Controlled Growth of SnO2 Nanocrystals in Eu3+-Doped SiO2

J. Phys. Chem. C 2009, 113, 21555–21559

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Controlled Growth of SnO2 Nanocrystals in Eu3+-Doped SiO2-SnO2 Planar Waveguides: A Spectroscopic Investigation B. N. Shivakiran Bhaktha,*,†,‡,§,| Christophe Kinowski,† Mohamed Bouazaoui,⊥ Bruno Capoen,⊥ Odile Robbe-Cristini,† Franck Beclin,# Pascal Roussel,∇ Maurizio Ferrari,‡ and Sylvia Turrell*,† LASIR (CNRS, UMR 8516) and CERLA, UniVersite´ Lille 1, Sciences et Technologies, 59655 VilleneuVe d’Ascq, France, CNR-IFN, CSMFO Laboratory, Via alla Cascata 56/c, 38050 PoVo-Trento, Italy, Dipartimento di Fisica, CSMFO Laboratory, UniVersita` di Trento, 38050 Trento, Italy, LPMC, CNRS UMR 6622, UniVersite´ de Nice-Sophia Antipolis, Parc Valrose, 06108, Nice Cedex 02, France, PhLAM (CNRS, UMR 8523) and CERLA, UniVersite´ Lille 1, Sciences et Technologies, 59655 VilleneuVe d’Ascq, France, LSPES (CNRS, UMR 8008), UniVersite´ Lille 1, Sciences et Technologies, 59655 VilleneuVe d’Ascq, France, and UCCS (CNRS, UMR 8181), ENSCL, 59652 VilleneuVe d’Ascq, France ReceiVed: August 11, 2009; ReVised Manuscript ReceiVed: October 6, 2009

We report on the fabrication of Eu3+-doped SiO2-SnO2 low-loss (0.8 dB/cm at 632.8 nm) glass-ceramic planar waveguides, fabricated by the sol-gel technique and dip-coating processing. The effects of heat treatments on the growth and evolution of SnO2 nanocrystals in the matrix were investigated using different spectroscopic tools. In situ high-temperature X-ray diffraction allowed for the determination of the crystallization temperature and confirmed the formation of tetragonal rutile SnO2 crystals. The effect of crystallization on the optical properties and on the photoluminescence of Eu3+ ions was also studied. Low-frequency Raman scattering was successfully used to determine the crystal size, and the results obtained were found to be consistent with transmission electron microscopy measurements. The breakage of Si-O-Sn linkages during the formation of SnO2 nanocrystals in the matrix was investigated by Fourier-transform infrared spectroscopy. 1. Introduction SnO2 is a wide-band-gap semiconductor (Eg ) 3.6 eV) that is studied for many applications1-3 such as gas sensors, solar energy cells, and transparent conductors. In addition, it has been found to have interesting properties suitable for photonics, such as a refractive index of 1.89 at 632 nm and a maximum phonon energy below 630 cm-1. SnO2 nanocrystals embedded in SiO2 glass matrices of bulk dimensions, activated with rare-earth (RE) elements, have been extensively researched, with a particular interest toward integrated optical (IO) devices.4-7 The fabrication of efficient, active, miniaturized IO devices as an alternative to those based on bulk optics and optical fibers has led to the need for high RE concentrations in a smaller volume. Unfortunately, high RE concentrations in glasses lead to the formation of clusters. The ions in the clusters interact, thus reducing the luminescence efficiency as a result of energy-transfer mechanisms coming from radiative and nonradiative processes.8 Glassceramic composite materials, with RE ions embedded in nanocrystals in order to avoid undesirable clustering effects, have risen as a valid alternative to the widely used glass hosts. It should be mentioned that these nanocomposite systems are of particular interest for photonic applications when the glassceramics can be prepared in a waveguiding configuration. Also, * Corresponding authors. E-mail: [email protected] (B.N.S.B.), sylvia. [email protected] (S.T.). † LASIR (CNRS, UMR 8516) and CERLA, Université Lille 1, Sciences et Technologies. ‡ CNR-IFN. § Universita` di Trento. | Universite´ de Nice-Sophia Antipolis. ⊥ PhLAM (CNRS, UMR 8523) and CERLA, Universite´ Lille 1, Sciences et Technologies. # LSPES (CNRS, UMR 8008), Universite´ Lille 1, Sciences et Technologies. ∇ UCCS (CNRS, UMR 8181), ENSCL.

