ions embedded in the vicinity of SnO2 nanocrystals could be controlled by the SnO2 concentration.. We give spectro-scopic evidence of energy transfer to erbium ions provided by SnO2nanoc
Trang 1Environment segregation of Er3+ emission in bulk sol–gel-derived
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
evolu-tion of the silica matrix caused by the formaevolu-tion and the
growth of SnO2nanocrystals Analysis of the
photolumi-nescence properties shows that the quantity of Er3? ions
embedded in the vicinity of SnO2 nanocrystals could be
controlled by the SnO2 concentration We give
spectro-scopic evidence of energy transfer to erbium ions provided
by SnO2nanocrystals in the silica matrix The 4I13/2level
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
wave-length range is of importance because of the minimum
absorption and dispersion in optical fibers The geometry of
integrated optical components needs a large gain achieved over a short distance; therefore, a high erbium concentra-tion 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 effi-ciency 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 concentra-tions 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 semicon-ductor 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, SnO2is 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 broad-band 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 SnO2is a well demonstrated matter of fact [8] An increase
T T T Van ( &) L Van Hieu
University of Science, Vietnam National University,
Ho Chi Minh City, Vietnam
e-mail: tttvan@hcmus.edu.vn
S Turrell L Boussekey C Kinowski
LASIR (CNRS, UMR 8516) and CERLA, Universite´ Lille 1,
59650 Villeneuve d’Ascq, France
B Capoen C Kinowski
PhLAM (CNRS, UMR 8523) and CERLA, Universite´ Lille 1,
59650 Villeneuve d’Ascq, France
M Ferrari D Ristic
CSMFO Lab., IFN-CNR, Via alla Cascata 56/c, 38050 Trento,
Italy
DOI 10.1007/s10853-014-8531-6
Trang 2in 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 SnO2nanocrystals
The bulk system 0.4 mol % SnO2doped 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 the4I13/2
level equal to 10 ms, and the decay curve presented a
non-single-exponential behavior More recently, S Brovelli et al
[10, 11] obtained bulk systems of Er-doped silica with
8 mol % of SnO2nanocrystals and Er3?ions concentration
up to 1 mol % They were the first to give evidence of energy
transfer from the SnO2nanocrystals 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–
5SnO2system doped with 0.4 mol % Er3?, J del-Castillo
et al [12] showed that the Er3?ions are partially dispersed
in the SnO2nanocrystals 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
SnO2crystals and the Er3?ions It is necessary to increase
the SnO2concentration in order to enhance energy transfer
However, the consequences on the form of the
photolu-minescence spectrum in the near-infrared region,
particu-larly the emission bandwidth around 1500 nm, have not
been discussed
In the present work, these questions will be addressed by
changing the SnO2and 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 %,
Sigma-Aldrich), ethanol, and de-ionized water with hydrochloric
acid (0.1 mol/l) as a catalyst This solution, with a molar ratio TEOS: H2O:Ethanol equal to 1:4:8, was pre-hydro-lyzed 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 tem-peratures 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) mea-surements, 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 temper-ature and then held at this tempertemper-ature 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–vis-ible absorption spectra were recorded using a Perkin Elmer UV/Vis/Nir spectrophotometer Lambda 19 Raman scat-tering 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 Peltier-cooled InGaAs detector
Trang 3For the lifetime measurements, experiments were
per-formed by far-field excitation using the 514.5 nm line of an
Ar? ion laser as source Si/InGaAs diode and a
photo-multiplier 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
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
mea-surement 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 % SnO2doped 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 SnO2crystal (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 SnO2nanocrystals
was also investigated by XRD measurement Fig 2 dis-plays 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 equa-tion 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 % SnO2doped 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 rutile-like SnO2
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
Trang 4Raman 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.4for the
sample 92SiO2–8SnO2doped 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 %
SnO2sample 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
con-centration 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
D1and D2rings, 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
sur-faces, and the calcination processes decrease the porosity
of the systems The profile of the band at 800 cm-1in 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,
