Physical Review B vol:72 issue:15 pages:155335-1-155335-15
Point defects in fumed similar to 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 center dot Si=O-3), EX, the peroxy radical (POR), the methyl radical, and an unknown closely axially symmetric center (g(parallel to)=2.0041, g(perpendicular to)=2.0027). The possible atomic nature of the latter is addressed. The E-' defects, occurring in a maximum density of similar to 1x10(-3) per nanoparticle, are monitored as a function of thermal treatment in vacuum in the range 850-1115 degrees 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'(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-' 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 similar to 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.