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Electron spin resonance probing of fundamental point defects in nanometer-sized silica particles

2005; American Physical Society; Volume: 72; Issue: 15 Linguagem: Inglês

10.1103/physrevb.72.155335

1550-235X

A. Stesmans, K. Clémer, V. V. Afanas’ev,

Photochemistry and Electron Transfer Studies

Point defects in fumed $\ensuremath{\sim}7\text{\ensuremath{-}}\mathrm{nm}$-sized silica nanoparticles have been studied by $K$- and $Q$-band electron spin resonance (ESR) following $10\text{\ensuremath{-}}\mathrm{eV}$ irradiation used to photodissociate H from passivated defects. Various types of ESR-active point defects are revealed, including the familiar ${E}^{\ensuremath{'}}$ center (generic entity $\mathbf{∙}\mathrm{Si}{\mathrm{O}}_{3}$), EX, the peroxy radical (POR), the methyl radical, and an unknown closely axially symmetric center (${g}_{\ensuremath{\Vert}}=2.0041$, ${g}_{\ensuremath{\perp}}=2.0027$). The possible atomic nature of the latter is addressed. The ${E}^{\ensuremath{'}}$ defects, occurring in a maximum density of $\ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ per nanoparticle, are monitored as a function of thermal treatment in vacuum in the range $850--1115\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ in order to assess specific physicochemical structural aspects of the particles. Experimental evidence is presented for the presence of two different systems of ${E}^{\ensuremath{'}}$ centers. The specific ESR parameters of the ${E}^{\ensuremath{'}}$ centers of one bath are found to be very similar to those of the $E^{\ensuremath{'}}{}_{\ensuremath{\gamma}}$ center in bulk fused silica, while the second bath exhibits a different zero crossing $g$ value and line shape, attributed to variations in local structure. It is inferred that the latter ${E}^{\ensuremath{'}}$ system pertains to the outer $\mathrm{Si}{\mathrm{O}}_{2}$ layers, exposing a structural nature different from bulk glassy $\mathrm{Si}{\mathrm{O}}_{2}$. Besides PORs, large numbers of other oxygen-hole type defects appear to be present also. The exhaustive number of all oxygen-hole centers, including the PORs, is determined at $\ensuremath{\sim}0.07$ defects/nanoparticle, making this kind of defect highly unlikely as playing a substantial role in narrowing of the optical bandgap, in contrast with previous suggestion.