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Science of Advanced MaterialsVol. 6, pp. 1–8, 2014

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Erbium-Doped Tin-Silicate Sol–Gel-DerivedGlass-Ceramic Thin Films: Effect of EnvironmentSegregation on the Er3+ EmissionTran T. T. Van1,2,∗, S. Turrell2, B. Capoen3, Lam Q. Vinh1, O. Cristini-Robbe3, M. Bouazaoui3,F. d’Acapito4, M. Ferrari5, D. Ristic5,6, A. Lukowiak5,7, R. Almeida8, L. Santos8, and C. Kinowski3

1University of Science HoChiMinh City, Vietnam2LASIR (CNRS, UMR 8516) and CERLA, Université Lille 1, 59650 Villeneuve d’Ascq, France3PhLAM (CNRS, UMR 8523) and CERLA, Université Lille 1, 59650 Villeneuve d’Ascq, France4CNR-IOM-OGG, c/o ESRF, 38043 Grenoble, France5CNR-IFN CSMFO Lab, Via alla Cascata 56/C, Povo, 38123 Trento, Italy6Institut Rud-er Boškovic, Bijenicka cesta 54, 10000 Zagreb, Croatia7Institute of Low Temperature and Structure Research, PAS, Okolna 2, 50-422 Wroclaw, Poland8Department of Chemical Engineering and ICEMS, Instituto Superior Técnico, University of Lisbon, Av. Rovisco Pais,1049-001 Lisboa, Portugal

ABSTRACT

A new class of glass-ceramic nanocomposite systems with a uniform distribution of SnO2 nanocrystals dis-persed within an amorphous silica phase is fabricated in the form of thin films by using sol–gel processing.In this work, experiments with (100-x)SiO2-xSnO2 nanocomposite thin films with (x= 25 and 30 mol%) dopedwith Er3+ ions (concentrations of 0.5, 1 and 2 mol%) are presented. The focus has been both to determinethe effect of rare-earth doping on the growth of SnO2 nanocrystals as well as the distribution of the Er3+ ionswithin the ceramic structure. Vibrational spectroscopic assessments have proven the glass-ceramic nature ofthe samples. EXAFS and photoluminescence spectra show that the rare-earth ions exist in both amorphousand crystalline phases. Moreover an energy transfer from SnO2 nanocrystals to erbium ions on excitation at351 nm is demonstrated.

KEYWORDS: SiO2–SnO2 Thin Films, EXAFS Measurement, Raman Spectroscopy, Er3+ Ions,Photoluminescence Spectroscopy.

1. INTRODUCTIONSilica-based materials doped with rare-earth (RE) ionspromise to find widespread application, for example, inlaser emitters, in high brightness displays and erbium-doped fiber amplifiers used in the main optical communi-cation window at 1.5 �m. However, advancing technologydemands increasingly smaller optical components, whichtranslates into a demand for higher concentrations of active(RE) ions contained in smaller volumes. One of the mainproblems to meet such demand is the fact that the dopingconcentration of these active ions is limited because abovea certain concentration, the RE ions tend to group andundergo energy transfers within the clusters, thus reducingluminescence efficiency. One way to alleviate this problem

∗Author to whom correspondence should be addressed.Email: tttvan@hcmus.edu.vnReceived: XX XX XXXXAccepted: XX XX XXXX

is to replace the glass matrix with a glass-ceramic one.In this way, the rare-earth ions can be embedded into thecrystalline phase which will then reduce their clusteringand the resulting energy transfers between them.1�2

Tin dioxide (SnO2) which is a high-refractive indexsemiconductor (Eg = 3�6 eV at 300 K), and is stable bothchemically and mechanically, is an excellent choice for thecrystalline component of the system. As it is also transpar-ent through the visible and infrared regions, it covers theemission range for active ions, like erbium. Compatibilitycan be enhanced by the fact that tin oxide nano-crystals(NCs) can be excited by a broad range of UV wavelengths,thus with the use of broad band arc lamps, while Er3+ ionshave very narrow excitation peaks.Studies on silica-tin bulk glass ceramics have shown

that the concentration of SnO2 has a definite effect on thestructure of the SiO2 glass phase.

