Revealing trap depth distributions in persistent phosphors with a thermal barrier for charging

Abstract

The performance of persistent phosphors under given charging and working conditions is determined by the properties of the traps that are responsible for these unique properties. Traps are characterized by the height of their associated barrier for thermal detrapping, and a continuous distribution of trap depths is often found in real materials. Accurately determining trap depth distributions is hence of importance for the understanding and development of persistent phosphors. However, extracting the trap depth distribution is often hindered by the presence of a thermal barrier for charging as well, which causes a temperature-dependent filling of traps. For this case, we propose a method for extracting the trap depth distribution from a set of thermoluminescence (TL) curves obtained at different charging temperatures. The TL curves are first transformed into electron population functions via the Tikhonov regularization, assuming first-order kinetics. Subsequently, the occupation of the traps as a function of their depth, quantified by the so-called filling function, is obtained. Finally, the underlying trap depth distribution is reconstructed from the filling functions. The proposed method provides a substantial improvement in precision and resolution for the trap depth distribution compared with existing methods. This is hence a step forward in understanding the (de)trapping behavior of persistent and storage phosphors.

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“…[159][160][161] Additionally there is also a lack of physical grounds behind the use of higher-order kinetics to fit TL glow curves that result from an underlying trap distribution and hence this should be avoided. [162,163] Multiple measurement and analysis protocols have been developed over the years to extract trap depths and frequency factors of the corresponding traps in the material. [153,164] One approach is due to Hoogenstraaten [165,166] who proposed the variable heating rate method which relies on the shift of the glow peak maximum to a higher temperature with increasing heating rates.…”

“…When adopting the latter approach the effect of thermal barriers for trapping should be carefully considered. [ 163 ]…”

“…Additionally, also a thermal barrier for trapping might introduce a temperature‐ and wavelength‐dependent behavior. [ 163 ] A detailed study on the wavelength and temperature dependence of the trap filling by Hagemann et al. [ 177 ] and Botterman et al.…”

“…When adopting the latter approach the effect of thermal barriers for trapping should be carefully considered. [163] Spectrally resolved measurements are of special interest for phosphors such as SrAl 2 O 4 :Eu 2+ that are characterized by multiple emissions, each with a different thermal behavior (Section 3.1) or to identify contributions from impurity phases such as Sr 4 Al 14 O 25 :Eu,Dy. However, quite often the detection is achieved using photomultiplier tubes or photodiodes and only some rudimentary spectral information is obtained through the use of bandpass filters.…”

“…Additionally, also a thermal barrier for trapping might introduce a temperature-and wavelengthdependent behavior. [163] A detailed study on the wavelength and temperature dependence of the trap filling by Hagemann et al [177] and Botterman et al [59] showed that trapping is to a large extent thermally activated. [171] The size of this effect depends, however, on which site Eu 2+ is incorporated: excitation with wavelengths higher than 420 nm, exclusively exciting Eu Sr2 , only induces trapping above 170 K whereas excitation with wavelengths lower than 420 nm, exciting both Eu Sr1 and Eu Sr2 , induces trapping at lower temperatures as well.…”