indicat-ing the gradual removal of solvent and precursor molecules
[9,14–16]
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 nanocrys-tals and the resulting reduction in the surface to volume (S/V) ratio [16,17] The band at 632 cm-1is due to the A1g volume phonon mode of SnO2in its rutile structure [14,18,
19] The increase in intensity of this band with increasing heat treatment temperatures from 600 to 1100°C is con-sistent with an increase in crystalline volume
Finally, for systems annealed at 1100°C, the relative intensity between D1and D2bands 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 SnO2nanocrystals 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 D1and D2bands to the Si–O-Si vibration with the percentage of SnO2indicates 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 A1gband with SnO2 concentration reflects either an increase in nanocrystals volume or in their number
Optical properties Absorption spectroscopy
Figure6a 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
Fig 4 Evolution of the Raman spectra of the 92SiO2–8SnO2doped
0.5 %Er 3? 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
Fig 5 Raman spectra of 4, 8, and 12 % SnO2doped with 0.5 %
Er3?samples heat treated at 1100 °C for 1 h
Trang 5attributed 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 SnO2and to the4I15/2–2H11/2 transition of Er3?ions, respectively
Figure7 shows emission spectra for the samples con-taining 4 mol % SnO2doped 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 split-ting of Er3? -ion energy levels caused by the SnO2crystal 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
Fig 6 Absorption spectra of 96 %SiO2–4 %SnO2doped with 1 %
Er 3? , heat treated at 600° (a) and 1000 °C (b) Inset Absorption
spectra of 4, 8, and 12 % SnO2doped with 0.5 % Er 3? samples heat
treated at 1000 °C
Fig 7 Photoluminescence upon different excitations of 4 %SnO2 samples doped with different erbium concentrations, annealed at
1100 °C : 0.5 mol % Er 3? (a) and 1 mol % Er3?(b)
Trang 6nanocrystals 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
exis-tence 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
wave-lengths are similar The observed narrow bands can be
attributed to Er3? ion located in the Sn4? sites of the
cassiterite structure
For higher SnO2concentrations, for example, 8 %SnO2
(Fig.8), in both cases regardless of the excitation
wave-length, the emission spectra are characteristic of Er3?ions
under the influence of the crystal field of SnO2
nanocrys-tals Comparison of Fig.5a, b suggests that 4 % of SnO2is
not enough to contain 1 mol % Er3? ions Thus, low
concentrations of SnO2would appear to ease the dispersion
of Er3?ions in a silica matrix On the other hand, with high
SnO2concentrations, the majority of Er3?ions are
incor-porated in or in the vicinity of SnO2nanocrystals, reflecting
the affinity of rare-earth ions for SnO2, and their low
sol-ubility in SiO2
Decay-time measurements
The lifetime of the metastable level4I13/2was measured at
1535 nm upon 514.5 nm excitation As seen in Fig.9, the decay curves of4I13/2-4I15/2were not single exponential In effect, these curves can be fitted using the double-expo-nential function:
IðtÞ Iðt ¼ 0Þ¼ A1exp
t
sf
þ A2exp t
ss
;
where sfis the decay time of the fast component, ssis 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 SnO2crystals or in the glassy phase The value of A1, A2permits 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 SnO2concentration makes a reduction of
Er3? ions clustering of nanocrystals as displayed in Table1, 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 concen-tration increases from 4 % to 8 mol % Moreover, the lengthening of the lifetime of the metastable level 4I13/2
Fig 8 Photoluminescence upon different excitations of 8 %SnO2
samples doped with different erbium concentrations, annealed at
1100 °C : 0.5 mol % Er 3? (a) and 1 mol % Er3?(b)
Fig 9 Decay curves of emission at 1535 nm (kex.= 514 nm) for an
8 %SnO2 sample annealed at 1100 °C and for different Er 3?
concentrations The solid lines represent double-exponential fits to the decay data (correlation coefficient R [0.99, for all the fittings)
Trang 7between the 8 % and 12 mol % SnO2samples is due to the
better solubility of Er3?ions in glassy matrix for the higher
SnO2concentration [8] (Table1)
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 SnO2content 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
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 SnO2concentration promotes the incorporation of Er3? ions in SnO2nanocrystals Energy transfer has been evi-denced 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 SnO2and 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
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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0.61 ms (59 %)
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8 % SnO2 6.9 ms (43 %)
0.93 ms (57 %)
3.57 ms (42 %) 0.49 ms (58 %)
12 % SnO2 7.84 ms (46 %)
1.17 ms (54 %)
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