1

Quantum phosphorsObservation of the photon cascade emission process for Pr3+-doped phosphors under vacuum ultraviolet

(VUV) and X-ray excitation

A.P. Vink1,2, E. van der Kolk1, P. Dorenbos1 and C.W.E. van Eijk1

1 Radiation Technology Group, Interfaculty Reactor Institute, Delft University of Technology,

Mekelweg 15, 2629 JB Delft, The Netherlands

2 Chemical Sciences, Netherlands Organisation for Scientific Research, P.O. Box 93470, 2509 AL

The Hague, The Netherlands

Radiation Technology, Interfaculty Reactor Institute

2

Outline 1. New generation lighting

2. Quantum cutting

3. Photon cascade emission with Pr3+

4. Selecting materials

5. Two types of emission in one material

6. Quantum cutting with X Rays

7. Energy transfer 1S0 emission

8. Conclusions

3

New generation lighting

120 130 140 150 160 170 180 190 2000,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

7.2eV

8.3eV

Inte

nsi

ty (

a.u

.)

Wavelength (nm)

• Commonly used TL lighting, mercury (254 nm emission) is used to excite a set of three phosphors

• Result: white light• Disadvantages: 1) mercury bad

for environment and 2) start-up time

• Alternative xenon-gas (emission around 172 nm)

• Result: new set of phosphors needed

4

New generation lighting

• In TL lighting: four lanthanides used: Y2O3:Eu3+ (red), BaMgAl10O17:Eu2+ (blue) and GdMgB5O10:Ce3+,Tb3+ (green)

• Also used in television: Y2O2S:Eu3+

• Partially filled 4f-shell, shielded from surrounding (host)

5

Quantum cutting

• Major disadvantage of Xe is low efficiency

• Comparison: • Hg 254 nm 50% energy loss

(4.9 eV) • Xe 172 nm 70% energy loss

(7.2 eV)• To increase quantum

efficiency:quantum cutting

• Excitation into high-energy state gives two step-emission to ground state: result two photons (visible region)

6

Photon cascade emission with Pr 3+

• Pr3+: [Xe] 4f2 (praseodymium)• Energy level scheme: 13 states

• Excitation into 1S0: two photons

• 1S0 level: weak absorption, excitation into 4f15d1 state, resulting in 1S0 → 1I6 (400 nm) and 3P0 → 3H4 (480 nm)

• Predicted by Dexter (1957), but discovered in 1974 by Sommerdijk (Philips) and Piper (GE) for YF3:Pr3+

0

10

20

30

40

50

60

4f15d

1

3F3,

3F4

3H6,

3F2

1G4

1D2

3P0

3P2

3P1,

1I6

1S0

3H5

3H4

Ene

rgy

(103 cm

-1)

7

Photon cascade emission with Pr 3+

• Material which shows PCE: SrAlF5:Pr3+

0

10

20

30

40

50

60

4f15d1

3F3,3F4

3H6,3F2

1G4

1D2

3P0

3P2

3P1,1I6

1S0

3H53H4

Ene

rgy

(103 cm

-1)

50 100 150 200 250 300 350 400 450 500 550 600 650 700

0,0

0,2

0,4

0,6

0,8

1,0

host

em

=404 nm exc

=189 nm

second order

Inte

nsi

ty (

a.u

.)

Wavelength (nm)

8

Photon cascade emission with Pr 3+

• Not only fluoride host show PCE, also oxides!

• Two situations: 4f15d1 below 1S0 (for CaSO4:Pr3+, above) and 4f15d1 above 1S0 (for BaSO4:Pr3+, below)

• What factors determine position of 4f15d1?

• Predict which material shows PCE?

100 150 200 250 300 350 400 450 500 550 600 650

0,0

0,2

0,4

0,6

0,8

1,0A

b

a

1D

2->

3H

4

+

a: 4f15d

1->

3H

4

4f15d

1->

1G

44f

15d

1->

1D

2

4f15d

1->

3F

3

4f15d

1->

3F

2

b: 4f15d

1->

3H

5

3P

0->

3F

24f15d

1->

3P

J,1I6 (J:0,1,2)

Excitation (em=230 nm) Emission (exc=190 nm)

Inte

nsi

ty (

a.u

.)

