Eugeniusz Zych

Luminescence properties of Ce-activated YAG optical ceramic scintillator materials

Abstract Scintillation and luminescence characteristics of a highly dense transparent YAG: Ce-ceramic are reported and compared to those of a single crystal. When excited with gamma-rays both types of materials display the same dominant ≈85 ns decay, but the ceramic also shows a new rapid component of ≈20 ns that is completely absent in the single crystal. The scintillation output from the ceramic reaches about 50% of that from the single crystal, having been diminished by unusual loss processes caused by the deformed lattice at the grain boundary interfaces. A phenomenological model involving distortion of the band structure is proposed to explain the results of kinetic measurements.

Kinetics of cerium emission in a YAG:Ce single crystal: the role of traps

We have measured the emission spectra and decay kinetics of Ce-doped YAG under both optical and gamma excitation, at temperatures ranging from 60 to 600 K. The decay traces under gamma excitation display an unusual pattern of temperature dependence. Beginning with the lowest temperatures, the characteristic time constant of the decay coincides with the intrinsic (radiative) decay time up to about 180 K, at which point it suddenly jumps by more than 30%. The decay time remains longer than radiative until well above room temperature, then gradually reverts to the intrinsic value. This behaviour can be explained in terms of an array of shallow traps, whose existence is confirmed by the presence of thermoluminescent glow peaks. Depending on temperature, one or more of these traps acts as a temporary way-station for the free carriers that carry the excitation to the Ce ion, delaying its emission beyond what would be required by radiative decay alone.

On the reasons for low luminescence efficiency in combustion-made Lu2O3:Tb

Nanocrystalline Tb-doped Lu2O3 was prepared via a combustion route. Its low luminescence efficiency could be dramatically increased through post-fabrication heat-treatment. Infrared spectra showed that the main reason for such a behavior, typical of combustion-made phosphors, was the presence of OH-vibrations in the as-made material. The OH-groups responsible for these vibrations could be completely removed only by heating the materials to at least 1500°C. A simple alteration of the synthesis procedure to assure evaporation of water before the combustion step led to a decrease in the OH-vibration intensities, mirrored by a noticeable increase in the emission efficiency of the resultant powder.

Quantum efficiency of europium emission from nanocrystalline powders of Lu2O3:Eu

Three series of nanocrystalline powders of Lu2O3:Eu with Eu concentration varying from 0.05 to 13% were prepared. All specimens were obtained through combustion synthesis using urea or glycine as the fuel. The powders of the first series consisted of materials with crystallites about 13 nm in size and were prepared with urea. For the next series of powders, made with glycine, the crystallites were about 30 nm in size. The powders of the third series differed from those of the second one in that they were co-doped with 1% of Ca. For the series made with urea the quantum efficiency was highest for 3% of Eu and never exceeded about 30%. For glycine-prepared specimens the highest quantum efficiency was about 90%. Without the Ca co-doping such a value could be obtained for specimens doped with 5% of Eu, while in the series co-doped with Ca 85–90% quantum efficiency could be maintained for all concentrations in th er ange 3–10%. A significant number of OH groups were proved to be left in th efi nal product obtained with urea. The low quantum efficiency of these powders is attributed to this effect. The results prove that properly prepared nanocrystalline phosphors can produce luminescence efficiently.

Properties of Tb-doped vacuum-sintered Lu2O3 storage phosphor

Tb3+-doped Lu2O3 sintered ceramics were prepared in vacuum and in air. It was shown that the vacuum-sintered disks are able to store energy when irradiated with 300-nm or shorter photons. A small part of the stored energy could be recovered with 980-nm light. A much more significant amount of the stored energy could be released with red 647-nm photons. However, recovering the total stored energy could be accomplished only upon heating up to about 300 °C. Changes in absorption of the raw materials upon ultraviolet irradiation and subsequent IR (980 and 647 nm) treatments or upon heating at 300 °C are presented and discussed. A model for energy storing and recovering through the various IR irradiations or through heating is presented. At least two distinct ways of hole trapping as Tb4+ or Vk-center as well as creation of F and F+ is suggested.

