1

Luminescence of lanthanides and phosphor applications

Koen Binnemans

KU Leuven - University of Leuven (Belgium)

2

Energy level structure of trivalent lanthanide ions

• Partly filled 4f shell – multiple configurations

Eu3+ 4f6 6 electrons for 7 f-orbitials (2 possible spin orientations)

14!/(6!(14-6)!) = 3003 different arrangments

• 4f shell well shielded from the environment by closed 5s2 and 5p6

shells

• Energy level splitting due to:

– Electrostatic interactions

– Spin-orbit coupling

– Crystal-field (CF) interactions

Splitting of 4f6 levels of Eu3+

3 Ref. J.C.G. Bünzli, Inorg. Chim. Acta 139 (1987) 2019.

Free-ion levels of Eu3+

4 Ref. : K. Binnemans, Bull. Soc. Chim. Belg. 105 (1996) 793.

5

Number of f

electrons

Number of

terms 2S+1L

Number of

levels 2S+1LJ

Number of LF

sublevels 2S+1Gx

1 13 1 2 14

2 12 7 13 91

3 11 17 41 364

4 10 47 107 1001

5 9 73 198 2002

6 8 119 295 3003

7 119 327 3432

Number of levels

(4 2) ! 14!

!(4 2 ) ! !(14- )!N N N N

6

Dieke (1968)

7

Ref.: R.T Wegh et al. J. Lumin. 66-68 (2000) 1002.

Extension to VUV

8

ANL report – Carnall, Crosswhite & Crosswhite energy levels in LaF3 - 1977

Types of transitions

• f-f transitions

• f-d transitions

• charge-transfer (CT) transitions

9

f-f transitions

• Narrow bands

• Weak intensities: e < 10 M-1cm-1

• Barycenters of CF sublevels are not much

dependent on the nature of the Ln3+ environment; therefore energy of

the transitions is more or less constant

(but not CF splitting!)

• Electric dipole transitions (ED) are forbidden

Magnetic dipole (MD) transitions are allowed, but very weak

• Number of components for a given (2S’+1)L’J’2S+1)LJ transition depends

on the site symmetry

• Some transitions are hypersensitive, i.e. very sensitive to small

changes in the Ln3+ environment.

10

Selection rules for f-f transitions

11

Electric dipole transitions Magnetic dipole transitions

Dl = ± 1 Laporte’s rule Dl = 0

DS = 0 spin rule DS = 0

DL 6 0, 2, 4, 6 DL = 0

DL 6 0, 2, 4, 6 DJ = 0, ± 1, except 0-0

Relaxed by J-mixing

Typical absorption spectrum of Eu3+

12

2.8

2.0

1.6

1.2

0.8

0.4

2.4

250 300 350 400 450 500 nm

EuCl3 0.05 M

in H2O

5G4

5D1 5D2

5L6

(2S+1)GJ 7F0,1

5G4

5G3

5K6 5F4

5H6

5HJ

6

4 3

5D4

5G6

5G2

5D3

e / M-1cm-1

Ref. : J.C.G Bünzli, Lanthanide course

13

f-d transitions

Allowed by Laporte’s rule, 100-1000 M-1cm-1

Highly energetic, except for Ce3+, Pr3+, and Tb3+

Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

30

40

50

60

70

80

4f to 5d transition

E / 103 cm

-1

Ref. : J.C.G Bünzli, Lanthanide course

f-d transitions

14

[Ce(H2O)9]3+, D3h symmetry

Ce3+ [Xe]5d1 generates two

levels, 2D3/2 and 2D5/2

40.0

E / 103 cm-1

0

2

44.0

48.0 225 250 300 nm

48 36 40 44 32

E / 103 cm-1

102 e 8

6

4

2

0

2F5/2

2F7/2

2D3/2

2D5/2

n.obs.