3�4 In addition, while REion concentrations as high as 1 mol% can be doped into

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the glass ceramic, Er3+ ions are found mainly in the glassphase, which results in high levels of quenching effects.5

While del-Castillo et al.6 presented data which showed thatfor RE ion concentrations of 0.4 mol%, only a small partwas embedded in the crystal phase, no analogous studieshave been performed on thin films. However, it is knownthat much larger quantities of SnO2 NCs can be incorpo-rated in thin films compared to bulk systems.7

In the present work, the aim was to determine theamount of Er3+ ions which can be incorporated in SiO2–SnO2 thin films. Using various techniques, the two mainquestions addressed are where the RE ions are located andwhether or not energy transfer between the RE ions andthe SnO2 NCs can be detected.

2. EXPERIMENTAL DETAILS2.1. Preparation Of SiO2–SnO2 Doped Er3+ FilmsThe present work concerns samples of nominal compo-sition xSnO2-(100-x)SiO2, with x = 25 and x = 30. Er3+

-doping was achieved by adding the nitrate salt of erbium,Er(NO3�3 · 5H2O (99.9%, Sigma-Aldrich), to an aqueoussolution containing SnCl2 · 2H2O, before mixing with asolution of TEOS in ethanol. The doping concentrationsof erbium were 0.5 mol%, 1 mol% and 2 mol%.Thin films were deposited on silica-buffered (thickness

of 2 �m) Si substrates, in a clean room, using the dip-coating process with a dipping rate of 60 mm/min. Eachlayer was annealed at 800 �C for 3 minutes in an opentubular oven prior to the application of the next coat. After20 coatings, the films were further annealed at 800 �Cfor 10 minutes in air, yielding transparent and crack-freefilms. Additional heat treatments, at temperatures rangingfrom 800 to 1200 �C for 1 hour, promoted the formationand growth of crystals in the films. Further details on thefabrication of tin-silicate glass ceramic planar waveguideshave been described in a previous work.4

2.2. CharacterizationThe microstructure of the film surfaces was studied byAFM in the tapping mode, using a Veeco DI CP-II AFMwith a silicon tip with a radius of 10 nm. The thickness(d) and the refractive indices (n) at 532 nm were mea-sured using a Metricon 2010 M-line apparatus based onthe prism coupling technique.Raman scattering measurements were performed using

the 488 nm line of an Ar+ ion laser in waveguide config-uration, using the prism-coupling technique.8

The scattered light was collected and analyzed using aT64000 Jobin Yvon spectrometer with a spectral resolu-tion of 1 cm−1. Fourier-transform infrared (FTIR) trans-mission measurements were performed using the ThermoNicolet Avatar 360 spectrometer with a spectral resolutionof 2.4 cm−1.The photoluminescence emission for the 4I13/2 →4I15/2

transition of trivalent erbium, was measured with an

InGaAs detector (near infrared spectrograph modelCDINIR/256 L, Control Development, USA), with a res-olution of 6 nm. Excitation wavelengths of an Argon ionlaser (Spectra-Physics, model 2016) were 351 nm and514.5 nm.The decay lifetimes of the 4I13/2 →4I15/2 transition of

trivalent erbium were also measured at 1535 nm uponexcitation at 514.5 nm. For this measurement, a rotatingchopper was included in the setup, with a frequency of30 Hz and an InGaAs photodetector (Germanium PowerDevices Corp., model GAP1000). The decay signal wasthen collected using a FLUKE PM 3370B oscilloscope.X-ray Absorption Spectroscopy (XAS) measurements at

