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J Mater Sci
DOI 10.1007/s10853-014-8531-6

Environment segregation of Er3+ emission in bulk sol–gel-derived
SiO2–SnO2 glass ceramics
Tran T. T. Van • S. Turrell • B. Capoen •
Le Van Hieu • M. Ferrari • Davor Ristic •
L. Boussekey • C. Kinowski

Received: 25 April 2014 / Accepted: 2 August 2014
Ó Springer Science+Business Media New York 2014

Abstract Er-doped (100-x) SiO2–x SnO2 glass–ceramic
monoliths were prepared using a sol–gel method. Raman
spectroscopic measurements showed the structural evolution of the silica matrix caused by the formation and the
growth of SnO2 nanocrystals. Analysis of the photoluminescence properties shows that the quantity of Er3? ions
embedded in the vicinity of SnO2 nanocrystals could be
controlled by the SnO2 concentration. We give spectroscopic evidence of energy transfer to erbium ions provided
by SnO2 nanocrystals in the silica matrix. The 4I13/2 level
decay curves present a double-exponential profile with two
lifetimes associated to rare-earth ions in two different
environments.

Introduction
For telecommunication application, the 1.55 lm wavelength range is of importance because of the minimum
absorption and dispersion in optical fibers. The geometry of

T. T. T. Van (&) Á L. Van Hieu
University of Science, Vietnam National University,
Ho Chi Minh City, Vietnam
e-mail:

S. Turrell Á L. Boussekey Á C. Kinowski
LASIR (CNRS, UMR 8516) and CERLA, Universite´ Lille 1,
59650 Villeneuve d’Ascq, France
B. Capoen Á C. Kinowski
PhLAM (CNRS, UMR 8523) and CERLA, Universite´ Lille 1,
59650 Villeneuve d’Ascq, France
M. Ferrari Á D. Ristic
CSMFO Lab., IFN-CNR, Via alla Cascata 56/c, 38050 Trento,
Italy

integrated optical components needs a large gain achieved
over a short distance; therefore, a high erbium concentration is required. Some studies have been carried out on the
1.55 lm luminescence of Er3?-doped glass systems such
as phosphate-, silicate-, and tellurite-based glasses [1–5].
However, the extremely low solubility of such active ions
in pure silica matrices leads to quenching effects due to
clustering of doping ions, even at low concentrations. For
example, significant Er3?–Er3? interactions have been
found in silica at concentrations as low as 100 ppm [6].
This grouping results in a reduction of luminescence efficiency due to energy transfers between ions, which then
results in non-radiative relaxations.
A solution to this problem is the use of glass–ceramics
because the incorporation of rare-earth ions in nanocrystals
not only prevents the aggregation even at high concentrations but also allows crystal-ion energy transfers, thus
enhancing the efficiency of ion luminescence, which
compensates for the small absorption cross section of these
ions [7].
In this work, the well-known wide-band gap semiconductor SnO2 (Eg = 3.6 eV at 300 K) was chosen as the
crystalline species. Tin dioxide is transparent through the
visible and infrared regions, which covers the emission

range for active ions like erbium. Moreover, with its very
low cutoff phonon energy of 630 cm-1, SnO2 is prone to
reduce the non-radiative decay of RE ion excited states.
Moreover, the tin oxide nanocrystals can be excited by a
broad range of UV wavelengths, as compared with the
narrow excitation peaks of the Er3? ions. Therefore, these
nanocrystals can be easily and efficiently excited by broadband arc lamps with UV emission. Hence, the SnO2-doped
silica glass–ceramic system should be an excellent host for
active ions. However, the low value of RE solubility in
SnO2 is a well demonstrated matter of fact [8]. An increase