Ref. : J.C.G Bünzli, Lanthanide course

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Charge-transfer (CT) transitions

Allowed by Laporte’s rule, » 200-500 M-1cm-1

Voor Eu3+: LMCT (formal reduction to Eu2+)

28

30

32

34

36

38

40

42

44

46

48

YPO4

YOF

Y2O3

LaPO4

La2O3

LaOCl

Y2O2S

EuIII

(CT state)

in 103 cm

-1

Host

E /

10

3 c

m-1

Ref. : J.C.G Bünzli, Lanthanide course

16

Red phosphor Y2O3:Eu3+

5D0→7F2

LMCT

f-f transitions

Excitation spectrum

230 280 330 380 430 480 530 580 630 680 730

l /nm

Emission spectrum

Ref. : J.C.G Bünzli, Lanthanide course

17

4f emission spectra

Pr Nd Sm Eu

Tb Dy Ho Er Tm Yb

0

5

10

15

20

25

30

35

40

0

5

10

15

20

25

30

35

40

E /

10

3 c

m -1

3H4 4I9/2

6H5/2 7F0,1

8S7/2 7F6

6H15/2 5I8

4I15/2 3H6

2F7/2

1D2

3P0

4F3/2

4G5/2 5D

0

1

2

6P7/2

5D4

5D3

4F9/2

5S2 4S3/2

1D2

1G4

2F5/2

Gd

1G4

4I13/2

3P0

3F4

5F5

Ref. : J.C.G Bünzli, Lanthanide course

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Characteristic of lanthanide luminescence

• Narrow emission lines (high chromatographic purity)

• Every spectroscopically active lanthanide ion has a typical

luminescence color

• Emission color is not very dependent on ion’s environment

• Large Stokes shift

• Long-lives excited states (several ms in inorganic matrices)

• Direct excitation in 4f-levels is not efficient

Sensitized luminescence is preferred

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Energy gap law

• The larger the gap between the excited state and the more energetic component of the ground state multiplet, the more intense the luminescence (less non-radiative deactivation).

• Gd3+ is the best emitter, but the 6P7/28S7/2 transition occurs in the UV

(around 310 nm).

• Eu3+, Tb3+ have often large intrinsic quantum yields and are used as luminescent probes.

Gd 32100 cm-1

Tb 14800 cm-1

Eu 12300 cm-1

Yb 10400 cm-1

Dy 7800 cm-1

Sm 7400 cm-1

Tm 6200 cm-1

Er 5900 cm-1

Nd 4400 cm-1

Luminescence of lanthanide ions

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Visible luminescence observed

for Eu3+, Tb3+, Sm3+, Dy3+

500 600 700 nm

Tb Eu

Dy Sm

Pr3+ (1.33 mm), Nd3+ (1.06 mm), Er3+ (1.54 mm), and Yb3+ (0.98 mm) have

interesting emission bands in the NIR range, some of them are in the

telecommunication window (1 – 1.6 mm)

NIR luminescence: telecommunication window

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NIR luminescence

22

12000 10000 8000

800 1000 1200 1400

Inte

nsity (

a.u

.)

wavenumber (cm-1)

a

b

c

wavelength (nm)

11000 10000 9000

900 1000 1100

Inte

nsity (

a.u

.)

wavenumber (cm-1)

wavelength (nm)

10000 8000 6000

1000 1200 1400 1600

Inte

nsity (

a.u

.)

wavenumber (cm-1)

a

b

c

wavelength (nm)

Nd3+

Yb3+

6800 6400 6000

1500 1600

Inte

nsity (

a.u

.)

Wavenumber (cm-1)

Wavelength (nm)

Er3+

Ho3+

Transitions in europium(III) luminescence spectra

23

Transitions with DJ = 0, 2 are hypersensitive, i.e. very

sensitive to minute changes in the environment of metal ion

Transitions from 5D0 to 7FJ for J =

0 ED 577-581 nm non-degenerate, highly forbidden

allowed in some symmetries

1 MD 585-600 nm allowed, weak intensity almost

independent from environment

2 ED 610-625 nm hypersensitive, forbidden if

inversion centre i is present

3 ED 640-655 nm forbidden, very weak

4 ED 680-710 nm allowed (except if i)

5 ED 740-770 nm forbidden, very weak

6 ED 810-840 nm allowed, weak (except if i)

Luminescence of [Eu(tta)3(phen)]

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O

F3C

O

S

3

Eu

N

N

575 600 625 650 675 700 725

0

5000

10000

15000

20000

25000

30000

7F

0

7F

1

7F

2

7F

4

7F

3

Inte

nsity /co

un

ts

Wavelength /nm

Europium spectra

25 Source: Tailorlux GmbH

Europium spectra

26 Source: Tailorlux GmbH

SnO2:Eu3+– centrosymmetric compound (D2h)

27 Ref.: M. Ma et al. J. Alloys Compds. 509 (2011) 3441

Other excited states

• Most transitions in Eu3+ luminescence spectra start from 5D0

• In inorganic matrices, transitions from 5D1 and 5D2 are possible

• Transitions from 5D3 are very rare

• In molecular compounds, often only transitions from 5D0 are observed,

because of stronger radiationless deactivation.