controlled growth of the nanocrystals in the glass matrix is necessary for the fabrication of devices. In this work, using diverse spectroscopic tools, we investigate the controlled, thermally activated growth of SnO2 nanocrystals in 75 SiO2-25 SnO2 waveguides (WGs) doped with 1 mol % Eu3+. 2. Experiments 2.1. Sample Preparation. Planar WGs consisting of 75 SiO2-25 SnO2 doped with 1 mol % Eu3+ were fabricated by the sol-gel technique using dip-coating processing. The molar content of SnO2 was chosen so as to have no phase separation and good optical and spectroscopic characteristics for a lowloss WG. The starting solution was obtained by mixing tetraethylorthosilicate (TEOS), ethanol (EtOH), deionized water, and hydrochloric acid (HCl) as a catalyst and was prehydrolyzed for 1 h at 65 °C. The TEOS:HCl:EtOH:H2O molar ratio was 1:0.01:37.9:2.9 An ethanolic colloidal suspension, prepared using SnCl2 · 2H2O and Eu(NO3)3 · 5H2O as precursors, was added to the solution containing TEOS. The Eu3+/[75 SiO2 + 25 SnO2] molar ratio was maintained at 0.01. The final mixture was left at room temperature under stirring for 16 h. The resulting sol was filtered and then deposited on pure vitreous SiO2 (v-SiO2) and silicon substrates by dip-coating, with a dipping rate of 60 mm/min. Each layer was annealed at 800 °C prior to the application of the next coat. The films resulting from 20 coatings were stabilized by a final treatment for 10 min in air, thus yielding crack-free and low-loss WGs. Optical characterizations were performed on the samples deposited on v-SiO2, and the thin films deposited on Si were used for structural characterizations. Formation and growth of nanocrystals in the thin films was observed with an additional heat treatment (HT) in air at temperatures ranging from 900 to 1100 °C. The samples were

10.1021/jp907764p  2009 American Chemical Society Published on Web 11/11/2009

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introduced into a furnace at ambient temperature and then heated at a ramp of 15 °C/min in order to avoid surface cracking. 2.2. Characterization. Transmission electron microscopy (TEM) was performed to determine the nanocrystal size and size distribution. Images of the nanocrystals in the planar WGs were obtained along their cross section. Samples on Si substrates were prepared by mechanical thinning and ion milling processes and then mounted on a copper grid for imaging using the Philips CM 30 electron microscope operating at 300 keV. X-ray diffraction (XRD) measurements were performed using Cu KR radiation at 1.5418 Å on a Bragg-Brentano geometry Bruker D8 Advance diffractometer, equipped with an Anton Paar HTK1200N furnace and a high-speed Vantec1 detector. In situ high-temperature X-ray diffractograms were recorded from 625 to 1050 °C at intervals of 25 °C. UV-visible absorption spectra were recorded using a Varian CARY 100 BIO UV-vis spectrophotometer. The WGs deposited on transparent v-SiO2 substrates were used for the absorption studies. The thickness (d) and the refractive index (n) at 543.5 nm were measured using a Metricon 2010 m-line apparatus based on the prism coupling technique. The propagation losses at 632.8 nm for the TE0 mode were evaluated by photometric detection of the light intensity scattered out of the WG plane. The ordering of the local environment around Eu3+ ions was studied by recording the photoluminescence spectra excited with the 351 nm line of a CW Coherent Ar+ laser, in air at room temperature. The emission was dispersed using a Jobin-Yvon U1000 double monochromator and collected by a Peltier-cooled photomultiplier tube. Raman scattering measurements were performed under two different excitation wavelengths. The 514.5 nm line of an Ar+ ion laser was used to collect the Raman spectra in the waveguiding configuration by exciting the TE0 mode using the prism-coupling technique. The scattered light was collected using a Coderg T800 triple monochromator. To separate the spectrum of SnO2 nanocrystals from that of the surrounding matrix, resonant Raman spectra were recorded using a LabRam HR 800 UV micro-Raman spectrometer with an excitation wavelength of 266 nm from a Coherent solid-state, diodepumped laser. Fourier-transform infrared (FTIR) transmission measurements were performed on silicon wafers using a Thermo Nicolet Avatar 360 spectrometer. 3. Results and Discussion 3.1. Structural Characterizations. TEM measurements were performed on the sample subjected to HT at 1100 °C for 5 h. Nanocrystals of around 4 nm were observed to be uniformly dispersed in the amorphous silica matrix throughout the film. In the image of Figure 1, the near-spherical nanocrystals are visible as dark spots, and the SnO2 lattice planes are clearly present. XRD was used to identify and study the structural evolution of the nanocrystals upon HT. In situ high-temperature XRD was performed on a thin film fabricated at 800 °C on a Si substrate. The XRD patterns were recorded at intervals of 25 °C from 625 to 1050 °C in order to determine the crystallization temperature. The resulting XRD patterns are shown in Figure 2. The diffractograms recorded between 650 and 850 °C do not exhibit observable changes in comparison to that recorded at 625 °C, and hence, they are not included in Figure 2. [It should be noted that the Si (200) peak at 2θ ) 33.3° observed in all of the diffractograms is due to the (100)-oriented Si substrate. In addition, other peaks marked with an asterisk that

