We described the positive (trapping and energy storage) and negative (trap depletion) effects of exciting persistent or glow-in-the-dark phosphors, by combining experimental input and modelling. The trapping process is clearly more dynamic and non-linear than previously realised.
Talk presented at the PRE'16 conference on the photoluminescence of rare earth ions, organized by Clemson U in Greenville, South Carolina, US (June 8-10, 2016)
Optical detrapping in persistent phosphors - talk at PRE'16
1. ugent.be lumilab.ugent.be nb-photonics.ugent.be
Optically stimulated detrapping in persistent phosphors
PRE ‘16 – Photoluminescence in Rare Earths: Photonic Materials and Devices
Greenville, SC – June 8 2016
Philippe F. Smet
Claude Tydtgat, Katrien Meert, Jonas Botterman, Katleen Korthout, Koen Van den
Eeckhout, Dirk Poelman, Mathias Kersemans, Simon Michels, Jonas J. Joos
philippe.smet@ugent.be
@pfsmet
1
and mechanically
V
2. Motivation
The disruptive SrAl2O4:Eu,Dy led to many products…
Van den Eeckhout K. et al, Materials 3 (2010) 2536-2566
Persistent Luminescence in Eu2+-Doped Compounds: A Review
4. I (cd/m²)
Time after sunset (h)
Motivation
It made us dream about a green, glowing world
DT = 0°C
DT = -10°C
DT = -20°C
Botterman et al, Optics Express 23 (2015) A868
Persistent phosphor SrAl2O4:Eu,Dy in outdoor conditions: saved by the trap distribution
5. Motivation
… totally new persistent phosphors were developed
(which are sometimes not even visible to the human eye)
T. Maldiney et al., Nat. Mater. 13, 418–426 (2014), “The in vivo activation of persistent
nanophosphors for optical imaging of vascularization, tumours and grafted cells”
6. Motivation
… novel applications were realized
Kersemans et al, Applied Physics Letters 107 (2015) 234102
Fast Reconstruction of a Bounded Ultrasonic Beam using Acoustically induced Piezoluminescence
Acoustically induced PiezoLuminescence (APL)
BaSi2O2N2:Eu
BaSi2O2N2:Eu
7. Motivation
… novel applications were realized
Acoustically induced PiezoLuminescence (APL)
Kersemans et al, Applied Physics Letters 107 (2015) 234102
Fast Reconstruction of a Bounded Ultrasonic Beam using Acoustically induced Piezoluminescence
BaSi2O2N2:Eu
10. The experiment for SrAl2O4:Eu,Dy...
Trapping and detrapping: the big question
Charging Afterglow TL
Cannot be fitted with
single exponentials, at any T
Build-up in signal Trap distribution!
Jonas Botterman et al, Physical Review B 90, 085147 (2014)
Trapping and detrapping in SrAl2O4:Eu,Dy persistent phosphors: Influence of excitation…
193K
353K
11. Maximizing trap filling in SrAl2O4:Eu,Dy?
Trapping and detrapping: the big question
lexc = 370nm
Jonas Botterman et al, Physical Review B 90, 085147 (2014)
Trapping and detrapping in SrAl2O4:Eu,Dy persistent phosphors: Influence of excitation…
afterglow
12. Maximizing trap filling in SrAl2O4:Eu,Dy at low T?
Trapping and detrapping: the big question
lexc = 370nm
Jonas Botterman et al, Physical Review B 90, 085147 (2014)
Trapping and detrapping in SrAl2O4:Eu,Dy persistent phosphors: Influence of excitation…
13. Trapping probability is very, very high.
Trapping and detrapping: the big question
Charging CaAl2O4:Eu,Nd
14. Trapping and detrapping: the big question
How can we unite
• an (initially) very high trapping efficiency
• a (relatively) low storage capacity
?
One possible approach:
Modelling trapping AND detrapping simultaneously
15. Eu2+ + RE3+ < --- > Eu3+ + RE2+ Trapping at non-RE defects
- strong evidence in e.g. YPO4:Ce,RE - - ‘intrinsic’ afterglow in SrAl2O4:Eu -
Models for Eu2+/Ce3+ doped persistent phosphors
P. Dorenbos,
J. Electrochem. Soc. 2005, 152, H107–H110
F. Clabau et al.
Chem. Mat. 2005, 17, 3904–3912
LOCAL
GLOBAL
16. Modeling trapping and detrapping
Setting up the model.
Eu2+
Trap + e
Eu3+
Empty trap
Detrapping
Claude Tydtgat et al, Optical Materials Express 6 (2016) 844-858
Optically stimulated detrapping during charging of persistent phosphors
17. Modeling trapping and detrapping
pnr
• Boundary conditions for charging and afterglow
• pe (excitation rate) is small
• Two solutions l1 and l2: two exponentials
Differential equations.