• In spectra with luminescence from 5D1 and 5D2, there can be an

overlap between the 5D0 7FJ and the 5D1,2 7FJ lines.

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Luminescence of Cs2Na(Y:Eu)Cl6 (Elpasolite): Oh

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5DJ 7FJ’

1 - 0 2 - 3 1 - 0 2 - 3 1 - 0 2 - 3 1 - 0 2 - 3 1 - 0 2 - 3 1 - 0 2 - 3

0 - 1 1 - 2 0 - 1 1 - 2 0 - 1 1 - 2 0 - 1 1 - 2 0 - 1 1 - 2 0 - 1 1 - 2

1 - 1 1 - 1 1 - 1 1 - 1 1 - 1 1 - 1 0 - 2 0 - 2 0 - 2 0 - 2 0 - 2 0 - 2

550 500 600 nm

5DJ 7FJ’

1 - 0 2 - 3 1 - 0 2 - 3 1 - 0 2 - 3 1 - 0 2 - 3 1 - 0 2 - 3 1 - 0 2 - 3

0 - 1 1 - 2 0 - 1 1 - 2 0 - 1 1 - 2 0 - 1 1 - 2 0 - 1 1 - 2 0 - 1 1 - 2

1 - 1 1 - 1 1 - 1 1 - 1 1 - 1 1 - 1 0 - 2 0 - 2 0 - 2 0 - 2 0 - 2 0 - 2

Ref. : J.C.G Bünzli, Lanthanide course

LaF3:Eu3+

30

20000 19000 18000 17000 16000 15000 14000

500 550 600 650 700

7F

37F

27F

1

7F

0

5D

1

7F

4

7F

3

7F

2

7F

0

Wavelength (nm)

Inte

nsi

ty (

a.u

.)

Wavenumber (cm-1

)

7F

1

5D

0

Ref.: K. Lunstroot et al., J. Phys. Chem. C 113 (2009) 13532.

Number of emission lines for Eu3+ in different symmetries

31

Ref. : P.A. Tanner, Chem. Soc. Rev. 42 (2013) 5090.

32 Ref. : J.C.G Bünzli, redrawn after K. Binnemans and C. Görller-Walrand, Journal of Rare Earths 14 (1996) 173.

Eu3+ as a probe for site symmetry determination

• In principle, it is possible to determine the point group symmetry of the

Eu3+ site by counting the number of CF components that can be

observed for the transitions 5D0 7FJ.

• In practice, an unambiguous assignment of the point group symmetry

on the basis of the number of observed CF components is difficult.

– Overlapping transitions (in case of small CF splitting or low

resolution)

– Extra lines due to vibronic transitions or crystal defects

– Overlap with transitions from 5D1 and 5D2

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34

Antenna effect

• Weak light absorption by forbidden f-f transitions; molar absorption

coefficients e are < 10 L-1 mol cm-1.

• Indirect excitation via attached ligands or via host lattice in order to get

sensitized luminescence.

• Excitation energy is transferred from ligand to lanthanide ion.

35

Antenna effect

S0

T1

S1

F

ISC

ET

P L

ligand lanthanide ion

A

A = absorption, F = fluorescence, P = phosphorescence,

L = lanthanide-centered luminescence, ISC = intersystem crossing,

ET = energy transfer; S = singlet, T = triplet.

