Photoluminescence characterization of sol–gel prepared low density silica samples C.M. Carbonaro * , D. Chiriu, R. Corpino, P.C. Ricci, A. Anedda Dipartimento di Fisica, Universita ` di Cagliari, Cittadella Universitaria, SP no. 8 Km 0.700, I-09042 Monserrato (CA), Italy Available online 9 February 2007 Abstract By exciting with synchrotron radiation in the 4–10 eV range the emission properties of sol–gel synthesized porous silica at room temperature and at 8 K are investigated. The modifications of the ultraviolet emission in the range 3.7–4.2 eV excited at different energies (5.64, 6.20, 6.53, and 6.89 eV) support the hypothesis of the contribution of two emitting centers. In particular the two emis- sions are spectrally resolved when exciting at 5.64 eV and a blue shift of the emission peak is reported as the excitation energy increases. The analysis of the decay times indicates that the two centers are temporally resolved with the 5.64 eV excitation both at room temperature and at 8 K and allows to estimate lifetimes of about 3 and 18 ns. When exciting at 6.53 eV, the larger relative con- tribution of the 4.0 eV band does not allow to efficiently resolve the two decay times and the estimated life time of the 3.8 eV band is shortened. Ó 2007 Elsevier B.V. All rights reserved. PACS: 78.30.Ly; 78.47.+p; 78.55.Àm; 78.55.Mb Keywords: Optical spectroscopy; Defects; Luminescence; Silica 1. Introduction The investigation of the optical properties of mesopor- ous silica in the visible and ultraviolet (UV) energy range has recently gathered attention because of the similarities with oxidized porous silicon and silicon nanostructure and because of the possible technological applications in the optoelectronic field for visible and UV emitting sili- con/silica based devices [1–3]. Three main different emis- sion bands were reported: a green band (around 2.2–2.4 eV) [2,4–6], a blue band (at about 2.8 eV) and an UV band (in the 3.7–4.2 eV range) [7–9]. Due to the huge surface-to-volume ratio, different models of surface centers have been hypothesized, including H- and C-related centers [2,6,10,11]. Recently, the UV emission has been associated to OH interacting silanol species [8,9] and the contribution of two different kinds of these surface centers has been sug- gested in order to explain the reported composite nature of the emission [12,13]. The aim of this work is to proceed further in the analy- sis of the UV band by investigating the UV emission under different excitation energies in the UV range: the observed modifications of the photoluminescence (PL) spectra support the hypothesis of the contribution of two different centers to the examined emission. Indeed the PL spectrum shows the presence of two spectrally resolved emission bands when the excitation is set at 5.64 eV and displays a blue shift of the peak emission from 3.8 to 4.1 eV as the excitation energy increases. The time decay curves collected at 3.8 and 4.0 eV at different excitation energies also indicate the presence of two contributions. Since the contribution of the two emissions depends on the excitation energy [13], the estimated decay times display a dependence of the resolved life times on the excitation energies. 0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.10.022 * Corresponding author. Tel.: +39 0706754823; fax: +39 070510171. E-mail address: cm.carbonaro@dsf.unica.it (C.M. Carbonaro). www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 353 (2007) 550–554