Bhaktha et al.

Figure 1. TEM image of the 1100 °C 5-h-heat-treated glass-ceramic waveguide. The circles highlight nanocrystals with diameters of about 4 nm.

Figure 2. In situ high-temperature X-ray diffractograms for a 75 SiO2-25 SnO2:1 mol % Eu3+ sample for annealing temperatures from 625 to 1050 °C. (The diffractograms have been offset along the y axis for clarity.) Whereas the asterisks indicate bands due to the corundum sample holder, the arrows at 2θ ) 26.9°, 34.1°, 38.2°, and 52.0° indicate the peaks due to tetragonal rutile SnO2 crystals.

also appear in all of the diffractograms are related to the corundum (Al2O3) high-temperature sample holder.] The effects of HT on the structure are quite evident (Figure 2). At the final temperature of 1050 °C, the prominent diffraction peaks at 2θ ) 26.9°, 34.1°, 38.2°, and 52.0° can be assigned, respectively, to the (110), (101), (200), and (211) planes of the tetragonal rutile SnO2 crystal.10 These peaks first appear at about 950 °C, and the steady evolution of their intensities with increasing temperature demonstrates the increase in grain size. The very broad band centered about 2θ ) 22° that appears in all of the spectra is attributed to the amorphous silica network surrounding the SnO2 nanocrystals. 3.2. Optical and Photoluminescence Studies. The UVvisible absorption spectra of thin films subjected to HT at different temperatures for 30 min are presented in Figure 3. The increase in the absorption shoulder in the range of 4-6 eV with

Controlled Growth of SnO2 Nanocrystals in WGs

Figure 3. UV-visible absorption spectra of 75 SiO2-25 SnO2:1 mol % Eu3+ samples recorded after various heat treatments ranging from 800 to 1100 °C for 30 min. The inset shows the transparency of the sample in the visible region.