the Er-LIII edge (8358 eV) were carried out at the GILDA-CRG beamline.9 at the European Synchrotron RadiationFacility in Grenoble, France. The monochromator wasequipped with a pair of Si(111) crystals and was run in thedynamically-focusing mode. The harmonic rejection wasachieved by using a pair of Pd-coated mirrors with a cut-off energy of 19 keV. The absorption coefficient of erbiumwas measured by collecting the intensity of the Er-L� linewith an energy resolving high purity germanium detec-tor. Data were quantitatively analyzed, for the Er-Si path,by taking the theoretical XAS paths from the Er2Si2O7

crystal10 and for the Er-O and Er-Sn paths, from a theo-retically calculated cluster of Er3+ substituting for Sn4+ inthe SnO2 structure.11 For this purpose the Feff8 code wasused.12 Data fitting (ARTHEMIS code) was carried out inR space in the interval 1–4 Å.13

The site of Er in SnO2 ErSn′ was simulated withinthe framework Density Functional Theory (DFT) usingthe VASP code.14 A 2*2*3 supercell of 72-atoms (sideof about 9.6 Å) derived from the rutile SnO2 structurewas considered, projector augmented wave (PAW) pseudopotentials were used together with the generalized gra-dient approximation (GGA) for the exchange-correlationfunctional.15 The energy cutoff for the plane waves was650 eV and the Brillouin zone was sampled with a2*2*2 k mesh following the Monkhorst-Pack scheme.16

The ionic positions were optimized until the Hellman-Feynman forces were below 10−3 eV/Å.

3. RESULTS AND DISCUSSION3.1. Microstructure StudyFigure 1 presents AFM images (5× 5 �m) obtained forthe 25 mol% SnO2 sample doped with 2 mol% Er3+ ions,subject to different heat treatments. Annealing at 800 �Cfor 1 hour (Fig. 1(a)) yielded a smooth surface, with anRrms (root mean square roughness) of 0.5 nm, indicat-ing that few SnO2 crystals had appeared at this temper-ature. Heat-treatment at 1100 �C (Fig. 1(b)), promoteda dense, homogenous distribution of crystals on the sur-face of the sample, with an Rrms of 3.8 nm. In fact, thegreater part of crystal formation began at around 900 �C

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(a)

(b)

(c)

Fig. 1. 3D AFM images of the surfaces of films of 25 SnO2–75 SiO2

doped with 2 mol% Er3+ heat treated for one hour at (a) 800 �C (b)1100 �C and (c) 1200 �C.

and continued steadily up to 1100 �C. However, upon fur-ther heat-treatment at 1200 �C (Fig. 1(c)) an almost totaldisappearance of surface crystals was observed (Rrms of0.2 nm).

In order to understand the reasons for this last observa-tion, the effect of Er3+ doping on the formation of SnO2

crystals was investigated, as shown in Figure 2. In effect,the AFM images of 30SnO2–70SiO2 samples (a) undoped,(b) doped with 0.5 mol% Er3+, respectively, and all heat-treated for one hour at 1200 �C, show a decrease ofsurface-crystal density with increase in concentration oferbium. In fact, all our work has shown that at an anneal-ing temperature of 1200 �C the presence of Er3+ inhibitsthe formation of surface crystals. It is possible that theerbium present on the surface might combine with tin toform one of the possible compositions (Er2Sn5, Er11Sn10,Er2SiO5 etc.).

17 Such a system would have a lower meltingpoint than the crystalline tin oxide and hence at 1200 �C

(a)

(b)

Fig. 2. AFM 3D images of the surface of films of 30 SnO2–70 SiO2

heat-treated for one hour at 1200 �C, (a) undoped and doped with (b)0.5 mol% Er3+.

the SnO2 crystals would tend to be reabsorbed into thefilm.