Eu2+ trap
18. Modeling trapping and detrapping
• pe 0 : Eigenvalues for charging and afterglow are
• Solution for charging:
charging
afterglow
19. Modeling trapping and detrapping
Simplified system (one site) – Sr2MgSi2O7:Eu,Dy
Charging | Afterglow | TL analysis
20. Modeling trapping and detrapping
Adding distribution for trap depths (no single exp. decay)
Claude Tydtgat et al, Optical Materials Express 6 (2016) 844-858
Optically stimulated detrapping during charging of persistent phosphors
21. Modeling trapping and detrapping
Problem #1
Step in charging curve
<> Step for afterglow
Charging | Afterglow | TL analysis
22. Modeling trapping and detrapping
Problem #2
Eigenvalues are identical for different pe
<> Charging dynamics strongly dependent on pe
Charging | Afterglow | TL analysis
23. Modeling trapping and detrapping
Problem #3
Absorption pe(M-me-m)
<> Absorption increases, depends on pe
24. Introducing OSL
Influence of excitation rate pe not negligible, on the contrary!
Large influence, yet pe is low
Effect is proportional to pe x a
Claude Tydtgat et al, Optical Materials Express 6 (2016) 844-858
Optically stimulated detrapping during charging of persistent phosphors
28. Introducing OSL
It makes sense for charging and decharging…
Intensity
a = 200
Claude Tydtgat et al, Optical Materials Express 6 (2016) 844-858
Optically stimulated detrapping during charging of persistent phosphors
29. Introducing OSL
… and for the influence of the excitation intensity
Claude Tydtgat et al, Optical Materials Express 6 (2016) 844-858
Optically stimulated detrapping during charging of persistent phosphors
30. Introducing OSL
‘Proving’ OSL
0
10000
20000
30000
-50 -25 0 25 50 75 100
TLintensity(arb.units)
Temperature (°C)
(0. All traps emptied)
1. Excitation at 0°C (2500s)
2. Cooling to -60°C
3. Waiting
4. Collecting TL glow curve
Claude Tydtgat et al, Optical Materials Express 6 (2016) 844-858
Optically stimulated detrapping during charging of persistent phosphors
31. Introducing OSL
‘Proving’ OSL
0
10000
20000
30000
-50 -25 0 25 50 75 100
TLintensity(arb.units)
Temperature (°C)
(0. All traps emptied)
1. Excitation at 0°C (2500s)
2. Cooling to -60°C
3. Waiting
4. Collecting TL glow curve
(0. All traps emptied)
1. Cooling to -60°C
2. Excitation at -60°C (2500s)
3. Collecting TL curve
Claude Tydtgat et al, Optical Materials Express 6 (2016) 844-858
Optically stimulated detrapping during charging of persistent phosphors
32. Introducing OSL
‘Proving’ OSL
0
10000
20000
30000
-50 -25 0 25 50 75 100
TLintensity(arb.units)
Temperature (°C)
(0. All traps emptied)
1. Excitation at 0°C (2500s)
2. Cooling to -60°C
3. Waiting
4. Collecting TL glow curve
(0. All traps emptied)
1. Cooling to -60°C
2. Excitation at -60°C (2500s)
3. Collecting TL curve
(0. All traps emptied)
1. Excitation at 0°C (2500s)
2. Cooling to -60°C
3. Excitation at -60°C (2500s)
4. Collecting TL curve
Claude Tydtgat et al, Optical Materials Express 6 (2016) 844-858
Optically stimulated detrapping during charging of persistent phosphors
33. Introducing OSL
The numbers…
a = 200, reasonable cross-section compared to Eu2+.
Temperature (K)
Energy(eV)
Shallower trap
Deeper trap
s = 1E14Hz
thermal quenching barrier
thermal barrier for trapping
34. Conclusions
OSL (at charging l) is compatible with:
• Different initial rise and drop after charging
• Exponentials for charging and afterglow are different
• Absorption increases during charging
• Influence of excitation intensity
• Charging behaviour depends on wavelength (via abs)
• Different trap filling for different wavelengths
This is worrying for applications…
… and requires careful study!
35. Resources
Feature issue on Persistent and Photostimulable Phosphors
in Optical Materials Express (published)
http://tiny.cc/OMEXPPP
37. Resources & Acknowledgments
Thank you for your attention !
(and your feedback…)
Presentation can be found at http://www.slideshare.net/pfsmet
Editor's Notes
Comparison of afterglow characteristics measured after 10 min exposure to 200 lx of D65 light. A: SrAl2O4:Eu2+, B: SrAl2O4:Eu2+,Dy3+, C: SrAl2O4:Eu2+,Nd3+, D: ZnS:Cu,Co. (Reprinted with permission from [5]. Copyright 1996, The Electrochemical Society).
Comparison of afterglow characteristics measured after 10 min exposure to 200 lx of D65 light. A: SrAl2O4:Eu2+, B: SrAl2O4:Eu2+,Dy3+, C: SrAl2O4:Eu2+,Nd3+, D: ZnS:Cu,Co. (Reprinted with permission from [5]. Copyright 1996, The Electrochemical Society).
Comparison of afterglow characteristics measured after 10 min exposure to 200 lx of D65 light. A: SrAl2O4:Eu2+, B: SrAl2O4:Eu2+,Dy3+, C: SrAl2O4:Eu2+,Nd3+, D: ZnS:Cu,Co. (Reprinted with permission from [5]. Copyright 1996, The Electrochemical Society).