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J Mater Sci

in the SnO2 concentration should serve to increase the
solubility of RE-ions, which prevents the non-radiative
processes due to ion–ion interactions. In addition, this
increase can enhance the emission efficiency of Er3? ions
through energy transfer from SnO2 nanocrystals.
The bulk system 0.4 mol % SnO2 doped with 0.5 mol %
Er3? in silica was prepared by N. Chiodini et al. [9] and under
excitation at 514 nm, they obtained a spectrum of Er3? ions
in an amorphous environment with a lifetime of the 4I13/2
level equal to 10 ms, and the decay curve presented a nonsingle-exponential behavior. More recently, S. Brovelli et al.
[10, 11] obtained bulk systems of Er-doped silica with
8 mol % of SnO2 nanocrystals and Er3? ions concentration
up to 1 mol %. They were the first to give evidence of energy
transfer from the SnO2 nanocrystals to the Er3? ions in bulk

systems. However, these authors showed that an increase of
Er3? concentration from 0.05 to 1 mol % induces a decrease
of photoluminescence decay time at 1.5 lm from 3 to 0.5 ms
due to quenching effects.
Using visible photoluminescence data for the 95SiO2–
5SnO2 system doped with 0.4 mol % Er3?, J. del-Castillo
et al. [12] showed that the Er3? ions are partially dispersed
in the SnO2 nanocrystals and that the efficiency of energy
transfer can be improved by changing both the SnO2 or
Er3? concentrations, as well as the thermal treatment.
All these works have been focused first on increasing the
ion solubility so as to avoid quenching effects and secondly
on improving the efficiency of energy transfer between the
SnO2 crystals and the Er3? ions. It is necessary to increase
the SnO2 concentration in order to enhance energy transfer.
However, the consequences on the form of the photoluminescence spectrum in the near-infrared region, particularly the emission bandwidth around 1500 nm, have not
been discussed.
In the present work, these questions will be addressed by
changing the SnO2 and Er3? concentrations and observing
the effects on the form of the emission spectrum in the
infrared and on the lifetimes of the 4I13/2 level of the
erbium ion, both being consequences of the change in
environment of the rare-earth ion.

Experimental
Sample preparation
(100-x)SiO2–xSnO2 (x = 4, 8, 12 mol %) glass–ceramics
doped with 0.1, 0.5 and 1 mol % Er3? were prepared using
the sol–gel technique with a process similar to that of
Hayakawa et al. [13]. The starting solution was obtained by

mixing tetra-ethyl-orthosilicate (TEOS 99.9 %, SigmaAldrich), ethanol, and de-ionized water with hydrochloric

123

acid (0.1 mol/l) as a catalyst. This solution, with a molar
ratio TEOS: H2O:Ethanol equal to 1:4:8, was pre-hydrolyzed for 2 hours at room temperature. Separately,
SnCl2.2H2O (98 %, Alfa Aesar) and Er(NO3)3.5H2O
(99.9 %, Sigma-Aldrich) dissolved in ethanol were added
to the solution containing TEOS. After stirring for 2 hours
at room temperature (RT), the resulting solution was placed
in sealed polypropylene containers, first at ambient temperatures for 2 weeks and then at 55 °C for another
2 weeks, so as to obtain monolithic gels. To complete the
hydrolysis and polymerization of terminal : Si–OH
groups, the dried gels were heated in water vapor at 80 °C
for 2 days. Finally, the resulting xerogels were annealed at
temperatures ranging from 600 to 1100 °C for 1 hour in
air, with a ramp of 0.5 °C/min, thus forming a stiff glass
network. Crack-free and pinkish transparent cylindrical
samples were obtained with dimensions of 5 mm in
diameter and 10 mm in height.

Characterization
For high temperature X-ray diffraction (HTXRD) measurements, the diffractometer was equipped with an Anton
Paar HTK1200 N high temperature chamber, which was
coupled to a high speed Vantec1 detector. After being
placed in this chamber, the samples were subjected to a
temperature increase of 5 °C/min up to a desired temperature and then held at this temperature for the duration of
the recording of the diffractogram. The diffractograms
were recorded at temperatures ranging from 650 to
1050 °C at intervals of 25 °C.