Preparation, X-ray analysis and spectroscopic investigation of nanostructured Lu2O3:Tb

Abstract Formation of Lu 2 O 3 through combustion of Lu(NO 3 ) 3 and CO(NH 2 ) 2 (urea) at various temperatures was investigated. From XRD patterns analysis, the optimal reaction temperature was found to be ∼600–700°C. For that range products with crystallites of ∼7–13 nm were obtained, as found from TEM pictures. With the same technique Tb-doped Lu 2 O 3 was successfully fabricated and its basic spectroscopic properties were characterized. Emission of the phosphors was found to be typical for Tb 3+ , with main components at 490 and 550 nm, resulting from radiative relaxation of 5 D 4 level. Undoped material produced broad-band luminescence peaking in blue. Raw Tb-doped samples were brownish, which was attributed to incorporation of some of the activator as Tb 4+ . Such assignment was derived from reflection, excitation and EPR spectra. The colour could be completely removed through post-fabrication vacuum heat-treatment at 1100°C.

Structural and spectroscopic characterization of Lu2O3:Eu nanocrystalline spherical particles

Spherical particles of Lu2O3:x% Eu, with x varying from 0% to 10% with respect to Lu, were prepared by precipitating hydroxides with urea at 80 °C and subsequently decomposing these hydroxides to oxides at 650 °C. TEM pictures revealed that the spherical particles were very uniform in size and their diameters were about 130 nm. Each of the particles consisted of crystallites about 20 nm in diameter. Luminescence and excitation spectra contained all the characteristic features of the Eu3+ ion. The most intense line in the emission was located around 611 nm. Energy transfer was observed from the Eu3+ ions occupying the S6(C3i) centrosymmetric site to the Eu3+ located in the non-centrosymmetric position of C2 symmetry. The decay kinetics were slightly non-exponential, especially for the lowest dopant concentrations. At liquid nitrogen temperature the average decay time for the 0.2% powder was shorter by about 40% compared to the 3–10% materials. At room temperature the average decay time varies only slightly. Rise times were observed for all concentrations at room temperature but only for higher concentrations at liquid nitrogen temperature. This effect is in contrast to that of nanoparticles of Lu2O3:Eu prepared using different synthesis procedures.

Analysis of Eu3+ emission from different sites in Lu2O3

Abstract Lu 2 O 3 doped with Eu of varying concentrations was prepared using a convenient combustion technique. Emissions from two different crystallographic sites, C 2 and S 6 , were found and analyzed. Using various spectroscopic techniques, luminescence of Eu 3+ exclusively from site C 2 or site S 6 could be detected. Kinetics of both emissions was characterized and time constants of 1.5 and 4.3 ms were found, respectively. Energy transfer from Eu 3+ in site S 6 to Eu 3+ in site C 2 was proved to take place when the dopant content reaches about 1% or higher. For rising activator concentration the transfer becomes more efficient and for heavily doped samples the emission from S 6 site lasts in a vestige form only. The only emission transitions detected from Eu in centrosymmetric site S 6 were the 5 D 0 – 7 F 1 three-line band.

Temperature dependence of Ce-emission kinetics in YAG:Ce optical ceramic

We have measured the scintillation decay kinetics of the Ce-emission from a YAG:Ce optical ceramic material over a temperature range between 110 and 450 K. Above room temperature the decay can be well described by the sum of two exponential terms, but below room temperature a third term is needed. This new component has a characteristic time shorter than the radiative lifetime of the Ce ion, varying from 33 ns at 110 K to 18 ns at 300 K. The presence of the short component can be explained in terms of the distortion of the YAG host band structure and the introduction of surface-boundary states into the otherwise forbidden energy region, which drains the emitting 5d levels of Ce ions located in the vicinity of the grain-boundaries.

Size effects on optical properties of Lu2O3:Eu3+ nanocrystallites

The size effect of nanoparticles on optical properties of Lu2O3:Eu3+ was investigated. No pronounced effect was observed for structure of emission spectra. It was found that effect is mainly manifested in the decay profiles of Eu3+ emission. We have noticed that the decay times were almost the same for low concentrations of Eu3+ ions. It is concluded that the size effect is associated with agglomeration of Eu3+ in nanorange particles.