Transcript VUV spectroscopy of rare earth ions in solids: V.N. Makhov

VUV spectroscopy of rare earth ions in solids:
recent studies and possible applications
V.N. Makhov
P.N. Lebedev Physical Institute,
Russian Academy of Sciences, Moscow, Russia
F
I
Tartu Ülikool · FÜÜSIKA INSTITUUT
INSTITUTE OF PHYSICS · University of Tartu
Institute of Physics,
University of Tartu, Tartu, Estonia
Outline
1.
2.
3.
4.
5.
General optical properties of trivalent rare earth ions in solids: intraconfigurational
4f  4f and interconfigurational 5d  4f transitions; spin-allowed and spinforbidden 5d  4f transitions.
Prospects for applications of rare earth containing materials: quantum cutting (multiphoton) phosphors for high-efficiency Hg-free fluorescent lamps and plasma display
panels; new fast and efficient scintillators for medical imaging (PET).
VUV luminescence from Gd3+ ions: spectral properties, decay kinetics, thermal
quenching; assignment to Gd3+ 5d-4f luminescence; vibronic structure; the strength
of electron-phonon coupling.
VUV luminescence from Lu3+ ions: spectral and timing properties; assignment to
Lu3+ 5d-4f luminescence; vibronic structure; the strength of electron-phonon
coupling; spin-forbidden and spin-allowed Lu3+ 5d-4f luminescence: interplay with
temperature; thermal quenching.
Concluding remarks.
General optical properties
of trivalent rare earth ions in solids
Rare earth elements
Energy level structure for 4fn electronic configuration of
trivalent rare earth ions (Dieke diagram)
Crystal field splitting for 4f electronic configuration
Because of the shielding effect of the outer 5s and 5p shell electrons, the crystalfield interaction with inner 4f electrons is weak and can be treated as a perturbation
(Stark effect) of the free-ions states. Accordingly, the energies of the corresponding
levels of 4fn configuration are only weakly sensitive to the type of the crystal host.
Splitting of energy levels of 4fn electronic configuration due to: I – Coulomb interaction;
II – spin-orbit interaction; III – crystal-field interaction
Crystal field splitting for 4fn-15d electronic configuration
The 5d electrons are not effectively shielded by other electrons, and the crystal field
influence on the energy levels of 4fn-15d electronic configuration is strong.
Accordingly, crystal field splitting of 5d levels is large and the energies of levels within
4fn-15d electronic configuration can strongly differ for different crystal hosts.
Crystal-field splitting of 5d1 configuration for tetragonal Ce3+ center:
I – free ion, II – Oh, III – Oh + spin-orbit, IV – С4V
4f and 5d energy levels of Ce3+ in tetragonal environment
3+
Ce :LiYF4
55
Site symmetry S4
50

45
3
Energy, 10 cm
-1
40
35
5d
1
30
25
20
15
10
5
0
2
4f
1
F7/2
F5/2
2
SO
Energies of the lowest 4fn-15d levels for RE3+ ions
doped into LiYF4 crystal
3+
90
RE :LiYF4
SA
70
3
-1
Energy (10 cm )
80
VUV
60
SF
50
LS
40
HS
30
Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
20
1
2
3
4
5
6
7
8
9
10
Number of 4f electrons
11
12
13
14
Schematic electron configurations for the ground state
(GS) 4f8, the lowest energy high-spin (HS) 4f75d state and
the lowest energy low-spin (LS) 4f75d state for Tb3+
Single configuration-coordinate diagram of the 4f and 5d
states and of 4f – 4f and 4f – 5d transitions in rare earth ion
High-efficiency
VUV-excited phosphors
Why we need VUV-phosphor efficiency > 100% ?
85
6
0.25/0.17 = 1.47
We need phosphor with Q > 100%
Quantum splitting (quantum cutting) schemes
Visible quantum cutting by two-step energy transfer
upon excitation in the 6GJ levels of Gd3+
1 violet photon absorbed on
Gd3+ 8S7/2→6GJ transitions,
2 red photons emitted on
Eu3+ 5D0→7F1 transitions
LiGdF4:Eu3+
GdF3:Eu3+
Visible quantum cutting via down-conversion
in LiGdF4:Er3+,Tb3+
1 VUV photon absorbed on
Er3+ 4f11 – 4f105d transition,
2 photons emitted on :
1) Er3+ 4S3/2→4I15/2 transition;
2) Tb3+ 5D3,4→7FJ transitions;
Scintillators for medical applications
(PET)
Principles of PET
Ring of Photon
Detectors
 Patient injected with drug
having + emitting isotope.