3.2. Optical StudiesM-line measurements were used to obtain values for therefractive indices (n) and thicknesses (d) of undoped andEr3+-doped 30SnO2–70SiO2 waveguides. The evolution ofthese values as a function of the annealing temperatureis shown in Figure 3. For all samples, an increase of therefractive index, as well as a decrease of film thicknesswith increasing annealing temperature can be ascribed tothe densification of films by calcination. In addition, it canbe seen that not only does the presence of Er3+ inducean augmentation of n, but this effect increases with theconcentration of erbium. The increase in refractive indexcan be attributed to the fact that rare-earth doping of vit-reous thin films tends to limit crystal growth. The result isa decrease of film porosity with a densification of the sys-tem. Earlier studies have shown that the concentration andthe properties of the optically active ion affect the densi-fication and the optical properties of the final glass, buta detailed explanation for the effects of the different rareearth ions is not yet established.18

3.3. Structural Evolution of the Silica Matrix andSnO2 Nanoparticles

3.3.1. X-Ray DiffractionFigure 4 shows the XRD patterns of undoped and0.5 mol% Er3+ doped 30 mol% SnO2 glass ceramic filmsobtained after heating at 1200 �C. In both cases, thediffraction peaks at 2� = 26�9�, 34.1�, 38.2� and 52.0�

can be assigned respectively to the (110), (101), (200),(211) and (112) planes of the tetragonal rutile SnO2 crystal

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800 850 900 950 1000 1050 1100

1.54

1.55

1.56

1.57

1.58

1.59

Thi

ckne

ss d

/mm

n 30% SnO2

n 30% SnO2 – 0.5% Er3+

n 30% SnO2 – 1% Er3+

Ref

ract

ive

inde

x n

Annealing temperature/°C

0.7

0.8

0.9

d 30% SnO2 – 1% Er3+

λ = 532 nm

Fig. 3. M-line values showing the effect of erbium concentration (x=0, 0.5 and 1%) and annealing temperature (800 �C to 1100 �C) on therefractive index values (n) and on the film thickness (d) for 30 SnO2–70SiO2 thin films.

(JCPDS 41-1445). Using the Scherrer equation, the crys-tallite size is calculated to be 10 nm. The erbium dopinghas no effect on the size of particles because the widthsof these diffraction peaks are unchanged after doping at0.5 mol% Er3+. However, the appearance of new peaks at30� and 32� can be related to the new phase, which is com-posed of erbium and silicon. After consulting the structureassociated with the file JCPDS 40-0384, it is possible thatthese new peaks correspond to (402) and (022) planes ofa crystalline erbium silicate (Er2SiO5).

3.3.2. FTIR SpectroscopyFigure 5 shows the infrared absorption spectra for the30SnO2–70SiO2 film doped with 0.5 mol% Er3+, heat-treated for one hour at temperatures ranging from 800 �Cto 1200 �C. At 800 �C, the broad band observed around3500 cm−1and the band at 1628 cm−1 are assigned, respec-tively, to stretching and bending vibrational modes involv-ing O H groups of residual water. The disappearance

24 28 32 36 50 60

Inte

nsity

/a.u

.

2θ (°)

(b)

*(022)

*(402)

(112)

(211)

(200)

(101)

(110)

(a)

Fig. 4. X-ray diffraction patterns of (a) undoped and (b) 0.5 mol% Er3+

doped 30 SnO2–70 SiO2 thin films heated at 1200 �C.

500 1000 1500 2000 2500 3000 3500

1628

1231

1095

810673

461

1200 °C 1 h

1100 °C 1 h

1000 °C 1 h900 °C 1 h800 °C 1 h

Tra

nsm

issi

on

Wavenumber/cm–1

Fig. 5. Infrared spectra of the 0.5 mol% Er3+-doped 70 SiO2–30 SnO2

film, heat-treated for one hour at temperatures ranging from 800 �C to1200 �C.

of these bands for the sample heat-treated at 900 �C for1 hour confirms the elimination of hydroxyl groups.18�19

