Talk "Squeezing the energy out of the traps" presented at the 18th International Conference on Luminescence (ICL), August 31 2017. Describes a validated method to estimate the number of traps in storage phosphors, by embedding the phosphor in a polymer layer and counting the emitted photons during the afterglow decay. Work performed at LumiLab (Ghent University) and Concordia University.

1. ugent.be lumilab.ugent.be
Squeezing the energy out of the traps
18th International Conference on Luminescence – ICL2017
João Pessoa – Aug 27-Sep 01 2017
Philippe F. Smet, David Van der Heggen, Jonas J. Joos,
Diana C. Rodríguez Burbano, John A. Capobianco
philippe.smet@ugent.be
@pfsmet
1
@UGentLumiLab

2. The game changing persistent phosphor: SrAl2O4:Eu,Dy
Van den Eeckhout K. et al, Materials 3 (2010) 2536-2566
Persistent Luminescence in Eu2+-Doped Compounds: A Review
1995: SrAl2O4:Eu,Dy paper
by Matsuzawa et al.

3. TRAPS
@pfsmet
Density?
Number of traps per (g - mol - cm³ - dopant ion)
Energy storage capacity ( J / g )

4. Whatever the application ( ),
energy storage capacity is crucial.
Electrical battery
407 J/g (alkaline)
Limited number of cycles
Optical battery
?? J/g (persistent phosphor)
No degradation

5. Materials 10 (2017) 867
Counting the photons: determining the absolute trapping capacity of persistent phosphors

6. Materials 10 (2017) 867
Counting the photons: determining the absolute trapping capacity of persistent phosphors

7. Materials 10 (2017) 867
Counting the photons: determining the absolute trapping capacity of persistent phosphors
All particles excited
Low intensity
Low mass
Only top particles excited
High intensity
High (overestimated) mass
All particles excited
Moderate intensity
Moderate mass

8. Materials 10 (2017) 867
Counting the photons: determining the absolute trapping capacity of persistent phosphors
SrAl2O4:Eu2+,Dy3+ in a polymer layer on glass substrate
20 wt% solution of kraton FG1901X in 3:1 toluene-to-methyl ethyl ketone (MEK)

9. Materials 10 (2017) 867
Counting the photons: determining the absolute trapping capacity of persistent phosphors
Determining the number of photons
Luminous efficacy
(lum/W)
Average photon
energy
AreaLuminance decay
(cd/m²)

10. Materials 10 (2017) 867
Counting the photons: determining the absolute trapping capacity of persistent phosphors
Extrapolating the decay curves.
1h – 65%
10h – 80%

11. Materials 10 (2017) 867
Counting the photons: determining the absolute trapping capacity of persistent phosphors
Measuring 22 phosphor disks with different loading.

12. Materials 10 (2017) 867
Counting the photons: determining the absolute trapping capacity of persistent phosphors
Influence of the excitation intensity?

13. Materials 10 (2017) 867
Counting the photons: determining the absolute trapping capacity of persistent phosphors
Determining the linear regime: diffuse transmission
Ruleofthumb:50%T

14. Materials 10 (2017) 867
Counting the photons: determining the absolute trapping capacity of persistent phosphors
On the role of the excitation source and the binding material.

15. Materials 10 (2017) 867
Counting the photons: determining the absolute trapping capacity of persistent phosphors
The numbers!
(1.57 ± 0.03) × 1017 photons per gram

16. Adv. Opt. Mater. 2015, 3, 551–557
Persistent and Photostimulated Red Emission in CaS:Eu2+,Dy3+ Nanophosphors
A CaS:Eu nanophosphor.
7.3 × 1014 photons per gram
(+OSL: 1.2 × 1015 photons per gram)

17. Materials 10 (2017) 867
Counting the photons: determining the absolute trapping capacity of persistent phosphors
The numbers for SrAl2O4:Eu2+,Dy3+
(1.57 ± 0.03) × 1017 photons per gram
OR
Number of traps*: 1,6% of the europium ions
*Including deep traps (via TL) and QE correction for afterglow

18. Blasse G., Journal of Luminescence 72-74 (1997) 129
Classical phosphors: A Pandora’s box
So there is hope!

19. The numbers into context.
XANES
(X-ray Absorption Near Edge Spectroscopy)
Energy (eV)
Charged - Uncharged
2+ 3+
europium
Similar change in Eu2+ - Eu3+
Physical Review B 84 (2011) 85140
Luminescence and X-ray absorption measurements of persistent SrAl2O4:Eu,Dy powders: Evidence for valence state changes

20. A hopeful start…
cao trapping
Charging CaAl2O4:Eu,Nd Afterglow
… > 72 hours
Initially,
every excitation leads to trapping

22. … undone by a competiting detrapping mechanism.
Optically stimulated
detrapping
2 photons absorbed,
only 1 emitted!
Optically stimulated detrapping during charging of persistent phosphors, Tydtgat et al,
Opt. Mater. Express 6 (2016) 844-858

23. Materials 10 (2017) 867
Counting the photons: determining the absolute trapping capacity of persistent phosphors
The method in brief.
weigh A small amount of the phosphor
disk Prepare homogeneous phosphor disks with an appropriate binder
T% Measure the diffusion transmission to verify the linear regime
decay Measure the persistent luminescence curve
-bkg Carefully correct for the background
calc Extrapolate, integrate & divide

24. A call.

25. Glowing roads
I (cd/m²)
Time after sunset (h)
∆∆∆∆T = 0°C
∆∆∆∆T = -10°C
∆∆∆∆T = -20°C
Botterman et al, Optics Express 23 (2015) A868
Persistent phosphor SrAl2O4:Eu,Dy in outdoor conditions: saved by the trap distribution

26. @pfsmet
C.-N. Xu et al. N. Terasaki and C.-N. Xu, IEEE Sens. J., 2013, 13, 3999.
Mechanoluminescence
BaSi2O2N2:Eu

27. 2cm
Visualization of ultrasound pressure fields
Kersemans et al, Applied Physics Letters 107 (2015) 234102
Fast Reconstruction of a Bounded Ultrasonic Beam using Acoustically induced Piezoluminescence
BaSi2O2N2:Eu2+

28. Resources and upcoming events.
Persistent Phosphors
Handbook on the Physics and Chemistry of Rare Earths
Volume 48, Chapter 274 (2015) 1 - 108
4th PPP International Workshop
April 4-8, 2018 – Beijing, China.
http://physics.buaa.edu.cn/ppp_4/home/index.htm

29. Thank you for your attention
and feedback!
Presentations can be found at http://www.slideshare.net/pfsmet
@pfsmet @UGentLumiLab
… and samples?
philippe.smet@ugent.be