The crystal size and morphology were determined by
transmission electron microscopy (TEM) using a Philips
CM30 microscope. For these measurements, the specimens
were ground in ethanol. A droplet of the resulting fine
powder suspension was placed on a copper microscope
grid.
The samples to be analyzed were annealed for 1 hour at
a desired temperature between 600 and 1100 °C. UV–visible absorption spectra were recorded using a Perkin Elmer
UV/Vis/Nir spectrophotometer Lambda 19. Raman scattering measurements were performed using the 488 nm
line of an Ar? ion laser. The scattered light was collected
and analyzed using a T64000 JobinYvon spectrometer with
a spectral resolution of 1 cm-1.
Room-temperature photoluminescence spectra were
obtained with a specially designed Jobin–Yvon micro
photoluminescence spectrometer using the 351 nm and
514 nm excitation lines of a CW Coherent Ar? laser. The
emission light was dispersed using a monochromator with a
spectral resolution of 1 nm and collected by a Peltiercooled InGaAs detector.


J Mater Sci

For the lifetime measurements, experiments were performed by far-field excitation using the 514.5 nm line of an
Ar? ion laser as source. Si/InGaAs diode and a photomultiplier tube were used as detectors. The excitation laser
was modulated using a 70 Hz chopper, and the spectra
were recorded using a standard lock-in technique. A part of
the exciting beam was deviated to a diode detector to use as
the trigger for the lock-in. Decay curves were obtained
using a standard oscilloscope, the same chopper used for
the modulation of the signal, and the lock-in technique

being used to chop the excitation beam.

Results and discussions

was also investigated by XRD measurement. Fig. 2 displays the XRD patterns of glass–ceramic monoliths doped
with 0.5 mol % Er3? annealed at 1100 °C for 1 h in air.
The mean crystal size estimated using the Scherrer equation ranges from 4.6 to 5.4 nm for 4 % and 12 mol %
SnO2, respectively.
A high-resolution TEM (HRTEM) image of the 88 %
SiO2–12 % SnO2 doped with 1 mol % Er3? sample heat
treated at 1100 °C for 1 h is presented in Fig. 3, showing
both spherical crystallites and others, which are slightly
oblong. The average size of crystals is found to be around
5 nm. Measurements yield interplanar spacings of
0.34 nm, which correspond to the (110) planes of rutilelike SnO2.

Structural properties
High temperature X-ray diffraction (HTXRD) and TEM
In order to study the evolution of the structure of the SnO2
nanocrystals upon heat treatment, in situ HTXRD measurement were performed. The diffractograms were
recorded at temperatures varying from 650 to 1050 °C.
Fig. 1 presents the HTXRD patterns of the sample 88 %
SiO2–12 % SnO2 doped with 1 mol % Er3? pre-heated at
600 °C. The appearance of peaks at 2h = 26.4, 33.5, 37.7,
51.5, 54.6, 57.5, and 64.9° can be assigned to the (110),
(101), (200), (211), (220), (002), and (112) planes of the
tetragonal rutile-type SnO2 crystal (International Centre of
Difraction Data (JPCD) file 41–1445). The width of the
diffraction peaks is virtually independent of annealing
temperature indicating that the heat treatment has a little

effect on the growth of crystals. In addition, the effect of
percentage of tin dioxide on the size of SnO2 nanocrystals

Fig. 1 In situ HTXRD patterns of the 88SiO2–12SnO2 doped
1 %Er3? samples with different annealing temperatures

Fig. 2 XRD patterns of 4, 8, and 12 % SnO2 doped with 0.5 %
Er3? samples heat treated at 1100 °C for 1 h

Fig. 3 HRTEM image of an 88 SiO2–12 SnO2 doped 1 % Er3?
glass–ceramic sample heat treated at 1100 °C for 1 h

