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

Point defects in fumed 7-nm sized silica nanoparticles have been studied by K- and Q-band electron spin resonance (ESR) using vacuum ultra-violet irradiation to photo-dissociate H from passivated defects. Various types of ESR-active point defects are revealed, including the familiar E′ center (generic entity SiO3), EX, the peroxy radical, the methyl radical, and a newly observed approximately axially symmetric center (g∥ = 2.0041, g⊥ = 2.0027). A possible atomic structure of the latter is discussed. The E′ defects are monitored as a function of thermal treatment in vacuum in the range 850-1175 °C in order to assess specific physical and chemical 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 system are found to be very similar to those of the Eγ′ center in bulk fused silica, while the second system has an enhanced statistical distribution of pertinent ESR principal values of the g matrix, primarily g⊥, related to variations in local structure. It is inferred that the latter E′ system is located in the outer layers of the SiO2 particles, indicating a structure different from the interior of the particle, which appears similar to that of macroscopic silica samples.