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

Point defects in fumed ~7-nm-sized silica nanoparticles have been studied by K- and Q-band electron spin resonance (ESR) following 10-eV irradiation used to photodissociate H from passivated defects. Various types of ESR-active point defects are revealed, including the familiar E^' center (generic entity ∙SiO₃), EX, the peroxy radical (POR), the methyl radical, and an unknown closely axially symmetric center (g_=2.0041, g_⊥=2.0027). The possible atomic nature of the latter is addressed. The E^' defects, occurring in a maximum density of ~1×10^-3 per nanoparticle, are monitored as a function of thermal treatment in vacuum in the range 850--11150.2em0ex^∘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^' centers. The specific ESR parameters of the E^' centers of one bath are found to be very similar to those of the E^'_γ 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^' system pertains to the outer SiO₂ layers, exposing a structural nature different from bulk glassy SiO₂. 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 ~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.