Plenary talk presented at the PRE19 workshop (Photoluminescence in Rare Earths: Photonic Materials and Devices) in Nice, France, on September 4. Dealing with persistent luminescence, afterglow, mechanoluminescence, traps, defects and thermoluminescence. Overview of the activities of the LumiLab research in the past 10 years.

1. Philippe Smet
philippe.smet@ugent.be
@pfsmet
http://LumiLab.UGent.be
@UGentLumiLab
Defects in energy storage phosphors:
friends or enemies?
Presented at PRE’19, Nice, September 4 2019.
#PRE19conf

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

3. Today’s programme
Part 1 – Defects, your friends in energy storage phosphors
Part 2 – Or… enemies? It’s complicated.
Part 3 – Know Your Defects!

4. Jiaren – NIR persistent phosphors
Ying – thermoluminescence
David – Metrology, TL
Simon – Ultrasound imaging
Jieqi - OSL
José – persistent phosphors
Ang - ML
Prof. Dirk PoelmanCredits

5. Lisa – SEM-CL spectroscopy Jonas – ab initio calculations
Robin – ALD coating of QDsReinert – Phosphor stability
Andreas – SmS thin films
Ewoud – photocatalysis

6. Trailer 1 – Red Mn4+ based fluoride phosphors
Excitation
Moon et al. , Opt. Mater. Express, 2016, 6, 782.
Sijbom et al., Opt. Mater. Express, 2017, 7, 3332.
Optical properties are excellent… But what about stability?
Emission

7. Trailer 1 – Red Mn4+ based fluoride phosphors
Inorganic coatings using
Atomic Layer Deposition
for long term stability
Stabilizing Fluoride Phosphors: Surface Modification by Atomic Layer Deposition, Verstraete et al., Chem. Mater. 2019.

8. Trailer 2 - Cathodoluminescence for phosphor research
Temperature
stage
EDX detector
Electron detectors
Spectrograph
ICCD
SEM chamber
Optical fiber
Beam blanker
Pulse
generator

9. 51200 spectra
Trailer 2 - Cathodoluminescence for phosphor research
Poelman and Smet, Physica B 439 (2014) 35–40
Time resolved microscopic cathodoluminescence spectroscopy for phosphor research

10. 5µm
Total CL intensity
Peak emission wavelength (nm) FWHM nm)
SrGa2S4:Eu2+
Trailer 2 - Cathodoluminescence for phosphor research

11. 5µm
Recording consecutive CL mappings, at different temperature
Trailer 2 - 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

12. Part 1 – Defects, your friends in
energy storage phosphors

13. 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

14. Inside energy storage phosphors
Eu2+
exc
em trap/defect
thermal barrier DE

15. A very simple picture…
A reservoir to store energy
Charge carriers
fill the traps

16. A very simple picture…
More traps filled
=
higher water level

17. A very simple picture…
A temperature controlled plug
Release, recombination… and emission of light!

18. A very simple picture…
When the reservoir empties,
the emission intensity reduces.

19. A very simple picture…
When temperature drops,
emission goes down.

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

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

22. 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

23. Botterman et al, Optics Express 23 (2015) A868
Persistent phosphor SrAl2O4:Eu,Dy in outdoor conditions: saved by the trap distribution
T(°C)
TLintensity
7h wait, TL
immediate TL
7h wait, TL
immediate TL
Tcharge
0°C
30°C
SrAl2O4:Eu,Dy - Saved by the trap distribution

