Role of sol-gel networking and fluorine doping in the silica Urbach energy

Abstract

We present the results of the analysis of the ultraviolet (UV) absorption edge of fluorine-modified sol-gel silica. UV transmission data, obtained by means of synchrotron radiation, have been analyzed in the spectral range 7.5 − 8.5 eV, with a spectral resolution of about 10 meV. Data on silica samples with different F content (from 0 to few 10 −1 mol %) have been analyzed and compared with literature data on quartz and pure synthetic commercial silica. The analysis allows us to discriminate between the effects of the fluorine addition and those ascribable to structural peculiarities of the sol-gel networking. The estimated Urbach energy E U (T = 0) ranges between 45 and 55 meV, higher that in crystalline quartz and lower than in commercial synthetic silica. The study of the temperature dependence of E U (T ) shows that the fluorine modification of the silica network causes the lowering of the static disorder and the widening of the energy gap. However, there is also a relevant effect of the production process, since sol-gel silica samples show lower E U values with respect to other types of silica, quite independently of the fluorine content. The analysis of the Raman spectra however shows that the starting amount of fluorine-modified molecular precursor influences the network condensation process, independently of the final fluorine content into the matrix.

Light-emitting Ga-oxide nanocrystals in glass: a new paradigm for low-cost and robust UV-to-visible solar-blind converters and UV emitters

“…36,38 Larger values are only found in systems with defective short range order, lowered by coordination defects and/or network modier ions, as in hydrogenated Si, uori-nated silica, Si-doped AsGa, and Cu-doped CdS (X ¼ 2-8, 2-3, 17, and 8, respectively). 35,36,[38][39][40][41] The occurrence of such a high X value in g-Ga 2 O 3 NCs strongly points out the role of the interphase, which is likely responsible for a large amount of localized states caused by coordination mismatch and stoichiometry deviation. In fact, it is quite reasonable that band-to-band transitions are strongly inuenced by interphase properties, since the surface-to-volume ratio is very large.…”

Hydrodechlorination of trichloroethylene over Pd supported on swellable organically-modified silica (SOMS)

“…Several other peaks appeared around the lower wavelength region below 1000 cm -1 . The bands at 484 and 603 cm -1 are possibly related to threeor four-membered rings of the Si-O-Si symmetric stretching mode, which are typically found in silica Raman spectra [43,44]. However, the intensity of these peaks were much lower.…”

Sol-gel synthesis of fluorine-doped silica glasses with low SiOH concentrations

“…It is also unclear how liquid-phase fluorine doping can reduce the concentration of SiOH groups in solgel derived silica glasses, although samples whose SiOH concentrations are as low as 4 ppmw have been prepared by oxygen sintering of fluorinemodified gels. 18) In the present study, we report solgel preparation of fluorinedoped, chlorine-free silica glasses with low SiOH concentrations, from solutions catalyzed by HF. The SiOH concentration is found to be decreased to as low as ³10 17 cm ¹3 (³1 ppmw) by densification under a helium atmosphere, without employing post-dehydration during sintering and vacuum sintering.…”