Post on 21-Feb-2021
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
19
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
20
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
21
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)]
24
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.
28
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
33
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)