Luminescence of CsBr:Eu films grown by liquid-phase epitaxy

Abstract

By liquid-phase epitaxy from an aqueous alcoholic solution, we have obtained films of the well-known storage phospor CsBr:Eu, and we have studied their cathodoluminescence and photoluminescence (PL) spectra compared with the undoped CsBr films. We have established that the structure of the photoluminescence centers of the CsBr:Eu films when excited by laser radiation in the absorption band of the Eu 2+ ions (λ = 337 nm) includes Eu 2+ -V Cs isolated dipole centers and CsEuBr 3 aggregate centers, and also luminescence centers based on inclusions of hydroxyl group OH − with the corresponding emission bands in the 440 nm, 520 nm, and 600 nm regions. We have studied the dependence of the spectra and the intensity of the photoluminescence for CsBr:Eu films on annealing temperature in air at 423-483 K, compared with analogous dependences for CsBr:Eu single crystals obtained from the melt. We have shown that annealing the films at T = 423-463 K leads to rapid formation of CsEuBr3 aggregate luminescence centers, while for T > 473 K thermal degradation of these centers occurs. We conclude that the observed differences between the photoluminescence spectra of CsBr:Eu films and CsBr:Eu single crystals may be due to additional doping of the films with OH − ions.Introduction. Optically isotropic CsBr:Eu crystals are well-known storage phosphors for digital luminescence x-ray radiography [1]. At the moment, there have been studies of the characteristic features of crystallization and the luminescent properties of these phosphors in the form of single crystals grown from the melt [2, 3] and powders prepared by solid-phase synthesis [1,4], and also NIPs 1 (needle image plates), consisting of needle-shaped close-packed crystals obtained by thermal evaporation under vacuum [5]. At the same time, no information is available about the luminescent properties of CsBr:Eu phosphors grown from solutions.It has been established [1-4] that regardless of the chemical composition of europium impurity added to the mix (EuBr 3 ⋅8.9H 2 O, EuOBr, Eu 2 O 3 , Eu met ), incorporation of europium ions into CsBr crystals occurs in the divalent state. A characteristic feature of incorporation of Eu 2+ ions into CsBr crystals is also the fact that along with Eu 2+ -V Cs isolated dipole centers, for impurity concentrations 0.05%-0.50%, aggregate luminescence centers are also formed as CsEuBr 3 nanocrystals [2, 3]. When CsBr:Eu crystals are excited by UV light in the absorption bands of the Eu 2+ ions with λ max = 250 nm and 350 nm, the isolated dipole centers and the aggregate centers emit in the blue (λ max = 440 nm) and green (λ max = 520 nm) regions of the spectrum respectively. It has been shown [3] that aggregate centers are formed both during growth of the CsBr:Eu crystals and during their heat treatment at 443-473 K, which is accompanied by increased intensity of emission from the aggregate centers in the green region of the spectrum. At temperatures above 473 K, degradation of these luminescence centers occurs. Such characteristic features of th...

Luminescent and scintillation properties of CsI:Tl films grown by the liquid phase epitaxy method

“…Apart from the thermal deposition, the liquid phase epitaxy (LPE) methods can be also successfully applied for producing high‐quality single‐crystalline film scintillators of oxide 9, 10 and alkali‐halide compounds 11 for application in medical imaging and nondestructive testing 1. Such scintillating films are used in environmental monitoring for detection of α and β particles as well 12.…”

“…It is important to note that the luminescence and scintillation properties of well‐known phosphors usually substantially depend on the methods of their preparation. Specifically, single crystals and thin crystalline films of oxide 9, 10 and alkali‐halide compounds 11, grown from melt by the Czochralski or Bridgman methods and melt solution by the LPE method, respectively, show the significantly different luminescent properties. These differences are caused by: (i) the highest content of intrinsic substitution‐type and vacancy‐type defects in crystals, grown from the melt at high temperatures 13, 14 in comparison with the films, crystallized from melt solutions at significantly lower temperatures 9, 10; (ii) the presence of flux impurities in the case of crystallization of the films 9, 10; (iii) the differences in the segregation coefficient of activators for both methods of crystallization 10.…”