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S.A. Klimin et al. / Physics Letters A 323 (2004) 159–163
Table 1
Samples used for spectroscopic studies
Sample
Electrolyte
Anodizing duration
(hours)
Average pore
diameter (nm)
Xerogel composition
Number of spin-on
depositions
Er O /TiO (wt.%)
2
3
2
B4N1
B4N2
B4N3
B4N4
Oxalic acid
12
18
12
18
40
80
80
80
20/80
20/80
50/50
50/50
9
9
7
5
Phosphoric acid
Phosphoric acid
Phosphoric acid
luminescence of the embedded Er [3], Tb [4], and
Eu [5] trivalent ions. The 1.53 µm emission from Er-
doped titania obtained via sol-gel route [3] and laser
ablation [6,7] was reported, whereas less attention has
repeated from 5 to 9 times. Finally, the annealing at
900 ◦C for 30 min in ambient atmosphere was applied
to activate the Er ions. The description of samples used
in experiments is presented in Table 1.
The visible luminescence of the samples at room
temperature was excited either by the third harmonic
of the YAG-Nd laser (λexc = 355 nm) or by the out-
put radiation of an optical parametric oscillator (OPO)
(λexc = 488 nm) pumped by the third harmonic of a
Q-switched Nd:YAG laser. An intensified optical mul-
tichannel analyzer (OMA) was used for the detection
of the luminescence. Luminescence spectra were not
corrected for the sensitivity of the detector.
been payed to the green emission from Er3+
.
In this Letter we report on our spectroscopic studies
in a wide spectral range of both luminescence and
absorption of titania xerogels, highly doped with
erbium, confined in porous anodic alumina membrane.
Visible luminescence of Er3+ is discussed.
2. Experimental
To detect the absorption spectra, we combined
together up to six samples of freestanding films
which enhanced an effective thickness of an erbium-
containing material. Such a stack was put into a
helium-vapor cryostat with variable temperature (4.2–
300 K). The spectra were obtained in the spec-
tral region 6000–13000 cm−1 with resolution up to
0.5 cm−1, using a Fourier-transform spectrometer
BOMEM DA3.002. Liquid nitrogen cooled InSb and
Si detectors for different spectral regions were used.
The mechanically and electrochemically polished
aluminum foils (99.999% purity) were used to prepare
porous anodic alumina (PAA) freestanding films. An-
odizing was performed in 0.24 M phosphoric acid so-
lution at a constant current 6 mA/cm2, and in 0.3 M
oxalic acid solution at a constant voltage 40 V. The sat-
urated HgCl2 solution was further used to separate the
PAA film from the aluminum foil. Typically the film
thickness of anodic alumina membrane was found to
be 125 and 90 µm after anodizing within 12 hours in
oxalic acid solution and 11 hours in phosphoric acid
solution, respectively.
3. Experimental results and discussion
Coatable colloidal solution of tetraethilorthotitanate
Ti(OC2H5)4 in a homogeneous phase with the mixture
of water and 96% ethanol (1 : 6 by volume) was used
as a precursor for fabrication of Er-doped TiO2 films.
The concentration of TiO2 in the prepared sols was
27 mg/ml. Further, the certain amounts of erbium ni-
trate were added to sols (see [3] for the details) for
fabrication of TiO2 xerogel films containing 20 and 50
wt.% Er2O3. Deposition of sol into pores of anodic
alumina was performed by spin-on route at 2700 rpm
for 30 s followed by drying at 200 ◦C in air for 10 min.
To achieve more complete filling of the pore volume
with xerogel [8,9], the spinning and drying steps were
Fig. 1 shows the spectra of a visible room-tempera-
ture Er luminescence from three different samples
under the 488 nm excitation into the level 4F7/2. Three
different bands centered at 521, 544, and 658 nm are
observed. They correspond to the radiative transitions
from the levels 2H11/2, 4S3/2, and 4F9/2 to the ground
4
2
4
level I15/2. The emission band H11/2 → I15/2
freezes out at decreasing the temperature, due to
2
a rapid phonon relaxation from the H11/2 level
4
to the level S3/2 that lies 800 cm−1 lower. At
2
ambient temperature, the level H11/2 is thermally
4
populated from the level S3/2. The intensity of the
Klimin
