determined by conditions |qI| ≪ 1 and, respectively, |q*| ≪ 1. The TSL and TSC curves and the temperature dependences of qI, q*, γ the recombination lifetime and the occupancies of active traps and recombination centres are numerically calculated for five sets of kinetic parameters and different heating rates. These calculation results show that (1) the upper limit of the heating rate for the presence of the QS state appears at a higher heating rate than that for the QE state when the retrapping process is present, and (2) the TSL (TSC) curves in the QS state have properties similar to those for the TSL (TSC) curves in the QE state. Approximate formulae for calculation of the parameters qI and q* in the initial range of the TSL and TSC curves are derived and used in the heating-rate methods, proposed in this work, for determination of those parameters from the calculated TSL curves.

Local Trapping Centers in the Ceramics of Eulytines

Thermally stimulated luminescence (TSL) of ceramics having the structure of eulytine Bi4X3O12 (X = Si, Ge, Zr) on exposure to x-ray irradiation in the temperature region 80–400 K is studied. An analysis of the forms of the TSL curves implies that the recombination processes in the TSL peaks at 149 and 212 K in Bi4Si3O12 ceramics and at 143 and 230 K in Bi4Ge3O12 ceramics can be described by the linear kinetics. The spectral composition of TSL of the ceramics obtained is investigated, and the activation energy and frequency factors which correspond to the TSL maxima are determined by various methods. Based on common spectral and energy characteristics of TSL, a conclusion concerning the connection of TSL with recombination processes in common structural complexes of BiO69− is drawn.

Interactive kinetics in thermally stimulated luminescence in insulators and semiconductors

The interactive kinetics in thermally stimulated luminescence (TSL) in an insulator is considered using the recombination model with two kinds of active traps having different depths, one kind of inactive (deep) trap, and one kind of recombination centre. Complex TSL curves with two peaks and other TSL characteristics are numerically calculated by solving the kinetic equations describing the model. The characteristics suggest that during the generation of the high-temperature TSL peak electrons thermally released from the active deeper traps partly recombine and partly are captured by the shallower traps from which they are released again and recombine. Consequently, the high-temperature TSL peak is composed of two partial peaks that are due to the recombination of electrons released from two different kinds of traps.

Interactive kinetics in thermally stimulated luminescence and conductivity and its effect on the trap depth determination

Thermally stimulated luminescence (TSL) and conductivity (TSC) complex glow curves having overlapping peaks are calculated for a system with two kinds of interactive electron traps, one kind of inactive trap, and one kind of recombination center. The densities of electrons localized on the interactive traps and the rates of change of these densities during the TSL and TSC runs also are calculated. The calculation results show that electrons thermally released from the traps of one kind during the sample heating are partly trapped by the traps of another kind from which they are next released. These processes of transfer of electrons influence the properties (the peak-positions, shape, and intensity) of the complex TSL and TSC curves. Consequently, the accuracy of trap depths evaluated from the calculated TSL and TSC curves using eight methods known from literature is, in general, lowered. This is found, particularly, for the initial-rise and the peak-shape methods.

Interpretation of thermoluminescence and thermally stimulated conductivity experiments: Part I: Extensions of the conventional model

The conventional model that one often uses for the interpretation of thermally stimulated luminescence (TSL) and conductivity (TSC) experiments does usually not describe adequately the actual situation in phosphors since in most of the practical cases there is a significant difference between the shapes of measured and calculated TSL and TSC curves. We calculated the influence of several model extensions such as recombination via excited states, the presence of a trap distribution, donor-acceptor pair recombination, the presence of additional centres, temperature dependent rate parameters, thermal quenching of luminescence and scattering due to ionized defect centres on TSL and TSC curves. It is concluded that such extensions may strongly alter these curves. We also found that trap depths determined after application of some methods from the literature are unreliable if the model that can be used for the interpretation of experimental results is not accurately known. Only the method of Hoogenstraaten appears to be rather insensitive for most of the extensions considered in this paper.

Interpretation of thermoluminescence and thermally stimulated conductivity experiments: Part II: Application to CdGa 2S 4

Results of thermally stimulated luminescence (TSL) and conductivity (TSC) experiments on CdGa 2S 4 are reported. In the lower temperature region of the TSl curve a decay of luminescence is observed that is probably due to donor-acceptor pair recombination since no accompanying conduction is measured. The TSL and TSC curves of the undoped material further consist essentially of two peaks. If trap depths are calculated with various methods from the literature it turns out that for one peak different values are obtained. These differences cannot be explained with the conventional model consisting of one trapping level and one recombination level. If the results are compared with more complex models it follows that thermal quenching, the presence of a trap distribution, thermally disconnected traps and recombination via excited states, are important. This is most clearly demonstrated in the cases with excess Ga and with Ag or In as dopant. Although this approach probably yields valuable information about CdGa 2S 4, no definite conclusions may be drawn since some essential questions remain unsolved.

Evaluation of the model for thermally stimulated luminescence and conductivity; reliability of trap depth determinations

A review of the basic theory on thermally stimulated luminescence (TSL) and conductivity (TSC) based on a certain simple model is given. Approximate analytical expressions for the shapes of the TSL and TSC curves are derived. Methods from the literature for trap depth determination, some of which can easily be derived from these expressions, are applied to numerically calculated TSL and TSC curves. It turns out that the methods of Bube, Haering and Adams, Hoogenstraaten and Unger yield a correct value for the trap depth independent of the values of the retrapping ratio and the frequency factor. The method of Garlick and Gibson is reliable if applied below 15% of the maximum TSL intensity. Some methods, as most of those derived by Chen, yield a correct trap depth for a specific value of the retrapping ratio in the case of TSL only.