Talk presented at the ICIQ seminar in Tarragona, June 10 2022. Abstract: Persistent phosphors are typically impurity doped luminescent materials that feature an energy storage step between the excitation (e.g. by exposure to daylight or UV) and the emission of light. This energy storage, which can last from seconds to days, is rooted in charge carriers being trapped at particular defects in the material. Thermal energy is then required to release the charge carriers, after which recombination and light emission occurs. In this seminar, I will highlight the state-of-the-art in the field, by showing the unique characteristics of current applications (like in safety signage or bioimaging) and the remaining challenges. The physico-chemical mechanisms of the trapping and detrapping processes are discussed, along with the key analytical and spectroscopic techniques applied to those materials. In the second part, starting from those recent insights, persistent phosphors are turned into a versatile sensing platform for mechanical pressure, ultrasound, temperature, or optical and X-ray dosimetry. For each application, the relevant mechanisms are highlighted, as well as the remaining hurdles.

1. Philippe Smet
philippe.smet@ugent.be
@pfsmet
http://LumiLab.UGent.be
@UGentLumiLab
Upgrading glow-in-the-dark phosphors
to a platform for sensing
Presented at ICIQ Seminar, Tarragona, June 10 2022

2. Today’s programme
Part 1 – What are persistent luminescent materials?
(aka glow-in-the-dark phosphors)
Part 2 – Defects are key.
Part 3 – A platform for sensing.

3. Phosphors are great!
(… and full of defects)

4. What are luminescent materials?
Ce3+ in YAG (Y3(1-x)Ce3xAl5O12)
Ce3+ ([Xe]4f)
Ce3+ ([Xe]5d)

5. Tunability of Eu2+ based phosphors
Luminescent materials
Journal of Luminescence (2003) 104, p. 239
Green phosphor SrGa2S4:Eu2+
excitation
emission

6. Applications
• Solid state lighting (fluorescent, LEDs…) and displays
• Lasers (including fiber lasers, Nd3+, Er3+…)
• Scintillators (radiation detection, medical imaging)
• X-ray dosimetry and imaging
• Upconversion, luminescence thermometry
• …

7. Red phosphors for displays and lighting
Excitation
Moon et al. , Opt. Mater. Express, 2016, 6, 782.
Sijbom et al., Opt. Mater. Express, 2017, 7, 3332.
Emission
Eu2+ decay time: approx. 1 µs, Mn4+ decay time: approx. 10 ms

8. Brief intermezzo
Lisa Martin

9. SEM-Cathodoluminescence for phosphor research
Temperature
stage
EDX detector
Electron detectors
Spectrograph
ICCD
SEM chamber
Optical fiber
Beam blanker
Pulse
generator

10. 51200 spectra
SEM-Cathodoluminescence for phosphor research
Poelman and Smet, Physica B 439 (2014) 35–40
Time resolved microscopic cathodoluminescence spectroscopy for phosphor research

11. 5µm
Total CL intensity
Peak emission wavelength (nm) FWHM nm)
SrGa2S4:Eu2+
SEM-Cathodoluminescence for phosphor research

12. 5µm
Recording consecutive CL mappings, at different temperature
SEM-EDX-Cathodoluminescence for phosphor research
Impact on phosphor research: doping homogeneity in SrGa2S4:Eu2+
Martin et al., ECS Journal of Solid State Science and Technology, 7 (1) R3052-R3056 (2018)
Microscopic Study of Dopant Distribution in Europium Doped SrGa2S4: Impact on Thermal Quenching and Phosphor Performance
Elevated Eu concentration via EDX

13. Part 1 – What are persistent
luminescent materials?

14. Inside energy storage phosphors
Eu2+
exc
em trap/defect
thermal barrier DE
Persistent phosphors for the future: Fit for the right application
Journal of Applied Physics 128, 240903 (2020); https://doi.org/10.1063/5.0032972

15. The early days – ZnS:Cu,Co
The disruptive compound
SrAl2O4:Eu,Dy ...
From LEGO’s Black Knight’s Castle (1992)
(Patent Nemoto 1994)
An example of energy storage phosphors: persistent phosphors

16. Inside energy storage phosphors
s = 1012 Hz
E = 0,8 eV
T = 263…293K

17. Glow-in-the-dark road marks
(Oss, NL, 2013, by Studio Roosegaarde)
Large scale applications of energy storage phosphors
Glowing bicycle path
(2017, Poland)

18. Botterman et al, Optics Express 23 (2015) A868
Persistent phosphor SrAl2O4:Eu,Dy in outdoor conditions: saved by the trap distribution
SrAl2O4:Eu,Dy - Brightness curve
0.3 mcd/m²
(100x limit eye sensitivity)
T = 20°C
T = 30°C
T = 0°C

19. Botterman et al, Optics Express 23 (2015) A868
Persistent phosphor SrAl2O4:Eu,Dy in outdoor conditions: saved by the trap distribution
T(°C)
TL
intensity
7h wait, TL
immediate TL
7h wait, TL
immediate TL
Tcharge
0°C
30°C
SrAl2O4:Eu,Dy - Saved by the trap distribution
• Range of defects
• Each type has own
trap depth
= Trap depth range
• Interaction?

