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TSL studies for the engineering of optical materials
Anna Vedda
Department of Materials Science University of Milano‐Bicocca
Success Meeting 2012 – Lyon
Main investigation fields:
1.Optical properties of rare earth ions in silica and silica based glass‐ceramics with crystalline nano‐phases, to obtain materials suitable to be used as scintillators in the detection of ionizing radiations and looking at the optical properties useful in photonics
2.Crystalline scintillators, such as tungstates, rare earth–doped perovskites, garnets and complex fluorides
3.Micro‐and nano structured luminescent materials (SrHfO3 , Lu4Hf3O12 , HfO2)
RESEARCH GROUP: Inorganic materials for sensing and photonics
Composition3 permanent staff members: N. Chiodini, A. Paleari, A. Vedda
3 post‐doc, 1PhD student, 2 undergraduates
Experimental facilities:
Synthesis laboratoryInorganic chemistry laboratory for sol‐gel preparations. Film deposition by spin‐coating. Furnaces for densification processes, instrumentation for optical finishing.
Physical characterization laboratory: optical absorptionphoto‐ thermo‐ and radio‐luminescence spectroscopy (10‐800 K)micro‐Raman scatteringrefractive index measurementprofilometrycomplex impedance spectroscopyFTIR absorptionICP‐Mass with laser ablationXRF
Within collaborations:SEM, TEM, XRD
The effect of Ga3+ dopingon the band structure
of Lu3GaxAl5-xO12 garnets
Outline
• Experimental data– UV‐Vis Absorption spectra– VUV Excitation spectra– TSL (initial rise technique)
• DFT calculations (VASP code) (Los Alamos)
Band gap
C.B. and V.B.
Lu3GaxAl5-xO12 Crystals
• Lu3GaxAl5-xO12 (LuGAG) single crystals grown by micro pulling down (m-PD) technique at the University of Sendai (Japan)
• Crystals size: 6 x 4 x 1 mm3 approximately
LuGAG:CeCe: 0.7 mol% (in the melt)Ga: 0%, 10%, 20% and 40%
LuGAG:EuEu: 0.1 mol% (in the melt)Ga: 0%, 20%, 60% ,100%
Band Gap shrinkingGa substitution induces a band gap shrinking as evidence by a low energy shift of the absorption edge in UV‐VIS absorption and inVUV‐PLE measurements.
1 2 3 4 5 6
LuGAG:Eu
Ga 0%Ga 20%Ga 60%Ga 100%
0
20
40
60
80
100
120
Abs
orp
tion
coef
ficie
nt (c
m-1
)
Energy (eV)
Eu3+ CT
Band edge
6.2 6.4 6.6 6.8 7 7.2 7.4
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Nor
ma
lized
PLE
am
plit
ude
Energy (eV)
LuGAG:CeGa 60%
LuGAG:CeGa 40%
LuGAG:CeGa 20%
LuGAG:CeGa 10%
LuGAG:PrGa 5%
LuGAG:CeGa 0%
Em. 248-310 nmat 8-10 K
Abs spectra PLE (at DESY)
Band Gap shrinking
• The band gap reduction was evaluated by combining PLE and Abs data.
• In LuGG (with 100% of Ga) the band gap is 1.6 eV smaller than in LuAG with no Ga content
0 20 40 60 80 100-2
-1.5
-1
-0.5
0
0.5
Band
gap
shift
(eV
)
Ga concentration (%)
From PLE spectra (Desy)
From Abs spectra
TSL mechanism (Ce, Eu)Ce3+ + h→ Ce4+
Ce4+ + e→ (Ce3+)* → Ce3+ + hν
Eu3+ + e→ Eu2+
Eu2+ + h→ (Eu3+)* → Eu3+ + hν
e trap
C.B.
V.B.
(Ce3+)*
Ce3+ h trap
C.B.
(Eu3+)*
Eu3+
V.B.
Irradiation
Heating
Released from an e trap Released from a h trap
TSL (initial rise technique) allows the evaluation of the thermal activation energy E of the e traps (in Ce‐doped LuGAG) and andh traps (in Eu –doped LuGAG)
ET
ET
Working assumption• We assume the e and h traps as fixed.• A change in the TSL thermal activation energy of an e trap (h trap) can thus be an approximate evaluation of the C.B. (V.B.) shift.
C.B.
V.B.
C.B.