HT can be assigned to the precipitation of SnO2 nanocrystals.11 The inset spectra show that the samples are transparent across the entire visible region. The thickness (d) and the refractive index (n) at 543.5 nm under TE polarization were measured to be d ) 0.89 µm and n ) 1.541 for the WG subjected to HT at 800 °C and d ) 0.76 µm and n ) 1.579 for the sample subjected to HT at 1100 °C for 5 h. The observed increase in n and the accompanying decrease in d with increasing HT temperature are attributed to the densification of the WGs. The propagation loss at 632.8 nm in the sample subjected to HT at 800 °C was 0.5 ( 0.2 dB/cm. Despite the formation and growth of the nanocrystals, the losses in the glass-ceramic WG subjected to HT at 1100 °C for 5 h remained around 0.8 ( 0.2 dB/cm. Although it is not possible to make a direct comparison of this value because of lack of publications on SnO2-SiO2 systems, an assessment of the quality of the fabricated system can be obtained by examining the attenuation coefficient values reported in the literature for top-down-fabricated glass-ceramic waveguides. In effect, Jestin et al.12 reported an attenuation coefficient of about 1 dB/cm at 1.5 µm for silica-hafnia-based glass-ceramic waveguides and Pe´ron et al.13 measured losses of about 2 dB/ cm at 633 nm for fluoride glass-ceramic waveguides. In this context, an attenuation coefficient of 0.8 dB/cm can be considered quite low for glass-ceramic waveguides fabricated by the top-down approach, particularly taking into account the novelty of the system. The modifications in the emission spectra presented in Figure 4 are attributed to the formation and growth of SnO2 nanocrystals with temperature and time. It can be deduced that the observed narrowing of the emission peaks is due to a limiting of the inhomogeneous broadening typical of glassy structural environments, which results in an ordering of the local environment around Eu3+.14 A second well-known point is that the 5D0 f 7F1 transition is magnetic dipole in character and is independent of the crystal fields, unlike the 5D0 f 7F0, which is a weak electric dipole transition. Although the 5D0 f 7F0 transition is forbidden in a centrosymmetric environment, it is allowed in low-symmetry sites. This effect is evidenced by the complete disappearance of the 5D0 f 7F0 transition in samples subjected to HT at higher temperatures. Furthermore, the ratio of the relative intensities of the 5D0 f 7F2 and 5D0 f 7F1 transitions, understood to be a reliable spectroscopic probe for the degree of symmetry around the Eu3+ ion, decreases with HT from 3.77 for the 800 °C heat-

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Figure 4. Room-temperature emission spectra of Eu3+ ions in the asprepared (at 800 °C) and 1100 °C 5-h-heat-treated samples, collected with an excitation wavelength of 351 nm.

Figure 5. Waveguide Raman spectra of samples after various heat treatments collected with an excitation wavelength of 514.5 nm. The spectra have been offset along the y axis for clarity, and the vertical dashed lines are guide lines.

treated sample to 1.76 for the 1100 °C 5-h-heat-treated sample. This result suggests that the Eu3+ ions are embedded in their crystalline surroundings.15 Finally, the presence of more than three components in the 5 D0 f 7F1 emission band is indicative of the existence of at least two sites for the Eu3+ ions.6 Together with the disappearance of the electric dipolar 5D0 f 7F0 transition, one can deduce that the most prominent site would correspond to the substitution of Eu3+ for Sn4+ ions in the C2h sites of the cassiterite structure.14 3.3. Raman Spectroscopy. Figure 5 shows the Raman spectra obtained in the WG configuration with a 514.5-nm excitation for the samples after various HTs. The spectrum for the sample subjected to HT at 800 °C is, to a large extent, similar to that of v-SiO2.16 The broad band centered at 440 cm-1 is assigned to the T-O-T bending modes of the polymerized structure, where T ) Si or Sn. The sharp band at 490 cm-1 and the shoulder observed at about 606 cm-1 are generally assigned the silica network defect bands D1 and D2, respectively.17 The D1 band is assigned to the symmetric breathing mode of regular four-membered silica rings, whereas the D2 band is assigned

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Figure 6. Micro-Raman spectra of samples after various heat treatments collected with an excitation wavelength of 266 nm. The spectra have been offset along the y axis for clarity, and the vertical dashed lines are guide lines.