24. A typical persistent phosphor has a trap depth distribution
At low temperature…
SHALLOW TRAPS DEEP TRAPS

25. A typical persistent phosphor has a trap depth distribution
At high temperature…
DEEP TRAPSSHALLOW TRAPS

26. Trap distributions are cool.

27. Factors driving the trap release:
 Trap depth
 Temperature
 IR radiation (OSL)
 Pressure

28. 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

29. Materials 2018, 11, 484

30. 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%
30
[SiON3]
Eu Ba

31. Spatial distribution of ML agrees
with the Von Mises stress
x
y
61.2-0.79
σxx
8.93-18.0
σyy
τxy
-15.1 15.4 59.66.98
σVM
𝜎 𝑉𝑀 = 3/2𝑠𝑖𝑗 𝑠𝑖𝑗
𝜎 𝑉𝑀 = 1/2 𝜎1 − 𝜎2
2 + 𝜎2 − 𝜎3
2 + 𝜎1 − 𝜎3
2
𝑠𝑖𝑗 = 𝜎𝑖𝑗 − Σ𝜎𝑖𝑖/3
𝜎𝑖 principal stress
Load = 1000 N,
Dimension: length 100 mm, thickness 2 mm, width 25 mm,
hole diameter: 3 mm
Materials: density 1.1 g/cm3, Young’s Modulus = 2.7 GPa
Finite Element Simulation
31

32. Mechanoluminescence: a memory effect
Drag

33. Mechanoluminescence: a memory effect (P-MEM)
WRITE READINITIALIZE (fill traps) < - - up to 72 h - - >
Drag

34. P-MEM

35. P-MEM

36. P-MEM

37. P-MEM

38. WRITE
READ
P-MEM

39. Mechanoluminescence: a memory effect

40. IRML
Mechanoluminescence: a memory effect
Transfer

41. b-Na(Gd,Lu)F4:Tb3+
Van der Heggen, Cooper, Tesson, Joos, Seuntjens, Capobianco, Smet,
Nanomaterials 2019, 9, 1127.

42. Nanomaterials 2019, 9, 1127.

43. Part 2
Or… enemies? It’s complicated.
@pfsmet

44. The problem: We want more light!
SrAl2O4:Eu,Dy as an example.

45. Botterman et al, Optics Express 23 (2015) A868
Persistent phosphor SrAl2O4:Eu,Dy in outdoor conditions: saved by the trap distribution
Overnight temperature drop?
I (cd/m²)
Time after sunset (h)
DT = 0°C
DT = -10°C
DT = -20°C
Traps become
thermally
inaccessible

46. Botterman et al, Optics Express 23 (2015) A868
Persistent phosphor SrAl2O4:Eu,Dy in outdoor conditions: saved by the trap distribution
Total brightness?
I (cd/m²)
Time after sunset (h)
DT = 0°C
DT = -10°C
DT = -20°C
1h
x10 is sufficient

47. Or… how many traps can be filled?
How to increase the storage capacity?
(by a factor of 10 !?)

48. For the same number of filled traps…
Limited number of traps, but all filled
High number of traps, but only few filled
MAX. LEVEL

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

50. A fully charged persistent phosphor can store per gram
about 200,000,000,000,000,000 electrons in traps
(i.e. ONLY a few % of the number of Eu2+ ions)
Number of traps?
Materials 10 (2017) 867
Counting the photons: determining the absolute trapping capacity of persistent phosphors

51. UV excitation ON
Persistent luminescence
… > 72 hours!
Emission
intensity
CaAl2O4:Eu2+,Nd3+
All traps empty.
Every excitation leads to trapping.
Many traps filled.
Most emission is PersL
UV excitation OFF

52. Trapping and detrapping
Excitation of Eu2+ centers
Photoluminescence
(5d – 4f)

53. J. Lumin. 131 (2011) 1465
Steady state
PL excitation
Trap filling

54. In Eu2+ doped persistent phosphors,
trap filling via excitation of Eu2+ is efficient.
Where is the bottleneck?

55. Quantum efficiency of SrAl2O4:Eu,Dy
Internal quantum efficiency:
𝜂𝑖𝑛 = 𝑁𝑒𝑚/𝑁𝑎𝑏𝑠
ACS Photonics 5 (2018) 4529–4537
Importance of Evaluating the Intensity Dependency of the Quantum Efficiency: Impact on LEDs and Persistent Phosphors