V.B.
e trap
h trap
e trap
h trap
But let’s not forget that:
• The C.B. and V.B. are not flat.• The thermal activation energy is not directly comparable with the optical transition energy.
• The traps themselves could be affected by Ga substitution.
no Ga with Ga
TSL glow curves Ga substitution induces a low temperature shift of the TSL peaks in both Ce‐ and Eu‐doped LuGAG samples.
100 200 300 400
Nor
mal
ized
TSL
ampl
itud
e
Temperature (°C)
Ga 0%
Ga 10%
Ga 20%100 200 300 400
Nor
mal
ized
TSL
ampl
itud
e
Temperature (°C)
Ga 0%
Ga 60%
Ga 20%
Ga 100%
LuGAG:Ce LuGAG:Eu
Wavelength resolved TSL
LuGAG:Ce LuGAG:Eu
The dopant trivalent ion (Ce3+, Eu3+) is the recombination centre for all of the observed TSL peaks with the exception of the 450 °C peak in LuGAG:Eu.
Initial rise technique
101
102
103
104
105
106
0 100 200 300 400 500
LuGAG:CeGa 0% T
SL a
mpl
itud
e (a
rb. u
nits
)
Temperature (°C)
A
BC D
EF ?
101
102
103
104
105
106
0 100 200 300 400 500
LuGAG:CeGa 10%TS
L am
plitu
de
(arb
. uni
ts)
Temperature (°C)
A
B
C DE
101
102
103
104
105
106
0 100 200 300 400 500
LuGAG:CeGa 20%TS
L am
plitu
de
(arb
. uni
ts)
Temperature (°C)
B C
D
E
• Several TSL peaks can be identified in the glow curves.
• The thermal activation energies of the TSL peaks was evaluated by means of the initial rise technique
C.B. Shift - LuGAG:Ce
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0 10 20
Peak APeak BPeak CPeak DPeak F
Ener
gy sh
ift (e
V)
Ga concentration (%)
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 10 20
Ener
gy (e
V)
Ga concentration (%)
Peak A
Peak B
Peak C
Peak D
Peak E
Thermal activation energy Energy Shift
The thermal activation energy af all the TSL peaks shifts of approximately the same amount suggesting that our assumption that the traps are fixed is a good approximation.
101
102
103
104
105
106
0 100 200 300 400 500
LuGAG:EuGa 0%T
SL a
mpl
itud
e (a
rb. u
nits
)
Temperature (°C)
A B CD
E
F
101
102
103
104
105
0 100 200 300 400 500
LuGAG:EuGa 20%
TSL
ampl
itud
e (a
rb. u
nits
)
Temperature (°C)
AC
D
E
F
101
102
103
104
105
106
107
0 100 200 300 400 500
LuGAG:EuGa 60%
TSL
ampl
itud
e (a
rb. u
nits
)
Temperature (°C)
AC
D
E
101
102
103
104
105
0 100 200 300 400 500
LuGAG:EuGa 100%
TSL
ampl
itud
e (a
rb. u
nits
)
Temperature (°C)
D
E
0.8
1
1.2
1.4
1.6
1.8
0 20 40 60 80 100
Ener
gy (e
V)
Ga concentration (%)
Peak D
Peak C
Peak B
Peak A-0.2
0
0.2
0.4
0.6
0.8
0 20 40 60 80 100
Peak APeak CPeak D
Ener
gy sh
ift (e
V)
Ga concentration (%)
Thermal activation energy Energy Shift
V.B. Shift - LuGAG:Eu• The thermal activation energy shift of the h‐traps looks slightly more dispersed.• The trap related to the most intense TSL peak (D) shifts up to about 0.7 eV.
4 4.5 5 5.5 6
LuGAG:Eu 0.1%Em = 592 nm
Nor
mal
ized
Inte
nsity
Energy (eV)
Ga 100% Ga 60%Ga 20%
Ga 0%
Charge transfer band (CT)
C.B.
V.B.
CT transition
Eu2+
• The CT band allows the monitoring of the V.B. shift independent from TSL data.
CT band vs. TSL
CT band shift is consistent with the V.B. shift evidenced by TSL data of peak D.
-0.2
0
0.2
0.4
0.6
0.8
0 20 40 60 80 100
Peak APeak CPeak DCT - Band
Ener
gy sh
ift (e
V)
Ga concentration (%)
0 20 40 60 80 100
DFTAbs and PLE
-2
-1.5
-1
-0.5
0
0.5
Band
gap
shrin
king
(eV
)
Ga concentration (%)
• DFT calculations, as expected, underestimate tha band gap value (3.85 eV).