The strongest band at around 1100 cm−1 is assignedto the asymmetric stretching vibration of ≡Si–O–Si≡bonds linking the SiO2 tetrahedra. This broad band con-sists of two components: a strong peak at 1095 cm−1

which corresponds to the transverse optical mode (TO)and a broad shoulder at 1231 cm−1 which is character-istic of the longitudinal optical mode (LO). The band at810 cm−1 has been assigned to Si–O–Si symmetric stretch-ing, with simultaneous Si cation motion, while the bandat 461 cm−1 corresponds to the � (Si–O–Si) vibrationalmode between adjoining SiO4 ring.20–22 Finally, the weak,broad band centered at 673 cm−1,which is assigned to� (O–Sn–O) vibrations of the crystalline phase of SnO2,appears initially at 900 �C and increases in intensity withannealing temperature, reflecting the increase of crystalvolume.19�23

The absence of bands in the spectra of Figure 5 at 882and 1048 cm−1, which could be assignable to Si–O–Snnetwork vibrations,24 suggests that there is a rather com-plete phase separation in these systems, with nanocrys-talline SnO2 domains isolated within an SiO2 amorphousmatrix.

3.3.3. Raman SpectroscopyThe structural evolution of the silica matrix and the for-mation of SnO2 crystals with annealing were examined byanalyzing the waveguided Raman spectra. Figure 6 showsthe TE0 polarized Raman spectra of 30SnO2–70SiO2 glass-ceramic thin films, doped with 0.5% Er3+as a function ofannealing temperature, varying from 800 �C to 1100 �C.The spectrum at 800 �C presents the bands characteristicof vitreous silica: the band at ∼440 cm−1, attributed tothe bending mode � (Si–O–Si) of the polymerized struc-ture of amorphous SiO2� the D1 and D2 bands at 489and 600 cm−1 are assigned to the symmetric breathingmodes of Si–O–Si bonds in four-fold and three-fold rings

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0 200 400 600 800

41

52

58

69

Inte

nsity

/a.u

.

Raman shift/cm–1

Si substrate

Si–O–Si

D1

D2

A1g

T–O–T

Eu

1100 °C-1 h1000 °C-1 h900 °C-1 h

800 °C-1 h

Fig. 6. Raman spectra of the 0.5 mol% Er3+-doped 70 SiO2–30 SnO2

glass-ceramic thin film as a function of annealing temperature, varyingfrom 800 �C to 1100 �C. Spectra were collected in VV polarization withexcitation of the TE0 mode.

of SiO2 tetrahedra, respectively.25 Finally, the band around

800 cm−1 can be assigned to a motion of silicon againstits tetrahedral oxygen cage.26–28 (The sharp peak observedat 521 cm−1 is due to the Si substrate).

After annealing at 900 �C for one hour, new weak fea-tures appear at 312 cm−1 and 632 cm−1 which can beassigned to the Eu transverse optical mode and the A1g

phonon mode, respectively, of tin dioxide in its rutile struc-ture. Both features increased in intensity with annealingtemperature, indicating an increasing crystal volume. Nor-mally, the Eu band should be Raman-inactive. However, adisorder at the interface between crystals, or an effect ofthe nanoparticle size can relax the selection rules.29�30

Comparing the Raman spectra obtained from 800 �Cto 1100 �C, we observe a gradual increase of the ratioof the intensity of D1 to that of the T–O–T band. It iswell accepted that the D1 ring is associated with the poresurface.31 Therefore, these results indicate an increase inthe porosity of the system, caused by the appearance ofSnO2 crystals.

Finally, consideration of the low-wavenumber regionof the spectrum provides additional information aboutthe crystal formation. Following the Duval model,32 the“boson peak” which occurs in this region of the Ramanspectrum of glasses is due to the acoustic vibrations ofcohesive domains separated by less cohesive interfacialzones. The presence of nanocrystals of well-defined size inan amorphous matrix induces a stronger coupling betweenlight and acoustic phonons, which leads to a dramaticchange in the shape of the boson peak, as it becomesnarrower and more intense. With increasing temperature,this now-called "nanocrystal peak" continues to change inwidth and intensity, as well as shifting to lower wavenum-bers. The size of crystals in the matrix can be estimatedfrom the position of this band using the elastic sphere

model.3�33–35 Using the following formula:

2r =D = Sl�nv

c

where v is one of the two sound velocities (vl for longi-tudinal and vt for transversal modes). Sn, l is a coefficientwhich depends on the choice of the boundary conditions(stress-free or rigid) as well as on the angular momen-tum l and on the harmonic index n of the vibration. Duvalet al.33�36 showed that only the l= 0 and l= 2 of spheroidalmodes are Raman-active. D is the diameter of the parti-cle, c is the speed of light in vacuum and is the valueof the low-wavenumber observed in the Raman spectrum.In the present work the estimated average crystal size wasfound to vary from 3 to 4.5 nm, for annealing temperaturesranging from 900 �C to 1100 �C.In a previous work for undoped systems,4 this value

ranged between 3.5 and 4.6 nm. This result makes it pos-sible to conclude that Er3+ doping has little or no effect oneither the silica structure or on the average size of SnO2

nanocrystals. However, the presence of erbium ions tendsto reduce the density of SnO2 crystals, especially at thesurface of the thin films.

3.3.4. EXAFS SpectroscopyThe EXAFS spectra of the 0.5 mol% Er3+ doped 70 SiO2–30 SnO2 thin films annealed at 800 and 1200 �C, aswell as those of a model Er:SnO2 compound in powderform (POW) are shown in Figure 7 whereas the relatedFourier Transforms (FT) are shown in Figure 8. The sam-ple annealed at 800 �C has a first peak at higher R valuesthan is found for samples 1200 and POW. In addition, thepeaks in the region ∼3–4 Å are much weaker and lesswell-defined. Samples 1200 and POW present commonfeatures, like in-phase oscillations in the k space and peaksin the ∼3–4 Å region in the FT. For these two samples,the data fit was carried out considering a first Er–O shellfollowed by two Er–Sn shells as found in the structuralsimulations by DFT.

Fig. 7. EXAFS spectra of the 0.5 mol% Er3+ doped 70 SiO2–30 SnO2

glass-ceramic thin film samples annealed at temperatures of 800 �C and1200 �C. (POW is for the model Er3+: SnO2 compound in powder form.)Dots=data; Line=best fit.

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Fig. 8. Fourier transforms of the EXAFS spectra of Figure 6. Trans-forms were carried out over the intervals k = 3–8 Å−1 for sample 800and k = 3–12 Å−1 for POW and sample 1200) using a k2 weight and aHanning window. (Dots= data; Line= best fit).

The model for the 800 sample is dominated by an Er–O first shell (1–2 Å region), while the second coordina-tion shell (3–4 Å), may consist of Sn and/or Si atoms.However, the low signal level does not permit a reliabledetermination of the geometric parameters. Comparisonbetween the EXAFS results for samples 1200 and POWand the respective DFT calculations (see namely the bonddistances) confirms the location of Er as substitutionalfor Sn in the SnO2 matrix. In both cases the substitu-tion is not complete, as only a fraction of Er enters thecrystals while the rest remains in the amorphous phase.The passage to a lower-coordinated phase is also con-firmed by the shrinkage of the first coordination shellradius with respect to that for sample 800. This last sam-ple retains the parameters typical for Er in an amorphoussilica structure, whereas the others exhibit Er–O distancevalues more appropriate for 6 coordinated Er. Er–O bondlengths calculated with the Bond Valence Method37 usingthe IUCR 2011 parameters38 yield R7−coord

Er−O = 2�30 Å andR6−coord

Er−O = 2�24 Å, in good agreement with the experi-mental data. Table I presents the EXAFS results togetherwith the fractions of Er atoms present in the crystallineSnO2.

Table I. Results of the quantitative analysis of the EXAFS data. Fits were carried out in R space over the interval R= 1–4 Å. POW and 1200 samplesstrictly follow the structure found by DFT i.e., an Er3+ ion substituted for a Sn4+ ion in the SnO2 structure. The last column indicates the fraction ofEr atoms present in the crystalline SnO2 phase and is derived from the apparent coordination number Er–Sn. Errors on the last digit are indicated inbrackets.