56. Trap induced optical absorptions (OSL)

57. But then we expect an intensity
dependent absorption!
Trap induced optical absorption, causing OSL

58. Trap induced optical absorption
𝝀 𝒆𝒙 (nm) 𝒂 𝑬𝒖 𝟐+ (cm²) 𝒂 𝒕𝒓 (cm²)
375 3.6 x 10-18 2.2 x 10-17
445 4.4 x 10-19 1.2 x 10-17
ACS Photonics 5 (2018) 4529–4537
Importance of Evaluating the Intensity Dependency of the Quantum Efficiency: Impact on LEDs and Persistent Phosphors

59. Local trapping, with OSL as detrapping Eu2+
Trap
Absorption
cross section
Cluster

60. Local trapping, with OSL as detrapping Eu2+
Trap
Absorption
cross section
Cluster
Absorption
& trapping

61. Local trapping, with OSL as detrapping Eu2+
Trap
Absorption
cross section
Cluster
Detrapping
& emission

62. Local trapping, with OSL as detrapping Eu2+
Trap
Absorption
cross section
Cluster
Fully charged phosphor,
this does not occur!
Why?

63. Local trapping, with OSL as detrapping Eu2+
Trap
Absorption
cross section
Cluster
OSL very likely,
due to large absorption
cross-section

64. Local trapping, with OSL as detrapping Eu2+
Trap
Absorption
cross section
Cluster
2-for-1 process

65. Huge reservoir… but it can’t be fully filled.

66. A road map for glowing roads
One key message:
Tailor phosphor design to application (e.g. charging wavelengths and efficiency)
Sustainable solution
Large phosphor quantities (!)
Limits light
pollution
Brightness increase needed
Limit losses
Optical design
Make full use of solar spectrum

67. On a side note:
This “non-radiative” path (2 for 1) is elusive,
yet impacting other phosphors

68. Loss mechanism in (high brightness) LED phosphors.
Flicker reduction in AC LEDs.
ACS Photonics 5 (2018) 4529–4537
Importance of Evaluating the Intensity Dependency of the Quantum Efficiency: Impact on LEDs and Persistent Phosphors

69. Part 3 – Know your defects.
(and question the obvious)
@pfsmet

70. What is the nature of the traps?
Example: Sr4Al14O25:Eu2+,Dy3+

71. X-ray Absorption Near Edge Spectroscopy (XANES)
0
1
2
6940 6965 6990 7015 7040
X-rayabsorption
Energy (eV)
Eu-LIII
Eu2+
Eu3+
71

72. 72

73. XANES on phosphor with “empty” traps
XANES on phosphor with “filled” traps
73

74. 2+3+ + 2+ 3++
Eu2+-Eu3+ Intervalence Charge Transfer (IVCT)
CaF2
Jonas Joos
Today, 14h20
Fundamentals & Theory
César Klein
Know your defects! (Advertisement)

75. Question the obvious.
Example: ultrasound induced luminescence in BaSi2O2N2:Eu2+

76. Imaging ultrasound pressure fields
BaSi2O2N2:Eu2+ (ML phosphor)
(APL)

77. 20
BaSi2O2N2:Eu2+ sample

78. Semi-transparent samples are needed
21
BaSi2O2N2:Eu2+ sample

79. 79

80. 27
0

81. 29
A quantitative reconstruction can be made by setting up
a calibration curve

82. 31
Hydrophone scan Calibrated APL image
Point measurement
~ 30 min
Full field imaging
~ 90 sec
4 mm 4 mm
A quantitative reconstruction can be made by setting up
a calibration curve

83. 29
But can we explain the calibration curve based on mechanoluminescence?

84. Thermographic camera
Phosphor disk sample
Transducer

85. Is APL caused by heat or by deformation?
42
𝐼 𝑈𝑆 = 1.9
𝑊
𝑐𝑚²
𝑇(𝑡

86. Is APL caused by heat or by deformation?
44
𝐼 𝑈𝑆 = 0.2
𝑊
𝑐𝑚²

87. Acoustically Produced Luminescence (APL)
Acoustically induced Piezo-Luminescence (APL)
via thermoluminescence

88. Conclusions

90. The importance (and complexity) of trap distributions
OSL as a common loss mechanism (2 for 1)
(Emerging) applications
Number and nature of defects

91. 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 smart
(and use them as advanced sensors)

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