Optical detrapping in persistent phosphors - talk at PRE'16
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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)
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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
- 3. Motivation
- 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
- 8. Motivation Whatever the application ( ), energy storage capacity is crucial. Where is the bottleneck?
- 9. The experiment... Variation 193K to 353K Trapping and detrapping: the big question
- 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
- 25. Introducing OSL Absorption cross-section Phosphor particle Eu2+ (ground state) Electron trap (empty) Excitation light
- 26. Introducing OSL Absorption cross-section Phosphor particle Eu3+ Electron trap Excitation light Eu2+ (ground state) Electron trap (empty)
- 27. Introducing OSL Absorption cross-section Phosphor particle Eu3+ Electron trap Eu2+ (ground state) Electron trap (empty)
- 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
- 36. Chapter Persistent Phosphors Handbook on the Physics and Chemistry of Rare Earths Volume 48, Chapter 274 (2015) 1 - 108
- 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).