Luminescence of aggregate centers in CsBr:Eu2+ single crystals

Abstract

We have used the Bridgman method to grow CsBr:Eu 2+ single crystals, adding an activator to the mix in the form of Eu 2 O 3 in amounts of 0.0125, 0.0250, and 0.0500 mole %. At T = 300 K, we studied the absorption spectra, the photoluminescence (PL) spectra, and the photostimulated luminescence (PSL) spectra of the grown crystals. We have established that the structure of the photoluminescence and photostimulated luminescence centers in crystals grown from the CsBr:Eu 2 O 3 mix includes isolated dipole centers Eu 2+ -V Cs , emitting in bands with maxima at 432 nm and 455 nm respectively, and in crystals grown at activator concentrations of 0.025 and 0.050 mole % they also include aggregate centers (AC) based on CsEuBr 3 nanocrystals with emission bands at 515 m and 523 nm. We have shown that the maximum concentration of aggregate centers of the CsEuBr 3 nanocrystal type in CsBr:Eu 2+ crystals is achieved for an activator content in the mix within the range 0.01-0.05 mole %.Introduction. CsBr:Eu 2+ crystals, having intense photostimulated luminescence (PSL), are promising storage phosphors for visualization of x-ray images [1,2]. The optical and luminescent properties of CsBr:Eu 2+ crystals synthesized by different technologies have been studied in [1][2][3][4][5]. It has been shown that regardless of the chemical composition of the activator (EuBr 2 , EuBr 3 ⋅8.9 H 2 O, EuOBr, Eu met ) in the CsBr mix, the europium ions are incorporated only in the divalent state into single crystals grown from the melt or into powder samples obtained by solid-phase synthesis. Another feature of incorporation of Eu 2+ ions into a CsBr crystal lattice is both formation of Eu 2+ -V Cs isolated dipole centers (IDC) [3] and, under certain conditions (activator concentrations and annealing temperatures), formation of aggregate centers (AC) of the CsEuBr 3 nanocrystal type [4,5].Upon excitation of CsBr:Eu 2+ crystals by UV light from the absorption region of Eu 2+ ions (λ max = 250 nm and 350 nm), isolated dipole centers and aggregate centers luminesce in bands with maxima at 434 nm and 523 nm. Typically the concentration of aggregate centers in CsBr:Eu 2+ crystals depends both on the activator concentration in the mix and on the crystal annealing temperature [4,5]. For annealing temperatures of 180 o C-200 o C, the concentration of aggregate centers in CsBr:Eu +2 crystals increases; while for T ≥ 250 o C degradation of the aggregate centers occurs [5]. This result correlates well with the data in [2] on the effect of annealing temperature for CsBr:Eu 2+ powders on the intensity of their photostimulated luminescence. According to [2], annealing of powders at T = 180 o C-200 o C doubles the intensity of photostimulated luminescence and thus improves the storage properties of the phosphor. Obviously such a result is due to formation of aggregate luminescence centers in the CsBr:Eu 2+ crystals.Aggregate luminescence centers in the form of CsPbCl 3 nanocrystals in a CsCl:Pb matrix were identified and studied for the first time in [6]. Later...

Bright Blue Emission Lead-Free Halides with Narrow Bandwidth Enabled by Oversaturated Europium Doping

“…The absorption peaks observed at around 340 nm can be attributed to the exciton absorption of the Eu 2+ -V Cs dipole. 40 When the reaction time was constant (see NC-10, NC-10-2, and NC-10-6), the intensity of exciton absorption increased with more europium oleate added, indicating that a larger amount of europium oleate could promote doping of Eu 2+ in NCs. When Eu(OA) 3 feeding ratio was fixed (see NC-1, NC-5, NC-10, NC-20, and NC-30), the intensity of exciton absorption reached a maximum value at 10 min and then gradually declined as the reaction extended.…”

“…The UV–vis spectra are shown in Figure b. The absorption peaks observed at around 340 nm can be attributed to the exciton absorption of the Eu 2+ -V Cs dipole . When the reaction time was constant (see NC-10, NC-10-2, and NC-10-6), the intensity of exciton absorption increased with more europium oleate added, indicating that a larger amount of europium oleate could promote doping of Eu 2+ in NCs.…”