to a similar motion of three-membered planar rings. The shoulder appearing at 800 cm-1 corresponds to the motion of silicon against its tetrahedral oxygen cage.18 Finally, in the lowwavenumber region, the strong band centered at 82 cm-1 is the boson peak, characteristic of the disordered glassy structure. The heat treatments result in strong modifications of the spectra and, thus, provide a spectroscopic view into the crystallization process. In the high-wavenumber region, the intensities of the T-O-T and 800 cm-1 bands decrease with increasing HT. The initial apparent increase in νTOT and the final disappearance of the 800 cm-1 “cage” mode are observations consistent with the idea of depolymerization of the silica matrix structure. The resulting band mass in the 350-700 cm-1 region is visibly broadened with discernible features around 634, 568, and 486 cm-1, which could correspond to modes of rutile SnO2.19,20 To verify these bands, it was necessary to facilitate the extraction of the spectrum of SnO2 from that of the vitreous silica matrix. As can be seen in Figure 3, SnO2 nanocrystals exhibit better absorption under UV excitation. Hence, resonance Raman spectra were recorded on the samples using an excitation

Bhaktha et al. wavelength of 266 nm. (The measurements were performed on the samples deposited on Si substrates in order to avoid ambiguity of signals that could arise from the silica substrate as well as from the silica-based thin films.) As seen in Figure 6, under UV excitation, one obtains an improved Raman scattering spectrum with a classical evolution of nanocrystalline SnO2 with increasing HT. The observed bands, although quite broad, are in good agreement with those of rutile SnO2: the band around 632 cm-1 can be assigned to the A1g mode, whereas that at 475 cm-1 is due to the Eg mode.20 These values are very close to those suggested in the Raman spectra of Figure 5. The sharp feature observed at 521 cm-1 in all of the spectra is due to the Si substrate. One can observe a gradual increase in relative intensity in the low-wavenumber region of the spectrum of Figure 5. The accompanying narrowing of the band, which starts around 1000 °C, indicates that this band changes from a vitreous boson peak to a crystalline “size peak” associated with the nanocrystals. The shift of this band to lower wavenumbers (from 65 to 43 cm-1) attests to the growth of SnO2 nanocrystals, as the peak position can be used to calculate the grain size,21,22 using the relation

ω)

Sl,nν dc

In this equation, ν is one of the two sound velocities, depending on the type of vibration (νl for spheroidal or longitudinal and νt for transversal or torsional); Sl,n is a coefficient depending on the sound velocities and the choice of boundary conditions (stress-free or rigid), as well as on the angular momentum l and the harmonic n of the vibration; d is the diameter of the spherical particle; and c is the velocity of light in a vacuum.20 Also, it is well-known that, while the symmetric l ) 0 modes give polarized Raman spectra, the l ) 2 modes result in depolarized spectra.23 In our case, only the depolarized quadrupolar l ) 2 vibrations were found to be Raman active. The particle size calculated for the samples subjected to HT at different temperatures, reported in Table 1, were obtained using the values of νt ) 3.12 × 105 cm s-1 and S ) 1.987 from the work of Die´guez et al.20 for l ) 2, considering the case of a rigid environment. TEM measurements showed the average

Figure 7. FTIR spectra of samples after various heat treatments: (A) 800 °C, (B) 900 °C, 30 min; (C) 1000 °C, 30 min; (D) 1100 °C, 30 min; (E) 1100 °C, 1 h; (F) 1100 °C, 5 h. The spectra have been offset along the y axis for clarity, and the vertical dashed lines are guide lines.

Controlled Growth of SnO2 Nanocrystals in WGs TABLE 1: Diameters Calculated for SnO2 Nanocrystals Formed in Samples Heat-Treated at Different Temperaturesa heat treatments

low-wavenumber peak position (cm-1)

calculated nanocrystal size (nm)

900 °C, 30 min 1000 °C, 30 min 1100 °C, 30 min 1100 °C, 1 h 1100 °C, 5 h

65 61 53 48 43

3.18 3.39 3.90 4.31 4.81

a Calculations based on the position of the low-wavenumber Raman peak observed in the spectra of Figure 5.