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J Mater Sci

Raman spectroscopy
The evolutions of a given silica matrix structure with heat
treatment and doping concentrations were studied by
Raman spectroscopy. An example is given in Fig. 4 for the
sample 92SiO2–8SnO2 doped with 0.5 mol %-Er3? and for
annealing temperatures ranging from 600 °C to 1100 °C.
For comparison, the top spectrum is that of an 8 mol %
SnO2 sample without erbium and heat treated at 1100 °C. It
can be noted that this latter spectrum is essentially identical
to that of the Er3?-doped sample heat treated at the same
temperature. Both spectra are basically characteristic of
amorphous silica with additional bands due to SnO2 but
with no bands which can be related to erbium oxide or

erbium mixed tin oxide phases. When the erbium concentration is increased to 1 mol %, there are still no
changes in the Raman spectrum, thus indicating that the
presence of Er3? has very little effect on the final structure
of the silica matrix.
The gradual broadening of the T-O-T band (attributed to
d(Si–O-Si) bending mode) around 440 cm-1 with
increased temperature is a well-known characteristic of the
densification process of a silica matrix. The ratio of the
intensities of the bands at 490 and 603 cm-1, assigned to
D1 and D2 rings, to that of the T-O-T band decreases with
increasing temperature. This behavior is to be expected as
these two types of rings are associated with the pore surfaces, and the calcination processes decrease the porosity
of the systems. The profile of the band at 800 cm-1 in the
spectra of the samples treated at 1100 °C is characteristic
of densified silica. Finally, the band at 980 cm-1, which is
assigned to vibrations of Si–OH groups, decreases in
intensity with increasing annealing temperatures, indicating the gradual removal of solvent and precursor molecules
[9, 14–16].

Fig. 5 Raman spectra of 4, 8, and 12 % SnO2 doped with 0.5 %
Er3? samples heat treated at 1100 °C for 1 h

The decrease in intensity of the surface phonon mode of
SnO2 at 348 cm-1 for annealing temperatures above
600 °C demonstrates the increase in size of the nanocrystals and the resulting reduction in the surface to volume
(S/V) ratio [16, 17]. The band at 632 cm-1 is due to the A1g
volume phonon mode of SnO2 in its rutile structure [14, 18,
19]. The increase in intensity of this band with increasing
heat treatment temperatures from 600 to 1100 °C is consistent with an increase in crystalline volume.
Finally, for systems annealed at 1100 °C, the relative

intensity between D1 and D2 bands and Si–O-Si vibration is
much greater than would be expected for a densified silica
system. This observation supports the proposition that the
presence of SnO2 nanocrystals induces a residual porosity
in the matrix [20].
The influence of the concentration of SnO2 on the
structural evolution of the matrix and on the formation of
the particles for systems annealed at 1100 °C has also been
examined (See Fig. 5). At this temperature, a slight
increase of the relative intensities of the D1 and D2 bands to
the Si–O-Si vibration with the percentage of SnO2 indicates
that an increase of SnO2 concentrations from 4 % to
12 mol % has very little effect on the matrix structure of
silica. The increase in intensity of the A1g band with SnO2
concentration reflects either an increase in nanocrystals
volume or in their number.
Optical properties
Absorption spectroscopy

Fig. 4 Evolution of the Raman spectra of the 92SiO2–8SnO2 doped
0.5 %Er3? samples as a function of increasing annealing temperature.
The Raman spectrum of an undoped sample heat treated at 1100 °C is
added as a reference for comparison

123

Figure 6a presents absorption spectra for the 4 %SnO2
system doped with 1 % Er3?, in which the transition from
the 4I15/2 fundamental level to excited levels of Er3? ions
can be observed. In addition, the band around 1365 nm is

J Mater Sci

Fig. 7 Photoluminescence upon different excitations of 4 %SnO2
samples doped with different erbium concentrations, annealed at
1100 °C : 0.5 mol % Er3? (a) and 1 mol % Er3? (b)
Fig. 6 Absorption spectra of 96 %SiO2–4 %SnO2 doped with 1 %
Er3?, heat treated at 600° (a) and 1000 °C (b). Inset Absorption
spectra of 4, 8, and 12 % SnO2 doped with 0.5 % Er3? samples heat
treated at 1000 °C

