1 Quantum phosphors Observation of the photon cascade emission process for Pr 3+ - doped phosphors under vacuum ultraviolet (VUV) and X-ray excitation A.P. Vink 1,2, E. van der Kolk 1, P. Dorenbos 1 and C.W.E. van Eijk 1 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 Pr Selecting materials 5. Two types of emission in one material 6. Quantum cutting with X Rays 7. Energy transfer 1 S 0 emission 8. Conclusions

3 New generation lighting 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: Y 2 O 3 :Eu 3+ (red), BaMgAl 10 O 17 :Eu 2+ (blue) and GdMgB 5 O 10 :Ce 3+,Tb 3+ (green) Also used in television: Y 2 O 2 S:Eu 3+ 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+ Pr 3+ : [Xe] 4f 2 (praseodymium) Energy level scheme: 13 states Excitation into 1 S 0 : two photons 1 S 0 level: weak absorption, excitation into 4f 1 5d 1 state, resulting in 1 S 0 → 1 I 6 (400 nm) and 3 P 0 → 3 H 4 (480 nm) Predicted by Dexter (1957), but discovered in 1974 by Sommerdijk (Philips) and Piper (GE) for YF 3 :Pr 3+

7 Photon cascade emission with Pr 3+ Material which shows PCE: SrAlF 5 :Pr 3+

8 Photon cascade emission with Pr 3+ Not only fluoride host show PCE, also oxides! Two situations: 4f 1 5d 1 below 1 S 0 (for CaSO 4 :Pr 3+, above) and 4f 1 5d 1 above 1 S 0 (for BaSO 4 :Pr 3+, below) What factors determine position of 4f 1 5d 1 ? Predict which material shows PCE?

9 Selecting materials Other lanthanide: Ce 3+ ([Xe] 4f 1 ) 4f 1 → 4f 0 5d 1 transition at lower energy and two 4f 1 states Scintillator material: position 4f 1 → 4f 0 5d 1 known in many compounds 5d 1 split in five states Pr 3+ 4f 2 → 4f 1 5d 1 single 5d electron splits into 5 states remaining 4f 1 (Pr 4+ or Ce 3+ )

10 Selecting materials 4f n-1 5d 1 structure of Ce 3+ similar as Pr 3+, also crystal field splitting is roughly the same (CaSO 4 :Ce 3+ /CaSO 4 :Pr 3+ Energy difference is about cm -1 (Dorenbos) In principle: extrapolate Pr 3+ from Ce 3+ data (scintillator data) Differences: splitting of first band is observed for Pr 3+ Only 4f 1 and 5d 1 splitting: two lines, ΔE~ cm -1 4f 1 5d 1 electrostatic interaction

11 Selecting materials In general: which materials show quantum cutting? Determined by position lowest 4f 1 5d 1 state Position 5d 1, centroid energy E C (determined by type of ligands) and crystal field splitting ε cfs (mainly by CN) Quantum cutters: high E C and small ε cfs Host materials: mainly fluorides (>E C ) and some oxides (<ε cfs ) Example: KY 3 F 10 :Pr 3+ (low CN)

12 Two types of emission in one material BaSO 4 :Pr 3+ both different emissions can be found Low temperatures: PCE and high temperatures both PCE and 4f 1 5d 1 emission Expected: only one emission from one site, but 4f 1 5d 1 near to 1 S 0 perhaps thermal population?

13 Two types of emission in one material Decay time 1 S 0 emission becomes shorter (190 to 56 ns) (4f 1 5d 1 → 4f 2 : 10 ns): extra decay channel Equations thermal population: intensity and decay time Determine energy barrier

14 Two types of emission in one material Results on intensity measurements straightforward Lifetime measurements: fitting A f =6.24*10 6 s -1, A d =62.24*10 6 s -1 (16 ns) Determining ΔE: eV (intensity) and eV (decay time) ΔE: energy barrier, not ΔE ( 1 S 0, 4f 1 5d 1 )!

15 Two types of emission in one material Effect is also found for other lanthanides with low 4f n-1 5d 1 bands (Eu 2+, Sm 2+ ), but not for trivalent lanthanides

16 Quantum cutting with X Rays Ce 3+ [Xe] 4f 1 configuration Excitation over the band gap: direct recombination and Self Trapped Exciton (STE) formation Both emissions give the same 4f 0 5d 1 emission to 2 F 7/2, 2 F 5/2 Scintillator applications: STE formation is unwanted; makes the scintillator slower Increase of temperature: more Ce 3+ emission, less STE Increase of Ce 3+ concentration, less STE: more efficient transfer

17 Quantum cutting with X Rays Pr 3+ [Xe] 4f 1 configuration Excitation over the band gap: direct recombination and STE formation Band gap can be reached with X rays and VUV (λ exc =111 nm) SrAlF 5 :Pr 3+ at low temperatures

18 Quantum cutting with X Rays SrAl 12 O 19 :Pr 3+ material: quantum cutter Concentration dependence of STE emission! (a: 0.05 %, b: 0.1 %, c: 0.5% and d: 1.0 %) At room temperature 1 S 0 emission is present: PCE process

19 Quantum cutting with X Rays Two processes: direct recombination (PCE) and formation of STE transferring its energy to Pr 3+ Studied SrAlF 5 :Pr 3+ under X ray excitation STE: nm < 403 nm 1 S 0 emissions > 487 nm 3 P 0 and 1 D 2 emissions STE does not overlap with 1 S 0 level (~215 nm)

20 Quantum cutting with X Rays 3 P 0 and 1 D 2 are fed by both STE energy transfer and second step PCE process: quench from 300K Is the energy transfer STE-Pr 3+ efficient? Measurements on NaMgF 3 :Pr 3+ at room temperature

21 Quantum cutting with X Rays

22 Quantum cutting with X Rays Direct recombination is dependent on temperature: rate determining step Which sequence? First Pr 3+ + h + → Pr 4+ then Pr 4+ + e - → Pr 3+ (4f 1 5d 1 ) First Pr 3+ + e - → Pr 2+ then Pr 2+ + V K → Pr 3+ (4f 1 5d 1 ) Measured Intensity ( 1 S 0 → 1 I 6 ) as function of temperature for SrAlF 5 :Pr 3+ Arrhenius behavior: lnI versus 1/T Analysis: ΔE= 0.06 eV, 455 cm -1, 2.2kT (RT)

23 Quantum cutting with X Rays Energy value is small, a typical value for a shallow electron trap, too small for a V K center So: first Pr 3+ + h + → Pr 4+ then Pr 4+ + e - → Pr 3+ (4f 1 5d 1 ) PCE process is determined by the recombination rate of electron trap with Pr 4+

24 Energy transfer 1 S 0 emission 1 S 0 → 1 I 6 (400 nm, UV) emission step not suitable for lamp applications Possible solution: co-doping with other lanthanides or with transition metal ions Possible candidate: Mn 2+ (3d 5 ): 1 S 0 → 1 I 6 overlaps with 6 A 1 → 4 A 1, 4 E (around 400 nm)

25 Energy transfer 1 S 0 emission SrAlF 5 :Mn 2+ and SrAlF 5 :Pr 3+,Mn 2+ (excitation into Pr 3+ at 190 nm) No Mn 2+ emission visible X Ray excitation: Mn 2+ built in!

26 Conclusions Discussed quantum cutting for Pr 3+ in a large number of hosts Can predict properties Pr 3+ from Ce 3+ data (scintillation) Pr 3+ in some hosts can show both 4f 1 5d 1 emission and 4f 2 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 Pr 3+ -Mn 2+ not visible up till now