Transcript The Effects of Annealing Conditions and Concentration o

The Effect of Annealing Conditions and Concentration on
5D  7F Emission in Terbium-doped Sol-gel Glasses *
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J
Colleen Gillespie and Dan Boye, Davidson College, Davidson, NC
Ann Silversmith, Hamilton College, Clinton, NY
Abstract
Sol-gel Recipe
Theory:
Annealing conditions
Theory: Concentration
Dependence
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Dissolve 11.8 mg of terbium nitrate (Tb(NO3)3*5H2O) and 20.3 mg of
aluminum nitrate (Al(NO3)3*9H2O) in 7.8 mL deionized water.
Add 20 μL concentrated nitric acid and 4.00 mL TMOS
(tetramethylorthosilicate, 99%) to solution and stir for 10 minutes
Put sol into 4 tightly capped polystyrene disposable test tubes
Gel at room temperature for 72 hours
Ramp at 5ºC/hr to 60ºC, then sit for 48 hours
Ramp at 5ºC/hr to 90ºC, then sit for 48 hours
Ramp at 2ºC/hr to 110ºC, then sit for 48 hours
Let cool to room temperature
The sample on the left has a concentration of 0.5%, and the
one on the right is 0.1%. The 0.5% sample has very little
emission from the 5D3 level (blue light) while the 0.1%
sample has a substantial amount of emission from this
level, and therefore looks blue-ish.
5D
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5D
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Exposure to Humidity
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When the samples are taken to 750ºC, the 5D3 peak becomes
detectable. Taking the samples to higher temperatures and for
longer dwell times at those temperatures, the intensity of the
5D peak becomes stronger. However, if the samples are
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annealed at too high a temperature, they can start to crystallize
or crack. Therefore, we wanted to find an ideal annealing
temperature and time to have a high 5D3 emission without any
damage to the sample.
Which sample looks bluer? So, which sample has the lower
concentration?
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Hydroxyl groups are present in the sol-gel materials. They
have an absorption band between 2000 and 4000 cm-1. This
allows electrons in the 5D3 level to relax to the 5D4 level by
losing energy to another hydroxyl group combined with a
lattice vibrational mode. This means that there is virtually no
fluorescence from the 5D3 level. However, the number of
hydroxyl grounds in the material can be significantly reduced
by annealing the samples at a high temperature.
Consequently, the 5D3 emission becomes comparable to 5D4
emission.
Picture of Samples
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2
0
7F
6.0E+07
0
1
2
3
4
5
7F
5.0E+07
0 mins
13 mins
25 mins
3 mins in H2O
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4.0E+07
A cross-relaxation process involving two Tb3+ ions depopulates the
5D level and reduces 5D emission intensity. At higher terbium
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concentrations, the terbium atoms are closer together, so there is
stronger cross-relaxation. Therefore, we predict that as terbium
concentration increases, the intensity of the 5D3 peak will decrease.
Fluorescence
Sol-gel synthesis is a low temperature was to prepare optically
transparent materials. Because the sol-gel is prepared at room
temperature, it is easy to include metallic, organic, and inorganic
additives. The optical properties of a sol-gel that has been heated to
~1000ºC are similar to those of traditional melt glasses, but since this
temperature is below the melting point of the material, the sol-gels
can hold more dopants than melt glasses. Sol-gels with rare earth
ions as dopants are used in phosphors, solid-state lasers, and
amplifiers. We can use the optical properties of these materials to
characterize the interactions between the dopants and the
surrounding material.
Energy (1000cm-1)
Motivation:
Sol-gel glasses
Sol-gel glasses have optical properties similar to those of traditional melt glasses, but are appealing because they can hold a higher
concentration of dopants due to their lower processing temperatures. In silicate sol-gel doped with trivalent terbium, the intensity of
fluorescence from the 5D3 level to the 7FJ (J = 0…6) ground state manifold levels is highly dependent on both terbium concentration and
annealing conditions. 5D3 emission is observed in glasses annealed at 750C and increases in intensity with increasing annealing time
and with higher annealing temperature. The relative intensity of emission from the 5D3 state decreases with increasing Tb3+
concentration. A cross-relaxation process involving two nearby Tb3+ ions depopulates the 5D3 level and causes this concentration
quenching.
3.0E+07
2.0E+07
1.0E+07
0.0E+00
400
Results: Concentration
Dependence
3.5
1000
5D
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New sample
2.5
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concentration dependence
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5D
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Ratio
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1.5
590nm
620nm
545nm
490nm
436nm
460nm
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414nm
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16
1
12
0.5
10
8
6
4
2
0
This is a partial energy level diagram of trivalent terbium, with labeled
transitions corresponding to observed fluorescence lines. The
samples were excited with 240 nm excitation light. The 5D4  7F5
transition dominates the emission spectrum and produces a green
color.
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After the sample is taken out of the oven, it begins to reabsorb
hydroxyl groups, and the 5D3 peak begins to shrink. Here is a
close up of the 5D3 peaks at three consecutive times. This
decay is enhanced by increasing the hydroxyl concentration by
putting the sample in water for a few minutes.
0.10%
Conclusions
0.20%
1.9E+08
0.50%
1.4E+08
9.0E+07
4.0E+07
7F
0
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2
3
4
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7F
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480
0.05%
2.4E+08
Fluorescence (arb. units)
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460
900
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Energy (1000cm-1)
2.9E+08
800
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440
wavelength (nm)
Results:
Annealing Conditions
Terbium Energy Levels
420
0
0
2
4
6
8
10
12
14
Dwell time at highest temperature
-1.0E+07
400
450
500
550
600
650
• We determined that heating the samples to 900ºC with a 12
hour dwell gives optimal optical properties without damaging
the sample.
• We confirmed that the 5D3 emission intensity decreases
with increasing terbium concentration
• Now that we have determined the best protocol for
processing these samples, we can use our knowledge to
begin working with erbium-doped sol-gels.
Wavelength (nm)
I heated the sample to 800, 900, and 1000ºC for various dwell
times. I then plotted the ratio of the 5D3  7F4 transition to the
5D  7F transition. At higher temperatures and for higher
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dwell times, the ratio increases. The sample shattered after
being at 1000ºC for 4 hours.
Plotting spectra of four different concentrations, it is clear that
5D emission decreases as terbium concentration increases.
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•Project supported by a grant
from the NSF-NRI program