Spectroscopic Analysis of Tb3 Doped Sol-Gels

1
Spectroscopic Analysis of Tb3 Doped Sol-Gels
Brendan W. Sullivan 07, Ann J. Silversmith,
Karen S. Brewer Departments of Chemistry and
Physics, Hamilton College
One phenomenon that we examined and found to be
consistent with current literature is the effect
of Aluminum co-doping on the fluorescence of
rare-earth ions in sol-gels. It is believed that
the addition of Al3 ions to the matrix will
increase the inter-ion separation between RE ions
and thus reduce the effects of clustering. Our
experimentation confirmed this idea, as show in
the two graphs on the left. Figure D is an
emission spectra of a sol-gel pellet containing
0.02 Tb and 0 Al (all percentages are by molar
ratio relative to TMOS). Figure E is an emission
spectra where the pellet also contains 2 Al. The
5D3 emission of the sol-gel containing 2 Al is
clearly much stronger, thus showing that
co-doping with Al3 ions helps reduce quenching
effects on 5D3 emission.

• In recent years, there has been much research in
  the field of rare-earth (RE) dope sol-gel
  glasses. These glasses are formed by mixing
  water, hydrochloric acid, and either TMOS
  (tetramethylorthosilicate) or TEOS
  (tetraethylorthosilicate), and gradually heating
  over the span of a few days. This allows the
  mixture to gel, and then by letting it cool, a
  solid piece of glass is formed.
• During the formation of the gels, we can add
  ions of rare-earth metals from the lanthanide
  series of the periodic table. These elements
  frequently have very strong fluorescence in the
  visible spectrum.
• By exciting the ions electrons with ultraviolet
  light, the electrons will jump to a higher energy
  level, and subsequently drop down to a lower
  level, in the process emitting a photon. These
  photons are what cause the gels to glow.
• Figure A shows a Dieke diagram depicting energy
  levels for all the RE elements