 Drug localizes in patient.
 Isotope decays, emitting +.
 + annihilates with e– from
tissue, forming back-to- back
511 keV photon pair.
 511 keV photon pairs detected
via time coincidence.
 Positron lies on line defined by
detector pair (a chord).
Produces planar image of a “slice” through patient
Scintillators for PET based on 5d – 4f transitions in Ce3+
Density
(g/cm3)
Atten. length
(mm) at 511 keV
Phot. eff.
%
Light yield
(phot/MeV)
Dec. time
(ns)
Wavel.
max. (nm)
Bi4Ge3O12
(BGO)
Lu2SiO5:Ce
(LSO)
LuAlO3:Ce
(LuAP)
Lu2Si2O7:Ce
(LPS)
Lu2S3:Ce
7.1
10.4
40
9000
300
480
7.4
11.4
32
26000
40
420
8.3
10.5
30
12000
18
365
6.2
14.1
29
20000
30
380
6.2
13.8
28000
32
590
Gd2SiO5:Ce
(GSO)
LaCl3:Ce
6.7
14.1
25
8000
60
440
3.86
27.8
14
46000
25
330
Requirements to new scintillators:
Lifetime of the emitting state (scintillation decay time): τ  λem3  shorterwavelength emission is needed for increasing time resolution of scintillation
detector: Pr3+, Nd3+, … activator ions with shorter-wavelength (UV/VUV)
and faster 5d – 4f transitions can be used instead of Ce3+.
Experimental setup for VUV spectroscopy
with synchrotron radiation
SUPERLUMI station at HASYLAB (DESY)
Primary monochromator
3 secondary monochromators
Position-sensitive detectors
Mechanical chopper
In-situ cleaving
4 to 900 K
G. Zimmerer, Radiation Measurements 42 (2007) 859
5d – 4f luminescence from Gd3+
The scheme of radiative and nonradiative transitions in Gd3+
90
7
6
4f
81.0
4f 5d
80
80.5
FJ
6
G
6 J
DJ 6
50
40
6
30
PJ
80.0
3
6
IJ
6
79.5
4f 5d
?
(2S+1=8)
79.0
78.5
20
78.0
10
0
8
7
4f
Gd
3+
S7/2
Nonradiative relaxation
(intersystem crossing)
is heavily spin-forbidden
(2S+1=2,4)
-1
60
Energy (10 cm
3
-1
Energy (10 cm )
)
70
VUV emission spectra of GdF3, LiGdF4 and CaF2:Gd3+(0.1%)
LiGdF4
GdF3
CaF2:Gd
3+
Emission intensity (a.u.)
300
200
S>5
S~1
100
0
122
124
126
128
130
132
Wavelength (nm)
M. Kirm, J.C. Krupa, V.N. Makhov, M. True, S. Vielhauer, G. Zimmerer,
Phys. Rev B 70, 241101(R) (2004)
Counts/channel
Decay curves of VUV luminescence from Gd3+-containing samples
10
4
10
3
3+
CaF2:Gd (0.1%)  = 8.5 ns
3+
LiGdF4:Ce (0.05%)  = 2.8 ns
LiGdF4:pure
}
LiYF4:Gd (10%)  = 2.5 ns
3+
10
2
3+
YF3:Gd (1%)  = 1.1 ns
GdF3  = 0.97 ns
10
1
SR
10
0
0
5
10
15
20
Time (ns)
25
30
35
Temperature dependence of VUV luminescence from GdF3
10 K
Emission intensity (a.u.)
ex = 117 nm
Emission intensity (a.u.)
30 K
60 K
-1
a~ 400 cm
Mott law
0
90 K
40
80
120
160
Temperature (K)
120 K
150 K
122
123
124
125
126
127
Wavelength (nm)
128
129
130
Temperature dependence of decay kinetics for Gd3+ 4f65d
– 4f7 emission from CaF2:Gd3+(0.1%),Ce3+(0.05%) crystal
in the range of 8 – 149 K
10
8
-1
6
hem=77820 cm
3
 (ns)
4
-1
Mott law
4
Ea=680 cm
2
0
0
50
-1
100
150
Temperature (K)
10
2
10
1
10
0
T=8K
T =149 K
Counts/channel
10
hex=80320 cm
0
10
20
Time (ns)
30
40
Comparison of Gd3+ 5d – 4f emission spectrum from LiGdF4 and
Ce3+ 4f – 5d excitation (absorption) spectrum from LiGdF4:Ce3+
3
-1
Energy (10 cm )
34,6
34,4
34,2
34,0
33,8
33,6
33,4
Absorption coefficient
calculations
3+
Emission intensity
LiGdF4:Ce (0.05%)
experiment
M. Kirm, G. Stryganyuk,
S. Vielhauer, G. Zimmerer,
V.N. Makhov, B.Z. Malkin,
O.V. Solovyev, R.Yu. Abdulsabirov,
S.L. Korableva, Phys. Rev. B 75,
075111 (2007)
LiGdF4
78,4
78,6
78,8
79,0
3
79,2
-1
Energy (10 cm )
79,4
79,6
Charge compensation of RE3+ ion in CaF2 by interstitial ions
If optically active RE3+ ions substitute for other (optically non-active) RE3+ ions of the
same charge state: Y3+, Sc3+, La3+, the site symmetry for optical centers will be the same
as for the ions in the host crystal. If the charge state of the cation in the host crystal is
different (e.g. +2) the charge compensation is necessary, which is reached usually by
neighboring interstitial ions which reduce the local symmetry of optical center.