particle dimension for the sample subjected to HT at 1100 °C for 5 h to be 4 nm, thus validating our values based on the low-wavenumber Raman data. 3.4. FTIR Spectroscopy. The FTIR spectra were obtained from samples deposited on Si substrates. Figure 7a shows the complete spectra for samples subjected to various HTs. For the as-prepared sample at 800 °C, the bands at 1625 and 3450 cm-1 are assigned to vibrational modes of water (O-H bands). The disappearance of these bands with further HT confirms the absence of hydroxyl groups in our samples above 900 °C. The FTIR spectra in Figure 7b focus on the low-wavenumber range, where the Sn-O vibrational modes can be observed. The assignments for the spectra of the as-prepared sample are as follows: 1074 cm-1, (νSi-O-Sisym); 802 cm-1, (νSi-O-Siasym); and 452 cm-1, (δSi-O-Si). Upon HT, a band appears at 673 cm-1 that becomes more intense at higher temperatures and can be assigned to ν(Sn-O-Sn) of the SnO2 crystalline phase.24 Another aspect to be noted is the gradual shift of the bands at 1074 and 452 cm-1 toward higher wavenumbers with increasing HT. Thus, one observes changes from 1074 and 452 cm-1 at 800 °C to 1092 and 462 cm-1 at 1100 °C (5 h). Similar shifts of these bands have been reported in the literature, but toward lower wavenumbers. In effect, Hua et al.25 reported these spectral changes after the introduction of metal oxides such as ZrO2, TiO2, and Al2O3, into silica-gel glasses. The shifts were attributed to the formation of Si-O-metal heterolinkages. In our case, the growth of SnO2 nanocrystals with increasing HT severity would result in the destruction of Si-O-Sn heterolinkages and a corresponding increase in the number of Si-O-Si linkages, which would lead to band shifts toward higher wavenumbers. This proposition of increased numbers of Si-O-Si linkages with increasing HT severity can be substantiated by the fact that the band at 462 cm-1 for the 1100 °C 5-h-heattreated sample is of higher intensity than its counterpart at 452 cm-1 for the 800 °C heat-treated sample. Furthermore, the intensity of the band at 802 cm-1 associated with (νSi-O-Siasym) increases as a function of HT severity, which can also be related to the formation of a larger number of Si-O-Si linkages with increasing HT severity. 4. Conclusions We have successfully fabricated low-loss (0.8 ( 0.2 dB/cm at 632.8 nm), glass-ceramic planar WGs of SiO2-SnO2, doped with 1 mol % Eu3+ following the sol-gel technique. Thermal treatments were performed on the samples in order to induce the growth of SnO2 crystals. In situ high-temperature XRD studies showed the formation of nanocrystal in the glass matrices

