Specificity and Response of the Photoluminescence of Irradiated Nanophosphors …telab.vuse.vanderbilt.edu/docs/DTRA_program-review_7_18-11.pdf · Thermal Engineering Laboratory,

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ity Specificity and Response of the Photoluminescenceof Irradiated Nanophosphors

Topic Per4-G: Nanoscale Radiation Indicators (Thrust 5)

DTRA Basic Research Technical Review, Washington DCJuly 18, 2011

Vanderbilt team: Greg Walker (ME, PI)Bridget Rogers, (ChemE, co-PI)Ron Schrimpf (EE, co-PI)Sarah Gollub (MatSci, PhD student)Rachael Hansel (MatSci, post-doc)Bobby Harl (ChemE, PhD student)Stephanie Weeden-Wright (EE, PhD student)

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ityProgram Objective (1/2)

Grand Challenge

To detect radiation exposure remotely, long after the exposure occurred anddiscreetly. The detection should be sensitive to dose and specific to the typeof radiation.

Project Scope

Test the photoluminescence (PL) of thermographic phosphors (TGP) beforeand after irradiation and (hopefully) identify a correlation between radiationdisplacement damage and PL emission.

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ityProgram Objective (2/2)

Alternative title: Non-scintillator radiation detection using nanophase phosphors

Consider existing research in the area:

Radiation in phosphors (mostly scintillators)

Radiation effects in nanocrystals

Radiation effects nano-phosphors (very few)

Luminescence in nanophase phosphors

Radiation-induced damage detection using luminescentnanophase phosphors

• There are very few articles (none?) in the literature that considerradiation-induced displacement damage in nano-phosphors.

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ityBackground and significance (1/3)

CdSe nanocrystals

increasing 1 MeV proton dose −→

-7

-6

-5

-4

-3

-2

-1

0

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4

ln(P

L/P

L 0)

dose (1015 cm-2)

4.8 nm2.2 nm

NC size theoretical experimental

(nm) σNC (A2) σNC (A

2)

2.2 1.14 5.19

4.8 0.11 2.58

(R.C. Feldman, unpublished, used with permission)

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ityBackground and significance (2/3)

1. Naturally inert materials, so environmental effects (“in the field”) won’t alterphosphor’s response to radiation exposure

2. Fabrication is easy and scalable, so mass production will be likely if the rightmaterial can be designed.

3. Doping can tune properties to control sensitivity range, response characteristics,excitation frequency, luminescence, etc.

4. Indefinite shelf life

5. Once damage occurs, the material will contain a record of the damageessentially indefinitely.

6. Response can be queried at a distance.

7. We have a good deal of experience with these materials, so we have a basis forcomparison.

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ityBackground and significance (3/3)

• YAG: screening material because we already have a great deal of experienceand understanding of its characteristics This will allow us to establish a robustexperimental protocol.

Material Molecular massYAG (Y3Al5O12) 593

YGG (Y3Ga5O12) 807LAG (Lu3Al5O12) 851

LGG (Lu3Ga5O12) 1066

• pyrochlores: experience plus larger stopping power, which should provide moresensitivity to radiation. Plus fabrication technique produces smaller particles

4% Eu

4% Eu4% EuA2 B2 O7

EuA−site

NdLa

Zr

HfB−

site 4% Eu

• strontium alumina: readily available doped with boron. Boron is known to havean exceptionally large cross-section, so interaction with radiation should also bequite large.

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ityTechnical Approach (1/1)

• To determine the (I) spectral photoluminescence modification of (II) nanophase(III) YAG, pyrochlores and strontium alumina doped with boron as a function ofsize when exposed to (IV) x-rays, protons, alphas and neutrons.I

– Relative intensity

– Decay time/quenching

– Peak shift

II– separation technologies

– size distribution

– synthesis options

III– change doping levels to

adjust molecular weight(radiation cross-section)

– characterization

IV– vary energy, dose rate and

dose to find thresholds ofsensitivity

• Results will provide selectivity and sensitivity of selected rare earth phosphors inradiation environments.

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ityResults—Nano-sizing techniques

• TEM gives us reason to believe we have nanocrystallites.

• Nano-sizing the samples

– Filter: TFF filters capable of filtering 200 nm and 500 nm particles have beenused. No particles were seen below 200 nm. Few particles below 500 nm.