1st shell 2nd shell Cryst.

Sample NO REr-O (Å) 2Er-O (Å2) NSn REr-Sn (Å) 2

Er-Sn (Å2) fraction(%)

DFT 6 2�21 – 2 3�26 –8 3�81

POW 6 2�23�2� 0�012�2� 0�5�1� 3�21�4� 0�007�2� 27(5)2�2�4� 3�74�4�

1200 6 2�20�2� 0�013�2� 0�3�1� 3�24�4� 0�007�2� 13(5)1�0�4� 3�77�4�

800 7(1) 2�30�2� 0�016�3� – –

3.4. Energy Transfer from SnO2 Particlesto Er3+ Ions

In order to investigate any possible energy transferbetween erbium ions and SnO2 nanocrystals, the environ-ment of the Er3+ ions was studied via infrared photolumi-nescence (PL) measurements which were recorded using351 nm and 514.5 nm excitation wavelengths. These twolines correspond to the excitation of the SnO2 crystals andto the excitation of the 4I15/2−2H11/2 - transition of Er3+

ions across their bandgap, respectively.Figure 9 shows the photoluminescence spectra of the

4I13/2 →4I15/2 transition of Er3+ ion for the samples con-taining 30 mol% SnO2, doped with 0.5 mol% Er3+ andheat-treated at 1200 �C. Under excitation at 514.5 nm, oneobtains an emission spectrum characteristic of Er3+ ionsin an amorphous medium, with a broad band (full width athalf maximum, FWHM, equal to 35 nm) centered around1535 nm. However, excitation at 351 nm results in a com-pletely different spectrum, in which the presence of narrowbands at 1521, 1531, 1549 and 1571 nm are assigned tothe Stark transitions between 4I13/2 and

4I15/2 levels. Whenthe erbium ion occupies the Sn4+ sites in the cassiteritestructure, the electrostatic field of the surrounding envi-ronment of ions removes degeneracy of the free ionJ levels and makes them split into doubly degeneratedstark level.39–41 This emission results from energy trans-fer between SnO2 nanoparticles and the rare-earth ionsby Forster’s mechanism. Accordingly, this spectrum indi-cates that most part of Er3+ ions are embedded in SnO2

nanocrystals. Consideration of these spectral features indi-cates that the Er3+ ions exist in two types of sites: thoseembedded in tin oxide nanocrystals and those within theamorphous silica matrix.24�42 The role of the local structureon the luminescence properties is very clearly described inGosh et al.42 and in Chowdhury et al.43 In these papers theauthors demonstrate that it is the local structure of the ionswhich plays the most important role on the modificationsof radiative and nonradiative relaxation mechanisms, andconsequently, of the overall photoluminescence properties.It is worthy to note that Chowdhury et al.43 also discuss the

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1450 1500 1550 1600 1650

λex = 351 nm

Inte

nsity

/a.u

.

Wavelength/nm

λex = 514 nm

Fig. 9. Photoluminescence spectra of the 30 mol% SnO2 sample, dopedwith 0.5 mol% Er3+ and heat-treated at 1200 �C, upon excitations at 351and 514.5 nm.

relative energy transfer efficiency from SnO2 nanocrystalsto Eu3+ ions with a specifically interesting approach thatis not possible to directly translate in the present research.

The influence of the incorporation of erbium in the tinoxide nanoparticles on luminescence are studied by thephotoluminescence measurements of films with differentEr concentrations. With increasing erbium concentration,the near-infrared emission intensity at 1540 nm is steadilyenhanced as revealed in Figure 10. The PL intensity ofthe 2 mol% Er-doped sample is more than three orders ofmagnitude greater than that of the 0.5 mol% Er-doped one.This behavior indicates that quenching of luminescencedid not occur in the 2 mol% erbium doped systems.