Wavelength (nm)

100 150 200 250 300 350 400 450 500 550 600 650

0,0

0,2

0,4

0,6

0,8

1,0B

1S

0->

3F

4

+

1D

2->

3H

4

3P

0->

3F

2

3P

0->

3H

4

1S

0->

1G

4

1S

0->

1D

2

1S

0->

1I6

Excitation (em

=403 nm) Emission (

exc=187 nm)

Inte

nsi

ty (

a.u

.)

Wavelength (nm)

9

Selecting materials

• Other lanthanide: Ce3+ ([Xe] 4f1) 4f1 →4f05d1 transition at lower energy and two 4f1

states• Scintillator material: position

4f1 → 4f05d1 known in many compounds

• 5d1 split in five statesPr3+ 4f2 →4f15d1

• single 5d electron splits into 5 states remaining 4f1 (Pr4+ or Ce3+) 0

10

20

30

404f05d1

2F7/2

2F5/2

Ene

rgy

(103 cm

-1)

10

Selecting materials

• 4fn-15d1 structure of Ce3+ similar as Pr3+, also crystal field splitting is roughly the same (CaSO4:Ce3+/CaSO4:Pr3+

• Energy difference is about 12 240 cm-1 (Dorenbos)

• In principle: extrapolate Pr3+ from Ce3+ data (scintillator data)

• Differences: splitting of first band is observed for Pr3+

• Only 4f1 and 5d1 splitting: two lines, ΔE~ 2 000 cm-1

• 4f15d1 electrostatic interaction

60,0 57,5 55,0 52,5 50,0 47,5 45,0 42,5 40,0 37,5 35,0 32,5 30,0

A

Inte

nstit

y

Energy (103 cm-1)

72,5 70,0 67,5 65,0 62,5 60,0 57,5 55,0 52,5 50,0 47,5 45,0 42,5

B

Inte

nstit

y

Energy (103 cm

-1)

11

Selecting materials

• In general: which materials show quantum cutting?

• Determined by position lowest 4f15d1 state

• Position 5d1, centroid energy EC (determined by type of ligands) and crystal field splitting εcfs (mainly by CN)

• Quantum cutters: high EC and small εcfs

• Host materials: mainly fluorides (>EC ) and some oxides (<εcfs)

• Example: KY3F10:Pr3+ (low CN)

50 100 150 200 250 300 350 400 450 500 550 600 650 7000,0

0,2

0,4

0,6

0,8

1,0exc

=216.5 nmem

=285 nm

second order

Inte

nsi

ty (

a.u

.)

Wavelength (nm)

12

Two types of emission in one material• BaSO4:Pr3+ both different

emissions can be found• Low temperatures: PCE and

high temperatures both PCE and 4f15d1 emission

• Expected: only one emission from one site, but 4f15d1 near to 1S0 perhaps thermal population?

200 250 300 350 400 450 500

0,0

0,2

0,4

0,6

0,8

1,0 T= 10K T= 292K

3P

0 ->

3H

44f

15d

1 ->

1D2

1S

0 ->

1G

4

1S

0 ->

1D

2

1S

0 ->

1I6

4f15d1 -> 3H4 -1G4

Inte

nsi

ty (

a.u

.)

Wavelength (nm)

13

Two types of emission in one material• Decay time 1S0 emission

becomes shorter (190 to 56 ns) (4f15d1

→ 4f2: 10 ns): extra decay channel

• Equations thermal population: intensity and decay time

• Determine energy barrier

25 50 75 100 125 150 1750,01

0,1

1

A T= 10K T= 293K

Lo

g In

ten

sity

Time (ns)

kTE

CII

Rd

f exp

kTEkT

EAA

Adf

tot

exp

exp

1

1

14

Two types of emission in one material• Results on intensity

measurements straightforward• Lifetime measurements: fitting

Af=6.24*106 s-1, Ad=62.24*106 s-1 (16 ns)

• Determining ΔE: 0.041 eV (intensity) and 0.040 eV (decay time)

• ΔE: energy barrier, not ΔE (1S0, 4f15d1)!

