Trapping and detrapping inSrAl2O4:Eu,Dypersistent phosphors: Influence of excitation wavelength and temperature

Abstract

SrAl 2 O 4 :Eu,Dy is presumably the best known persistent luminescent phosphor. At room temperature, its green emission remains visible for hours after switching off the excitation. It is known that upon lowering the temperature of the phosphor a second photoluminescence emission band arises in the blue part of the visible spectrum, although its origin is still the subject of discussion. In this paper we thoroughly study the origin of both emission bands in SrAl 2 O 4 :Eu,Dy and we attribute this to europium ions substituting for the two different Sr sites in the phosphor's monoclinic host lattice. The photoluminescence properties, the thermal quenching behavior, and photoluminescence lifetime of both emission bands are investigated. A lanthanide energy level scheme is constructed for both sites. Using an integrated approach, i.e., combining charging, afterglow, and thermoluminescence measurements in the same run, we study the charging or trap filling processes in SrAl 2 O 4 :Eu,Dy upon excitation with site selective excitation wavelengths and at different temperatures. We show that trap filling is a thermally activated process when the green emitting center is excited at 435 nm.Furthermore, we also demonstrate that the distribution of filled traps after charging depends strongly on the excitation wavelength and thus on which Eu 2+ center has been excited. This suggests trapping of the electron close to the ionized Eu 2+ ion, without full delocalization to the conduction band during the trapping process. Finally, the quantum efficiency of the persistent luminescence is estimated at 65 (+/−10)%.

Thermal and concentration dependent energy transfer of Eu^2+ in SrAl_2O_4

“…Hölsä et al [6] proposed the possibility of anomalous low-temperature luminescence, but the reported experimental data and theoretical calculations allow to conclude that the emission comes indeed from two different sites [4,7]. Nakazawa et al [8] observed the thermal quenching of SrAl 2 O 4 :Eu 2+ (doped with 1 mol%) but at much lower temperatures (100 K less) than all other publications.…”

“…The band gap of SrAl 2 O 4 is 6.6 eV [12]. The positions of the ground states can be estimated by the Dorenbos model [13], as it was done in [7] or it can be measured [14]. The europium ions in the two sites can be excited with UV-light.…”

Local, Temperature-Dependent Trapping and Detrapping in the LiGa5O8:Cr Infrared Emitting Persistent Phosphor

“…TL was recorded in a homebuilt setup, details of which can be found in Botterman et al 26 Before each TL measurement, the sample was thermally cleaned by heating to 220…”

Eu2+luminescence in strontium aluminates

“…Some authors assigned the 445 and 520 nm emission bands to emission of the Eu 2+ ion located at two nonequivalent strontium sites in the SrAl 2 O 4 host. 53,61 However, the two strontium sites in the SrAl 2 O 4 are very similar and differ only by a slight distortion of their square planes. 51 The similar coordination for both sites is expected to give rise similar luminescence properties due to comparable crystal field strengths and centroid shifts.…”