attributed to the second harmonic vibrations of isolated Si–
OH groups, while the band at 1400 nm is associated to
hydrogen-bonded Si–OH silanol groups. Finally, the band
at 1900 nm is assigned to hydrogen-bonded water. The
appearance of these two features is due to the adsorption of
residual Si–OH groups on the pore surface of the sample
[21, 22]. Obviously, the presence of these OH groups has
detrimental effects on optical properties. However, with
higher heat treatment temperatures (at 1000 °C in Fig. 6b),
the disappearance of the band at 1365 nm reflects the more
efficient removal of isolated silanols, while the downshift
of the band 1400–1380 nm suggests a lengthening of the
Si–OH bonds, which correlates with their progressive
destruction. A decrease in intensity of the band at 1900 nm
correlates with the loss of water with annealing. However,
an increase of SnO2 concentration causes a residual OH
groups in higher SnO2 percentage samples despite a heat
treatment at 1000 °C as presented in the inset of Fig. 6b.

Photoluminescence measurements
In order to study the environment of the Er3? ions, infrared
photoluminescence measurements were undertaken using
351 and 514 nm as excitation wavelengths. These two lines
correspond to the band gap of SnO2 and to the 4I15/2–2H11/2
transition of Er3? ions, respectively.
Figure 7 shows emission spectra for the samples containing 4 mol % SnO2 doped with 0.5 and 1 mol % Er3?.
For systems doped with 1 mol % Er3? (Fig. 7b), upon
excitation at 514 nm, one obtains an emission spectrum
characteristic of Er3? ions in an amorphous medium with a
broad band (full width at half maximum: FWHM equal to
33 nm) centered around 1535 nm. However, excitation at
351 nm results in a completely different spectrum, in
which the presence of narrow bands at 1521, 1531, 1549,
and 1571 nm can be attributed to the Stark effect, a splitting of Er3? -ion energy levels caused by the SnO2 crystal
field. Hence, this emission results from an efficient energy
transfer between SnO2 nanoparticles and the rare-earth
ions. Therefore, this spectrum corresponds to that of Er3?
ions located within or in the close vicinity of SnO2

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J Mater Sci

Fig. 9 Decay curves of emission at 1535 nm (kex. = 514 nm) for an
8 %SnO2 sample annealed at 1100 °C and for different Er3?
concentrations. The solid lines represent double-exponential fits to
the decay data (correlation coefficient R [0.99, for all the fittings)

Decay-time measurements

Fig. 8 Photoluminescence upon different excitations of 8 %SnO2
samples doped with different erbium concentrations, annealed at
1100 °C : 0.5 mol % Er3? (a) and 1 mol % Er3? (b)

nanocrystals. In fact, Er3? can substitute for Sn4? in the
rutile crystal structure of SnO2, but it could also be within
the crystal without substitution or even on the nanocrystal
surface. Consideration of these spectra suggests the existence of two types of sites for Er3? ions: those within the
close vicinity of tin oxide nanocrystals and those within the
amorphous silica matrix. Nevertheless, the spectra of the
systems doped with 0.5 mol % under two excited wavelengths are similar. The observed narrow bands can be
attributed to Er3? ion located in the Sn4? sites of the
cassiterite structure.
For higher SnO2 concentrations, for example, 8 %SnO2
(Fig. 8), in both cases regardless of the excitation wavelength, the emission spectra are characteristic of Er3? ions
under the influence of the crystal field of SnO2 nanocrystals. Comparison of Fig. 5a, b suggests that 4 % of SnO2 is
not enough to contain 1 mol % Er3? ions. Thus, low
concentrations of SnO2 would appear to ease the dispersion
of Er3? ions in a silica matrix. On the other hand, with high
SnO2 concentrations, the majority of Er3? ions are incorporated in or in the vicinity of SnO2 nanocrystals, reflecting
the affinity of rare-earth ions for SnO2, and their low solubility in SiO2.