J. Phys. Chem. C, Vol. 113, No. 52, 2009 21559 to occur at about 950 °C. A uniform distribution of particles throughout the film was confirmed by TEM measurements, and particles with an average size of 4 nm were grown upon HT at 1100 °C for 5 h. Photoluminescence spectra showed that crystallization affects the local environment of the Eu3+ ion, forming about it a centrosymmetric environment. Raman spectroscopy was also shown to be a reliable probe for following the crystallization process, and the particle size calculated by the low-wavenumber Raman scattering (4.81 nm for the 1100 °C 5-h-heat-treated sample) was found to be consistent with the TEM results. In addition, the crystallization process was also tracked by FTIR spectroscopy, thus providing information on the dynamics of Si-O-Sn linkages during the heat treatments. Acknowledgment. B.N.S.B. acknowledges financial support from USTL. This research was performed in the framework of the ITPAR Phase II (2008-2011) research project area “Nanophotonics”. References and Notes (1) Kim, T. W.; Lee, D. U.; Yoon, Y. S. J. Appl. Phys. 2000, 88, 3759. (2) Del Castillo, J.; Rodrı´guez, V. D.; Yanes, A. C.; Ramos, J. M.; Torres, M. E. Nanotechnology 2005, 16, S300. (3) Martucci, A.; Buso, D.; De Monte, M.; Guglielmi, M.; Cantalini, C.; Sada, C. J. Mater. Chem. 2004, 14, 2889. (4) Chiodini, N.; Paleari, A.; Brambilla, G.; Taylor, E. R. Appl. Phys. Lett. 2002, 80, 4449. (5) Nogami, M.; Ohno, A.; You, H. Phys. ReV. B 2003, 68, 104204. (6) Yanes, A. C.; Del Castillo, J.; Torres, M.; Peraza, J.; Rodrı´guez, V. D.; Me´ndez-Ramos, J. Appl. Phys. Lett. 2004, 85, 2343. (7) Ningthoujam, R. S.; Sudarsan, V.; Godbole, S. V.; Kienle, L.; Kulshreshtha, S. K.; Tyagi, A. K. Appl. Phys. Lett. 2007, 90, 173113. (8) Quimby, R. S.; Miniscalco, W. J.; Thompson, B. J. Appl. Phys. 1994, 76, 4472. (9) Ribeiro, S. J. L.; Messaddeq, Y.; Gonc¸alves, R. R.; Ferrari, M.; Montagna, M.; Aegerter, M. A. Appl. Phys. Lett. 2000, 77, 3502. (10) Gueu, V.; You, H.; Hayakawa, T.; Nogami, M. J. Sol-Gel Sci. Technol. 2007, 41, 231. (11) Nogami, M.; Enomoto, T.; Hayakawa, T. J. Lumin. 2002, 97, 147. (12) Jestin, Y.; Armellini, C.; Chiappini, A.; Chiasera, A.; Ferrari, M.; Goyes, C.; Montagna, M.; Moser, E.; Nunzi Conti, G.; Pelli, S.; Retoux, R.; Righini, G. C.; Speranza, G. J. Non-Cryst. Solids 2007, 353, 494. (13) Pe´ron, O.; Boulard, B.; Jestin, Y.; Ferrari, M.; Duverger-Arfuso, C.; Kodjikian, S.; Gao, Y. J. Non-Cryst. Solids 2008, 354, 3586. (14) Morais, E. A.; Ribeiro, S. J. L.; Scalvi, L. V. A.; Santilli, C. V.; Ruggiero, L. O.; Pulcinelli, S. H.; Messaddeq, Y. J. Alloys Compd. 2002, 344, 217. (15) Bettinelli, M.; Speghini, A.; Ferrari, M.; Montagna, M. J. NonCryst. Solids 1996, 201, 211. (16) Gonc¸alves, R. R.; Carturan, G.; Montagna, M.; Ferrari, M.; Zampedri, L.; Pelli, S.; Righini, G. C.; Ribeiro, S. J. L.; Messaddeq, Y. Opt. Mater. 2004, 25, 131. (17) Nedelec, J. M.; Courtheoux, L.; Jallot, E.; Kinowski, C.; Lao, J.; Laquerriere, P.; Mansuy, C.; Renaudin, G.; Turrell, S. J. Sol-Gel Sci. Technol. 2008, 46, 259. (18) Okuno, M.; Zotov, N.; Schmu¨cker, M.; Schneider, H. J. Non-Cryst. Solids 2005, 351, 1032. (19) Loridant, S. J. Phys. Chem. B 2002, 106, 13273. (20) Die´guez, A.; Romano-Rodrı´guez, A.; Vila`, A.; Morante, J. R. J. Appl. Phys. 2001, 90, 1550. (21) Duval, E.; Boukenter, A.; Champagnon, B. Phys. ReV. Lett. 1986, 56, 2052. (22) Ristic´, M.; Ivanda, M.; Popovic´, S.; Music´, S. J. Non-Cryst. Solids 2002, 303, 270. (23) Montagna, M. Phys. ReV. B 2008, 77, 045418. (24) Kurihara, L. A.; Fujiwara, S. T.; Alfaya, R. V. S.; Gushikem, Y.; Alfaya, A. A. S.; de Castro, S. C. J. Colloid Interface Sci. 2004, 274, 579. (25) Hua, B.; Qian, G.; Wang, M.; Hirao, K. J. Sol-Gel Sci. Technol. 2005, 33, 169.

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