Counting the Photons: Determining the Absolute Storage Capacity of Persistent Phosphors

Abstract

The performance of a persistent phosphor is often determined by comparing luminance decay curves, expressed in cd/m2. However, these photometric units do not enable a straightforward, objective comparison between different phosphors in terms of the total number of emitted photons, as these units are dependent on the emission spectrum of the phosphor. This may lead to incorrect conclusions regarding the storage capacity of the phosphor. An alternative and convenient technique of characterizing the performance of a phosphor was developed on the basis of the absolute storage capacity of phosphors. In this technique, the phosphor is incorporated in a transparent polymer and the measured afterglow is converted into an absolute number of emitted photons, effectively quantifying the amount of energy that can be stored in the material. This method was applied to the benchmark phosphor SrAl2O4:Eu,Dy and to the nano-sized phosphor CaS:Eu. The results indicated that only a fraction of the Eu ions (around 1.6% in the case of SrAl2O4:Eu,Dy) participated in the energy storage process, which is in line with earlier reports based on X-ray absorption spectroscopy. These findings imply that there is still a significant margin for improving the storage capacity of persistent phosphors.

Broadband NIR photostimulated luminescence nanoprobes based on CaS:Eu2+,Sm3+ nanocrystals

“…Such a long PL rise time of CaS:Eu 2+ ,Sm 3+ NCs reveals the excellent energy storage capability of the NCs. 51,52 3.4. PSL properties of the annealed CaS:Eu 2+ ,Sm 3+ NCs Although the stored excitation energy in deep traps of CaS:Eu 2+ ,Sm 3+ NCs could not be thermally activated at RT, it can be effectively released upon irradiation with NIR light, resulting in intense PSL from the NCs.…”

Predicting the afterglow duration in persistent phosphors: a validated approach to derive trap depth distributions

“…This gives opportunities to use other techniques to empty these deeper traps, e.g. optically stimulated luminescence (OSL), 23,24 in order to increase the light output of the LGO:Cr phosphor. Conversely, the model shows that most of the stored energy is depleted by yielding afterglow.…”

“…Currently, the limited storage capacity of many persistent phosphors remains a significant drawback for their applicability in various fields. 24 Understanding the trap structure of persistent phosphors and modifying it deliberately can lead to a large increase of the amount of energy that can be stored and subsequently be accessed 'on demand' by e.g. photostimulation, 25 and hence, of the performance of these materials.…”

A Review of Mechanoluminescence in Inorganic Solids: Compounds, Mechanisms, Models and Applications

“…The load range in which the instantaneous ML intensity is linear to mechanical loads, or range of linearity in brief, is a good indicator of ML phosphors for the capability of sensing stress. Of course, the storage capacity (for example SrAl${}_2$O${}_4$:Eu${}^{2+}$, Dy${}^{3+}$ has a capacity of 1.6 × 10${}^{17}$ photons/g [60]) and the initial fraction of the traps filled by photon charging determine the intensity of the trap controlled ML. Saturation, which makes ML independent of trap filling, can be obtained by increasing the duration or the intensity of the excitation during the charging step.…”