123

The lifetime of the metastable level 4I13/2 was measured at
1535 nm upon 514.5 nm excitation. As seen in Fig. 9, the
decay curves of 4I13/2-4I15/2 were not single exponential. In

effect, these curves can be fitted using the double-exponential function:




IðtÞ
t
t
¼ A1 exp À
þ A2 exp À
;
Iðt ¼ 0Þ
sf
ss
where sf is the decay time of the fast component, ss is the
decay time of the slow component, A1 and A2 are the
amplitudes of the fast and slow components, respectively.
Such a behavior constitutes additional evidence for the
existence of two kinds of sites for the Er3? ions [23–26].
These ions can be located in SnO2 crystals or in the glassy
phase. The value of A1, A2 permits to roughly assign the
population ratio of erbium ions between the two sites.
In the present work, the fast decay component of glass–
ceramic monoliths is attributed to Er3? in the nanocrystals.
An increase in SnO2 concentration makes a reduction of
Er3? ions clustering of nanocrystals as displayed in
Table 1, thus leading to longer luminescence lifetimes
from 0.61 to 1.17 ms. The slow decay rate is thus related to
Er3? ions in the glass environment. As shown in Fig. 6b,
the residual OH groups in the higher SnO2 percentage

samples (8 % and 12 mol % SnO2) are more than those of
4 mol % sample. These OH groups, which are mainly
associated with the silica matrix, are known to quench the
erbium luminescence at 1.5 lm. This effect results a
reduction of long lifetime values when the SnO2 concentration increases from 4 % to 8 mol %. Moreover, the
lengthening of the lifetime of the metastable level 4I13/2


J Mater Sci
Table 1 Lifetime s for the 4I13/2 erbium level of 4, 8, and 12 % SnO2
doped with 0.1 and 0.5 mol % Er3? samples heat treated at 1100 °C
for 1 h
0.1 % Er3?
4 % SnO2

0.5 % Er3?

9.8 ms (41 %)

7.85 ms (35 %)

0.61 ms (59 %)

0.14 ms (65 %)

8 % SnO2

6.9 ms (43 %)
0.93 ms (57 %)

3.57 ms (42 %)
0.49 ms (58 %)

12 % SnO2

7.84 ms (46 %)

4.88 ms (52 %)

1.17 ms (54 %)

0.78 ms (48 %)

within a silica matrix makes it possible to limit their
growth, even at temperatures as high as 1100 °C.
Photoluminescence features have shown that an increase
in SnO2 concentration promotes the incorporation of Er3?
ions in SnO2 nanocrystals. Energy transfer has been evidenced between these nanocrystals and the rare-earth ions.
This transfer may serve wide-band pumping applications of
lasers. Nevertheless, for applications in telecommunications,
a compromise between SnO2 and Er3? concentrations must
be found in order to obtain a long luminescence lifetime at
1.5 lm and broad emission spectra in the infrared region.

The population ratio of each erbium site is given in brackets
Acknowledgement The authors would like to thank P. Russell
(UCCS-Lille1) for his help with HTXRD measurements. This
research is funded by Vietnam National Foundation for Science and
Technology Development (NAFOSTED) under Grant Number
103.06-2012.16.

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Fig. 10 Luminescence lifetime of the 4I13/2 level as a function of
SnO2 concentration and for two different Er3? concentrations in
samples annealed at 1100 °C

between the 8 % and 12 mol % SnO2 samples is due to the
better solubility of Er3? ions in glassy matrix for the higher
SnO2 concentration [8] (Table 1).
The shortening of the longer lifetime with an increase of
the erbium concentration, as presented in Fig. 10, suggests
a significant luminescence quenching.
Finally, even the fast component shows a lifetime
increase with the SnO2 concentration. This observation
reflects the fact that Er3? ions are less able to cluster with
fewer losses associated with ion–ion interactions

Conclusion
Using a sol–gel technique, Er-doped (100-x) SiO2–x SnO2
crack-free glass–ceramic monoliths have been successfully
fabricated with a SnO2 content as high as 12 mol %. The
calculated average size of particles using of XRD data is
about 4 nm, which correlates quite well with that deduced
from TEM analysis. The formation of SnO2 nanocrystals

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