0,012 0,010 0,008 0,006 0,004 0,0020

-1

-2

-3

-4 Data points (

exc=188 nm)

Data points (exc

=205 nm) Linear fit: ln(R)=a/T+b

ln (

R)

1/T (K-1)

0 50 100 150 200 250 3000

5

10

15

20

1/

(1

06 s-1)

Temperature (K)

15

Two types of emission in one material• Effect is also found for other

lanthanides with low 4fn-15d1

bands (Eu2+, Sm2+), but not for trivalent lanthanides

E 4f15d

1 (2)

4f15d

1 (1)

1S0

Q

16

Quantum cutting with X Rays

• Ce3+ [Xe] 4f1 configuration• Excitation over the band gap:

direct recombination and Self Trapped Exciton (STE) formation

• Both emissions give the same 4f05d1 emission to 2F7/2,2F5/2

• Scintillator applications: STE formation is unwanted; makes the scintillator slower

• Increase of temperature: more Ce3+ emission, less STE

• Increase of Ce3+ concentration, less STE: more efficient transfer 0

10

20

30

404f05d1

2F7/2

2F5/2

Ene

rgy

(103 cm

-1)

17

Quantum cutting with X Rays

• Pr3+ [Xe] 4f1 configuration• Excitation over the band gap:

direct recombination and STE formation

• Band gap can be reached with X rays and VUV (λexc=111 nm)

• SrAlF5:Pr3+ at low temperatures

200 300 400 500 6000.0

0.2

0.4

0.6

0.8

1.0

X r

ay e

xcita

tion

T=

100K

exc

=11

1 nm

T=

10K

Inte

nsity

(a.

u.)

Wavelength (nm)

18

Quantum cutting with X Rays

• SrAl12O19:Pr3+ material: quantum cutter

• Concentration dependence of STE emission! (a: 0.05 %, b: 0.1 %, c: 0.5% and d: 1.0 %)

• At room temperature 1S0

emission is present: PCE process

19

Quantum cutting with X Rays

• Two processes: direct recombination (PCE) and formation of STE transferring its energy to Pr3+

• Studied SrAlF5:Pr3+ under X ray excitation

• STE: 260-545 nm

• < 403 nm 1S0 emissions

• > 487 nm 3P0 and 1D2 emissions

• STE does not overlap with 1S0 level (~215 nm)

200 300 400 500 600 700 800

0.00

0.08

0.16

0.24

0.32

0.40 T=100K T=350K

Inte

nsity

(*1

E9)

Wavelength (nm)

0

10

20

30

40

50

60

4f15d1

3F3,3F

43H

6,3F

2

1G4

1D2

3P0

3P2

3P1,1I

6

1S0

3H53H4

Ene

rgy

(103 cm

-1)

20

Quantum cutting with X Rays

• 3P0 and 1D2 are fed by both STE energy transfer and second step PCE process: quench from 300K

• Is the energy transfer STE-Pr3+ efficient?

• Measurements on NaMgF3:Pr3+ at room temperature

100 150 200 250 300 350

0.0

0.1

0.2

0.3

0.4

0.5

1S0->1I

6

3P0->3H

4

1D2->3H

4

STE

Inte

nsi

ty

Temperature (K)

200 300 400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0 exc

=190 nm

exc

= 90 nm

3P0 3H

6

3P0 3H

5

1D2 3H

41S

0 3F

3

1S0 1G

4

1S0 1D

2

1S0 3H

4

3P0 3H

4

1S0 1I

6

Inte

nsity

(a.

u.)