• The band gap shrinking is nevertheless reliable and in good agreement with the experimental data obtained from Absorption and photoluminescence excitation spectra.
DFT vs Exper. - Band Gap
CB shift DFT vs Experiment
0 20 40 60 80 100
DFTExperim. (TSL)
-1.5
-1
-0.5
0
0.5
CB
shift
(eV
)
Ga concentration (%)
• DFT calculations and TSL data are in agreement on the C.B. shift.
VB shift DFT vs Experiment
• DFT calculations and TSL data on the V.B. shift are slightly less consistent.
• DFT predicts a lower band shift than observed.
0 20 40 60 80 100
VB shift TEOVB shift TSLVB shift CT
-0.2
0
0.2
0.4
0.6
0.8
VB
shift
(eV
)
Ga concentration (%)
Conclusions• Experimental data (Abs and PLE) evidence that Ga substitution
in LuGAG induces a band gap shrinking of approximately 1.6 eV.
• TSL analyses (initial rise technique) show that the band gap shrinking is due to a shift of both the C.B. and V.B.
• Ab intio DFT calculations (VASP code) predict similar results showing a good quantitative agreement with the experimental data.
M. Fasoli, A. Vedda, M. Nikl, C. Jiang, B.P. Uberuaga, D.A. Andersson, K.J. McClellan, and C.R.Stanek,“Band‐gap engineering for removing shallow traps in rare‐earth Lu3Al5O12 garnet scintillators using Ga3+ doping”, Phys. Rev. B 84, 081102(R) (2011).
Evaluation of the lanthanide excited state thermal ionization barrier in luminescent materials
Thermal ionizationof the luminescent center
• When the excited level of the luminescence center is close enough to the conduction band, thermal ionization can occur.
• Under photo excitation of the luminescence center the electron can escape from the excited level to the conduction band and, possibly, get trapped in localized defects.
• The thermal ionization energy barrier Eth can be evaluated by exploiting trapping states.
CB
VB
Pr3+
TrapThermal
ionization
Photo excitation
Lu2Si2O7:Pr (LPS:Pr)
1. Thermally Stimulated Luminescence (TSL)
2. Delayed Recombination Temperature Dependence (DRTD)
Eth
Exploiting stable TSL peaks• If we excite, at a given temperature,
Pr3+ in the 4f‐5d1 absorption band, the fraction of electrons thermally promoted to the CB is:
• The fraction of ionized electrons trapped in a stable defect can be evaluated from the TSL glow curve.
• By plotting the TSL peak intensity as a function of the illumination temperature the value of the thermal ionization barrier Eth can be evaluated.
• If no TSL peak is stable in a convenient temperature range we are forced to deal with unstable traps.
TkE
b
th
e−
LPS:Pr
Excitation: 240 nm (5 nm b.p.)Temperature range: 283‐353 K
Delayed recombination
TSL glow curves of LPS:0.5mol%Pr afterlight illumination (240 nm, 630 μW/cm2) at different temperatures. The arrow indicates temperature increasing.
Arrhenius plot of the integrals obtainedfrom the glow curves of Fig. 1 from 430 and 490 K after subtractingthe background signal.
Results
ConclusionsThe method can be applied also on powder materials and thin films.
It exploits the TSL glow curves to evaluate the fraction of electrons thermally ionized.
‐ Needs a stable TSL peak ‐ Fast and simple‐ No correction required for the dependence of quantum efficiency of the recombination center upon temperature
M. Fasoli, A. Vedda, E. Mihokova, and M. Nikl, “Optical methods for the evaluation of lanthanide excited state thermal ionization barrier in luminescent materials”, Phys. Rev. B 85, 085127 (2012).
The effect of Ga3+ doping�on the band structure�of Lu3GaxAl5-xO12 garnetsOutlineLu3GaxAl5-xO12 CrystalsBand Gap shrinkingBand Gap shrinkingTSL mechanism (Ce, Eu)Working assumptionTSL glow curves Wavelength resolved TSLInitial rise techniqueC.B. Shift - LuGAG:CeConclusionsEvaluation of the lanthanide excited state thermal ionization barrier in luminescent materialsThermal ionization� of the luminescent center