Revealing trap depth distributions in persistent phosphors

Abstract

Persistent luminescence or afterglow is caused by a gradual release of charge carriers from trapping centers. The energy needed to release these charge carriers is determined by the trap depths. Knowledge of these trap depths is therefore crucial in the understanding of the persistent luminescence mechanism. Unfortunately, the trap depths in persistent phosphors are often difficult to evaluate in an accurate and reliable way. The existing analysis methods are mostly based on single experiments, or they ignore the possibility of a continuous distribution of trap depths. We present a procedure to accurately probe the activation energies, even in the presence of a continuous distribution of energy levels. By performing a series of thermoluminescence experiments with varying excitation duration and at varying excitation temperature, and employing the initial rise analysis method, the depth and shape of such a distribution can be estimated. As an example, we investigated the trap system in the violet persistent phosphor CaAl 2 O 4 :Eu,Nd, and show that it consists of a Gaussian-shaped distribution of trap depths. The maximal density of traps lies in the region around 0.9 eV, but the distribution extends to 0.7 eV on the shallow side and 1.2 eV on the deep side. The described procedure can be used to obtain a clear view of the trap system in other persistent phosphors as well. This can lead to a better understanding of the nature of these trapping centers, and the role they play in the persistent luminescent mechanism.

Persistent luminescence in nitride and oxynitride phosphors: A review

“…When exciting Ba 2 Si 5 N 8 :Eu with ultraviolet light at 285nm, the thermoluminescence glow peaks shift to higher temperature for higher excitation temperature. This is compatible with the presence of a trap distribution or multiple overlapping traps, rather than a single discrete trap [21]. At higher excitation temperature, the shallower traps are continuously being emptied and a shift of the TL glow curves to higher temperature is observed.…”

“…By thermally assisted detrapping or quantum tunneling effects, the charges can then move to a recombination center to yield the delayed luminescence [17][18][19]. Several aspects of this trapping and detrapping process in persistent phosphors are still under debate, mainly on the (chemical) nature of the traps and the trapping/detrapping pathways [20,21]. Consequently, the optimization of persistent phosphors and the discovery of new ones is often still a matter of trial-and-error.…”

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

“…This specific decay behavior has been linked to either a detrapping mechanism from isolated traps via tunneling processes; 21 or an indication for the presence of a broad trap distribution in the phosphor. 23,24,27,28 In general, one can distinguish between the two processes by checking the temperature dependence of the afterglow and thermoluminescence behavior. Figure 3 shows a recorded TL curve for LGO:Cr after charging at 20…”

Lu2CaMg2(Si1−xGex)3O12:Ce3+solid-solution phosphors: bandgap engineering for blue-light activated afterglow applicable to AC-LED

“…S5a, c and e (ESI †). The measurement details and the involved physical pictures have been previously elucidated by Eeckhout et al 48 For each measurement, the sample was cooled/heated to a given T exc , irradiated with 460 nm light for 5 min, delayed at a time interval of 3 min, and then measured starting from T exc at a heating rate of 1 K s…”

“…This method is able to efficiently reveal the shallowest occupied electron trap depth in the host, regardless of the order of the kinetics involved in the detrapping processes. 48,51,52 Making assumptions that the concentration of trapped electrons on the low-temperature side of a TL glow curve remains relatively constant, TL intensity (I(T)) can be approximately expressed as:…”