Wavelength (nm)

21

Quantum cutting with X Rays

90

75

60

45

30

15

0

3 eV

11 eV

Pr3+

CB

VB

En

erg

y (1

03 cm

-1)

STE

4f15d1

3F3,3F

4

3H6,3F

2

1G4

1D2

3PJ (J:0,1,2), 1I

6

1S0

3H5

3H4

22

Quantum cutting with X Rays

• Direct recombination is dependent on temperature: rate determining step

• Which sequence? • First Pr3+ + h+→Pr4+ then Pr4+ +

e-→Pr3+ (4f15d1)• First Pr3+ + e-→Pr2+ then Pr2+ +

VK→Pr3+ (4f15d1)• Measured Intensity (1S0→1I6) as

function of temperature for SrAlF5:Pr3+

• Arrhenius behavior: lnI versus 1/T

• Analysis: ΔE= 0.06 eV, 455 cm-

1, 2.2kT (RT)

0.0025 0.0030 0.0035 0.0040 0.0045 0.0050 0.0055 0.0060-3.2

-2.8

-2.4

-2.0

-1.6

-1.2

-0.8 Data point Least squares fit

Ln

I

1/T (K-1)

kTE

CI exp*

23

Quantum cutting with X Rays

• Energy value is small, a typical value for a shallow electron trap, too small for a VK center

• So: first Pr3+ + h+→Pr4+ then Pr4+ + e-→Pr3+ (4f15d1)

• PCE process is determined by the recombination rate of electron trap with Pr4+

3a

6electron trap

7b

7a

4

5a

5b

3b

2b

2a

1

90

75

60

45

30

15

0

Pr3+

CB

VB

Ene

rgy

(103 c

m-1)

STE

4f15d1

3F3,3F

4

3H6,3F

2

1G4

1D2

3PJ (J:0,1,2), 1I

6

1S0

3H5

3H4

24

Energy transfer 1S0 emission

• 1S0 → 1I6 (400 nm, UV)

emission step not suitable for lamp applications

• Possible solution: co-doping with other lanthanides or with transition metal ions

• Possible candidate: Mn2+ (3d5): 1S0 → 1I6 overlaps with 6A1 →4A1, 4E (around 400 nm)

0

10

20

30

40

50

60

Mn2+Pr3+

4T24T14A2

4T14E

4T2

4A1,

2E

4T2

6A1

4T1

4f15d

1

3F3,

3F4

3H6,

3F2

1G4

1D2

3P0

3P2

3P1,

1I6

1S0

3H5

3H4

Ene

rgy

(103 cm

-1)

25

Energy transfer 1S0 emission

• SrAlF5:Mn2+ and SrAlF5:Pr3+,Mn2+ (excitation into Pr3+ at 190 nm)

• No Mn2+ emission visible• X Ray excitation: Mn2+ built in!

200 300 400 500 600 700 8000,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

scaled

SrAlF5:Pr

3+,Mn

2+

SrAlF5:Pr

3+

SrAlF5:Mn

2+

Inte

nsi

ty (

*1E

9)

Wavelength (nm)

50 150 250 350 450 550 650 7500,0

0,2

0,4

0,6

0,8

1,0

Mn2+

STE

em

=523 nm exc

=113 nm

Inte

nsi

ty (

a.u

.)

Wavelength (nm)

250 300 350 400 450 500 550 600 650 700

0,0

0,2

0,4

0,6

0,8

1,0

second orderInte

nsi

ty (

a.u

.)

Wavelength (nm)

26

Conclusions

• Discussed quantum cutting for Pr3+ in a large number of hosts

• Can predict properties Pr3+ from Ce3+ data (scintillation)• Pr3+ in some hosts can show both 4f15d1 emission and 4f2

emission from the same spectroscopic site• Excitation with X Rays can also result in quantum cutting,

but is temperature dependent• Fluoride materials are the most promising materials, have

to be co-doped with another ion• Energy transfer Pr3+-Mn2+ not visible up till now