– Separation: Need a better way to break up airy foam from combustionsynthesis (ball milling)

– Direct fabrication: Thermochemical analysis for designing combustion control

∗ mass-controlled reactor∗ thermochemical properties of the precursor at various steps in combustion

are needed∗ TGA/DSC/MS has been performed to study these values

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ityResults—Thermochemical analysis

TGA-DSC for lanthanum zirconate (pyrochlore) made with glycine (fuel):

0

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4

6

8

10

12

-120

-100

-80

-60

-40

-20

0

20

0 10 20 30 40 50 60

Weight (mg)

Heat Flow (mW)

Time, minutes

• Initial mass loss and heat flow iswater evaporation

• Initial endothermic reaction unknown

• Combustion temperature is 282◦C(accompanied by a mass loss).

• Decrease in mass is due to residualcombustion products cooking off

• Combustion parameters lead to better combustion processes

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ityResults—Simulation

• SRIM provides penetration depths and stopping powers for materials selectionand experimental design

MRED SRIM

LAMMPS Marlowe

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ityResults—x-ray irradiation

We saw no change in the luminescence, decay time, XRD, XPS, ...

• However, this was expected. X-rays are ionizing and (in general) do not createphysical damage.

• We used these tests to prove out our mounting system and characterizationtechniques

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ityResults—proton irradiation of La2Zr2O5

Evidence of damage is less than 1 week oldPyrochlore sample was irradiated to a fluence of 2×1015 cm−2

• Physical appearance changed: discolored (yellowish gray) spot where beamstruck the sample

– Could be damage as in oxygen vacancies in yttrium oxide

– Could be carbon buildup on sample surface

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ityResults—proton irradiation of La2Zr2O5

• decrease in red emission when illuminated with UV (PL)pre post

– beam was D = 3mm, which is commensurate with the apparent damage

– extraction of “red” channel shows a decrease in color that corresponds to thebeam impact site. No corresponding decrease in green or blue channels.

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ityResults—proton irradiation of La2Zr2O5

• cathodoluminescence (CL) appears to degrade with time

120

140

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180

200

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280

0 20 40 60 80 100 120 140 160 180

inte

nist

y (a

.u.)

time (minutes)

intensityfit

– the green channel of one pixel from the series of images was collected andplotted.

– The fit is a standard exponential

I = I0 exp(

−tτ

)

+ Ioff

I0 = 150; τ = 47min; Ioff = 130.

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ityResults—proton irradiation of La2Zr2O5

• PL significantly altered after Φ = 2×1015 cm−2

-1

0

1

2

3

4

5

450 500 550 600 650 700 750

inte

nsity

(a.

u.)

wavelength (nm)

5D0->7F25D1->7F0

5D2->7F0

5D0 ->7F5

pre-irradiationpost-irradiation

– relative peaks shift—this is important because it suggests that thedegradation is not due to contamination on the surface

– background increase in blue region—could be Eu+3 → Eu+2 resulting fromdisplacement damage because Zr has similar oxidation states

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ityAccomplishments

• Publications: abstracts to MRS and NSREC

• Presentations: 2 abstracts for AVS meeting in Nashville

• Students (supported at various levels):

– Sarah Gollub (Materials Science, PhD 2014)

– Justin Collar (Materials Science, REU summer 2011)

– Stephanie Weeden-Wright (Electrical Engineering, PhD 2014)

– Bobby Harl (Chemical Engineering, PhD 2013)

– Rachael Hansel (Mechanical Engineering, postdoc)

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ityConclusions

• Ceramic rare-earth phosphors are viable candidates for stand-off, discrete,delayed, radiation sensors.

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ityFuture Directions

• Immediate goals (next year)

– Step-stress tests on pyrochlore and YAG with protons

– Vary concentration of gallium substituted YAG for subsequent proton testing

– alternative pyrochlore configurations

– detailed simulations of damage in our compounds

– non-isotopic alpha sources

• Long-term (one year away)

– neutrons

– boron-based phosphors

– effects of nano-particle size

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itySupplemental slides

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ityEu3+ transitions

E 5D3 5D2 5D1 5D025500 24200 21500 19000 17200

7F0 0 392 413 465 526 5817F1 400 398 420 474 538 5957F2 1000 408 431 488 556 6177F3 1900 424 448 510 585 6547F4 2500 435 461 526 606 6807F5 3900 463 493 568 662 7527F6 4800 483 515 599 704 806

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ityInternal wiki

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ityRadiation sources

type source energy beamx-ray Aracor 4100 10keV �3cmproton Van de Graaff ≤ 1.8MeV �3mm, 1cm scangamma 60Co 1.33MeV & 1.17MeV isotopicalpha 241Am 3.4MeV in air 1.5cm2 isotopicheavy ion tandem pelletron 1−4MeV 5µmneutron ORNL (SNS/HFIR) tbd tbd

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itySchedule

Irradiation matrix

YAG/YGG pyrochlores SA:boron∗

x-rays Y1 Y1 Y2protons Y1 Y1 Y2alphas Y1 Y1 Y2neutrons Y3 Y3 Y3other Y3 Y3 Y3

∗ strontium alumina doped with boron

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ityWhat are thermographic phosphors?

