ESTE2016 Detrapping in persistent phosphors

ugent.be lumilab.ugent.be nb-photonics.ugent.be

Persistent phosphors beyond the afterglow:optical and mechanical detrapping

ESTE2016 – 6th International Conference on Excited States of Transition Elements

Polanica Zdrój, Poland – Augustus 23 2016

Philippe F. SmetClaude Tydtgat, Dirk Poelman, Katrien W. Meert,

Simon Michels, Mathias Kersemans

philippe.smet@ugent.be

@pfsmet

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The game changing persistent phosphor: SrAl2O4:Eu,Dy

Van den Eeckhout K. et al, Materials 3 (2010) 2536-2566Persistent Luminescence in Eu2+-Doped Compounds: A Review

@pfsmet

@pfsmet

thermally, mechanicallyor optically drivenrelease

I (cd/m²)

Time after sunset (h)

Thermally driven (uncontrolled) release

DT = 0°CDT = -10°CDT = -20°C

Botterman et al, Optics Express 23 (2015) A868Persistent phosphor SrAl2O4:Eu,Dy in outdoor conditions: saved by the trap distribution

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Back of the envelope calculationAccessible traps: 10% of recombination centers2% molar doping by Eu in SrAl2O4

Alkaline battery: 407 J/g

Charging(filling of traps)

Decharging(emptying of traps)

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”

Botterman et al., Acta Materialia 60 (2012) 5494-5500Mechanoluminescence in BaSi2O2N2:Eu

BaSi2O2N2:Eu

Mechanically driven release

Whatever the application ( ),

energy storage capacity is crucial.

Where is the bottleneck?

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Charging CaAl2O4:Eu,Nd

The trapping probability is very, very high.

Afterglow

… > 72 hours

How can we unite

• a (relatively) low storage capacity

• a very high trapping efficiency

?

One possible approach:

Modelling trapping and detrapping simultaneously

The experiment... in order to understand (de)trapping

Variation 193K to 353K

Jonas Botterman et al, Physical Review B 90, 085147 (2014)Trapping and detrapping in SrAl2O4:Eu,Dy persistent phosphors: Influence of excitation…

Simple system (one Eu site) – Sr2MgSi2O7:Eu,Dy

Charging | Afterglow | TL analysis

TL cannot be explainedby single trap depth

Setting up the model: keep it simple & local

Eu2+

Trap + e

Eu3+

Empty trap

Detrapping

Claude Tydtgat et al, Optical Materials Express 6 (2016) 844-858Optically stimulated detrapping during charging of persistent phosphors

pnr

• Boundary conditions for charging and afterglow• pe (excitation rate) is small• Two solutions l1 and l2: two exponentials

Differential equations

Eu2+ trap

• pe 0 : Eigenvalues for charging and afterglow are

• Solution for charging:

charging

afterglow

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Adding distribution for trap depths

Claude Tydtgat et al, Optical Materials Express 6 (2016) 844-858Optically stimulated detrapping during charging of persistent phosphors

Eu2+ traps

Problem #1

Step in charging curve ≠ Step for afterglow

Charging | Afterglow | TL analysis

Simple system (one Eu site)

Sr2MgSi2O7:Eu,Dy

Charging | Afterglow | TL analysis

Problem #2

Eigenvalues are identical for different pe

≠ Charging dynamics strongly depend on pe

Problem #3

Absorption pe(M-me-m) should decrease<> Absorption increases, depends on pe

Reflectedexcitation

light

Influence of excitation rate pe not negligible, on the contrary!

Large influence, yet pe is lowEffect is proportional to pe x a

OSL : a pe

Claude Tydtgat et al, Optical Materials Express 6 (2016) 844-858Optically stimulated detrapping during charging of persistent phosphors

Eu2+ traps

It makes sense for charging and decharging…

Emission intensitya = 200

Claude Tydtgat et al, Optical Materials Express 6 (2016) 844-858Optically stimulated detrapping during charging of persistent phosphors

Charging | Afterglow

… and for the influence of the excitation intensity pe

Claude Tydtgat et al, Optical Materials Express 6 (2016) 844-858Optically stimulated detrapping during charging of persistent phosphors

25

-60°C

0°C

TL

HeatingEmpty all traps

0

10

20

30

-50 -25 0 25 50 75 100

TL in

ten

sity

(ar

b. u

nit

s)Temperature (°C)

100%

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-60°C

0°C

TL

HeatingEmpty all traps

100% 1% 60%

Thermal barrierfor trapping

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-60°C

0°C

TL

HeatingEmpty all traps

100% 1% 60%

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/hopeful for applications…

… and requires careful study!

Part 2: Mechanically driven release

C.-N. Xu et al.

N. Terasaki and C.-N. Xu, IEEE Sens. J., 2013, 13, 3999.

Kersemans et al, Applied Physics Letters 107 (2015) 234102Fast Reconstruction of a Bounded Ultrasonic Beam using Acoustically induced Piezoluminescence

Part 2: Mechanically driven release

BaSi2O2N2:Eu2+

Kersemans et al, Applied Physics Letters 107 (2015) 234102Fast Reconstruction of a Bounded Ultrasonic Beam using Acoustically induced Piezoluminescence

Part 2: Mechanically driven release

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Ultra-sound

Competition with thermal detrapping

Linear response to ultrasound power

Kersemans et al, Applied Physics Letters 107 (2015) 234102Fast Reconstruction of a Bounded Ultrasonic Beam using Acoustically induced Piezoluminescence

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Imaging of US pressure field by Acoustically induced Piezoluminescence

Kersemans et al, Applied Physics Letters 107 (2015) 234102Fast Reconstruction of a Bounded Ultrasonic Beam using Acoustically induced Piezoluminescence

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0

Transducer Increasing distance z (mm)

100 200 300

Kersemans et al, Applied Physics Letters 107 (2015) 234102Fast Reconstruction of a Bounded Ultrasonic Beam using Acoustically induced Piezoluminescence

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Simulation

APL

Fast (10”)

Hydrophone

Slow (10’)

2cm

Towards a fast, full 3D reconstructionof ultrasound beams

Kersemans et al, Applied Physics Letters 107 (2015) 234102Fast Reconstruction of a Bounded Ultrasonic Beam using Acoustically induced Piezoluminescence

Conclusions.

1. In persistent phosphors, optically stimulateddetrapping occurs also at excitation wavelength, limiting the trapping capacity.

2. Mechanoluminescent phosphors can be used tomap ultrasound pressure.

Resources

Feature issue on Persistent and Photostimulable Phosphorsin Optical Materials Express (published March 2016)

http://tiny.cc/OMEXPPP

Chapter

Persistent PhosphorsHandbook on the Physics and Chemistry of Rare EarthsVolume 48, Chapter 274 (2015) 1 - 108

Resources & Acknowledgments

Thank you for your attention !(and your feedback…)

Presentation can be found at http://www.slideshare.net/pfsmet