This individual project was intended to examine
the quenching processes in Terbium (Tb) doped
sol-gel glasses. Neighboring Tb3 ions in the
sol-gel matrix can undergo a process called
cross-relaxation, where one ions electron drops
down in energy and donates this energy to the
other ions electron (Figure B). This is a
problem because that drop in energy of the first
electron yields a photon with a wavelength
outside of the visible spectrum, whereas other
energy level changes result in visible
fluorescence that can be examined easily (Figure
C).
Fig. D
Another problem has to do with residual hydroxyl
(OH-) groups that remain even after the annealing
process in sol-gel synthesis. These ion groups
can affect the fluorescence of the Tb3 ions by
releasing and absorbing energy into phonons, or
tiny vibrational energies, as opposed to light.
Fig. C Tb3 energy levels
These are the problems we sought to examine with
this project, in hopes of finding Tb to be a good
probe for learning about rare-earth clustering
and sol-gels in general. Along the way, we
discovered other interesting phenomena as well.
Fig. E
Fig. B cross-relaxation
(all spectroscopic data obtained on Fluoromax)
Another factor that affects 5D3 emission is the
concentration of Tb3 ions. We hypothesize that
higher concentrations of RE ions result in more
clustering and thus more quenching of
fluorescence, and that lower concentrations will
tend to show stronger peaks in the 5D3 range.
This can be seen in Figure F. However, we have
noticed that the differences between subsequent
concentration changes become smaller for very low
concentrations. The 5D3 to 5D4 peak ratios (see
Figure I below) are very similar for 0.01 Tb and
0.02 Tb, and both are typically much better than
any other concentration, so its possible that we
have found the lower limit of RE concentration,
meaning that the ions are dispersed through the
matrix well enough such that any further
reductions in concentration will not reduce
clustering effects.
Fig. G annealing temperature dependence
Fig. A Dieke diagram
Fig. F concentration dependence
Typically our sol-gels are annealed by gradually
heating them to 750C for 24 hours. As an
experiment, we tried reheating sol-gel samples to
higher temperatures for 6 hours. For instance, we
tried reheating to 900C, 950C, and 1000C for 6
hours. In terms of qualitative data, heating to
950C and 1000C yielded sol-gels that were
slightly more cloudy, and less transparent.
Heating to 900C, though, seemed like a perfect
temperature, as it yielded optically clear
samples, and also showed strong 5D3 emission when
tested. Figure G shows a comparison between
annealing at 750C and reheating to 900C.
The most interesting phenomenon we noticed was
the decay of 5D3 emission over time after
reheating. Heating to 900C yielded great results
with strong 5D3 peaks, but unfortunately, when
tested the next day, there was significantly less
fluorescence in that region. Figure H shows how a
sample containing 0.02 Tb and 2 Al decayed over
time after being heated to 900C for 6 hours.
We also experimented with other co-dopants in the
sol-gel recipe. One paper mentioned their results
when co-doping with both Al3 and Na ions,
saying that when both Al3 and Na ions are
co-doped at the same time, the composition
dependent PL spectrarevealed that one of them
compensates the spectral change due to the
other.1 We attempted to obtain similar results
by co-doping such that the total concentration of
Al3 and Na ions summed to 2. When annealed at
750C for 24 hours, the samples showed very
little activity. However, when reheated to 900C
for 6 hours, they showed significantly stronger
5D3 emission (see Figure J). The 2 Na and 1.5
Na samples, though, still showed little activity.
The best co-dopant amount that we tried appears
to be 0.5 Na and 1.5 Al. Perhaps in the future
other combinations of Al3/Na can be tested.
Fig. H 5D3 emission time decay
To compare the 5D3 emission of different samples,
we created a 5D3 to 5D4 ratio. We calculated the
heights of each 5D3 peak and one 5D4 peak, and
then found their ratio. Figure I shows the ratios
for 5 sets of samples that we tested. It is clear
that 2 Al shows more activity than 0 Al, and
900C/6hrs shows more activity than 750C/24hrs.
Also, the samples that had aged 3 days since
reheating to 900C show very little
activity. Finally, we tried reheating a set of
samples to 900C for 6 hours and then immediately
placing them in a secure environment. To do this
we used Schlenkware and a vacuum to create an
airtight environment and to keep the samples
unaffected by the atmosphere for 2 days. Then we
removed them and tested their fluorescence. The
results showed that they behaved almost as well
as completely fresh samples, and clearly much
better than other aged samples.
Fig. I 5D3 to 5D4 ratios
Conclusions Our project has examined many
characteristics of Tb3 sol-gels. We have
confirmed the idea that Al3 co-doping reduces
clustering effects and improves 5D3 emission.
Concentration effects, as well, have been
confirmed, and in fact we believe we have found
the lower limit of Tb3 concentration. Annealing
temperature dependence, and the effects of decay
over time after reheating, however, can be
explored much further. Our experiments have shown
that after reheating to 900C for 6 hours, 5D3
emission gradually decreases (see Figure K), but
we are unsure as to why exactly this happens.
Perhaps future experiments will be able to
examine the mechanism for this process and
discover ways to prevent it. Finally, the effects
of Al/Na co-doping can also be explored further
by trying other combinations of the two ions.
Fig. J Al3/Na co-doping
Fig. K 5D3 to 5D4 ratios over time
References 1. K. Itoh, N. Kamata, T. Shimazu, C.
Satoh, K. Tonooka, K. Yamada. Journal of
Luminescence 87-89 (2000) 676-678. 2. T.
Ishizaka, R. Nozaki, Y. Kurokawa. Journal of
Physics and Chemistry of Solids 63 (2002)
613-617. 3. R. Reisfeld, T. Saraidarov, E.
Ziganski, M. Gaft, S. Lis, M. Pietraskiewicz.
Journal of Luminescence 102-103 (2003)
243-247. Thanks to Profs. Silversmith and
Brewer, Greg Armstrong, Helena Grabo, Kate
Schirmer, Peter Burke