• Materials that emit light when excited and whose emission is temperaturedependent

• Usually ceramics doped with rare earth elements

• Not to be confused with phosphorus

• Excitation can be light, electrons, x-rays

Uses

• TV screens/CRT

• plasma display

• fluorescent lighting

• glow-in-the-dark toys

Thermal measurement

• Decay time

• Relative intensity peaks

• Line shift

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ityTGP Fabrication

Normally, fabrication involves diffusional process that 1) is time consuming, 2)requires lots of thermal energy, 3) is difficult to control, and 4) requirespost-processing to get luminescence.Combustion synthesis

1. Mix nitrates of constituentsstoiciometrically with urea

2. Heat in a muffle furnace or hot platefor a few minutes

3. Calcining can improve emission

Materials

• Gallium-substituted YAG:Ce(Y1−xCex)3(Al1−yGay)5O12. Widelyused in solid-state lighting andlasers.

• Pyrochlores (A2B2O7)Used as thermal barrier coating

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ityCombustion products

0

500

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2500

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Inte

nsity

(a.

u.)

2Θ

(b) La1.92Eu0.08Zr2O7

0

500

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1500

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2500

20 30 40 50 60 70

(a) La1.92Eu0.08Hf2O7

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ityIs nano important?

credit: ORNL SHaRE program

• Hypothesis: small crystallites provide more surface states that increasenon-radiative pathways for altered spectra and decay times.

• This effect appears not to be an issue for crystallites larger than 20nm, which iswhat we get.

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ityMaterials design

⇐ Dieke diagram

⇓ Configurational coordinate diagram

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ityYAG:Ce Characterization

Red shift with gallium in (Y1−xCex)3(Al1−yGay)5O12

x (%Ce) y (%Ga) λem (nm)0.01 0 5370.01 0.5 5140.02 0 5390.02 0.5 517

4f1

5d1

Eg2

5d1

Eg2

T2g2T2g

2

λem =520nmλem =540nm

Y3Al5 O12 :Ce Y3 O12Ga5 :Ce

E’’

E’

E’’

E’

En

erg

y

non−cubic structure cubic structure

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ityDecay mechanisms

Multi-phonon emissionTemperature-dependent decay has aradiative and non-radiative component.

1τ= Pr +Pnr

Weber’s model for non-radiativerecombination

Pnr(T) = Pnr(T = 0)

[

1− exp

(

Eph

kBT

)]−n

Results of fit to pyrochlore data

Sample Eph n(cm−1)

La1.92Eu0.08Zr2O7 785 24La1.92Eu0.08Hf2O7 745 21

Charge transfer state

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ityYAG:Ce Results

• x is the percent gallium

• gallium red-shifts the emission anddecreases the decay time

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60

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90

0 10 20 30 40 50 60 70 80

Dec

ay T

ime

(ns)

%Gallium

0 10 20 30 40 50 60

20 40 60 80 100 120

Dec

ay T

ime

(ns)

Temperature (ðC)

x=0

x=0.25

x=0.50

x=0.75

500 550 600 650

Inte

nsity

(a.

u.)

Wavelength (nm)

x=0

x=0.25

x=0.50

x=0.75

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ityPyrochlore Results

Excitation Emission

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3000

250 300 350 400 450 500 550 600

Inte

nsity

(a.

u.)

λ (nm)

La1.92Eu0.08Hf2O7

La1.92Eu0.08Zr2O7

0

500

1000

1500

2000

2500

560 580 600 620 640 660 680 700 720

Inte

nsity

(a.

u.)

λ (nm)

La1.92Eu0.08Hf2O7

La1.92Eu0.08Zr2O7

0.1

1

10

100

1000

10000

300 400 500 600 700 800 900 1000 1100

Lif

etim

e (µ

s)

Temperature (K)

La1.92Eu0.08Hf2O7La1.92Eu0.08Zr2O7

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