Destruction mechanism of color centers and recombination luminescence in NaCl-Cd crystal phosphors

The authors have studied the origin of integral thermoluminescence peaks and the destruction mechanism of color centers during thermal relaxation of X-irradiated NaCl-Cd phosphors. The luminescence spectra have also been studied, and previous similar results are discussed.     

Sputtering of zinc sulfide phosphors in excitation of radical-recombination luminescence by atomic hydrogen

Treatment of zinc sulfide phosphors in atomic hydrogen not only produces radical-recombination luminescence (RRL) but also sputtering of the phosphor. The sputtering rate has been estimated for FKP-03k powder phosphor at various temperatures. The atomic hydrogen produces defects of etch-pit type on the surface of a single crystal. The long-term response of the RRL has been recorded for the FKP-03k phosphor and it is ascribed to the sputtering.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 107–110, March, 1975.     

Investigation of luminescence and capture centers in magnesium and zinc tungstates

Electron capture centers and hole luminescence centers are considered. Models are proposed for the luminescence centers responsible for the radiation from the crystals investigated, in the region 480–650 nm, and also the corresponding radiation mechanisms.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 43–46, March, 1978.In conclusion, we express our deep gratitude to O. O. Glazunov and Yu. P. Tsaregorodtsev for obtaining the XL and KTL spectra and for interest in this work.     

Photostimulated luminescence of phosphors

The published data on photostimulated luminescence and some new results obtained by the authors with the KCl-Tl phosphor are analyzed. It is shown that for phosphors with associated donor-acceptor pairs photo stimulated luminescence appears as a consequence of an inter-impurity electronic transition. The kinetic aspects of the luminescence depend upon the Coulomb attraction between a quasi-free electron and a charged donor. The character of the radiative recombination is determined by the interaction energy between the two partners and by the trap depth. This enables us to consider various well-known phosphor models from a general point of view. It is emphasized that the coexistence of various mechanisms in the same phosphor must be taken into account to obtain a correct interpretation of the luminescence characteristics. It is also shown that, for the mechanism involving free holes, an exciton must be created before the recombination energy can be transferred to the luminescent center.     

Influence of surface quenching on luminescence dynamics of ion centers in nanocrystals

In this paper, the influence of surface quenching on decay curve and quantum efficiency of ion centers in nanoparticles were discussed by considering energy transfer between doped ions and surface quench centers.With isotropic and continuum approximation, energy transfer rate of an ion at position r within a spherical nanoparticle to all the surface quench centers was calculated. Quantum efficiency and decay curve under nonselective excitation were then calculated numerically by integrating over the sample. The calculated decay curve was fitted to the experimental one measured in Eu³⁺-doped YVO₄ nanoparticles. The ratio of the quantum efficiency of YVO₄:Eu³⁺ nanoparticle (R=13.5 nm) to that of the bulk is 17%, estimated with the parameters obtained from the fitting.     

Optical and luminescence spectroscopy of zinc gallate phosphors codoped with manganese and europium ions

Zinc gallate compounds codoped with manganese and europium ions were synthesized using the high-temperature solid-state reaction method. All samples were “characterized” by X-ray diffraction measurements, ultraviolet reflection spectroscopy, and photoluminescence spectroscopy. Structural investigations confirmed that incorporation of europium ions in zinc gallate host leads to distortion of the spinel unit cell and increase in lattice parameter. Manganese ions show the intense excitation in the region of band-to-band transitions. Zinc gallate spinel compounds codoped with manganese and europium ions exhibit emission in the whole spectral region. Among the manganese and europium activator emissions in the “green” and “red” spectral regions, the “blue” emission of spinel matrix was observed. Photoluminescence excitation spectra of europium ions and reflection spectra show similar results in the near-ultraviolet region of the spectra. The optimal concentration of europium activator was determined. Europium doping level leads to redistribution of excitation and emission intensities of zinc gallate codoped samples. The possible energy transfer mechanisms between matrix, manganese, and europium ions are discussed. Overlapping of excitation bands of all types of luminescence gives a possibility to obtain emission in the whole visible spectral range. The Commission Internationale de l’Eclairage chromaticity diagrams confirmed a possible application of the studied zinc gallate spinel compounds codoped with manganese and europium ions for the development of ultraviolet and near-ultraviolet phosphors with flexible emission color in the visible spectral range.     