C4V
C2V
C3V
compensation
Emission and absorption (excitation) spectra due to
4f  5d transitions in Ce3+ (C4v) doped into CaF2
3+
CaF2:Ce (0.02%)
5d – 4f 2F7/2
T=8 K
480 cm-1
Emission intensity
5d – 4f 2F5/2
4f 2F5/2 – 5d
Excitation spectrum
Emission spectrum
28
29
30
31
3
32
-1
Energy (10 cm )
33
High-resolution ( ~1 Å) VUV emission spectrum under 124.7
nm excitation and excitation spectrum of Gd3+ 4f65d – 4f7 emission
at 129 nm from CaF2:Gd3+(0.1%),Ce3+(0.05%) crystal
hex= 80190 cm
Luminescence intensity
-1
~1970 cm-1
*
3+
CaF2:Gd ,Ce
370 cm-1
3+
Spectral lines tentatively
ascribed to ZPLs are marked
by symbol “  ”, and to
dominating vibronic lines
by symbol “  “
T=9.5 K
*
hem= 77520 cm
*
-1
*
*
V.N. Makhov, S.Kh. Batygov,
L.N. Dmitruk, M. Kirm, G.
Stryganyuk, and G. Zimmerer,
*
76
78
80
82
3
84
-1
Energy (10 cm )
86
phys. stat. sol. (c) 4, 881 (2007)
Up-conversion excitation to
Gd3+ 4f65d configuration
by KrF excimer laser
D. Lo, V.N. Makhov, N.M. Khaidukov,
J.C. Krupa, J. Luminescence 119-120, 28
(2006)
5d – 4f luminescence from Lu3+
Lu3+ 4f135d – 4f14 emission and 4f14 – 4f135d excitation
spectra for several fluoride matrices
90
13
4f 5d
T=10 K
LS
LuF3
HS
80
Intensity
-1
60
3
Energy (10 cm )
70
50
LiLuF4
40
SA
SF
30
20
10
0
LiYF4:Lu
1
14
4f
Lu
78
80
82
84
3
-1
Energy (10 cm )
86
3+
S0
M. Kirm, J.C. Krupa, V.N. Makhov,
M. True, S. Vielhauer, G. Zimmerer,
Phys. Rev B 70, 241101(R) (2004)
Lu3+ d-f emission and f-d excitation spectra from CaF2:Lu3+(0.04%)
Luminescence intensity
hexc=82990 cm
-1
hem=79680 cm
T=8 K
-1
78
80
No zero-phonon line in spinforbidden transitions because of
extremely low probability for
pure electronic transitions: only
vibronic lines are observable
ZPL
ZPL ?
Pure electronic spin-forbidden
transitions (in emission):
82
84
3
86
-1
Energy (10 cm )
V.N. Makhov, S.Kh. Batygov, L.N. Dmitruk, M. Kirm, S. Vielhauer,
and G. Stryganyuk, Physics of the Solid State 50, 1565 (2008)
Appearance of emission band due to spin-allowed 5d – 4f
transitions in Lu3+ at higher temperatures due to thermal
population of the higher-lying low-spin 5d state
T=8.2 K
SF
-1
hem= 80320 cm
-1
Luminescence intensity
hexc= 84750 cm
SA
T = 8.3 - 258 K
T=168.9 K
-1
hem= 81700 cm
79
80
81
82
3
83
-1
Energy (10 cm )
