Optical activity of Sn-variants of oxygen deficient centers in fluorine-modified silica A. Paleari a, * , S. Brovelli b , F. Meinardi a , R. Lorenzi a , A. Lauria a , N. Mochenova a , B. Vodopivec a , N. Chiodini a a CNISM – Department of Materials Science, University of Milano-Bicocca, Via Cozzi 53, I-20125 Milano, Italy b Department of Physics and Astronomy, London Centre for Nanotechnology, University College London, London WC1E 6BT, United Kingdom article info Article history: Available online 8 May 2009 PACS: 78.55.Qr 81.05.Kf 81.20.Fw Keywords: Optical spectroscopy Defects Absorption Luminescence Infra-red properties Raman spectroscopy Silica Sol–gels (xerogels) Defects Water in glass abstract Photoluminescence in fluorine-modified Sn-doped silica has been analyzed by means of synchrotron radiation in the UV and vacuum-UV, from 120 to 330 nm, looking at the optical activity of oxygen-defi- cient-centers ODC(II) in Sn-substituted cationic sites. The comparison between F-modified Sn-doped samples and previous data on F-free Sn-doped material evidences differences in the intensity of the 3.2 eV emission band excited at 3.7 eV, and in the thermal dependence of the intensity of this emission excited via intersystem crossing. The role of fluorine in modifying the optical activity of ODC(II) and in the SnO 2 clustering is discussed, showing that an efficient excitation transfer may be activated from SnO 2 to the Sn-variant of ODC(II). Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Oxygen vacancy defects in twofold coordinated cation sites are among the best characterized defects in silica [1–3] and their opti- cal activity was proved to be a sensible probe of the environment [4]. Main features of the optical activity of this type of oxygen defi- cient center, often called ODC(II), are two emission bands at about 2.7–3.2 eV (b-band) and 4.2–4.4 eV (a-band) arising from radiative decay from triplet and singlet excited levels, respectively, to the singlet ground state. Technological applications of these ultraviolet (UV) emission bands in pure and Ge- or Sn-doped silica are cur- rently investigated to obtain efficient UV-emitting devices fully compatible with the Si technology [5]. The relative intensities of the two emission bands (b- and a- bands), when excited simultaneously through transitions from the ground state to the first excited singlet level or to higher ex- cited configuration (at 5 and above 7 eV, respectively), are regu- lated by branching ratios that are dependent on thermally activated intersystem crossing processes from excited singlet to triplet levels. This kind of processes are influenced by the struc- tural features of the defect environment and reflect a statistical distribution of local parameters [6,7]. On the other hand, direct excitation of triplet state is strongly forbidden and the consequent radiative decay is often undetectable in purely intrinsic systems. Nevertheless, a weak emission at 3.1– 3.2 eV may be observed exciting directly the triplet state at 3.7 eV in extrinsic variants of the defect, when silicon is substituted by germanium or tin atoms [8,9]. In these cases, the large spin–orbit coupling of the heavier Ge and Sn substitutional atoms causes the mixing of orbital features of the defect wave functions and makes the intensity of triplet-to-singlet transition detectable. As a results of the wave function mixing, also the effects of the envi- ronment on the wave functions may have an influence on the fea- tures of the forbidden transition. Therefore, both the thermal dependence of a and b relative intensities and the intensity of the directly excited b emission are sensible probes of the structural features of defect and environment. Here we show that the Sn-var- iant of ODC(II) evidences clear peculiarities in fluorine-modified silica with respect to F-free material. The results suggest of a role of fluorine in inducing the formation of SnO 2 nanoparticles able to transfer excitation to ODC(II) sites. The great enhancement of UV emission intensity by energy transfer gives a perspective for applications as UV emitting material. 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.12.015 * Corresponding author. Tel.: +39 02 64485164; fax: +39 02 64485400. E-mail address: alberto.paleari@mater.unimib.it (A. Paleari). Journal of Non-Crystalline Solids 355 (2009) 1024–1027 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol