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Oxygen-excess related defects in SiO2-based materials : coupling theory and experiments

  • Winkler, Blaz
Publication Date
May 07, 2019
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This work is primarily focused on application of standard first-principle compu-tational approaches to model oxygen excess related point defects in amorphoussilica. Atomic models with their respective electronic and optical properties areexplored together with some conversion mechanisms between defect models.The first chapter overviews extensive literature about the already knownproperties of oxygen related defects. Second chapter briefly introduces mainmethods that have been used in this research, in particular Density FunctionalTheory (DFT) as energy and force engine with short description of minimalenergy path (MEP) algorithm used for modeling chemical/migration reactions,GW approximation for charged electronic excitations (band structure) andBethe-Salpeter Equation (BSE) for neutral excitations (optical absorption andexcitonic structure including electron hole interaction).The third chapter is devoted to the presentation of results. Thanks to thecalculation of optical properties of peroxy bridge (POL), a correlation has beenfound between structural disorder, specifically dihedral angle dispersion, andlow coupling with light, which has been identified as main reason why no clearabsorption bands have been assigned to the POL. Structure and stability ofsome other defects, like interstitial ozone molecule (ozonyl) and dioxasilirane(silicon analogy of dioxirane), have been studied. These defects are usuallynot considered as most important species, however their calculated formationenergies are lower compared to some known defects, which indicates theymight be present in silica.From a detailed study on possible reaction mechanisms, it has been foundthat ozonyl might be one of the most important intermediate steps for oxygenexchange reactions. Results also show that dioxasilirane can be spontaneouslycreated during the interaction of oxygen with lone pair defects. By exploringdifferent reactions between oxygen and pre-existing oxygen deficiency centers(ODCs), calculations predict two kinds of passivation behaviors: single-barrierreversible mechanisms with the formation of dioxasilirane-like groups, forwhich the network keeps the memory of the precursory lone pair defects, andsingle or multiple-barrier mechanisms, for which the network loses its memory,either because of the high reverse barrier or because of a reconstruction.Final part of this research has been devoted to experimental characterizationof the response and tolerance of optical fibers loaded with oxygen under irra-diation. These include experiments on commercial fiber along with canonicalsamples (Optical fibers developed with the intention of studying correlationsbetween different fabrication parameters, dopant/impurity concentration andiidoping concentrations). Studied fibers also include rare-earth doped fibers.

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