Effect of magnesium isovalent impurity on the temperature stability of blue luminescence in zinc selenide

The effect of Mg isovalent impurity on photoluminescence in undoped and Al(donor)-doped ZnSe crystals is investigated. The anomalously weak temperature quenching of the blue band in Mg-containing samples is detected.     

The luminescence of heavily-doped KCl-Tl phosphors

Excitation and emission spectra of KCl : Tl phosphors having higher Tl content (Tl ≈ 0.01 m.f. and above) were examined with the help of Aminco-Bowman spectrophotofluorometer. Measurements also included the luminescence study of TlCl powder and TlCl solution. It is observed that there is parallelism in the excitation and emission spectra of heavily-doped KCl : Tl phosphor, TlCl powder and aqueous TlCl-solution. It is concluded that heavily-doped KCl : Tl phosphor contains undissociated TlCl molecules which act as luminescence centres.     

On the luminescence and absence of luminescence of F centers

A model, proposed originally by Dexter, Klick and Russell, is used to explain the occurence or non-occurence of F-center luminescence in ionic crystals, in terms of a simple two-state configuration coordinate diagram. In this model, which works for all known cases, luminescence is quenched by a competing non-radiative process whenever the intersection of the ground and excited state curves lies below the energy reached in absorption in a vertical (Franck-Condon) transition. The criterion for the occurence of luminescence is expressed as $\text{Λ <}\text{1}\text{4}$, where Λ (=excited-state lattice-relaxation energy/optical-absorption energy) is a parameter, related to the relative displacement of the two curves, which can be inferred from data on the temperature dependence of the F-band line width. Thus the possibility of observing luminescence can be predicted from optical absorption data alone. It is found emperically that Λ for alkali halides with rocksalt structure is independent of lattice parameter, and the observed dependence of Λ on the ratio of ionic radii in terms of ion-size effects. Values of Λ range from 0.009 for CsF to 0.831 for LiI; NaCl with Λ = 0.260 is a marginal case for luminescence.     

Excitation dependent quenching of luminescence in LED phosphors

Droop, the decrease of efficiency with increased power density, became a major topic with InGaN LEDs, after its introduction in 2007. This paper provides insight into droop in localized center luminescence phosphors, exemplified here by Eu²⁺ doped materials. This topic is of increasing importance, as high brightness blue LEDs have reached outputs >1 W/mm². The nonlinearities in phosphor quantum efficiency result in drive‐dependent color point shift and reduction of overall efficiency of phosphor converted white LEDs which utilize Eu²⁺ activated phosphors. The efficiency quenching can be traced back to two processes, well‐known in laser physics, excited state absorption or/and cross relaxation by Foerster/Dexter transfer. Both processes lead to reduction in phosphor efficiency, but they can be differentiated. Understanding the root cause of efficiency quenching opens ways to minimize the practical consequences. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Energy transfer,luminescence properties,and thermal stability of color tunable barium pyrophosphate phosphors

A series of barium pyrophosphate Ba_2P_2O_7 (BPO) phosphors doped with Ce~(3+)or Tb~(3+)ions is synthesized via a co-precipitation method under reducing atmosphere. The phase structures, photoluminescence(PL) properties, and thermal stabilities of the samples are characterized by using powder x-ray diffraction(PXRD) and PL spectra. The emission colors of samples can be tuned from blue(0.1544, 0.0310) to green(0.2302, 0.4229) by changing the doping concentrations of Tb~(3+)under ultraviolet excitation. The energy transfer mechanism between Ce~(3+)and Tb~(3+)in the BPO is dipole–dipole interaction with a critical distance of 25.86 °A and an energy transfer efficiency of about 85%, which are determined through the PL spectrum and the decay curve. Moreover, the Ce~(3+)/Tb~(3+)co-doped sample has good thermal stability for temperature quenching, and the emission intensity at 423 K is maintained at 95% measured at 298 K. The above results show that the BPO:Ce~(3+), Tb~(3+)can serve as a promising candidate of green emitting phosphor for solid-state lighting.     

Effects of IR illumination on the radical-recombination luminescence of crystal phosphors

The effects of infrared light have been examined for the radical-recombination luminescence characteristics of narrow-band and wide-band phosphors; it is found that the stored light sum is small near the surface space-charge region.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 66–71, October, 1973.