20. Applications
• Signage, night-long illumination
• Counterfeiting
• Solar blind phosphors (deep UV emission), photocatalysis…
• (Infra)red emitting phosphors (bio-imaging)
• …

21. Can we go nano?
ZnGa2O4:Cr3+
Richard et al.
Theranostics 2016; 6(13):2488-2524.
DOI: 10.1002/ejic.201700841

22. b-Na(Gd,Lu)F4:Tb3+
Van der Heggen, Cooper, Tesson, Joos, Seuntjens,
Capobianco, Smet, Nanomaterials 2019, 9, 1127.
Can we go nano?

23. Nanomaterials 2019, 9, 1127.

24. Part 2 – Defects are key!
(but how to study them?)
dr. Jonas Joos

25. Trapping defects: “intrinsic” defects, co-dopants…
• Driven by synthesis methods
(e.g. reducing atmosphere)
• Some guidelines for co-dopants
(systematics, Dy3+ in case of Sr2+…)
• A lot of ‘optimizations’

26. • XANES (HERFD) at ID26 (ESRF):
high x-ray intensity, small spot size
o X-rays ‘charge’ the sample within estimated 0.1s
o Beam damage for long measurements
Identification of Dy3+/Dy2+ as Electron Trap in Persistent Phosphors
Phys. Rev. Lett. 125, 033001 (2020)

27. Rotating sample (11mm diameter)

30. Adding a spectrometer
Identification of Dy3+/Dy2+ as Electron Trap in Persistent Phosphors
Phys. Rev. Lett. 125, 033001 (2020)

33. 33
Trap filling
Detrapping

34. Adding pump sources
Blue, near-UV
Infrared

36. vs.
Limited trapping (few %) Optically simulated detrapping
Identification of Dy3+/Dy2+ as Electron Trap in Persistent Phosphors
Phys. Rev. Lett. 125, 033001 (2020)

37. Part 3 – Sensing platform
Pressure, sound, light
dr. Ang Feng
Robin Petit

38. C.-N. Xu et al. N. Terasaki and C.-N. Xu, IEEE Sens. J., 2013, 13, 3999.
Mechanoluminescence
BaSi2O2N2:Eu2+
Adv. Mater. 2015, 27, 2324–2331

39. Materials 2018, 11, 484

40. Model systems: dog bone shaped coupon (epoxy + ML particles)
Uniform
Stress concentration
around the hole
100
2
12.5
25
BaSi2O2N2 :Eu2+
ML particles
(d < 30 μm)
ML particle 3 wt%
40
[SiON3]
Eu Ba

41. Mechanoluminescence: a memory effect
Drag

42. Mechanoluminescence: a memory effect (P-MEM)
WRITE READ
INITIALIZE (fill traps) < - - up to 72 h - - >
Drag
Adding memory to pressure-sensitive phosphors
Light: Science & Applications volume 8, Article number: 124 (2019)

43. P-MEM

44. P-MEM

45. P-MEM

46. P-MEM

47. WRITE
READ
P-MEM

48. Mechanoluminescence: a memory effect

49. Part 3 – Sensing platform
Pressure, sound, light
dr. Simon Michels

50. 50
Charging
Afterglow
Ultrasound

51. Detection
in transmission
The APL setup
(acoustically produced luminescence)
Transducer Optimal composition:
80 wt.% PDMS
20 wt.% BaSi2O2N2:Eu2+

52. APL (photocamera)
~1 minute
Hydrophone scanning
~30 minutes
Fast and High-Resolution Ultrasound Pressure Field Mapping Using Luminescent Membranes
Advanced Optical Materials, 9 (2021) 2100085

53. Thermographic camera
Phosphor disk sample
Transducer

54. Is APL caused by heat or by deformation?
42

55. Is APL caused by heat or by deformation?
44

56. Ultrasound
pressure
Heat APL (TL)
emission

57. Part 3 – Sensing platform
Pressure, sound, light
Verena Fritz
dr. David Van der Heggen

59. Key issues
• Stable and optically accessible traps
• No long term fading
• No cross-talks between traps
• High storage capacity

60. A Standalone, Battery-Free Light Dosimeter for Ultraviolet to Infrared Light
Advanced Functional Materials, 32 (2022) 2109635

61. Conclusions

62. The importance (and complexity) of traps (and the distributions)
OSL as a common loss mechanism (2 for 1)
(Emerging) applications
Nature of defects
v
v
v

63. Conclusion: energy storage phosphors are great!
Don’t expect the impossible
(e.g. for powering solar panels at night)
Many safety applications within reach
(with a little push and optimization)
Make energy storage phosphors nano/smart/…
(and use them as advanced sensors)

64. I am looking forward to
your feedback!
Presentations can be found at http://www.slideshare.net/pfsmet
@pfsmet @UGentLumiLab
philippe.smet@ugent.be