M. Kirm, G. Stryganyuk, S. Vielhauer, G. Zimmerer, V.N. Makhov, B.Z. Malkin, O.V.
Solovyev, R.Yu. Abdulsabirov, S.L. Korableva, Phys. Rev. B 75, 075111 (2007)
LuF3
ex= 116 nm
Luminescence intensity
SF
Normalized spectra of VUV
emission due to Lu3+ 5d – 4f
transitions in LuF3 measured
at different temperatures
80
8.1 K
5d (LS)
5d (HS)
70
80.5 K
60
200.5 K
240.3 K
79
80
81
82
3
40
30
20
SA
78
50
3
147.2 K
Energy (10 cm
-1
)
SF
-1
Energy (10 cm )
10
83
0
1
S0
SA
5.0
3+
T=790 K
LiYF4:Tm (1%)
4.5
slow
ex=140 nm
4.0
T=530 K
Normalized time-resolved
spectra of VUV emission due to
Tm3+ 5d – 4f transitions in
LiYF4:Tm3+
slow
3.5
3.0
T=530 K
fast
50
slow
(SF)
40
3
-1
)
2.5
3
3
3
1.5
H5
F4
H6
T=350 K
fast
1.0
T=350 K
3
30
3
F4
3
H6
20
10
60
65
3
-1
Energy (10 cm )
I6
D2
G4
3
3
0.0
55
1
1
slow
0.5
P2
P1 1
3
2.0
Energy (10 cm
dI/dE (arb.units)
60
5d (LS)
5d (HS)
0
F
F3 3 2
H4
3
H
3 5
F4
3
H6
fast
(SA)
Temperature dependence of
5d – 4f luminescence from
Er3+ doped into LiYF4:
time-resolved VUV
emission spectra
V.N. Makhov, N.M. Khaidukov,
N.Yu. Kirikova, M. Kirm, J.C.
Krupa, T.V. Ouvarova, G. Zimmerer,
J. Lumin. 87-89, 1005 (2000)
Energy splitting between low-spin (LS) and high-spin (HS)
5d states of heavy RE3+ ions (from Tb3+ to Lu3+) in LiYF4
LS
1500
Yb3+
4f13
HS
800
Lu3+
4f14
L. van Pieterson, R.T. Wegh, A. Meijerink, M.F. Reid, J. Chem. Phys. 115, 9382 (2001)
LS
HS
Luminescence intensity (rel.units)
Temperature dependence of integrated intensity of VUV
luminescence from LuF3, LiYF4:Tm3+ and LiYF4:Er3+
1.0
LiYF4:Er
0.8
LiYF4:Tm
0.6
3+
3+
LuF3
0.4
a=0.04 eV
a=0.50 eV
0.2
0.0
0
100
200
300
400
500
600
700
800
Temperature (K)
The curves are the best fits with the formula: I(T)/I(0) = (1+A exp(-a/kBT))-1 ,
a activation energy, A pre-exponent factor (fitting parameters), kBBoltzmann constant.
V.N. Makhov, T. Adamberg, M. Kirm, S. Vielhauer, G. Stryganyuk, J. Lumin. 128, 725 (2008)
Different mechanisms of thermal quenching
for RE3+ 5d – 4f luminescence
conduction band
5d
5d
Energy
4f
Energy
Energy
5d
4f
4f
valence band
0
Q
Multi-phonon relaxation
0
Q
Thermally activated
intersystem crossing
Thermally activated ionization
to conduction band
Position of 4f and 5d energy levels of RE3+ and RE2+ ions
in the band gap of the host crystal (CaF2)
Conduction band
Valence band
P. Dorenbos, J. Phys.: Condens. Matter 15, 8417 (2003)
, eV
Trends in 5d levels position with respect to conduction band
for RE3+ ions in the second half of lanthanide series
3
-1
Energy (10 cm )
2
Conduction band
0
LS
-2
HS
-4
-6
-8
Tb
8
3+
Dy
9
3+
Ho
3+
10
Er
3+
11
3+
3+
Tm
Yb
12
13
Lu
3+
14
Number of 4f electrons
V.N. Makhov, M. Kirm, S. Vielhauer, G. Stryganyuk, G. Zimmerer,
ECS Transactions 11, 1 (2008)
Concluding remarks
 High-resolution (~0.5 Å) VUV emission and excitation spectra as well as decay
kinetics of VUV luminescence obtained for LiGdF4, LiYF4:Gd3+(1.0, 10%), GdF3,
YF3:Gd3+(1.0%), CaF2:Gd3+(0.1%), LiLuF4, LiYF4:Lu3+(0.5%, 1.0%, 5.0%), LuF3
and CaF2:Lu3+(0.04%), evidently show that this VUV luminescence originates from
4f65d – 4f7 transitions in Gd3+ for Gd-containing materials and from 4f135d – 4f14
transitions in Lu3+ for Lu-containing crystals.
