Point defects in fumed ~7nm-sized silica nanoparticles were studied by K- and Q-band electron spin resonance following 10eV irradiation used to photodissociate H from passivated defects. Various types of electron spin resonance-active point defects were revealed, including the familiar E’ center (generic entity ·Si≡O3), EX, the peroxy radical, the methyl radical, and an unknown closely axially symmetric center (g|| = 2.0041, g = 2.0027). The possible atomic nature of the latter was addressed. The E’ defects, occurring in a maximum density of about 10–3 per nanoparticle, were monitored as a function of thermal treatment in vacuum at 850 to 1115C in order to assess specific physicochemical structural aspects of the particles. Experimental evidence was presented for the presence of two different systems of E’ centers. The specific electron spin resonance parameters of the E’ centers of one bath were found to be very similar to those of the Eγ’ center in bulk fused silica, while the second bath exhibited a different zero crossing g-value and line-shape, attributed to variations in local structure. It was inferred that the latter E’ system applied to the outer SiO2 layers; exposing a structural nature different from bulk glassy SiO2. Besides peroxy radicals, large numbers of other O-hole type defects also appeared to be present. The exhaustive number of all O-hole centers, including the peroxy radicals, was determined to be ~0.07 defects/nanoparticle, making this type of defect highly unlikely to play a substantial role in narrowing of the optical band-gap, in contrast with previous suggestion.

Electron Spin Resonance Probing of Fundamental Point Defects in Nanometer-Sized Silica Particles. A.Stesmans, K.Clémer, V.V.Afanasev: Physical Review B, 2005, 72[15], 155335 (12pp)