In order to explain for the existence of two types of sitesof erbium ions, the lifetime of the metastable level 4I13/2was measured at 1535 nm upon 514.5 nm excitation forthe 30 mol% SnO2 sample, doped with 0.5 mol% Er3+.The decay curve for this sample, presented in Figure 11,can be fitted using a double exponential function. This

1450 1500 1550 1600 16500

50

100

150

200

250

300

350

400

450

(c)

(b)

Inte

nsity

(a.

u.)

Wavelengh (nm)

(a)

Fig. 10. Photoluminescence spectra of (a) 0.5 mol%, (b) 1 mol% and(c) 2 mol% Er3+ doped 25 mol% SnO2 films heat-treated at 1100 �C,upon excitations at 514.5 nm.

0.00 0.01 0.020.01

0.1

1

τf = 1.2 msτs = 8.8 ms

Inte

nsity

/a.u

.

Time/s

Fig. 11. Lifetime decay curve of the metastable level 4I13/2, measuredat 1535 nm upon 514.5 nm excitation for the 30 mol% SnO2 sample,doped with 0.5 mol% Er3+ and heat-treated at 1200 �C. The solid linesrepresent the exponential fits to the decay data.

observation, which corresponds to two lifetimes, givesadditional evidence for the existence of two sites for theEr3+ ions. The maximum phonon energy of SnO2 is about630 cm−1, compared to that of silica at 1000 cm−1.42

Hence, the SnO2 crystal medium serves to reduce the prob-ability of non-radiative multiphonon transitions from the4I13/2 level of Er

3+ ions. As a result, the longer lifetime �sabout 8.8 ms can be associated with Er3+ ions in the crys-talline phase, while the shorter lifetime �f (1.2 ms) couldcorrespond to Er3+ ions in an amorphous environment.

4. CONCLUSIONSUsing a sol–gel technique, (100-x)SiO2- xSnO2 glass-ceramic thin films with a nominal SnO2 content as highas 30 mol% and doped with up to 2 mol% Er3+ havebeen successfully fabricated. Microstructural analyses haveshown that, while doping with Er3+ produces an increaseof index of refraction of the films, it inhibits the growthof SnO2 crystals on the film surface at 1200 �C.The FTIR and Raman spectra indicate that the SnO2

crystals appear in the heat-treated amorphous silica matrixat 900 �C, with sizes of about 4 nm. In addition, there isa separation of phases between the SiO2 and SnO2 sys-tems. Moreover, the formation of SnO2 crystals brings onan increase in porosity. This observation suggests that thepresence of tin oxide nanoparticles causes a pore expan-sion. Although Er3+ doping at 0.5 mol% has a negligibleeffect on both the structure of the silica matrix and on thegrowth of SnO2 particles, it leads to a reduction of crystaldensity on the film surface.The interpretation of the EXAFS spectra indicates that

the Er3+ ions are located in two different sites: one dis-persed in the amorphous silicate matrix and the otherincluded substitutionally as Er

′Sn in SnO2. The thermal

treatment increases the fraction of erbium hosted in the

Sci. Adv. Mater., 6, 1–8, 2014 7

Erbium-Doped Tin-Silicate Sol–Gel-Derived Glass-Ceramic Thin Films: Effect of Environment Segregation on the Er3+ Emission Van et al.

ARTICLE

crystals which reaches about 13% in the sample annealedat 1200 �C.Photoluminescence measurements support this conclu-

sion giving clear evidence that the Er3+ ions can be locatedboth inside the tin oxide nanocrystals and within the silicamatrix. In addition these measurements show energy trans-fer from SnO2 nanocrystals to the embedded Er3+ ions.

Acknowledgments: The authors would like to thankL. Boussekey of LASIR for his invaluable technicalsupport and Bruno Nunes of ICEMS for the AFMmeasurements. This research is funded by VietnamNational Foundation for Science and Technology Develop-ment (NAFOSTED) under grant number 103.06-2012.16.GILDA is a project jointly financed by CNR and INFN.A. Lukowiak thanks the Polish Ministry of Science andHigher Education for the financial support from the“Mobility Plus” Program.

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