36

Other routes to sensitization

• Sensitization is not only possible via energy transfer from triplet state

of organic ligands, but also by:

– Inorganic chromophores (e.g. MoO42-, VO4

2-)

– Charge-transfer bands

Lanthanide phosphors

37

38

Fluorescent lamps

39

Fluorescent lamps

Coating

W filament

DV

Fill gas: Ar UV (254 nm)

e-

Hg

UV photons excite phosphor coating. White light is emitted by

blend of phosphors

40

Producing white light: trichromatic stimuli

400 500 600 nm

80

40

60

20

0

-20

CRI / % There are three “prime”

colors corresponding to the

three spectral responses of

human vision

400 500 600 nm

Color rendering index obtained

by mixing the three prime colors

EuIII

TbIII

(EuII)

Ref. : J.C.G Bünzli, Lanthanide course

41

Lamp phosphors

Year Phosphors

1960 Ca5(PO4)3Cl:Sb3+,Mn2+ (white)

1974 BaMg2Al16O27:Eu2+ CeMgAl10O19:Tb3+ Y2O3:Eu

3+

1990 BaMgAl10O17:Eu2+

(Sr,Ca)5(PO4)3Cl:Eu2+

(La,Ce)PO4:Tb3+ CeMgAl10O19:Tb3+

(Gd,Ce)MgB5O10:Tb3+

Y2O3:Eu3+

2005 BaMgAl10O17:Eu2+ (La,Ce)PO4:Tb3+ Y2O3:Eu

3+

Ref. : J.C.G Bünzli, Lanthanide course

42

Red phosphor Y2O3:Eu3+

5D0→7F1

LMCT

f-f transitions

Excitation spectrum

230 280 330 380 430 480 530 580 630 680 730

l /nm

Emission spectrum Hg 254 nm

Ref. : J.C.G Bünzli, Lanthanide course

43

Green phosphor LaPO4:(Ce3+,Tb3+)

240 290 340 390 440 490 540 590 640 690

l (nm)

Excitation spectrum

4f-5d transition Emission spectrum

Hg 254 nm

Ce3+→Tb3+

transfer

Ref. : J.C.G Bünzli, Lanthanide course

44

Blue phosphor Sr4Al4O25:Eu2+

200 300 400 500 600 nm

excitation Emission (d-f transition)

Hg 254 nm

Ref. : J.C.G Bünzli, Lanthanide course

45

Tricolor lamps

80

60

40

20

0 600 700 500 400

P /mW·nm-1·lm-1 Spectral distribution

of a luminescent lamp

with the following

phosphors:

BaMg2Al16O27:Eu2+

CeMgAl11O19:Tb3+

Y2O3:Eu3+

Ref. : J.C.G Bünzli, Lanthanide course

Cathode ray tubes (CRTs)

46

Red: Y2O2S:Eu3+

Green: ZnS:Cu

Blue: ZnS:Ag

Phosphor-based white LEDs

47 Source: Merck + Wikipedia

Combination of blue LED and yellow

phosphor Y3Al5O12:Ce3+ (YAG:Ce)

gives (cold) white light

Different options to create white LEDs

48 Source: http://pglab.nolda.co.kr

Security inks

49

UV Euro bills

50

Under UV irradiation

Eu3+: red luminescence

Eu2+: green-yellow luminescence

Security inks

51

The euro is protected by the

luminescence from europium:

red from Eu3+

Spectrum resembles that

of a -diketonate complex

560 580 600 620 640 660 680 700 720

l / nm

l exc.

370.5 nm 5D0

7FJ

EuIII 2

J =

0 1 3 4

Ref. : J.C.G Bünzli, Lanthanide course

52

lexc= 375 nm

450 500 550 600 650 700

l/nm

Possibly Eu2+ ?

X-ray phosphors

• Designed to respond to X-rays re-emitting the energy as visible light

• Incorporated into a variety of X-ray imaging devices

Medical and security applications

• Most used phosphors:

Gd2O2S:Tb3+ green

La2O2S:Tb3+ green

Gd2O2S:Pr3+ green

Gd2O2S:Eu3+ red

53

Scintillation phosphors

• Used in the detection of alpha, beta and gamma radiation

• Need to have fast decay times (40-65 ns) and high densities

• Most used phosphors:

Lu2SiO5:Ce3+ peak: 400 nm

YAlO3:Ce3+ peak: 365 nm

Y3Al5O12:Ce3+ peak: 550 nm

• Use of Lu2SiO5:Ce3+ in PET scanners

is most important application of

lutetium

54

PET scanner

55 Source: http://www.cellsighttech.com

PET scanner

56

O

AcO

AcO18F

OAc

OH

Detects increased metabolic activity

Positron emitter (18F)