Download on the influence of sample preparation on the re

ON THE INFLUENCE OF SAMPLE PREPARATION ON THE RE-ABSORPTION IN
ABSOLUTE QUANTUM YIELD MEASUREMENTS BY INTEGRATING SPHERES
XVI International Symposium on Luminescence Spectrometry
Rhodes Island, Greece, September 2014
Robert Brüninghoff, David Enseling, and Thomas Jüstel
T OM
Tailored Optical Materials
Department of Chemical Engineering, Münster University of Applied Sciences, Stegerwaldstr. 39, Steinfurt, D-48565, Germany
Background: Quantum yield (QY) determination is presently done by different
techniques, i.e. by relative or absolute methods. A widely applied procedure to
measure the absolute quantum yield of organic solid state compounds by an
integrating sphere (Ulbricht sphere) were firstly described by Y. Kawamura [1].
However, for this method the effect of re-absorption has to be considered if the
luminescent material shows spectral overlap between the respective excitation and
emission band. Due to the measuring principle of an integration sphere, emitted
photons can partially be reabsorbed by the phosphor and subsequently re-emitted
with a probability determined by the quantum yield of the phosphor. This effect
leads to a lowered measured QY. We investigated the QY of YAG:Ce and the
influence of the re-absorption on the QY correlated with the phosphor
concentration. Furthermore, we applied a correction algorithm to the measured QY
as described by Ahn et al.[2] and reveal novel challenges in this context.
Excitation spectrum (Emis. monitored @ 560 nm)
Emission spectrum (Exc. @ 455 nm)
Reflection spectrum
5,5 5 4,5
4
3,5
3
2,5
(1)
Energy [eV]
2
1,0
100
0,8
80
1,0
Reference
Phosphor
60
0,4
40
0,2
0,8
Intensity [~ counts]
0,6
Reflection [%]
Intensity [a.u.]
0,9
20
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
0,0
400
0
250
300
350
400
450
500
550
600
650
700
750
800
500
600
700
Wavelength [nm]
Wavelength [nm]
Fig. 2: Measuring scheme of the integration sphere
for recording emission and reflection spectra
Fig. 1: Reflection spectra, normalised excitation- and
emission spectra of YAG:Ce
Fig. 3: Equation for QY calculation and schematic depiction of
the measurement
Principle: A concentration series of YAG:Ce (Philips U819) was prepared (Fig. 4). To this end, the phosphor was mixed with the reference white standard (BaSO4) in
1 %, 2 %, 5 %, 10 %, 20 %, 40 %, 60 % and 80 % by mass. The emission spectra (including the excitation band) (Fig. 5) were recorded with an Edinburgh Instruments FS
920 spectrometer using a Spectralon integration sphere. The measuring scheme is presented in Fig. 2. To calculate the observed QY, equation 1 (Fig. 3) was used.
For the correction of the observed quantum yields using the approach of Ahn et al. (eq. 3) the re-absorption parameter a is necessary. Re-absorption parameter a is
computed according to eq. 2. To determine the absorption the reflection spectra of the concentration sequence was recorded (Fig. 6) using the same spectrometer with
integrating sphere as described above.
Energy [eV]
2,8
2,6
2,4
2,2
2
1,8
4,5 4
1,6
107
10
5
U 819 1 %
U 819 2 %
U 819 5 %
U 819 10 %
U 819 20 %
U 819 40 %
U 819 60 %
U 819 80 %
U 819 100 %
0,7
104
0,6
0,5
0,4
0,3
0,2
0,1
0,0
500
600
700
300
800
400
Wavelength [nm]
800
0,02
0,01
0,02
0,10
0,14
0,23
0,22
0,33
0,43
QY corr.
98
99
97
96
95
95
95
95
95
(3)
QY obs.
QY corr.
100
98
QY [%]
QY obs. Re-Absorption
98
99
97
96
94
93
94
93
92
700
=
Tab. 1: Measured QY and corrected QY by approach of
Ahn et al. [2]
U 819 1 %
U 819 2 %
U 819 5 %
U 819 10 %
U 819 20 %
U 819 40 %
U 819 60 %
U 819 80 %
U 819 100 %
600
Fig. 6: Reflection spectra of the YAG:Ce (U819)/BaSO4
concentration sequence
Fig. 5: Emission spectra of the YAG:Ce (U819)/BaSO4
concentration sequence (with excitation band at 450 nm)
Sample
500
Wavelength [nm]
(2)
Results and Discussion: It turned out that a strong
influence of the phosphor concentration on the QY exist.
We observed an increase of the QY for a decrease of the
YAG:Ce (U819) concentration (Tab. 1, Fig. 7). This
phenomenon can be explained with a reduced reabsorption effect for low phosphor concentrations.
The correction of the measured QY values is more
essential for high YAG:Ce concentrations wich can be
explained with a higher re-absorption due to the high
phosphor concentration. However, the corrected values
should show a constant value for all concentrations. The
problem here is the correct determination of the absolute
absorbed photons. Due to scattering effects, the reabsorption of powder samples can not be so easily
determined as in non-scattering samples.
Energy [eV]
2
0,8
103
Fig. 4: Sample holder with YAG:Ce (U819)/BaSO4
– concentration series. Increasing YAG:Ce content
1 % (left, bottom), 2 %, 5 %, 10 %, 20 %, 40 %, 60
%, 80 % (right, bottom)
2,5
0,9
Reflection
Intensity [~ counts]
106
3
1,0
BaSO4
U 819 1 %
U 819 2 %
U 819 5 %
U 819 10 %
U 819 20 %
U 819 40 %
U 819 60 %
U 819 80 %
U 819 100 %
exc = 450 nm
3,5
96
94
92
90
0
10
20
30
40
50
60
70
80
90
100
w (YAG:Ce) [%]
Fig. 7: Measured QY and corrected QY by the approach of
Ahn et al. [2]
Conclusions: The effect of the phosphor concentration on the re-absorption and therefore on the QY is shown when using an integrating sphere. Furthermore, the lack of
optimised correction of the values is shown due to the lack of a correct determination of the absolute absorbed photons. By determination the QY using an integrating
sphere several parameters with respect to the re-absorption has to be considered. Not only the concentration of the phosphor but also the sample area and size of the
Ulbricht sphere are crucial for QY measurements.
Literature: [1] Y. Kawamura et al. Jpn. J. Appl. Phys., Vol. 43, No. 11A (2004); [2] Ahn et al. Rev. Sci. Instrum. 78, 086105 (2007)
Robert Brüninghoff, e-mail: [email protected]
web: http://www.fh-muenster.de/juestel