Controlled Growth of SnO 2 Nanocrystals in Eu 3+ -Doped SiO 2 -SnO 2 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 Eu 3+ -doped SiO 2 -SnO 2 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 SnO 2 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 SnO 2 crystals. The effect of crystallization on the optical properties and on the photoluminescence of Eu 3+ 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 SnO 2 nanocrystals in the matrix was investigated by Fourier-transform infrared spectroscopy. 1. Introduction SnO 2 is a wide-band-gap semiconductor (E g ) 3.6 eV) that is studied for many applications 1-3 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 . SnO 2 nanocrystals embedded in SiO 2 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 mecha- nisms coming from radiative and nonradiative processes. 8 Glass- ceramic 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 glass- ceramics can be prepared in a waveguiding configuration. Also, 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 SnO 2 nanocrystals in 75 SiO 2 -25 SnO 2 waveguides (WGs) doped with 1 mol % Eu 3+ . 2. Experiments 2.1. Sample Preparation. Planar WGs consisting of 75 SiO 2 -25 SnO 2 doped with 1 mol % Eu 3+ were fabricated by the sol-gel technique using dip-coating processing. The molar content of SnO 2 was chosen so as to have no phase separation and good optical and spectroscopic characteristics for a low- loss 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:H 2 O molar ratio was 1:0.01:37.9:2. 9 An ethanolic colloidal suspension, prepared using SnCl 2 · 2H 2 O and Eu(NO 3 ) 3 · 5H 2 O as precursors, was added to the solution containing TEOS. The Eu 3+ /[75 SiO 2 + 25 SnO 2 ] 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 SiO 2 (v-SiO 2 ) 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-SiO 2 , and the thin films deposited on Si were used for structural characteriza- tions. 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 * Corresponding authors. E-mail: bhaktha@unice.fr (B.N.S.B.), sylvia. turrell@univ-lille1.fr (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. J. Phys. Chem. C 2009, 113, 21555–21559 21555 10.1021/jp907764p  2009 American Chemical Society Published on Web 11/11/2009