 The fine structure due to zero-phonon and vibronic lines along with wide side bands
observed in VUV emission and excitation spectra of LiGdF4, LiYF4:Gd3+,
CaF2:Gd3+, LiLuF4, LiYF4:Lu3+ and CaF2:Lu3+ indicate intermediate electron-lattice
coupling (S ~1) between the 4fn-15d electronic configurations of the Gd3+ and Lu3+
ions and the lattice vibrations in these matrices, whereas the spectra of GdF3,
YF3:Gd3+ and LuF3 have a smooth shape and large Stokes shift because of strong
electron-lattice coupling (S > 5).
 The observation of Gd3+ 4f65d – 4f7 luminescence requires an assumption that a
dense 4f-level system behind the 5d-excitations not necessarily quenches 5demission. The influence of spin selection rules on energy relaxation should be taken
into account.
 Interplay with temperature of spin-allowed and spin-forbidden d-f luminescence
from rare earth ions in the second half of lanthanide series agrees with the common
trend in decreasing energy splitting between the lowest high-spin and low-spin 5d
levels towards heavier rare earth ions.
Thermal quenching of d-f luminescence agrees with the common trend in decreasing
energy gap between the lowest 5d level and the bottom of the conduction band of the
host crystal towards heavier rare earth ions.
Only fast spin-allowed d – f luminescence is observed from Gd3+ compounds, whereas
both spin-forbidden and spin-allowed d – f luminescence has been detected from Lu3+
compounds, the latter being observed only at high enough temperatures.
Many new observations were obtained during past years concerning fundamental
optical properties in VUV of RE ions in solids. However, possible practical application
of RE containing materials with optical activity in VUV is still under discussion.
Acknowledgements
Many thanks to all co-workers from P.N. Lebedev Physical Institute and
various Institutions from Russia and other countries for fruitful collaboration
when performing joint experiments with the use of synchrotron radiation.
Thank you for your attention !
Emission spectrum of LiYF4:Er3+ crystal due to spin-allowed
(fast component) and spin-forbidden (slow component)
4f105d – 4f11 transitions in Er3+
4
4f5d
I15/2
LiYF4:Er(5%)
2000
T=300 K
Intensity, a.u.
fast
slow
hex= 10 eV
1500
1000
4
4
I13/2
500
4
F9/2
11/2
4
F7/2,5/2,3/2( S3/2)
4
G11/2...7/2
9/2
0
140
*10
160
180
200
220
240
Wavelength, nm
260
280
Decay kinetics for different emission bands corresponding to
spin-allowed (S-A) and spin forbidden (S-F) 4f105d - 4f11
radiative transitions in Er3+doped into some fluoride crystals
BaY2F8, S-A
Counts/channel
10000
Er
3+
T=10 K
BaY2F8, S-F
1000
LiYF4, S-F
100
ErF3
LiYF4, S-A
10
SR
1
0
20
40
60
Time, ns
80
100
120
UV/ VUV excited phosphors in lighting devices
Schematic representation of one end of a fluorescent tube, illustrating the
process of the generation of visible light.
Schematic representation of a single plasma display cell, illustrating the process
of light generation.
Quantum cascades in Pr3+ doped materials
Photon #1
Photon #2
Table 1. Comparison between calculated and experimental
values of the lowest 4f5d excitation energies (zero-phonon
lines, cm-1) of Gd3+ ion in LiYF4:Gd3+, LiGdF4 and CaF2:Gd3+
matrix
LiYF4
LiGdF4
CaF2
Ce3+ 4f-5d ZPL
(exp)
33450
33615
31930
Gd3+ 4f-5d ZPL
(calc)
79250
79415
77730
Gd3+ 4f-5d ZPL
(exp)
79250
79377
77660
Gd
Ce
EZPL
 EZPL
 E Gd ,Ce , EGd,Ce = 45800 cm-1
P. Dorenbos, J. Luminescence 91, 91, 155 (2000)
Table 2. Comparison between calculated and experimental values
of the lowest S-A 4f5d excitation energies (zero-phonon lines,
cm-1) of Lu3+ ion in LiYF4:Lu3+, LiLuF4 and CaF2:Lu3+
matrix
LiYF4
LiLuF4
CaF2
Ce3+ 4f-5d ZPL
(exp)
33450
33130
31930
Lu3+ 4f-5d ZPL
(calc)
82620
82300
81100
Lu3+ 4f-5d ZPL
(exp)
81877
81777
82355
ELu,Ce = 49170 cm-1